Synthesis and utilization of poly (methylmethacrylate) nanocomposites based on modified montmorillonite

Ahmed M. Youssef; F.M. Malhat; A.A. Abdel Hakim; Imre Dekany

doi:10.1016/j.arabjc.2015.02.017

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Original article

10 (

); 631-642

doi:

10.1016/j.arabjc.2015.02.017

Synthesis and utilization of poly (methylmethacrylate) nanocomposites based on modified montmorillonite

Ahmed M. Youssef^{^a,^c,⁎}, F.M. Malhat^{^b}, A.A. Abdel Hakim^{^a}, Imre Dekany^{^c}

a Packing and Packaging Materials Department, National Research Centre, 33 El Bohouth St. (former El Tahrir st.), Dokki, Giza, P.O. 12622, Egypt

b Department of Pesticide Residues and Environmental Pollution, Central Agricultural Pesticide Laboratory, Agriculture Research Center, Dokki, Giza, Egypt

c Supramolecular and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, University of Szeged, Aradi Vertanuk Tere, Szeged, Hungary

⁎Corresponding author at: Packing and Packaging Materials Department,National Research Centre, 33 El Bohouth St. (former El Tahrir st.), Dokki, Giza, P.O. 12622, Egypt.

Received: 2014-1-17, Accepted: 2015-2-3, Published: 2015-07-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Poly (methylmethacrylate) nanocomposite was prepared via in-situ emulsion polymerization (PMMA/Mt-CTA). The modified montmorillonite (Mt-CTA) is used as hosts for the preparation of poly (methylmethacrylate) nanocomposites with basal distance 1.95 nm. Moreover, exfoliated nanocomposite was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The fashioned nanocomposites exhibited better thermal stability than pristine PMMA which make it suitable for packaging applications. Furthermore, this nanocomposite reveals tremendous affinity for removing pesticides from aquatic solutions. The data obtained from GC/ECD gas liquid chromatography illustrated that the removal efficiency of PMMA/Mt-CTA nanocomposites for organochlorine pesticides (OCPs) varied from 73.65% to 99.36% that make it as a new method for water treatment. Also, the antimicrobial activity of the Mt-CTA and PMMA/Mt-CTA nanocomposites was evaluated by the inhibitory zone tests and revealed good activity against Escherichia coli and Staphylococcus aureus, which makes it suitable materials for packaging applications.

Keywords

Montmorillonite

Exfoliation

Nanocomposites

Packaging

Organochlorine pesticides

Antimicrobial activity

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1 Introduction

Polymer nanocomposite using layer silicate appears to offer the greatest potential for industrial application. In the recent years, great progresses were made through the preparation of great number of high performance polymer nanocomposites based on clays, to match with the expansion of industrial and economic activities. Also, there has been substantial significance in the intercalation of organic molecules into layered structures (Tunney and Detellier, 1996; Komori et al., 1998; Hayashi, 1997; Mercier and Pinnavaia, 1998 ; Wand and Pinnavaia, 1998; Youssef et al., 2013a,b,c ). Nanocomposites as new family of composite materials in which, at least one of the dimensions of the filler, is in the nanometer range (i.e., clay mineral) support with the functionality of an intercalated organic compound is creating new types of materials for heterogeneous catalysis, nanocomposites, mesoporous materials, environmental chemistry, polymers, pharmaceuticals, packaging and chromatography (Ogawa and Kuroda, 1995; Guimaraes et al., 1998; Haroun and Youssef, 2011; Youssef et al., 2012a,b, 2013a,b,c; Youssef, 2013 ). It has been largely reported in the literature that the incorporation of nanofillers to a pure polymer matrix increases some appropriate material properties, such as mechanical properties, thermal stability (Kotsilkova et al., 2001; Nassar and Youssef, 2012 ; Youssef, 2014), and gas barrier properties, without significant reductions in toughness (Alexandre and Dubois, 2000) and transparency (Wan et al., 2003) which makes it suitable for packaging applications. The most widely utilized clay mineral is the montmorillonite (Mt) for its large cation exchange capacity (CEC) (Giannelis, 1996; Huang and Brittain, 2001; Abd El-Ghaffar et al., 2015 ). The synthesis of polymer/clay nanocomposites used different methods: in-situ polymerization, intercalation of the polymer from a solution, melts intercalation of the polymer, and sol–gel technique (Zanetti et al., 2000). The idea foremost which nanocomposites were built, is based on implementation of the polymerization process into the basal space of the clay, which is of a nanometric scale, the crowding of the growing polymer chains inside this limited space force at least expand the basal space of the clay (intercalation) and at maximum defoliate the ordered layered structure into many separate layers totally dispersed in the polymer matrix (exfoliation) (Dietsche and Muelhaupt, 1999; Fu and Qutubuddin, 2001; Li et al., 2003; Youssef, 2013 ). It was extremely done in two or more steps in montmorillonite, initially by substituting the Na⁺ in the space between the layered structure of the montmorillonite by either, a surfactant, monomer or the initiator which expands the basal space and facilitates for more penetration of monomer molecules inside before the polymerization process commences (Fischer et al., 1999; Tseng et al., 2002; Youssef et al., 2013a,b,c ). The layered structure materials in general can be applied as adsorbent for liquid mixture (Dékány, 1992), organic compounds (Dékány et al., 1996) and for several ions (Bujdosó et al., 2009) due to their high ionic exchange (Meyn et al., 1990; Lagaly and Beneke, 1991) even in wastewater treatment. Additionally, the successful preparation of PMMA/Mt-CTA nanocomposites opens new areas for basic strategic, and applied research in the field of water treatment, while the polymeric adsorbents such as sugarcane bagasse, green coconut shells, chitin and chitosan (Crisafully et al., 2008; Youssef et al., 2013a,b,c ) and organoclays (Lee et al., 2004) have been used as an alternative to activated carbon due to their economic viability, TiO₂ nanowires used for removing OCPs from wastewater (Youssef and Malhat, 2014), adsorption–regeneration properties and mechanical strength to purify water contaminated by pesticides and other hazardous chemicals. Besides, the prepared nanocomposites were used to remove the organochlorine pesticides from the wastewater; this is because water is a finite and vulnerable resource that is essential for sustaining life, development and our environment. Statistics indicate that over one billion of the world population lack access to safe water, and nearly two billion lack safe sanitation worldwide. Due to the extensive use of pesticides in industry and agriculture, more and more water sources are contaminated with pesticides. Attention is usually focused on contamination by organochlorine pesticides (OCPs), because of a number of disadvantages including environmental persistence, bioaccumulation and their toxic action upon the nervous system (Hardell et al., 1996; Moysich et al., 1998). Furthermore, OCPs may act as environmental estrogens by disrupting the normal functioning of the hormones and may cause breast cancer in humans. To date, the removal of OCPs and its degradation by-products to low levels remains a challenge to scientists, local governments and other section in industry. Pesticides can be eliminated from water in different ways, most frequently by adsorption and/or ozonization. In addition, pharmacology studies have revealed that sodium montmorillonite (Na⁺MMT) adsorbed bacteria such as Escherichia coli (E. coli), Staphylococcus aureus (S. aureus) and immobilized cell toxins (Zhou et al., 2004; Hu et al., 2005).

