Effect of plastic deformation on the character of micro-brownian motions in glassy poly(methyl methacrylate)

Abstract

Glassy PMMA samples are plastically deformed at room temperature in the uniaxial compression regime to residual strains of e res = 25%. Dielectric spectra of the initial and deformed samples are recorded via the method of broadband dielectric spectroscopy in the frequency range f = (5 × 10−4) − 107 Hz. The results are compared with the dynamic mechanical spectra of samples deformed under the same conditions. Dielectric and mechanical spectra are noticeably distorted by deformation. As a result, dielectric permittivity ɛ′ increases, shear modulus G′ decreases, and the intensity of dielectric β losses slightly increases, while dielectric and mechanical α losses increase appreciably. In addition, the “anomaly” of total dielectric Δɛtot and total mechanical ΔG tot dispersions (Δɛtot = ɛ0 − ɛ∞ ≈ Δɛα + Δɛβ and ΔG tot = G 0 − G ∞ ≈ ΔG α + ΔG β) occurs, that is, the polymer is transformed from the state with Δɛα ≪ Δɛβ and ΔG α Δɛβ and ΔG α > ΔG β. The described phenomenon is related to a strong gain in α dielectric and mechanical losses in the deformed material. It is found that α losses increase owing to an anelastic deformation component arising during glass loading. This component is responsible for an increase in the internal energy of the glass during its anelastic deformation. Possible causes of the observed effects are discussed.