Preparation and study of Polyvinyl Alcohol/Attapulgite nanocomposite fibers with high strength and high Young’s modulus by gel spinning

Polyvinyl alcohol (PVA)/Attapulgite (ATT) nanocomposite fibers with high strength and high Young’s modulus were prepared via gel spinning. The structures and properties of PVA/ATT nanocomposite fibers were investigated with Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimeter (DSC), thermo gravimetric analysis (TGA), x-ray diffraction (XRD), Environmental scanning electron microscope (ESEM), and mechanical testing. The results showed that ATT had a great influence on the structures and properties of PVA/ATT nanocomposite fibers. The melting temperatures, crystallinities, initial decomposition temperatures and maximum decomposition temperatures of PVA/ATT nanocomposite fibers increased firstly when ATT content was increased from zero to 3 wt%, however, they were then dropped when ATT contents were 5 wt% and 7 wt%. The highest melting temperature, crystallinity, initial decomposition temperature and maximum decomposition temperature of PVA/ATT-3 were 240.4 °C, 67.6%, 266.7 °C and 358.6 °C, respectively. Furthermore, ESEM observation indicated that ATT had good adhesion to PVA matrix. Mechanical tests showed that PVA/ATT-3 had the highest breaking tensile and Young’s modulus of 12.6 cN/dtex and 301.9 cN/dtex, respectively.


Introduction
Polyvinyl alcohol (PVA) has a planar zigzag structure like polyethylene and can be made into high-performance fibers with high strength and high modulus [1]. The repeating unit of PVA contains a hydroxyl group that easily forms intramolecular or intermolecular hydrogen bonds in PVA [2,3]. The theoretical assumption of a PVA fiber's tensile strength and Young's modulus calculated using bond energy can be as high as 27 GPa and 255 GPa, respectively [4]. However, the maximum tensile strength of a PVA fiber that has been reported was approximately nearly 10% of the theoretical calculation of strength [2]. Great interest in developing high-performance PVA fibers has been primarily driven by this significant discrepancy between the practical and theoretical strength.
Up to now, many attempts have been made to develop PVA fibers with high strength and high modulus, such as enhancing the polymerization degree and tacticity of PVA and improving spinning methods and spinning processes of PVA fibers. Yamaura et al [5] prepared a PVA fiber via a gel spinning using a binary solvent composing of 80 vol % DMSO and 20 vol% water, resulting in a high Young's modulus of 39 GPa. It is still not an easy job to reach the theoretical values in a majority of PVA fiber production. One major problem was associated with intermolecular hydrogen bonding commonly found in PVA macromolecules, resulting in challenges in molecular orientation development at both fiber spinning line and post drawing process [6,7]. Moreover, the addition of inorganic materials may provide an effective way to improve the mechanical properties of PVA fibers. Jo et al [8] obtained a PVA fiber with high tensile strength and Young's modulus of 23.1 g d −1 and 308.3 g d −1 , respectively, using boric acid to weaken the intramolecular and intermolecular hydrogen bonding of PVA.
Attapulgite (ATT) is a typical porous silicate fibrillar mineral containing ribbons [9][10][11][12]. These rod-like ribbons can be efficiently blended with polymer matrix, such as polyurethane [13], polyamide-6 [14] and polylactic acid [15]. The purposes of these studies are to improved properties, such as enhanced mechanical strength, crystallization and thermostability. Furthermore, Peng [16,17] et al reported on the effect of ATT content on the crystallization of PVA/ATT nanocomposite films and the effect of drawing radio on the mechanical properties of PVA/ATT nanocomposite fibers by wet spinning, however, the highest tensile strength of the PVA/ATT nanocomposite fiber was just 6.9 cN/dtex, this tensile strength limited its application and could not be used as a reinforcing material. Moreover, the effect of ATT content on the structures and mechanical properties of PVA nanocomposite fibers were not studied.
In this work, we prepared PVA/ATT nanocomposite fibers by gel spinning. The tensile strength and Young's modulus were much higher than other studies, the highest breaking tensile strength was about twice as that reported in reference [17], and the effects of ATT content on the structures and mechanical properties of PVA/ATT nanocomposite fibers were investigated.

