Electron spin resonance studies of spin-labelled polymers. Part 7.—Dependence of rotational correlation times on solvent properties and polymer concentration

Abstract

Rotational correlation times τ_c, for seven narrow fractions of spin-labelled polystyrene have been measured in cyclohexane and α-chloronaphthalene. Within the temperature range studied τ_c is greater in both of these solvents than in toluene, due to the higher viscosity of chloronaphthalene and the poor solvent properties of cyclohexane. In high molecular weight polymers, segmental reorientation is the main relaxation process having an activation energy of 25.1 ± 0.8 kJ mol^–1 in cyclohexane and 26.4 ± 1.3 kJ mol^–1 in chloronaphthalene, compared with our earlier figure of 18.0 ± 0.8 kJ mol^–1 in toluene. When the activation energies for viscous flow of the solvent are deducted from these figures the internal energy barriers to rotation are obtained. These barriers are 8.8 kJ mol^–1 in toluene and chloronaphthalene and at least 12.6 kJ mol^–1 in cyclohexane. The higher internal barriers to segmental rotation in cyclohexane are probably due to the more tightly coiled conformation of the polymer chains. τ_c increases slowly with polymer concentration up to a critical value beyond which τ_c rises steeply. The critical concentration is close to the “entanglement” concentration as obtained by viscosity measurements. The entanglement concept is discussed in the light of these results.