Pattern formation in thin polystyrene films induced by an enhanced mobility in ambient air

Thin polystyrene films (prepared in a sandwich geometry between two aluminum electrodes) develop a characteristic pattern in ambient air when kept above the glass-transition temperature, whereas they remain unchanged in vacuum or in a pure nitrogen atmosphere. Measurements by broadband dielectric spectroscopy, capacitive dilatometry, and AC calorimetry reveal that the pattern formation is initiated in ambient air by an increase in the average relaxation rate of the dynamic glass transition, which results in a corresponding reduction of the glass-transition temperature. Infrared measurements evidence that this is caused by a shift of the molecular weight distribution to lower values as a consequence of oxygen-induced chain scissions.