Low-frequency forced-vibration apparatus for the measurement of complex compliances of polymers in the rubberlike region

For studies of the low-frequency viscoelastic behavior in the rubberlike plateau zone of lightly cross-linked networks, an apparatus based on forced vibrations was developed, working over the frequency range from 2.5 × 10−4 to 1 cps. Sandwich-type samples were used, the stress imposed on the samples varied sinusoidally, both stress and strain were registered using an X-Y recorder, and the components of the complex compliance were calculated using a computer from the recorded ellipse. Log J′ ≥ −8 can be measured over the temperature range from −50° to +200°C. The viscoelastic data measured with this apparatus and the reduced curves are presented and discussed for uncross-linked poly-(n-butyl methacrylate) and poly(n-butylacrylate). For poly(n-butyl methacrylate), it was not possible to reduce the J'p and J''p values in the whole range using the same c1 and c2 parameters of the Williams-Landel-Ferry equation. Anomalous behavior found at temperatures T ≥ 110°C was the same as that found by Child and Ferry. Good agreement was obtained from the comparison of the calculated dynamic data with the results obtained by creep measurements. Preliminary compression of the samples inserted into the apparatus and variations of magnitude of shear strain in the range used did not influence the results.