Surface glass transition temperature of amorphous polymers. A new insight with SFM

The surface glass transition temperature, T-g, of several amorphous polystyrene (PS) samples each with a narrow molecular weight distribution covering the range of M-n from 3 900 to 1340 000 has been measured by force-distance measurements using a scanning force microscope (SFM). Hysteresis in the loading: unloading cycles of the force-distance curves is observed above T-g and is ascribed to viscoelasticity at the polymer surface. The hysteresis behavior for different molecular weight samples above T-g is described in terms of the contact mechanics of the tip-surface interaction, by consideration of the relative roles of bulk viscoelasticity and peeling viscoelasticity. Data on the molecular mass dependence of thick films of the surface T-g are presented, along with the dependence of T-g on film thickness for thin films, and the annealing of films with a Langmuir-Blodgett overlayer. The results are related to existing theories describing the T-g depression at lower Mn as a result of the increased free volume in the system. The change in T-g with molecular weight of PS is steeper for the surface than the bulk Fox-Flory relationship. We show that polymer chain entanglement variation rather than the end group. localization on the free surface is responsible for the observed surface T-g depression effect.