PSA Composite Fibers and Membranes

PSA Composite Fibers and Membranes Polysulfonamide/nano titanium dioxide (PSA/nano-TiO2) composite spinning solutions with various nano-TiO2  mass fractions were prepared using the solution blending method. The corresponding composite fibers were developed by wet-spinning technology and the composite membranes were prepared using the digital spin-coating technique. The properties of PSA/nano-TiO2 composite fibers and membranes were investigated by scanning electron  microscope, Fourier transform infrared spectroscopy and X-ray diffraction, etc. The effects of nano-TiO2 and its  mass fractions on the mechanical properties, thermal stability and ultraviolet resistance of PSA composites were  also analyzed. The experimental results showed that nano-TiO2 with low mass fractions can be dispersed evenly  in the PSA matrix; the blending of nano-TiO2 had no obvious influence on the molecular structure and the chemical composition of PSA fiber; the crystallization in PSA fiber was promoted at low nanoparticles mass f ractions  because it can act as a nucleation agent; the mechanical properties and the thermal stability of PSA/nano-TiO2  composites can be enhanced obviously by blending nano-TiO2 into PSA matrix. The ultraviolet resistance of PSA  composites can be improved significantly with the increasing nano-TiO2 mass fractions and the 7 wt.% specimen  showed the lowest UV transmittance. Polysulfonamide (PSA) fiber is a new kind of hightemperature resistant material and it has outstanding  heat resistance, flame retardancy, and thermal stability,  therefore, it can be used to develop protective products used in aerospace, high-temperature environments  and civil fields with the flame retardant requirements  (Ren, Wang, Zhang, 2007; Wang, 2009). However,  raw PSA generally demonstrates poor ultraviolet resistance and the amide groups in polymer molecular  chains are prone to break down under the ultraviolet  radiation; besides, the breaking tenacity of PSA fibers  is low; these properties lead to some difficulties in its  manufacturing procedures and limit its application in  developing functional textiles. Therefore, it is a challenging work to improve the mechanical properties  and ultraviolet resistance of PSA. It has been proved that nano-TiO2 is one of the  ideal nano-enhanced materials and it has attracted great  scientific attention because of its excellent mechanical  properties in significantly improved properties of composites (Ali, Shadi, Shirin, Seyedeh, Khademno,  2010; Han Yu, 2005). Moreover, nano-TiO2 is good  semiconductor oxides and it has excellent ultraviolet  scattering and absorption (Popov, Priezzhev, Lademann, Myllylà ¤, 2005). It is feasible to blend nanoTiO2 into PSA matrix to improve the mechanical properties and ultraviolet resistance of PSA composites. Experimental Materials The PSA polymer was used as spinning solution with  intrinsic viscosity of 2.0–2.5 dL/g and relative molecular  mass of 462. The rutile titanium dioxide (nano-TiO2)  was blended as functional particles with a diameter ranging from 30 to 50 nm and the rutile content of nano-TiO2  was about 99%. The dimethylacetamide (DMAC) was  selected as dissolvent in this study. The above materials  were provided by Shanghai Tanlon Fiber Co. Ltd. All the  chemicals used here were of reagent grade and they were  used without further purification. Preparation of PSA/nano-TiO2 composites A certain amount of nano-TiO2 was predispersed in  DMAC using ultrasonic vibration for 30min; and thenadded into the PSA solution. The PSA/nano-TiO2  composite spinning solutions with various mass fractions of nanoparticles was prepared after mechanical stirring for 1 h and ultrasonic vibration for 2 h. The  experimental data are shown in Table 1. The pure PSA fibers and PSA/nano-TiO2 composite fibers were developed by a small-scale and singlescrew wet spinning apparatus. Besides, the pure PSA  membrane and PSA/nano-TiO2 composite membranes  were prepared using the SJT-B digital spin-coating  instrument. The preparation procedures of nanofibers  and membranes can be referred to the previous studies  (Chen, Xin, Wu, Wang, Du, in press; Xin, Chen,  Wu, Wang, in press). Test methods The dispersion of nanoparticles in PSA composites  S-3400N scanning electron microscope (SEM) with a  resolution of 4 nm was used to characterize the dispersion of nano-TiO2 in PSA matrix. The machine was  operated at 5 kV. FTIR spectroscopy Thermo Nicolet AVATAR 370 Fourier transform infrared spectroscopy (FTIR) was used to characterize the  molecular structure and chemical composition of  fibers; each spectrum was collected by cumulating 32  