College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University , Chengdu 610065, China.
J Phys Chem B. 2017 Jun 15;121(23):5842-5852. doi: 10.1021/acs.jpcb.7b02241. Epub 2017 May 31.
In this work, we explored the crystallization of poly(lactic acid) (PLA) blended with poly(ethylene glycol) (PEG) under two inevitable processing fields (i.e., flow and pressure) that coexist in almost all processing for the first time. Here, the PEG was incorporated into PLA as a molecular chain activity promoter to induce PLA crystallization. A homemade pressuring and shearing device was utilized to prepare samples and necessary characterization methods, such as differential scanning calorimetry, scanning electron microscopy, and synchrotron radiation, and were used to investigated the joint effects of PEG, pressure, and shear flow on the crystallization behaviors and morphologies of PLA/PEG samples. The results reveal that adding 3-5 wt % PEG into PLA can significantly increase the PLA crystallinity due to the efficient plasticization effect of PEG, while the PEG content reaches 10 wt %, the PLA crystallinity decreases drastically as the phase separation between PEG and PLA occurs. We also find that applying a higher pressure (∼100 MPa) can facilitate the formation of thicker lamellae with fewer defects as well as higher crystallinity under an equal degree of supercooling compared to normal pressure or a low pressure condition because the slip of molecular chains during crystallization makes the lamellae thicker under higher pressures. The PLA crystalline structure in the PLA/PEG sample is not influenced by the shear flow, yet the crystallinity is largely enhanced by applying a shear flow with an appropriate intensity (0-3.5 s). It is worth noting that pressure and shear flow show a synergetic effect to fabricate PLA/PEG samples with high crystallinity. These meaningful results could beyond doubt help comprehend the relationship between crystallization conditions and crystallization behaviors of PLA/PEG samples and thus provide guidance to obtain high-performance PLA/PEG products via controlling crystallization conditions.
在这项工作中，我们首次探索了聚乳酸（PLA）与聚乙二醇（PEG）共混物在几乎所有加工过程中都共存的两种不可避免的加工领域（即流动和压力）下的结晶。在这里，PEG 被掺入 PLA 中作为分子链活性促进剂来诱导 PLA 结晶。利用自制的加压剪切装置制备样品，并采用差示扫描量热法、扫描电子显微镜和同步辐射等必要的表征方法，研究了 PEG、压力和剪切流对 PLA/PEG 样品结晶行为和形态的协同作用。结果表明，在 PLA 中添加 3-5wt%的 PEG 可以显著提高 PLA 的结晶度，这是由于 PEG 的有效增塑作用，而当 PEG 含量达到 10wt%时，由于 PEG 和 PLA 之间发生相分离，PLA 的结晶度急剧下降。我们还发现，与常压或低压相比，在相同过冷度下，施加较高的压力（约 100MPa）可以促进形成更厚的缺陷更少的片晶以及更高的结晶度，因为在结晶过程中分子链的滑移使得片晶在较高压力下变厚。剪切流对 PLA/PEG 样品的结晶结构没有影响，但在施加适当强度（0-3.5s）的剪切流时，结晶度会大大提高。值得注意的是，压力和剪切流对制备具有高结晶度的 PLA/PEG 样品具有协同作用。这些有意义的结果无疑有助于理解 PLA/PEG 样品的结晶条件与结晶行为之间的关系，从而为通过控制结晶条件获得高性能 PLA/PEG 产品提供指导。
