380 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju 2.0 343℃ 1.5 1.0 0.5 0.0 0 20040060080010001200 Aging Time (hrs) Fig.9.12.Measurement of surface crack density for specimens aged in air photomicrograph suggests that the cracks formed in several steps(primary, secondary,and tertiary cracks).As the cracks propagate into the surface and widen,new oxidative surfaces form around the crack front and sub- sequent cracking occurs.Thus,oxidants are able to penetrate further into the sample through extensive cracking. Figure 9.14 illustrates severe oxidation damage observed in PMR-15 neat resin specimens after 670 h of aging in air at 343C.Note the appear- ance of voids in the oxidized layer with a larger concentration near the surface.The voids form because inward diffusion of oxygen is slower than outward diffusion of degradation byproducts [63]. While optical microscopy techniques are successfully used to monitor oxidation in PMR-15 resin,the same is not true for other polyimide systems, such as the recently developed AFR-PE-4 resin,because the optical characteristics of the polyimide do not change when it is oxidized.Other techniques,such as dark-field imaging [95],polarized light microscopy, Tertary Crack 262.78wm Secondary Crack Primary Crack 50 Fig.9.13.Crack penetration depth after Fig.9.14.Damaged PMR-15 from 342 h of aging in air at 343C isothermal aging at 343C for 670 hphotomicrograph suggests that the cracks formed in several steps (primary, secondary, and tertiary cracks). As the cracks propagate into the surface and widen, new oxidative surfaces form around the crack front and sub￾sequent cracking occurs. Thus, oxidants are able to penetrate further into the sample through extensive cracking. 0.0 0.5 1.0 1.5 2.0 0 200 400 600 800 1000 1200 Crack density (cracks/mm) Aging Time (hrs) 343o C Fig. 9.12. Measurement of surface crack density for specimens aged in air Figure 9.14 illustrates severe oxidation damage observed in PMR-15 neat resin specimens after 670 h of aging in air at 343°C. Note the appear￾ance of voids in the oxidized layer with a larger concentration near the surface. The voids form because inward diffusion of oxygen is slower than outward diffusion of degradation byproducts [63]. While optical microscopy techniques are successfully used to monitor oxidation in PMR-15 resin, the same is not true for other polyimide systems, such as the recently developed AFR-PE-4 resin, because the optical characteristics of the polyimide do not change when it is oxidized. Other techniques, such as dark-field imaging [95], polarized light microscopy, 342 h of aging in air at 343°C isothermal aging at 343°C for 670 h G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju Fig. 9.13. Crack penetration depth after Fig. 9.14. Damaged PMR-15 from 380