Glass-transition cooperativity onset in a series of random copolymers poly(n-butyl methacrylate-stat-styrene)

Dielectric spectroscopy, heat capacity spectroscopy (HCS), and differential scanning calorimetry (DSC) investigations in the alpha beta relaxation splitting region of a series of random copolymers of n-butyl methacrylate with styrene are reported. A separate onset of the a relaxation is dielectrically observed, about one frequency decade below a continuous local ap component in the Arrhenius diagram. This splitting scenario shifts to higher frequencies and temperatures for increasing styrene content and does not qualitatively change from homo PnBMA. up to 54 mol % styrene. The logarithms of onset frequency, log omega(on), and of WLF asymptotic frequency, log Omega, change linearly with the styrene content, but their ratio is constant and remains large, log(10)(Omega/omega(on)) = 3.8 +/- 1. The log Omega approximate to 7 (rad/s) values for small styrene content are unusually low. Omega is explained as the frequency of local cooperativity chances in the concept of kinetic molecular randomness for the dynamic glass transition. The alpha dielectric intensity, Delta epsilon(alpha), the caloric intensity, Delta c(p), and the square root of cooperativity from a fluctuation formula, N-alpha(1/2), are linearly proportional to the temperature difference to the onset, e.g. Delta epsilon(alpha) similar to (T-on - T). The dielectric activities of the alpha process and a hypothetical gamma process (beyond the Johari Goldstein beta process) increase with increasing styrene content although the styrene unit is almost nonpolar. This is interpreted by dipole decompensation for the alpha and gamma relaxation modes caused by the random styrene units.