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首页> 外文期刊>Polymer: The International Journal for the Science and Technology of Polymers >Time-resolved FTIR spectroscopic study of the evolution of helical structure during isothermal crystallization of propylene 1-hexene copolymers. Identification of regularity bands associated with the trigonal polymorph
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Time-resolved FTIR spectroscopic study of the evolution of helical structure during isothermal crystallization of propylene 1-hexene copolymers. Identification of regularity bands associated with the trigonal polymorph

机译：丙烯1-己烯共聚物等温结晶过程中螺旋结构演变的时间分辨FTIR光谱研究。识别与三角形多态性相关的规则带

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摘要

Two different types of regularity bands are identified in a real time FTIR crystallization of a series of random propylene 1-hexene copolymers. The first is akin to the bands observed in the homopolymer, those associated with 3 _1 helices of isotactic sequences of different n length (n, number of monomer units). The second type corresponds to vibrational coupling of short sequences of the chain that include the 1-hexene comonomer. Among the latter are absorbances at 910 and 1025 cm~(-1) which are markers for the formation of a trigonal phase in these copolymers. They remain unchanged prior to and during crystallization in copolymers with the 1-hexene units rejected from the crystallites (<13 mol% 1-hexene) and increase in intensity when the comonomer is an integral part of the crystallites (>13 mol% 1-hexene). Analysis of the real time evolution of IR regularity bands during isothermal crystallization of these copolymers confirms the beginning of crystallization at a critical helical sequence length (n) of ～12 isotactic units (841 cm~(-1)), and enables details of the early and final stages of crystallization. In the homopolymer and copolymers, the intensity of regularity bands with n ≤ 10 is constant in the initial undercooled melt, and increases simultaneously with the appearance of helices with n ≥12, in support of a classical crystallization mechanism of nucleation and growth. Due to density fluctuations in the initial melt, the short helices eventually collapse in aggregates or precursors that spontaneously (within the experimental macroscopic time frame) extend to stable nuclei (n ≥ 10). Stable nuclei further extend and grow cooperatively dragging additional short sequences as inferred by the simultaneous temporal evolution of helices with n = 10 and greater. The intensity of the 998 cm-1 (n = 10) band prior to nucleation, correlates directly with the isotactic sequence length of the copolymer and is independent of the final structure that evolves, either monoclinic or trigonal. This feature infers a nucleation event driven preferentially by the initial steady-state content of short helices in iPP and iPP-based copolymers. The temporal evolution of the 841 cm~(-1) band is an excellent avenue to study the crystallization kinetics of copolymers, including those with very low crystallinities. Via FTIR, the mechanism of the formation of mesomorphic crystallites in copolymers with ～10 mol% 1-hexene at low temperatures is contrasted with the formation of alpha crystallites at higher temperatures in the nucleation driven range. The intensity of the 841 cm~(-1) band (n = 12) at the end of the transformation correlates linearly with the degree of crystallinity obtained by WAXD.
机译：在一系列无规丙烯1-己烯共聚物的实时FTIR结晶中鉴定出两种不同类型的规则带。第一个类似于在均聚物中观察到的带，这些带与不同n长度（n，单体单元数）的3 _1个全同立构序列的螺旋相关。第二类对应于包括1-己烯共聚单体的链的短序列的振动偶联。后者是在910和1025 cm-1处的吸光度，这些吸光度是这些共聚物中形成三角相的标志。当共聚单体是微晶的组成部分（> 13 mol％1-）时，它们在结晶前和结晶过程中保持不变，而1-己烯单元被微晶排斥（<13 mol％1-己烯），强度增加。己烯）。对这些共聚物等温结晶过程中IR规则带的实时演化进行分析，可以确定在临界螺旋序列长度（n）为约12个全同立构单元（841 cm〜（-1））处开始结晶，并提供了详细的信息。结晶的早期和最终阶段。在均聚物和共聚物中，n≤10的规则带强度在初始过冷熔体中是恒定的，并随着n≥12的螺旋出现而增加，以支持经典的成核和生长结晶机制。由于初始熔体中的密度波动，短螺旋最终坍塌成聚集体或前体，这些聚集体或前体自发地（在实验宏观时间范围内）扩展到稳定的核（n≥10）。稳定的原子核进一步扩展并协同拖动其他短序列，如n = 10或更大的螺旋的同时时间演化所推断的。成核之前998 cm-1（n = 10）谱带的强度与共聚物的等规序列长度直接相关，并且与所形成的最终结构（单斜晶或三角晶）无关。此功能推断成核事件是由iPP和iPP基共聚物中短螺旋的初始稳态含量优先驱动的。 841 cm〜（-1）带的时间演化是研究共聚物（包括结晶度非常低的共聚物）的结晶动力学的绝佳途径。通过FTIR，与成核驱动范围内，在约10 mol％1-己烯的共聚物中，在低温下形成同晶微晶的机理与在较高温度下形成α-微晶的机理相反。转变结束时841 cm〜（-1）谱带的强度（n = 12）与WAXD获得的结晶度呈线性关系。

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