Conformation of a Comb-like Chain Free in Solution and Confined in a Nanochannel: From Linear to Bottlebrush Structure

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 σ s N g ≈ 3.5, where σ s is the grafting density of side chains, N g is the degree of polymerization of side chains, and σ s N g is the stretching factor quantifying the degree of overlap of side chains. The graft–graft excluded-volume interaction can be ignored only when σ s N g < 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 ( l p / l p0 ) quasi-linearly increases with σ s N g, and the ratio of the radius of gyration of the comb-like chain ( R g ) and the backbone itself ( R g0 ) satisfies a quantitative relationship, that is, R g / R g0 – 1 ∼ (σ s N g ) 0.66±0.11, which effectively validates the previous experimental results. In the confined state, the relative degree of confinement ( H / l p ) is defined as the diameter of the nanochannel ( H ) divided by the persistence length of the comb-like chain ( l p ). Our results show that as H / l p increases, the ratio between the component of the radius of gyration parallel to the nanochannel axial and its radius of gyration ( R g x 2 / R g 2 ) 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 ( l p,con / l p0 ), a plateau appears when H / l p > 2.0. Only when N g 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 R g x 2 / R g 2 and an increase in l p,con / l p0 . For a high grafting density, with an increase of H / l p, the value of R g x 2 / R g 2 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.