The physical properties
of glassy polymer films change as they
become confined. These changes are often attributed to increased average
molecular mobility and reduction in entanglement density. Both are
known to alter mechanical behavior, including the formation of strain
localizations, e.g., crazing and shear deformation zones. Here, we
determine how the entanglement density and surface mobility change
the mechanical behavior of a glassy polymer film when it becomes confined.
We utilize a custom-built uniaxial tensile tester for ultrathin films
and dark-field optical microscopy to characterize the complete stress–strain
response and the associated strain localizations for ultrathin polystyrene
films of varying thickness (h
F = 20–360
nm). These experiments provide direct measurement of the stress in
a craze as well as the stresses involved through the transition from
crazing to shear deformation zones. Most significantly, we observe
a transition in strain localization from crazing to shear deformation
zones as film thickness changes from 30 to 20 nm, providing new insights
into how the surfaces alter the mechanical behavior in confined polymer
films.
The density profile in a thin cross-linked polyisoprene (PI) film spin-coated on a quartz substrate in n-hexane was examined by specular neutron reflectivity. We found two layers with different PI densities at the substrate interface. The characteristics of the layers are discussed in terms of bound rubber typically associated with the field of rubber materials and adsorbed layers typically associated with the field of glassy materials.
The thin film stabilities of ω-N-(3-(dimethylamino)propyl)propylamide-terminated polystyrene (PS-N) and its blends with conventional polystyrene (PS-H) supported on silicon wafers with a native oxide layer were examined. Whereas a 20-nm-thick film of PS-H with a number-average molecular weight of ∼ 50k decomposed at 423 K, a comparable PS-N film and blended films containing a PS-N fraction of 440 wt% were stable. Although the local conformation of chains at the substrate interface was not the same for PS with and without the functional end group, the glass transition temperature at the interface was identical for both PS-H and PS-N. The residual adsorbed layer on the substrate after washing the films with a good solvent was thicker for PS-N than for PS-H. This implies that end functionalization, rather than segmental dynamics, affects chain movement on a large scale.
The adhesion of fibroblast on polymer bilayers composed of a glassy polystyrene (PS) prepared on top of a rubbery polyisoprene (PI) was studied. Since the top PS layer is not build on a glassy, or firm, foundation, the system becomes mechanically unstable with decreasing thickness of the PS layer. When the PS film was thinner than 25 nm, the number of cells adhered to the surface decreased and the cells could not spread well. On a parallel experiment, the same cell adhesion behavior was observed on plasma-treated PS/PI bilayer films, where in this case, the surface was more hydrophilic than that of the intact films. In addition, the fluorescence microscopic observations revealed that the formation of F-actin filaments in fibroblasts attached to the thicker PS/PI bilayer films was greater than those using the thinner PS/PI bilayer films. On the other hand, the thickness dependence of the cell adhesion behavior was not observed for the PS monolayer films. Taking into account that the amount of adsorbed protein molecules evaluated by a quartz crystal microbalance method was independent of the PS layer thickness of the bilayer films, our results indicate that cells, unlike protein molecules, could sense a mechanical instability of the scaffold.
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