Some researchers found that natural clay minerals showed no antibacterial effect, but could adsorb and kill bacteria when materials with antimicrobial achievement were intercalated. There are a certain number of reports about modified Na⁺MMT with antibacterial activity, such as cetylpyridinium-exchanged Na⁺MMT, MMT-carrying copper and silver ions as effective bacteriostasis materials (Jo et al., 2007). To the best of our knowledge, there have been few reports on the antibacterial activity of CTA-MMTs. Predominantly, this study was designated to synthesize and characterize a completely new class of polymer nanocomposites which allow us to consider the novel possible applications in water treatment for removal of pesticides as well as packaging applications

2 Experimental section

2.1

2.1 Materials

Methylmethacrylate (MMA) monomer (98% purity, Aldrich) was distilled using aqueous solution sodium hydroxide (10%) and stored at low temperature prior to use, and cetyltrimethyl ammonium chloride (CTAC) and sodium dodecylsulfate (SDS) were provided by Merck, Darmstadt, Germany, as emulsifier. Sodium montmorillonite (Na⁺-Mt) with cation exchange capacity (CEC) of 90 mEq/100 g, under trade name Mineral colloid BP, was purchased from Southern Clay Products Inc. Potassium persulphate (PPS) (SRI, Mumbai-India) was used. All organic solvents used in this study were of HPLC grade and were purchased from Alliance BIO, USA. Water of high quality was obtained by deionization through a Milli-Q system (Millipore water). The mixture of OCP reference standard was provided by Dr. Ehrenstorfer, Augsburg, Germany. The main physical and chemical structure data of the tested pesticides are given in Tables 1 and 2.

Table 1 Main physical and chemical data of the tested organochlorine pesticides.

Pesticides	Formula	Molecular weight (g mol⁻¹)	Solubility in water (mg/L at 25 °C)	Log K_ow
α-HCH	C₆H₆Cl₆	287.86	2.00	3.08
γ-HCH	C₆H₆Cl₆	287.86	10.00	2.67
δ-HCH	C₆H₆Cl₆	287.86	31.40	4.14
Heptachlor	C₁₀H₅Cl₇	373.30	0.180	4.40
Hept. epoxide	C₁₀H₅Cl₇	369.82	0.056	5.44
Aldrin	C₁₂H₈Cl₆	364.90	0.027	5.60
Endrin	C₁₂H₈Cl₆O	380.90	0.230	4.56
Dieldrin	C₁₂H₈C_l6O	377.87	0.190	3.69
P,p′-DDD	C₁₄H₁₀Cl₄	320.05	0.090	6.02
P,p′-DDE	C₁₄H₈Cl₄	318.03	0.260	6.96
P,p′-DDT	C₁₄H₉Cl₅	354.49	0.025	6.91

K_ow: Octanol water portion coefficient.

Table 2 IUPAC Name and chemical structure of some organochlorine pesticide.

Common name	IUPAC name	Chemical structure
α-HCH	1α, 2α, 3β, 4α, 5β, 6β-Hexachlorocyclohexane
γ-HCH	1α, 2α, 3β, 4α, 5α, 6β-Hexachlorocyclohexane
δ-HCH	1α, 2α, 3α, 4β, 5β, 6β-Hexachlorocyclohexane
Heptachlor	1,4,5,6,7,8,8-Heptachloro-3a,4,7,7a-tetrahydro-4,7-methano-1H-Indene
Aldrin	(1R,4S,4aS,5S,8R,8aR)-1,2,3,4,10,10-hexachloro-1,4,4a,5,8,8a-hexahydro-1,4:5,8-dimethanonaphthalene
Heptachlor epoxide	(1aα, 1bβ, 2α, 5aβ, 6β, 6aα)-2,6-Methano-1H-Indene
Dieldrin	(1R,4S,4aS,5R,6R,7S,8S,8aR)-1,2,3,4,10,10-hexachloro-1,4,4a,5,6,7,8,8a-octahydro-6,7-epoxy-1,4:5,8-dimethanonaphthalene
p,p′-DDE	1,1-Dichloro-2,2-bis(4-chlorophenyl)ethylene
Endrin	(1R,4S,4aS,5S,6S,7R,8R,8aR)-1,2,3,4,10,10-hexachloro-1,4,4a,5,6,7,8,8a-octahydro-6,7-epoxy-1,4:5,8-dimethanonaphthalene
p,p′-DDD	1,1-Dichloro-2,2-bis(4-chlorophenyl)ethane
p,p′-DDT	1,1,1-Trichloro-2,2-bis(4-chlorophenyl)ethane

2.2

2.2 Methods

2.2.1

2.2.1 Surface modification of Na⁺-montmorillonite

20 g of Na⁺-montmorillonite was (1000 ml) dispersed in water containing a stoichiometric quantity of the cationic surfactant, CTAC (6.8 g), which leads to full cation exchange of Na⁺ by CTA at room temperature, sodium chloride was produced as side product as shown in Scheme 1, and subsequently the temperature was increased to 70 °C under vigorous stirring for 8 h. The aggregated organo-clay suspension was separated by filtration and washing five times with distilled water before vacuum dried at 60 °C for 24 h. Complete cation exchange was established on the filtrate by the addition of sliver nitrate (AgNO₃). Finally the clay was grinded with a mortar and sieved into fine powder. The abbreviation, CTA-Mt refers to the treated Na⁺-montmorillonite with the CTAC cationic surfactants.

