Preparation, Structural Characterization And Properties Of Long Chain Branched Polylactides And Toughened Polylactide Blends

Polylactide (PLA) is a type of aliphatic thermoplastic polyester and possesses a number of interesting properties including biodegradability, biocompatibility, sufficient mechanical properties, and processability, which make it become one of the most competitive and promising candidates to substitute some petroleum-based polymers in future applications. However, PLA shows some inherent drawbacks including poor melt strength, low crystallization kinetics and brittleness. In this thesis, a series of studies is presented in an attempt to overcome these drawbacks of PLA and extend its application. Meanwhile, the thesis provides further scientific insights into the roles of chain topological modification, external flow field and toughening modifier on the properties of PLA. Main content includes four aspects:1. An easy procedure was applied to prepare high melt strength polylactide (PLA), which involved gamma radiation induced free radical reactions to introduce long chain branched structure on linear PLA precursor with addition of a trifunctional monomer, trimethylolpropane triacrylate (TMPTA). Various rheological plots including viscosity curve, storage modulus, loss angle and Cole-Cole plot are used to distinguish the improved melt strength for LCB PLA samples. The effect of LCB structure on elongational rheological properties is further investigated. The LCB PLA samples demonstrate the enhancement of strain-hardening under elongational flow. The enhanced melt strength substantially improves the foaming performance of LCB PLA samples.2. The topological structures of LCB PLA were investigated by SEC-MALLS and rheological analysis. SEC-MALLS measurements show that LCB PLA exhibits not only the increased weight-average molecular mass but also a bimodal architecture with a short linear chain fraction and a LCB fraction. By the analysis of the thermorheological behaviors and determination of activation energies, the bimodal architecture is confirmed. A conclusion with respect to the tree-like topography for LCB PLA samples is drawn from the molecular mass dependences of zero-shear viscosity (ηo-Mw, plot). An explanation to these findings is provided under the consideration of the radiation dose rate for the gamma radiation.3. The effects of long chain branching on the nucleation density enhancements and morphological evolution for polylactide (PLA) materials during shear-induced isothermal crystallization process were thoroughly investigated by using rotational rheometer and polarized optical microscopy (POM). The results of shear-induced isothermal crystallization kinetics show that the crystallization process under shear is greatly enhanced compared to the quiescent conditions and the crystallization kinetics is accelerated with the increases in shear rate and/or shear time. LCB PLA crystallizes much faster than linear PLA under the same shear condition. A saturation effect of shear time on crystallization kinetics is observed for both linear PLA and LCB PLA. In-situ POM observations demonstrate that LCB PLA not only possesses higher nucleation density under the identical shear time and a constant lower value of spherulitic growth rate compared with that of linear PLA but also forms the shish-kebab structure after sheared for sufficient time. A saturation of nucleation density under shear can be reached for both linear PLA and LCB PLA. The enhancement of nucleation ability and the morphological evolution from the spherulitic to shish-kebab structures induced by shear flow can be ascribed to the broadened and complex relaxation behaviors of LCB PLA.4. Super-tough biocompatible and degradable binary blends of polylactide (PLA) and crosslinked poly(ethylene glycol) diacylate (CPEGDA) were fabricated by applying a novel and facile method involving reactive blending of PLA with PEGDA monomer with no addition of exogenous radical initiators. Torque analysis and FT-IR spectra suggest that crosslinking reaction of acylate groups occurs in melt blending process according to the free radical polymerization mechanism. Differential scanning calorimetry (DSC) and phase contrast optical microscopy (PCOM) results indicate the in-situ polymerization of PEGDA leads to a phase separated morphology with crosslinked PEGDA as the dispersion phase domains and PLA matrix as the continuous phase. The blends show increasing viscosity and elasticity with increasing crosslinked PEGDA content with a rheological percolation crosslinked PEGDA content of15wt%. Introduction of crosslinked PEGDA shows little effect on crystallinity of PLA in the blends. Mechanical properties of these blends are improved significantly. The effective interfacial compatibility is achieved by the dangling PEG chains and transesterification reactions at the interfaces between crosslinked PEGDA particles and PLA matrix.