Grafted Dipolar Chains: Dipoles and Restricted Freedom Lead to Unexpected Hairpins

We use atomistic molecular dynamics simulations to investigate the structure of oligohydroxybutyrate (OHB) and oligolactide (OLA) chains grafted onto cellulose nanocrystals (CNCs) in melts of polyhydroxybutyrate and polylactide chains, respectively. We observed that, contrary to the established theory for neutral chains, the more flexible OHB grafts extend further from the CNC surface, whereas the stiffer OLA ones tend to fold back toward the surface by forming hairpin-like structures. Both OHB and OLA grafts have dipoles but are charge-neutral. To resolve this apparent contradiction, interactions between the grafts' monomers as well as their flexibilities were evaluated. The interaction analysis, however, showed insignificant differences. We examined this further by systematically switching off the dipolar interactions and dihedrals from the OLA grafts. The observed counterintuitive behavior is due to a subtle combination of the dipole dipole interactions and the additional methylene group in OHB that is critical in allowing the OHB grafts to sample more conformations than the more restricted OLA chains: the steric restrictions are critical and lead to hairpin-like structures and enable dipole dipole interactions to effectively form a zipper that maintains the hairpin-like conformations.