Phase behavior of side-chain liquid-crystalline polymers containing biphenyl mesogens with different spacer lengths synthesized via miniemulsion polymerization

Abstract

The synthesis of a series of methacrylate side-chain liquid crystal polymers (SCLCPs) bearing biphenyl mesogen with different spacer lengths and a fixed tail, poly[ethyl 4′-((n-(methacryloyloxy)alkyl)oxy)-[1,1′-biphenyl]-4-carboxylate]s (n-PMLCM, n = 3, 4, 5, 6), in aqueous media by free radical miniemulsion polymerization is described. This method offers the advantage of producing high molar masses (>10⁵ Da) and full monomer conversion, not possible to achieve with conventional routes (solution polymerization). The resulting n-PMLCMs proved to have high thermal stability. The phase behaviors of the polymers were investigated by a combination of techniques including differential scanning calorimetry, polarized light microscopy, and small and wide angle X-ray scattering. The results show mesomorphic liquid crystalline behavior with a monolayer structure where the side-groups on both sides of the backbone would be interpenetrated. The liquid crystal phase transition of n-PMLCM follows the sequence smectic E (smectic E or smectic C for 4-PMLCM) ↔ smectic A ↔ isotropic liquid. The transition temperatures and the associated entropy changes exhibit a distinct odd–even effect as the length and parity of the spacer are varied, with the odd members exhibiting the higher values. The high molar masses achievable using miniemulsion polymerization translate into a more perfect and stable ordering, characterized by larger lamellar domains and higher transition temperatures, than in low molar mass SCLCPs. Compared to polymeric liquid crystals with similar mesogens but shorter tails, we found that longer tails facilitate the ordering of the mesogens and allow more efficient packing around the backbones, imparting a high stability to the smectic phases formed.