Probing Glass Transition Temperature of Polymer Thin\nFilms under Static Shear Stress

Accurate\ncharacterization of the glass transition temperature (<i>T</i><sub>g</sub>) in polymer thin films is pivotal for their\napplication in nanotechnology. Here, we introduced a novel and simple\nmethod to measure <i>T</i><sub>g</sub> based on the observation\nof creep flow in response to static shear stress. The technique employs\na polydimethylsiloxane (PDMS) pad placed on top of the polymer thin\nfilm. The sample is held isothermally at 2 K intervals upon heating.\nAt each temperature, steady shear is applied with constant normal\nand lateral forces for a constant duration of time. <i>T</i><sub>g</sub> is identified by the onset temperature where PDMS displacement\nis observed at the polymer/PDMS interface in optical microscopy. The\nmeasured <i>T</i><sub>g</sub>s strongly correlate with those\nmeasured by spectroscopic ellipsometry for various polymers with various\nmolecular weights and film thicknesses. Furthermore, we demonstrate\nthat this approach can be employed in conditions where <i>T</i><sub>g</sub> measurements using other methods may be challenging.\nFor example, in polymer-infiltrated nanoparticle films, the <i>T</i><sub>g</sub> of the highly confined polymer in the composite\ncan be accurately measured. This facile and inexpensive technique\ncan be readily adopted in various industries, where alternative techniques,\nsuch as ellipsometry, can be costly and require extensive expertise.