Chapter 9:Predicting Thermooxidative Degradation 367 time-temperature superposition models [21].Inherent in the use of physical aging models is the assumption that the aging process is thermoreversible, therefore nonreversible chemical aging cannot be properly modeled using this approach.That is,the effects of irreversible processes,e.g.,chemical decomposition,hydrolytic degradation,oxidation,etc.,are neglected in these models or are assumed to be implicitly accounted for in determining the physical aging parameters.However,chemical aging can be the primary life-limiting degradation process for HTPMCs used at temperatures near their T.Neglecting the modeling of the chemical degradation for com- posites in an oxidative environment will likely result in inaccurate and nonconservative predictions of performance. 9.1.2 Chemical Aging of Polymers and PMCs Chemical aging,unlike physical aging,is typically not thermoreversible. Hydrolysis (the chemical reaction of the polymer with water)and oxi- dation(the chemical reaction of the polymer with oxygen)are the primary forms of chemical degradation in HTPMCs.The chemical changes occurring during oxidation include chemical bond breaks that result in a reduction in molecular weight,mechanical response changes,and a local loss of mass associated with outgassing of oxidation byproducts.Although the rate of oxidative chemical reactions is,in part,governed by the avail- ability of reactive polymer,the oxygen concentration,and the reaction temperature for high Ts glassy polymers,the rate of oxidation can be greater than the rate of diffusion of oxygen into the polymer.For such circum- stances,the oxidative process is diffusion rate limited.Diffusion rate-limited oxidation of neat polymer specimens typically results in the development of an oxidative layer or graded oxidative properties near the free surfaces of the specimen.Within the oxidized region of the polymer,it is typical that the tensile strength,strain to failure,flexural strength,density,and toughness decrease while the modulus increases.The effect of oxidation on changes in the Ts is dependent on the specific polymer system.Some polymers initially have a decrease and then an increase in T3,others may have only a decrease,and still others may only have an increase in T.This may be due to competing chemical and physical aging phenomenon [56]or differences in the oxidation reaction mechanisms.Since HTPMCs typi- cally operate at temperatures near their initial design T3,any changes in the local or global T can have detrimental effects on performance. Determination of the primary,secondary,and tertiary chemical degrada- tion mechanisms in high-temperature polyimide composites is an extremely challenging task [92]but can yield substantial benefits for designing newtime–temperature superposition models [21]. Inherent in the use of physical aging models is the assumption that the aging process is thermoreversible, therefore nonreversible chemical aging cannot be properly modeled using this approach. That is, the effects of irreversible processes, e.g., chemical decomposition, hydrolytic degradation, oxidation, etc., are neglected in these models or are assumed to be implicitly accounted for in determining the physical aging parameters. However, chemical aging can be the primary life-limiting degradation process for HTPMCs used at temperatures near their Tg. Neglecting the modeling of the chemical degradation for com￾posites in an oxidative environment will likely result in inaccurate and nonconservative predictions of performance. 9.1.2 Chemical Aging of Polymers and PMCs Chemical aging, unlike physical aging, is typically not thermoreversible. Hydrolysis (the chemical reaction of the polymer with water) and oxi￾dation (the chemical reaction of the polymer with oxygen) are the primary forms of chemical degradation in HTPMCs. The chemical changes occurring during oxidation include chemical bond breaks that result in a reduction in molecular weight, mechanical response changes, and a local loss of mass associated with outgassing of oxidation byproducts. Although the rate of oxidative chemical reactions is, in part, governed by the avail￾ability of reactive polymer, the oxygen concentration, and the reaction temperature for high Tg glassy polymers, the rate of oxidation can be greater than the rate of diffusion of oxygen into the polymer. For such circum￾stances, the oxidative process is diffusion rate limited. Diffusion rate-limited oxidation of neat polymer specimens typically results in the development of an oxidative layer or graded oxidative properties near the free surfaces of the specimen. Within the oxidized region of the polymer, it is typical that the tensile strength, strain to failure, flexural strength, density, and toughness decrease while the modulus increases. The effect of oxidation on changes in the Tg is dependent on the specific polymer system. Some polymers initially have a decrease and then an increase in Tg, others may have only a decrease, and still others may only have an increase in Tg. This may be due to competing chemical and physical aging phenomenon [56] or differences in the oxidation reaction mechanisms. Since HTPMCs typi￾cally operate at temperatures near their initial design Tg, any changes in the local or global Tg can have detrimental effects on performance. Determination of the primary, secondary, and tertiary chemical degrada￾tion mechanisms in high-temperature polyimide composites is an extremely challenging task [92] but can yield substantial benefits for designing new Chapter 9: Predicting Thermooxidative Degradation 367