Glass transition temperature


The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a crystalline phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). A material’s glass transition temperature, T<sub>g</sub>, is the temperature below which molecules have little relative mobility. T<sub>g</sub> is usually applicable to wholly or partially amorphous phases such as glasses and plastics. For inorganic or mineral glasses, such as common silicon dioxide (SiO<sub>2</sub>) glass, it is the mid-point of a temperature range in which they gradually become more viscous and change from being liquid to solid. Thermoplastic (non-crosslinked) polymers are more complex because, in addition to a melting temperature, T<sub>m</sub>, above which all their crystalline structure disappears, such plastics have a second, lower T<sub>g</sub> below which they become rigid and brittle, and can crack and shatter under stress. Small molecular weight pure substances such as water have just one such condensed-phase temperature, below which they are solid crystals (or amorphous ice if cooled below T<sub>g</sub> fast enough) and above which they are liquids.