Stress effects on the elastic properties of amorphous polymeric materials

Brillouin light scattering measurements have been used to study the stress induced modification in the elastic properties of two glass forming polymers: polybutadiene and epoxy-amine resin, prototypes of linear and network polymers, respectively. Following the usual thermodynamic path to the glass transition, polybutadiene has been studied as a function of temperature from the liquid well into the glassy phase. In the epoxy resin, the experiments took advantage of the system ability to reach the glass both following via the chemical vietrification processroute, i.e. by increasing the number of covalent bonds among the constituent molecules, as well as via the physical thermal wayroute, i.e. by decreasing the temperature. Independently from the particular way choosen to reach the glassy phase, the measurements reveal the signature of long range tensile stresses development in the glass. The stress presence drives the system out of the linear elastic regime modifying both the value of the sound velocities and their mutual relationshipatio breaking the generalized Cauchy-like relation. In particular, when long range stresses, by improvise sample cracking, are released, clear changes of the sound velocities occurs: the frequency of longitudinal acoustic modes increases more than 10% in polybutadiene while the sound velocities increase of  4% in the epoxy resin., the sound velocities increase of about 4%. The data analysis suggests the presence of at least two different mechanisms acting on different length scales which strongly affect the overall elastic behaviour of the systems: i) the development of long tensile stress acting as a negative pressure and ii) the development of anisotropy which increases its importance deeper and deeper cooling in the glassy state.