Hypersound propagation in oriented polymethylmethacrylate

Abstract

AbstractThe hypersonic elastic properties of oriented polymethylmethacrylate (PMMA) have been studied using high contrast (>108) multipass Fabry‐Perot techniques. Four samples of different degrees of orientation (birefringence Δn = −4, −9, −15, −24 × 10−4) were prepared by drawing at 130°C. Measurements of longitudinal sound velocity, VL, as a function of ϕ, the angle between the draw and propagation directions, show an anisotropy which scales with Δ and amounts to 6% in the sample having Δn = 24 × 10−4. At ϕ = 0, VL = 2.80 kilometers per second, (KMS−1), the same as the value in an isotropic sample. The elastic constants C11 and C33 are measured as a function of Δn. Data as a function of sound wavelength, Λ, show dispersive effects in both VL and attenuation αΛ, for Λ > 0.7 μ. Over the range 0.7 μ < Λ < 1.3 μ, αΛ increases from 0.1 to 0.5 and VL increases by 8%. Attenuation is consistently higher for ϕ = 0. The data are interpreted in terms of densification (up to 15%) accompanied by void formation on orientation, the structure having characteristic dimensions of approximately 1.5 μ. The structure may be asymmetric with the longer dimension perpendicular to the draw direction. Additional Brillouin peaks may result from sound propagation in the voids (VL = 5.8 KMS−1) in which case the density is approximately 0.3 g cm−3. Measurements of the Landau‐Placzek ratio support these ideas, but evidence from electron micrographs is inconclusive. Dispersion measurements should in principle help determine details of the structure shape.