An Optical Method of Observing Stress Relaxation in Transparent Solids

The relaxation of birefringence produced in two organic glasses after they have been sheared in a modified coaxial cylinder viscometer has been followed at different temperatures by the use of a photocell. For the low rates of loading used it has been found that the relaxation of both the glasses can be represented by a single exponential function, which shows that there is one predominant relaxation mechanism. The relaxation times were measured at different temperatures and cover a range from about 20 seconds to ½ millisecond. The viscosity has also been measured over the same temperature range and, from the values of the viscosity and relaxation time at a given temperature, the rigidity modulus of elasticity has been determined. Of the two materials used, one, glycerol sextol phthalate, was more chemically associated than the other, 2́-hydroxy-2: 4: 4: 6: 5́-pentamethylflavan. This difference was reflected in the temperature variation of the quantities measured.