Preparation And Properties Of High-molecular-weight Polylactide Fibers With Completely Stereo-complexed Crystallites Based On Thermal Induction

Currently,commercial poly(lactic acid)(PLA)fibers are primarily developed from PLLA.With poor hydrolytic stability and low softening point,PLLA fibers cannot withstand textile wet processes,which are usually under high temperature.Thus,PLLA fibers have limited applications in textile industry.In this study,melt spun fibers(sc-PLA fibers)from equimolar poly(L-lactic acid)(PLLA)and poly(D-lactic acid)(PDLA)were thermally treated for stereo-complexation,and mechanism of stereo-complexation of PLLA and PDLA polymers via a thermal treatment was proposed.Enantiomeric PLAs both with MW up to 6.0×10~5 were completely stereo-complexed via the thermal treatment,and the relationship between MW and fiber properties was investigated.Preparation of sc-PLA fibers with equiweight mixtures of low-molecular-weight PDLA(L-PDLA)and high-molecular-weight PLLA(H-PLLA)was also proposed.Finally,dyeing behavior of sc-PLA fibers was also investigated.The main research contents and conclusions are shown as below.(1)PLLA and PDLA both with molecular weight of 3.0×10~5 were mixed and directly melt spun into fibers.Different degrees of stereo-complexation(%sc)were achieved via thermally treating at different temperatures.The effect of%sc on thermal properties and hydrolysis resistance of sc-PLA fibers were characterized and analyzed by WAXD,DSC and DMA.Fibers from equimolar PLLA/PDLA with the same molecular weight showed%sc of37%,75%and 100%after treated under 160°C,180°C and 200°C,respectively.The higher the%sc,the better the hydrolytic stability,and the higher the softening point.For example,the sc-PLA fibers stereo-complexed under 200°C had softening point 61°C higher than their PLLA counterparts.After treated in water with p H 5 at 130°C for 120 min,a common textile wet processing condition,PLLA fibers became too brittle for testing,while sc-PLA fibers retained 92%,91%and 96%tensile strength,breaking elongation and Young's modulus,which qualified them for textile processing.Finally,a mechanism of complete stereo-complexation via thermal induction was proposed.(2)To verify the universal applicability of the mechanism of complete stereo-complexation via the thermal induction,melt spun fibers from equimolar PLLA and PDLA with the same MW ranging from 1.0×10~5 to 6.0×10~5 were completely stereo-complexed via the thermal induction.Effects of MW on the%sc,and the relationship between%sc and hydrolytic and thermal stability of sc-PLA fibers,were investigated.Stereo-complexation via thermal treatment could be employed to prepare high-MW sc-PLA with completely stereo-complexed crystallites(sc-crystallites).For PLLA fibers with MW of6.0×10~5,percentage retention of MW and percentage retention of tensile strength after hydrolysis under simulated textile dyeing condition increased from 39%to 93%and from 20%to 89%,respectively,after complete stereo-complexation.The melting point and softening point of sc-PLA fibers were up to 51°C and 59°C higher than that of PLLA fibers with the same MW,respectively.Comparing to sc-PLA fibers with MW of 1.0×10~5,sc-PLA fibers with MW of 6.0×10~5 showed better hydrolytic stability,and much better thermal stability with softening point and melting point increased by 22°C and 10°C,respectively.(3)To lower the cost of PDLA,equal-weight mixtures of L-PDLA and H-PLLA were melt spun into sc-PLA fibers,and then completely stereo-complexed via thermal induction.The relationships between MW combination and fiber properties were investigated.Complete stereo-complexation via thermal induction could be applied not only to prepare sc-PLA fibers with equivalent MWs but also to prepare sc-PLA fibers with non-equivalent MWs.The hydrolysis resistance of L3/D1 fibers(PLLA(M_v=3.0×10~5)/PDLA(M_v=1.0×10~5))was similar to that of L3/D3 fibers(PLLA(M_v=3.0×10~5)/PDLA(M_v=3.0×10~5)),but much higher than that of L3 fibers(PLLA(M_v=3.0×10~5)).Melting temperature and softening temperature of L3/D1 fibers(223°C and 126°C)were also similar to those of L3/D3 fibers(224°C and 131°C),but higher than that of L3 fibers(172°C and 72°C).(4)Adsorption kinetic and thermodynamic studies of sc-PLA fibers dyed with disperse dyes were first reported.It was proved that sc-PLA fiber had enhanced high-temperature dyeing behaviors.The adsorption kinetic and thermodynamic properties of sc-PLA fibers were investigated using PLLA fibers as the control.K/S values and colorfastness to laundering and light of sc-PLA fibers dyed with C.I.Disperse Blue 56 and C.I.Disperse Blue 79 were measured,and hydrolysis resistance of sc-PLA fibers after dyeing was analyzed.The adsorption kinetic and thermodynamic properties of sc-PLA fibers were similar to that of PLLA fibers.Thus,stereo-complexation did not influence the adsorption behavior of sc-PLA fibers.The sorption of disperse dyes on sc-PLA fibers showed good agreement with the second-order kinetic model.Equilibrium dye concentration in the fibers,dyeing rate constant and diffusion coefficient increased with the increase of dyeing temperature.Langmuir isotherm agreed best for the disperse dyes adsorption on sc-PLA fibers.Adsorption capacity and adsorption affinity of sc-PLA fibers increased with the increase of dyeing temperature.After dyeing at 130°C,PLLA fibers became too poor to test,while sc-PLA fibers retained 90%tensile strength and breaking elongation.Dye exhaustion and K/S of sc-PLA fibers dyed with C.I.Disperse Blue 79 at 130°C were 91%and 24.35,while that of PLLA fibers dyed with C.I.Disperse Blue 79 at 110°C were 67%and 16.68.sc-PLA fibers dyed with C.I.Disperse Blue 79 at 130°C also showed good colorfastness to laundering and light.sc-PLA fibers dyed with disperse dyes at 130°C exhibited improved dye exhaustion and residual tensile properties,which showed great potential to facilitate the application of PLA fibers in the textile industry on a large scale.The method of complete stereo-complexation via thermal induction proposed in this study was low-cost and environmentally friendly.High-molecular-weight PLA fibers with enhanced hydrolytic and thermal stability could be obtained via complete stereo-complexation.This study provided theoretical foundation and technical support for the application of sc-PLA fiber in the textile industry.