Deformation of glassy polymers

Thermally stimulated shrinkage of amorphous PET and polystyrene (PS) after their deformation via uniaxial compression at room temperature was studied by direct microscopic observations. Experimental evidence allows one to conclude that the deformation of a glassy polymer may be conditionally divided into two stages. At the first stage (below the yield point), the polymer is transformed into thinnest layers localized in shear bands which are separated by the blocks of initial undeformed polymer. In such layers, the glass transition temperature is reduced. This fact is responsible for the appearance of a low-temperature component of the thermal shrinkage of an oriented glassy polymer. At the second stage (after they yield point), an overwhelming volume transition of polymer to its oriented state takes place. This process is not accompanied by any marked volume changes and is similar to the deformation of an amorphous polymer at temperatures above the glass transition temperature. Thermal shrinkage related to this deformation mode occurs in the glass transition region of a bulk polymer.