Highly birefringent polymer films from the photo-crosslinking polymerisation of bistolane-based methacrylate monomers

Abstract

The photo-polymerisation of mixtures of mono- and di-methacrylates, containing a bistolane moiety with a central fluorine-substituted benzene ring, resulted in the formation of highly birefringent polymer films (Δn = 0.40), which were obtained in a nematic liquid crystal (NLC) phase. While the dimethacrylate forms enantiotropic NLCs at T = 110–138°C, smectic phases at T = 50–138°C and crystallises at T = 50°C, whereas the monomethacrylate forms NLCs at a wider temperature range (T = 98–185°C) and crystallises at a lower temperature (T = 98°C). These methacrylates were infinitely miscible and a 20/80 (w/w) mixture of the dimethacrylate/monoacrylate was able to form an NLC phase over a broad temperature range (T = 73–179°C). The mixed NLC phase exhibited a Δn value of 0.36, even though the Δn values of the mono- and di-methacrylates were determined as 0.35 and 0.25, respectively, suggesting that the Δn of the mixture follows an additivity rule. Furthermore, the Δn of the NLC phase could be increased to 0.40 by photo-polymerisation.