09-27-2021-2030 - glass–liquid transition, or glass transition

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The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubbery state as the temperature is increased.^[1][2] An amorphous solid that exhibits a glass transition is called a glass. The reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification.

The glass-transition temperature T_g of a material characterizes the range of temperatures over which this glass transition occurs. It is always lower than the melting temperature, T_m, of the crystalline state of the material, if one exists.

Hard plastics like polystyrene and poly(methyl methacrylate) are used well below their glass transition temperatures, i.e., when they are in their glassy state. Their T_g values are both at around 100 °C (212 °F). Rubber elastomers like polyisoprene and polyisobutylene are used above their T_g, that is, in the rubbery state, where they are soft and flexible; crosslinking prevents free flow of their molecules, thus endowing rubber with a set shape at room temperature (as opposed to a viscous liquid).^[3]

Despite the change in the physical properties of a material through its glass transition, the transition is not considered a phase transition; rather it is a phenomenon extending over a range of temperature and defined by one of several conventions.^[2][4][5] Such conventions include a constant cooling rate (20 kelvins per minute (36 °F/min))^[1] and a viscosity threshold of 10¹² Pa·s, among others. Upon cooling or heating through this glass-transition range, the material also exhibits a smooth step in the thermal-expansion coefficient and in the specific heat, with the location of these effects again being dependent on the history of the material.^[6] The question of whether some phase transition underlies the glass transition is a matter of continuing research.^{[4][5][7][when?]}

IUPAC definition

glass transition (in polymer science): Process in which a polymer melt changes on cooling to a polymer glass or a polymer glass changes on heating to a polymer melt.^[8] Note 1: Phenomena occurring at the glass transition of polymers are still subject to ongoing scientific investigation and debate. The glass transition presents features of a second-order transition since thermal studies often indicate that the molar Gibbs energies, molar enthalpies, and the molar volumes of the two phases, i.e., the melt and the glass, are equal, while the heat capacity and the expansivity are discontinuous. However, the glass transition is generally not regarded as a thermodynamic transition in view of the inherent difficulty in reaching equilibrium in a polymer glass or in a polymer melt at temperatures close to the glass-transition temperature.

Note 2: In the case of polymers, conformational changes of segments, typically consisting of 10–20 main-chain atoms, become infinitely slow below the glass transition temperature.

Note 3: In a partially crystalline polymer the glass transition occurs only in the amorphous parts of the material.

Note 4: The definition is different from that in ref.^[9]

Note 5: The commonly used term “glass-rubber transition” for glass transition is not recommended.^[10]

^{https://en.wikipedia.org/wiki/Glass_transition}