Multi-wavelength interferometric engineering of TiO₂-functionalized polyethylene terephthalate fibers: Electronic structure, optical properties, and antimicrobial performance

Mona A. Alhasani; Gadeer R. Ashour; Oumr A. Osra; Wajdy S. Qattan; Hatun H. Alsharief; Alia A. Alfi; Ahmed S. Badr El-din; Nashwa El-Metwaly

doi:10.25259/AJC_1295_2025

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Original Article

ARTICLE IN PRESS

doi:

10.25259/AJC_1295_2025

Multi-wavelength interferometric engineering of TiO₂-functionalized polyethylene terephthalate fibers: Electronic structure, optical properties, and antimicrobial performance

Mona A. Alhasani^a, Gadeer R. Ashour^a, Oumr A. Osra^a, Wajdy S. Qattan^b, Hatun H. Alsharief^a, Alia A. Alfi^a, Ahmed S. Badr El-din^c,d, Nashwa El-Metwaly^a,*

aDepartment of Chemistry, Faculty of Science, Umm Al-Qura University, Makkah, Saudi Arabia

bDepartment of Architecture, Faculty of Engineering and Architecture, Umm Al-Qura University, Makkah e 1, Saudi Arabia.

cDepartment of Chemistry, Faculty of Science, University of Tabuk, Tabuk, Saudi Arabia

dDepartment of Chemistry, Faculty of Science, Tanta University, Tanta, Egypt

*Corresponding author: E-mail address: n_elmetwaly00@yahoo.com (N. El-Metwaly)

Received: 2025-10-27, Accepted: 2025-12-11, Epub ahead of print: 2026-03-18,

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Abstract

The incorporation of titanium dioxide (TiO₂) nanoparticles into polyethylene terephthalate (PET) fibers represents a promising strategy for developing multifunctional materials combining enhanced optical properties with antimicrobial activity. This study employed variable wavelength automated interferometry (VAWI) with the Fourier transform method (FTM) across the visible spectrum (400-700 nm) to comprehensively characterize PET fibers functionalized with 2.0 wt% TiO₂ nanoparticles. The nanocomposite fibers demonstrated substantial property enhancements compared to pristine PET: 18.5% birefringence increase, 15.3% enhancement in parallel dispersion, 18.4% improvement in molecular orientation, and 68% increase in stress-optical coefficient. Cauchy dispersion analysis revealed enhanced wavelength-dependent optical response with dispersion anisotropy ratio increasing from 1.63 to 1.82. Antimicrobial efficacy testing via shaking flask methodology demonstrated robust broad-spectrum activity with 75.8-85.4% growth reduction against diverse pathogens, attributed to photocatalytic reactive oxygen species generation. These findings establish quantitative structure-property-performance relationships essential for rational design of multifunctional fiber materials with tailored optical, mechanical, and biological properties for advanced textiles, photonic devices, and biomedical applications.

Keywords

Antimicrobial properties

Cauchy dispersion

Electronic polarizability

Optical birefringence

PET/TiO₂ nanocomposite fibres

Stress-optical coefficient

Variable wavelength automated interferometry

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1. Introduction

The development of multifunctional polymer fiber materials has emerged as a frontier in advanced materials science, driven by the increasing demand for textiles and fiber-based products that combine enhanced mechanical performance with specialized functional properties [1-3]. Among synthetic polymers, polyethylene terephthalate (PET) occupies a position of paramount industrial importance, with global production exceeding 30 million metric tons annually for applications spanning textile manufacturing, packaging materials, automotive components, and technical fabrics [3-5]. The widespread adoption of PET in these diverse sectors stems from its exceptional combination of mechanical strength, dimensional stability, chemical resistance, thermal durability, and cost-effectiveness [6,7]. However, conventional PET fibers lack certain functional properties that are increasingly demanded in modern applications, particularly antimicrobial activity, ultraviolet protection, and self-cleaning capabilities [3,8]. These limitations have motivated extensive research into the development of functionalized PET composites that retain the polymer’s inherent advantages while incorporating additional performance characteristics.

The incorporation of inorganic nanoparticles into polymer matrices represents one of the most promising strategies for engineering multifunctional fiber materials with tailored properties [9]. Among various nanofillers, titanium dioxide (TiO₂) has attracted considerable attention due to its unique combination of photocatalytic activity, ultraviolet absorption, chemical stability, biocompatibility, and commercial availability [10,11]. TiO₂ nanoparticles exhibit well-documented antimicrobial properties arising from the generation of reactive oxygen species (ROS) under light exposure, including hydroxyl radicals (•OH), superoxide anions (O₂•⁻), and hydrogen peroxide (H₂O₂), which induce oxidative damage to bacterial cell membranes, proteins, and nucleic acids [11-14]. This photocatalytic mechanism provides broad-spectrum antimicrobial activity against both Gram-positive and Gram-negative bacteria, as well as fungi, without the limitations of conventional antimicrobial agents such as the development of microbial resistance or chemical leaching [15,16]. Additionally, TiO₂ nanoparticles provide effective ultraviolet radiation shielding through both absorption and scattering mechanisms, making TiO₂-functionalized fibers particularly suitable for outdoor textiles, sun-protective garments, and applications requiring long-term UV stability [16]. The self-cleaning properties imparted by TiO₂’s photocatalytic degradation of organic contaminants further expand the potential applications of these composite fibers to medical textiles, hygiene products, air filtration systems, and architectural fabrics [16,17]. The incorporation of titanium dioxide (TiO₂) nanoparticles into the antimicrobial, electronic, and optical functions of PET fibers expands their potential in architectural textiles and surface coatings. PET fibers would be able to transform façades, shading systems, and even the interior of buildings by providing antimicrobial, light-diffusing, and self-cleaning properties to the environment, thus fostering healthier and sustainable buildings. The photocatalytic property of TiO₂, active under light and climate responsive to pollutants, is crucial in sustainable architecture, as seen in modern self-cleaning façades. In addition, the fibers’ enhanced birefringence, improved molecular orientation, and potential use in adaptive architectural membranes, where light and performance of material hinges are critical. This work advances architecture in the integration of innovative design and health-focused environmental nanotechnology.

Traditional methods for optical characterization of polymer fibers, including ellipsometry, refractometry, and single-wavelength interferometry, typically provide limited information at discrete wavelengths and often require complex sample preparation or cannot simultaneously measure direction-dependent properties in anisotropic materials [18,19]. Recent advances in interferometric microscopy have overcome many of these limitations through the development of sophisticated techniques capable of quantitative phase imaging with high spatial resolution [18]. Among these advanced methods, variable wavelength automated interferometry (VAWI) represents a particularly powerful approach that combines phase-shifting interferometry with automated wavelength scanning and duplicated-image polarization analysis [20]. The VAWI technique, based on a modified Pluta polarizing interference microscope equipped with computer-controlled motorized wavelength selection and specialized beam-splitting optics, enables the simultaneous acquisition of orthogonally polarized interference patterns in a single measurement [20]. This duplicated-mode operation captures the optical response for light polarized both parallel and perpendicular to the fiber axis without requiring specimen rotation or repositioning, thereby eliminating systematic errors associated with sequential measurements and dramatically reducing acquisition time [20,21]. Through systematic wavelength scanning across the visible spectrum, VAWI provides comprehensive characterization of wavelength-dependent refractive indices, birefringence, dispersion properties, and derived structural parameters, including density, crystallinity, molecular orientation, and stress-optical coefficients [20].

Despite the growing body of research on polymer-TiO₂ nanocomposites and their antimicrobial properties, a significant knowledge gap exists regarding the comprehensive optical characterization of these materials and the fundamental relationships between nanoparticle incorporation, molecular structure, and optical properties. While numerous studies have demonstrated enhanced functional performance through TiO₂ addition, relatively few investigations have systematically examined how nanoparticle incorporation influences the fundamental optical constants, molecular orientation, crystalline structure, and wavelength-dependent behavior of polymer fibers [21,22]. Understanding these structure-property relationships is essential for several reasons. First, the optical properties of fibers directly impact their appearance, color, transparency, and aesthetic qualities, which are critical for textile and consumer product applications. Second, optical characterization provides a non-destructive, highly sensitive probe of molecular organization, crystallinity, and orientation that complements traditional structural characterization techniques. Third, the refractive index, birefringence, and dispersion characteristics determine the fiber’s performance in optical and photonic applications, including smart textiles, optical sensors, and light-management systems. Fourth, a quantitative understanding of how nanoparticle incorporation modifies the polymer’s electronic structure, molecular orientation, and stress-optical response is essential for rational design and optimization of nanocomposite fibers with tailored multifunctional properties.

Hence, the paper aims to establish a comprehensive understanding of how titanium dioxide nanoparticle incorporation fundamentally alters the optical, structural, and molecular organization characteristics of PET fibers through systematic multi-wavelength interferometric analysis. While the antimicrobial and UV-protective benefits of TiO₂-functionalized polymer composites have been demonstrated in previous studies, the precise mechanisms by which nanoparticles modify the polymer’s intrinsic optical constants, molecular orientation, crystalline morphology, and electronic polarizability across the electromagnetic spectrum remain inadequately characterized. This knowledge gap significantly hinders the rational design and optimization of multifunctional fiber materials, as optical properties serve as sensitive indicators of molecular-level structural organization and directly influence material performance in textile, photonic, and sensing applications. By employing the advanced VAWI technique combined with rigorous Fourier transform method (FTM) phase extraction and comprehensive theoretical analysis, this research seeks to quantitatively elucidate the structure-property relationships governing nanocomposite fiber behavior. Specifically, the study aims to determine how 2.0 wt% TiO₂ nanoparticle incorporation affects wavelength-dependent refractive indices in both parallel and perpendicular directions, optical birefringence and anisotropy, dispersion characteristics quantified through Cauchy parameters, density and crystallinity derived from optical measurements, macroscopic and molecular polarizabilities reflecting electronic structure modifications, molecular orientation quantified through Hermans orientation function, and stress-optical coupling coefficients. Furthermore, the research aims to correlate these optical property modifications with the demonstrated antimicrobial efficacy against diverse pathogenic microorganisms, thereby establishing connections between nanoparticle-induced structural changes and functional performance enhancement. The comprehensive dataset and analytical framework developed through this investigation will provide fundamental insights into polymer-nanoparticle interactions, interfacial phenomena, and orientation mechanisms during fiber formation, ultimately enabling the predictive design of next-generation multifunctional fiber materials with precisely tailored combinations of optical, mechanical, and biological properties for advanced applications in smart textiles, medical devices, environmental remediation systems, and integrated photonic platforms.

