酯交换法制备长链支化聚乳酸及其流变和结晶行为

Rheological and Crystalline Behaviour of Long Chain Branched Polylactic Acid Prepared by Transesterification Reaction

在促进剂纳米氧化锌、三官能度单体季戊四醇三丙烯酸酯(PETA)的存在下,对聚乳酸(PLA)进行熔融支化改性。研究表明,PETA成功地引入到PLA分子链上,由此获得了具有长链支化(LCB)结构的聚乳酸,且未产生凝胶,熔融支化制备LCBPLA的机理为酯交换反应,并使得聚乳酸的相对分子质量分布变宽。纳米氧化锌对LCBPLA的支化有明显的促进作用,当促进剂含量为0.2 phr时,聚乳酸呈现出更低的相位角和星形支化特征,产生更明显的应变硬化,熔体强度得到显著提高,由纯PLA的0.02 N升高到0.11 N。改性PLA为线型结构与支化结构的混合物,产生了双重熔融峰,长链支化结构的引入没有改变PLA的晶体结构。虽然链段的规整性遭到了破坏,但支化大分子易于成核,等温结晶过程中晶核数量大幅提高,球晶生长速率降低,在变温结晶过程中降低了起始结晶温度,提高了结晶速率和结晶度。

Melt-branching modification of linear polylactic acid (PLA) was carried out in the presence of nano-zinc oxide (nano-ZnO) as accelerant, and tri-functional monomer pentaerythritol triacrylate (PETA) as branching center via a method of transesterification reaction. Results show that PLA with long branching chains without gel is obtained and the reaction mechanism of the melt branching reaction is ester exchange. The addition of nano-ZnO could promote the branching reaction of PLA and PETA obviously, and in addition, the molecular weight distribution becomes wider compared with the virgin linear PLA. A shoulder generated owing to the introduction of long branches in the molecular weight distribution curve. When the nano-ZnO content is 0.2 phr, the melt strength of the prepared LCBPLA is increased from 0.02 N to 0.11 N. And the nano-ZnO also promotes the degradation, the melt strength of PLA decreases with the increase of accelerant content. Differential scanning calorimetry (DSC) results reveal the faster crystallization rate and lower crystallization onset temperature in LCBPLA than that of PLA, which are due to the presence of LCB, promoting the crystallization of the system, resulting in the increase of crystallinity and the decrease of spherulites growth rate. The modified PLAs are the mixtures of linear chains and branched macromolecules, which form a double melting peak in the DSC curves, and the branching parts of which the completeness is poor would melt first.