Dynamics in polymer-liquid crystal microcomposites studied by low frequency mechanical spectroscopy

Polymer-liquid crystal (LC) microcomposites (LC droplets dispersed in a polymeric matrix) exhibit optical properties associated with the mesogenic molecules together with the mechanical properties of the polymeric matrix. The raw materials considered in this study are (i) the UV curable prepolymer and (ii) the LC molecules (cyanobiphenyl or benzoates mixtures). The morphology and thus the physical properties induced by the phase separation depend on the polymerization conditions. This morphology was observed by electron microscopy and the transition temperatures were determined by calorimetric analysis. The dynamic properties of these microcomposites were studied by low frequency mechanical spectroscopy. Different phenomena were observed, namely (i) at low temperature the secondary relaxation of the polymeric matrix and a process attributed to a liquid glass like transition in the LC domains and (ii) a strong relaxation effect near the room temperature attributed to the glass transition of the polymeric matrix. The characteristics of these phenomena were deduced from temperature and frequency scanning. The elastic modulus becomes proportional to the temperature above the main transition of the polymeric matrix, which is the classical effect of entropic elasticity. Moreover, it is shown that the glass transition of the polymeric matrix is depressed by the presence of LC molecules that remain dissolved in the matrix and act as a plasticizer.