Dynamic mechanical properties of poly- n-octyl methacrylate

Abstract

The real ( J′) and imaginary ( J″) components of the complex compliance have been measured between 15 and 3600 cycles/sec. in the temperature range from −14° to 130°C. for a fractionated poly- n-octyl methacrylate with molecular weight 3.62 × 10 6. The method of reduced variables gave superposed curves for J′ and J″ except for an anomaly at the lowest temperatures where J″ passes through a minimum and fails to superpose, suggesting the onset of a β mechanism. The temperature dependence of relaxation times follows the WLF form with f g (relative free volume at the glass transition temperature, T g ) = 0.027 and α f (thermal expansion coefficient of free volume) = 2.5 × 10 −4 deg. −1. The relaxation and retardation spectra, reduced to 100°C., resemble in shape those for the other methacrylates studied except for minor features. The logarithm of the monomeric friction coefficient (in dynes-sec./cm.) is −5.78 at 100°C. and 2.54 at −20°C. ( T g ). The relaxation and retardation spectra for the ethyl, n-butyl, n-hexyl, and n-octyl polymers at 100°C. can be brought into corresponding states (choosing the butyl as reference) by adjusting the magnitudes with a dilution factor representing the number of monomoles per unit volume and the time scales with a factor derived from the friction coefficients. Their positions then agree very closely at short times and moderately well in the transition zone, but near the maximum of the retardation spectrum there are residual differences, probably associated with the Bueche entanglements. The time scale adjustments can be fairly well accounted for by the differences in free volume at 100°C. as deduced by Rogers and Mandelkern.