A multi-physics approach on modeling of hygrothermal aging and its effects on constitutive behavior of cross-linked polymers

Abstract

A multi-physics model is proposed to predict the changes in the constitutive behavior of cross-linked polymeric systems due to damages induced by deformation, oxidation, moisture, and temperature. Coupling the concept of network evolution, hydrolysis, and thermo-oxidation aging, we hypothesize that the synergized effects of deformation-induced damage, as well as different environmental factors such as humidity, temperature, and oxygen, can be taken into account through a generic model. Deformation-induced damage is due to the chain breakage while hygrothermal aging results from the interaction of elastomeric components with moisture in the presence of oxygen and temperature. Results indicate that hygrothermal aging can be considered as a consequence of damage accumulation of two concurrent aging mechanisms, namely; (i) thermo-oxidative, and (ii) hydrolytic aging. In order to capture the mutual effects of thermo-oxidative and hydrolytic aging, the assumption has been made that each of the aging phenomena can be superimposed upon each other. In fact, each of them acts independently, and as a result, they can propagate hygrothermal aging in collaboration. In view of multiple kinetics involved in hygrothermal aging, its effect has been induced by the concurrent effects of three micro-mechanisms; (i) chain scission due to the presence of temperature, (ii) reduction of cross-links attributed to the attendance of water, and (iii) formation of cross-links as a repercussion of oxygen present in the environment. Utilizing the theory of network decomposition, all phenomena and their correlation were modeled, and thus, the strain energy function of the polymer matrix is written with respect to four independent mechanisms; (i) the shrinking original matrix that has neither been attacked by water nor oxygen, (ii) conversion of the first network to two new networks due to reduction and formation of cross-links, and (iii) energy loss from network degradation due to water attrition to polymer active agents. The proposed model cannot consider variation in oxygen density since it has been derived for cases where there is abundant oxygen. In this respect, the model is valid for a slow aging process that occurs in super-thin samples. Finally, the model is validated with respect to extensive sets of our experimental data. In view of its interoperability, precision, and deep insight it provides into the nature of the aging phenomenon, the model is a good choice for advanced implementation in FE applications.