The surface roughness of polymer gels was investigated by tapping mode atomic force microscopy (AFM). The spongelike domains on N-isopropylacrylamide (NIPA) gels were directly observed in water. It was found that the surface domain structure was strongly affected not only by the degree of the homogeneities of polymer networks but also by the bulk phase transition in response to the change in the external temperature. The domain size of homogeneous gels prepared at the ice temperature was found to be much smaller than that of the inhomogeneous gels prepared at a temperature above the cloud point of NIPA polymers. The surface structure was reproducibly observed at exactly the same position below and above the volume phase transition temperature, and its change was found to be reversible with temperature. The surface roughness due to the spongelike domains was discussed in terms of the autocorrelation function, the root-mean-square roughness, and the power spectral density, which were calculated from the AFM images.

We report here, for the first time, the direct observation of the submicron structure of gel surfaces in water by using an atomic force microscope ͑AFM͒. We present also its change in response to external stimuli; we investigated, among the variables that affect the topography of the gel surface, the effect of the network density of poly͑acrylamide͒ gels and the effect of the temperature change of poly N-isopropylacrylamide gels. Gels were prepared with disklike shape of thickness ranging from 10 to 50 m, and one of the gel surfaces was chemically adhered onto a glass plate. Spongelike domains of submicrometer scale were found here on the gel surfaces, which was strongly affected by the cross-linking density ͑nature of the gel network͒ as well as the osmotic pressure ͑environmental condition͒, and also thickness ͑condition of constraint͒. The qualitative properties of the surface microscopic structure of gels are discussed in relation to a hypothetical model of two-dimensional gels based on the Flory-Huggins theory. These results disclose that the surface microstructures of polymer gels in solvent as well as the nanometer scale structural changes are associated with the gel phase transition. Moreover, they indicate that the potential for a new technology to control the domain size of the gel surface as well as its function by external stimuli could emerge, which would find a variety of applications in many fields, such as engineering, medicine, and biology.

We investigated the effects of guest molecules of the gel particles on the volume phase transition properties of bulk gels. The guest molecules are the microspheres of poly(N-isopropylacrylamide) (NIPA) gel with submicron diameter, synthesized by the emulsion-polymerized reaction in water. We prepared several cylindrical NIPA gels with sub-millimeter diameter by the incorporation of the submicron NIPA gel particles of various concentrations at gelation, where the total NIPA monomer concentration (from the host networks and the guest particles) was fixed. In order to make clear the distribution of the gel particles immobilized in the bulk polymer networks, the fractured surfaces of the gels were observed by tapping mode atomic force microscopy in water. It was found that the gel particles aggregate in the host homogeneous networks to form domains with densely connected structure, which become larger with increasing particle concentration. We measured the macroscopic swelling curves, which indicate that the presence of guest NIPA particles with lower concentration does not strongly affect the transition temperature as well as the discontinuity of the phase transition in the bulk gels. On the other hand, when the amount exceeds a threshold concentration, the transition becomes continuous, and the swollen diameter rapidly increases. These results have been compared with the network inhomogeneities in pure NIPA gels (without particles) introduced at gelation, which depend on the gelation temperature below or above the cloud point of the linear polymer. We discussed qualitatively the macroscopic swelling behavior of the NIPA gels in terms of the inhomogeneities introduced artificially by the incorporation of the submicron gel particles.

We have measured the static contact angle of sessile air bubbles in water on poly (N-isopropylacrylamide) (NIPA) gel surfaces. The temperature dependence of the contact angle was measured on two different kinds of samples, consisting of homogeneous networks (transparent) and of phase-separated networks (opaque). It was found that the transparent gel exhibits a sharp but continuous decrement in the contact angle upon heating in the vicinity of the transition point, while the opaque gel does not show any appreciable change. The absolute value of the contact angle of the transparent gel in the collapsed state is evidently smaller than that of the opaque gel. These results were qualitatively discussed in terms of not only the chemical but also the physical surface properties in response to the temperature change; the former is the balance of the hydrophobic-hydrophilic interactions in this system, and the latter includes the surface polymer density, the surface roughness, and the network inhomogeneity.

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