Casting solvent effects on molecular dynamics of weak dynamic asymmetry polymer blend films via broadband dielectric spectroscopy

The effects of solvent casting parameters on the molecular dynamics of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blend films was investigated using broadband dielectric spectroscopy. Through changing the solvent type and solution concentration, blend film samples with different entanglement intensity were prepared, and they can significantly affect the glass transition temperature (Tg) and segmental dynamics of the films. In films cast from a methyl ethyl ketone (MEK) solution at various concentrations, Tg and relaxation time (τmax) increase with increasing solution concentration due to an increased entanglement density, decreased molecular mobility and entanglement recovery. No obvious distribution broadening is observed due to the unchanged heterogeneous dynamics. In the case of films cast from chloroform, MEK and tetrahydrofuran solution, Tg and τmax of the resultant films are hardly affected, while Tg and τmax of films cast from a N, N-Dimethylformamide (DMF) solution are much higher than the other three due to a higher entanglement degree and strong interaction contributions. Moreover, the poor dissolving capacity of DMF may result in more heterogeneous dynamics and subsequently a larger dc conductivity process and broader and more symmetric α-relaxation spectra. Neither the dynamics nor the distribution width of the subglass relaxation processes is affected by the casting solvent or solution concentration, indicating little change in the local environment of the segments.