Scale-Dependent Miscibility of Polylactide and Polyhydroxybutyrate: Molecular Dynamics Simulations

Miscibility of polylactide
(PLA) and polyhydroxybutyrate (PHB)
is studied by the microsecond atomistic molecular dynamics (MD) simulations
for the first time. The model and the simulation protocol were confirmed
through comparison of the glass transition temperature (Tg) with experimental data. It was established that PLA
and PHB are miscible on the basis of the Flory–Huggins theory.
Analysis of the mobilities of PLA and PHB subchains revealed that
the blends have two transitions to a glassy state at the length scale
of a few Kuhn segments, which is in line with the predictions of the
self-concentration model. At the same time at the larger length scale
a single transition to a glassy state was observed, suggesting scale
dependence of PLA and PHB miscibility. This scale dependence was confirmed
through the evaluation of the interchain pair correlation functions.

本研究首次采用微秒级全原子分子动力学（atomistic molecular dynamics, MD）模拟，探究聚乳酸（polylactide, PLA）与聚羟基丁酸酯（polyhydroxybutyrate, PHB）的共混相容性。通过将模拟所得玻璃化转变温度（glass transition temperature, Tg）与实验数据比对，验证了所采用的模拟模型与方案的合理性。基于弗洛里-哈金斯（Flory–Huggins）理论，本研究证实聚乳酸与聚羟基丁酸酯具有共混相容性。对二者子链迁移率的分析结果表明，在数个库恩段（Kuhn segment）的长度尺度下，该共混体系存在两次玻璃态转变过程，这与自浓度模型（self-concentration model）的预测结果一致。而在更大的长度尺度下，仅观测到一次玻璃态转变过程，提示聚乳酸与聚羟基丁酸酯的相容性具有尺度依赖性。通过对链间成对相关函数的计算评估，进一步验证了这一尺度依赖性。