Hydrogen-Bonding Assembly of Rigid-Rod Poly(p-sulfophenylene

Publication Date (Web): November 17, 2014 ... the PVA/sPPTA composite exhibits the best mechanical properties, with a tensile strength of 169 ± 13 MP...
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Hydrogen-Bonding Assembly of Rigid-Rod Poly(p‑sulfophenylene terephthalamide) and Flexible-Chain Poly(vinyl alcohol) for Transparent, Strong, and Tough Molecular Composites Mao Peng,* Guohua Xiao, Xinglei Tang, and Yang Zhou MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Molecular composites comprising poly(p-sulfophenylene terephthalamide) (sPPTA), a sulfonated polyaramid rigidrod polyelectrolyte, and flexible-chain poly(vinyl alcohol) (PVA) were prepared by a green and easy-to-scale-up water casting method. Influence of sPPTA on the microstructure and properties of the molecular composites was systematically investigated. Fourier transform infrared spectroscopy confirms the existence of hydrogen bonding between sPPTA and PVA. Wide-angle X-ray diffraction patterns do not show the characteristic of neat sPPTA crystalline aggregates in the composites even when the sPPTA content is as high as 33 wt %, suggesting that the strong interaction between sPPTA and PVA prevents the self-aggregation of sPPTA and leads to the formation of PVA/sPPTA complexes inside the composites. Transmission electron microscopy shows that sPPTA has good compatibility with PVA, and nanoscale fibril-like supramolecular assemblies dispersing uniformly in the composites become observable with the increase of sPPTA content. Moreover, the PVA/sPPTA complexes have a strong effect on the melt point, crystallinity, mechanical properties, and thermal stability of PVA. The PVA/sPPTA composites exhibit both high strength and high ductility. When the content of sPPTA is 5 wt %, the PVA/sPPTA composite exhibits the best mechanical properties, with a tensile strength of 169 ± 13 MPa, which is 54% higher than that of neat PVA (110 ± 10 MPa). Surprisingly, the reinforcement factor is even superior to that of multiwalled carbon nanotubes, vapor grown carbon fibers, and nanodiamonds previously reported for the reinforcement of PVA nanocomposites. Moreover, the PVA/sPPTA molecular composites have a relatively low modulus but a much larger elongation at break than prefabricated nanocomposites, showing good ductility. The strong and tough PVA/sPPTA molecular composites can be potentially used as high performance membranes or fibers in the future.

1. INTRODUCTION 1

A serials of water-soluble rigid-rod polyelectrolytes based on sulfonated polyaramid, having a Kevlar-like rigid conjugated main chain and numerous sulfonic acid side groups imparting them with water solubility, have been synthesized.10−17 Poly(psulfophenylene terephthalamide) (sPPTA) is one of the most extensively investigated sulfonated polyaramids. Different from flexible polyelectrolytes, sulfonated polyaramids exhibit nematic needle-like supramolecular aggregates in water and form anisotropic liquid crystal phase and even gels at low concentrations. The driving force for the aggregation of sPPTA is the intramolecular hydrogen bonding between sulfonic acid group and the proton of the amide bond and the strong hydrophobic interaction of unsubstituted aromatic rings leading to the strong packing between the main chains. The rigid−flexible polyelectrolyte complexes of sPPTA and cationic polyelectrolytes have been investigated. For example, the effect of cationic poly(allylamine hydrochloride) (PAH) on

2

Since Helminiak and Takayanagi presented the concept of molecular composites more than three decades ago, a variety of polymeric composite systems with rigid-rod polymers as the reinforcing agents and flexible polymers as the matrices have been produced for evaluation. The major obstacle for the production of true molecular composites is to molecularly disperse the stiff, rigid-rod polymers in the flexible matrices because of the unfavorable enthalpy of mixing even for polymers with similar chemical structure.3 Many studies reported phase separation and limited increase of the mechanical properties of the composites prepared by direct blending rigid and flexible polymers. To resolve this problem, researchers have synthesized the copolymers of the rigid and flexible polymers or introduced certain interactions, such as ionic,4 acid−base interaction,5,6 and hydrogen bonding7,8 between the two components. For example, Robers et al.9 reported the preparation of molecular composites of poly(pphenylene-2,6-benzobisthiazole) and nylon-66 from their soluble coordination complexes in organic solvents. © XXXX American Chemical Society

