Glass transition temperature (Tg) and dynamics of thin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) are investigated using dielectric relaxation spectroscopy and related methods. In case of thin films of polystyrene, the glass transition temperature could successfully be obtained as a function of film thickness and the obtained results are consistent with those observed using other techniques such as ellipsometry. In accordance with the decrease in the glass transition temperature in thin films, the dynamics of the α-process becomes faster with decreasing film thickness and the distribution of the α-relaxation times becomes broader. In case of thin films of poly(methyl methacrylate), the thickness dependence of the α- and β-processes could be observed using the dielectric relaxation spectroscopy. Both the temperatures Tα and Tβ, at which the dielectric losses due to the α- and β-processes show a maximum at a given frequency, decrease with decreasing film thickness. Below a critical thickness, both Tα and Tβ begin to decrease rapidly with decreasing thickness. In order to investigate the positional dependence of the dynamics of the α-processes for polystyrene thin films, we utilize bilayer films, where a thin film of PS labeled with a dye is located on top of the film of unlabeled PS. For such systems, we could extract the contribution only from the upper labeled layer of PS. The glass transition temperature of the upper labeled layer between unlabeled PS layer and aluminum electrode is lower than that of the bulk labeled polystyrene. This result supports that the sample geometry commonly used for dielectric measurements of ultra thin films is the same to that used for other methods as far as the glass transition dynamics are concerned.
Living cells respond to external mechanical stimuli and exhibit various functions such as adhesion, motility and division. Thus, it is important to elucidate the mechanical and rheological properties in their physiological conditions. The atomic force microscope (AFM) is one of the useful tools for measuring the viscoelastic properties of living cells at nanoscale. In this article, we overviewed the basic principle of measuring the viscoelastic properties of living cells with AFM. The relation between the local elastic properties and the stress relaxation processes as well as the dependence of the shape of indenter on the stress relaxation of living cells were presented and discussed in terms of the structures of cells.
Scanning viscoelasticity microscopy (SVM) with a mechanical model analysis was applied to polystyrene (PS) surfaces. When the surface was in a glassy state, that is, in an elastic regime, the SVM vibrational system can be simply expressed by a series model composed of two springs such as a cantilever and the sample surface. Once the surface reaches in a viscoelastic regime, a damping factor must be incorporated into the model surface. The spring constants for the PS surfaces being in glassy and transition states were successfully extracted by the analysis. The surface value even for the glassy state was lower than the corresponding bulk value by a few decades.
We study the evaporation process which causes phase separation using the dissipative particle dynamics. To simulate the evaporation, we modify the method introduced by Yamamoto et al: we exchange the solvent particle with the gas particle not in the region near the surface, but in the region far from the surface. This method allows us to control the evaporation rate, and to analyze the solvent distribution in the gas phase. For the evaporation of the polymer solution of binary polymer blend, we found two types of structure in the dried film, one is that phase separated domains is orthogonal to the film, and the other parallel to the film. We found that in addition to the difference in the interaction between polymer and substrate, and that between polymer and gas, the interaction between polymer and solvent can cause such structural change.
Nanomechanical mapping by atomic force microscopy (AFM) has been developed as the useful application to measure the physical properties of soft materials at nano-meter scale. To date, the Hertz theory was used for analyzing force-distance curves as the simplest model of contact mechanics between elastic bodies. However, the preexisting methods based on Hertz theory do not consider the adhesive interaction in principle, which cannot be neglected in the ambient condition. First, we introduce a new analysis to estimate the elasticity and adhesive energy simultaneously by means of the JKR theory, describing adhesive contact between elastic materials. Secondly, poly(dimethylsiloxane), PDMS, and butyl rubber, isobutene-co-isoprene rubber (IIR), were analyzed to verify the validity of the JKR analysis, the method mentioned above. For elastic samples such as PDMS, the force-deformation (F-δ) plots obtained experimentally were consistent with JKR theoretical curves. Meanwhile, for viscoelastic samples, especially for IIR, the F-δplots revealed deviations from JKR curves depending on scan velocity and indentation depth. To elucidate the limit of the JKR method, we compared the characteristic behaviors of elastic and viscoelastic materials observed from contact measurement at nano-meter scale.
The hydrophilic polymer brushes with 25-35 nm thick were prepared by surface-initiated atom transfer radical polymerization of (2,2-dimethyl-1,3-dioxolan-4-yl)methyl methacrylate (DMM), 2-methacryloyloxyethyl phosphorylcholine (MPC), 2-dimethylaminoethyl methacrylate (DMAEMA), and vinyl acetate (VAc) on the initiator-immobilized silicon wafer. PDMM, PDMAEMA, and PVAc were converted to poly(2,3-dihyroxypropyl methacrylate) (PDHMA), poly(2-methacryloxyethyltrimethylammonium iodide) (PMETA), and poly(vinyl alcohol) (PVA), respectively. Macroscopic frictional properties of the hydrophilic polymer brushes were characterized by sliding a glass ball probe in air, water, and toluene under the load of 0.49 N at a sliding velocity of 90 mm/min. The dynamic frictional coefficient of the surfaces immobilized with PDHMA, PMPC, PMETA and PVA brushes were lowered in water (good solvent) compared with in dry air condition, while they increased in toluene (poor solvent). Extremely low friction coefficient of PMPC brush was observed in humid atmosphere because water molecules adsorbed into brush surface acted as a lubricant. Adhesion force measurement of high-density PMPC brush was also carried out by AFM using a colloidal probe. Larger adhesion force was observed in a poor solvent (ethanol/water = 75/25, v/v) compared with good solvent (pure water), because of strong interaction between a brush and a probe in a poor solvent. These results indicate that the frictional properties largely depend on the solvent quality and the adhesion force between the substrate and the sliding probe.
Interdiffusion of cyclic polystyrene (c-hPS) / cyclic deuterated polystyrene (c-dPS) laminated film whose molecular weights are ca.50 k was examined by neutron reflectivity and dynamic secondary ion mass spectroscopy. It has been found that time evolution of the interfacial thicknesses of (c-hPS/c-dPS) multi-layer films are similar to those of the corresponding linear hPS/linear dPS (l-hPS/l-dPS) bilayer films at short time region. However, at the annealing time longer than 5×103 s, the interfacial thicknesses of (c-hPS/c-dPS) multi-layer films are evidently larger than those of the (l-hPS/l-dPS) films at each observing time. These results can be explained in terms of the less entanglement nature of cyclic polymers than that of the linear ones.