2. Materials and Methods

2.1. Theoretical considerations, phase demodulation using Fourier transform method

The optical characterization of tested materials begins with the recording of interference patterns using the VAWI device in duplicated mode. This advanced technique simultaneously captures both parallel and perpendicular interference patterns in a single acquisition. For the duplicated mode operation employed in VAWI, two separate intensity patterns are recorded simultaneously on different regions of the detector, corresponding to orthogonal polarization states [23] (equation 1):

(1)

\begin{matrix} I^{∥} (x, y) = a^{∥} (x, y) + b^{∥} (x, y) \cos (ϕ^{∥} (x, y) + 2 π f_{0} x) \\ I^{⊥} (x, y) = a^{⊥} (x, y) + b^{⊥} (x, y) \cos (ϕ^{⊥} (x, y) + 2 π f_{0} x) \end{matrix}

Here, the superscripts $∥ & ⊥$ indicate light polarized parallel and perpendicular to the fiber axis, respectively. In this equation, $a (x, y)$ represents the background intensity distribution, while b $(x, y)$ describes the modulation amplitude, which depends on the fringe visibility and local contrast. The term $φ (x, y)$ contains the phase distribution information related to the optical path difference introduced by the fiber sample, and $f_{0}$ is the carrier frequency introduced by the interferometer configuration. The extraction of phase information from the recorded intensity patterns is accomplished using the FTM [24,25]. The procedure begins with the computation of the 2D Fourier transform of the intensity distribution, which decomposes the spatial intensity pattern into its frequency components that appear at different locations in the frequency space [24]. In the frequency domain, the spectrum consists of three well-separated components: a zero-order term centered at the origin that corresponds to the DC component, a positive first-order term centered at $(+ f^{0}, 0)$ containing the desired phase information, and a negative first-order term centered at $(- f^{0}, 0),$ which is the complex conjugate of the positive term [25]. The positive first-order spectrum $B (u - f_{0}, v) e^{(i φ (x, y))}$ is isolated by applying a carefully designed bandpass filter. The filtered spectrum is then shifted to the origin to facilitate subsequent processing, and an inverse Fourier transform ( $F^{- 1} {.}$ ) is applied to return to the spatial domain, yielding complex-valued function ( $c (x, y)$ ) that contains both amplitude and phase information as described in the following equation [24] (Eqs. 2 and 3):

(2)

c (x, y) = F^{- 1} (B (u - f^{0}, v) e^{(i φ (x, y))}) = b (x, y) e^{(i φ (x, y))}

The wrapped phase map ( $φ_{w} (x, y)$ ) is obtained by computing the complex argument through [24,25],

(3)

φ_{w} (x, y) = \arctan (\frac{Im (c (x, y))}{Re (c (x, y))})

where $Im (\cdot)$ and $Re (\cdot)$ denote the imaginary and real parts of the complex function, respectively. The arctangent function constrains the extracted phase values to the principal range of (-π, π], which introduces 2π discontinuities at locations where the true continuous phase exceeds these bounds. To recover the true continuous phase distribution across the fiber sample, phase unwrapping process is implemented [26]. The unwrapping process is performed separately and independently for both polarization configurations, yielding $Δ φ^{∥} (x, y)$ representing the unwrapped phase map for $∥$ polarization and $Δ φ^{⊥} (x, y)$ representing the unwrapped phase map for $⊥$ polarization. These continuous distributions contain complete information about the optical path differences introduced by the fiber for each polarization state, and they form the foundation for all subsequent quantitative analysis.

2.2. Determination of principal and derived refractive indices and optical anisotropy from interferometric phase distribution

The refractive indices parallel and perpendicular to the fiber axis, are calculated directly from the unwrapped phase maps using the relationship between optical path difference and change in phase as described in the following equation [27] (equation 4),

(4)

\begin{matrix} n^{∥} = \frac{(λ Δ φ^{∥})}{(2 π t)} + n_{L} \\ n^{⊥} = \frac{(λ Δ φ^{⊥})}{(2 π t)} + n_{L} \end{matrix}

In these expressions, $λ$ represents the wavelength of the incident light used in the interferometric measurement, $t$ is the fiber thickness or diameter measured along the direction of light propagation, and $n_{L}$ is the refractive index of the surrounding medium. The optical birefringence is calculated directly from the difference between parallel and perpendicular refractive indices through [28] (Eq. 5),

(5)

Δ n = n^{∥} - n^{⊥}

For an optically anisotropic material, where the refractive index varies with direction due to molecular orientation, the isotropic refractive index ( $n_{i s o}$ ) must first be calculated as an appropriate average of the directional values. This isotropic refractive index is given by [29] (Eq. 6)

(6)

n_{i s o} = \frac{(n^{∥} + 2 n^{⊥})}{3}

Where the perpendicular component is weighed twice as heavily as the parallel component to account for the fact that there are two independent perpendicular directions but only one parallel direction in the cylindrically symmetric fiber geometry.

2.3. Multi-wavelength analysis and dispersion properties

By repeating the complete measurement and analysis procedure at multiple wavelengths using the variable wavelength capability of the VAWI system, the wavelength-dependent optical properties of the fiber are systematically characterized. This multi-wavelength approach provides critical information about the dispersion behavior of the material, which reflects the underlying electronic structure and the frequency-dependent response of the polymer to electromagnetic radiation [30]. The dispersion properties are essential for understanding how the fiber will behave across the electromagnetic spectrum and for applications requiring broadband optical performance. The wavelength dependence of the refractive index is described by Cauchy’s empirical dispersion relation, a power series expansion that is highly effective for characterizing normal dispersion in transparent materials away from absorption edges [30]. For the visible spectrum, a two-term form is often sufficient (Eq. 7):

(7)

n (λ) = A + \frac{B}{λ 2}

where $n (λ)$ is the refractive index at wavelength $λ$ , $A$ is Cauchy’s constant representing the limiting refractive index at infinite wavelength, and $B$ is the dispersion coefficient that describes the strength of the wavelength dependence. The constant $A$ relates to the material’s low-frequency electronic polarizability, while $B$ is influenced by the strength and density of electronic absorption transitions in the UV region. This model is widely applied in optical material science for its simplicity and physical interpretability, particularly for polymers and nanocomposites in the transparent spectral window [31]. The Cauchy constants A and B are determined separately for both parallel and perpendicular orientations through linear regression analysis, plotting measured refractive indices versus inverse wavelength squared. The y-intercept provides the A constant, while the slope yields the B coefficient. Correlation coefficients R² > 0.999, typically observed for polymer fibers, validate the Cauchy equation’s applicability across the visible spectrum [31]. For anisotropic materials, the dispersion anisotropy ratio quantifies directional differences in wavelength-dependent response, providing information complementary to birefringence measurements about molecular orientation effects and, in nanocomposites, potential nanoparticle orientation or anisotropic interfacial regions.

2.4. Density determination using Lorentz-Lorenz equation

The density of the fiber material is calculated using the fundamental Lorentz-Lorenz equation, which relates the refractive index to the material density through the molar refraction. This relation connects this isotropic refractive index to the material density through [29,31] (Eq. 8)

(8)

\frac{(n^{2}_{i s o} - 1)}{(n^{2}_{i s o} + 1)} = K \times ρ

Where ρ is the bulk density in g cm^-3, and K is a constant equal to R/M, where R is the molar refraction, and M is the molecular weight of the repeating unit (192 g mol^-1 for PET). The constant K is determined using reference values for amorphous PET (n_am = 1.576, ρ_am = 1.335 g cm^-3), yielding K = 0.2479. The application of the Lorentz-Lorenz equation to PET/TiO₂ nanocomposites is justified, given the homogeneous dispersion of nanoparticles at the 2.0 wt% level and the effective medium behavior observed in our optical measurements. The equation provides an ‘optical density’ derived from the composite’s overall polarizability. However, it is important to note the limitations of this approach: (1) it assumes a homogeneous internal field, which may be perturbed at polymer-nanoparticle interfaces, and (2) the calculated density represents a weighted average of the polarizabilities of both PET and TiO₂ phases. Therefore, the derived density and subsequent crystallinity values should be interpreted as ‘apparent’ properties that reflect the composite’s effective optical response rather than the absolute physical density measured by techniques like density gradient columns.

2.5. Crystallinity and morphological parameters

The mass fraction crystallinity of the fiber is determined from the calculated density using the two-phase model, which assumes that the fiber consists of a mixture of crystalline and amorphous regions, each with characteristic density values, and that the overall density represents a weighted average of these two phases. The crystallinity is calculated through [29,32] (Eq. 9)

(9)

χ_{m} = \frac{(ρ - ρ_{a})}{(ρ_{c} - ρ_{a})}

where $ρ_{a}$ = 1.335 g/cm³ is the density of the amorphous phase, and $ρ_{c}$ = 1.455 g cm^-3 is the density of the crystalline phase. The calculated mass fraction crystallinity $χ_{m}$ represents the proportion of the fiber mass that exists in the crystalline state, while the complementary mass fraction of amorphous regions is simply. $χ_{a}$ = 1 - $χ_{m}$ .

2.6. Polarizability parameters

The macroscopic polarizability per unit volume is calculated using the Lorentz-Lorenz relation expressed as [29] (Eq. 10)

(10)

Φ = \frac{(n^{2} - 1)}{(n^{2} + 2)}

This equation is applied separately for $n^{∥}$ , $n^{⊥}$ and $n_{i s o}$ to obtain $Φ^{∥}$ , $Φ^{⊥}$ and $Φ_{i s o}$ , respectively. These parameters describe the macroscopic polarization response of the material to an applied electric field. The molecular polarizability of individual monomer units ( $α_{i}$ ) is determined through the relationship [29] (Eq. 11):

(11)

α_{i} = (\frac{3}{4 π N}) \times \frac{(n_{i}^{2} - 1)}{(n_{i}^{2} + 2)}

where N is the number of molecules per unit volume, calculated as N = (ρ × Nₐ)/M, with Nₐ = 6.022 × 10²³ mol⁻¹ being Avogadro’s number. The molecular polarizabilities $α^{∥}$ and $α^{⊥}$ are calculated for the parallel and perpendicular directions, respectively, and are reported in cubic Angstroms (Å³). The optical configuration parameter, $Δ α = α^{∥} - α^{⊥}$ , quantifies the anisotropy in molecular polarizability. The polarizability constant $[Δ α / 3 α_{0}]$ , where $α_{0} = (α^{∥} + 2 α^{⊥}) / 3$ is a structure constant that depends on the molecular orientation and provides insight into the electronic polarizability behavior.

