Ordering of cylindrical microdomains in thin films of hybrid isotropic/liquid crystalline triblock copolymers

We have investigated the roles of liquid crystallinity (LC) and substrate i nteractions on the ordering of isotropic cylindrical microdomains in thin f ilms of two hybrid isotropic/LC triblock copolymers. Each copolymer consist s of polystyrene (PS) end blocks and a side-chain LC midblock that exhibits either a nematic or smectic mesophase up to temperatures beyond the glass transition temperature (T-g) of PS. In thin films measuring on the same ord er as the lattice period, the orientation of the cylinders relative to the substrate is sensitive to the interactions of the mesogens at internal micr odomain boundaries and external interfaces. Upon annealing such copolymer f ilms at temperatures below the LC --> isotropic transition but above the PS T-g on a neutral NaCl substrate, the PS cylinders within the nematic matri x adopt a long-range parallel orientation, whereas those within the smectic matrix lie perpendicular to the substrate and exhibit liquidlike order. Af ter long annealing times, the cylinders residing near the film surface with in the smectic matrix reorganize from perpendicular to parallel orientation due to surface tension at the film/air interface. On a polished Si substra te that promotes homeotropic mesogen anchoring, the perpendicular cylindric al orientation within the smectic mesophase remains stable.