The local conformation of polymer chains in a film at
a substrate
interface was examined by sum-frequency generation spectroscopy. When
a polystyrene (PS) film was prepared on a quartz substrate by a spin-coating
method, the chains were aligned in the interfacial plane of the substrate.
A dissipative particle dynamics simulation revealed that a spinning
torque induced the chain orientation during the film preparation process
and the extent of the orientation was a function of the distance from
the interface. This interfacial orientation of chains was not observed
for a PS film prepared by a solvent-casting method. Interestingly,
the local conformation of chains at the substrate interface was unchanged
even at a temperature that was 80 K higher than the bulk glass transition
temperature (T
g). This observation means
that polymer chains at the substrate interface can be only partially
relaxed under conditions where the bulk chains are fully relaxed.
On the other hand, interfacial chains could be easily relaxed by solvent
annealing.
The glass transition temperature at the surface of polymer film is studied by the coarse-grained
molecular dynamics simulation. By the analysis of the segmental motion, the glass transition temperatures at the
surface region and that in the bulk region are determined separately. The glass transition at the surface region is
found to be lower than that in the bulk region. The molecular weight dependence of the glass transition temperature
obtained by the simulation agrees well with that obtained by the scanning force microscopic measurements.
The segmental mobility of a typical amorphous polymer, polystyrene, at the interfaces with solid substrates was noninvasively examined by fluorescence lifetime measurements using evanescent wave excitation in conjunction with coarse-grained molecular dynamics simulation. The glass transition temperature (T(g)) was discernibly higher at the interface than in the internal bulk region. Measurements at different incident angles of excitation pulses revealed that T(g) became higher closer to the interface. The gradient became more marked with an increasing difference in the free energy at the interface between the polymer and solid substrate. The T(g) value at the interface decreased with decreasing molecular weight. However, the decrement for the interfacial T(g) was not as much as that for the bulk T(g), due to the restriction of chain end portions by the substrate. Finally, it was observed that when a film became thinner than 50 nm, the depressed mobility at the interface coupled with the enhanced mobility induced by the presence of the surface. The experimental and simulation results were in good accord with each other.
Using the dynamic density functional method, we studied the structural evolution of a thin film of polymer blends placed between the solid substrate and free surface. We observed various types of structural instabilities such as the spinodal wave and the roughening of the free surface due to droplet formation. We proposed a simple theoretical argument based on the Neumann triangle condition among the interfacial tensions to construct a phase diagram for the instability of the free surface and confirmed this by a series of dynamic density functional simulations.
We performed coarse-grained MD simulations of polymer nanocomposites with spherical nanoparticles (NPs) on the basis of the Kremer−Grest model. Cases of "aggregated" and "dispersed" NPs are compared for repulsive and attractive interactions among the NPs and the polymers. Although a clear difference in the stress−strain relations for the interactions can be seen, a small dependence of morphological changes of positions of NPs is observed for a small mesh cross-linked polymer network. It is confirmed that the attractive interaction enhances the stress values as well as the degree of NP aggregation. For large elongation ratios, the bond orientation orders to the elongation axis are increasing. For the "aggregated" NPs, this increase is significant. We also calculated twodimensional scattering patterns of NPs during the elongations. For strain >50%, we observed a spot pattern on structure factor S(q x ,q yz ) and a two-point bar pattern on scattering intensity I(q x ,q yz ), which have been reported in the experiments. A small dependence of the two-dimensional scattering patterns on the interactions can be observed as well as that of the NP configurations.
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