2.7. Molecular orientation

The degree of molecular orientation in the fiber, which is the primary factor determining its optical and mechanical anisotropy, is quantified through Hermans’ orientation factor expressed as [33] (Eq. 12)

(12)

f_{Δ} = \frac{Δ n}{Δ_{0}}

Where $Δ n$ is the measured birefringence and Δ₀ = 0.23 is the intrinsic birefringence of perfectly aligned PET. The orientation angle θ represents the average angle between polymer chain segments and the fiber axis, providing a geometric interpretation of molecular alignment that is often more intuitive than the orientation factor. This angle is calculated from the orientation factor through [34] (Eq. 13)

(13)

θ = \sin^{- 1} (\sqrt{\frac{2}{3}} (1 - f_{Δ}))

For perfectly aligned chains, θ = 0°, indicating that chain segments are parallel to the fiber axis, while for a random, unoriented sample, θ = 54.7°, corresponding to the tetrahedral angle.

2.8. Optical stress coefficient

The optical stress coefficient provides a quantitative relationship between mechanical stress applied to the fiber and the resulting change in optical birefringence, establishing a connection between the mechanical and optical properties of the material. This coefficient is calculated using [29] (Eq. 14),

(14)

C_{s t r e s s} = (\frac{3 π}{45 K T}) \times (\frac{({\bar{n}}^{2} + 2)}{2}) \times (α^{∥} - α^{⊥})

where K = 1.381 × 10⁻²³ J K^-1 is Boltzmann’s constant, T = 298.15 K is the absolute temperature, and $\bar{n} = (n^{∥} + n^{⊥}) / 2$ is the average refractive index. The complete analytical workflow has been schematically illustrated in Figure 1, which provides an overview of the sequential processing steps from raw interferogram acquisition through final parameter extraction.

Schematic workflow diagram illustrating the complete multi-wavelength interferometric analysis methodology.

2.9. Methodology and materials

To perform a full characterization of the tested fiber through wavelength-dependent interferometric analysis, the VAWI system was set up. The experimental apparatus is built around a Pluta polarizing interference microscope (developed by Professor Maksymilian Pluta at the Polish Academy of Sciences in the 1970s-1980s [20,21] configured for transmitted light operation. This sophisticated instrument, described comprehensively in Pluta’s foundational treatises [18], employs shearing interference microscopy principles specifically adapted for quantitative phase analysis of anisotropic materials. The optical train begins with a white light source that provides broad-spectrum illumination, which subsequently passes through a computer-controlled motorized interference filter assembly. This filter system, driven by precision stepper motor technology, enables automated wavelength scanning across the entire visible spectrum from 400 to 700 nm, eliminating the need for manual intervention and ensuring highly reproducible measurements at precisely selected wavelengths. The monochromatic light emerging from the filter is then linearly polarized by the first polarizer before entering the condenser lens system, which focuses the illumination onto the specimen stage, where the fiber sample is mounted in a refractive index-matching medium. The distinctive capability of the VAWI technique lies in its implementation of specialized beam-splitting optics integrated within the microscope optical path, specifically employing a Wollaston prism that functions as a birefringent beam divider [20]. This critical optical component simultaneously splits the transmitted light into two orthogonally polarized beams, creating duplicated images of the fiber specimen within a single field of view. One image corresponds to light with its electric vector oscillating parallel to the fiber axis, while the second image simultaneously captures the optical response when the light vector oscillates perpendicular to the fiber axis [20].

This dual-image configuration represents a significant advancement over conventional interferometric methods, as it enables the simultaneous acquisition of directionally dependent optical properties in a single measurement without requiring specimen rotation or repositioning, thereby substantially reducing measurement time and eliminating potential sources of systematic error associated with sequential measurements. The objective lens collects the transmitted light from the specimen and collimates it before the beam enters the Wollaston prism for polarization-dependent splitting. After passing through the beam splitter, both orthogonally polarized beams traverse an analyzer (crossed polarizer) that is oriented to maximize interference contrast, allowing the characteristic fringe patterns to develop. These interference fringes arise from the optical path differences introduced by the fiber material relative to the surrounding medium, with the fringe displacement being directly related to the refractive indices along the respective polarization directions. The resulting microinterferograms are captured by a high-resolution CCD camera that is interfaced with a computer-based image acquisition and analysis system. The digital imaging system records the interference patterns at each wavelength position during the automated spectral scan. The computer control system orchestrates the entire measurement sequence by synchronizing the motorized filter movement with image capture, ensuring that interferograms are systematically recorded at predefined wavelength intervals throughout the visible spectrum.

The experimental investigation utilized two distinct fibrous materials: pristine PET fibers and titanium dioxide-functionalized PET composite fibers (PET/TiO₂). The baseline PET fibers were manufactured through melt-spinning extrusion of virgin PET polymer chips (semicrystalline thermoplastic polyester). Conventional PET fibers are extensively utilized across diverse industrial sectors, including textile manufacturing for apparel and home furnishings, packaging materials, automotive interior components, and industrial fabrics for filtration and geotextile applications [2,3]. Their widespread adoption stems from their excellent mechanical strength, dimensional stability, chemical resistance, and cost-effectiveness [5]. In contrast, the PET/TiO₂ composite fibers were fabricated by incorporating titanium dioxide (TiO₂) nanoparticles into the polymer matrix prior to the melt-spinning operation. The nanoparticles, of the rutile phase, had the following specifications: a primary particle size of 20–50 nm (as per manufacturer’s data), a specific surface area of 45–55 m² g^-1, and a purity of ≥99.0%. To enhance compatibility with the PET matrix and ensure a uniform dispersion, the nanoparticles were surface-treated with aluminum oxide and an organic coating. A concentration of 2.0 wt% of these functionalized TiO₂ nanoparticles was used relative to the total mass of the PET polymer chips. The melt-spinning parameters were subsequently optimized to achieve a homogeneous distribution of nanoparticles throughout the fiber structure and to prevent aggregation. These modified fibers exhibit enhanced ultraviolet (UV) protection capabilities, making them highly suitable for outdoor textiles, sun-protective garments, and awning materials [10,11]. Additionally, the photocatalytic activity of TiO₂ enables self-cleaning and antimicrobial functionalities, rendering these fibers particularly valuable for medical textiles, hygiene products, air filtration systems, and environmental remediation applications. Titanium dioxide also contributes to improved whiteness and opacity, which is advantageous for decorative and industrial textile applications [14,16]. For each fiber type (blank PET and PET/TiO₂ composite) and each wavelength, measurements were performed on five independent regions of the fiber specimen (n=3) to assess spatial uniformity and measurement reproducibility. The reported refractive indices and derived parameters represent mean values. The standard deviation for refractive index measurements was typically ±0.0008-0.0015, reflecting the high precision of the interferometric technique.

While this study focuses on comprehensive optical characterization through multi-wavelength interferometry, we acknowledge that complementary morphological and structural characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) would provide valuable independent validation of nanoparticle dispersion quality, crystallinity measurements, and morphological features. Future investigations will incorporate these techniques to provide comprehensive multi-method characterization. The current study deliberately focuses on demonstrating the power of VAWI as a non-destructive, highly sensitive optical probe for rapid characterization of polymer nanocomposite fibers, with optical constants serving as indicators of molecular-level structural organization.

3. Results and Discussions

3.1. Assessment of antimicrobial efficacy of PET/TiO₂ composite fibers via shaking flask methodology

The antimicrobial properties of PET/TiO₂ composite fibers were assessed employing the shaking flask technique, a well-established protocol for quantifying bacterial inhibition [10]. This methodology enables the determination of microbial growth reduction rates (Rp %) when exposed to fibrous materials. The experimental protocol involved placing 0.6 g aliquots of both test specimens (PET/TiO₂ composite fibers) and control samples (pristine PET fibers) into separate Erlenmeyer flasks containing 20 mL of sterile physiological saline solution. Each flask was subsequently inoculated with 1 mL of standardized microbial suspension. The inoculated systems were then subjected to continuous agitation in a rotary shaking incubator maintained at 37°C ± 1°C for 24 h. Microbial proliferation was quantified spectrophotometrically by measuring optical density (O.D.) at 600 nm. It is important to note that O.D. at 600 nm represents light scattering (turbidity) caused by suspended microbial cells rather than true molecular absorbance. The O.D. value is directly proportional to cell concentration and biomass: higher O.D. indicates greater cell density (more turbidity), while lower O.D. indicates fewer viable cells due to growth inhibition or cell death. Therefore, effective antimicrobial activity manifests as reduced O.D. values in treated samples compared to controls. Higher absorbance values correlate with increased turbidity and cellular biomass concentration.

The antimicrobial efficacy assessment was conducted with an experimental design and statistical analysis to ensure reproducibility and reliability of results. Each experimental condition (control PET fibers and PET/TiO₂ composite fibers) was tested in triplicate (N = 3) against each of the five test microorganisms, using freshly prepared microbial suspensions for each replicate to minimize batch-to-batch variation. Optical density measurements at 600 nm were performed using a UV-Vis spectrophotometer (Shimadzu UV-1800, Shimadzu Corporation, Kyoto, Japan) with 1 cm path length quartz cuvettes, with baseline correction using sterile physiological saline solution as blank. The antimicrobial activity data were analyzed using one-way analysis of variance (ANOVA) to compare the mean optical density values between control and composite fiber groups for each microorganism. The null hypothesis tested was that there is no significant difference in microbial growth (as measured by O.D.) between control PET and PET/TiO₂ composite treatments. Statistical analyses were performed using MATLAB R2023b with a significance threshold set at α = 0.05. The F-statistic and associated p-values were calculated to assess the statistical significance of observed differences. The experimental data presented in Figure 2 demonstrate substantial antimicrobial efficacy of the TiO₂-incorporated PET fibers against a diverse panel of clinically relevant microorganisms, encompassing both Gram-positive and Gram-negative bacterial species, as well as a representative fungal pathogen. Error bars in this figure represent standard deviation from triplicate measurements (N=3).

Antimicrobial efficacy of PET/TiO₂ composite fibers assessed by shaking flask methodology.

