Polymers and 'strange' liquids

Abstract

intensively studied since the beginning of polymer science and these investigations contributed significantly to our understanding of polymer structure, conformation and thermodynamics. There are two classes of liquids, however, which may raise new questions and promote new research leading to new understanding in this field, namely ionic liquids and supercritical fluids, more specifically supercritical water. Both classes of ‘strange liquid’ have been developed, studied and used widely in the socalled ‘green chemistry’ movement in the last decades of the 20th century. Ionic liquids are true ionic compounds with bulky, non-symmetric (mostly nitrogen based organic) cations and special (mostly perfluorinated inorganic) anions having a melting point below, or in the neighborhood of room temperature. They have extremely low vapor pressure, can be relatively easily recovered, they are non-flammable and heat resistant. Ionic liquids can be used as reaction media for polymerization, but they can dissolve and/or swell certain polymers and can be used as plasticizers even with medium polarity polymers as e.g. poly(methyl methacrylate). Polymer gels swollen with ionic liquids may be used as solid polymer electrolytes. Ionic liquids can be trapped in crosslinked polymers by in situ polymerization or even built into the polymer network. As polymerization media, they can be used in living polymerization or in polymer-analogous reactions. Supercritical (sc) fluids, as special kinds of low viscosity liquids with adjustable solution power have already been with us for decades and the uses of sc CO2 (pcr = 73 bar, Tcr = 304 K) as reaction or extraction medium have become well known in the ‘polymer community’. As patents show sc CO2 may provide an alternative solution to melt or solution blending in the preparation of nanocomposites as well. Here, however, I would like to draw the readers’ attention to sc H2O, supercritical water (pcr = 221 bar, Tcr = 647 K), as a special medium for controlled polymer recycling and degradation. The dielectric permittivity and the ion-product of sc H2O can be varied within wide limits by adjusting the pressure and temperature which makes possible the degradation of biomass and polymers within seconds even in the absence of catalysts! This feature is especially important with crosslinked resins, rubbers, which cannot be recycled in the molten state. According to the Chemical Abstracts the patenting and publication activity of Asian countries is especially high in this area.