An x‐ray diffraction study of nonlinear polyethylene. I. Room‐temperature observations
An x‐ray diffraction study of nonlinear polyethylene. I. Room‐temperature observations
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非线性聚乙烯的 X 射线衍射研究 I. 室温观察。
DOI：
10.1002/pol.1985.180231210
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发表时间：
1985
期刊：
影响因子：
--
通讯作者：
A. Pijpers
中科院分区：
文献类型：
--
作者：
C. G. Vonk;A. Pijpers
本文用广角x射线散射法（WAXS）测定了高、低密度聚乙烯和乙烯-醋酸乙烯共聚物样品的结晶度；从小角度x射线散射（SAXS）中直接或结合WAXS数据获得了以下特征：结晶度ϕS的值或极限值；晶体密度ρcS；扩散边界层的厚度；结晶层和非晶层的数平均厚度；以及长时间的数量和重量平均值。结果表明，在规整的片层外存在较大的非晶态区域，这不能归因于两者之间的差异；假设WAXS结晶度属于晶片的核心，而部分弥散边界层则包含在ϕS值中，从而使数据一致。非线性样品的ρcW和ρcS数据显示出系统差异，这是由于晶体区域中部分侧基的掺入，其浓度估计为总浓度的20-40%。随着侧基浓度的增加，晶片核心的厚度接近侧基之间的线性链段的平均长度。在这些观察的基础上，提出了一个非线性聚乙烯结晶的方案，根据该方案，许多侧基被生长的晶体包裹起来。这些数据可以通过假设在生长的晶面上提供的所有链都直接结晶来解释，而不管结晶茎是否带有侧基。进一步的结晶将在第一茎的两端进行链折叠，直到遇到不可结晶的单元。在这个方案中，允许晶体中大约一半的茎通过褶皱连接；考虑到“过度拥挤”的影响，这是必要的。最后，讨论了冷却速率和分子量对结晶和非晶层厚度的影响，并指出了高密度聚乙烯和低密度聚乙烯的非晶密度的差异。
In this investigation on samples of high- and low-density polyethylene and ethylene-vinyl acetate copolymers, crystallinities ϕW and crystalline densities ρcW were obtained with the aid of wide-angle x-ray scattering (WAXS) methods. From small-angle x-ray scattering (SAXS) the following characteristics were obtained either directly or by combination with the WAXS data: values, or limiting values, of the crystallinity ϕS; crystal densities ρcS; thicknesses of the diffuse boundary layer; number-average thicknesses of the crystalline and amorphous layers; and both number and weight averages of the long periods. It was shown that a discrepancy between ϕS and ϕW cannot be attributed to the occurrence of large amorphous regions outside the regular stacks of lamellae; the data were reconciled by assuming that the WAXS crystallinities pertain to the cores of the crystalline lamellae, whereas part of the diffuse boundary layers is comprised in the values of ϕS. The ρcW and ρcS data of the nonlinear samples show systematic differences, which were attributed to partial incorporation of side groups in the crystalline regions at a concentration estimated to be of the order of 20–40% of the overall concentration. With increasing side-group concentration, the thickness of the core of the crystalline lamellae was found to approach the average length of the linear chain segments between side groups. On the basis of these observations a scheme for the crystallization of nonlinear polyethylene is proposed according to which a number of side groups is encapsulated by the growing crystal. The data can be explained by assuming that all chains, offered at a crystal face where growth takes place, crystallize directly, irrespective of whether the crystallizing stem carries a side group. Further crystallization would then proceed by chain folding at both ends of the first stem, until a noncrystallizable unit is met. In this scheme, allowance is made for about half the stems in the crystals to be connected by folds; this is required in view of the “overcrowding” effect. Finally, the effect of cooling rate and molecular weight on the thicknesses of the crystalline and amorphous layers is discussed, and differences between the amorphous densities of high-and low-density polyethylene are noted.