Study of the side‐chain relaxation of methacrylate homopolymers and copolymers by the dielectric method

Dielectric constant and dielectric loss of copolymers of methyl methacrylate (MMA) with n‐butyl methacrylate (nBMA) and isobutyl methacrylate (iBMA) have been measured in the frequency range 30 cps to 1 Mcps at temperatures from 70°K to 370°K. Results lead, together with those of previously published investigations on copolymers of MMA, to the following conclusions. (1) The loss‐peak temperature attributed to side‐chain relaxation (β peak) of PMMA varies with the comonomer ratio when the comonomer does not have an α‐methyl group, but remains almost unchanged for comonomers having an α‐methyl group. (2) In both cases, the β peak height of PMMA decreases with increasing ratio of comonomer B and completely vanishes for poly‐B, and the loss peak temperature plotted against the fraction of B does not extrapolate to the β peak of poly‐B. It is suggested on the basis of the above facts that the moving unit in the side‐chain relaxation consists of a single side chain with a segment of the backbone chain and that the change in mobility of the side chain upon copolymerization results from the distortion of the helical structure of the backbone chain due to random distribution of α‐methyl groups. Dielectric studies of the low‐temperature side‐chain relaxation (β2 peak) in PnBMA, poly(n‐octyl methacrylate), and poly(n‐dodecyl methacrylate) (130°K at 1 kcps) have been made and an interpretation is offered for the molecular nature of this relaxation.