Received: August 1, 2014 Revised: November 2, 2014

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Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All materials were used without further purification. 2.2. Synthesis of SPPTA. sPPTA was synthesized by direct polycondensation following a procedure described in the literature with some modifications.7 1.0 g (5.3 mmol) of 2,5-DABSA and 0.55 g (5.3 mmol) of Na2CO3 were dissolved in 100 mL of H2O in a 250 mL one-necked flask. 1.08 g (5.3 mmol) of TPC in 50 mL of benzene was then added dropwise to the flask. After being mechanically stirred for 10 h at room temperature, the reaction mixture was poured into methanol, and the precipitation in the form of a yellow powder was collected. The product was further washed with 1 M hydrochloric acid and then purified by repeated filtration and washing with water and methanol to get rid of sodium ions in sPPTA. The weight-average molecular weight of sPPTA was 21 211 g mol−1, as measured by gel permeation chromatography (GPC) (Figure S1). 2.3. Preparation of the PVA/SPPTA Composites. The PVA/ sPPTA composite films were fabricated by simple water casting. sPPTA was first added into an aqueous solution of PVA (1 wt %) at various ratios. The mixtures were mechanically stirred gently at room temperature for 1 h to obtain uniform and clear solutions and then poured into polystyrene molds and kept at 60 °C for 24 h until an equilibrium weight was reached. The thickness of the composite films for mechanical measurement was about 50 μm. 2.4. Characterization. Fourier transform infrared (FTIR) measurements were performed on a Bruker Vector-22 FTIR spectrophotometer (Germany) using KBr pellets. TG analysis was performed on a TA SDT Q600 thermal analyzer (TA Instruments) at a heating rate of 20 °C/min in N2 from room temperature to 700 °C. Differential scanning calorimetry (DSC) measurements were performed using a TA Q200 instrument (TA Instruments) in the temperature range from 40 to 240 °C in N2. The crystallinity of the composite films (Xc) was calculated according to the result of DSC. Because sPPTA has no contribution to melt therapy, the crystallinity of PVA inside the composite (XPVA) is calculated as XPVA = Xc/(1 − x), in which x is the weight fraction of sPPTA in the composites. The crystallinity for the composites was calculated using the enthalpy of 138.6 J/g for a theoretical 100% crystalline PVA.33 The crystallization structures of pristine PVA and the composites were characterized by wide-angle X-ray diffraction (XRD) using a PANalytical X′Pert PRO MPD diffraction system (The Netherlands) with Cu Kα radiation (λ = 1.542 Å). The optical transmittance of the composite films with a thickness of 10 μm was measured by Varian CARY 100 Bio UV−vis spectrophotometer. The dispersion of sPPTA in PVA matrix was observed by polarized optical microscopy (POM) on an OLYMPUS BX51 microscope (Japan) and transmission electron microscopy (TEM) with a JEM-1200EX electron microscope (JEOL, Japan) at an accelerating voltage of 120 kV. The mechanical properties of the composite films were measured with an RTW10 electronic universal testing machine (Reger, Shenzhen, China) at 25 °C and an environmental humidity of about 50%, with a crosshead speed of 10 mm/min and an initial gauge length of 10 mm. The sample films, with a thickness of about 50 μm, were cut into strips with a length of 30 mm and a width of 1 mm. Six strips were measured for each sample. The thickness of the samples was measured using a CH-1 B film thickness gauge with a resolution of 1 μm (Shanghai Liuling Instrument Co. Ltd., China). The toughness (K) values of the composites were determined as the area surrounded by the stress (σ)− strain (ε) curves according to the method reported in the literature.29 Dynamic mechanical analysis (DMA) experiments were performed using a DMA Q800 instrument (TA Instruments) in tension mode at the heating rate of 3 °C/min under a dry nitrogen atmosphere.

the structural and conducting properties of sPPTA membranes has been investigated.18 The strong electrostatic interaction between the ionic functional groups leads to material uniformity and the improved mechanical and chemical stability. Chen et al.19 reported the molecular composites of sPPTA and poly(4-vinylpyridine) (PVP), in which the ion−dipole interactions between sPPTA and PVP leads to good miscibility at low concentrations of sPPTA. The sPPTA/PVP molecular composites are transparent, the storage modulus is enhanced over a wide temperature range, and the glass transition temperature is increased to higher values. On the other hand, it is well-known that polymers can be reinforced by strong and stiff nanofillers. Much of the research focus has been on the reinforcement of poly(vinyl alcohol) (PVA), a water-soluble polymer, by carbonaceous nanomaterials, such as single-walled carbon nanotubes (SWNTs),20,21 multiwalled carbon nanotubes (MWNTs),22,23 VGCFs,23,24 few-walled carbon nanotubes (FWNTs),25 graphene oxide (GO),26−28 and nanodiamond,29 due to their extremely high values of strength and modulus. PVA exhibits excellent mechanical properties and chemical stability and has been demonstrated to be an idea modal polymer to study the mechanism of the reinforcement of polymers by nanofillers. It is also an attractive material for high performance fibers, films, and polymer electrolyte membranes of fuel cells because of its good methanol barrier properties.30,31 Here, we present an alternative approach for the reinforcement of PVA by rigid-rod sPPTA and demonstrate that transparent, strong, and tough molecular composites of sPPTA and PVA can be prepared by water casting, a simple and green method. We characterized the hydrogen bonding between sPPTA and PVA and carefully investigated the influence of sPPTA content on the phase structure, crystallization behavior, thermal stability, and mechanical properties of the PVA/sPPTA composite films in order to provide some insight into the relationship between the microstructure and properties. Surprisingly, neat sPPTA crystalline aggregates cannot be detected by wide-angle X-ray diffraction even when the sPPTA content is as high as 33 wt %, but transmission electron microscopy shows the appearance of anisotropic, nanoscale fibril-like inclusions in the composites when the sPPTA content is only about 12 wt %. We also compared the mechanical properties of the PVA/sPPTA molecular composites and PVA nanocomposites reinforced by carbonaceous nanofillers (such as carbon nanotubes, VGCFs, GO, and nanodiamonds) and found, interestingly, that at similar filler content the tensile strength of our molecular composites is superior to that of many PVA nanocomposites, and the elongation at break is much larger. Furthermore, compared with prefabricated PVA/ PAH,18 PVA/PVP,19 and PVA/poly(styrene sulfonic acid) (PSSA)32 mixtures, the PVA/sPPTA composites exhibit superior strength, toughness, and processability; therefore, we believe that the molecular composites are more appropriate for applications in proton exchange membranes materials and solid catalytic membranes/fibers in the future.

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction. X-ray diffraction patterns of neat sPPTA, PVA, and PVA/sPPTA composite films with various sPPTA contents are presented in Figure 1. The sharp diffraction peaks centered at 2θ = 19.6° of neat PVA correspond to the (101) plane of PVA crystals.34 The shoulder peak at 2θ = 22.9° and weak diffraction peak at 2θ = 40.8°

2. EXPERIMENTAL SECTION 2.1. Materials. Terephthaloyl chloride (TPC) was supplied by Aladdin Chemistry Co. Ltd. (Shanghai, China). 2,5-Diaminobenzenesulfonic acid (2,5-DABSA) was obtained from Sigma-Aldrich. Sodium carbonate, benzene, and poly(vinyl alcohol) (average molecular weight ranges of 77 000 g mol−1, 98% hydrolyzed) were supplied by B

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To illuminate the interaction between sPPTA and PVA, FTIR spectra of sPPTA, PVA, and the PVA/sPPTA composites are measured as shown in Figure 2. The O−H stretching

Figure 1. X-ray diffraction patterns of sPPTA, PVA, and PVA/sPPTA composite films with various sPPTA contents.

