Thermophysical Measurements and Interfacial Adhesion Studies in Ultrathin Polymer Films Using Homodyne Photothermal Spectrometry

Abstract

Homodyne laser photopyroelectric spectrometry (PPES) is a new photothermal detection scheme which uses electronic mixing to downshift a high-frequency swept wave photothermal response into a bandwidth of approximately one kilohertz. A rapid photopyroelectric impulse response is recovered from the downshifted component at high time resolution and is used, here, in two different geometries, to probe the thermal and interfacial properties of assemblies composed of submicron-thick-ness polymer films deposited on the metallized surface of a pyroelectric thin-film detector. In the thermal transmission mode geometry, the sample is excited by irradiation in a very thin opaque surface layer, and the transit time of a heat pulse through the sample to the pyroelectric transducer can be used to obtain a thickness or thermal diffusivity measurement. By the use of an inverse mode geometry, a metal/polymer interface is excited by irradiation of an optically transparent polymer thin-film sample attached to the metallized surface of the pyroelectric transducer. Heat flow at short distances from the bond interface can be monitored by the pyroelectric effect through the high time resolution of the homo-dyne photopyroelectric measurement. The combination of information obtained in these two geometries with an analysis of the thermal wave reflections which occur at interfaces between materials of dissimilar thermal effusivity can be used to evaluate relative interfacial adhesion between the metal and polymer layer.