Atomistic structure of bottlebrush polymers: Simulations and neutron scattering studies

We have used small angle neutron scattering (SANS) measurement and atomistic molecular dynamics (MD) simulations to investigate the conformation of bottlebrush polymers with poly(norbornene) (PNB) backbone and different sizes of poly(lactide) (PLA) side chains (PNB₂₅-g-PLA₅, PNB ₂₅-g-PLA₁₀, and PNB₂₅-g-PLA₁₉). At early stage of simulations, stretched side chains with visible spatial-correlations of about 30 Å were observed. The experimentally measured SANS data, on the other hand, does not exhibit any correlation peaks in the corresponding length scale indicating a compact form rather than a stretched-hairy polymer conformation. As the simulation continued, the spatial correlations between side chains disappeared after about 40 ns of chain relaxation, and the scattering intensity calculated for the simulated structure becomes reasonably close to the measured one. Statistical approach is used to overcome the time scale limitation and search for optimal conformation space, which also provides a good agreement with the experimental data. Further coarse-grained simulation results suggest that the side chain conformation strongly depends on the solubility competition among side chain, backbone, and solvent. Significant changes of backbone dynamics due to the side chain encapsulation have been revealed and discussed.