Numerical evidences for a free energy barrier in starlike polymer brushes

Abstract

The existence of a free energy barrier, which prohibits the upward motion of retracted molecules into the surface region of starlike polymer brushes, is analyzed through molecular dynamics simulations in good solvent. This barrier emerges at moderate and high grafting densities, as a result of a density-discontinuity at the branching points of the highly stretched starlike molecules. The vertical force profiles of brushes of varying densities are taken with the help of a probe-particle that is gradually moved into the brush, and the results are compared with the density profiles and their negative gradients which generate the local osmotic pressures. Chain expulsion simulations, supported by scaling theory, are conducted to understand the dynamics of individual molecules inside the brushes. We prove that the flip-rates between retracted and extended states, being of relevance for the generation of efficiently switchable, environment-responsive brush layers, are determined by the elastic tension of the stretched molecules.

Theoretical approaches to starlike polymer brushes in Θ-solvent

“…Consequently, in order to keep the monomer density of the brush's lower parts sufficiently high, a certain fraction of molecules had to retract into a collapsed state. This proposal has been confirmed in subsequent applications of the SF-SCF formalism [21,22] and in MD simulations of brushes in good solvent [29][30][31] as well as at the Θ point [37]. …”

“…A common feature of these approaches is that every single molecule of the brush is treated equally, but it has been suspected early on that this might not reflect reality: Inside a dense brush, branched polymers have to segregate into several populations of molecules, each of which featuring different degrees of vertical elongation, in order to assume a homogeneous monomer density [24]. Numerical ScheutjensFleer [28] (SF-SCF) simulations on a lattice have verified the existence of such a segregation [21,22], and in molecular dynamics (MD) simulations, its features have been analyzed in detail [29][30][31][32]. It therefore seemed necessary to search for simple mean-field approaches which were able to deal with this multi-population structure of branched brushes, and also to predict the partition of polymers among their different populations [32][33][34][35].…”

From polymer configuration and dynamics to protein folding

“…带电高分子刷由于其带电行为而产生很多不同 于中性聚合物刷的物理特性, 因此越来越多的人开 始关注这个研究领域. 早在50年前, Kuhn等人 [15] 就 描述了带电聚合物链的Flory行为, 早期的带电聚 合物刷理论研究则由Miklavic等人 [16] 以及Misra等 人 [17] [18,19] 限伸展弹性势, Cui等人 [20,21] 建立了简单方便、 适 用于连续空间的平均场模型来预测良好溶剂中星形 聚合物刷的主要特性, 并进一步开发一种适用于良 好溶剂中的第二代树状分子刷的平均场理论. 不同 种类的聚合物末端单体也会随溶液的性质发生改 变, 聚合物的分子结构能够实现对外界环境快速反 应的"化学开关" [22,23] , 这些特质值得我们进一步深 入考虑, 比如在纳米粒子上吸附聚合物, 以形成球基 面上的聚合物刷, 改变聚合物的亲疏水性, 从而实现 药物传递, 这里吸附密度和聚合物链长度和数目都 会影响药物的传送, 这也会是今后研究的重要课题 方向.…”

Arm Retraction Dynamics in Dense Polymer Brushes

“…(I. 10) where γ is an effective dimensionless spring constant of order unity, k the Boltzmann constant, and T the absolute temperature. Note that because of this effective potential we have to consider now "stretched" chains with reduced dynamic fluctuations instead of "swollen" ones.…”

“…These brushes can be used in various applications ranging from drug delivery, colloid stabilization, , and lubrication − to switchable amphiphilic surfaces . In recent years, brushes made of more complex architectures like stars, cyclic polymers, dendrimers along with cross-linked brushes, , or systems containing nanoinclusions , were analyzed.…”