Styrene–diene block copolymers as embedding matrices for polymer-dispersed liquid crystal films

Composite films made of the nematic mixture E7 or the nematic single component K15 embedded in styrene–diene triblock copolymers have been prepared by the solvent cast method from homogeneous solutions; their phase separation behaviour has been investigated by calorimetric and morphological analyses as a function of mixture and matrix compositions, diene nature, casting solvent and evaporation time. Three copolymeric samples of comparable molecular weight have been used; two of them, based on polybutadiene, differ in their styrene content (30 and 55% by weight), in the third the major fraction of soft segments (75%) is made of polyisoprene. Films based on homopolymeric polystyrene and polybutadiene have been also prepared and tested as reference materials. The results reveal a substantially different solubility of the liquid crystal (LC) in the two phases of the copolymer micro-heterogeneous structures. The droplet size, shape and distribution that characterise the film morphology change considerably upon increasing the LC concentration in the blends, varying the casting solvent or lowering the evaporation rate; moreover a significant role is played by the nature of the elastomeric fraction. The dependence of morphological features on the various parameters is tentatively discussed in terms of both the affinity between the casting solvent and the matrix blocks and the critical surface tensions of LC and copolymer components.