From this figure, the antimicrobial efficacy data demonstrate robust broad-spectrum activity of PET/TiO₂ composite fibers across phylogenetically diverse microorganisms, with growth reduction ranging from 75.8% to 85.4%. Escherichia coli exhibited the highest susceptibility (85.4%), attributed to its thinner peptidoglycan layer, rendering it more vulnerable to reactive oxygen species (ROS) generated by TiO₂ photocatalytic activity. Gram-positive bacteria (S. aureus and B. subtilis) demonstrated substantial inhibition (82.6% and 80.5%, respectively), confirming that ROS effectively penetrates thick peptidoglycan layers to oxidize cellular components. Pseudomonas aeruginosa showed comparatively lower reduction (77.6%) due to its intrinsic resistance mechanisms, including impermeable outer membrane, biofilm formation capacity, and robust antioxidant defense systems. Candida albicans exhibited the lowest yet clinically significant reduction (75.8%), reflecting the structural complexity of eukaryotic cells with rigid chitinous walls, larger cell dimensions, and compartmentalized organization. The antimicrobial mechanism involves TiO₂-mediated generation of hydroxyl radicals (•OH), superoxide anions (O2•−), and hydrogen peroxide (H₂O₂) upon light exposure, inducing non-specific oxidative damage to membrane lipids, proteins, and DNA. Statistical results for each microorganism demonstrated highly significant antimicrobial activity: Bacillus subtilis (F = 243.7, p < 0.001); Escherichia coli (F = 387.6, p < 0.001); Staphylococcus aureus (F = 298.4, p < 0.001); Pseudomonas aeruginosa (F = 218.9, p < 0.001); and Candida albicans (F = 142.8, p < 0.001). The exceptionally high F-statistics (ranging from 142.8 to 387.6) and very low p-values (all p < 0.001) provide strong statistical evidence that PET/TiO₂ composite fibers exhibit highly significant antimicrobial activity compared to control PET fibers across all tested organisms.

The shaking flask O.D. measurement approach employed in this study represents a well-established screening method for antimicrobial activity assessment, providing quantitative evaluation of microbial growth inhibition. However, this method has inherent limitations that should be acknowledged. Optical density measurements reflect total cell biomass (including both viable and non-viable cells) rather than directly quantifying viable cell counts. Therefore, O.D. reduction could result from either bactericidal effects (cell death), bacteriostatic effects (growth inhibition without killing), or cell lysis and fragmentation. To definitively establish the antimicrobial mechanism and confirm cell viability, future work should incorporate complementary techniques. These include colony-forming unit (CFU) enumeration for direct quantification of culturable cells, Live/Dead fluorescence staining to distinguish membrane-intact from compromised cells, metabolic activity assays to assess cellular function, and SEM to visualize morphological damage. Despite these limitations, the substantial and statistically significant O.D. reductions observed (75.8-85.4%, p < 0.001) across all tested organisms provide strong evidence of robust antimicrobial efficacy.

3.2. Multi-wavelength optical characterization and molecular orientation of undrawn PET fiber

3.2.1. Phase extraction and refractive index and birefringence calculations for blank PET fiber

The optical characterization of undrawn blank PET fiber commenced with the acquisition of interference patterns using the VAWI system configured in duplicated mode. Figure 3a shows one of the captured interference patterns at a selective wavelength of 620 nm. The duplicated interference patterns, captured simultaneously for both parallel and perpendicular polarization orientations relative to the fiber axis. The upper portion of the interferogram corresponds to light polarized parallel to the fiber axis, revealing the optical response along the molecular chain direction, while the lower portion corresponds to perpendicular polarization, capturing the optical properties in the transverse direction.

(a) Duplicated interference patterns of blank PET fiber captured, at selective wavelength 620 nm, simultaneously in VAWI mode, showing upper portion corresponding to light polarized parallel to fiber axis and lower portion corresponding to perpendicular polarization; (b) Wrapped phase maps for both polarization orientations; (c) Unwrapped phase maps.

The extraction of quantitative phase information from the recorded interference patterns was accomplished using the FTM. The computational procedure began with the application of a 2D Fast Fourier Transform to the digitized intensity distributions for both polarization states, decomposing the spatial fringe patterns into their constituent frequency components. Through selective bandpass filtering, the positive first-order spectrum was isolated. The filtered spectral component was subsequently shifted to the origin, and the inverse Fourier transform was applied to reconstruct the complex-valued spatial distribution as expressed in Eq. (2). From this complex function, the wrapped phase map was calculated according to Eq. (3), yielding phase values constrained to the principal range of -π to +π radians as displayed in Figure 3b. To recover the true continuous phase distribution required for quantitative refractive index calculations, a phase unwrapping algorithm was implemented. This computational process systematically identifies and corrects the 2π discontinuities. The resulting unwrapped phase maps have been presented in Figure 3c.

The principal refractive indices parallel and perpendicular to the fiber axis were calculated directly from the unwrapped phase distributions using Eq. (4). The complete spectral characterization of the blank PET fiber was achieved by systematically repeating the measurement procedure at multiple wavelengths spanning the visible spectrum from 400 nm to 700 nm, with wavelength increments of 20 nm. This multi-wavelength approach was facilitated by the automated motorized interference filter assembly of the VAWI system, which enabled precise wavelength selection and scanning without manual intervention, ensuring high reproducibility and minimizing systematic errors associated with wavelength-dependent optical alignment variations. The optical birefringence was subsequently calculated, for each wavelength, as the difference between parallel and perpendicular refractive indices according to Eq. (5). According to Eq. (6), the isotropic refractive index of blank PET was calculated. The wavelength dependence of the parallel refractive index ( $n^{∥}$ ), perpendicular refractive index ( $n^{⊥}$ ), isotropic refractive index ( $n_{i s o}$ ), and birefringence ( $Δ n$ ) for the blank PET fiber have been presented in Figure 4. The spectral dispersion curves exhibited normal dispersion behavior, with both $n^{∥}$ and $n^{⊥}$ decreasing monotonically from 400 nm to 700 nm. The spectral dispersion curves exhibited normal dispersion behavior, with both parallel and perpendicular refractive indices decreasing monotonically from 400 nm to 700 nm. At 400 nm, the parallel and perpendicular indices were 1.728 and 1.572, respectively (birefringence: 0.156). At the standard wavelength of 620 nm, these values were 1.693 and 1.552 (birefringence: 0.141), while at 700 nm they decreased to 1.692 and 1.550 (birefringence: 0.142). This represents a 2.1% decrease for parallel and 1.4% for perpendicular indices across the spectrum.

Wavelength-dependent optical properties of undrawn blank PET fiber across visible spectrum (400-700 nm): parallel refractive index, perpendicular refractive index, optical birefringence, and isotropic refractive index (Connecting lines serve as guides to the eye).

The isotropic refractive index decreased from 1.624 at 400 nm to 1.597 at 700 nm (1.599 at 620 nm). The birefringence showed modest wavelength dependence (0.142-0.156), indicating both indices decrease at similar rates, maintaining relatively constant optical anisotropy. The stronger dispersion in the blue-violet region reflects proximity to UV absorption bands. The parallel direction exhibited greater absolute dispersion than the perpendicular direction, correlating with anisotropic chromophoric group arrangement along oriented polymer chains. The significant birefringence (parallel exceeding perpendicular by ∼9.1% at 620 nm) reveals considerable molecular alignment along the fiber axis, even in undrawn PET fiber, resulting from extensional flow during melt spinning and subsequent cooling under tension.

3.2.2. Spectral dispersion and Cauchy analysis of optical constants for blank PET fiber

To quantitatively describe the wavelength-dependent refractive index behavior and extract fundamental dispersion parameters, Cauchy’s empirical dispersion equation (Eq. 7) was applied to the measured spectral data. This classical relation expresses refractive index as a power series function of wavelength. Cauchy constants were determined for both principal orientations through linear regression, plotting measured refractive indices against inverse wavelength squared (λ⁻²) and fitting a straight line using least-squares optimization as displayed in Figure 5a. The excellent linearity (correlation coefficients >0.999 for both orientations) confirms that the Cauchy equation accurately represents dispersion behavior across the visible spectrum. Linear regression yielded A^∥ = 1.675 and B^∥ = 8,500 nm² for the parallel direction. The constant A^∥ represents the high-frequency refractive index limit, directly related to electronic polarizability at low photon energies, where electromagnetic radiation interacts weakly with the electronic structure without approaching resonant absorption transitions. This value characterizes the intrinsic optical density of oriented PET chains along their longitudinal axis, reflecting cumulative electronic oscillator contributions at sub-resonance frequencies. The dispersion coefficient B^∥ = 8,500 nm² quantifies wavelength-dependent strength and relates to UV electronic transition distribution and oscillator strengths. The large positive value indicates substantial normal dispersion, with refractive index increasing significantly toward the blue-violet region as photon energies approach electronic transitions.

(a) Cauchy dispersion analysis for undrawn blank PET fiber showing linear relationship between measured refractive indices and inverse wavelength squared (λ⁻2) for both principal orientations (Solid lines represent linear regression fits). (b-d) Wavelength-dependent bulk density, mass fraction crystallinity and amorphous content, macroscopic polarizability per unit volume of undrawn blank PET fiber, respectively (Connecting lines serve as guides to the eye).

In the perpendicular direction, Cauchy parameters were A^⊥ = 1.540 and B^⊥ = 5,200 nm². The lower A^⊥ reflects reduced transverse optical density from lower polarizability perpendicular to oriented polymer chains. The aromatic rings and carbonyl groups dominating PET’s optical properties possess inherent molecular-level polarizability anisotropy, with higher polarizability along the conjugated backbone, which manifests as macroscopic optical anisotropy when chains align along the fiber axis. The lower B^⊥ indicates weaker perpendicular wavelength dependence. This dispersion strength difference, visible as different slopes in Figure 5a, demonstrates that both average refractive index and dispersion characteristics are anisotropic. The ratio B^∥/B^⊥ = 1.63 exceeds A^∥/A^⊥ = 1.088, indicating molecular orientation affects wavelength dependence more strongly than average refractive index. This suggests electronic transitions responsible for visible-region dispersion are themselves anisotropic, with oscillator strengths and transition energies depending on incident light polarization relative to molecular orientation. Molecularly, this reflects transition dipole moment orientation: π-π* transitions of aromatic rings have in-plane dipole moments that become direction-dependent when rings align through chain orientation.

3.2.3. Derived structural and electronic properties: Density, crystallinity, and polarizability for blank PET fiber

The bulk density of blank PET fiber was calculated from measured refractive indices using the Lorentz-Lorenz relation (Eq.8). This required calculating the isotropic refractive index from Equation (6), where the perpendicular component is weighted twice to account for cylindrical fiber symmetry. The Lorentz-Lorenz equation relates refractive index squared to density through a proportionality constant depending on molar refractivity and molecular weight. For PET (repeat unit: 192.17 g/mol), K = 0.2479 was determined using reference values for amorphous PET. Figure 5b presents the wavelength-dependent calculated density, showing a systematic decrease from 1.3805 g cm^-³ at 400 nm to 1.3784 g/cm^-³ at 700 nm (1.3792 g cm^-³ at 620 nm), representing ∼0.15% variation. The calculated density falls between fully amorphous (ρ_a= 1.335 g cm^-³) and fully crystalline (ρ_c= 1.455 g cm^-³) values, indicating a two-phase mixture of crystalline and amorphous regions characteristic of semicrystalline polymers.

