Effect of spacer lengths on the interplay of intrachain and interchain in polynorbornene homopolymers embedded with polyhedral oligomeric silsesquioxane (POSS): A molecular simulation approach

Abstract

Molecular models of polynorbornene homopolymers, with POSS (polyhedral oligomeric silsesquioxane) group linked to the side chain via various numbers of methylene spacers, were constructed and named as P-m-POSS (where m represents the number of methylene units that connect amide and POSS end group on the side chain, m = 0, 4, and 9), respectively. By molecular mechanics (MM) and molecular dynamics (MD) simulation methods, molecular geometry, chain mobility and packing characteristics were investigated to elucidate the complex interplay of intrachain and interchain in homopolymers dependent on various methylene spacer lengths of the side chain. With short spacer length containing less number of methylene units, the synergetic mobility of the main chain and the side chain can be observed in P-0-POSS by analysis of mean square displacement (MSD), denoting an intrachain coupling restraint within molecule. It is also noticed that the disorganization of twisted side chains surrounding the main chain results in a loose and irregular packing in P-0-POSS, validated by the amorphous morphology in experiment. As the spacer length increased with more methylene units in P-4-POSS, side chains with the highest mobility are partially stretched and independent of the main chain, suggesting the invalidation of intrachain coupling restraint. Furthermore, with the longest spacer length containing 9 methylene units, fully stretched side chains are decoupled from the main chain and have a tendency to be close to each other in P-9-POSS. The interchain correlation between side chains in P-9-POSS facilitates a distinctively regular interdigitation in bulk structure, resulting in the highest crystallinity in experiment.