The combined influence of molecular weight and temperature on the physical aging and creep compliance of a glassy thermoplastic polyimide

• Published in: Mechanics of time-dependent materials Vol. 5; no. 3; pp. 199 - 227
• Main Authors: NICHOLSON, Lee M, WHITLEY, Karen S, GATES, Thomas S
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer 2001
• Subjects: Applied sciences; Exact sciences and technology; Mechanical properties; Physical properties; Polymer industry, paints, wood; Properties and testing; Technology of polymers; Isothermal condition; Polyimide; Thermoplastics; Aging test; Creep test; Temperature effect; High temperature; Experimental study; Glass transition temperature; Molecular weight; Glassy polymer; Properties of materials; Performance analysis; Constant load
• ISSN: 1385-2000; 1573-2738
• DOI: 10.1023/A:1017911509777

The effect of molecular weight on the viscoelastic performance of an advanced polymer (LaRC exp TM-SI) was investigated through the use of creep compliance tests. Testing consisted of short-term isothermal creep and recovery with the creep segments performed under constant load. The tests were conducted at three temperatures below the glass transition temperature of five materials of different molecular weight. Through the use of time-aging-time superposition procedures, the material constants, material master curves and aging-related parameters were evaluated at each temperature for a given molecular weight. The time- temperature superposition technique helped to describe the effect of temperature on the timescale of the viscoelastic response of each molecular weight. It was shown that the low molecular weight materials have higher creep compliance and creep rate, and are more sensitive to temperature than the high molecular weight materials. Furthermore, a critical molecular weight transition was observed to occur at a weight-average molecular weight of M sub w approx 25,000 g/mol below which, the temperature sensitivity of the time-temperature superposition shift factor increases significantly. The short-term creep compliance data were used in association with Struik's effective time theory to predict the long-term creep compliance behavior for the different molecular weights. At long timescales, physical aging serves to significantly decrease the creep compliance and creep rate of all the materials tested. Long-term test data verified the predictive creep behavior. Materials with higher temperature and lower molecular weights had greater creep compliance and higher creep rates.