The manner in which the dynamics of a polymer are affected by thin film confinement is one of technological significance, impacting thin film applications such as lubricants, adhesives, and chemically amplified photoresists. In this manuscript we use specular X-ray reflectivity (SXR), beam positron annihilation lifetime spectroscopy (PALS), and incoherent neutron scattering (INS) to study the influence of thin film confinement on the apparent glass transition temperature Tg and the thermal expansion coefficients in an identical set of thin polycarbonate (PC) films. Both the SXR and PALS indicate a significant suppression of Tg when the film thickness becomes less than 200 . However, the INS measurements suggest an increase in the apparent Tg below this same length scale. These disparate estimates of the thin film Tg are discussed in terms of what appears as a broadening of the glass transition, with the each technique sensitive to different aspects of the glass formation process. All three techniques agree that thin film confinement results in reduced thermal motion, regardless of the temperature designated as the thin film Tg. This is quantified in terms of an immobilized region that extends from the rigid substrate approximately 9 to 116 , depending on the measurement technique and temperature range.
Issue: No. 8
Pub Type: Journals
dynamics, glasstransition, neutron scattering, positron annihilation, thin films, x-ray reflectivity