Experimental
Materials PVA that had the degree of polymerization of 2400 and the degree of hydrolysis of 99 mol % was received from Sinopec Sichuan Vinylon Works Group. ATT was obtained from Jiangsu Junda ATT Material Co., Ltd. The ATT was further purified using hydrochloric acid before use. Dimethyl sulfoxide was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Other reagents supplied by Sinopharm Chemical Reagent Co., Ltd were analytical grade and used as received.

Preparation of PVA/ATT spinning dope
In the preparation of spinning dopes, for example, PVA/ATT-1 (ATT content was 1 wt% of PVA), 0.1 g ATT was dispersed in 5.0 g DMSO for about 30 min using ultrasonic. 9.9 g PVA was dissolved in another 35.0 g DMSO at approximately 100°C. Then, the ATT-DMSO dispersion solution was gradually added into the PVA solution at stirring. These mixtures were subjected to further stirring for 3 h at around 100°C. The desired concentrated PVA/ATT-1 spinning dope was obtained after defoaming in an oven at around 80°C for 24 h.

Fiber spinning
Fiber spinning was done using a customized spinning apparatus [18]. A series of PVA nanonanocomposite fibers with ATT fraction form 0 wt% to 1 wt%, 3 wt%, 5 wt% and 7 wt% were prepared. The spinning dopes were extruded at 30 ml h −1 through a 0.3 mm diameter spinneret at 70°C by circulating water, resulting in a fine flow of the fiber. The fine flow of the fiber continued to flow into an anhydrous ethanol bath at room temperature and then was stretched in the air at room temperature after it flowed out of the coagulation bath. And then, the air-stretched fiber was fully extracted in an anhydrous methanol bath. The as-spun fibers were obtained after they were dried in an oven at around 50°C for 24 h. Then, the as-spun fibers were stretched at 220°C in a heat pipe. The resulting samples were defined as PVA/ATT-X, where X were the ATT content in nanonanocomposite fibers. The hot-drawing ratio was determined by the ratio of the linear speed of the winding roller to the linear speed of the injection roller, which was the maximum drawing degree for each fiber that was continuously collected without breaking, resulting in 8, 9.5, 10, 9 and 8.5 for PVA/ATT-0, PVA/ATT-1, PVA/ATT-3, PVA/ATT-5, PVA/ATT-7, respectively.

Characterization
Attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) ATR-FTIR measurements were performed using a Nicolet 6700 (Thermo Electron, United States). Spectra were obtained in the range 600-4000 cm −1 by averaging 32 scans at a resolution of 4 cm −1 .
Differential scanning calorimeter (DSC) DSC was measured on a Q20 DSC instrument (TA, America) at a heating rate of 10°C min −1 , under continuous nitrogen flowing. In each experiment, the sample (∼3 mg) was heated from room temperature to 250°C.
Thermogravimetric analysis (TGA) TGA were conducted with a TG 209 F1 Iris thermogravimetric analyzer (Netzsch-Geraetebau GmbH, Germany) under nitrogen flow. In each experiment, the sample (∼3 mg) was heated from room temperature to 650°C at the rate of 20°C min −1 under nitrogen.
Environmental scanning electron microscope (ESEM) ESEM images were observed on a quanta-250 (FEI, Czech) at an accelerating voltage of 10 kV.

Tensile stress testing
Tensile strength and Young's modulus were measured on XQ-1A (China) yarn strength tester. The length of fixture was 2 cm and the stretching speed was 10 mm min −1 . The reported tensile strength and Young's modulus were the average value of 20 samples.