The mass fraction crystallinity of blank PET fiber was determined from calculated density using the two-phase model (Eq. 9). Figure 5c presents wavelength-dependent crystallinity and amorphous content. Crystallinity decreased systematically from 37.92% at 400 nm to 36.17% at 700 nm (36.83% at 620 nm), averaging ∼37%. Amorphous content increased from 62.08% to 63.83% (63.17% at 620 nm), averaging ∼63%. The ∼1.75 percentage point wavelength variation is an artifact from wavelength-dependent calculated density combined with assumed wavelength-independent phase densities. The average 37% crystallinity is physically meaningful, representing the actual crystalline content of the undrawn fiber. This modest crystallinity is characteristic of as-spun PET fibers undergoing strain-induced crystallization during spinning without additional heat treatment or drawing. During melt spinning, molten polymer rapidly cools under extensional stress from take-up, inducing molecular orientation and crystallization, but short crystallization time during rapid cooling limits crystal formation. The structure consists of small, imperfect crystallites (tens of nanometers) embedded in a continuous amorphous matrix.

The macroscopic polarizability per unit volume, quantifying electron distribution distortion by applied electric fields, was calculated using the Lorentz-Lorenz relation (Eq.10) for parallel, perpendicular, and isotropic orientations. Figure 5d presents wavelength-dependent polarizability for all three orientations. At 620 nm, parallel polarizability was Φ^∥ = 0.36423, perpendicular was Φ^⊥ = 0.32145, and isotropic was Φ_iso = 0.33568. These dimensionless quantities describe fractional electron distribution change per unit electric field, with higher values indicating greater polarizability and stronger electromagnetic interaction. The substantial parallel-perpendicular difference reflects oriented molecular group arrangement, with polymer chains, aromatic rings, and carbonyl groups preferentially aligned along the fiber axis. The wavelength dependence shows expected increasing values toward shorter wavelengths, consistent with normal dispersion. Parallel polarizability increased from 0.36423 at 620 nm to 0.37165 at 400 nm (∼2.0%), perpendicular polarizability increased from 0.32145 to 0.32754 (∼1.9%), and isotropic polarizability increased from 0.33568 to 0.34224. This systematic increase toward shorter wavelengths reflects proximity to UV electronic resonances. As photon energy approaches aromatic π-π* and carbonyl n-π* transition energies, polarizability increases through resonance enhancement, with electrons more easily displaced at near-resonance frequencies.

3.2.4. Quantification of molecular orientation and stress-optical response for blank PET fiber

Molecular orientation within blank PET fiber was quantified using the Hermans orientation function (Eq. 12), providing a normalized chain alignment measurement from zero (random) to unity (perfect parallel alignment). The orientation angle θ, representing the average angle between polymer segments and fiber axis, was calculated from the Hermans function using Eq. (13). Figure 6a presents the wavelength-dependent Hermans orientation function and the corresponding orientation angle. At 620 nm, f_Δ = 0.6130, indicating ∼61.3% of the theoretical maximum orientation achieved in the undrawn fiber.

(a) Wavelength-dependent hermans orientation function, corresponding orientation angle (θ), and (b) optical stress coefficient for undrawn blank PET fiber, respectively (Connecting lines serve as guides to the eye).

This substantial orientation, remarkable for fiber without intentional drawing, reflects significant molecular alignment developing naturally during melt spinning from extensional flow in spinneret capillary, rapid cooling/solidification, and mechanical tension during take-up. Values ranged from f_Δ = 0.6233 at 400 nm to 0.6117 at 700 nm (∼1.9% variation), arising from wavelength-dependent birefringence combined with assumed wavelength-independent intrinsic birefringence. The orientation angle θ was 24.09° at 620 nm, indicating average chain segments tilted ∼24° from perfect parallel alignment. This small angle confirms substantial orientation (0° = perfect alignment; 54.74° = random). Chain segments are distributed in an orientation cone centered on the fiber axis, with the majority within ±24° of the axial direction. Values ranged from 23.25° at 400 nm to 24.19° at 700 nm (∼1° increase), inversely related to the orientation function wavelength dependence. The slight wavelength dependence suggests that different electronic transitions contributing preferentially at different wavelengths may experience subtly different orientation degrees. Aromatic ring π-π* transitions dominating at shorter wavelengths may exhibit slightly stronger orientation-induced anisotropy compared to carbonyl n-π* and aliphatic ethylene glycol σ-σ* transitions contributing more at longer wavelengths. This reflects PET’s complex electronic structure and different orientational distributions of chromophoric groups. The substantial orientation affects mechanical, thermal, and transport properties beyond optical properties.

The optical stress coefficient, quantifying the relationship between applied mechanical stress and resulting birefringence change, was calculated using Eq. (14) to characterize the blank PET fiber’s stress-optical behavior. Figure 6b presents the wavelength-dependent optical stress coefficient. At 620 nm, C_stress = 3.5477x10^-12 TPa⁻¹, with inverse terapascal units reflecting birefringence change per applied stress. This positive value indicates tensile stress along the fiber axis increases birefringence, corresponding to enhanced molecular orientation in the stress direction. The large coefficient magnitude reflects high optical property sensitivity to mechanical deformation, arising from ease of polymer chain segment orientation under stress, particularly in amorphous regions. The stress-optical coefficient decreased from 3.6142 × 10^-12 TPa⁻¹ at 400 nm to 3.5389 × 10^-12 TPa⁻¹ at 700 nm (∼2.1% variation), reflecting the combined wavelength-dependent influence of mean refractive index and molecular polarizability anisotropy in the theoretical expression. The decrease toward longer wavelengths follows from these constituent parameters’ wavelength dependence. The smooth systematic variation confirms the internal consistency of optical measurements and derived parameters.

3.3. Multi-wavelength optical characterization and molecular orientation of undrawn PET fiber

3.3.1. Phase extraction and refractive index and birefringence calculations for PET/TiO₂ nanocomposite fiber

The optical characterization of the PET/TiO₂ nanocomposite fiber (2.0 wt% titanium dioxide nanoparticles uniformly distributed) followed the identical protocol established for the blank PET fiber. Figure 7a presents duplicated interference patterns captured simultaneously for parallel and perpendicular polarizations using the VAWI system at a selective wavelength of 620 nm. Duplicated imaging captured both polarization states simultaneously (upper: parallel, lower: perpendicular). Phase extraction for PET/TiO₂ followed the Fourier Transform Method. Figures 7b and 7c show the obtained wrapped phase and unwrapped phase maps of the PET/TiO₂ nanocomposite fiber, respectively. The parallel refractive index, perpendicular refractive index, birefringence, and isotropic refractive index for the PET/TiO₂ composite fiber were calculated from unwrapped phase distributions using Eqs. (4-6). Systematic multi-wavelength measurements (400-700 nm) enabled comprehensive characterization of nanocomposite wavelength-dependent optical properties. Figure 8 presents wavelength-dependent $n^{∥}$ , $n^{⊥}$ , $Δ n$ and $n_{i s o}$ . Spectral dispersion curves reveal pronounced normal dispersion with both principal indices decreasing monotonically from blue-violet to red.

(a) Duplicated interference patterns of PET/TiO₂ nanocomposite fiber captured simultaneously, showing parallel (upper) and perpendicular (lower) polarization orientations at selective wavelength 620 nm; (b) Wrapped phase maps extracted; (c) Unwrapped continuous phase maps.

Wavelength-dependent optical properties of PET/TiO₂ nanocomposite fiber across visible spectrum (400-700 nm): parallel refractive index, perpendicular refractive index, optical birefringence, and isotropic refractive index (Connecting lines serve as guides to the eye).

At 400 nm, the parallel index reached 1.7452, and the perpendicular index attained 1.5617 (birefringence: 0.1835). At 620 nm, parallel index was ∼1.7087, perpendicular ∼1.5416 (birefringence: ∼0.1671), representing 0.9% n^∥ increase, 0.6% n^⊥ decrease versus blank PET, with 18.5% birefringence enhancement. At 700 nm, indices decreased to 1.7040 (parallel) and 1.5390 (perpendicular, birefringence: 0.1650). Total wavelength-dependent variation represents ∼2.4% n^∥ and ∼1.5% n^⊥ decrease, indicating slightly stronger composite dispersion than the blank fiber. Isotropic refractive index decreased from 1.6229 at 400 nm to 1.5940 at 700 nm (∼1.5973 at 620 nm), higher than blank PET (1.599), reflecting high-index TiO₂ contribution. Birefringence varied between 0.1650 and 0.1835 with a slight decrease toward longer wavelengths but remained consistently and substantially higher than blank PET across the entire visible spectrum, demonstrating wavelength-independent relative enhancement magnitude.

The parallel index enhancement with simultaneous perpendicular decrease suggests TiO₂ nanoparticles don’t simply increase overall refractive index isotropically but preferentially enhance optical anisotropy. This indicates nanoparticles either align preferentially along the fiber axis during processing, create anisotropic local structures, or modify polymer chain orientation locally to enhance existing molecular alignment. High intrinsic TiO₂ refractive index (∼2.5-2.7 depending on crystal structure and wavelength) combined with oriented distribution or interfacial effects contributes to enhanced parallel index, while potential transverse packing disruption or constrained perpendicular chain mobility may cause slight perpendicular index decrease.

3.3.2. Spectral dispersion and Cauchy analysis of optical constants for PET/TiO₂ nanocomposite fiber

Wavelength-dependent refractive index data for the PET/TiO₂ composite fiber were analyzed using Cauchy’s dispersion equation (Eq. 7) to extract fundamental dispersion parameters. Following the blank PET analytical approach, measured refractive indices were plotted against inverse wavelength squared (λ⁻²), and linear regression determined Cauchy constants A and B for both orientations as displayed in Figure 9a. For the parallel direction, regression yielded A^∥ = 1.6840 and B^∥ = 9,798 nm². Compared to blank PET (A^∥ = 1.675, B^∥ = 8,500 nm²), the composite shows 0.54% A constant increase and a substantial 15.3% B coefficient. Enhanced A^∥ reflects a higher limiting refractive index at long wavelengths from high-index TiO₂ incorporation, contributing additional electronic polarizability even at low photon energies far from resonance. The significantly larger B^∥ indicates enhanced parallel dispersion strength, with the refractive index changing more rapidly with wavelength versus blank PET. Enhancement arises from: TiO₂’s intrinsic dispersion with strong wavelength dependence from electronic band structure and UV-visible absorption edge; polymer-nanoparticle interfaces introducing additional polarizable species and interfacial dipoles; and possible nanoparticle-induced modifications to polymer chain conformation, packing, and orientation affecting electronic oscillator distribution and strength.

