Article Moisture absorption and hydrothermal aging of phenylethynyl-terminated pyromellitic dianhydride-type asymmetric polyimide and composites Yixiang Zhang 1 , Masahiko Miyauchi 2 and Steven Nutt 1 Abstract The effects of moisture on a polymerized monomeric reactant (PMR)-type polyimide (TriA X) and associated composites were investigated. Water uptake tests were performed on the polyimide at various temperatures and relative humidity levels to investigate moisture absorption behavior. Two-stage moisture absorption was observed, in which the first stage was diffusion controlled, whereas the second stage was moisture plasticization controlled. As exposure temperature increased, the equilibrium moisture content of the polyimide decreased, indicating an exothermic absorption process. The Arrhenius temperature dependence and moisture saturation as functions of temperature and humidity in the neat polymer were determined using curve fitting based on the published mathematical models. Long-term hydrothermal aging at 95  C was conducted on the neat polyimide and associated carbon fiber composites. Reversible hydrolytic reactions and a trace of irreversible hydrolysis were observed in the long-term exposure. The tensile ductility of the neat polyimide and the short-beam shear strength of the composites decreased with increasing aging time, while the tensile strength and modulus and thermal properties of the polyimide exhibited little change after 2000-h aging, demonstrating hydro- thermal stability. The decrease in the ductility of the neat polymer after long-term moisture exposure was attributed to the network structure change, driven by hydrolysis and moisture plasticization. Keywords Polyimide, moisture absorption, hydrothermal aging, thermal properties, mechanical properties, plasticization, hydrolysis, composite, -relaxation Introduction Because of the high glass-transition temperatures (T g ) and thermal stability of polyimides, carbon fiber composites with polyimide matrices are deployed in high-service- temperature applications, such as aeroengines and super- sonic aircraft. 1,2 However, polyimide–matrix composites are more expensive than conventional epoxy–matrix com- posites because of resin costs (200–900 US$ kg 1 ) and the complexity of the fabrication/cure process. 3,4 Thus, long lifetime is expected for polyimide–matrix composites, and stability issues must be addressed before adopting polyi- mide resins in new engineering applications. Hydrothermal stability of polyimide systems has been a primary concern for aerospace applications, because poly- imides are hydrophilic polymers and thus susceptible to moisture. 5,6 The general effects of moisture include the hydrolysis of imide units and water plasticization. 5–7 Hydrolysis can depolymerize polyimides by opening imide rings to form polyamic acids, followed by chain scission and regeneration of monomers (or even demonomeriza- tion), resulting in degradation of dry mechanical proper- ties. 5 For example, after 1000-h hydrothermal exposure at 160  C and subsequent drying, one polyimide (AFR700B) manifest approximately 70% strength loss and approxi- mately 85% strain-to-failure decrease, while another (K3B) showed approximately 18% strength loss and an approximately 21% decrease in strain-to-failure. 5 In 1 Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, USA 2 Kaneka U.S. Material Research Center, Kaneka Americas Holding, College Station, TX, USA Corresponding author: Yixiang Zhang, University of Southern California, 3651 Watt Way, Los Angeles, CA 90089, USA. Email: zhangyix@usc.edu High Performance Polymers 1–10 ª The Author(s) 2018 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0954008318816754 journals.sagepub.com/home/hip