Study On Dewetting And Glass Transition Of Nanoconfined Polystyrene Chains

In numerous scientific and technological fields, a fundamental understanding of the influence of nanoconfinement effects on the equilibrium and dynamic properties of polymeric materials proves to be of importance. Nanoparticles of polystyrene (PS) with different molecular weights adsorbed on mica substrates were prepared by spin coating from extremely dilute solutions, and dimensions and dewetting phenomenon of these samples were characterized by atomic force microscopy (AFM). Isolated single- or pauci-chain PS nanoparticles were obtained by fast evaporation from dilute solutions, and the glass transition phenomenon of these samples was characterized by differential scanning calorimeter (DSC).The AFM investigation on the PS nanoparticles adsorbed on mica showed that in the formation of nanoparticles, the PS molecules gathered in large clusters at the early stage, then the aggregates were separated or spun off, and more and more particles emerged, finally the clusters vanished and the particles formed on the substrate. The dimensions of the particles increased with molecular weight. The nanoparticles dewetted after annealing and the particle-height was elevated and the distribution was broadened. Numerous small particles appeared newly on the mica substrate after annealing. The volumes of these particles measured by AFM and the calculated volumes were in reasonable agreement, suggesting that the new-particles contained only one polymer chain. The higher the molecular weight of single-chain particle, the height increased more after annealing. After the height value were normalized, the extents of the chain contractions were the same, indicating PS single chains had reached equilibrium on mica, although height changes of single chains of three molecular weights were different.The DSC investigation on the PS nanoparticles obtained by fast evaporation method showed that the higher the molecular weight of PS, the glass transition happened at the higher temperature. The relationship between Tg and reciprocal of molecular weight coincided basically with Flory-Fox equation. Tgs of different molecular weight PS nanoparticles were decreased with different degrees, the higher molecular weight, the greater the degree of Tg declined. This was because that the degree of topological entanglements of PS by rapid evaporation method reduced greatly, which promoted the movements of chain segments, thus Tg was decreased. Through the melting or redissolving method, Tgs of these samples rised which showed that chains interlocked and formed new topological entanglements.