Effect of Molecular Chain Architecture on Dynamics of Polymer Thin Films Measured by the Ac-Chip Calorimeter

It was reported that glass transition temperature (T-g) measured by differential alternating current (ac) chip calorimetry showed little thickness dependence for polymer films. Here we demonstrate the detection of T-g in thin films by ac-chip calorimeter and show that T-g is decreased as the thickness is reduced for oligomers and star-shaped polymers, as compared with their long linear analogues. The deviation range is a few to more than ten Kelvin. Such a depression in T-g is quite pronounced for ac-chip calorimetric measurement at a high frequency of 10 Hz. We argue that the perturbation in the increased interfacial free volume for spin-cast oligomers and dendrimers is the major reason for increasing segmental dynamics for ultrathin films.