Glass Transition of Polymers

the transition of a polymer from a high-elastic state to a solid glassy state. In its physical nature, the glass transition of polymers does not differ from the vitrification of low-molecular-weight liquids, although the mechanism of the process has features that reflect the specific nature of the thermal molecular motion in the glassy and high-elastic states of the polymer.

In the glassy state, the atoms of a polymer are fixed to points in an irregular space lattice and do not undergo translational displacements in response to external forces as do the atoms in ordinary solids. In the high-elastic state, group translational movement of segments of long-chain macromolecules is possible, as is a change in the relative positions of the segments in response to external forces, that is, a change in the structure of the polymer. The rate of rearrangement of the structure is characterized by the relaxation time; it decreases with cooling and below a certain temperature becomes so low that the structure is “frozen,” that is, converted to the glassy state. Thus, the glass transition of polymers has a kinetic nature inasmuch as it results from a gradual loss of mobility of atoms and groups of atoms.

Glass transition occurs in a range of temperatures characterized by an arbitrary value, the glass-transition temperature T_g, which is determined graphically on curves for the temperature changes of certain physicochemical properties of the polymer. The value for T_g, depends on the polymer’s chemical composition and structure, thermal history, and rate of thermal or mechanical action. At a given temperature, a polymer may be highly elastic in response to slow mechanical action and rigid in response to rapid action. The effect of increasing T_g, with an increase in the rate of the mechanical action is often called the mechanical glass transition.