Physical aging in polymers and polymer composites: An analysis and method for time-aging time superposition

The ability to include the effects of physical aging is critical to the design of engineering structures involving advanced polymers and polymer composites. Physical aging, a phenomenon caused by material evolution towards thermodynamic equilibrium, is typically described by reducing short-term momentary compliance curves, via time-aging time superposition, into a predictive reference curve and shift factor function. This paper presents an analytic method based on error minimization for this data reduction task, and introduces the concept of a total reference curve as an alternative to choosing a single momentary curve for the reference. Four different predictive approaches are considered to determine the efficacy of the total reference concept, and whether vertical shifting is beneficial. For most of the materials studied, the total reference using horizontal shifting alone is the recommended method; in the few remaining cases, the total reference procedure with both horizontal and vertical shifting resulted in the best outcome.