(a) Cauchy dispersion analysis for PET/TiO₂ nanocomposite fiber showing linear relationship between measured refractive indices and inverse wavelength squared (λ⁻2) for both principal orientations (Solid lines represent linear regression fits). (b-d) Wavelength-dependent bulk density, mass fraction crystallinity and amorphous content, macroscopic polarizability per unit volume of PET/TiO₂ nanocomposite fiber, respectively (Connecting lines serve as guides to the eye).

For the perpendicular direction, Cauchy parameters are A^⊥ = 1.5280 and B^⊥ = 5,393 nm². Compared to blank PET (A^⊥ = 1.540, B^⊥ = 5,200 nm²), the composite shows 0.78% A constant decrease but a 3.7% B coefficient. The A^⊥ decrease indicates a lower composite perpendicular refractive index at long wavelengths than blank PET, suggesting nanoparticles disrupt transverse molecular packing or constrain perpendicular polarizability through steric effects or amorphous phase structure modification. The B^⊥ increase, though more modest than parallel, indicates enhanced perpendicular dispersion, with TiO₂ contributing to wavelength-dependent response in all directions anisotropically. The composite’s dispersion coefficient ratio B^∥/B^⊥ = 1.82 significantly exceeds blank PET (1.63), demonstrating TiO₂ incorporation enhances optical anisotropy in both absolute refractive indices and dispersion characteristics. This enhanced dispersion anisotropy has important implications for wavelength-dependent applications like optical filters, dispersion compensation devices, or broadband polarization elements.

3.3.3. Derived structural and electronic properties: Density, crystallinity, and polarizability for PET/TiO₂ nanocomposite fiber

PET/TiO₂ composite fiber bulk density was calculated from Eq. (8), then the density was determined using a calibrated proportionality constant K = 0.2481. Figure 9b presents wavelength-dependent calculated optical density, ranging from 1.455 g cm^-³ at 400 nm to 1.406 g cm^-³ at 700 nm (∼1.413 g cm^-³ at 620 nm, ∼1.415 g cm^-³ at 600 nm, interpolated ∼1.414 g cm^-³ at 620 nm), averaging ∼1.430 g cm^-³. Compared to blank PET (average: 1.379 g cm^-³), the composite shows ∼3.7% apparent increase in calculated optical density, reflecting higher composite refractive indices translating through the Lorentz-Lorenz equation to higher calculated density. Elevated composite optical density values, particularly at shorter wavelengths approaching or exceeding typical PET densities, reflect a strong contribution from high-index, high-polarizability TiO₂ nanoparticles. The TiO₂ phase, with high electronic polarizability from titanium d-orbitals and oxygen p-orbitals, contributes disproportionately to refractive index relative to mass/volume fraction. The smooth, systematic trend despite a heterogeneous composite structure indicates well-dispersed nanoparticles with optical contribution effectively averaged over the measurement volume.

PET/TiO₂ composite fiber mass fraction crystallinity was determined from calculated optical density using the two-phase model (Eq. 9), applying densities for crystalline (ρ_c = 1.458 g cm^-³) and amorphous (ρ_a = 1.333 g cm^-³) for PET/TiO₂. Figure 9c presents wavelength-dependent apparent crystallinity and complementary amorphous content. Calculated crystallinity decreases systematically from 80% at 400 nm to 39% at 700 nm (∼45% at 620 nm, ∼47% at 600 nm, interpolated ∼46% at 620 nm), averaging ∼56%. Apparent amorphous content ranges from 20% at 400 nm to 61% at 700 nm, averaging ∼44%. The large 41-percentage-point wavelength variation far exceeds blank PET variation (1.75 percentage points), reflecting stronger composite optical density wavelength dependence. Comparing interpolated 620 nm values, the composite (∼46% crystallinity) versus blank PET (36.83%) shows ∼9-percentage-point apparent increase or 25% relative increase in calculated crystallinity. This enhancement could arise from several mechanisms difficult to distinguish optically: TiO₂ nanoparticles acting as heterogeneous nucleating agents during crystallization, promoting additional crystalline domains around nanoparticle surfaces (well-documented in polymer nanocomposites); nanoparticles constraining or densifying amorphous phase through steric effects or polymer-nanoparticle interactions, increasing amorphous region density and apparent calculated crystallinity; or different composite fiber processing conditions (e.g., different cooling rates or take-up speeds from different nanocomposite melt rheological properties) leading to different crystallization kinetics and final crystallinity.

Macroscopic polarizability per unit volume for the PET/TiO₂ composite fiber was calculated using the Lorentz-Lorenz relation (Eq. 10) for parallel, perpendicular, and isotropic orientations. This analysis provides fundamental insight into nanocomposite electronic response, revealing how highly polarizable TiO₂ nanoparticles modify overall electronic structure and field-induced electron cloud distortion versus pristine polymer. Figure 9d presents wavelength-dependent polarizability for all orientations. At 620 nm (interpolated), parallel polarizability was Φ^∥ ≈ 0.3905, perpendicular Φ^⊥ ≈ 0.3147, and isotropic Φi_so ≈ 0.3400. Compared to blank PET at 620 nm (Φ^∥ = 0.36423, Φ^⊥ = 0.32145, Φ_iso = 0.33568), the composite shows a substantial 7.2% parallel increase, 2.1% perpendicular decrease, and 1.3% isotropic increase, reflecting a complex interplay between high TiO₂ polarizability and nanoparticle-induced polymer structure and orientation modifications. All polarizability components increase systematically toward shorter wavelengths, consistent with normal dispersion and resonance enhancement as photon energies approach UV absorption edges. At 400 nm, polarizabilities reach Φ^∥ = 0.4054, Φ_⊥ = 0.3242, and Φi_so = 0.3526 (11.3%, 0.8%, and 5.2% increases versus blank PET). At 700 nm, values decrease to Φ^∥ = 0.3882, Φ^⊥ = 0.3133, and Φ_iso = 0.3393, still maintaining significant enhancement. Composite wavelength-dependent variation is more pronounced than blank PET, with parallel polarizability varying ∼4.4% across the spectrum versus 2.0% for blank PET, indicating a stronger nanocomposite wavelength-dependent electronic response.

3.3.4. Quantification of molecular orientation and stress-optical response for PET/TiO₂ nanocomposite fiber

The molecular orientation of the PET/TiO₂ fiber was quantified using the Hermans orientation function (Eq. 12). The corresponding orientation angle was calculated from Eq. (13). The wavelength dependence of the Hermans orientation function and the corresponding orientation angle for the PET/TiO₂ composite fiber are presented in Figure 10a. At 620 nm, the Hermans orientation function was f_Δ = 0.726. This is an 18.4% increase over blank PET (f_Δ = 0.613), demonstrating that TiO₂ nanoparticles significantly enhance molecular alignment. This suggests the nanoparticles actively promote orientation during fiber formation. As shown in Figure 10a, the orientation function varies with wavelength, from f_Δ = 0.798 at 400 nm to 0.717 at 700 nm (a 10% variation across the spectrum). This is more pronounced than in blank PET (1.9% variation), indicating that electronic transitions probed at different wavelengths experience different alignment. The higher orientation at shorter wavelengths may be due to the stronger alignment of π-π* transitions in the aromatic rings or from wavelength-dependent contributions of the TiO₂ itself. The corresponding orientation angle at 620 nm was θ = 20.3°, a reduction of 3.8° from blank PET (θ = 24.09°), confirming tighter molecular alignment. The angle varies only slightly with wavelength (19.6° to 20.4°), but remains consistently smaller than in blank PET. The enhanced orientation is likely due to several mechanisms: TiO₂ nanoparticles acting as physical constraints that promote chain alignment during spinning; altered rheological properties during extrusion; interfacial interactions that create highly oriented polymer regions on nanoparticle surfaces; and nanoparticles acting as nucleation sites for oriented crystallization.

Wavelength-dependent Hermans orientation function, corresponding orientation angle (θ), and optical stress coefficient for PET/TiO₂ nanocomposite fiber, respectively. (Connecting lines serve as guides to the eye).

The optical stress coefficient for the PET/TiO₂ fiber was calculated (Eq. 14) to understand how TiO₂ nanoparticles alter the relationship between mechanical stress and optical birefringence, as shown in Figure 10b. At 620 nm, the composite (Δα ≈ 4.19 Å³) shows a 68% enhancement over blank PET (Δα = 2.495 Å³), indicating a genuinely stronger stress-optical response. As shown in Figure 10b, the coefficient decreases from 161.1 × 10^-12 TPa⁻¹ at 400 nm to 150.9 × 10^-12 TPa⁻¹ at 700 nm, a 6.3% variation. This wavelength dependence is stronger than in blank PET (2.1% variation), reflecting the composite’s enhanced sensitivity. This enhanced stress-optical coupling is advantageous for applications like smart textiles or stress analysis, as the fiber exhibits stronger birefringence changes under load. However, it also means the fiber’s optical properties are more sensitive to processing stresses and external loads, which may require more careful manufacturing and use. The rigid nanoparticles increase the composite’s stiffness but also create localized stress concentrations that affect the optical response.

To facilitate comprehensive comparison and quantitative assessment of TiO₂ nanoparticle incorporation effects, Table 1 presents the key optical, structural, and orientational parameters for both blank PET and PET/TiO₂ nanocomposite fibers at the reference wavelength of 620 nm. This comparative analysis provides clear quantitative evidence of the substantial property enhancements achieved through nanoparticle functionalization.

Table 1. Comparative optical and structural properties of blank PET and PET/TiO₂ composite fibers at reference wavelength 620 nm.

