| Most recently,the demand for high performance fiber increased rapidly due to the higher demand of national security and the worsening situation of anti-terrorism.Carbon fiber(CF),aramid fiber(AF)and ultrahigh molecular weight polyethylene fiber(UHMWPE)were the three most important kinds of high performance fiber in the world today.In which,UHMWPE fibers have attracted lots of attention and have become one of the research hotspots owing to its good properties,such as,low density,excellent tenacity,flexibility,bio-compatibility properties,ultraviolet and chemical stability.They have been extensively used for various application areas,such as,military,aerospace,civil fields and other field of safety production.However,the improvement of the ultradrawing and ultimate tensile properties of UHMWPE is the most critical and challenging one for the applications of UHMWPE fibers.In order to obtain high strength and high modulus UHMWPE fibers,appropriate spinning process was have to adopted to prepare the beginning of silk,and then extend in an appropriate extension,so that the folded molecular chain of HMWPE molecule will be converted into an ideal stretched molecular chain structure maximally,which is beneficial to improve its strength and modulus fundamentally.This dissertation aims to improve the ultradrawing and ultimate tensile properties of UHMWPE fibers by incorporating very small amounts of nanofillers with high specific surface area during gel spinning process of UHMWPE fibers.The well dispersed nanofillers can serve as effective nucleation sites and accelerate crystallization of UHMWPE molecules into crystals with lower crystallization temperature(Tm)and/or evaluated lamellar thickness(Lc)but higher crystallinity percentage(Wc)during their crystallization processes.The lamellar crystals with lower Tm and/or evaluated lc values are expected to melt and/or pull out of folded lamellar crystals comparatively easily during the ultradrawing processes,and hence,causes more demarcated"microfibril" characteristics,higher achievable draw ratos(Dra),orientation factor(fo)and tensile strength(?f)values of the UHMWPE/nanofillers composite fibers.In this dissertation,UHMWPE/nanofillers composite fibers were prepared by incorporating very small amounts of nanofillers with specific surface areas of 100 m2/g(cellulose nanofiber,CNF)to 1400 m2/g(activated nanocarbon,ANC)in UHMWPE gel solution.The surface morphology and oriented Shish-Kebab structure after and before drawing,crystalline microstructure,dynamic mechanical,molecular orientation,thermal,ultradrawing and ultimate tensile properties of UHMWPE/nanofillers composite fibers were studied systematically.The main results obtained in this doctoral dissertation are summarized as follows:1.CNF nanofillers with a specific surface area at 120 m/g and a nanofiber diameter of 20 nm were successfully prepared by proper acid hydrolysis of cotton fibers using sulfuric acid solution.Modified cellulose nanofiber(MCNFx)nanofillers were prepared by grafting various contents of maleic anhydride grafted polyethylene(PE-g-MAH)onto CNF nanofillers.UHMWPE/CNF and UHMWPE/MCNFx as-prepared nanocomposite fibers were successfully prepared by gel-spinning UHMWPE/CNFy and UHMWPE/MCNFxy gel solutions,respectively.The specific surface area of MCNFx reached a maximal value(c.a.157 m2/g)as weight ratios of PE-g-MAH to CNF increased to an optimal value at 6.The Wc value.obtained for the best prepared UHMWPE/MCNF60.05 as-prepared fiber specimen with optimal contents of MCNFx nanofillers at 0.05 part per hundred parts of UHMWPE resin(phr)prepared reached a maximal value(c.a.70.2%).The Tm and lc value obtained for the best prepared UHMWPE/MCNF60.05 as-prepared fiber specimen with optimal contents of MCNFx nanofillers at 0.05 phr prepared reached a minimal value(c.a,136.7? and 10.2 nm).The Dra value obtained for the best prepared UHMWPE/MCNF60.05 as-prepared fiber specimen with optimal contents of MCNFx nanofillers at 0.05 phr prepared using one-stage drawing process at 95? reached a maximal value(c.a.170).Many demarcated drawn "micro-fibrils" were found paralleling to the drawing direction of the drawn UHMWPE and UHMWPE/nanofiller composite fiber specimens as their draw ratios increased,wherein the thicknesses of these drawn micro-fibrils reduced significantly as the draw ratios increased.In which,"micro-fibrils" found on etched surfaces of UHMWPE/MCNF60.05 as-prepared and/or drawn fibers were significantly more and thinner than those of corresponding UHMWPE and UHMWPE/CNF0.025 as-prepared and/or drawn fibers with the same draw ratios.The best prepared UHMWPE/MCNF60.05 dawn fibers filled with MCNF6 with high specific surface area exhibited an relatively high fo and ?fvalue at 92%and 53.5 g/d,respectively.This ?fvalue is about 67%higher than that of the best prepared UHMWPE drawn fiber specimen,that were prepared at the same optimal UHMWPE concentration and drawing condition but without addition of nanofillers.2.Acid treated