New poly(ether-amide-imide) reinforced layer silicate nanocomposite: Synthesis and properties

2.1 Materials

Trimellitic anhydride, 4,4′-diamino diphenyl ether, acetic acid, triphenyl phosphite (TPP), CaCl₂, pyridine and N-methyl-2-pyrrolidone (NMP) were purchased from Merck Chemical Company and used without further purification. The organically modified Cloisite® 20A supplied by Southern Clay Products (TX), was used as polymer nano reinforcement. The organic modifier and the interlayer distance of the clays are shown in Table 1 to account for the structural modifications of the functionalizations.

Table 1 Organic modifiers and interlayer distance of the clays.

Type of clay	Organic modifier	Concentration of organic modifier (meq/100 g clay)	Interlayer distance g/cc
Cloisite® 20A		95	1.77

HT = Hydrogenated Tallow (∼65% C₁₈; ∼30% C₁₆; ∼5% C₁₄).

2.2 Monomer synthesis

2.3 Polymer synthesis

A mixture of 1.1 g (2 mmol) of N,N'-(4,4′-diphenylether)bistrimellitimide, 0.4 g (2 mmol) of 4,4′-diamino diphenyl ether 2, 0.2 g of CaCl₂, 0.6 mL of Pyridine, 2 mL of TPP, and 2 mL of NMP were heated while being stirred at 120 °C for 5 h. The viscosity of the reaction solutions increased after 30 min, and additional NMP was added to the reaction mixture. At the end of the reaction, the obtained polymer solution was trickled into stirred methanol. The yellow, stringy polymer was washed thoroughly with hot water and methanol, collected by filtration, and dried at 100 °C under reduced pressure. The resulting polymer 3 was dried under vacuum to leave 0.13 g (97%) of solid polymer. The inherent viscosity of this soluble PEAI 3 was 0.42 dL/g. IR (KBr): 3235 (m), 3064 (m), 1776 (w), 1726 (s), 1672 (s), 1605 (m), 1508 (m), 1421 (m), 1380 (m), 1302 (s), 1220 (m), 1141 (m), 794 (w), 756 (w), 725(w).

2.4 PEAI-nanocomposite synthesis of 3a and 3b

PEAI-nanocomposites 3a and 3b were produced by the solution intercalation method, two different amounts of organoclay particles (5 and 10 wt.%) were mixed with appropriate amounts of PEAI solution in N-methyl-2-pyrrolidone (NMP) to yield particular nanocomposite concentrations. To control the dispersibility of organoclay in poly(amide-imide) matrix, constant stirring was applied at 25 °C for 24 h. Nanocomposite films were cast by pouring the solutions of each concentration into Petri dishes placed on a leveled surface followed by the evaporation of solvent at 70 °C for 12 h. Films were dried at 80 °C under vacuum to a constant weight. Scheme 1 shows the flow sheet diagram and the synthetic scheme for PEAI-nanocomposite films 3a and 3b.

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Scheme 1

Synthetic route of N,N′-(4,4′-diphenylether)bistrimellitimide.

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2.5 Measurements

IR spectra were recorded on a Galaxy series FTIR 5000 spectrophotometer (England). Band intensities are assigned as weak (w), medium (m), strong (s) and band shapes as shoulder (sh), sharp (s) and broad (br). UV–Vis absorptions were recorded at 25 °C in the 190–700 nm spectral regions with a Perkin-Elmer Lambda 15 spectrophotometer on NMP solutions by using cell path lengths of 1 cm. Inherent viscosity was measured by a standard procedure using a Technico® viscometer. Thermogravimetric analysis (TGA) data were taken on a Mettler TA4000 System under N₂ atmosphere at a rate of 10 °C/min. The morphology of nanocomposite film was investigated on a Cambridge S260 scanning electron microscope (SEM).

3.1 Monomer synthesis

Diacid 1 was synthesized by the condensation reaction of two equimolars of trimellitic anhydride with one equimolar of 4,4′-diamino diphenyl ether in acetic acid solution (Scheme 1).

The chemical structure of diacid 3 was confirmed by FT-IR and ¹H-NMR spectroscopy.

3.2 PEAI-nanocomposite films

PEAI-Nanocomposites were prepared by the appropriate amounts of Cloisite® 20A and PEAI in NMP (Scheme 2). PEAI-nanocomposite films were transparent and yellowish brown in color. The incorporation of organoclay changed the color of films to dark yellowish brown. Moreover, a decrease in the transparency was observed at higher clay contents. Scheme 2 shows the flow-sheet diagram and the synthetic scheme for PEAI-nanocomposite films 3a and 3b.

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Scheme 2

Flow sheet diagram for the synthesis of PEAI-nanocomposite films 3a and 3b.

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3.3 FT-IR spectroscopy analysis

FT-IR data of PEAI-nanocomposite films 3b and 3b showed the characteristic absorption bands of the Si-O and Mg-O moieties at 1019 and 1018 cm⁻¹ respectively. The incorporation of organic groups in PEAI-nanocomposite films was confirmed by the presence of peaks around 1776, 1726, 1380, 725 (imide rings) and 1650 (amide carbonyl group) (Fig. 1).

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Figure 1

FT-IR spectra of PEAI, nanocomposites 3a and 3b.

