Wang Yaotao, Jiang Zhiyong, Fu Lianlian, Lu Ying, Men Yongfeng
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Changchun, People's Republic of China.
PLoS One. 2014 May 12;9(5):e97234. doi: 10.1371/journal.pone.0097234. eCollection 2014.
Polybutene-1 (PB-1), a typical semicrystalline polymer, in its stable form I shows a peculiar temperature dependent strain-whitening behavior when being stretched at temperatures in between room temperature and melting temperature of the crystallites where the extent of strain-whitening weakens with the increasing of stretching temperature reaching a minima value followed by an increase at higher stretching temperatures. Correspondingly, a stronger strain-hardening phenomenon was observed at higher temperatures. The strain-whitening phenomenon in semicrystalline polymers has its origin of cavitation process during stretching. In this work, the effect of crystalline lamellar thickness and stretching temperature on the cavitation process in PB-1 has been investigated by means of combined synchrotron ultrasmall-angle and wide-angle X-ray scattering techniques. Three modes of cavitation during the stretching process can be identified, namely "no cavitation" for the quenched sample with the thinnest lamellae where only shear yielding occurred, "cavitation with reorientation" for the samples stretched at lower temperatures and samples with thicker lamellae, and "cavitation without reorientation" for samples with thinner lamellae stretched at higher temperatures. The mode "cavitation with reorientation" occurs before yield point where the plate-like cavities start to be generated within the lamellar stacks with normal perpendicular to the stretching direction due to the blocky substructure of the crystalline lamellae and reorient gradually to the stretching direction after strain-hardening. The mode of "cavitation without reorientation" appears after yield point where ellipsoidal shaped cavities are generated in those lamellae stacks with normal parallel to the stretching direction followed by an improvement of their orientation at larger strains. X-ray diffraction results reveal a much improved crystalline orientation for samples with thinner lamellae stretched at higher temperatures. The observed behavior of microscopic structural evolution in PB-1 stretched at different temperatures explains above mentioned changes in macroscopic strain-whitening phenomenon with increasing in stretching temperature and stress-strain curves.
聚丁烯-1（PB-1）是一种典型的半结晶聚合物，其稳定的I型在室温至微晶熔点之间的温度下拉伸时，表现出一种特殊的温度依赖性应变发白行为，应变发白程度随拉伸温度升高而减弱，达到最小值后在更高拉伸温度下又增加。相应地，在较高温度下观察到更强的应变硬化现象。半结晶聚合物中的应变发白现象源于拉伸过程中的空化过程。在这项工作中，通过同步加速器超小角和广角X射线散射技术相结合的方法，研究了结晶片层厚度和拉伸温度对PB-1中空化过程的影响。拉伸过程中的三种空化模式可以被识别，即对于片层最薄的淬火样品，只有剪切屈服发生，为空化模式“无空化”；对于在较低温度下拉伸的样品和片层较厚的样品，为空化模式“有空化且重排”；对于在较高温度下拉伸的片层较薄的样品，为空化模式“有空化但无重排”。“有空化且重排”模式发生在屈服点之前，由于结晶片层的块状亚结构，板状空洞在片层堆叠中开始形成，其法线垂直于拉伸方向，在应变硬化后逐渐重排至拉伸方向。“有空化但无重排”模式出现在屈服点之后，椭圆形空洞在那些法线平行于拉伸方向的片层堆叠中产生，随后在更大应变下其取向得到改善。X射线衍射结果表明，在较高温度下拉伸的片层较薄的样品的结晶取向有了很大改善。在不同温度下拉伸的PB-1中观察到的微观结构演变行为解释了上述宏观应变发白现象随拉伸温度和应力-应变曲线增加而发生的变化。
