Dynamics of thin polymer films: Recent insights from incoherent neutron scattering

Abstract

AbstractIncoherent neutron scattering is presented as a powerful tool for interpreting changes in molecular dynamics as a function of film thickness for a range of polymers. Motions on approximately nanosecond and faster timescales are quantified in terms of a mean‐square atomic displacement (〈u2〉) from the Debye–Waller factor. Thin‐film confinement generally leads to a reduction of 〈u2〉 in comparison with the bulk material, and this effect becomes especially pronounced when the film thickness approaches the unperturbed dimensions of the macromolecule. Generally, there is a suppression (never an enhancement) of 〈u2〉 at temperatures T above the bulk calorimetric glass‐transition temperature (Tg). Below Tg, the reduction in the magnitude of 〈u2〉 depends on the polymer and the length scales being probed. Polymers with extensive segmental or local mobility in the glass are particularly susceptible to reductions of 〈u2〉 with confinement, especially at the Q vectors probing these longer length scales, whereas materials lacking these sub‐Tg motions are relatively insensitive. Moreover, a reduced 〈u2〉 value correlates with reduced mobility at long time and spatial scales, as measured by diffusion in these thin polymer films. Finally, this reduced thin‐film mobility is not reliably predicted by thermodynamic assessments of an apparent Tg, as measured by discontinuities or kinks in the T dependence of the thermal expansion, specific volume, index of refraction, specific heat, and so forth. These measurements illustrate that 〈u2〉 is a powerful and predictive tool for understanding dynamic changes in thin polymer films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3218–3234, 2004