Intercalating of CTAC onto Na+ Mt followed by in situ emulsion polymerization of MMA.

2.2.2

2.2.2 Preparation of PMMA/Mt-CTA nanocomposites via in situ emulsion polymerization

In three neck round flask, 1.5 g of the hydrophobic clay (Mt-CTA 10 wt%) count on the monomer concentration was left to swell in the monomer phase (10 ml) for 30 min and 0.025 g of sodium dodecylsulphate (SDS) as emulsifier, was dissolved in aqueous phase. Then the mixture was added to an aqueous phase containing 0.1 g of potassium persulphate (PPS) as an initiator. The polymerization process was carried out under stirring at 80 °C for 8 h to assure complete polymerization of methylmethacrylate monomer. Subsequently, the emulsion was precipitated using ethyl alcohol and washing with water in addition to, the prepared nanocomposites was dried in oven at 70 °C.

2.2.3

2.2.3 Method for removing pesticides from wastewater

PMMA-montmorillonite nanocomposites were tried to determine their efficacy to remove OCP from water by column elution technique. Glass column (30 cm × 1.8 cm) was taken and plugged with cotton. A PMMA-montmorillonite nanocomposite (1 g) was loaded into the column, and was washed with 25 ml distilled water. Water sample (500 ml) was fortified with OCP at 0.1 mg l⁻¹ level and passed through the column. Flow rate was adjusted at 5 ml min⁻¹. The elute was collected in a beaker. 100 ml fraction was taken in a 250 ml separatory funnel and was extracted twice with 60 ml of 10% methylene chloride in n-hexane. The combined extracts were dried over anhydrous sodium sulfate and concentrated to about 1 ml in a rotating evaporator. The extracts were concentrated and injected into GC (Agilent 6890) equipped with a Ni⁶³ ECD. GC analysis was conducted on a HP-5MS (Agilent, Folsom, CA) capillary column of 30 m, 0.25 mm id., 0.25 μm film thickness. The oven temperature was programed from an initial temperature 160 (2 min hold) to 260 °C at a rate of 5 °C min⁻¹ and was maintained at 260 °C for 12 min. Injector and detector temperatures were maintained at 300 and 320 °C, respectively. Nitrogen was used as a carrier at flow rate of 3 ml min⁻¹.

2.2.4

2.2.4 Antibacterial activity assay

The agar disk diffusion method was employed for the determination of antimicrobial capabilities of the PMMA/Mt-CTA nanocomposites film. The bacteria (S. aureus or E. coli) were subcultured to nutrient agar and incubated overnight at 37 °C. Afterward, the cells were dispersed in the same medium. The agar plates were streaked with a sterile swab moistened with the bacterial suspension. The PMMA/Mt-CTA film (diameter 8 mm) also placed over the surface of the agar plates and then incubated. All the test plates were incubated overnight at 37 °C. The reaction of the microorganisms with the PMMA/Mt-CTA film was determined by the size of the inhibitory zone. When the materials have an excellent antibacterial activity, the inhibitory zones are very large.

3 Characterizations

The XRD patterns of the PMMA-montmorillonite nanocomposites were carried out on a Diano X-ray diffractometer using Co Kα radiation source energized at 45 kV and a Philips X-ray diffractometer (PW 1930 generator, PW 1820 goniometer) with Cu K radiation source (λ = 0.15418 nm). The basal spacing (d_L) was calculated from the (0 0 1) reflection via the Bragg’s equation. The FTIR Spectra were recorded on a Nexus 821 spectrophotometer, Medison, USA. The thermal stability was evaluated on a thermal gravimetric analyzer (TGA), Perkin Elmer using about 20 mg of the samples at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere. Differential scanning calorimeter (DSC), Perkin Elmer with a heating rate of 10 °C/min under nitrogen atmosphere was used for the determination of the glass transition temperatures. The structure and surface morphology of the prepared nanocomposites were examined using JEOL JEM-1230 transmission electron microscope (TEM) with acceleration voltage of about 80 kV. The microscopy probes of the nanocomposites were prepared by adding a small drop of the emulsion of polymer nanocomposites onto a Lacey carbon film-coated copper grid and allowed to dry initially in air then by applying high vacuum. The SAXS technique was used to provide information regarding the effect of synthesis route, the MMT content and the organophilic modifier on the dispersion level of the silicate layers in the polymer matrix. SAXS curves were recorded with a slit-collimated Kratky compact small-angle system (KCEC/3 Anton-Paar KG, Graz, Austria) equipped with a position-sensitive detector (PSD 50 M from Mbraun) containing 1024 channels 55 μm in width. Cu Kα radiation was generated by a Philips PW1830 X-ray generator operating at 40 kV and 30 mA. All powder samples were filled into 2 mm diameter glass capillaries and introduced to the beam. The Kratky camera was calibrated by using silver behenate with a well-defined lamellar structure (d = 5.848 nm). The optics and the sample cell were both held under vacuum to minimize the scatter from air. A moving slit device was used to measure the intensity of the beam. The raw scattering functions were normalized and corrected by the normalized scattering function of the background. The fractal dimensions were determined from the slope (p) of the linear region of the log I–log h scattering curves. The mass (D_m) and the surface fractal dimension (D_s) were calculated via the relations D_m = |p₁| + 1 and D_s = p₂ + 5. The surface area per unite volume (S/V) and the specific surface area (S_p) values were determined as in Eqs. (1) and (2).

