The GLASS TRANSITION In Rubbery MATERIALS

A characteristic of rubbers that is unique among polymers is the similarity of their dynamic behavior in the glass transition zone to that of molecular liquids. Near T_g materials become very energy dissipative, and spectacular changes in mechanical properties can be induced by minuscule changes in temperature or pressure. Various applications of elastomers, as diverse as skid-resistant tires, acoustic tiles, military armor, and mammalian blood vessels, exploit this behavior in order to function. Although glass formation is as old as the planet itself, a viable theory or even predictive model for the phenomenon does not exist.

            We have been engaged in an experimental program to understand the glass transition – its distinctive properties, the mechanisms underlying those properties, and the relationship to chemical structure. Three aspects will be discussed: (i) the relative contributions of temperature and volume to the slowing down of molecular motions as a material is cooled toward T_g; (ii) the invariance of certain properties to temperature, pressure, and volume when the structural relaxation time is maintained constant; and (iii) the collapse of dynamic properties such as the viscosity and diffusion constant onto a single master curve when plotted versus the ratio of temperature to density, with the latter raised to a material constant. The validity of this superpositioning extends from T_g to high temperatures, a range over which the conventional time-temperature superposition principle breaks down.