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

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 (n=0.40), which were obtained in a nematic liquid crystal (NLC) phase. While the dimethacrylate forms enantiotropic NLCs at T=110-138 degrees C, smectic phases at T=50-138 degrees C and crystallises at T=50 degrees C, whereas the monomethacrylate forms NLCs at a wider temperature range (T=98-185 degrees C) and crystallises at a lower temperature (T=98 degrees 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 degrees C). The mixed NLC phase exhibited a n value of 0.36, even though the n values of the mono- and di-methacrylates were determined as 0.35 and 0.25, respectively, suggesting that the n of the mixture follows an additivity rule. Furthermore, the n of the NLC phase could be increased to 0.40 by photo-polymerisation.