In-situ X-ray reflectivity measurement of polyvinyl acetate thin films during glass transition

Thin films quite often have different properties from the bulk. In the case of polyvinyl acetate, it is known that glass transition takes place at around room temperature (∼30 °C), and the thermal expansion coefficient is well determined for the bulk. However, with a ∼0.1 micron thin film, such measurements are not generally easy. The present paper reports the results of in-situ X-ray reflectometry, which is extremely sensitive to slight changes in layer thickness. It was found that thickness increases from 968 to 985 Å when the temperature is scanned from 10 to 55 °C, but the slope changes from 0.22 to 0.72 Å/ °C at around 35 °C. As the thickness change is due to thermal expansion, it is possible to discuss the mechanical properties below and above the transition temperature.