Mohottalalage Supun S, Saab Andrew P, Maiti Amitesh
Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
Polymers (Basel). 2023 Nov 28;15(23):4560. doi: 10.3390/polym15234560.
Ionomers are associative polymers with diverse applications ranging from selective membranes and high-performance adhesives to abrasion- and chemical-resistant coatings, insulation layers, vacuum packaging, and foamed sheets. Within equilibrium melt, the ionic or associating groups are known to form thermally reversible, associative clusters whose presence can significantly affect the system's mechanical, viscoelastic, and transport properties. It is, thus, of great interest to understand how to control such clusters' size distribution, shape, and stability through the designed choice of polymer architecture and the ionic groups' fraction, arrangement, and interaction strength. In this work, we represent linear associating polymers using a Kremer-Grest type bead-spring model and perform large-scale MD simulations to explore the effect of polymer chain-length (l) and fraction (fs) of randomly placed associating groups on the size distribution and stability of formed clusters. We consider different chain-lengths (below and above entanglement), varying fractions of associating groups (represented by 'sticky' beads) between 5 and 20%, and a fixed sticky-sticky nonbond interaction strength of four times that between regular non-associating beads. For all melts containing associating groups the equilibrium structure factor S(q) displays a signature ionomer peak at low wave vector q whose intensity increases with increasing fs and l. The average cluster size Nc increases with fs. However, the effect of chain-length on Nc appears to be pronounced only at higher values of fs. Under extensional flows, the computed stress (and viscosity) is higher at higher fs and l regardless of strain rate. Beyond a critical strain rate, we observe fragmentation of the associative clusters, which has interesting effects on the stress/viscous response.
离聚物是一种缔合聚合物，具有多种应用，从选择性膜、高性能粘合剂到耐磨和耐化学涂层、绝缘层、真空包装以及泡沫片材。在平衡熔体中，离子基团或缔合基团会形成热可逆的缔合簇，其存在会显著影响体系的机械、粘弹性和传输性能。因此，了解如何通过设计聚合物结构以及离子基团的分数、排列和相互作用强度来控制这些簇的尺寸分布、形状和稳定性，具有重要意义。在这项工作中，我们使用Kremer-Grest型珠簧模型来表示线性缔合聚合物，并进行大规模分子动力学模拟，以探究聚合物链长（l）和随机放置的缔合基团分数（fs）对形成的簇的尺寸分布和稳定性的影响。我们考虑了不同的链长（低于和高于缠结长度）、5%至20%之间不同分数的缔合基团（由“粘性”珠子表示）以及固定的粘性-粘性非键相互作用强度，其为常规非缔合珠子之间相互作用强度的四倍。对于所有含有缔合基团的熔体，平衡结构因子S(q)在低波矢q处显示出特征性的离聚物峰，其强度随fs和l的增加而增加。平均簇尺寸Nc随fs增加。然而，链长对Nc的影响似乎仅在较高的fs值时才明显。在拉伸流动下，无论应变率如何，计算得到的应力（和粘度）在较高的fs和l时都更高。超过临界应变率后，我们观察到缔合簇的破碎，这对应力/粘性响应有有趣的影响。
