S. Kahle, J. Korus, E. Hempel, R. Unger, S. Höring, K. Schröter, E. Donth:
Glass-Transition Cooperativity Onset in a Series of Random Copolymers
S. Kahle, J. Korus, E. Hempel, R. Unger, S. Höring ‡,
K. Schröter, E. Donth:
Glass-Transition Cooperativity Onset in a Series of Random Copolymers
Poly(n-butyl methacrylate-stat-styrene)
Macromolecules 30 (1997) 7214–7223
DOI: 10.1021/ma961933b
Fachbereich Physik and Fachbereich Chemie (‡),
Universität Halle, D-06099 Halle (Saale), Germany
Dielectric spectroscopy, heat capacity spectroscopy (HCS), and differential scanning calorimetry (DSC) investigations in the αβ relaxation splitting region of a series of random copolymers of n-butyl methacrylate with styrene are reported. A separate onset of the α relaxation is dielectrically observed, about one frequency decade below a continuous local aβ 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 ωon, and of WLF asymptotic frequency, log Ω, change linearly with the styrene content, but their ratio is constant and remains large, log10(Ω/ωon) = 3.8 ± 1. The log Ω ≈ 7 (rad/s) values for small styrene content are unusually low. Ω is explained as the frequency of local cooperativity chances in the concept of kinetic molecular randomness for the dynamic glass transition. The α dielectric intensity, Δεα, the caloric intensity, Δcp, and the square root of cooperativity from a fluctuation formula, Nα1/2, are linearly proportional to the temperature difference to the onset, e.g. Δεα ~ (Ton − T). The dielectric activities of the α process and a hypothetical γ process (beyond the Johari Goldstein β process) increase with increasing styrene content although the styrene unit is almost nonpolar. This is interpreted by dipole decompensation for the α and γ relaxation modes caused by the random styrene units.