Influence of alkoxy tail length on the phase behaviors of side-chain liquid crystalline polymers without the spacer
A series of end-on side-chain liquid crystalline polymers (SCLCPs) based on the biphenyl mesogen which were directly attached to the polymer backbone without the flexible spacer, poly(4,4′-alkoxybiphenylyl methacrylate) (PMBi-m, m = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16) were successfully synthesized by free radical polymerization. The chemical structures of the monomers were confirmed by 1H NMR and mass spectrometry. The molecular characterizations of the polymers were performed with 1H NMR, GPC and TGA. The phase behaviors were investigated by a combination of techniques including DSC, POM and 1D/2D WAXD. The experimental results showed that the alkoxy tail played an important role in the phase behaviors of the SCLCPs without the spacer. Firstly, all polymers form the smectic phase. Secondly, the clearing temperatures decrease with a small odd–even effect as the length of the alkyl tail increases and then increase slightly. Lastly, compared with the influence of the alkyl spacer length on liquid crystal properties of end-on SCLCPs with the biphenyl mesogen, the end-on SCLCPs without the spacer and with the different alkyl tail lengths (PMBi-m) exhibit a high glass transition temperature and a stable LC phase.