Experimental characterization and modeling of isothermal and nonisothermal physical aging in glassy polymer films.

Abstract

Fiber-reinforced polymer matrix composites (PMCs) using high-temperature thermoplastics as the matrix material represent a lightweight design solution for applications above 250°F. This is often required for aircraft structures subjected to supersonic airflows. While thermoplastic matrix PMCs may be successfully designed for such temperatures, they generally exhibit a time-dependent material response which must be well understood for successful design. One important aspect of this response is physical aging, which causes the viscoelastic behavior of amorphous polymers below their glass transition temperature to change with time. While isothermal physical aging has been extensively studied, physical aging during a varying temperature history has received less scrutiny. This dissertation focuses on nonisothermal physical aging of polymers from both experimental and theoretical aspects. The study concentrates on pure polymers rather than fiber-reinforced composites; this step removes several complicating factors to simplify the study. It is anticipated that the findings of this work can then be applied to composite materials applications. The physical aging tests in this work are performed using a dynamic mechanical analyzer (DMA). The viscoelastic response of glassy polymers under various loading and thermal histories are observed as stress-strain data at a series of time points. The first stage of the experimental work involves the characterization of the isothermal physical aging behavior of two advanced thermoplastics. The second stage conducts tests on the same materials with varying thermal histories and with long-term test duration. This forms the basis to assess and modify a nonisothermal physical aging model (KAHR- a te model). Based on the experimental findings, the KAHR- a te model has been revised by new correlations between aging shift factors and volume response; this revised model performed well in predicting the nonisothermal physical aging behavior of glassy polymers. In the work on isothermal physical aging, short-term creep and stress relaxation tests were performed at several temperatures within 15-35°C below the glass transition temperature ( T g ) at various aging times, using the short-term test method established by Struik. Stress and strain levels were such that the materials remained in the linear viscoelastic regime. These curves were then shifted together to determine momentary master curves and shift rates. In order to validate the obtained isothermal physical aging behavior, the results of creep and stress relaxation testing were compared and shown to be consistent with one another using appropriate interconversion of the viscoelastic material functions. Time-temperature superposition of the master curves was also performed. The temperature shift factors and aging shift rates for both PEEK and PPS were consistent for both creep and stress relaxation test results. Nonisothermal physical aging was monitored by sequential short-term creep tests after a series of temperature jumps; the…