Blockiness reduces the packing of styrene/methyl methacrylate copolymers.

The influence of monomer sequence on the bulk packing and thermodynamics remains a substantial open question in polymer science. We use all-atom molecular simulations to address this problem for styrene/methyl methacrylate (S/M) copolymers. We examine eight copolymer sequences along with the two homopolymers, and report a strong density-sequence correlation, which is found to be enthalpic in origin. Optimal packing efficiency at the atomic-scale is observed for sequences with minimal block character, corresponding to a more statistically homogeneous monomer distribution. We establish correlations among nanoscale morphology, interaction and copolymer sequence. These findings imply that the sequence as a key variable that can be used to subtly tune the atomic-scale interaction, packing and resulting macroscopic density of S/M copolymers, which are primary descriptors of other macroscopic properties of polymeric materials.