Discrepancies in the characterization of the glass transition in thin films of hyperbranched polyesters

The dynamic glass transition and the dilatometric glass transition temperature are simultaneously characterized in thin films of hyperbranched aromatic polyesters by broadband dielectric spectroscopy and capacitive scanning dilatometry. A diverging thickness dependence is detected: while the temperature position of the alpha relaxation peak T-alpha decreases by similar to 30 K, the dilatometric T-g increases by similar to 10 K with decreasing film thickness. This emphasizes the subtle character of the glass transition phenomenon-as manifested in the molecular dynamics and in the (structural) thermal expansion-and proves that, in contrast to the bulk, different experimental techniques do not necessarily deliver similar results in confinement. (c) 2006 Wiley Periodicals, Inc.