Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS
Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS
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DOI：
10.1021/acs.macromol.8b00397
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发表时间：
2018-09-25
期刊：
影响因子：
5.5
通讯作者：
Telling, Mark T. F.
中科院分区：
文献类型：
--
作者：
Arrighi, Valeria;Gagliardi, Simona;Telling, Mark T. F.
我们提出了一个详细的调查当地的动力学的线性和环状聚（二甲基硅氧烷）（PDMS）涵盖了广泛的摩尔质量。为了帮助解释实验数据，QENS测量的时间尺度从2到200 ps和Q = 0.3到1.8埃（-1）的理论计算补充。这些利用我们在其他地方开发的方法适用于简单的链模型和真实的链，并在这里应用，第一次，环状PDMS。在T < T-m处的非相干动态结构因子的分析表明，甲基基团的旋转运动不受聚合物拓扑结构的影响。在较高的温度下，QENS数据描述的模型，包括两个动态的贡献：甲基旋转和链段运动，后者描述的拉伸指数函数。线性和环状PDMS的弛豫时间都随着摩尔质量的增加而增加。理论预测的几个特征也被实验数据重现。我们表明，毫不含糊地，环具有更高的弛豫时间的链段运动相比，相同数量的单体单元的线性链。理论计算支持这样的想法，即局部动力学的减慢是由于环闭合所施加的拓扑约束，对于非常大的摩尔质量，这种约束变得可以忽略不计。我们的计算表明，由于其虽然小的构象刚性，环状PDMS经历了一个额外的约束，这进一步增加了松弛时间，产生一个浅的最大值N约50个重复单元。在实验QENS数据中也观察到类似的特征。活化能，E-a，的值来自分析的准弹性加宽的温度依赖性，并被发现与文献中报道的粘度测量结果一致。虽然线性PDMS的E-a的明显摩尔质量依赖性肯定与移动的链端的存在有关，但对于环状聚合物，行为似乎比预期的更复杂。
We present a detailed investigation of local dynamics of linear and cyclic poly(dimethylsiloxane) (PDMS) covering a wide range of molar masses. To aid interpretation of the experimental data, QENS measurements in the time scale from 2 to 200 ps and at Q = 0.3 to 1.8 angstrom(-1) are complemented by theoretical calculations. These make use of a methodology developed by us elsewhere applicable to both simple chain models and real chains and applied here, for the first time, to cyclic PDMS. Analysis of the incoherent dynamic structure factor at T < T-m, shows that the rotational motion of the methyl groups is unaffected by polymer topology. At higher temperatures, the QENS data are described by a model that consists of two dynamic contributions: methyl group rotation and segmental motion, the latter described by a stretched exponential function. Relaxation times of both linear and cyclic PDMS increase with increasing molar mass. Several features predicted by theory are also reproduced by the experimental data. We show, unambiguously, that rings have higher relaxation times for the segmental motion compared to linear chains of the same number of monomer units. Theoretical calculations support the idea that such slowing down of local dynamics is due to the topological constraint imposed by the ring closure, a constraint which becomes negligible for very large molar masses. Our calculations suggest that due to its albeit small conformational rigidity, cyclic PDMS undergoes an additional constraint which further increases the relaxation time, producing a shallow maximum for N approximate to 50 repeat units. A similar feature is also observed in the experimental QENS data. Values of activation energy, E-a, are derived from analysis of the temperature dependence of the quasi-elastic broadening and are found to be in agreement with viscosity measurements reported in the literature. Although the pronounced molar mass dependence of E-a for linear PDMS is certainly linked to the presence of mobile chain ends, for the cyclic polymers the behavior appears to be more complex than anticipated.