(1)

\frac{S}{V} = \frac{4 w_{1} w_{2} K_{p}}{Q}

(2)

S_{p} = \frac{S \cdot 1000}{Vd}

where Q is the invariant and d is the apparent density, while w₁ and w₂ are the volume fractions of the solid phase and pores, respectively. K_p and Q were determined as in Eqs. (3) and (4).

(3)

K_{p} = \lim_{h \to \infty} I (h) h^{3}

(4)

Q = \int_{0}^{\infty} I (h) h dh

The average intersection lengths characteristic of the individual phases of the two-phase system were calculated, as follows: l₁ = 4w₁V/S, l₂ = 4w₂V/S. The scattering vector (h) was defined as $h = \frac{4 π}{λ} \cdot \sin θ$ , where θ is one-half of the scattering angle. The SAXS measurements were preformed in a slit geometry and no desmearing correction was applied; therefore, the aforementioned relationships for D_s, K_p and Q refer to slit-smeared scattering curves. The calculation details are presented in Dékány et al. (1999), Kratky and Stabinger (1984).

4 Results and discussion

4.1

4.1 Structural properties and thermal stability

The idea upon which nanocomposites were built, is based on carrying out the polymerization process into the inner space of lamella of hydrophobic montmorillonite, which is 1 nm thicker of layer silicate is crowding by the growing polymer chains inside this confined space, might at least expand the inter lamellar space of the clay almost defoliate the ordered layered structure into many separate layers dispersed in the polymer matrix. The exchange of the clay to the organophilic form by replacing the Na⁺ with an organic cation is the key factor to enhance the compatibilization of layered structure materials (e.g. clay) and polymer matrices. This organic cation may be in the form of monomer, initiator or surfactant as in our case (CTAC) and the driving force of the “in situ-emulsion polymerization” technique is associated with the polarity of the monomer molecules and is thought to be the following. During the swelling period, the high surface energy of the layered structure of (Na⁺-Mt) clay attracts (MMA) as polar monomer molecules consequently that they distribute between the clay lamellar. After certain time, equilibrium is reached, the diffusion stops and the clay is swollen in the monomer to a convinced extent (d space = 3.3 nm) as shown in Scheme 1 and evidenced by X-ray diffraction pattern Fig. 1c. When the polymerization process is initiated, the MMA monomer starts to convert to polymer and this reaction lowers the overall polarity of the intercalated molecules and displaces the thermodynamic equilibrium so that more polar molecules are ambitious between the clay lamellar. As this mechanism occurs, the organic molecules can ultimately exfoliate the clay.

XRD of: (a) Na+-Mt, (b) Mt-CTA, (c) Mt-MMA/CTA, and (d) PMMA/Mt-CTA nanocomposites.

Scheme 1 shows the intercalation of CTAC onto the interlayer gallery of Na⁺-Mt followed by the polymerization process.

The X-ray diffraction pattern was revealed in Fig. 1a and b where the pure Na⁺-Mt illustrated the d-spacing of 1.2 nm expanded after organophilized to 1.95 nm. It is obvious that CTAC can expand the basal space of montmorillonite more efficiently and provide great opportunity for the monomer molecule to penetrate between the gallery of the organo-clay and then the polymerization process takes place after the addition of the initiator. Moreover, the XRD profile clearly shows that basal spacing is set in a random fashion in the case of CTAC. This results in extensive intercalation of methylmethacrylate monomer onto the clay gallery and expansion of the interlayer distance, which is likely to afford easier exfoliation upon accomplishing the in-situ polymerization; thus, CTAC was selected for the achievement of the work in this study. Fig. 1c showed that there is no peak at XRD profile which confirmed that the formation of exfoliated nanocomposite, leads to complete dispersion of layer silicate in the PMMA matrix.

The structure of the prepared nanocomposites was investigated in detail by scanning electron microscope as well as transmission electron microscopy (TEM). The SEM photographs of the prepared PMMA nanocomposites containing hydrophobic montmorillonite (10% CTA-MMT) were shown in Fig. 2a and b, confirming the nanostructure of the composites of poly (methylmethacrylate) and hydrophobic montmorillonite. For the sake of clarity, each photograph is shown in two magnifications. The Mt-CTA completely dispersed in the PMMA matrix is clearly evident in the SEM image. In addition, the accomplishment of the intercalation development was examined based on the morphological investigation of the intercalated products. The pure Na⁺-Mt is composed of platelets-like structure as shown in (Fig. 3a), and the montmorillonite after being converted to Mt-CTA form (Fig. 3b) still presents some thicker particle aggregates, but the bulk of the sample is composed of thin platelets and elongated units. The intercalation of CTAC produced products with higher stacking disorder, nominated by broadened reflections of the XRD patterns, is a key factor for penetration of PMMA chains between the layered structures of Mt-CTA. Moreover, Fig. 3c showed the TEM photographs of the PMMA/Mt-CTA which fully match with the assumption of the predominance of exfoliated microstructures in the case of using the simultaneous emulsion methods for the formation of the exfoliated form of nanocomposites which is also proven by XRD (Fig. 1d). In addition the phenomenon which is known as the “mass-thickness contrast” is employed to observe the repartition of the clay layers. The clay has a higher electron density than the polymer. Accordingly, the clay layers will appear darker than the polymer matrix. If the sample is too thick, the polymer appears darker and this reduces the contrast with the clay.

SEM images of PMMA/Mt-CTA nanocomposites prepared via emulsion polymerization using 10% hydrophobic Mt-CTA.

TEM image of (a) Na+-Mt, (b) Mt-CTA, (c) PMMA/Mt-CTA nanocomposites prepared by in situ emulsion polymerization.

