Crystallization Behavior And Physical Property Of Biodegradable Polyesters Having High Melting Points

Aliphatic polyesters are a representative group of biodegradable thermoplastics that can be frequently produced from the renewable sources.However,most of the biodegradable polyesters have the low melting points and thermal resistance;this restricts their industrial applications.Among the family of biodegradable polyesters,there are two kinds of materials,i.e.,stereocomplexed poly(lactic acid)(SC-PLA)and poly(glycolic acid)(PGA)that have the high melting points and thermal resistance.The high melting points and thermal resistance of SC-PLA and PGA are attributed to their unique crystalline structure.However,the high-molecular-weight(HMW)PLA are usually difficult to crystallize in SC form and the relationships between physical performances and crystalline structure of PGA is not clear.In this work,we proposed two methods(i.e.,immiscible blending and random polymerization)to promote the SC crystallization and prepared the HMW SC-PLA material.The crystalline structure of PGA was also investigated to explain its unique physical properties.First,we used the immiscible blending to enhance the SC crystallization of HMW poly(L-lactic acid)/poly(D-lactic acid)(PLLA/PDLA)blends.Effects of poly(vinylidene fluoride)(PVDF)on the crystallization kinetics,competing formations of homocrystallites(HCs)and SCs,polymorphic crystalline structure,and HC-to-SC crystalline reorganization of HMW PLLA/PDLA racemic mixture were investigated.Even though the PLLA/PDLA/PVDF blends are immiscible,blending with PVDF enhances the crystallization rate and SC formation of PLLA/PDLA components at different temperatures that are higher or lower than the melting temperature of the PVDF component;it also facilitates the HC-to-SC melt reorganization upon heating.Crystallization rate and degree of SC crystallinity(Xc,SC)of PLLA/PDLA components in nonisothermal crystallization increase after immiscible blending with PVDF.At different isothermal crystallization temperatures,the crystallization half-time of PLLA/PDLA components decreases;its spherulitic growth rate and Xc,sc increase as the mass fraction of PVDF increases from 0 to 0.5 in the presence of either a solidified or a molten PVDF phase.The HCs formed in primary crystallization of PLLA/PDLA components melt and recrystallize into SCs upon heating;the HC-to-SC melt reorganization is promoted after blending with PVDF.We proposed that the PVDF-promoted crystallization,SC formation,and HC-to-SC melt reorganization of PLLA/PDLA components in PLLA/PDLA/PVDF blends stem from the enhanced diffusion ability of PLLA and PDLA chains.Furthermore,we also used the random polymerization to improve the SC crystallization of HMW PLLA and PDLA.HMW poly(L-lactide-co-glycolide)(LG)and poly(D-lactide-co-glycolide)(DG)were synthesized by ring-opening polymerization,and then equivalent LG and DG were blended in solution.Crystallization kinetics,crystalline structure,spherulitic morphology,and mechanical properties of LG/DG blends were investigated.Crystallize ability of LG/DG blend is much stronger and its crystallization rate is significantly enhanced compared to the enantiopure LG or DG.Spherulitic morphology of LG/DG blends gradually changes from the typical Maltess-cross spherulite to dendritic spherulite with the content of glycolide in LG and DG increases,because of the dilution effects of glycolide units.Besides,the elongation-at-break of LG/DG blends is improved as the content of glycolide in LG and DG increases.These results open up a method to prepare the SC-PLA materials and crystalline poly(lactide-co-glycolide)through SC crystallization.Finally,we systematically investigated the crystalline structure,spherulitic morphology,and structural evolution of PGA in the crystallization and melting processes by time-resolved wide-angle X-ray diffraction and FTIR spectrometry.PGA exhibits fast crystallization and it completely crystallizes in the fast cooling process.The melting point,degree of crystallinity,and long period of PGA increase as the crystallization temperature increases.PGA forms the unique hedrites but not the normal spherulites in crystallization.Because of the thermal expansion of the crystal lattice,the a and b axis of PGA crystal cells increase linearly with temperature in the heating process.The band splitting in FTIR spectrum of PGA is observed in C=O stretching and CH2 bending banding regions and the spectral splitting intensifies with cooling,indicating the existence of intermolecular dipolar interactions in PGA crystals.It is speculated that the C-H…O=C H-bonds are formed in the crystalline phase of PGA,as indicated by the significant red shifts of C=O stretching band during crystallization.