Comparative specular X-ray reflectivity, positron annihilation lifetime spectroscopy, and incoherent neutron scattering measurements of the dynamics in thin polycarbonate films

There are many technological applications using thin polymer films that would be crucially influenced by confinement-induced changes in the transport properties of the film. In the present, we utilize specular X-ray reflectivity (SXR, to measure film thickness h), beam positron annihilation lifetime spectroscopy (PALS, to measure the nanometer-sized domains of unoccupied volume v), and incoherent neutron scattering (INS, to measure the mean-square atomic displacements <u2>) to quantify the influence of film thickness in thin polycarbonate (PC) films. The thermal expansion coefficients of both h and v, as well as the amplitudes of <u(2)>, indicate that thin film confinement affects reduced molecular mobility in PC. This reduced mobility is not necessarily reflected in the apparent glass transition temperature (Tg) derived from the same techniques. Specifically, SXR and PALS indicate a weak suppression of apparent Tg (the kink in the thermal expansion curve) when the film thickness becomes less than 200 A, which should not be interpreted to imply enhanced mobility. The INS measurements confirm this by demonstrating that the kink in <u(2)> vs T (designated operationally as T-g) tends to disappear, possibly even increasing to higher T for film thinner than 500 Angstrom. The reduced thermal motion can nominally be parametrized in terms of an immobilized region next to the rigid substrate, extending approximately 40-130 Angstrom into the film, depending on the measurement technique and temperature range.