Mechanical Relaxations of Free-standing Polymer Films

Young’s modulus of free-standing polystyrene films with thicknesses, h, from 48 nm to 100 μm was studied as a function of temperature and relaxation time. For the h ≤ 115 nm or h ≥ ~3.5 mm films, data could be fit to a single Kohlrausch-Williams-Watts (KWW) function, with the former showing relaxation times shorter than the bulk (denoted t_s) and the latter similar to the bulk (t_bulk). For the films with thicknesses in between, data could be described by a double KWW function with relaxation times of t_s and t_bulk, respectively. We found that t_s is similar to the fast relaxation time observed in previous optical anisotropy relaxation experiment. However, the apparent thickness found here of the film portion where fast relaxations take place is ~100 times larger. Our result shows that mechanical relaxations are much more susceptible to perturbations by the free surface than molecular relaxations are. Confinement effects may have far bigger impact on the properties of polymers than previously contemplated.