Effect of localized control of cross-link density on mechanical properties of bicontinuous cubic lyotropic networks via copolymerization with different singly-polymerizable monomers

Abstract

Two singly-polymerizable analogs of a bicontinuous cubic (Q) phase, lyotropic liquid crystal (LLC) (i.e., amphiphilic) cross-linking monomer were designed and synthesized as phase-compatible, non-cross-linkable co-monomers. These were copolymerized with the parent, single-head/single-tail cross-linking monomer to locally control the cross-link density in either the hydrophobic tail or hydrophilic headgroup regions of the Q phase and study their effect on the mechanical properties of the resulting polymer networks. Structure-property relationships were elucidated: (1) Blends with smaller amounts of the parent cross-linking monomer relative to non-cross-linkable analog exhibit progressively lower endpoint stress and Young's modulus values. (2) A decrease in cross-link density also influences the pliability of each sample as there appears to be a threshold (i.e., minimum) cross-link density that must be exceeded before stretchability is significantly affected. (3) Each LLC-based structural motif tolerates different levels of non-cross-linkable analog, shifting the stress-strain behavior of these cross-linked Q-phase materials.