activated nanocarbon(ATANC)were successfully prepared by proper acid modification of activated nanocarbon(ANC)with specific surface areas higher than 1000 m2g-1using H2SO4/HNO3(1:3 v/v)solution.Functionalized activated nanocarbon(FANC)were prepared by grafting various contents of PE-g-MAH onto ATANC particles.UHMWPE/ATANC and UHMWPE/FANCX as-prepared nanocomposite fibers were successfully prepared by gel-spinning UHMWPE/ATANCy and UHMWPE/FANCxy gel solutions,respectively.The specific surface area of FANCX reached a maximal value(c.a.1098 M2/g)as weight ratios of PE-g-MAH to ATANC increased to an optimal value at 12.5.The W,value obtained for the best prepared UHMWPE/FANC1250.075 as-prepared fiber specimen with an optimal content of FANCx nanofillers at 0.075 phr reached a maximal value(c.a.73.9%).The Tm and lc value obtained for the best prepared UHMWPE/FANC12.50.075 as-prepared fiber specimen with an optimal content of FANCx nanofillers at 0.075 phr reached a minimal value(c.a.136.1? and 9.5 nm).By using one-stage drawing at 95?,the Dra value obtained for the best prepared UHMWPE/FANC12.50.075 as-prepared fiber specimen with an optimal content of FANCx nanofillers at 0.075 phr reached a maximal value(c.a.398).The temperature dependence of the storage modulus(E)and tan ? of UHMWPE and UHMWPE/nanofiller composite fiber specimens were analysed.Three distinct transitions(i.e.?-?-and ?-transitions)were observed at temperatures near 90°C to 130?,-45? to-15?and-120? in the tan ? curves of UHMWPE and UHMWPE/nanofiller composite fiber specimens,respectively.The best prepared UHMWPE/FANC12.5 0.075 dawn fibers filled with FANC12.5 exhibited an relatively high f,and fvalue at 95%and 93.5 g/d,respectively.This?fwhich is about 190%higher than that of the best prepared UHMWPE drawn fiber specimen,that were prepared at the same optimal UHMWPE concentration and drawing condition but without addition of nanofillers.3.Acid treated activated nanocarbon particles with different specific surface areas were successfully prepared by proper acid modification of activated nanocarbon with specific surface areas at 100,500,1000 and 1400 m2/g(i.e.ANC100,ANC500,ANC1000 and ANC1400)using H2SO4/HNO3(1:3 v/v)solution,respectively.A series of functionalized activated nanocarbon particles with a wide range of specific surface areas were successfully prepared by grafting various contents of PE-g-MAH onto ATANC100,ATANC500,ATANC1000 and ATANC14oo,respectively.Optimal value of weight ratios of PE-g-MAH to ATANC100,ATANC500,ATANC1000 and ATANC 1400 increased from 3,7.5,12.5 to 13 and optimal additive content value of the corresponding prepared FANC3100,FANC7.5500,FANC12.51000? FANC131400 reduced from 0.1,0.0875,0.075 to 0.0625 phr as specific surface areas of ANC increased from 100,500,1000 to 1400 m2/g,respectively.It is interesting to note that the specific surface area of FANCx1400 reached a maximal value(c.a.1513 m2/g)as weight ratios of PE-g-MAH to ATANC1400 increased to an optimal value at 13.The W,value obtained for the best prepared UHMWPE/FANC130.0625 as-prepared fiber specimen with an optimal content of FANCx nanofillers at 0.0625 phr reached a maximal value(c.a.74.4%).The Tm and lc value obtained for the best prepared UHMWPE/FANC1 0.0625 as-prepared fiber specimen with an optimal content of FANCx nanofillers at 0.0625 phr reached a minimal value(c.a.135.6? and 9.0 nm).By using one-stage drawing at 95?,the D,a value obtained for the best prepared UHMWPE/FANC130.0625 as-prepared fiber specimen with an optimal content of FANCx nanofillers at 0.0625 phr reached a maximal value(c.a.404).Many demarcated drawn"micro-fibrils" were found paralleling to the drawing direction of the drawn UHMWPE and UHMWPE/nanofiller composite fiber specimens as their draw ratios increased,wherein the thicknesses of these drawn micro-fibrils reduced significantly as the draw ratios increased.In which,"micro-fibrils" found on etched surfaces of UHMWPE/FANC 130.0625 as-prepared and/or drawn fibers were significantly more and thinner than other UHMWPE and UHMWPE/FANC fibers series with the same draw ratios.The best prepared UHMWPE/FANC130.0625 dawn fibers filled with FANC131400 exhibited an relatively high fo and?f value at 97%and 95.5 g/d,respectively.This ?fis the highest value ever reported for one-stage drawn UHMWPE fibers.4.The well dispersed nanofillers with higher specific surface areas can serve as effective nucleation sites and accelerate crystallization of UHMWPE molecules into crystals with lower melting temperature(Tm)and/or evaluated lamellar thickness(lc)during their crystallization processes.The lamellar crystals with lower Tm and/or evaluated lc values are expected to activate,melt and/or pull out of folded lamellar crystals comparatively easily during the ultradrawing processes,and hence,causes more demarcated "microfibril"characteristics,higher achievable Dra,fo and ?f values of the UHMWPE/nanofillers composite fibers. |
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