Changes of Viscoelastic Properties of Polymethyl Methacrylate Soaked in Various Organic Solvents

The viscoelastic properties of polymethyl methacrylate have already been studied in detail and made clear by A. V. Tobolsky, A. E. Woodward, J. D. Ferry and others. They studied only its behavior in the atmospheric surroundings while in dental usages it is used selectively in oral cavity. The mechanical behavior in the air is surmised to be considerably different from that in saliva. To make clear such difference, the author measured the stress relaxation of specimens soaked in water as well as various organic solvents and examined their influence. The results obtained are as follows : 1) In course of dissolving of polymethyl methacrylate in organic solvents there can be found two different types of behavior, i.e. swelling and cracking, according to the solvents used. 2) The stress relaxation modulus of the soaked in acetone, benzene, xylene, eugenol is lower in the room temperature range than that of the unsoaked, while the glass transition temperature is not apparently changed by soaking. 3) The glass transition temperature of the specimen soaked in chloroform, which belongs to the swelling type, is lower than that of the unsoaked. 4) The specimen soaked in ethyl alcohol, which belongs to the swelling type, has more of temporary, physical cross-links provided by molecular chain entanglements than that of the unsoaked. 5) The stress relaxation behavior of the specimen soaked in water (37℃) is similar to that of the unsoaked. 6) The specimen prepared directly by polymerization of pure monomer (MM specimen) and the specimen prepared by the dental method starting from monomer liquid and polymer powder (MP specimen) can not be relaxed so easily as the specimen prepared by heat pressing of polymer powder (PP specimen).

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