SAXS measurements were carried out to illuminate the structural properties for Na⁺-Mt and PMMA/Mt-CTA nanocomposites samples as shown in (Figs. 4 and 5). The scattering curves in log–log representation (Fig. 4) demonstrated significant structural differences for the Na⁺-Mt and PMMA/Mt-CTA nanocomposites. A small shoulder observed in the range of high scattering vectors (h) shifted toward smaller h values in the presence of clay into the polymer matrix after the polymerization process occurring using 10% loading of modified clay (Mt-CTA). Even as, the I (h)-h3-h3 scattering curves (Porod plots) of Na⁺-Mt and PMMA/Mt-CTA nanocomposites are presented in Fig. 5. These curves offered the Porod constant (K_p) for determination of the specific surface areas (S_p) of the samples. The SAXS results confirmed that S_p increased notably (from 43 to 72 m²/g) when the emulsion polymerization of methylmethacrylate monomer took place in the presence of 10% of modified clay. Dispersity on level of Mt-CTA lamellae in PMMA/Mt-CTA nanocomposite was investigated by SAXS measurement. In Fig. 6, the scattering curves of Mt-CTA and PMMA/Mt-CTA nanocomposites were illustrated in log–log representation. Mt-MMT shows basal reflections at h = 3.21 nm⁻¹. The reflection peak disappears in composite contains PMMA. This means, that in the nanocomposite the MMT lamellae are completely exfoliated and encapsulated in polymer matrix. The slopes (p₁ and p₂) from which the fractals were determined are marked in Fig. 7. The p₁ value of −2.26 indicates (according to D_m = |p₁| + 1 relation) that the samples do not show mass fractal behavior. The D_s value of PMMA/Mt-CTA calculated from the slop in the higher h (p₂) range was 2.35, which shows moderate surface roughness of the polymer nanocomposite. The specific surface areas of the samples were calculated from the Porod plots presented in Fig. 7. The calculated parameters (the Porod constant K_p, the specific surface area S_p, the correlation length l_c, and the characteristic length of the solid-phase l₁) are given in Table 3. The surface area of Mt-CTA and PMMA/Mt-CTA determined from SAXS measurement was 18.7 m²/g and 2468 m²/g, respectively as shown in Table 3. The average intersection length characteristic of the individual phases in a two-phase system was also calculated. In this case, l_l gives the approximate diameter of the Mt-CTA packages. The l₁ value was 124.2 nm and 12.7 nm for Mt-CTA and Mt-PMM/CTA, respectively. This means, that the organophilized montmorillonite packages are dispersed in the polymer matrix.

Small-angle X-ray scattering curves in log–log representation of (a) Na+-Mt, and (b) PMMA/Mt-CTA nanocomposites.

Small-angle X-ray scattering curves in Porod representation for Na+-Mt and PMMA/Mt-CTA nanocomposites.

SAXS curves of Mt-CTA and PMMA/Mt-CTA in log–log representation.

Table 3 SAXS parameters, BET surface (a^SBET) and density (d) of Mt-CTA and PMMA/Mt-CTA nanocomposites.

Sample	a^SBET (m²/g)	D_s	K_p (cps/nm³)	S_p (m²/g)	l₁ (nm)	l_c (nm)	d (g/cm³)
Mt-MMT	1.1	–	4.37	18.7	124.2	36.2	1.7172
PMMA/Mt-MMT	23.2	2.35	2.29	246.8	12.7	17.5	1.2779

The FT-IR spectrum of MMT (Fig. 8) shows the vibration bands at 3640 cm⁻¹ for O—H stretching, 3482 cm⁻¹ due to interlayered O—H stretching (H bonding), at the 1660 and 1502 cm⁻¹ for H—O—H bending, 954 and 898 cm⁻¹ for Si—O stretching, 602 cm⁻¹ for Al—OH, 896 cm⁻¹ due to (Al, Mg)—OH vibration modes and 530 and 470 cm⁻¹ for Si—O bending (Liu et al., 2002). Moreover, the intercalation of the CTAC was confirmed by FT-IR which recorded in Fig. 8, in addition to some new bands appeared in the case of Mt-CTA, 1486 cm⁻¹ (—CH₂), 2860 cm⁻¹ (—CH aliphatic) and 3495 cm⁻¹ for interlayer O—H stretching. This was confirmed as well from the XRD shown in Fig. 1. For pure PMMA sample the absorption bands at 2950 and 2846 cm⁻¹ signify CH₂ stretching, and the bands at 1453 and 752 cm⁻¹ correspond to the bending and rocking vibration of CH₂, respectively. The characteristic absorption band at 1733 cm⁻¹ represents stretching vibration of C⚌O of pure PMMA. After the polymerization process took place and PMMA/Mt-CTA nanocomposites were formed, it can be seen that as the effect of MT-CTA, the stretching vibration of the C⚌O groups appearing at the frequency of 1730 cm⁻¹ for pure PMMA moves to lower positions (1697 cm⁻¹) (Kuo et al., 2003; Qian et al., 2005). The characteristic peak at 1733 cm⁻¹ can be assigned to free carbonyl groups of PMMA, while the peak at 1725 cm⁻¹ may be owned by hydrogen-bonded carbonyl groups (Liu et al., 2002). Furthermore, the absorption peak at 3641 cm⁻¹ assigned to for OH stretching shifts to 3495 cm⁻¹. Consequently, there is interaction between CTA-MMT and PMMA molecular chains.

FTIR spectra of untreated (Na+-Mt) as well as treated montmorillonite (Mt-CTA), PMMA and Mt-PMMA/CTA nanocomposites.

