A SURFACE-INDUCED TRANSITION IN POLYMERIC NEMATICS

New polymeric liquid crystals can be treated as standard nematic liqui d crystals when only their bulk properties are at issue, but they exhi bit peculiar surface properties. The most striking one is that biaxial distributions may be induced on a confining surface. On continuously varying the surface anchoring conditions, we find a first-order phase transition from planar to homeotropic alignment in the bulk. Moreover, the decay towards these uniaxial states is radically different in the two cases: it is asymptotically exponential in the former, whereas it happens abruptly at a finite depth in the latter. There is precisely one surface biaxial distribution that induces bistability between thes e decay modes: it depends on the elastic constants in the Landau-de Ge nnes free energy functional. The analysis of the model we propose can prove useful in detecting the sign of the difference between splay and bend constants.