Dynamic mechanical properties of poly- n-hexyl methacrylate

Abstract

The real ( J′) and imaginary ( J″) components of the complex compliance have been measured between 15 and 2400 cycles/sec. in the temperature range from 5° to 125°C. for a fractionated poly- n-hexyl methacrylate with molecular weight 4.0 × 10 6. The method of reduced variables gave superposed curves for J′ and J″ except for the same two anomalies observed for poly- n-butyl methacrylate: the maximum in J″ increased slightly with increasing temperature, and near the bottom of the dispersion of J′ the data corresponded to two additive mechanisms with different temperature dependences. For the principal (α) mechanism, the temperature dependence of retardation times follows the WLF form with f g (relative free volume at the glass transition temperature, T g ) = 0.025 and α f (thermal expansion coefficient of free volume) = 1.9 × 10 −4 deg. −1. For the β mechanism, the temperature dependence of retardation times corresponds to an apparent activation energy of 13 kcal. The retardation and relaxation spectra of the α mechanism resemble those for other methacrylate polymers, but certain progressive effects of increasing side-chain length on the position and detailed shape of the spectra are apparent. The logarithm of the monomeric friction coefficient (in dynes-sec./cm.) is −5.21 at 100°C. and 2.63 at −5°C. ( T g ). At their respective T g ′s, the friction coefficients of polyethyl, poly- n-butyl, and poly- n-hexyl methacrylates are in the ratios 4000:13:1, though the relative free volume at T g as deduced from the WLF equation is the same for all three polymers.