A temperature- and molar mass-dependent change in the crystallization mechanism of poly(1-butene): Transition from chain-folded to chain-extended crystallization?
A temperature- and molar mass-dependent change in the crystallization mechanism of poly(1-butene): Transition from chain-folded to chain-extended crystallization?
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DOI：
10.1021/ma0015875
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发表时间：
2001-04-10
期刊：
影响因子：
5.5
通讯作者：
Thomann, Y
中科院分区：
文献类型：
--
作者：
Fu, Q;Heck, B;Thomann, Y
采用偏光显微镜（PLM）、原子力显微镜（AFM）、广角X射线散射（WAXS）、X射线衍射仪（XRD）、时间分辨和温度分辨小角X射线散射（SAXS）等方法研究了聚1-丁烯（P1 b）的结晶行为。PLM中的观察表明结晶机制的温度依赖性变化。当超过一定的临界结晶温度时，其形貌由球晶转变为方形、片状单晶。不同摩尔质量的样品的研究表明，转变温度是摩尔质量依赖性的;在降低摩尔质量的转变转移到较低的温度。WAXS结果表明，球晶和单晶均为亚稳晶型。在样品快速冷却至室温后获得的AFM图像中也观察到形态变化;形态外观的差异通过从II型转化为I型而得以保留。根据衍射测量，结晶机制的变化导致结晶速率的温度依赖性的变化，并且还导致结晶度的阶梯式增加。SAXS实验结果表明，P1 b微晶的形成是由相同的一般规律为以前研究的其他聚合物。结晶温度T-c和熔化温度Tf均与结晶层厚度倒数d（c）（-1）呈线性关系，但d（c）（-1）的斜率和极限温度不同-->0。这些观察结果再次表明晶体的发展分为两个步骤：首先出现初始形式，然后转变为最终的层状微晶。作为一个新的特征，直接对应于在显微镜下观察到的两种不同的结晶机制，显示出两种不同的结晶线（d（c）（-1）vs T-c），表明存在两种不同的初始状态。另一方面，只发现一条共同的熔化线（T-f vs d（c）（-1）），这意味着两种结晶机制产生具有相似表面自由能的微晶。通过比较熔体中链的回转半径R-g与晶体厚度d（c），讨论了P1 b的特殊结晶性质，并提出当d（c）大于R-g时，结晶机制的变化可能是由于从折叠链结晶转变为扩链结晶。
The crystallization behavior of poly(1-butene) (P1b) was investigated by polarized light microscopy (PLM), atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), dilatometry, and also by time- and temperature-resolved small-angle X-ray scattering experiments (SAXS). Observations in the PLM indicate a temperature-dependent change in the mechanism of crystallization. When crossing a certain critical crystallization temperature, the morphology changes from spherulites to quadratic, platelike single crystals. Investigations of samples with different molar mass show that the transition temperature is molar mass-dependent; on decreasing the molar mass the transition shifts to lower temperatures. As proved by WAXS, both the spherulites and the single crystals are of the metastable form II. The morphological change is also observed in AFM images obtained after a rapid cooling of the samples to room temperature; the difference in the morphological appearance is preserved through the transformation from form II to form I. According to dilatometric measurements, the change in the crystallization mechanism leads to variations in the temperature dependence of the crystallization rate and also to a steplike increase in the crystallinity. The results of SAXS experiments show that the formation of P1b crystallites is governed by the same general laws as for other polymers studied before. Both the crystallization temperature, T-c, and the melting temperature, Tf, are linearly dependent on the reciprocal crystalline layer thickness, d(c)(-1), but with different slopes and different limiting temperatures for d(c)(-1)-->0. The observations are again indicative for a crystal development in two steps: First an initial form appears which then transforms into the final lamellar crystallites. As a new feature, in direct correspondence to the two different crystallization mechanisms observed microscopically, two different crystallization lines (d(c)(-1) vs T-c) show up, indicating the occurrence of two different initial states. On the other hand, only one common melting line (T-f vs d(c)(-1)) is found, which means that the two crystallization mechanisms produce crystallites with similar surface free energies. We discuss the peculiar crystallization properties of P1b by comparing the radius of gyration R-g of the chains in the melt with the crystal thickness d(c) and propose that the change in the crystallization mechanism could be due to a change from folded-chain to chain-extended crystallization, taking place when d(c) gets larger than R-g.