Figure 2. FTIR spectra of (a) sPPTA, (b) PVA, and the PVA/sPPTA composites with a sPPTA content of (c) 5, (d) 8, (e) 16, and (f) 33 wt %.

vibration peak at 3457 cm−1 for sPPTA and 3401 cm−1 for neat PVA shifts to lower wavenumbers for the PVA/sPPTA composites, suggesting the formation of hydrogen bonding between PVA and sPPTA. Furthermore, the absorption peak at around 1019 cm−1 is ascribed to the intramolecular hydrogen bonding between the sulfonic acid and the hydrogen of the neighboring amide groups of sPPTA macromolecules.38 When the content of sPPTA is low, it is impossible to check the character of sulfonic acid group at around 1019 cm−1 because it is obscured by the adsorption peak of PVA between 1000 and 1150 cm−1. However, for PVA/sPPTA-16 wt % and PVA/ sPPTA-33 wt %, the adsorption peak shifts to 1026 and 1025 cm−1, respectively, suggesting that the sulfonic acid group is less involved in the hydrogen bonding with the hydrogen of neighboring amide groups of sPPTA.37 This result confirms the formation of hydrogen bonding between the sulfonic acid groups of sPPTA and the hydroxyl groups of PVA, which is consistent with previous reports on the formation of hydrogen bonding between PVA and PSSA32 or P3TASH.35 As aforementioned, XRD analysis shows that there are no neat sPPTA crystalline aggregates in the mixtures of PVA and sPPTA; therefore, it is clearly evident that the strong hydrogen bonding between PVA and sPPTA leads to the formation of crystalline PVA/sPPTA complexes and prevents the formation of self-assembled sPPTA crystalline aggregates in the mixtures. 3.3. Differential Scanning Calorimetry (DSC). DSC was performed to further analyze the crystallization behavior of PVA/sPPTA composites over a wide composition range, as shown in Figure 3. Because neat sPPTA does not show noticeable endothermic or exothermic peaks in the range between 120 and 270 °C, the obvious endothermic peaks in the DSC traces at above 200 °C should result from the melting of PVA crystals. The melting temperature (Tm) and crystallinity of the samples are tabulated in Table 1. When sPPTA content is increased from 0 to 8 wt %, the melting enthalpy increases, indicating the increase of PVA crystallinity. Moreover, addition of sPPTA also increases the melting point (Tm) of PVA; for example, Tm increases 229.6 °C for sPPTA/PVA-8 wt %, 9.5 °C higher than that of neat PVA. This phenomenon is similar to previously observed crystallization behavior of PVA in the existence of some inorganic fillers. For example, it has been

correspond to the (200) and (102) plane of near PVA, respectively. For sPPTA, a large diffraction peak centered at 2θ = 25.3° and two small peaks centered at 2θ = 17.3° and 2θ = 8.2° are observed. For PVA/sPPTA composites with sPPTA content below 8 wt %, the XRD patterns are almost the same with that of neat PVA, and no crystalline peaks of sPPTA can be identified. The diffraction peaks at 2θ = 19.6° corresponding to PVA are sharpened by the addition of sPPTA, suggesting a decrease in the amorphous character of the composites. For PVA/sPPTA-16 wt % and even PVA/sPPTA-33 wt %, the characteristic diffraction peak of neat sPPTA at 2θ = 25.3° is still invisible. The diffraction peak at 2θ = 19.3° for neat PVA slightly shifts to 2θ = 19.8°, and the diffraction peaks for the (200) and (102) plane of PVA crystals disappear. Moreover, new diffraction peaks appear at 2θ = 15.8° and 7.9°. Disappearance of the crystalline peaks of neat sPPTA presents clear evidence for molecular dispersion of sPPTA in PVA matrix. Any sPPTA aggregates formed can only consist of very few of sPPTA molecules; otherwise, the diffraction peak can be identified. The appearance of new crystal phase in the composites suggests that strong interaction between sPPTA and PVA leads to the formation of PVA/sPPTA crystalline complex and prevents the formation of pure sPPTA crystalline aggregates in the composites. 3.2. Fourier Transform Infrared (FTIR) Spectroscopy. In previous studies, sulfonic acid group containing polymers, such as PSSA32 and poly[2-(3′-thienyl)ethanesulfonic acid] (P3TASH),35 have been blended with PVA because as polymer electrolyte membranes of fuel cells, PVA itself does not provide a source of protons for conduction as compared with Nafion membrane.36 Investigation on the interaction between the sulfonic acid groups and hydroxyl groups of PVA demonstrates the formation of strong hydrogen bonds between the two groups. The PVA/PSSA blend membranes with an interpenetrating microstructure display a high proton conductivity, reduced methanol permeability, good water retention capacity, and mechanical stability.32 In the blends of PVA and a self-aciddoped conjugated conducting polymer P3TASH, strong hydrogen bonding between the −SO3H groups of P3TASH and the −OH groups of PVA leads to the formation of P3TASH/PVA complexes in the blends.35 C