Results and discussions FT-IR analysis
The ATR-FTIR spectra of pure ATT and PVA/ATT nanocomposite fibers are shown in figure 1. In the spectra of pure ATT, the characteristic absorption peaks at 3553, 1655, 1196 and 982 cm −1 were assigned to hydroxyl groups of coordinated water in the tunnels of ATT, bending vibration of zeolite water, Si-O vibrations and asymmetric stretching of Si-O-Si bonds [19], respectively. In the spectra of PVA/ATT nanocomposite fibers, the absorption peaks near 3313∼3327 cm −1 were characteristic for the -OH asymmetric stretching vibration band. In addition, the absorption peaks at about 2937 cm −1 , 2908 cm −1 , 1660 cm −1 , 1428 cm −1 and 1091 cm −1 were caused by -CH 2 asymmetric and symmetric stretching vibration, C-C framework stretching vibration, -CH flexural vibration and C-O stretching vibration, respectively [20,21]. In a comparison of the spectra of PVA/ATT nanocomposite fibers with different ATT contents, although the characteristic absorption peaks of other groups had little change, the OH stretching vibration absorption peaks at about 3298 cm −1 moved toward higher wavenumber with the increase of ATT content, indicating that hydrogen bonding in the PVA/ATT nanocomposite fibers was weaker than that in the pure PVA fiber. It is well-known that the presence of Si-OH groups on ATT surface would allow the formation of hydrogen-bonding interaction with the hydroxyl groups in the PVA nanocomposite fibers [16,17].

DSC analysis
DSC measurements were used to study the thermal properties of the PVA composite fiber [22]. Figure 2 shows the DSC curves of PVA/ATT nanocomposite fibers. An endothermic peak of melting temperature at around 225°C-245°C was found on the DSC curve of pure PVA (PVA/ATT-0). With the addition of ATT, the melting point of PVA/ATT nanocomposite fibers was increased from 234.1°C to 240.0°C when ATT content was increased from 0 wt% to 3 wt%. However, the melting temperatures were then dropped at high ATT content (5 wt% and 7 wt%). The melting temperatures suggested a critical change when ATT content was 3 wt%. Table 1 showed melting temperature, heat of fusion, and degree of crystallinity of PVA/ATT nanocomposite fibers. The is the melting enthalpy for 100% crystalline PVA, and ω is the weight fraction of the PVA component in the nanocomposite fibers. Both the heat of fusion and the crystallinities of PVA/ATT nanocomposite fibers were increased when the ATT was increased from zero to 3 wt%, however, they were decreased when the ATT contents continued to increase from 3 wt% to 7 wt%. The highest crystallinity of PVA/ ATT-3 was 67.7%, which was 8.1% higher than that reported in reference [17]. The hydroxyl groups distributed in the surfaces of ATT might work as nucleation sites [16,23]. The strongest nucleating effect exists at an optimal ATT content that was most likely about 3 wt%. Nevertheless, for higher ATT contents, phase separation may occur, leading to ATT agglomerates and affecting fiber integrity.

TGA analysis
The TGA and DTG curves were used to obtain ATT content in nanocomposite fibers and the detailed information of degradation process of PVA/ATT nanocomposite fibers ( figure 3). The thermograms data were summarized in table 2. The temperature at which fiber weight loss is 5 wt% is defined as the initial decomposition temperature (T i ). The temperature at which the degradation rate reaches a maximum is defined  as the maximum decomposition temperature (T m ). As shown in figure 3, a small weight loss at the beginning of the ATT was assigned to the loss of absorbed water in the air [24]. In the high temperature region even above 650°C, ATT showed little weight loss and there was about 86.5 wt% residue at 650°C, indicating that the ATT had a superior thermal stability. The DTG curve of neat PVA fiber shows two peaks at 272.7°C and 342.7°C that can be attributed to the decomposition of side chain and main chain of PVA fiber [25], respectively. The peak at 272.7°C disappeared in the DTG curves of PVA/ATT nanocomposite fibers when ATT content was 3 wt% and more, indicating that there were no decomposition of side chain in PVA/ATT-3, PVA/ATT/5 and PVA/ ATT-7. As shown in table 2, T i of PVA/ATT nanocomposite fibers was increased from 239.2°C to 266.7°C when the ATT content was increased from 0 wt% to 3 wt%. However, T i was then dropped at high ATT content (5 wt% and 7 wt%). The T i and T m of PVA/ATT-3 were improved 27.5°C and 15.9°C, respectively, compared with those of pure PVA fiber, suggesting that PVA/ATT-3 had the best thermal stability, these were consistent with the results of DSC. The enhancement of thermal stability was due to the strong interfacial interactions between ATT and PVA chains [16].

