Interfacial tension of a liquid crystalline polymer in an isotropic polymer matrix

We investigated the interfacial tension of a thermotropic liquid crystalline polymer (LCP) embedded in a flexible isotropic matrix, poly(dimethylsiloxane) (PDMS), using a modified deformed droplet retraction method (MDDR). The thermotropic polymer employed, PSHQ-6,12, was nematic and featured a well-defined nematic - isotropic clearing transition temperature so that MDDR experiments could be conducted over a temperature range spanning nematic, isotropic, or biphasic regions for the droplet phase. Our results showed that the interfacial tension for temperatures in the nematic range of the droplet is ∼5 mN/m and increases slightly with temperature. Within the isotropic phase, the interfacial tension is significantly higher, starting at a value greater than 20 mN/m and decreasing with temperature with a slope of -0.3 mN/(m K). On crossing the transition from a nematic to isotropic phase for the LCP, the interfacial tension increases in a nearly stepwise manner. Imaging of both retardance and orientation with compensated polarizing optical microscopy clearly revealed homeotropic orientation of the LCP near the LC/isotropic interface, especially below the bulk isotropization temperature, but also for temperatures near, but above, this temperature. We conclude that the interfacial tension for this system is sensitive to the level of nematic ordering at the interface, with homeotropic anchoring yielding significantly lower values than an interface with no preferential anchoring.