Decoupling of glassy dynamics from viscosity in thin supported poly(n-butyl methacrylate) films

We utilized fast scanning calorimetry to characterize the glass transition temperature (Tg) and intrinsic molecular mobility of low-molecular-weight poly(n-butyl methacrylate) thin films of varying thicknesses. We found that the Tg and intrinsic molecular mobility were coupled, showing no film thickness-dependent variation. We further employed a unique noncontact capillary nanoshearing technique to directly probe layer-resolved gradients in the rheological response of these films. We found that layer-resolved shear mobility was enhanced with a reduction in film thickness, whereas the effective viscosity decreased. Our results highlight the importance of polymer–substrate attractive interactions and free surface-promoted enhanced mobility, establishing a competitive nanoconfinement effect in poly(n-butyl methacrylate) thin films. Moreover, the findings indicate a decoupling in the thickness-dependent variation of Tg and intrinsic molecular mobility with the mechanical responses (shear mobility and effective viscosity).