A mobile surface layer exists near the free surface of polymers. Some studies have found that the elastic modulus of thin polymer films (E) decreased with decreasing film thickness (h) and ascribed the observation to the mobile surface layer being rubbery. But others have found that E was constant. By studying the strain rate (γ̇) dependence of E, we found that E varied with h when γ̇ was small but was a constant when γ̇ was sufficiently high. This shows that the surface mobile region is rubbery at low γ̇, but it freezes and recovers the bulk elastic modulus when γ̇ is high.

The thickness of the mobile surface layer, h_t, has been reported to be both nanometers and micrometers. By performing E(γ̇) and E(t) measurements on freestanding polymer films wi...[ Read more ]

A mobile surface layer exists near the free surface of polymers. Some studies have found that the elastic modulus of thin polymer films (E) decreased with decreasing film thickness (h) and ascribed the observation to the mobile surface layer being rubbery. But others have found that E was constant. By studying the strain rate (γ̇) dependence of E, we found that E varied with h when γ̇ was small but was a constant when γ̇ was sufficiently high. This shows that the surface mobile region is rubbery at low γ̇, but it freezes and recovers the bulk elastic modulus when γ̇ is high.

The thickness of the mobile surface layer, h_t, has been reported to be both nanometers and micrometers. By performing E(γ̇) and E(t) measurements on freestanding polymer films with and without metal coating (which obliterates the mobile surface layer), we determined that h_t was micrometers. However, within a nanoscale outer region, the relaxation time of the polymer was further reduced. Therefore, experiments with time scales longer than the relaxation time of the slower portion of the mobile surface layer may find h_t to be micrometers, others may find h_t to be nanometers.

Polymer chains are prone to adsorption to a solid substrate due to formation of multiple bound contacts. We addressed whether polymer adsorbed layers (AL) are irreversible by studying the mean-square displacements of the adsorbed chains from the substrate with thermal annealing by using dynamic secondary ion mass spectrometry and deuterium-labeling. Our result showed that the 'outer loosely adsorbed layer' of an AL could come off completely. In contrast, very few chains in the "inner flattened layer (FL)" could. A detailed investigation of the morphological evolution of the FL showed that the adsorbed chains therein moved noticeably, revealing that FL may be a metastable state.