Density Scaling of the Structural and Johari–Goldstein Secondary Relaxations in Poly(methyl methacrylate)

Abstract

Dielectric spectra were obtained on a low molecular weight poly(methyl methacrylate) (PMMA) over a range of temperatures (331 < T (K) < 386) at pressures approaching 0.8 GPa. The α relaxation times, τα, superpose when plotted versus T/ργ, where ρ is density and γ a material constant, in accord with results for many other van der Waals liquids and polymers. However, the Johari–Goldstein (JG) relaxation times, τJG, do not conform to this density scaling for the same value of the exponent γ. Likewise, the frequency separation of the α and JG loss peaks in the spectrum increases with pressure for constant τα; that is, state points having the same α relaxation time and same peak breadth have different τJG. Similar results were obtained on a lower molecular weight PMMA, for which there was less overlap of the two peaks. The implication is that density scaling of the segmental relaxation times originates in the glass transition dynamics, not, as recently suggested, in higher frequency secondary processes.