PLLA/TAIC Blends:Research On Crystallization Behavior And Thermal Stability

Polymer blending is of great theoretical and practical significance in the research of multi-phase/multi-component polymer materials since it is an efficient way to improve their performance based on the synergistic effect.In the blend system containing at least one crystalline component,the amorphous or lower-meltingtemperature component(e.g.,B)can enrich in “inter-lamellar”,“inter-fibrillar” and“inter-spherulitic” regions of the other component(e.g.,A),which is dominated by the ratio between the crystal growth rate of A and the diffusion coefficient of B.There have been some reports concerning the distribution of B during the crystallization of A.Most of them,however,focus on polymer/polymer blends.Little attention has been paid to polymer/small molecule blends.On the contrary,small molecules have always been blended with polymers to enhance the processability and performance of the product.In this work,therefore,PLLA/TAIC has been taken as a model system to investigate the segregation of TAIC,the influence of crosslinked structures on the crystallization kinetics of PLLA,and the strategy to improve the thermal stability of PLLA based on crosslinking with the help of differential scanning calorimetry(DSC),scanning electron microscope(SEM),polarized light microscopy electron microscopy(POM)and small angle X-ray scattering(SAXS).The main contents of the thesis are as follows:(1)Inclusion/Exclusion behaviors of small molecule during crystallization of polymer in miscible PLLA/TAIC blend: The inclusion and exclusion of TAIC during the crystallization of PLLA in their miscible blend have been investigated.Our results indicate that the growth rate and diameter of PLLA spherulites dominate the localization of TAIC.On the one hand,crystallization temperature plays an important role.Crystallization at higher temperatures corresponds to higher growth rates and greater diameter of PLLA spherulites.The former improves the ability of PLLA crystals to trap TAIC while the latter leads to a lower volume fraction of space among neighboring PLLA spherulites.The combination of them contributes to enhanced inclusion behaviors.On the other hand,relative to melt crystallization,cold crystallization results in much smaller spherulites and sufficient space among spherulites,which accounts for the enrichment of TAIC in inter-spherulitic regions and its enhanced exclusion.(2)Effects of cross-linking/pre-ordered structure on crystallization kinetics of PLLA/TAIC blends: PLLA/TAIC blend upon isothermal crystallization at 100°C was crosslinked via γ-ray irradiation,followed by thermal treatment above its melting temperature and then cooling to a certain temperature for isothermal crystallization for the second time.Our results indicate that the kinetic of re-crystallization is under the control of favored crosslinked structures in amorphous regions.When the irradiation dose increases from 15 k Gy to 50 k Gy,stable crosslinked structures with higher gel content have been obtained.They depress the mobility of polymer chain during melting,resulting in the so-called pre-ordered structures.This is the reason for the accelerated kinetics during re-crystallization.(3)Enhancement of thermal stability of PLLA/TAIC blends by crosslinking:In the blend of PLLA/TAIC under γ-ray irradiation,there are several effects including“plastification of TAIC”,“degradation of PLLA because of γ-ray irradiation” and“crosslinking of PLLA via TAIC”.In this work,the plastification and degradation effects have been depressed via “optimization of irradiation dose” and “removal of free TAIC via extraction”,respectively.Then,both glass transition temperature and thermal degradation temperature of PLLA have been improved significantly based on the restriction of PLLA chain in 3-D network structures.In this way,a novel strategy has been developed to enhance the thermal stability of PLLA.