Synthesis and characterization of spin-labelled and spin-probed side-chain liquid crystal polymers

A synthetic methodology is reported for the preparation of spin-labelled side-chain liquid crystal polymers in which the label is covalently attached to the polymer backbone. Thus, a mesogenic methacrylate, 6-(4-methoxy-4′-oxy-azobenzene) hexyl methacrylate, was copolymerized with methacryloyl chloride and the resulting copolymer esterified using 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (4-hydroxy TEMPO). Unreacted acid chloride groups were subsequently esterified using methanol. The analogous 6-(4-methoxy-4′-oxy-azobenzene) hexyl methacrylate/methyl methacrylate copolymer not containing the spin-label was also prepared and doped with a nitroxide radical, 4-oxo-2,2,6,6-tetramethylpiperidinyloxy (4-oxo-TEMPO). The thermal behaviour of the labelled and unlabelled polymers was identical, both exhibiting nematic behaviour. The electron spin resonance (e.s.r.) spectra of the labelled and probed systems have been recorded as a function of temperature. The temperature at which the extrema separation in the e.s.r. spectrum is 50G (T_50G) for the spin-probed polymer was estimated to be approximately equal to the onset temperature of the glass transition as determined by differential scanning calorimetry. This unusual result is rationalized by proposing that the spin probe is preferentially dissolved in the side-chains. Thus T_50G correlates with a relaxation process of the side-chains. By comparison, T_50G for the labelled polymer is equal to the onset temperature of the nematic-isotropic transition suggesting that T_50G may correlate with a confinement of the polymer backbone by the nematic field.