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peak appears between 120 and 230 °C, which should result from the release of volatile products from the dehydration reaction between the hydroxyl groups of PVA and the sulfonic acid groups of sPPTA.39,40 The melting peak of pure PVA crystals disappears completely at high sPPTA content. However, it should be noted that strong diffraction peak can still be observed in the XRD pattern at 2θ = 19.8° of PVA/ sPPTA-16 wt % and for PVA/sPPTA-33 wt %, suggesting the existence of crystalline phase in the composites. Therefore, it is safe to conclude that the new crystalline phase in the PVA/ sPPTA composites is composed of the complexes of PVA and sPPTA, and the amount of new crystalline complexes increases with the addition of sPPTA content at the expense of neat PVA crystals in the composites. 3.4. Thermogravimetric (TG) Analysis. The TG and differential TG (dTG) curves of the sPPTA/PVA composite films are presented in Figure 4. When the sPPTA content is below 8 wt %, the decomposition temperature of the PVA/ sPPTA composites obviously shifts to higher temperatures with the increase of sPPTA content. For neat PVA, the 7 wt % weight loss from 100 to 160 °C is associated with the loss of absorbed water. The weight loss of about 68 wt % between 200 and 350 °C is mainly due to the dehydration of PVA accompanied by the formation of volatile products. At temperatures above 350 °C, the polyene residues are further degraded to yield carbon and hydrocarbons.40,41 A small decomposition peak centered at 442 °C is observed in the dTG curve. For neat sPPTA, when temperature is increased from 50 to 360 °C, as denoted by the arrow in Figure 4A, noticeable drop in mass is observed, which is ascribed to the loss of absorbed water followed by the departure of sulfonic acid group.12,39 The TG and dTG curves for PVA/sPPTA-3 wt % composites are almost the same as that of pristine PVA. When the content of sPPTA is increased to 5 wt %, the main decomposition peak at about 264 °C for neat PVA remarkably decreases in size and shifts to 269 °C. Moreover, there appears a shoulder peak in the dTG curve at about 350 °C. When sPPTA content is 6.5 wt %, the main decomposition peak shifts

Figure 3. DSC curves of (1) neat sPPTA, (2) neat PVA, and PVA/ sPPTA composites with a sPPTA content of (3) 5 wt %, (4) 8 wt %, (5) 10 wt %, (6) 12 wt %, (7) 14 wt %, (8) 16 wt %, and (9) 33 wt %.

Table 1. Parameters Obtained from DSC Curves of PVA and the PVA/sPPTA Composites with SPPTA Content from 5 to 14 wt % sample

enthalpy (J/g)

melting point (°C)

XPVA (%)

PVA PVA/sPPTA-5 wt % PVA/sPPTA-8 wt % PVA/sPPTA-10 wt % PVA/sPPTA-12 wt % PVA/sPPTA-14 wt %

46.5 53.5 59.0 57.9 54.9 51.5

226 229.2 229.1 228.3 226.6 215.9

33.5 38.9 42.6 41.8 39.6 37.2

shown that montmorillonite can induce crystallization of PVA on filler surface and lead to the formation of new crystal phase with higher melting point.38 Our result suggests that sPPTA, at low contents, also behaves like montmorillonite and acts as a nucleating agent for PVA crystallization due to the strong intermolecular interaction between sPPTA and PVA. However, when the sPPTA content is increased to above 10 wt %, both melting enthalpy and melting point decrease. Surprisingly, the melting peak at above 200 °C disappears for PVA/sPPTA-16 wt % and PVA/sPPTA-33 wt %, and a very broad endothermic

Figure 4. (A) TG and dTG curves of neat PVA, sPPTA, and PVA/sPPTA composites with a sPPTA content of 3, 5, 6.5, and 8 wt %. (B) TG and dTG curves of neat PVA, sPPTA, and PVA/sPPTA composites with a sPPTA content of 12, 16, and 33 wt %. D

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to 270 °C with a further decreased size, and a new peak appears at about 370 °C. When sPPTA content is further increased to 8 wt %, the decomposition peak at about 264 °C for neat PVA completely disappears, and the peak at about 370 °C becomes much stronger. This result indicates that at low sPPTA content the PVA/sPPTA complexes may act as a barrier to hinder the volatile decomposition products throughout the composites and improve the thermal stability of the composites. However, when sPPTA content is further increased to above 12 wt %, decomposition peak below 300 °C appears again and shifts to lower temperatures with the increase of sPPTA content, as shown in Figure 4B. It is noted that the decomposition peak appears at 177 °C for PVA/sPPTA-33 wt %, which is in fact the same with the endothermic peak at 176 °C in the DSC curve (Figure 3). This confirms that apparent dehydration reaction of the PVA/sPPTA-33 wt % composites occurs at temperature below 200 °C. Belaineh et al.42 investigated the solid-state esterification reaction between p-toluenesulfonic acid (TSA) and PVA. It was reported that after heat treatment at 120 °C TSA exists primarily in the dehydrated form, as anhydrous sulfonic acid, a strong acid that can be expected to catalyze the dehydration of the PVA to give ether and sulfonate ester formation. After further heat treatment at 170 °C, TSA exists primarily in the sulfonate ester state. Therefore, it can be speculated that there is also esterification reaction between sPPTA and PVA. At low sPPTA content, only small amount of water is generated by the esterification reaction, but when sPPTA content is high, a large amount of water is released, resulting in significant mass loss. This explains why at high sPPTA content the decomposition peak below 300 °C and even lower temperatures appears again. 3.5. Polarized Optical Microscopy (POM). Previous studies have shown that sPPTA forms supramolecular assemblies that behavior as the building blocks of the nematic liquid crystalline gel in water. The minimum concentration for gelation is as low as 0.6 wt %, and birefringence can be observed at the sPPTA concentration above 0.8 wt %.10,11 The strong intramolecular interaction between the sulfonic acid groups of sPPTA and the protons of the amide bonds is thought to reduce the solubility of sPPTA in water and leads to the formation of supramolecular sPPTA aggregation in water.14,43 The strong hydrophobic interaction of unsubstituted aromatic ring is also suspected to contribute to the aggregation of sPPTA chains in water.14,43 After sPPTA was added to the 1 wt % aqueous solution of PVA, homogeneous, transparent, and stable solutions were obtained by mild stirring at room temperature, as shown in Figure S2. The solutions were heated to 60 °C. With the evaporation of water, uniform and smooth PVA/sPPTA composite films were obtained. Apparently, the concentrations of sPPTA and PVA increase with the evaporation of water. We observed the birefringence of the solutions during the drying process by POM. Interestingly, the sPPTA/PVA aqueous solutions remain nonbirefringent throughout the drying process when sPPTA content in the final composite films is from 3 to 8 wt %. Transparent yellow and uniform PVA/sPPTA composite films were obtained. At sPPTA content above 12 wt %, birefringence appears after most water evaporates. The final composite films remain transparent but become slightly cloudy (Figure S3). The POM images of PVA and PVA/sPPTA films are shown in Figure 5 and Figure S4. The PVA films are nonbirefringent when viewed under crossed polarized optical microscope (Figure S4), although PVA is a semicrystalline

