Tailoring The Morphology And Properties Of Poly(lactic Acid)by A Self-Assembly Nucleator Multiamide And Poly(ethylene Oxide)

Poly(lactic acid)(PLA)is considered as a promising green alternative to traditional plastics.However,its practical applications have been significantly limited by its slow crystallization rate.Adding nucleating agent is one of the most effective ways in enhancing the crystallization rate of PLA.In this thesis,the influence of a self-assembly nucleating agent,N,N',N''-tricyclohexyl-1,3,5-benzenetricarboxylamide(BTCA),on the crystallization behavior and morphology evolution of PLA has been investigated.Moreover,the influence of a flexible polymer,poly(ethylene oxide)(PEO),together with BTCA on the crystallization behavior and mechanical properties of PLA was also investigated.The main results are as follows:(1)The effect of BTCA on the crystallization behavior of PLA.At first,the influence of BTCA on the crystallization behavior of PLA was investigated by differential scanning calorimeter(DSC),polarizing optical microscope(POM)and wide angle X-ray diffraction(WAXD).DSC results show that BTCA can significantly shorten the half-crystallization time t1/2,increase the crystalliation peak temperature Tp and the crystallinity Xc.POM results show that BTCA can provide a large number of nucleation sites,where PLA can epitaxially crystallize on their surfaces.The WAXD results indicate that the addition of BTCA results in the formation of ? crystals in PLA.Afterwards,the dissolution and self-assemble behavior of BTCA in PLA melt was investigated.It reveals that the annealing temperature Tf,the selected pretreatment temperature Tpre and the cooling rate all have an effect on the morphology of BTCA.The solubility of BTCA in PLA melt depends on the Tf,which plays a significant role on the self-assemble behavior of BTCA,making it grows into short rod-like,fine needle-like and dendrite-like structures.Under the multi-step isothermal process,it is found that BTCA is prone to form an aggregated thick dendritic-like structure when the pretreatment temperature Tpre of BTCA is higher.The specific surface area of this thick structure is relatively small,providing less nucleation sites for PLA.As a result,the crystallization promotion effect of PLA is limited.When the Tpre is lower,BTCA tends to form adispersed needle-like fiber structure.The specific surface area of this structure is relatively large,and the number of nucleation sites is increased.Different morphologies of BTCA can form during the cooling process via controlling of the cooling rate,making it possible to regulate the crystallization behavior and crystal morphology of the PLA matrix.(2)The effect of PEO together with BTCA on the crystallization behavior and mechanical properties of PLA.The nonisothermal crystallization behavior and kinetics were examined by using Avrami and Jeziorny models.The results show that PEO and BTCA have a synergistic effect on enhancing the crystallization rate of PLA.POM results show that PEO and BTCA have an opposite effect on the size and density of PLA spherulites.The addition of PEO increases the size of spherulites and decreases the nucleation density of PLA,while the presence of BTCA decreases the size of spherulites and increases the nucleation density of PLA.WAXD results show that neat PLA crystallized at a rate of 2.5 °C/min from the melt forms ?' crystals,while PLA formulations comprising PEO with BTCA form ? crystals.Moreover,dynamic thermomechanic analysis(DMA)results show that BTCA revealed prominent reinforcement effect on both neat PLA and PLA/PEO blends in the glassy and rubbery regions,indicating the improvement on the heat resistance of the materials.Tensile testing results show that the elongation at break value of PLA blended with PEO is dramatically increased.This investigation is helpful for designing high-performance PLA with enhanced crystallization rate and toughness.