Self-Consistent Field Analysis of Molecular Bottle-Brushes with Primary and Secondary Side Chains: Induced Persistence Length and Lateral Thickness

Macromolecules may feature liquid crystalline behavior when the chain aspect ratio, that is the ratio between the chain persistence length and the cross-sectional thickness of its segments, is large. Scaling theory suggests that for molecular bottle-brushes under good solvent condition the induced persistence length does not depend on the chain architecture but only on the amount of segments in the side chains per unit length of the backbone. Numerical self-consistent field results are presented for macromolecular bottle-brushes with side chains of variable architecture attached at regular intervals along the backbone chain. We consider side chains (flexible “flagstaffs”), which feature one branch-point onto which secondary side chains (the “flag”) emanate. We varied the distance of this branch-point from the main chain as our tuning parameter, resembling the “raising of the flag.” For a given height of the flags, that is when the mass-distribution along a side chain is architecturally fixed, the induced persistence length is a function of the branching parameter (ratio between the length of the longest path and the total number of segments in the graft). We found that the induced persistence length and hence the chain aspect ratio, vary non-monotonically with the height of the flags. The lowest aspect ratio is found when the flag is raised to about a quarter of the full height of the flagstaff. The induced persistence length is independent of this height when upon bending the translocation of the side chains from compressed to expanded regions is artificially suppressed, which leads us to speculate that the non-monotonic behavior of aspect ratio is related to the efficiency of side chains to partly translocate themselves upon bending.