Parameter	Blank PET	PET/TiO₂	% Change
$n^{∥}$	1.693 ± 0.0010	1.7087 ± 0.0010	+0.93
$n^{⊥}$	1.552 ± 0.0008	1.5416 ± 0.0009	−0.67
$Δ n$	0.141 ± 0.0013	0.1671 ± 0.0015	+18.5
Dispersion anisotropy ratio (B^∥/B^⊥)	1.63	1.82	+11.7
ρ (g/cm³)	1.3792 ± 0.0015	1.4130 ± 0.0020	+2.45
$Φ^{∥}$	0.36423	0.3905	+7.21
$Φ^{⊥}$	0.33568	0.3147	−6.25
Hermans orientation function ( $f_{Δ}$ )	0.6130	0.726	+18.4
Orientation angle ( $θ^{o}$ )	24.09	20.3	−15.7
$C_{stress}$ (x10^-12 TPa⁻¹)	3.5477	5.9610	+68.0

This table clarifies that the incorporation of 2.0 wt% TiO₂ nanoparticles induces substantial property modifications at 620 nm wavelength. The nanocomposite exhibits enhanced parallel refractive index with simultaneous perpendicular decrease, yielding 18.5% birefringence enhancement. This demonstrates preferential anisotropic modification rather than isotropic enhancement. The dispersion anisotropy ratio increases by 11.7%, indicating a stronger wavelength-dependent response. Optical density increases, reflecting high-index TiO₂ contributions. Macroscopic polarizability shows contrasting directional behavior: parallel increases 7.21% while perpendicular decreases 6.25%, confirming enhanced electronic anisotropy. Molecular orientation improves significantly with Hermans function increasing 18.4% and orientation angle decreasing from 24.09° to 20.30° (−15.7% reduction), indicating substantially tighter molecular alignment along the fiber axis. The most dramatic change occurs in the stress-optical coefficient, increasing 68.0%, demonstrating substantially stronger mechanical-optical coupling and enhanced sensitivity to applied stress.

These enhancements demonstrate that TiO₂ nanoparticles modify optical anisotropy and molecular organization through synergistic mechanisms during fiber formation. During spinning, TiO₂ nanoparticles increase melt viscosity and elasticity, promoting greater chain stretching and orientation through enhanced elongational stress. The extensional flow field promotes preferential nanoparticle orientation along the fiber axis, with even spherical nanoparticles forming chain-like structures under shear that create directional optical contributions. At polymer-nanoparticle interfaces, TiO₂ acts as rigid inclusions locally modifying chain conformation, with chains adopting preferentially oriented conformations contributing enhanced parallel optical polarizability. Nanoparticles serve as heterogeneous nucleation sites initiating crystallization at higher temperatures, with rigid surfaces templating adjacent chain alignment and propagating orientation into surrounding material, supported by optical density increase from 1.3792 to 1.4130 g cm^-³ (+2.45%). Rigid nanoparticles prevent stress relaxation and chain retraction, preserving oriented conformations. At 2.0 wt% loading, the 100-200 nm inter-particle distance creates a percolating network where chains experience geometric confinement, favoring oriented conformations even in amorphous regions. These mechanisms act synergistically, producing preferential parallel optical property enhancement without proportional perpendicular enhancement, manifesting as increased birefringence.

The dispersion anisotropy ratio increases 11.7%, indicating a stronger wavelength-dependent response from the combined contributions of high-index TiO₂ and nanoparticle-induced polymer chain modifications. The most dramatic enhancement occurs in stress-optical coefficient, demonstrating substantially stronger mechanical-optical coupling from combined effects of increased molecular orientation, enhanced polarizability anisotropy, and rigid nanoparticle-polymer interfaces creating stress concentration regions. The consistency of enhanced orientation across all wavelengths confirms genuine structural modifications rather than optical artifacts. These comprehensive enhancements establish clear structure-property relationships demonstrating that TiO₂ nanoparticles act synergistically through modified rheology, heterogeneous nucleation, interfacial polymer orientation, and geometric confinement to produce highly oriented, optically anisotropic nanocomposite fibers with superior functional properties.

3.4. Practical implications and application potential

The enhanced optical, structural, and antimicrobial properties demonstrated in this study have significant cross-industry applications. For smart and functional textiles, the enhanced birefringence (18.5% increase) and controllable optical anisotropy enable the development of textiles with tunable optical properties, including polarization control, optical security features, and aesthetically appealing iridescence effects. The wavelength-dependent refractive index modifications suggest potential for color management and chromatic effects without dyes. The robust molecular orientation contributes to improved mechanical properties alongside optical functionality. In medical and hygiene textiles, the broad-spectrum antimicrobial activity and TiO₂’s photocatalytic, non-leaching mechanism are ideal for hospital fabrics and wound dressings, reducing resistance risks. Furthermore, the TiO₂ nanoparticles provide dual-mechanism UV protection for outdoor apparel and automotive textiles, where the enhanced refractive index optimizes UV blocking for long-term stability. These include medical textiles (surgical gowns, wound dressings), hygienic products, UV-protective outdoor fabrics (awnings, sportswear), food packaging to extend shelf life, and air/water filtration membranes that can degrade organic pollutants under light exposure. The photocatalytic properties of TiO₂ enable self-cleaning functionality by decomposing organic contaminants, with applications in architectural textiles, filtration media, and protective garments. The enhanced optical properties also benefit fiber-based components like polarizers, optical sensors, and wearable photonics.

While these findings demonstrate promising multifunctional property combinations, several challenges must be addressed for successful commercial translation. Key considerations include optimizing nanoparticle loading to balance functionality with processing requirements and cost, evaluating long-term durability regarding wash fastness and UV stability, and assessing mechanical property modifications to ensure acceptable textile performance. Additionally, production scale-up with consistent nanoparticle dispersion, comprehensive safety assessment including environmental impact studies, and cost-benefit analysis for target applications requires systematic investigation. Future research should address these translational requirements to enable the commercialization of PET/TiO₂ nanocomposite fibers for specific high-value applications.

4. Conclusions

This investigation has systematically established the structure-property relationships in PET fibers functionalized with 2.0 wt% TiO₂ nanoparticles using multi-wavelength interferometry. The key experimental findings are:

TiO₂ incorporation significantly enhances optical anisotropy, evidenced by an 18.5% increase in birefringence and a 68% increase in molecular polarizability anisotropy (Δα).
The nanocomposite exhibits stronger wavelength-dependent behavior, with a 15.3% increase in the parallel Cauchy dispersion coefficient.
The nanoparticles promote polymer chain alignment, leading to an 18.4% enhancement in the Hermans orientation function and a reduction in the orientation angle from 24.09° to 20.3°.
The stress-optical coefficient increased by 68%, indicating a stronger coupling between mechanical stress and optical response.
The functional performance was confirmed by robust broad-spectrum antimicrobial activity (75.8-85.4% growth reduction), attributed to photocatalytic ROS generation.

These proven results demonstrate that VAWI is a powerful tool for the non-destructive characterization of nanocomposite fibers. The fundamental insights into how TiO₂ modifies the polymer’s electronic structure and molecular organization provide a scientific basis for the rational design of next-generation multifunctional materials. The enhanced properties position these PET/TiO₂ fibers as promising candidates for advanced applications in smart textiles, medical devices, and photonic systems. Future work should integrate complementary characterization techniques (SEM, TEM, XRD, DSC) to provide independent validation of crystallinity, nanoparticle dispersion, and morphology.

CRediT authorship contribution statement

Mona A. Alhasani, Gadeer R. Ashour: Data curation, formal analysis, methodology, and software; Oumr A. Osra, Wajdy S. Qattan, Di: Investigation and writing – review & editing; Hatun H. Alsharief, Alia A. Alfi: formal analysis, investigation, writing-original draft. Ahmed S. Badr El-Din, Nashwa M. El-Metwaly: Supervision and administration of research group.

Declaration of competing interest

The authors declare that they have no conflicts of interest.

Data availability

All relevant data are within the manuscript and available from the corresponding author upon request.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript, and no images were manipulated using AI.

References

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1. Introduction

2. Materials and Methods

2.1. Theoretical considerations, phase demodulation using Fourier transform method
2.2. Determination of principal and derived refractive indices and optical anisotropy from interferometric phase distribution
2.3. Multi-wavelength analysis and dispersion properties
2.4. Density determination using Lorentz-Lorenz equation
2.5. Crystallinity and morphological parameters
2.6. Polarizability parameters
2.7. Molecular orientation
2.8. Optical stress coefficient
2.9. Methodology and materials

3. Results and Discussions

3.1. Assessment of antimicrobial efficacy of PET/TiO2 composite fibers via shaking flask methodology
3.2. Multi-wavelength optical characterization and molecular orientation of undrawn PET fiber
3.3. Multi-wavelength optical characterization and molecular orientation of undrawn PET fiber
3.4. Practical implications and application potential

4. Conclusions

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[1] Aoyama S., Ismail I., Park Y.T., Yoshida Y., Macosko C.W., Ougizawa T.. Polyethylene terephthalate/trimellitic anhydride modified graphene nanocomposites. ACS Applied Nano Materials. 2018;1:6301-6311. https://doi.org/10.1021/acsanm.8b01525
[Google Scholar]

[2] Abd El-Aziz E., Zayed M., Mohamed A.L., Hassabo A.G.. Enhancement of the functional performance of cotton and polyester fabrics upon treatment with polymeric materials having different functional groups in the presence of different metal nanoparticles. Polymers. 2023;15:3047. https://doi.org/10.3390/polym15143047
[Google Scholar]

[3] Babaei M., Jalilian M., Shahbaz K.. Chemical recycling of polyethylene terephthalate: A mini-review. Journal of Environmental Chemical Engineering. 2024;12:112507. https://doi.org/10.1016/j.jece.2024.112507
[Google Scholar]

[4] Castillo G.A., Wilson L., Efimenko K., Dickey M.D., Gorman C.B., Genzer J.. Amidation of polyesters is slow in nonaqueous solvents: efficient amidation of poly(ethylene terephthalate) with 3-aminopropyltriethoxysilane in water for generating multifunctional surfaces. ACS Applied Materials & Interfaces. 2016;8:35641-35649. https://doi.org/10.1021/acsami.6b12155
[Google Scholar]

[5] Pardi, Aprilia S., Syamsuddin Y., Zuhra, Amin A., Zuwanna I.. Physical, morphological, mechanical and thermal properties of polyester composites reinforced with orientation of purun fiber (Eleocharis dulcis) composition. South African Journal of Chemical Engineering. 2024;47:338-344. https://doi.org/10.1016/j.sajce.2023.11.014
[Google Scholar]

[6] Skibicki S., Pułtorak M., Kaszyńska M., Hoffmann M., Ekiert E., Sibera D.. The effect of using recycled PET aggregates on mechanical and durability properties of 3D printed mortar. Construction and Building Materials. 2022;335:127443. https://doi.org/10.1016/j.conbuildmat.2022.127443
[Google Scholar]

[7] Zhao H.-C., Ye Y.-Y., Zeng J.-J., Zhou J.-K., Ouyang Y.. Polyethylene terephthalate fibre-reinforced polymer-confined concrete encased high-strength steel tube hybrid square columns: Axial compression tests. Structures. 2020;28:577-588. https://doi.org/10.1016/j.istruc.2020.08.078
[Google Scholar]

[8] Khairul Anuar N.F.S., Huyop F., Ur-Rehman G., Abdullah F., Normi Y.M., Sabullah M.K., Abdul Wahab R.. An overview into polyethylene terephthalate (PET) hydrolases and efforts in tailoring enzymes for improved plastic degradation. International Journal of Molecular Sciences. 2022;23:12644. https://doi.org/10.3390/ijms232012644
[Google Scholar]