Morphology and structure
The surface morphologies of PVA nanocomposite fibers were demonstrated in ESEM images. As shown in figures 5(a)-(c), the surfaces of PVA/ATT nanocomposite fibers were smooth and uniform when ATT contents were no more than 3 wt%. However, the defects and voids on the surfaces of PVA/ATT-5 and PVA/ATT-7 were found in figures (d) and (e), indicating that phase separation may occur when ATT contents were 5 wt% and 7 wt%. It is well known that the properties of PVA/ATT nanocomposite fibers depend on the size and dispersion of ATT. Figure 5(f) was the ESEM image of the cross section of PVA/ATT-3 without hot stretching, large amounts of bright dots which were the ends of the embedded ATT can be found in figure 5(f). It can be found that ATT was enwrapped in the matrix of PVA, which was irregularly distributed, but some agglomerates can be observed. In addition, the interface between ATT and PVA matrix was not so clear, which means that ATT have good adhesion to PVA. The excellent adhesion between PVA matrix and ATT can be attributed to be the formation of hydrogen bonding between them [17].

Mechanical properties
The mechanical properties of PVA/ATT compostie fibers were studied. The stress-strain curves of PVA/ATT nanocomposite fibers were shown in figure 6. Table 3 showed maximum hot-drawing ratio, breaking tensile strength, Young's modulus and breaking elongation of PVA/ATT nanocomposite fibers. The breaking tensile strength and Young's modulus were increased when ATT content was increased from zero to 3 wt%. However, they were then dropped at high ATT content (5 wt% and 7 wt%), resulting in highest breaking tensile strength and Young's modulus measured in PVA/ATT-3 as shown in figure 6 and table 3. In addition, the breaking elongation of PVA/ATT-3 was relatively low. In general, PVA/ATT-3 showed the best performance among these PVA nanocomposite fibers. Furthermore, the breaking tensile strength of all PVA/ATT composite fibers were more than 10 cN/dtex, and PVA/ATT-3 had the highest strength of 12.6 cN/dtex, which was about twice as that reported in the reference [17]. From the structural characterization, it can be concluded that ATT work as rigid supports in the PVA matrix and the nucleating effect enhanced the X c value of PVA/ATT nanocomposite fibers. All this is a response to a significant improvement of the mechanical properties of PVA/ATT nanocomposite fibers [16].

Conclusions
The structures and properties of a group of PVA/ATT nanocomposite fibers with different ATT contents were investigated using chemical and physical analysis. The results showed that ATT content had a great influence on Figure 6. The stress-strain curves of the PVA/ATT nanocomposite fibers. the structures and properties of PVA/ATT nanocomposite fibers. The melting temperatures, crystallinities, initial decomposition temperatures and maximum decomposition temperatures of PVA/ATT nanocomposite fibers increased firstly when ATT content was increased from zero to 3 wt%, however, they were then dropped when ATT contents were 5 wt% and 7 wt%. Furthermore, ESEM observation indicated that ATT were embedded in PVA matrix and combined closely, mechanical tests showed that the highest breaking tensile and Young's modulus were PVA/ATT-3, which were 12.6 cN/dtex and 301.9 cN/dtex, respectively. These suggested that the strongest reinforcement exists at an optimal ATT content of 3 wt%.