Carbon-13 Relaxation of Methyl Derivatives of Polystyrene in Solution

Abstract

Carbon-13 spin-lattice relaxation times and line-widths have been measured for solutions of o-, m-, and p-methyl derivatives of polystyrene at temperatures ranging between 30 and 60°C. Spin-lattice relaxation times of these polymers are intimately related with molecular motions, namely, the segmental motion and two internal motions of the phenyl and methyl groups. With respect to the internal rotation of the phenyl group, a transition change was observed in the vicinity of 50°C for poly(o-methylstyrene), 45°C for poly(m-methylstyrene) and 40°C for poly(p-methylstyrene), respectively. In comparison with the model of a freely rotating methyl group, it is concluded that the mobility of the methyl group decreases from the para to the ortho position of the substituent. This result suggests that the trend of the steric hindrance between the hydrogens bonded to the methyl carbon and those in the backbone has an effect on the segmental motion and the rotation of the phenyl group. The finding that the activation energies for the segmental motion and the rotation of the phenyl group in poly(m-methylstyrene) were larger than those in poly(o-methylstyrene) and smaller than those in poly(p-methylstyrene) is in accord with the above suggestion.