scans at a resolution of 4 cm_1 X-ray diffraction X-ray diffraction (XRD) measurements of the crystalline structure of fibers were recorded on k780  FirmV_06 X-ray diffraction using the CuKÃŽ ± radiation  (ÃŽ » = 0.15406 nm). The spectra were obtained at 2h  angles range of 5o –60o with a scanning speed of 0.8 s/ step. Mechanical properties test YG006 electronic single fiber strength tester was used  to investigate the mechanical properties of fibers. The  sample gage length was 10mm. The elongation speed  was set at 20mm/min. The measurements for each  sample were carried out 10 times and the average wasThe thermal stability test  The thermal stability of fibers was measured by Germany STA PT-1000 Thermal Gravimetric Analyzer  (Linseis Inc., New Jersey, USA); the experiment was  conducted under nitrogen atmosphere with a gas flow  of 80–100ml/min; the samples were heated up to  700 °C from the room temperature at a heating rate of  20 °C/min. Ultraviolet resistance test Labsphere UV-1000F Ultraviolet Transmittance Analyzer (Labsphere, Inc., North Sutton, NH, USA) was  used to test the UV transmittance of membranes. The  instrument parameters were described as below: the  absorbance was 0–2.5A; scanning time was about 5 s;  data interval was 1 nm and the diameter of beam was  10mm. The measurements for each sample were carried out for 10 times and the average was used for the  result discussion. Results and discussion The distribution of nano-TiO2 in PSA composites As demonstrated in Figure 1, 1 wt.% of nano-TiO2 can  be dispersed evenly throughout the PSA matrix and the  size of nanoparticles is about 50–60 nm; with the nanoTiO2 mass fraction increased to 3 wt.%, a little aggregation can be observed; when the mass fraction of nanoTiO2 increased to 5 or 7 wt.%, its dispersion in PSA  becomes inhomogeneous because of their large specific  surface and high surface polarity, and the aggregation  size is about 100–300 nm. It is difficult for nano-TiO2  with high mass fractions to distribute uniformly in the  PSA blending system. FTIR analysis of PSA/nano-TiO2 composite fibers   As shown in Figure 2, the position and shape of characteristic peaks of PSA composites blending with nanoTiO2 did not change obviously compared with the pristine PSA. The characteristic peaks of PSA composites  exhibiting at about 3338.99 cm_1  can be attributed to the amide N–H stretching vibration and the peaks are  flattened slightly with the mass fractions of nano-TiO2  increased from 1 to 7 wt.%. It ascribes to the quantum  size effect of nanoparticles (Zhang Mou, 2001). In  conclusion, it shows no significant changes to the  molecular structure and chemical composition of PSA  fibers with the addition of nano-TiO2. XRD analysis of PSA/nano-TiO2 composite fibers  As depicted in Figure 3, the PSA composite fibers have  diffraction peaks at 27.54 °, 36.15 °, 41.35 °, and 54.40 °,  this is because of the blending of nano-TiO2 (Chen,  Liu, Zhang, Zhang, Jin, 2003; Xia Wang, 2002). In addition, all the specimens have diffraction peaks at  about 11.85 ° and 21.25 °. The sharp diffraction peaks  corresponding to 11.85o  indicate that there are crystalline structures in PSA/nano-TiO2 composite fibers  (Yang, 2008). Besides, the sharpness of the diffraction  peaks at about 11.85 ° of composites enhances gradually with the nano-TiO2 mass fractions increased from  1 to 5 wt.%. It suggests that the crystallization in PSA  can be improved with the blending of nano-TiO2,  because it can act as a nucleation agent. Moreover, the  shape of diffraction peaks exhibiting at 21.25 ° of PSA  composites broadens significantly with the increasing  nano-TiO2 mass fractions and it proves that the size of  crystal region becomes smaller (Meng, Hu, Zhu,  2007). The mechanical properties of PSA/nano-TiO2 composite fibers As illustrated in Table 2, the breaking tenacity of PSA  composite fiber with 1 wt.% nano-TiO2 improved  obviously; however, the improving degree of breaking  tenacity begins to decrease with the continuous  increase in mass fractions of nano-TiO2 and the value  of the 7 wt.% sample is lower than the pure PSA. This is because nano-TiO2 is an ideal nano-enhancedmaterial; the blending of it into PSA can improve the  mechanical properties of composites to some extent. Moreover, nano-TiO2 with low mass fractions can be  distributed evenly in PSA matrix and it can form a  good interface with PSA molecular chains. As can be seen in Table 2, the composite fibers have  low elongation at break which is lower than the raw  PSA; simultaneously, the initial modulus of composites  increased significantly, however, the improvement  begins to decrease with the mass fractions of nano-TiO2  increased from 1 to 5 wt.