The thermal stability of the prepared nanocomposites can be monitored by TGA, and the behavior is shown in Fig. 9 and Table 4 for pure PMMA and the nanocomposites. It was reported elsewhere that two main reaction stages occur through degradation of PMMA in nitrogen atmosphere (Hirata et al., 1985). The first stage, which can be divided into two steps, represents decomposition of weak head-to-head linkages and impurities for the range between 160 and 240 °C, and decomposition of PMMA chain ends around 290 °C. The second stage, between 300 and 400 °C, represents random scission of the polymer chains. In Fig. 9, the pure PMMA decomposition demonstrates two reaction stages, whereas the nanocomposites display only the second stage indicating random scission decomposition. PMMA/Mt-CTA nanocomposites showed enhanced thermal stability when compared to the pure polymer. TGA signified an increase of 60 °C in the 20 wt% decomposition temperature for the nanocomposites prepared by in situ emulsion polymerization. Moreover, the DTG profile of pure PMMA and PMMA/Mt-CTA nanocomposites (Fig. 9) showed a small endothermic peak centered at 79 °C in both images. A major mass loss occurred between ∼264 °C and ∼382 °C, convoyed by a strong exothermic peak at 339 °C and 351 °C for pure PMMA and PMMA/Mt-CTA respectively. This indicates that the bound polymer chains near silica particles (clay) might have higher thermal stability and/or nanosilica may afford a barrier which prevents release of evolved degradation products, trapping the products which recombine to form thermally stable residues. This enhancement in thermal stability of the prepared nanocomposites makes it as appropriate material for packaging applications (Meneghetti and Qutubuddin, 2006).

TGA thermograms and derivatives TGA (SDTA) of pure PMMA as well as PMMA/Mt-CTA nanocomposites prepared by in situ emulsion polymerization.

Table 4 Thermogravimetric results of PMMA and PMMA/Mt-CTA nanocomposites.

Sample name, description	T₁ (°C)	Δm₁ (%)	T₂ (°C)	Δm₂ (%)	T₃ (°C)	Δm₃ (%)	T₄ (°C)	Δm₄ (%)	ΣΔm (%)
PMMA	122.9	−0.57	264.1	−6.83	385.9	−73.9	721.8	−5.52	−86.66
PMMA/Mt-CTA	381.9	−80.3	–	–	–	–	–	–	−80.30

The reason of the improvement of the crystallization rate is that Mt surface nanostructure itself can help the PMMA molecules stack on each other to mature into crystallites, thus leading to the higher crystallization rate. A different possible reason is that Mt can act as an effective heterogeneous nucleating agent. Thus the nanocomposites award higher rate of crystallization than pure PMMA. For heating rate, pure PMMA shows quite small exothermic peak around 184 °C, while PMMA/Mt-CTA nanocomposites still show large crystallization peak around 180 °C. It indicated the crystallinity of the nanocomposites is increased compared with pure PMMA. We suppose that this behavior can be accredited to the interaction between the newly formed crystals on the clay surface at the early stage of crystallization to facilitate the dissipative capability of the nanocomposites. In other terms, DSC result also shows that the glass transition temperature (T_g) of the prepared nanocomposites increases by the addition of 15 wt% of Mt-CTA during the polymerization process from 95 to 134 °C for pure PMMA and Mt-PMM/CTA, respectively.

Differential scanning calorimetry (DSC) curves obtained from PMMA and PMMA/Mt-CTA nanocomposites with 15 wt% of Mt-CTA are showed in Fig. 10a and b. PMMA/Mt-CTA nanocomposites have high crystallization temperature in the presence of Mt-CTA. In DSC, crystallization temperature from melt (T_mc) and its half peak width (width of the peak at half height) are two parameters that can characterize the crystallization rate. When T_mc is higher and half peak width is less, the rate of crystallization is higher. Fig. 10b confirms that adding Mt will increase T_mc, increase the sharpness of crystallization peak, and decrease the half peak width. Consequently, the PMMA/Mt-CTA nanocomposites offer higher crystallization rate than pure PMMA.

(a) DSC thermograms of PMMA prepared via in situ emulsion polymerization and (b) DSC thermograms of PMMA/ Mt-CTA (10 wt%) nanocomposites prepared via in situ emulsion polymerization.

4.2

4.2 Adsorption of pesticides from aquatic solution on PMMA/Mt-CTA

Removal of organochlorine pesticides residues from water by PMMA/Mt-CTA nanocomposites is shown in Table 5. The data revealed that the removal efficiency of PMMA/Mt-CTA nanocomposites for OCPs varied from 73.65% to 99.36%. The adsorption of OCPs was due to high specific surface area of modified montmorillonite (18.7 m²/g) besides the complete exfoliated structure of the layer silicate into the polymer matrix in nanometer level scale and the specific surface area increased to 246.80 m²/g as revealed by SAXS measurements, that enhance the capability of nanocomposites to absorb molecules such as organochlorine pesticides from wastewater and also maximum interaction between the individual layer silicate facilitates the absorption of OCPs. In addition, the surface area of PMMA/Mt-CTA nanocomposites is very large, making it effective for adsorbing chemical compounds.

Table 5 The efficiency of PMMA/Mt-CTA nanocomposites for removing organochlorine pesticide residues from water.

Pesticides	Concentration of OCP (mg/L)		$\frac{(C_{0} - C)}{C_{0}} \times 100$ Removal (%)
Pesticides	Before treatment (C₀)	After treatment (C)	$\frac{(C_{0} - C)}{C_{0}} \times 100$ Removal (%)
α-HCH	0.256	0.031	87.89
γ-HCH	0.320	0.034	89.37
δ-HCH	0.320	0.014	95.50
Heptachlor	0.320	0.052	83.75
Hept. epoxide	1.024	0.006	99.36
Aldrin	0.256	0.015	94.02
Endrin	0.320	0.084	73.65
Dieldrin	0.256	0.046	81.67
P,p′-DDD	0.320	0.018	94.15
P,p′-DDE	0.320	0.026	91.87
P,p′-DDT	0.320	0.055	82.71

It is clear that from Scheme 2 the understanding of the adsorption efficiency of any system studied depends on the nature of interaction between the nanocomposites containing layer silicate nanostructure of adsorbent and the adsorbate (organochlorine pesticides) in the aqueous medium. According the swelling phenomenon, it can conclude that the factors that enhance the swellability of nanocomposites adsorbent are the driving forces for enhancing the adsorption efficiency. All pesticides tested on the column with PMMA/Mt-CTA nanocomposites at saturation point, confirming that the adsorption on PMMA/Mt-CTA nanocomposites is very efficient technique for removal of OCPs from water. However, it is not destructive process and the adsorption efficiency depends on both the surface properties and porosity of the PMMA/Mt-CTA nanocomposites, as well as on the chemical assists and geometry of pesticides. These observations indicate that the PMMA/Mt-CTA nanocomposite material can be used for the treatment of wastewater.

