Tensile creep of thermoplastics:: Time-strain superposition of non-iso free-volume data

The dimensional stability of thermoplastics is characterized by their tensile compliance D(t,sigma,T) as a function of time t, stress sigma, and temperature T. Creep retardation times are controlled by the free volume available for underlying molecular (segmental) motions. Tensile deformation of polymeric materials, whose Poisson ratio is smaller than 0.5, is accompanied by volume dilatation that can be identified with an increase in available free volume. Consequently, a steady increase in strain with time during tensile creep experiments accounts for shortening of the retardation times. The superposition of as-received tensile compliance curves is difficult because any point of a curve requires a shift factor along the time axis that differs from those of other points. In this article, tensile creep at a constant stress and temperature is viewed as a non-iso free-volume process. A procedure is proposed to transform as-received data to a pseudo-iso free-volume state that eliminates this deficiency and permits construction of a generalized compliance curve for the pseudo-iso free-volume state. This curve can be used for calculation of real-time-dependent compliance for any selected stress in the range of reversible deformations. As the superposed curve can be generated with several short-term creep tests (e.g., 100 min) for a series of stresses, the proposed procedure saves experimental time. The effects of physical aging on tensile compliance (observed previously by other researchers) are interpreted in terms of the proposed approach in appendix A. (C) 2003 Wiley Periodicals, Inc.