Thermodynamic, conformational, and structure properties of bottlebrush polymer melts are investigated with molecular dynamics simulations and compared to linear, regular star and unknotted ring polymer melts to gauge the influence of polymer topology on polymer melt properties. We focus on variation of the backbone chain length, the grafting density along the backbone, and the length of the side chains at different temperatures above the melt glass transition temperature. Based on these comparisons, we find that the segmental density, isothermal compressibility, and isobaric thermal expansion of bottlebrush melts are quantitatively similar to unknotted ring polymer melts and star polymer melts having a moderate number (f = 5 to 6) of arms. These similarities extend in the mass scaling of the chain radius of gyration. These results together indicate that bottlebrush polymers in their melt state are more similar to randomly branched polymers than linear polymer chains. We also find that the average shape of bottlebrush polymers having short backbone chains with respect to side chain length is also similar to unknotted ring and moderately branched star polymers in their melt state. As a general trend, the molecular shape of bottlebrush polymers becomes more spherically symmetric when the length of the side chains has a similar length as the backbone chain. Finally, we calculate the partial static structure factor of the backbone segments and we find the emergence of a peak at the length scales that characterizes the average distance between the backbone chains. This peak is absent when we calculate the full static structure factor. We characterize the scaling of this peak with parameters characterizing the bottlebrush molecular architecture to aid in the experimental characterization of these molecules by neutron scattering.
Journal of Chemical Physics
Bottlebush, polymers, melt state, thermodynamics, scattering, mass scaling, molecular architecture