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3.4 X-ray diffraction analysis

The XRD is most useful for the measurement of interlayer spacing of the organoclay upon the formation of the nanocomposites. It supplies information on the change of d-spacing of ordered immiscible and ordered intercalated nanocomposites. Fig. 2 shows the XRD patterns of PEAI-nanocomposite films 3a and 3b containing 5 and 10 wt.% of silicate particles. The Cloisite® Na gives a distinct peak around 2θ equal to 8.93, which corresponds to a basal spacing of around 1.00 nm. The organically modified Cloisite® 20A employed for the preparation of nanocomposites has a typical peak at 2θ equal to 6.56 increased d-spacing, when the amount of organoclay increased (5–10 wt.%) in the nanocomposites. These results indicated a significant expansion of the silicate layer after the insertion of PEAI chains. The shift in the diffraction peaks of PEAI-Nanocomposite films confirms that intercalation has taken place. This is direct evidence that PEAI-Nanocomposites have been formed as the nature of intercalating agent also affects the organoclay dispersion in the polymer matrix. Usually there are two types of nanocomposites depending upon the dispersion of clay particles. The first type is an intercalated polymer clay nanocomposite, which consists of well ordered multi layers of polymer chain and silicate layers a few nanometers thick. The second type is an exfoliated polymer–clay nanocomposite, in which there is a loss of ordered structures due to the extensive penetration of polymer chain into the layer of silicate. Such part would not produce distinct peaks in the XRD pattern (Krishnan et al., 2007).

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Figure 2

X-ray diffraction patterns of organoclay, PEAI-nanocomposites 3a and 3b.

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3.5 Scanning electron microscopy

In order to investigate the morphology, fractured surfaces of PEAI-nanocomposite films were studied using SEM. The micrographs of the nanocomposites containing 5 and 10 wt.% silica in the matrix are shown in Fig. 3. The results show a fine dispersion of silica particles in the matrix when the concentration of inorganic phase is increased. Nanocomposite films have a homogeneous distribution with no preferential accumulation of silica in any region across the films. The micrographs also indicate the presence of interconnected silica domains in the continuous polyamide phase, which demonstrates better compatibility between smaller silica nanoparticles and the PEAI in the nanocomposite films.

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Figure 3

SEM micrographs of the PEAI-nanocomposites with various silica contents (wt%): 5 and 10.

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3.6 Optical clarity of PEAI-nanocomposite films

Optical clarity of PEAI-nanocomposite films containing 5–10 wt.% clay platelets and neat PEAI was compared by UV–Vis spectroscopy in the region of 260–800 nm. Fig. 4 shows the UV–Vis transmission spectra of pure PEAI and PEAI-nanocomposite films containing 5 and 10 wt.% clay platelets. These spectra show that the UV–Visible region (250–800 nm) is affected by the presence of clay particles and exhibiting low transparency reflected to the primarily intercalated composites. Results show that pure PEAI and PEAI-nanocomposite films with various amounts of silica are transparent. The maximum transmittance was found for the PEAI. The transparency of these naocomposites depends upon the size and spatial distribution of silica particles in the PEAI matrix. Nanocomposite films were transparent because the average size of ceramic particles is smaller than the wavelength of light, and the distribution of particles is relatively uniform. Ultimately the tendency for the agglomeration of small particles into larger ones may increase, which decreases the homogeneity of the system. As particle size becomes larger, the transmittance values decrease.

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Figure 4

UV–Vis spectra of PEAI 5, PEAI-nanocomposite films 3a–3b.

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3.7 Thermal properties

The thermal properties of PEAI-nanocomposite films containing 5 and 10 wt.% clay platelets and neat PEAI were investigated by TGA in a nitrogen atmosphere at a heating rate of 10 °C/min (Fig. 5). Initial decomposition temperature, 5% and 10% weight loss temperatures (T₅, T₁₀) and char yields are summarized in Table 2. These samples exhibited good resistance to thermal decomposition. T₅ for neat PEAI and PEAI-nanocomposite films containing 5 and 10 wt.% clay platelets ranged from 270 to 337 °C and T₁₀ for them ranged from 388 to 418 °C, and residual weights at 800 °C ranged from 38% to 44.5% in nitrogen respectively. Incorporation of organoclay into the PEAI matrix also enhanced the thermal stability of the nanocomposites. Thus, we can speculate that interacting PEAI chains between the clay layers serve to improve the thermal stability of nanocomposites. The addition of organoclay in polymeric matrix can significantly improve the thermal stability of PEAI.

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Figure 5

TGA–DTG curve for (a) PEAI, (b) PEAI-nanocomposite 3a, and (c) PEAI-nanocomposite 3b.

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3.8 Water absorption measurements

The water absorption of PEAI-nanocomposite films was carried out using a procedure under ASTM D570-81 (Zulfiqar and Sarwar, 2008). The results showed a monotonic maximum water uptake for the pure polyamide (16.01%) but an asymptotic decrease thereafter (Table 2). The exposure of polar groups to the surface of polymer where water molecules develop secondary bond forces with these groups. The clay platelets obviously restrict the access of water to the hydrogen-bonding sites on the polymer chains. The weight gain by the films gradually decreased as the clay content was increased. It is apparently due to the mutual interaction between the organic and inorganic phases. This interaction resulted in the lesser availability of polar groups to interact with water. Secondly, the impermeable clay layers mandate a tortuous pathway for a permeant to transverse the nanocomposite. The enhanced barrier characteristics, chemical resistance and reduced solvent uptake of PEAI-nanocomposites all benefit from the hindered diffusion pathways through the nanocomposite.