Figure 5. Polarized optical micrographs of (A) PVA/sPPTA-8 wt % and (B) PVA/sPPTA-12 wt %.

polymer with a crystallinity of 33.7% as measured by DSC. This is because PVA crystals do not have adequate birefringence.44 As shown in Figure 5A, the PVA/sPPTA composite films remain nonbirefringent even when the content of sPPTA is as high as 8 wt %. This demonstrates that sPPTA disperses very uniformly in PVA matrix and does not form birefringent sPPTA supramolecular assemblies in PVA matrix. This is in sharp contrast with the strong self-assembling behavior of sPPTA in water, in which case sPPTA forms nematic liquid crystalline gel in water at a content as low as 0.8 wt %.44 This phenomenon further confirms that there exists strong interactions between sPPTA and PVA that prevents the self-assembly of sPPTA in PVA. When the content of sPPTA is further increased to 12 wt %, birefringent texture is observable, as shown in Figure 5B. As aforementioned, XRD results show that there is no neat sPPTA crystalline phase inside the composite films even when the sPPTA content is as high as 33 wt % and neat PVA crystals are nonbirefringent. Therefore, the appearance of optically anisotropic domains in the PVA/sPPTA-12 wt % composite film is related to the crystalline PVA/sPPTA complexes. 3.6. Transmittance Electron Microscopy (TEM). The microstructure of PVA/sPPTA composites was further investigated by TEM. The typical TEM images of the cryosectioned PVA/sPPTA samples with a thickness of about 70 nm are shown in Figure 6 and Figure S5. For neat PVA, the sample is featureless. For PVA/sPPTA-3 wt % and PVA/ sPPTA-5 wt %, dark inclusions appear in the samples, which should be the sPPTA-rich reinforcement phase; however, it is difficult to distinguish the domain size and shape because the contrast between the dark and bright regions is too low and no distinct interface can be determined. We have tried to observe the samples with a high-resolution TEM and at higher magnifications but failed to obtain better results. With the increase of sPPTA content, the dark domains become more and more distinct, confirming the occurrence of microphase separation and coarsening of sPPTA-rich phase. For PVA/ sPPTA-12 wt % and PVA/sPPTA-16 wt %, anisotropic fibrillike inclusions dispersing uniformly in the composites become visible, and the size of fiber diameter increases with the increase of sPPTA content. Figure 6F shows that the diameter of the E

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Figure 6. TEM images for (A) PVA, (B) PVA/sPPTA-3 wt %, (C) PVA/sPPTA-5 wt %, (D) PVA/sPPTA-8 wt %, (E) PVA/sPPTA-12 wt %, and (F) PVA/sPPTA-16 wt %.

fibril-like inclusions in PVA/sPPTA-16 wt % is below 30 nm, as denoted by the arrow. The existence of nanoscale fibril-like reinforcing structure in the PVA/sPPTA composites at relatively high sPPTA contents is apparently different from the sea−island or bicontinuous (or in other words, interpenetrating networks) phase structure of flexible polymer blends, but similar to the microphase-separated molecular composites of stiff polymer and flexible polymers,45 in which the stiff-chain macromolecules aggregate and align themselves parallel in nanometer or coarser scale domains that coalesce to form larger fibrillar structure in the presence of shear forces or during elongation deformations. Another example with similar microstructure is chitin nanofibers dispersing in protein matrix in nacres.46 The formation of nanoscale fibril-like microstructure in PVA/sPPTA composites should be related to the rigid-rod structure of sPPTA macromolecules and the strong intermolecular interaction between sPPTA and PVA that gives rise to the formation of anisotropic PVA/sPPTA complexes. Different from thermoplastic molecular composites, the structure of PVA/sPPTA composites is stable and does not change upon heating. 3.7. UV−vis Transmittance. Figure 7 presents the UV−vis transmittance spectra of PVA/sPPTA composite films with a thickness of about 10 μm. The transmittance of neat PVA is almost 100% in the visible region. After the addition of sPPTA, the transmittance of the composites is obviously decreased in