The adsorption of organochlorine pesticides onto the surface of PMMA/Mt-CTA.

4.3

4.3 Antibacterial activity of Mt-CTA and PMMA/Mt-CTA nanocomposites

The PMMA film did not demonstrate clear microbial inhibition zones for E. coli and S. aureus (Fig. 11a), reflecting no antibacterial activity for these materials. The PMMA/Mt-CTA nanocomposites revealed microbial inhibition zones against the two microorganisms in the disk method. The Mt-CTA based films obsessed good antimicrobial activity (as shown in Fig. 11b). This property will be very favorable to the applications of the novel antimicrobial material. The mechanism involves the dissociation of the antibacterial agents from the montmorillonite surface and exertion of their antibacterial effects on bacteria in suspension. The mechanism of the antibacterial activity of nanomaterials includes (a) adsorption onto the bacterial cell surface; (b) diffusion throughout the cell wall; (c) binding to the cytoplasmic membrane; (d) disruption of the cytoplasmic membrane; (e) release of the cytoplasmic constituents; (f) finally, the cell death (Youssef and Abdel-Aziz, 2013; Youssef et al., 2014).

Representative results of antimicrobial activity of (a) Mt-CTA as well as, (b) PMMA/Mt-CTA nanocomposites films containing 10% Mt-CTA against Staphylococcus aureus and Escherichia coli.

5 Conclusion

PMMA/Mt-CTA nanocomposites were successfully prepared via in-situ emulsion polymerization using hydrophobic montmorillonite that was expended from 11.48 Å to 19.49 Å. Furthermore, the Mt-CTA form was used as hosts for the preparation of poly (methylmethacrylate) nanocomposites. Exfoliated nanocomposites were obtained after polymerization process took place and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The fashioned nanocomposites exhibited better thermal stability in comparison with the pure polymethylmethacrylate. Adsorptions from aquatic solution of pesticides were investigated on the PMMA/Mt-CTA nanocomposites partially in column experiments. The affinity of the prepared nanocomposites demonstrated high efficiency for elimination of organochlorine pesticides (OCPSs) from wastewater and the removal percent diverse from 73.65% to 99.36%. The prepared Mt-CTA and PMMA/Mt-CTA were representative antimicrobial activity against E. coli and S. aureus. Consequently, the PMMA/Mt-CTA nanocomposites can be used as packaging materials.

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Introduction

Experimental section

Materials
Methods

Characterizations

Results and discussion

Structural properties and thermal stability
Adsorption of pesticides from aquatic solution on PMMA/Mt-CTA
Antibacterial activity of Mt-CTA and PMMA/Mt-CTA nanocomposites

Conclusion

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[1] Abd El-Ghaffar, M.A., Youssef, A.M., Abdelhakim, A.A., 2015. Polyaniline nanocomposites via in-situ emulsion polymerization based on montmorillonite; preparation & characterization. Arab. J. Chem. 8, 771–779.

[2] Alexandre M., Dubois P., . Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater. Sci. Eng.: R: Rep.. 2000;28:1-63.
[Google Scholar]

[3] Bujdosó T., Patzkó A., Galbács Z., Dékány I., . Structural characterization of arsenate ion exchanged MgAl-layered double hydroxide. Appl. Clay Sci.. 2009;44:75-82.
[Google Scholar]

[4] Crisafully R., Milhome M.A.L., Cavalcante R.M., Silveira E.R., Keukeleire D.D., Nascimento R.F., . Removal of some polycyclic aromatic hydrocarbons from petrochemical wastewater using low-cost adsorbents of natural origin. Bioresour. Technol.. 2008;99:4515-4519.
[Google Scholar]

[5] Dékány I., . Liquid adsorption and immersional wetting on hydrophilic/hydrophobic solid surfaces. Pure Appl. Chem.. 1992;64:1499-1509.
[Google Scholar]

[6] Dékány I., Farkas A., Király Z., Klumpp E., Narres H.D., . Interlamellar adsorption of 1-pentanol from aqueous solution on hydrophobic clay mineral. Colloids Surf.. 1996;119:7-13.
[Google Scholar]

[7] Dékány I., Turi L., Fonseca A., Nagy J.B., . The structure of acid treated sepiolites: small-angle X-ray scattering and multi MAS-NMR investigations. Appl. Clay Sci.. 1999;14:141-160.
[Google Scholar]

[8] Dietsche F., Muelhaupt R., . Thermal properties and flammability of acrylic nanocomposites based upon organophilic layered silicates. Polym. Bull.. 1999;43:395-402.
[Google Scholar]

[9] Fischer H.R., Gielgens L.H., Koster T.P.M., . Nanocomposites from polymers and layered minerals. Acta Polym.. 1999;50:122-126.
[Google Scholar]

[10] Fu X., Qutubuddin S., . Polymer-clay nanocomposites: exfoliation of organophilic montmorillonite nanolayers in polystyrene. Polymer. 2001;42:807-813.
[Google Scholar]

[11] Giannelis E.P., . Polymer layered silicate nanocomposites. Adv. Mater.. 1996;8:29-35.
[Google Scholar]

[12] Guimaraes J.L., Peralta-Zamora P., Wypych F., . Covalent grafting of phenylphosphonate groups onto the interlamellar aluminol surface of kaolinite. J. Colloid Interface Sci.. 1998;206:281-287.
[Google Scholar]

