Preparation and characterization of new polyamide/montmorillonite nanocomposites containing azo moiety in the main chain

2.1 Materials

4-Nitrobenzoic acid, 4,4′-diaminodiphenyl ether, acetic acid, triphenyl phosphite (TPP), CaCl₂, pyridine and N-methyl-2-pyrrolidone (NMP) were purchased from Merck Chemical Company and used without previous purification. Purified organically-modified Cloisite 20A (Where HT is Hydrogenated Tallow) supplied by Southern Clay Products (TX), were used as polymer nano reinforcement. The organic modifiers and the interlayer distance of the clays are shown in Table 1 to account the structural modifications of the functional points.

Table 1 Organic modifiers and interlayer distance of the clays.

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

2.2 Equipment

¹H-NMR spectrum was recorded on a Bruker 300 MHz instrument (Germany). Fourier transform infrared (FTIR) spectra were recorded on Galaxy Series FTIR 5000 spectrophotometer (England). UV–vis spectra were recorded at 25 °C in the 250–700 nm spectral regions with a Perkin–Elmer Lambda 15 spectrophotometer in NMP solution using cell lengths of 1.0 cm. Thermal gravimetric analysis (TGA and DTG) data were taken on a Mettler TA4000 System under N₂ atmosphere at a rate of 10 °C/min. The morphology and structure of nanocomposite film were investigated on Cambridge S260 scanning electron microscope (SEM). X-ray diffraction (XRD) were performed on Philips X-Pert (Cu–Ka radiation, λ = 0.15405 nm). The water absorption of PA-nanocomposite films were carried out using a procedure under ASTM D570-81 (Zulfiqar and Sarwar, 2008).

2.3 Monomer synthesis

4,4′-Azobenzoic acid 2 was prepared according to a typical procedure shown in Scheme 1 (Faghihi and Hagibeygi, 2003).

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Scheme 1

Synthetic route of 4,4′-azobenzoic acid 2.

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2.4 Polymer synthesis

Into a 100 mL round bottomed flask was placed a mixture of 4,4′-azobenzoic acid 2 (0.002 mol), 4,4′-diaminodiphenyl ether 3 (0.002 mol), 0.60 g of calcium chloride, 1.0 mL of triphenyl phosphite, 1.0 mL of pyridine and 4.0 mL NMP. The mixture was heated for 1 h at 60 °C, 2 h at 90 °C and then refluxed at 140 °C for 8 h until a viscous solution was formed. Then it was cooled to room temperature and 30 mL of methanol was added to reaction mixture. The precipitate was formed, filtered off and washed with methanol. The resulting polyamide 4 was dried under vacuum. The inherent viscosity of this polymer in a solution of DMF was 0.55 dL/g. Elemental analysis for C₂₆H₁₈N₄O₃, Calcd. C, 71.88; H, 4.18; N, 12.90; found, C, 70.99; H, 4.15; N, 12.82.

2.5 PA-nanocomposite synthesis 4a and 4b

PA-nanocomposites 4a and 4b were produced by solution intercalation method. Two different amounts of organoclay particles (10 and 20-wt%) were mixed with appropriate amounts of PA solution in N-methyl-2-pyrrolidone (NMP) to yield particular nanocomposite concentrations. To control the dispersibility of organoclay in polyamide matrix, constant stirring was applied at 25 °C for 24 h. Nanocomposite films were cast by pouring the solutions for 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 synthetic scheme for PA-nanocomposites film 4a and 4b.

2.6 The water absorption analysis

The water absorption of PA-nanocomposite films was carried out using a procedure under ASTM D570-81 (Zulfiqar and Sarwar, 2008). The films were dried in a vacuum oven at 80 °C to a constant weight and then weighed to get the initial weight (W₀). The dried films were immersed in deionized water at 25 °C. After 24 h, the films were removed from water and then they were quickly placed between sheets of filter paper to remove the excess water and the films were weighed immediately. The films were again soaked in water. After another 24 h soaking period, the films were taken out, dried and weighed for any weight gain. This process was repeated again and again till the films almost attained the constant weight. The total soaking time was 168 h and the samples were weighed at regular 24 h time intervals to get the final weight (W_f). The percent increase in weight of the samples was calculated by using the formula (W_f − W₀)/W₀ 100%.

3.1 Monomer synthesis

4,4′-Azobenzoic acid 2 was synthesized from the reduction of 4-nitrobenzoic 1 acid by glucose in a basic solution (Scheme 1).

The chemical structure of diacid 2 was confirmed by FTIR and ¹H-NMR spectroscopy. In the FT-IR spectrum of diacid 2 peaks appearing at 2500–3400 cm⁻¹ (acid O–H stretching) and 1709 cm⁻¹ (C⚌O acid) confirmed the presence of carboxylic groups in this compound (Fig. 1). The ¹H-NMR spectrum of diacid 2 shows H(a) protons relevant to O–H carboxylic groups at 12.0 ppm and peaks between 8.2 and 8.19 ppm related to aromatic protons (Fig. 2).

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Figure 1

FT-IR spectrum of diacid 2.

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Figure 2

¹H-NMR spectrum of diacid 2.

