Effects of guest microparticles on the phase transition of bulk polymer gels

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.