Preparation,Structure And Properties Of Polylactic Acid-Based Multiphase Systems And Their Foams

In this dissertation,polylactide(PLA)/thermoplastic polyurethane(TPU)blends with 5TPU contents(10,20,30,40 and 50 wt%)were prepared by using a single-screw extruder with a chaotic mixing screw developed in our group.The relationship of“phase morphologies–rheological properties–properties”was analysed.A series of PLA/talc(5,10,and 20 wt%)and PLA/talc(20 wt%)/PLA-g-MAH(5 and 10 wt%)composites were prepared by using the same extruder.The effects of talc and PLA-g-MAH contents on the melt strength,crystallization behavior and mechanical properties of PLA were investigated.Then,PLA based multiphase systems were foamed,and the relationship of“phase morphology–cellular structure–oil adsorption property”of foamed PLA/TPU samples and the effect of foaming temperature on cellular structure of foamed PLA/talc samples were studied.Two phases in the PLA/TPU blends were partially compatible and the phase morphologies of blends were changed from sea-island to co-continuous structure as the TPU content increased from 10 to 50 wt%.Storage modulus and complex viscosity of the PLA/TPU blends were increased as the TPU content increased,and the blends with higher TPU content also exhibited more obvious shear thinning behavior.The melt strength of the PLA was improved due to the higher tensile strength of TPU,so the PLA/TPU blends had better foaming performance.The chaotic mixing in the extruder effectively improved the dispersion of the talc in the PLA matrix.For the PLA/talc(20 wt%)composite samples,theαcrystal content of the PLA was increased obviously and its X_c was increased to 25.1%.At 175°C,the melt strength for the PLA sample was relatively low(3.6 mN),and it was obviously enhanced(11.6 mN)for the composite sample with 20 wt%talc.The explanation for this was as follows.The well dispersed talc had reinforcing effect on the PLA melt and increased the crystallization behavior of PLA;moreover,the draw-down force improved both effects of the orientation-induced PLA crystallization and the increased PLA crystallization behavior by the added talc.PLA based multiphase systems were foamed in a batch process.For foamed PLA/TPU blend samples,the mean cell diameter and cell density were gradually reduced and increased,respectively,as the TPU content increased.Investigation on the oil-sorption performance of foamed PLA/TPU samples showed that the foamed PLA/TPU(10 wt%)sample had higher oil-sorption capacities[29.9 g/g(cyclohexane)],while the foamed PLA/TPU(50 wt%)sample had higher oil-sorption rate and better reusability.For foamed PLA/talc(20 wt%)composites,nanocells were foamed by using the low temperature foaming(foaming temperature:100~130°C).Whereas foaming temperature was 150°C,the cellular shape was polygon and most of the cells presented a closed cell structure,and cellular size was relatively uniform and in micron level.