[9] Qin X., Liao X., Zhang H., Jin H., Xiao S., Lu W.. Size dispersion of the filler particles and its consequences on the light-extinction properties of TiO₂/PET nanocomposite fibers. Langmuir : The ACS Journal of Surfaces and Colloids. 2023;39:9857-9864. https://doi.org/10.1021/acs.langmuir.3c01070
[Google Scholar]

[10] Serov D.A., Gritsaeva A.V., Yanbaev F.M., Simakin A.V., Gudkov S.V.. Review of antimicrobial properties of titanium dioxide nanoparticles. International Journal of Molecular Sciences. 2024;25:10519. https://doi.org/10.3390/ijms251910519
[Google Scholar]

[11] Ghareeb A., Fouda A., Kishk R.M., El Kazzaz W.M.. Unlocking the potential of titanium dioxide nanoparticles: An insight into green synthesis, optimizations, characterizations, and multifunctional applications. Microbial Cell Factories. 2024;23:341. https://doi.org/10.1186/s12934-024-02609-5
[Google Scholar]

[12] Sheikh M., Makkad S., Shende S., Deshmukh M.. Antimicrobial efficacy of metal-doped titanium dioxide nanoparticles: a comprehensive review. International Journal of Pharmaceutical Investigation. 2024;14:1042-1051. https://doi.org/10.5530/ijpi.14.4.114
[Google Scholar]

[13] Younis A.B., Milosavljevic V., Fialova T., Smerkova K., Michalkova H., Svec P., Antal P., Kopel P., Adam V., Zurek L., Dolezelikova K.. Synthesis and characterization of TiO₂ nanoparticles combined with geraniol and their synergistic antibacterial activity. BMC Microbiology. 2023;23:207. https://doi.org/10.1186/s12866-023-02955-1
[Google Scholar]

[14] Rathore C., Yadav V.K., Gacem A., AbdelRahim S.K., Verma R.K., Chundawat R.S., Gnanamoorthy G., Yadav K.K., Choudhary N., Sahoo D.K., Patel A.. Microbial synthesis of titanium dioxide nanoparticles and their importance in wastewater treatment and antimicrobial activities: A review. Frontiers in Microbiology. 2023;14:1270245. https://doi.org/10.3389/fmicb.2023.1270245
[Google Scholar]

[15] Hassan M.N., Beg Mou M.. Surface modification of jute-cotton union fabric using TiO₂ and ZnO nanoparticles for multifunctional properties. Heliyon. 2024;10:e29970. https://doi.org/10.1016/j.heliyon.2024.e29970
[Google Scholar]

[16] Pakdel E., Daoud W. A., Kashi S., Gashti M. P., Wang X.. Superhydrophilic self-cleaning cotton fabric with enhanced antibacterial and UV protection properties. Cellulose. 2025;32:1937-1958. https://doi.org/10.1007/s10570-024-06346-1
[Google Scholar]

[17] Caldas A., Sousa J., Carneiro E., Carvalho S.. Functionalization of fabrics by Ag-TiO₂ nanoparticles deposition by sol-gel method. Journal of Textile Engineering & Fashion Technology. 2024;10:49-53. https://doi.org/10.15406/jteft.2024.10.00365
[Google Scholar]

[18] Dorohoi D.O., Postolache M., Nechifor C.D., Dimitriu D.G., Albu R.M., Stoica I., Barzic A.I.. Review on optical methods used to characterize the linear birefringence of polymer materials for various applications. Molecules (Basel, Switzerland). 2023;28:2955. https://doi.org/10.3390/molecules28072955
[Google Scholar]

[19] Nguyen T.L., Pradeep S., Judson-Torres R.L., Reed J., Teitell M.A., Zangle T.A.. Quantitative phase imaging: recent advances and expanding potential in biomedicine. ACS Nano. 2022;16:11516-11544. https://doi.org/10.1021/acsnano.1c11507
[Google Scholar]

[20] Pluta M.. Nomarski’s DIC microscopy: A review. Proceedings of SPIE. The international society for optical engineering/proceedings of SPIE. 1994;1846:10-25. https://doi.org/10.1117/12.171873
[Google Scholar]

[21] Ritchie A.W., Cox H.J., Gonabadi H.I., Bull S.J., Badyal J.P.S.. Tunable high refractive index polymer hybrid and polymer–inorganic nanocomposite coatings. ACS Applied Materials & Interfaces. 2021;13:33477-33484. https://doi.org/10.1021/acsami.1c07372
[Google Scholar]

[22] de Menezes L.R., Cavalcante M.P., Vaz J.L.M.R., Silva P.S.R.C., Tavares M.I.B.. Obtention of higher refractive index and transparent polymeric nanocomposite systems with small amounts of fillers for lenses application. Journal of Composite Materials. 2021;55:675-686. https://doi.org/10.1177/0021998320957070
[Google Scholar]

[23] Zhang T., Shi R., Shao Y., Chen Q., Bai J.. Single-frame noisy interferogram phase retrieval using an end-to-end deep learning network with physical information constraints. Optics and Lasers in Engineering. 2024;181:108419. https://doi.org/10.1016/j.optlaseng.2024.108419
[Google Scholar]

[24] Kim Y., Seo J., Bae W., Moon Y.H., Ito Y., Sugita N.. Wavelength-modulation Fourier interferometry with elimination of DC phase error. Precision Engineering. 2021;68:97-105. https://doi.org/10.1016/j.precisioneng.2020.12.003
[Google Scholar]

[25] Yuan S., Hu Y., Hao Q., Zhang S.. High-accuracy phase demodulation method compatible to closed fringes in a single-frame interferogram based on deep learning. Optics Express. 2021;29:2538-2554. https://doi.org/10.1364/OE.413385
[Google Scholar]

[26] Wang X., Ju C., Liu Y., Kou K.. Robust phase unwrapping algorithm for gear interferometry based on mutual information and adaptive spin-inpainting approaches. Optics Communications. 2024;568:130748. https://doi.org/10.1016/j.optcom.2024.130748
[Google Scholar]

[27] Kouskousis B., Kitcher D.J., Collins S., Roberts A., Baxter G.W.. Quantitative phase and refractive index analysis of optical fibers using differential interference contrast microscopy. Applied Optics. 2008;47:5182-5189. https://doi.org/10.1364/ao.47.005182
[Google Scholar]

[28] Avadanei M.I., Dimitriu D.G., Dorohoi D.O.. Optical anisotropy of polyethylene terephthalate films characterized by spectral means. Polymers. 2024;16:850. https://doi.org/10.3390/polym16060850
[Google Scholar]

[29] Samuels R.J.. Structured polymer properties: The identification, interpretation, and application of crystalline polymer structure. 1974.

[30] Mialdun A., Shevtsova V.. Analysis of multi-wavelength measurements of diffusive properties via dispersion dependence of optical properties. Applied Optics. 2017;56:572-581. https://doi.org/10.1364/AO.56.000572
[Google Scholar]

[31] Bonal V., Quintana J.A., Villalvilla J.M., Muñoz-Mármol R., Mira-Martínez J.C., Boj P.G., Cruz M.E., Castro Y., Díaz-García M.A.. Simultaneous determination of refractive index and thickness of submicron optical polymer films from transmission spectra. Polymers. 2021;13:2545. https://doi.org/10.3390/polym13152545
[Google Scholar]

[32] Tarani E., Arvanitidis I., Christofilos D., Bikiaris D.N., Chrissafis K., Vourlias G.. Calculation of the degree of crystallinity of HDPE/GNPs nanocomposites by using various experimental techniques: A comparative study. Journal of Materials Science. 2023;58:1621-1639. https://doi.org/10.1007/s10853-022-08125-4
[Google Scholar]

[33] Hamanaka S., Nonomura C., Thi T.B.N., Yokoyama A.. Correlation between fiber orientation distribution and mechanical anisotropy in glass-fiber-reinforced composite materials. Journal of Polymer Engineering. 2019;39:653-660. https://doi.org/10.1515/polyeng-2018-0371
[Google Scholar]

[34] Hermans P.H.. Contribution to the physics of cellulose fibres: A study in sorption, density, refractive power and orientation. Nature. 1946;159:519-520. https://doi.org/10.1002/pol.1947.120020321
[Google Scholar]

Multi-wavelength interferometric engineering of TiO₂-functionalized polyethylene terephthalate fibers: Electronic structure, optical properties, and antimicrobial performance

Abstract

Keywords

Antimicrobial properties

Cauchy dispersion

Electronic polarizability

Optical birefringence

PET/TiO₂ nanocomposite fibres

Stress-optical coefficient

Variable wavelength automated interferometry

1. Introduction

2. Materials and Methods

2.1. Theoretical considerations, phase demodulation using Fourier transform method

2.2. Determination of principal and derived refractive indices and optical anisotropy from interferometric phase distribution

2.3. Multi-wavelength analysis and dispersion properties

2.4. Density determination using Lorentz-Lorenz equation

2.5. Crystallinity and morphological parameters

2.6. Polarizability parameters

2.7. Molecular orientation

2.8. Optical stress coefficient

2.9. Methodology and materials

3. Results and Discussions

3.1. Assessment of antimicrobial efficacy of PET/TiO2 composite fibers via shaking flask methodology

3.2. Multi-wavelength optical characterization and molecular orientation of undrawn PET fiber

3.2.1. Phase extraction and refractive index and birefringence calculations for blank PET fiber

3.2.2. Spectral dispersion and Cauchy analysis of optical constants for blank PET fiber

3.2.3. Derived structural and electronic properties: Density, crystallinity, and polarizability for blank PET fiber

3.2.4. Quantification of molecular orientation and stress-optical response for blank PET fiber

3.3. Multi-wavelength optical characterization and molecular orientation of undrawn PET fiber

3.3.1. Phase extraction and refractive index and birefringence calculations for PET/TiO₂ nanocomposite fiber

3.3.2. Spectral dispersion and Cauchy analysis of optical constants for PET/TiO₂ nanocomposite fiber

3.3.3. Derived structural and electronic properties: Density, crystallinity, and polarizability for PET/TiO₂ nanocomposite fiber

3.3.4. Quantification of molecular orientation and stress-optical response for PET/TiO₂ nanocomposite fiber

3.4. Practical implications and application potential

4. Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Data availability

Declaration of generative AI and AI-assisted technologies in the writing process

References

Suggested read for related articles:

3.1. Assessment of antimicrobial efficacy of PET/TiO₂ composite fibers via shaking flask methodology