Physical aging studies were made using commercial and laboratory samples of isotactic polypropylene. Linear and nonlinear viscoelastic responses were measured after quenching the glass from above the glass transition temperatures to below the glass transition. Results show that aging is not eliminated by large mechanical deformations; rather, the time required for the glass to age into equilibrium is independent of the applied stress. Results obtained are purely kinetic and interpreted as the effects accompanying the process of glass formation in a semicrystalline polymer.
Ibhadon, A. O. (1996). Physical aging in isotactic polypropylene. Journal of Applied Polymer Science, 62(11), 1843-1846. https://doi.org/10.1002/%28SICI%291097-4628%2819961212%2962%3A11%3C1843%3A%3AAID-APP8%3E3.0.CO%3B2-Q