Study On Graft Copolymerization And Reactive Blending Of Biodegradable Polyester

Due to the good biodegradability and biocompatibility, biodegradable polyesters have been considered as the green and ecofriendly polymeric materials. However, compared to the petroleum-based plastics, the biodegradable polyesters have some drawbacks such as low mechanical and barrier, properties and high cost. Therefore, the modification is necessary for the industrial application of biodegradable polyesters. In this thesis, we studied the grafting copolymerization and reactive blend systems of biodegradable polyester. We first prepared the graft copolymer of biodegradable polyester through the control of macromolecular architecture. Then, we modified the barrier property of biodegradable polyester via the reactive blending. Meanwhile, we studied the crystallization kinetics and crystalline structure of the grafting polymers and reactive blends.We first proposed a new method to prepare the grafting copolymer of poly(L-lactide)(PLLA) with controllable graft length and graft density and investigated the effects of branching structure on the crystallization behavior of PLLA segments. Graft copolymers of poly(vinyl acetate)-<graft-PLLA (PVAc-g-PLLA) comprising the biodegradable graft chains were synthesized through the ring-opening polymerization (ROP) of lactide. The partially hydrolyzed PVAcs, that is, poly(vinyl acetate-co-alcohol)[P(VAc-co-VA)], having degrees of hydrolysis of5.2and9.6%were used as the macromolecular multifunctional initiator of ROP. Graft length and density in PVAc-g-PLLA can be controlled by varying the lactide/P(VAc-co-VA) feed ratio and the degree of hydrolysis of P(VAc-co-VA). The graft length of PLLA chain and molecular weight of graft copolymers increased with the lactide/P(VAc-co-VA) feed ratio. The graft length and density of PVAc-g-PLLA had a significant effect on its crystallization behavior. The crystallization and spherulitic growth of PVAc-g-PLLA were decelerated with decreasing the length of PLLA graft chain or increasing the graft density, due to the branching effects and dilution effects of PVAc. The graft copolymer with shorter graft length and higher graft density showed disordered spherulites during crystallization, due to the structural defects in the crystal growth caused by branching.In order to improve the barrier property of biodegradable polyester, poly(butylene succinate)(PBS) was blended with the ethylene-vinyl alcohol copolymer (EVOH), which is a typical polymer with excellent barrier property. The compatibilizer was used to improve the compatibility and physical properties of the blend systems. A new compatibilizer, that is, PBS-g-maleic anhydride (PBS-g-MAH), was prepared from PBS and maleic anhydride. Then, PBS was blended with EVOH in the melt state using the PBS-g-MAH as the compatibilizer. The effects of PBS-g-MAH content on the thermal behavior, crystallization, mechanical and barrier properties of PBS/EVOH blends were investigated. PBS and EVOH are thermodynamically immiscible. The miscibility of the blend pairs were improved by the introduction of PBS-g-MAH. The content of PBS-g-MAH had a significant effect on the crystallization behavior of blend component. In the PBS-rich blend, the crystallizability of EVOH component increased as the PBS-g-MAH content increased. The introduction of PBS-g-MAH did not affect the crystalline structure of PBS and EVOH. The tensile strength and barrier property of PBS/EVOH blends was improved by PBS-g-MAH. The barrier property to water vapor of PBS increased after blending with EVOH.