Conformation of a Comb-like Chain Free in Solution and Confined in a Nanochannel: From Linear to Bottlebrush Structure
Zengxian Tang, Xuejun Pan, Heng-Wei Zhou, Lianwei Li, Mingming Ding
Abstract
We study the conformation of a comb-like polymer chain in dilute solution from being in a free state to being confined in a nanochannel by using Brownian dynamics simulations. In the free state, our results demonstrate that for the evaluation of the radius of gyration, there is a transition region around σsNg ≈ 3.5, where σs is the grafting density of side chains, Ng is the degree of polymerization of side chains, and σsNg is the stretching factor quantifying the degree of overlap of side chains. The graft–graft excluded-volume interaction can be ignored only when σsNg < 3.5; otherwise, its effect becomes increasingly significant. The ratio of the persistence length of the backbone grafted by side chains and the backbone itself (lp/lp0) quasi-linearly increases with σsNg, and the ratio of the radius of gyration of the comb-like chain (Rg) and the backbone itself (Rg0) satisfies a quantitative relationship, that is, Rg/Rg0 – 1 ∼ (σsNg)0.66±0.11, which effectively validates the previous experimental results. In the confined state, the relative degree of confinement (H/lp) is defined as the diameter of the nanochannel (H) divided by the persistence length of the comb-like chain (lp). Our results show that as H/lp increases, the ratio between the component of the radius of gyration parallel to the nanochannel axial and its radius of gyration (Rgx2/Rg2) gradually decreases until it tends to be stable, but for the ratio of the persistence length of the backbone with grafting side chains and the backbone itself (lp,con/lp0), a plateau appears when H/lp > 2.0. Only when Ng is longer than the Kuhn length, the effect of grafting density can be observed; that is, an increase of grafting density leads to a decrease in Rgx2/Rg2 and an increase in lp,con/lp0. For a high grafting density, with an increase of H/lp, the value of Rgx2/Rg2 does not always decrease, but a plateau appears. Our work lays a foundation for characterizing the conformation of comb-like polymers in different states, which is an indispensable precondition for various applications.