% and the 7 wt.% sample has  the minimum value of the initial modulus. It suggests  that the blending of nano-TiO2 with low mass fractions  can improve the mechanical properties of PSA composite fibers to a certain extent. The thermal stability of PSA/nano-TiO2 composite  fibers TG curves and derivative thermogravimetric analysis  (DTG) curves of PSA/nano-TiO2 composite fibers are  demonstrated in Figures 4 and 5, respectively. The  main parameters of the curves are presented in Table 3. In Figure 4, the thermal decomposition behaviors of  specimens are divided into three regions. The first region is a stage of small mass loss ranging from room temperature to 400 °C. As depicted in  Figure 4, each TG curve has a sharp decrease in the  beginning and then reaches a platform with the temperature heating up to 350 °C. However, the mass loss  of PSA composites blending with nano-TiO2 is always  lower than the pure PSA during this process. As  shown in Table 3, the T10wt of each PSA composite is  high, whereas the mass loss of pure PSA reached 10%  at 170.19 °C. This suggests that it is hard for the PSA  composites to decompose and the thermal stability is  significantly higher than PSA. The second region is a stage of thermal decomposition process ranging from 400 to 600 °C. According  to the analysis of bond energy (Zhang, Cheng, Zhao, 2000), the C–N section of amide in PSA macromolecular chains decomposes at 500–600 °C (Broadbelt, Chu, Klein, 1994a, 1994b) and the mass loss  of PSA at this stage is attributed to the gases released  such as SO2,NH3, and CO2. In addition, as illustrated  in Table 3, the To of PSA composites blending with 1  and 3 wt.% nano-TiO2 can be increased; therefore, its  thermal stability can be improved correspondingly.   As exhibited in Figure 4, the mass loss of specimens accelerates steadily with the increasing temperature and each TG curve presents a rapid  decomposition at about 500 °C. Corresponding to the  rapid decomposition, there is a peak in DTG curve  shown in Figure 5 and the Tmax can be determinedaccording to the value of the maximum peak (Yang,  2008). The third region is a high-temperature phase of  carbon formation ranging from 600 to 700 °C. As  demonstrated in Figure 4, the PSA composites still  show a slight decomposition during this stage;  besides, the mass loss of pure PSA decreases obviously. As illustrated in Table 3, the residual mass of  composites at the terminal temperature is higher than  the pure PSA. Therefore, it is concluded that the thermal stability  of PSA composites blending with nano-TiO2 can be  improved significantly. The ultraviolet resistance As exhibited in Figure 6, the ultraviolet transmittance of specimens ranging from 390 to 400 nm  decreases gradually with the increase in mass fractions of nano-TiO2. This suggests that the nanoTiO2 can improve the ultraviolet resistance of PSA  composites significantly. This is because the refraction index (RI) of nano-TiO2 is extremely high  (2.73) and it has excellent ultraviolet scattering  properties (Liu, Tang, Zhang, Sun, 2007). In  addition, electrons in nano-TiO2 are transited from  the valence band to the conduction band under the  ultraviolet radiation; therefore, the nano-TiO2 has  outstanding ultraviolet absorption properties. Conclusions The PSA composite fibers and membranes with different mass fractions of nano-TiO2 were developed. The experimental results can be summarized as follows: (1) The nano-TiO2 with low mass fractions (1 or 3  wt.%) can be distributed evenly in the PSA  blending system; however, it is difficult for  nano-TiO2 with high mass fractions (5 or 7 wt.  %) to disperse homogeneously throughout the  PSA matrix. (2) The blending of nano-TiO2 showed no obvious  changes to the molecular structure and chemical  composition of PSA composite fibers. (3) The crystallization of PSA composite fibers can  be improved by blending with low mass fractions of nano-TiO2, because it can act as a  nucleation agent. (4) The breaking tenacity and initial modulus of 4 5 ance % (a) (b) (c) PSA composite fibers can be improved obviously by blending with low mass fractions of  nano-TiO2; whereas the elongation at break  of PSA composite was decreased with the  particles mass fractions increased from 1 to 7  wt.%. (5) The thermal stability of PSA composites can be  increased significantly and the nano-TiO2 has  some influences on the To, T10wt, and Tmax of  PSA composites compared with the pure PSA. (6) The blending of nano-TiO2 can improve the  ultraviolet resistance of PSA composites signifi-  cantly and the 7 wt.% specimen had the lowest  UV transmittance.