Above, below, and in-between the two glass transitions of ultrathin free-standing polystyrene films: Thermal expansion coefficient and physical aging

ABSTRACT In previous work we observed two simultaneous transitions in high molecular weight (MW) free‐standing polystyrene films that were interpreted as two thickness‐dependent reduced glass transition temperatures (Tgs). The weaker lower transition agreed well with the MW‐dependent Tg(h) previously reported, while the much stronger upper transition matched the MW‐independent Tg(h) previously observed in low‐MW free‐standing films. Here, we investigate the nature of these two transitions by inspecting the temperature dependence of the films' thermal coefficient of expansion (TCE) and present physical aging measurements using ellipsometry both below and in‐between the two transitions. TCE values indicate approximately 80 to 90% of the film solidifies at the upper transition, while only 10 to 20% remains mobile to lower temperatures, freezing out at the lower transition. Physical aging is observed at a temperature below the upper transition, but above the lower transition, indicative of the upper transition being an actual glass transition associated with the α‐relaxation. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 64–75 Ultrathin high molecular weight free‐standing polystyrene films exhibit two transitions in the thermal expansion coefficient, where 80 to 90% of the film solidifies at the upper transition, while 10 to 20% remains mobile till the lower transition. Previously interpreted as two reduced glass transition temperatures (Tgs), the nature of the two transitions are investigated by physical aging measurements below and in‐between the two transitions, indicating the upper transition is an actual glass transition associated with the α‐relaxation.