Donostia International Physics Center, Paseo Manuel de Lardizabal 4, E-20018 San Sebastián, Spain.
J Chem Phys. 2011 Jan 14;134(2):024523. doi: 10.1063/1.3525147.
By means of computer simulations and solution of the equations of the mode coupling theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of nonentangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results, and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However, a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed deviations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and nonmonotonous wavelength dependence of the mode correlators.
我们通过计算机模拟和模式耦合理论（MCT）方程的求解，研究了分子内势垒对非缠结聚合物几个动力学方面的作用。研究的动力学范围从玻璃形成体的笼蔽区域扩展到链罗塞模式的弛豫。我们回顾了这个问题的最近工作，提供了新的结果，并对理论的局限性进行了批判性讨论。MCT 对结构弛豫的解再现了模拟对于弱和中等势垒的定性趋势。然而，随着刚性链极限的接近，逐渐出现了差异。这种差异似乎与动态不均匀性无关，而实际上，增加势垒强度并不会增强动态不均匀性。它也与三点静态相关的卷积近似对于刚性链的失效无关，该近似在刚性链中仍然有效。这些发现表明需要改进聚合物熔体的 MCT 方程。关于链自由度的弛豫，MCT 为从链重取向到笼蔽区域的时间尺度提供了微观基础。它从第一性原理出发，解释了观察到的随着势垒强度增加而偏离罗塞模型的现象。这些现象包括弛豫时间的异常标度、长时间平台和模式相关函数的非单调波长依赖性。
