Synthesis and supramolecular liquid crystalline structure modulation of side-chain polynorbornenes with asymmetrical substituent mesogenic groups

The side-chain polymer is one of the most familiar polymer architectures due to its structural diversity and functionality. In this work, we designed and synthesized a series of side-chain polynorbornenes (PNB-n-2, PNB-n-3 and PNB-n-23) with dodecyloxy biphenyl mesogenic groups attached at the unconventional asymmetrical substituent positions on the central phenyl group. Their phase behaviours and self-organized structures have been systematically investigated in combination with SAXS, WAXS, GISAXS, DSC, POM, and AFM. The substituent position has an important influence on the supramolecular liquid crystalline structure evolution. When the mesogenic group is attached to the 3 position for PNB-n-3 (n = 2, 6), a lamellar phase exists. No LC phase could be formed for PNB-n-2 (n = 2, 6) with a mesogenic group at the 2 position. When both the 2 and 3 positions are attached with mesogenic groups for PNB-n-23 (n = 2, 6, 8), a thermally stable supramolecular two-dimensional ordered structure with an oblique symmetry and lattice parameter larger than 10 nm has been found. It originates from the frustration of the lamellar structure. During a heating process, the frustration could be diminished and the lamellar phase appears at a higher temperature.