Understanding the decreased segmental dynamics of supported thin polymer films reported by incoherent neutron scattering

Incoherent neutron scattering (INS) has commonly reported a suppression of segmental dynamics for supported thin polymer films as thickness is decreased, which is counter to expectations based on other measurement techniques such as ellipsometry and fluorescence. Here INS is utilized to measure the dynamics of thin films of comb polystyrene (PS) from 50 to 525 K. There is a significant suppression in dynamics as determined from the ∼5 ns Debye-Waller factor, 〈u²〉, as measured via INS for films as thick as 213 nm, while there is no change in the glass transition temperature (T_g) as determined by ellipsometry for films as thin as 20 nm. This poor correlation between T_g from ellipsometry and dynamics as measured by 〈u²〉 is attributed to contamination of nanosecond 〈u²〉 by incipient relaxation processes, differences in sensitivity to the postulated dynamically dead layer near the substrate due to the relative weighting of the distribution of dynamics between the two techniques, differences in the time scales probed, and possible decoupling between fast and slow dynamics under nanoconfinement. These results suggest that branching of PS significantly increases the interactions with the substrate to suppress the dynamics. Both technique-specific sensitivity to time scales and its weighing of the average over the gradient in dynamic properties present at the interfaces are important to consider when qualitatively different phenomena are inferred from different measurements.