Nao Hosaka

Friction behavior of high-density poly(2-methacryloyloxyethyl phosphorylcholine) brush in aqueous media

Super-hydrophilic polymer brushes were prepared by surface-initiated atom transfer radical polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) on initiator-immobilized silicon wafers. The graft density was estimated to be 0.22 chains nm based on the linear relationship between and the layer thickness. The contact angle against water was very low, and air bubbles in water hardly attached onto the brush surface, indicating a super-hydrophilic surface. Neutron reflectivity measurements of the poly(MPC) brush showed that the grafting polymer chains extended a fair amount in the vertical direction from the substrate in a good solvent such as water, while they shrunk in a poor solvent. Frictional properties of the poly(MPC) brushes were characterized by sliding a glass ball probe in air and various solvents under a load of 0.49 N at a sliding velocity of 90 mm min. An extremely low friction coefficient of the poly(MPC) brush was observed in humid atmosphere because water molecules adsorbed into the brush layer acted as a lubricant.

Control of dispersion state of silsesquioxane nanofillers for stabilization of polystyrene thin films.

The influence of the dispersion states of the nanofillers on the dewetting behavior of the polymer thin film was investigated. Polyhedral oligomeric silsesquioxanes (POSS) with various substituents were added into polystyrene (PS) thin films as the nanofillers. The dewetting rate of the films drastically changed with the surface substituents of POSS additives. Neutron reflectivity measurements indicated that the difference of the dewetting rate was associated with the dispersion state of POSS additives in the films. POSS with phenethyl groups (PhPOSS), which homogeneously dispersed into the films, resulted in the decrease of the glass transition temperature of PS and the enhancement of the dewetting of the films. POSS with a fluoroalkyl group (CpPOSS-R f) segregated to the film surface and showed the retardation of the dewetting by the decrease of the surface energy of the film. POSS with hydroxyl groups (CpPOSS-2OH) segregated to the film surface and film-substrate interface and led to the elimination of the dewetting, suggesting the importance of the interfacial segregation for the inhibition of dewetting. These results revealed the strong relationship between the dispersion state of the nanofillers and the dewetting of the nanofilled films.

Influence of the addition of silsesquioxane on the dewetting behavior of polystyrene thin film

A strategy to suppress the dewetting of polystyrene (PS) thin films by the addition of octacyclopentylsilsesquioxane (cPOSS) as a nanofiller was proposed. PS thin films with cPOSS were prepared by spin-coating. The bulk glass transition temperature of PS was not changed with an addition of the nanofiller up to 10 wt%. On the other hand, the addition of cPOSS to the PS thin films led to a great inhibition of dewetting. After annealing for 3 h at 373 K, no appreciable dewetting was observed by optical microscopy in the PS film with 15 wt% cPOSS, in contrast, the PS film without cPOSS was completely dewetted. Holes formed on the PS films with 10 wt% cPOSS. However, in that case, the growth of the holes stopped before reaching the final stage of the dewetting. This suggests that the increase of the cPOSS concentration per unit area at the rim of the holes prevents further growth of the holes, and this inhibition effect can be attributed to the interaction between cPOSS and the substrate accompanying modification of the PS-substrate interface.