[13] Hardell L., Lindström G., Liljegren G., Dahl P., Magnuson A., . Increased concentrations of octachlorodibenzo-p-dioxin in cases with breast cancer – results from a case-control study. Eur. J. Cancer Prev.. 1996;5:351-357.
[Google Scholar]

[14] Haroun A.A., Youssef A.M., . Synthesis and electrical conductivity evaluation of novel hybrid poly (methyl methacrylate)/titanium dioxide nanowires. Synth. Met.. 2011;161:2063-2069.
[Google Scholar]

[15] Hayashi S., . NMR study of dynamics and evolution of guest molecules in kaolinite/dimethyl sulfoxide intercalation compound. Clays Clay Miner.. 1997;45:724-732.
[Google Scholar]

[16] Hirata T., Kashiwagi T., Brown J., . Thermal and oxidative degradation of poly(methyl methacrylate): weight loss. Macromolecules. 1985;18:1410-1418.
[Google Scholar]

[17] Hu C.H., Xu Z.R., Xia M.S., . Anti-bacterial effect of Cu²⁺-exchanged montmorillonite on Aeromonas hydrophila and discussion on its mechanism. Vet. Microbiol.. 2005;109:83-88.
[Google Scholar]

[18] Huang X., Brittain W.J., . Synthesis and characterization of PMMA nanocomposites by suspension and emulsion polymerization. Macromolecules. 2001;34:3255-3260.
[Google Scholar]

[19] Jo S.C., Rim A.R., Park H.J., Yuk H.G., Lee S.C., . Combined treatment with silver ions and organic acid enhances growth-inhibition of Escherichia coli O157:H7. Food Control. 2007;18:1235-1240.
[Google Scholar]

[20] Komori Y., Sugahara Y., Kuroda K., . A kaolinite-NMF-methanol intercalation compound as a versatile intermediate for further intercalation reaction of kaolinite. J. Mater. Res.. 1998;13:930-934.
[Google Scholar]

[21] Kotsilkova R., Petkova V., Pelovski Y., . Thermal analysis of polymer-silicate nanocomposites. J. Therm. Anal. Calorim.. 2001;64:591-598.
[Google Scholar]

[22] Kratky O., Stabinger H., . X-ray small angle camera with block-collimation system an instrument of colloid research. Colloid Polym. Sci.. 1984;262:345-360.
[Google Scholar]

[23] Kuo S.-W., Kao H.-C., Chang F.-C., . Thermal behavior and specific interaction in high glass transition temperature PMMA copolymer. Polymer. 2003;44:6873-6882.
[Google Scholar]

[24] Lagaly G., Beneke K., . Intercalation and exchange reactions of clay minerals and non-clay layer compounds. Colloid Polym. Sci.. 1991;269:1198-1211.
[Google Scholar]

[25] Lee S.Y., Kim S.J., Chung S.Y., Jeong C.H., . Sorption of hydrophobic organic compounds onto organoclays. Chemosphere. 2004;55:781-785.
[Google Scholar]

[26] Li Y., Zhao B., Xie S., Zhang S., . Synthesis and properties of poly (methyl methacrylate)/montmorillonite (PMMA/MMT) nanocomposites. Polym. Int.. 2003;52:892-898.
[Google Scholar]

[27] Liu Y., Lee J.Y., Hong L., . Synthesis, characterization and electrochemical properties of poly (methyl methacrylate)-grafted-poly(vinylidene fluoride-hexafluoropropylene) gel electrolytes. Solid State Ionics. 2002;150:317-326.
[Google Scholar]

[28] Meneghetti P., Qutubuddin S., . Synthesis, thermal properties and applications of polymer-clay nanocomposites. Thermochim. Acta. 2006;442:74-77.
[Google Scholar]

[29] Mercier L., Pinnavaia T.J., . A functionalized porous clay heterostructure for heavy metal ion (Hg²⁺) trapping. J. Microporous Mesoporous Mater.. 1998;20:101-106.
[Google Scholar]

[30] Meyn M., Beneke K., Lagaly G., . Anion-exchange reactions of layered double hydroxides. Inorg. Chem.. 1990;29:5201-5207.
[Google Scholar]

[31] Moysich K.B., Ambrosone C.B., Vena J.E., Shields P.G., Mendola P., Kostyniak P., Greizerstein H., Graham S., Marshall J.R., Schisterman E.F., Freudenheim J.L., . Environmental organochlorine exposure and postmenopausal breast cancer risk. Biomarkers Prev.. 1998;7:181-188.
[Google Scholar]

[32] Nassar M., Youssef A.M., . Mechanical and antibacterial properties of recycled carton paper coated by PS/Ag nanocomposites for packaging. Carbohydr. Polym.. 2012;89:269-274.
[Google Scholar]

[33] Ogawa M., Kuroda K., . Photofunctions of intercalation compounds. Chem. Rev.. 1995;95:399-438.
[Google Scholar]

[34] Youssef A.M., . Polymer nanocomposites as a new trend for packaging applications. Polym.-Plast. Technol. Eng.. 2013;52:635-660.
[Google Scholar]

[35] Youssef A.M., . Morphological studies of polyaniline nanocomposite based mesostructured TiO₂ nanowires as conductive packaging materials. RSC Adv.. 2014;4:6811-6820.
[Google Scholar]

[36] Youssef A.M., Abdel-Aziz M.S., . Preparation of polystyrene nanocomposites based on silver nanoparticles using marine bacterium for packaging. Polym.-Plast. Technol. Eng.. 2013;52:607-613.
[Google Scholar]

[37] Youssef A.M., Malhat F., . Selective removal of heavy metals from drinking water using titanium dioxide nanowire. Macromol. Symp.. 2014;337:96-101.
[Google Scholar]

[38] Youssef A.M., El- Samahy M.A., Abdel Rehim M., . Preparation of conductive paper composites based on natural cellulosic fibers for packaging applications. Carbohydr. Polym.. 2012;89:1027-1032.
[Google Scholar]

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Synthesis and utilization of poly (methylmethacrylate) nanocomposites based on modified montmorillonite