Probing Glass Transition Temperature of Polymer Thin Films under Static Shear Stress

Abstract

Accurate characterization of the glass transition temperature (T_g) in polymer thin films is pivotal for their application in nanotechnology. Here, we introduced a novel and simple method to measure T_g based on the observation of creep flow in response to static shear stress. The technique employs a polydimethylsiloxane (PDMS) pad placed on top of the polymer thin film. The sample is held isothermally at 2 K intervals upon heating. At each temperature, steady shear is applied with constant normal and lateral forces for a constant duration of time. T_g is identified by the onset temperature where PDMS displacement is observed at the polymer/PDMS interface in optical microscopy. The measured T_gs strongly correlate with those measured by spectroscopic ellipsometry for various polymers with various molecular weights and film thicknesses. Furthermore, we demonstrate that this approach can be employed in conditions where T_g measurements using other methods may be challenging. For example, in polymer-infiltrated nanoparticle films, the T_g of the highly confined polymer in the composite can be accurately measured. This facile and inexpensive technique can be readily adopted in various industries, where alternative techniques, such as ellipsometry, can be costly and require extensive expertise.