Complex Smectic Phases in Polymer-Grafted Shape-Uniform Cellulose Nano-Crystals

Abstract

The effect of short chain grafting on the liquid crystalline (LC) ordering of nanocrystals is investigated using molecular dynamics simulations of a coarse-grained grafted nanorod model. Monodisperse nanorods, with aspect ratios typical of cellulose nanocrystals (CNCs), are grafted randomly with oligomers at different grafting densities. LC ordering depends nontrivially on grafting density as the effective nanorod shape and softness are modified. Ungrafted rods exhibit Nematic and tilted Smectic-C phases. At 25% grafting, the addition of a few side chains entirely suppresses Smectic order, and instead, a persistent Nematic phase is favored. Intermediary grafting, around 50%, results in a Nematic and the reappearance of tilted Smectic-F phases. Heavier grafting facilitates direct transitions to either Smectic-I with extreme tilt (75%) or an untilted Smectic-B (100%). Such behavior falls outside of current hard or soft-rod descriptions of phase transitions in rod-like LC systems and points to undiscovered LC behavior in both shape-purified grafted/ ungrafted CNCs.