Institute for Physical Chemistry, Bulgarian Academia of Sciences, 1113 Sofia, Bulgaria.
Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55128 Mainz, Germany.
J Chem Phys. 2018 Nov 7;149(17):174909. doi: 10.1063/1.5049630.
Coarse-grained models of lyotropic solutions of semiflexible polymers are studied by both molecular dynamics simulations and density functional theory calculations, using an implicit solvent bead-spring model with a bond-angle potential. We systematically vary the monomer density, persistence length, and contour length over a wide range and explore the full range from the isotropic-nematic transition to the nematic-smectic transition. In the nematic regime, we span the entire regime from rigid-rod like polymers to thin wormlike chains, confined in effective straight tubes caused by the collective nematic effective ordering field. We show that the distribution of bond angles relative to the director is well described by a Gaussian, irrespective of whether the chains are rod-like or rather flexible. However, the related concept of "deflection length" is shown to make sense only in the latter case for rather dilute solutions since otherwise the deflection length is of the order of about two bond lengths only. When the solution is semi-dilute, a substantial renormalization of the persistence length occurs, while this effect is absent in the isotropic phase even at rather high monomer densities. The effective radii of the "tubes" confining the chains in the related description of orientational ordering are significantly larger than the distances between neighboring chains, providing evidence for a pronounced collective character of orientational fluctuations. Hairpins can be identified close to the isotropic-nematic transition, and their probability of occurrence agrees qualitatively with the Vroege-Odijk theory. The corresponding theoretical predictions for the elastic constants, however, are not in good agreement with the simulations. We attribute the shortcomings of the theories to their neglect of the coupling between local density and orientational fluctuations. Finally, we detected for this model a transition to a smectic phase for reduced monomer densities near 0.7.
通过分子动力学模拟和密度泛函理论计算，使用具有键角势的隐式溶剂珠-弹簧模型，研究了溶致性半柔性聚合物的粗粒模型。我们系统地改变单体密度、持久长度和轮廓长度的范围很广，并探索了从各向同性-向列相转变到向列-层状相转变的整个范围。在向列相中，我们跨越了从刚性棒状聚合物到薄的蠕虫状链的整个范围，这些链被集体向列有效有序场产生的有效直管限制。我们表明，相对于指向矢的键角分布很好地由高斯描述，无论链是棒状还是相当灵活。然而，只有在后者情况下，“偏转角”的相关概念才有意义，因为否则偏转角的大小只有大约两个键长。当溶液为半稀溶液时，持久长度会发生实质性的重整化，而在各向同性相中，即使在相当高的单体密度下，也不会出现这种效应。在相关的取向有序描述中，限制链的“管”的有效半径明显大于相邻链之间的距离，为取向波动的明显集体特征提供了证据。可以在各向同性-向列相转变附近识别发夹，并定性地与 Vroege-Odijk 理论一致。然而，对于弹性常数的相应理论预测与模拟结果并不一致。我们将理论的缺点归因于它们忽略了局部密度和取向波动之间的耦合。最后，我们发现对于这个模型，在单体密度接近 0.7 时，存在向层状相转变的趋势。
