Dynamic mechanical behavior of lightly crosslinked networks of poly(n‐butyl methacrylate) and poly(n‐butyl acrylate) in the rubberlike region

An investigation was made of the dynamic mechanical behavior in the rubberlike region of poly(n‐butyl methacrylate) (PBuMA) and poly(n‐butyl acrylate) (PBuA) networks lightly crosslinked with ethylene dimethacrylate to concentrations from 10−6 to 10−4 mole/cm3. The measurements were carried out by use of an apparatus for low‐frequency forced vibrations working in the frequency range 2.5 × 10−4 to 1 Hz. With parameters c1 and c2 of the Williams‐Landel‐Ferry equation, obtained from data in the main transition region, the data did not reduce in the rubberlike region for the poly(butyl methacrylate) networks; the spread of the deviations decreases with increasing concentration of the crosslinking agent. Superposition could be achieved in all cases when a shift factor was used on the vertical axis. At sufficiently low reduced frequencies and at high temperatures the storage compliance decreases in both series of polymers with increasing concentration of the crosslinking agent as expected. At higher reduced frequencies and at higher temperatures of measurement, however, anomalous behavior was observed with uncrosslinked samples having a lower compliance than those crosslinked to a very low degree. This finding was explained as due to very long relaxation times of the untrapped entanglements present in the noncrosslinked polymer, which are absent in the same polymer crosslinked already to very low degrees. The retardation spectra of both PBuMA and PBuA exhibited secondary relaxation mechanisms which were shifted by four logarithmic decades toward higher retardation times in comparison with the primary retardation maximum.