Radiation Synthesis of Binary Hydrogels with Thermoresponsive Pores

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Two types of hydrogels composed of poly(ethylene glycol) (PEG) and poly(N-isopropyl acrylamide) (PNIPAm) were synthesized via γ-ray-induced gelation. One type gel was a solid binary hydrogel. Gelation of PEG aqueous solutions was performed by γ-irradiation and homogeneous PEG matrix gels impregnated with NIPAm was also irradiated with γ rays. The other type gel was a porous binary hydrogel. It was also synthesized by γ-irradiation to a porous PEG matrix gel impregnated with NIPAm. The porous matrix gel was made by mixing silica microparticles (ca. 1.0 μm in diameter) into a PEG aqueous solution before gelation and then chemically decomposing them after gelation. These hydrogels show different thermoresponsive behavior. The swelling ratio of both hydrogels is reduced when the temperature is raised from 12 to 40 °C. However, the swelling ratio of the porous binary hydrogels was smaller than that of the solid binary hydrogels. In the solid gel, the PNIPAm chains are uniformly distributed and therefore the contraction of PNIPAm chains causes the shrinkage of the whole gel. In contrast, the PNIPAm chains of the porous gel are more localized in the pore spaces of PEG matrix gels, and exert less influence on the volume change of the whole gel.
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