Study On The Structures And Properties Of Long Chain Branched Poly(lactic Acid) Prepared Via Functional Group Reactions

As one of the biodegradable polyesters produced from non-renewable resources, poly(lactic acid)(PLA) is a promising one of wide potential applications. However, its abroad applications are currently limited by several main shortcomings, such as its low melt strenghth, low crystallinity and crystallization rate. This study focuses on the approach to get long chain branched PLA via functional group reactions by processing, as well as the evaluation and characterization on its topological structures and properties.In our work, it was found out that the effective methods to get some polyester such as PET long chain branched do not work for PLA, because of the severe thermal degradation during the processing. Therefore, a preliminary study was done on the effect of antioxidants, catalyzers and multifunctional monomers on thermal degradation of PLA, as well as the processing flow effects. Then the most possible thermal degradation mechanism under our study conditions was concluded to be the"back-bitten"mechanism.After discovering that independently unsing one kind of multifunctional monomers can not effectively get PLA branched, we found out a new approach to obtain highly branched PLA with high molecule weight by reacting successively with PMDA then TGIC for the first time. The existence of long chain branching were fully proved through several methods such as FTIR, GPC, linear and non-linear rheology. What's more, the quantitative information on the chain topology was abtained by simulating their viscoelasticities using BOB model. Finally, this structural information was correlated with its foaming ability, with guidance on its potential applications in polymeric foam or film.In addition, the effects of long chain branching and flow on the crystallization properties of PLA were also studied. During the investigation on the influence of long chain branching on crystallinity and crystallization rate of PLA under quiescent conditions, it was found that the existence of long chain branching decreases a little the crystallinty, but could largely increase the crystallization rate of the system. The higher degree of long chain branching, the more prominent this phenomenon was. And then the study was done on the effect of temperature rate and flow parameters on PLA crystallization behavior under steady shear flow. As it was showed in the results, under steady flow, long chain branching PLAs began to crystallize at relatively higher temperatures than linear ones, and both decreasing temperature rate and increasing shear rate, could raise the crystallization temperature. Coppola model could well predict the crystallization temperature of linear PLA during temperature cooling process under steady flow.