Study On Static Crystallization And Flow-induced Crystallization Of Poly (L-iactic Acid) Telechelic Lonomers

Polylactic acid(PLA)is a biodegradable thermoplastic aliphatic polyester,derived from renewable materials(plant starch,etc.),and has good biocompatibility,high permeability and easy processing and molding.Therefore,it is considered to be one of the ideal substitutes for petroleum-based polymer materials.The development and application of polylactic acid is conducive to protecting the ecological environment and reducing the consumption of petroleum resources.However,usually polylactic acid(here refers to PLLA)has poor heat resistance and mechanical properties due to its large spherulite morphology and low crystallinity,which limits the application of PLLA.Flow-induced crystallization(FIC)is a cost-effective method to accelerate crystallization kinetics,increase crystallinity,and improve crystal morphology,which has attracted great attention from academia and industry.However,most of the current FIC studies on PLLA focus on highly entangled systems,and there are few studies on associated systems,and previous studies are limited to the high-temperature region near Tm dominated by "thermodynamics".Therefore,in order to better understand the FIC behavior and mechanism of the association system PLLA in the near Tg low temperature region dominated by "kinetics",so as to provide theoretical guidance for the molecular design and processing of PLLA,we designed and synthesized a PLLA telechelic ionomer model system,the main research results and conclusions are as follows:We prepared short-chain PLLA telechelic ionomer with sodium sulfonate ion group at the end,and confirmed the existence of ion cluster structure in the system by small-angle X-ray scattering(SAXS).The linear viscoelastic curve shows a wide platform,and the strong association of ions(the association energy Ea=36kJ/mol=12kBT at 80℃)makes the system form a reversible physical crosslinking network,which greatly delays the terminal relaxation time.Studies have shown that due to the restriction of chain segment movement by terminal ions,the dual effect of affecting stereoregularity hinders crystallization,making the static crystallization kinetics of ionomer samples very slow.The study results show that the static crystallization kinetics of ionomer samples is very slow due to the dual effect of limiting the movement of chain segments by terminal ions and affecting tacticity,which provides an experimental window for studying the FIC behavior of PLLA in the low temperature region near Tg dominated by "kinetics".The FIC behavior and mechanism of PLLA telechelic ionomer under different flow fields were studied by rotational and extensional rheometers.Strain hardening(i.e.,induced crystallization)of ionomer samples under shear flow occurs before or after reaching steady-state stress(shear-induced ion association and disassociation reaches equilibrium);similarly,under extensional flow,the strain hardening of ionomer samples occurs either before or after stress overshoots(corresponding to sites where straininduced ions undergo severe dissociation),which lead to rich rheological behavior of the system.Studies have shown that strain hardening is related to critical work,and the critical work of strain hardening after reaching a steady state under a shear flow field and after stress overshoot under an extensional flow field is significantly higher than the opposite case.This is attributed to the fact that the chain rebound after ion dissociation reduces the degree of orientation of the polymer chains in the system,resulting in energy dissipation,which requires a greater critical work to crystallize,resulting in a further lag in strain hardening.In addition,the corresponding critical work under the shear flow field is significantly larger than that under the extensional flow field,which indicates that the extensional flow field is more likely to induce crystallization.