the UV region. At low sPPTA content, the transmittance in the visible region remains high as the result of good compatibility between sPPTA and PVA. When sPPTA content is above 8 wt %, light transmission above 400 nm decreases, apparently suggesting the inhomogeneity in the optical properties of the composite films as a result of phase separation, which is consistent with the result obtained by TEM. 3.8. Tensile Properties. The typical stress−strain curves for PVA and PVA/sPPTA composite films are presented in Figure 8. Compared with that of neat PVA, the tensile strength of PVA/sPPTA composites is remarkably increased with the addition of sPPTA. PVA/sPPTA-5 wt % shows a tensile strength of 169 ± 13 MPa, which is about 54% higher than that of neat PVA (110 ± 10 MPa), suggesting that sPPTA is a very effective molecular reinforcing agent for PVA. When sPPTA content is further increased, the tensile strength is decreased because of the coarsening of reinforcement phase in PVA matrix as demonstrated by TEM observation. For example, the tensile strength of PVA/sPPTA-16 wt % is even lower than that of neat PVA, and the sample displays a brittle fracture without yielding at very low elongation at break. It is noted that the tensile strength of PVA/sPPTA molecular composites is even superior to those of many prefabricated nanocomposites of PVA filled with SWNTs,20,21 MWNTs,22,23 VGCFs,23,24 FWNTs,25 GO,26−28 and nanodiamonds.29 Many PVA/carbonaceous nanofiller nanocomposites exhibit a tensile strength below 150 MPa. However, because the mechanical properties of PVA and its nanocomposites strongly depend on the molecular weight and hydrolysis degree of PVA, the content of residual water, as well as the temperature and humidity at which the tensile strength measurement was conducted, it is not justified to directly compare the mechanical properties obtained by different authors. Therefore, we calculated the reinforcement factor of the nanofillers by dividing the increase of tensile strength and modulus of the nanocomposites by those of corresponding neat PVA to extract the reinforcement upon addition of nanofillers. As shown in Table 2, the reinforcement factor for tensile strength, (σ − σm)/σm, of sPPTA is somewhat lower than those of SWNTs and GO but superior to those of MWNTs, VGCFs, and nanodiamonds, demonstrating that sPPTA is a very effective molecular reinforcing agent for PVA. SWNTs and GO exhibit greater

Figure 7. UV−vis transmission spectra of sPPTA/PVA composite films with various sPPTA contents. The thickness of the films was all about 10 μm. F

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Figure 8. Typical stress−strain curves (A), tensile strength values (B), Young’s modulus (C), and elongation at break and toughness values (D) of PVA/sPPTA composites with various sPPTA contents.

Table 2. Tensile Strength (σ), Young’s Modulus (E), and Reinforcement Factor, (σ − σm)/σm and (E − Em)/Em, for Previously Reported Nanocomposites and PVA/sPPTA Molecular Composites of This Study, in Which σm and Em Are the Tensile Strength and Modulus of PVA Matrices for Corresponding Composites filler type

filler content (wt %)

sPPTA SWNTs SWNTs MWNTs MWNTs VGCFs VGCFs FWNTs GO GO reduced GO sulfonated GO nanodiamond

5 5 0.8 5 5 5 5 1 0.7 20 20 3 5

σ (MPa) 169 148 107 54 137.9 177.5 73.1 132.57 87.6 118 188.9 133 124

(σ − σm)/σm (%)

± 13 ±8 ±4

54 73 45 9.7 12.7 45 44.9 65 76 75.4 182 177 30.5

± 4.6 ± 9.8 ± 3.82 ±9 ± 16 ± 5.4

E (GPa) 1.72 4.0 4.3 3.1 4.4 4.6 4.8 7.1 3.45 11.4 10.4 5.3 10.6

± 0.12 ± 0.15 ± 0.1 ± 0.4 ± 0.028

± 0.8

(E − Em)/Em (%)

ref

9.6 110 79 121 47 43 242 64 62 178 154 89 186.5

this study 20 21 22 23 23 24 25 26 27 27 28 29

On the other hand, the Young’s modulus of our PVA is relatively low because of the relatively high humidity (50%) in extensile measurement and high content of residual water (7 wt %) in the samples. Furthermore, sPPTA does not increase the Young’s modulus of PVA as significantly as nanofillers. As shown in Figure 8C, PVA/sPPTA-8 wt % only exhibits a Young’s modulus 19% higher than that of neat PVA. In contrast, carbonaceous nanofillers can remarkably increase the Young’s modulus of PVA. As shown in Table 2, 5 wt % nanodiamonds can increase the modulus of PVA by 186%,29 and 5 wt % VGCFs and MWNTs functionalized by PVA can increase the modulus of PVA by about 50%. 23 This phenomenon is most likely related to the relatively low modulus of sPPTA, a polymeric reinforcing agent, compared with carbonaceous nanofillers. The accurate Young’s modulus of sPPTA is not available in the literature; we note that the modulus of commercial Kevlar fiber made from poly(pphenylene terephthalamide), with a chemical structure similar to sPPTA, is 70.5−112.4 GPa,45 which is much lower than that of GO, CNTs (∼1000 GPa),47 and VGCFs (680 GPa).48

reinforcement factor because of their extremely large aspect ratio and high mechanical properties. It should be noted that the synthesis of sPPTA is very simple, and sPPTA can be directly dissolved in aqueous PVA solutions by mild stirring. At relatively low content, sPPTA disperses very uniformly in PVA matrix after drying. The strong intermolecular interaction between PVA and sPPTA not only retards phase separation but also ensures effective stress transfer to the reinforcement phase upon being stretched. Furthermore, sPPTA increases the crystallinity of PVA when sPPTA weight fracture is below 8 wt %. All these factors contribute to the remarkable enhancement of the mechanical properties of PVA/sPPTA composites. On the contrary, surface modification involving complicated and time-consuming chemical reactions and ultrasonication are necessary to improve the dispersion of the intrinsically hydrophobic CNTs, VGCFs, and graphene in PVA and to improve the interfacial interaction. However, ultrasonication can introduce defects to CNTs and even break them, which is obviously negative to the reinforcement factor. G

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be the Tg of PVA.50 With the addition of sPPTA, the tan δ curves show Tg peaks shifting to higher temperatures, a decreased peak intensity, and broadening glass transition. There is also the secondary relaxation of PVA at about 120 °C with a broader relaxation peak for neat PVA, which has been ascribed to the molecular motion of PVA within the crystalline phase by Takayanagi.51 With the increase of sPPTA contents, the second relaxation temperature also shifts to higher temperature remarkably with a decreased intensity. This suggests that the PVA/sPPTA complexes in the composites restrict the thermal motion of the PVA molecular segments due to the strong hydrogen bonding between sPPTA and PVA and the increased crystallinity of PVA by the introduction of sPPTA, which restricts the motion of PVA macromolecular chains.

