Regulation Of Crtstal Structure And Perfomances Of Poly (Lactic Acid) Fibers

Poly(lactic acid)(PLA)is one of the bio-based polyesters with the highest degree of industrilization at present.It has good processing properties,biodegradability and biocompatibility,so it is named as one of the most promising green fibers and has a broad application prospect in the fields like clothing,home textiles,medical devices,industrial textiles,etc.Due to the limitation of weak nucleation ability,PLA fibers suffer problems like low tensile strength,poor heat resistance and physical aging during melt-spinning.To solve the problems mentioned above,the modification of heterogeneous nucleation,long chain branching and optimization of drawing process is applied to PLA fibers,and the effects of heterogeneous nucleation,long chain branching and stress-induced crystallization on the crystal structure,tensile mechanical properties and heat resistance of PLA fiber are emphatically studied.The main contents and progresses are as follows:1.The effects of heterogeneous nucleating agent and tensile stress on the crystal structure and properties of PLA fiber were studied by controlling the content of organic nucleating agent and the drawing process parameters.The diphenylhydrazide succinate(TMC-306)is selected as heterogeneous nucleating agent,and PLA/TMC-306(PLA/T)composites and fibers are prepared by melt-blending and melt-spinning respectively.The effect of TMC-306 content on the spherulite morphology and non-isothermal crystallization behavior of PLA is discussed,and the synergistic effect of heterogeneous nucleation and stress-induced crystallization on the microstructure and properties of PLA/T composite fibers is investigated in detail.It is found that the crystallization rate and crystallization temperature of PLA are promoted with the heterogeneous nucleation of TMC-306.Moreover,the orientation and conformation rearrangement of PLA are induced by TMC-306 as well.The tensile strength of PLA/T composite fiber first increases and then decreases with the increase of TMC-306 content,draw ratio.The nucleus induced by TMC-306 plays the role like“physical crosslink”,which promoting the crystallization and orientation the PLA with the synergistic effect of stress and temperature.Compared with neat PLA fibers,the tensile strength and storage modulus of PLA/0.3T composite fiber are increased from 2.44 c N/dtex and 32.98 GPa to3.46 c N/dtex and 46.11 GPa respectively,and the glass transition temperature is increased by 16 °C.2.The effects of chain entanglement and melt strength on the crystallization behavior and properties of PLA fibers were studied by introducing long chain branching structure into the main chain of PLA through reactive processing.The dicumyl peroxide(DCP)is used as the free radical initiator,and the long-chain branched PLA(LCB-PLA)is successfully prepared by reactiveprocessing with adequate temperature,residence time of twin-screw extruder and content of DCP.LCB-PLA fibers are fabricated by melt-spinning.The effects of long-chain branching on the chemical structure,rheology,crystallization behavior and performances of PLA fibers are discussed systematically.The degree of long chain branching is positively correlated with the content of DCP with the same reaction conditions.The introduction of long chain branched structure improves the degree of chain entanglement and melt strength of PLA,so the nucleation rate and crystallization rate of PLA are significantly improved.Therefore,the mechanical strength and storage modulus of LCB-PLA-0.05 D fibers increase from 2.44 c N/dtex and 32.98 GPa to 3.25 c N/dtex and 38.00 GPa respectively,and the glass transition temperature increase by 13.5 °C simultaneously.3.Based on heterogeneous nucleation modification and long chain branching modification,the synergetic method is applied to further improve the performance of PLA fiber,and effect of synergetic method on PLA crystal structure and performances is discussed.The DCP and TMC-306 are used as free radical initiator and heterogeneous nucleating agent,and the LCB-PLA/T composite fibers were prepared.Compared with PLA/T and LCB-PLA,the crystallization performance of LCB-PLA/T fiber is further improved with the synergistic effect of heterogeneous nucleation,stressinduced crystallization and long chain branching.Hence,the crystal size and spacing of planes(110/200)are of LCB-PLA/0.3T composite fibers reduce from 0.540 nm to 0.537 nm and 9.59 nm to 8.15 nm respectively.Therefore,the mechanical strength and glass transition temperature of LCBPLA/0.3T composite fibers further improve from 3.25 c N/dtex to 3.59 c N/dtex and 76.5 °C to 87 °C compared with LCB-PLA fibers.