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3.2 Polymer synthesis

Polyamides 4 were synthesized by the direct polycondensation reaction of an equimolar mixture of diacid 2, and diamine 3 by using triphenyl phosphite (TPP) and pyridine as condensing agents (Scheme 2). PA 4 was obtained in good yield (99%) and had an inherent viscosity (0.55 dL/g) in a solution of DMF. The structure of the resulting polymer 4 was confirmed as PA by using FTIR spectroscopy and elemental analyses. The resulting polymer has an absorption band at 1680 cm⁻¹ due to amide carbonyl groups. Absorption bands between 3350 and 3500 cm⁻¹ demonstrated the presence of the amide N–H stretching absorption in this polymer (Fig. 3). The elemental analysis value of the resulting polymer was in good agreement with the calculated values for the proposed structure.

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Scheme 2

Synthetic route of PA 4.

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Figure 3

FT-IR spectrum of: (a) Organoclay, (b) PA 4, (c) PA-nanocomposite film 4a, (d) and PA-nanocomposite film 4b.

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3.3 PA-nanocomposite films

PA-nanocomposite films were transparent and have light orange color. The incorporation of organoclay changed the color of films to dark orange color. Moreover, a decrease in the transparency was observed at higher clay contents.

3.4 FT-IR spectroscopy analyses

FT-IR spectrum of PA-nanocomposite films 4a and 4b showed the characteristic absorption bands of the Si–O moieties at 1040 and 1035, Mg–O moieties at 465 and 454 cm⁻¹, respectively. The incorporation of organic groups in PA-nanocomposite films was confirmed by the presence of peaks at 3280 and 1650–1662 (amide N–H stretching and carbonyl group) (Fig. 3).

3.5 X-ray diffraction analysis

Fig. 4 shows the XRD patterns of PA-nanocomposite films 4a and 4b containing 10 and 20-wt% of silicate particles. These results indicated significant expansion of the silicate layer after insertion of PA chains. The shift in the diffraction peaks of PA-nanocomposite films confirms that intercalation has been taken place. This is direct evidence that PA-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 the polymer chain into the silicate layer. Such parts would not produce distinct peaks in the XRD pattern (Bharadwaj, 2001). In our PA-nanocomposite films there are coherent XRD signal at 5.80° and 4.21° related to 10 and 20 wt% nanocomposite films, respectively.

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Figure 4

X-ray diffraction patterns of: (a) Organoclay, (b) PA-nanocomposite films 4a, (c) and 4b.

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3.6 Scanning electron microscopy

The surface morphology of the PA-nanocomposite films prepared by solution intercalation technique is compared by SEM analyses. Fig. 5 shows the morphological images of 10 and 20 wt% nanocomposite films, respectively. The SEM images show that the PA matrix has a smooth morphology. Also SEM micrographs of PA-nanocomposite containing 10 and 20 wt% clay platelets were uniformly distributed without agglomeration.

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Figure 5

Scanning electron micrographs of PA-nanocomposite films 4a and 4b.

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3.7 Optical clarity of PA-nanocomposite films

Optical clarity of PA-nanocomposite films containing 10 and 20 wt% clay platelets and neat PA was compared by UV–vis spectroscopy in the region of 300–800 nm. Fig. 6 shows the UV–vis transmission spectra of pure PA and PA-nanocomposite films containing 10 and 20 wt% clay platelets. This spectrum shows that the UV–visible region (500–800 nm) is affected by the presence of the clay particles and exhibiting low transparency reflected to the primarily intercalated composites. Results shows that the optical clarity of PA-nanocomposite films system is significantly lower in the neat PA system.

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Figure 6

UV–vis spectra of PA 4, PA-nanocomposite films 4a and 4b.

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3.8 Thermogravimetric analysis

The thermal properties of PA-nanocomposite films containing 10 and 20 wt% clay platelets and neat PA were investigated by using TGA in nitrogen atmosphere at a rate of heating of 10 °C/min, and thermal data are summarized in Table 2. These samples exhibited good resistance to thermal decomposition, up to 265–380 °C in nitrogen, and began to decompose gradually above this temperature. T₅ for these polymers ranged from 190 to 220 °C and T₁₀ for them ranged from 240 to 290 °C (temperature at which 5% and 10% weight loss was recorded by TGA at heating rate of 10 °C/min in N₂, respectively), and residual weights at 600 °C ranged from 53.20% and 62.31% in nitrogen, respectively (Fig. 7). Incorporation of organoclay into the PA matrix also enhanced the thermal stability of the nanocomposites. Fig. 5 shows the TGA thermograms of PA-nanocomposites under nitrogen atmosphere. Thus, we can speculate that interacting PAs 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 PA.

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Figure 7

TGA thermograms of neat PA 4 and PA-nanocomposite films 4a and 4b.

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3.9 Water absorption measurements

The PA under investigation contains polar amide groups in the backbone that have the tendency to uptake water through hydrogen bonding. Thus water absorption measurements become necessary for neat PA 4 and PA-nanocomposite films 4a and 4b and data are shown in Table 2. In the water permeability studies, we found that the incorporation of clay platelets into PA matrix results in a decrease of water uptake relative to pure PA by forming the tortuous path of water permeant. Water permeability depends on length, orientation and degree of delamination of layered silicate (Zulfiqar and Sarwar, 2008). It should be noted that a further increase in clay concentration resulted in an enhanced barrier property of nanocomposites which may be attributed to the plate-like clays that effectively increase the length of the diffusion pathways, as well as decrease the water permeability.