The ductility of PVA nanocomposites has also attracted the interests of researchers.49 It has been found that carbonaceous nanofillers greatly reduce the ductility of PVA. For examples, our previously studied PVA-functionalized MWNTs and VGCFs reduce the elongation at break of PVA to below 7% and toughness to below 10 J/g when the filler content is only 5 wt %.21 As to the nanodiamond filled PVA,29 when the nanodiamond content is 5 wt %, the elongation at break is about 16.9% and the toughness is only 11 J/g. In sharp contrast, the PVA/sPPTA molecular composites exhibit much higher ductility. As shown in Figure 8D, the elongation at break remains 44 ± 15% and 32 ± 8% for PVA/sPPTA-5 wt % and PVA/sPPTA-6.5 wt %, respectively. The toughness of PVA/ sPPTA-5 wt % and PVA/sPPTA-6.5 wt % is 54 ± 19 and 37 ± 9 J/g, respectively, much higher than those of prefabricated PVA nanocomposites. Such a high ductility is desirable for certain applications, especially when PVA is used as the adhesive of inorganic fillers in composites with high filler contents. 3.9. Dynamic Mechanical Properties. To further understand the influence of sPPTA on the mechanical properties of PVA, PVA/sPPTA-5 wt % and PVA/sPPTA-8 wt % were selected to conduct the DMA measurement. Figure 9 shows the

4. CONCLUSIONS We have demonstrated that PVA/sPPTA molecular composites can be easily prepared by water casting. Strong hydrogen bonding between the two components is found to prevent the self-aggregation of sPPTA in PVA matrix and lead to the formation of crystalline PVA/sPPTA complexes in the composites. No characteristic of neat sPPTA crystalline aggregates was observed even when the sPPTA content is as high as 33 wt %. At relatively high sPPTA contents, nanoscale fibril-like inclusions appear in the composites. The structure and properties of the composites are strongly dependent on sPPTA content. At low sPPTA contents, the crystallinity, melting point, mechanical properties, and thermal stability of PVA are obviously increased. When sPPTA content is 5 wt %, the composite film exhibits the best mechanical properties with a tensile strength of 169 ± 13 MPa, which is 54% higher than that of neat PVA, and an elongation at break of 44 ± 15%. When sPPTA content is above 6.5 wt %, the mechanical properties of the composites decrease due to the agglomeration of the sPPTA-rich reinforcement phase. The mechanically strong PVA/sPPTA composite films can be potentially used as high performance membranes or fibers in the future.

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ASSOCIATED CONTENT

S Supporting Information *

GPC result of sPPTA, optical images of the aqueous solution of PVA/sPPTA mixture and the solid PVA/sPPTA composite films, POM images of neat PVA and PVA/sPPTA-5 wt %, TEM images of PVA/sPPTA-5 wt %, PVA/sPPTA-8 wt %, PVA/sPPTA-12 wt %, and PVA/sPPTA-16 wt % at low magnifications. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. (A) Temperature dependence of storage modulus (E′) and (B) tan δ of neat PVA, PVA/sPPTA-5 wt %, and PVA/sPPTA-8 wt % composite films measured at 5 Hz.

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variation of storage modulus (E′) and tan δ with temperature for neat PVA film, PVA/sPPTA-5 wt %, and PVA/sPPTA-8 wt %. The storage modulus of the PVA/sPPTA composite films is obviously higher that of neat PVA and increases with the increase of sPPTA content, which further confirms that sPPTA is able to enhance the mechanical properties of PVA, because of its rigid-rod molecular structure, uniform dispersion in the composites, and its strong interaction with PVA matrix. The storage modulus above the Tg of PVA is also increased remarkably, suggesting the enhanced heat resistance of PVA/ sPPTA composites. Figure 9B presents the temperature dependence of loss tangent (tan δ) for neat PVA and PVA/sPPTA composites. The peak of tan δ curve between 50 and 70 °C is considered to

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax +86-571-87953712; Tel +86-571-87953712 (M.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (51173158 and 50773066). H

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(39) Zhu, M. L.; He, B. Q.; Shi, W. Y.; Feng, Y. H.; Ding, J. C.; Li, J. X.; Zeng, F. D. Fuel 2010, 89, 2299. (40) Nitanan, T.; Akkaramongkolporn, P.; Rojanarata, T.; Ngawhirunpat, T.; Opanasopit, P. Polym. Bull. 2013, 70, 1431. (41) Schotman, A. H. M.; Weber, T.; Mijs, W. J. Macromol. Chem. Phys. 1999, 200, 635. (42) Belaineh, D.; Png, R. Q.; McGuiness, C. L.; Mathai, M.; Seshadri, V.; Ho, P. K. H. Chem. Mater. 2014, 26, 4724. (43) Chu-Evan, Y.; Xu, Z. S.; Lee, C. M.; Sek, C. K. F.; Okamoto, Y.; Pearce, E. M.; Kwei, T. K. J. Polym. Sci., Part B 1995, 33, 71. (44) Cho, J. D.; Lyoo, W. S.; Chvalun, S. N.; Blackwell, J. Macromolecules 1999, 19, 21. (45) Aharni, S. M. J. Appl. Polym. Sci. 1994, 53, 1615. (46) Weiner, S.; Traub, W.; Parker, S. B. Philos. Trans. R. Soc., B 1984, 304, 425. (47) Yu, M. F.; Louire, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Science 2000, 287, 637. (48) Jacobsen, R. L.; Tritt, T. M.; Guth, J. R.; Ehrlich, A. C.; Gillespie, D. J. Carbon 1995, 33, 1217. (49) Song, P. A.; Xu, Z. G.; Guo, Q. P. ACS Macro Lett. 2013, 2, 1100. (50) Chen, L.; Zheng, K.; Tian, X. Y.; Hu, K.; Wang, R. X.; Liu, C.; Li, Y.; Cui, P. Macromolecules 2010, 43, 1076. (51) Takayanagi, M. Mem. Fac. Eng. 1963, 23, 41.

