Woo Eamor M, Lugito Graecia
Department of Chemical Engineering, National Cheng Kung University, No. 1, University Road, Tainan 701-01, Taiwan.
Polymers (Basel). 2016 Sep 2;8(9):329. doi: 10.3390/polym8090329.
This article reviews possible mechanisms of various crack forms and their likely correlations with interior crystal lamellae and discontinuous interfaces in spherulites. Complex yet periodically repetitive patterns of cracks in spherulites are beyond attributions via differences in thermal expansion coefficients, which would cause random and irregular cracks in the contract direction only. Cracks in brittle polymers such as poly(l-lactic acid) (PLLA), or poly(4-hydroxyl butyrate) (PHB), or more ductile polymers such as poly(trimethylene terephthalate) (PTT) are examined and illustrated, although for focus and demonstration, more discussions are spent on PLLA. The cracks can take many shapes that bear extremely striking similarity to the ring-band or lamellar patterns in the same spherulites. Crack patterns may differ significantly between the ring-banded and ringless spherulites, suggesting that the cracks may be partially shaped and governed by interfaces of lamellae and how the lamellar crystals assemble themselves in spherulites. Similarly, with some exceptions, most of the cracks patterns in PHB or PTT are also highly guided by the lamellar assembly in either ring-banded spherulites or ringless spherulites. Some exceptions of cracks in spherulites deviating from the apparent crystal birefringence patterns do exist; nevertheless, discontinuous interfaces in the initial lamellae neat the nuclei center might be hidden by top crystal over-layers of the spherulites, which might govern crack propagation.
本文综述了各种裂纹形式的可能机制，以及它们与球晶内部晶体片层和不连续界面的可能关联。球晶中复杂但周期性重复的裂纹模式无法通过热膨胀系数的差异来解释，因为热膨胀系数的差异只会在收缩方向上导致随机和不规则的裂纹。本文研究并说明了脆性聚合物（如聚（L-乳酸）（PLLA）、聚（4-羟基丁酸酯）（PHB））或更具韧性的聚合物（如聚对苯二甲酸丙二醇酯）（PTT）中的裂纹，不过为了重点突出和便于展示，对PLLA的讨论更多。裂纹可以呈现多种形状，与同一球晶中的环带或片层图案极为相似。有环带球晶和无环带球晶的裂纹模式可能有显著差异，这表明裂纹可能部分由片层界面以及片层晶体在球晶中的组装方式所塑造和控制。同样，除了一些例外情况，PHB或PTT中的大多数裂纹模式在有环带球晶或无环带球晶中也受到片层组装的高度引导。球晶中的裂纹确实存在一些偏离明显晶体双折射图案的例外情况；然而，靠近核中心的初始片层中的不连续界面可能被球晶顶部的晶体覆盖层所掩盖，这可能控制着裂纹的扩展。
