Vitrifying star-shaped liquid crystals: synthesis and application in cholesteric polymer networks

Abstract

In this paper, the formation of glass-forming reactive mesogens, that do not crystallize upon cooling, but vitrify and form supercooled LC phases, is described. These molecules exhibit broad range LC phases and enable us to carry out photopolymerization in a broad range of temperatures. From such reactive mesogens densely crosslinked networks in which the liquid crystalline order is permanently fixed are formed by photopolymerization. For this purpose eight novel low molecular mass LC materials with photopolymerizable acrylate groups have been synthesized and the detailed experimental procedures are given. The molecules have a star-shaped topology with three and four arms. The mesogenic units were varied by the addition of lateral groups in different positions. Comparing the twin molecules which we have described before with the novel three- and four-armed stars, we found that the supercooled LC phase in the three-armed stars has a stability superior to that in both twin molecules and four-armed stars. In the three-armed star triple-4 with a suitable substituent pattern, the supercooled nematic phase is stable at room temperature for at least nine months. Photo-DSC experiments show that the final conversion after 10 min of UV-irradiation for the threearmed star molecule triple-4 is as high as for the smaller molecules twin-4 and mono-4 over the whole temperature range. Doped with a suitable chiral molecule the novel nematics formed cholesteric phases which were used to make cholesteric polymer networks by photopolymerization.