REFERENCES

(1) Helminiak, T. E. ACS Div. Org. Coat. Plast. Chem. Prepr. 1979, 40, 475. (2) Takayanagi, M.; Ogata, T.; Morikawa, M.; Kai, T. J. Macromol. Sci. Phys. 1980, B17, 591. (3) Moscicki, J. K.; Williams, G. Polymer 1982, 4, 558. (4) Bayer, A.; Datko, A.; Eisenbach, C. D. Polymer 2005, 46, 6614. (5) Eisenbach, C. D.; Hofmann, J.; Göldel, A.; Noolandi, J.; Shi, A. C. Macromolecules 1999, 32, 1463. (6) Winter, D.; Eisenbach, C. D.; Pople, J. A.; Gast, A. P. Macromolecules 2001, 34, 5943. (7) Robeson, L. M.; Hale, W. F.; Merriam, C. N. Macromolecules 1981, 14, 1644. (8) Painter, P. C.; Tang, W. L.; Graf, J. F.; Thomson, B.; Coleman, M. M. Macromolecules 1991, 24, 3929. (9) Roberts, M. F.; Jenekhe, S. A. Chem. Mater. 1994, 6, 135. (10) Sarkar, N.; Kershner, L. D. J. Appl. Polym. Sci. 1996, 62, 393. (11) Yun, H. C.; Chu-Evan, Y.; Han, Y. K.; Lee, J. L.; Kwei, T. K.; Okamoto, Y. Macromolecules 1997, 30, 2185. (12) Viale, S.; Jager, W. F.; Picken, S. J. Polymer 2003, 44, 7843. (13) Mendes, E.; Viale, S.; Santin, O.; Heinrich, M.; Pichen, S. J. J. Appl. Crystallogr. 2003, 36, 1000. (14) Viale, S.; Best, A. S.; Mendes, E.; Jager, W. F.; Picken, S. J. Chem. Commun. 2004, 14, 1596. (15) Viale, S.; Best, A. S.; Mendes, E.; Picken, S. J. Chem. Commun. 2005, 12, 1528. (16) Wu, Z. L.; Furukawa, H.; Yang, W.; Gong, J. P. Adv. Mater. 2009, 21, 4696. (17) Wu, Z. L.; Kurokawa, T.; Liang, S.; Gong, J. P. Macromolecules 2010, 43, 8202. (18) Every, H. A.; van der Ham, L. V.; Picken, S. J.; Mendes, E. J. Phys. Chem. B 2008, 112, 16403. (19) Chen, W. C.; Sauer, J. A.; Hara, M. J. Polym. Sci., Part B 2002, 40, 1110. (20) Zhang, X. F.; Liu, T.; Streekumar, T. V.; Kumar, S.; Moore, V. C.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2003, 3, 1285. (21) Liu, L. Q.; Barber, A. H.; Nuriel, S.; Wagner, H. D. Adv. Funct. Mater. 2005, 15, 975. (22) Pogue, R. T.; Ye, J.; Klosterman, D. A.; Glass, A. S.; Chartoff, R. P. Composites, Part A 1998, 29, 1273. (23) Sun, L. N.; Guo, H. L.; Zhu, Z. M.; Guo, T. H.; Peng, M. RSC Adv. 2013, 3, 10983. (24) Bin, Y.; Mine, M.; Koganemaru, A.; Jiang, X.; Matsuo, M. Polymer 2006, 47, 1308. (25) Hou, Y.; Tang, J.; Zhang, H. B.; Qian, C.; Feng, Y. Y.; Liu, J. ACS Nano 2009, 3, 1057. (26) Liang, J. J.; Huang, Y.; Zhang, L.; Wang, Y. Y.; Ma, F.; Guo, T. Y.; Chen, Y. S. Adv. Funct. Mater. 2009, 19, 2297. (27) Li, Y. Q.; Yu, T.; Yang, T. Y.; Zheng, L. X.; Liao, K. Adv. Mater. 2012, 24, 3426. (28) Layek, R. K.; Samanta, S.; Nandi, A. K. Carbon 2012, 50, 815. (29) Morimune, S.; Kotera, M.; Nishino, T.; Goto, K.; Hata, K. Macromolecules 2011, 44, 4415. (30) Pivovar, B. S.; Wang, Y.; Cussler, E. L. J. Membr. Sci. 1999, 154, 155. (31) Higa, M.; Sugita, M.; Maesowa, S.; Endo, N. Electrochim. Acta 2010, 55, 1445. (32) Sahu, K.; Selvarani, G.; Pitchumani, S.; Sridhar, P.; Shukla, A. K.; Narayanan, N.; Banerjee, A.; Chandrakumarc, N. J. Electrochem. Soc. 2008, 155, B686. (33) Hassan, C. M.; Peppas, N. A. Macromolecules 2000, 33, 2472. (34) Shao, C. L.; Kim, H. Y.; Gong, J.; Ding, B.; Lee, D. R.; Park, S. J. Mater. Lett. 2003, 57, 1579. (35) Chen, S. A.; Hua, M. Y. Macromolecules 1996, 29, 4919. (36) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242. (37) Viale, S.; Li, N.; Schotman, A. H. M.; Best, A. S.; Picken, S. J. Macromolecules 2005, 38, 3647. (38) Strawhecker, K. E.; Manias, E. Macromolecules 2001, 34, 8475. I

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