N-15 SOLID-STATE NMR-STUDIES ON THE CURE AND DEGRADATION OF POLYIMIDEFILMS UNDER TEMPERATURE AND HUMIDITY STRESS

An investigation of the thermal cure reactions and the hydrolysis reac tions involved in the degradation of polyimide films under temperature and humidity stress using nitrogen-15 solids nuclear magnetic resonan ce (NMR) is herein reported. Nitrogen-16 labeling was used in combinat ion with dipolar decoupled, cross polarization magic angle spinning (C PMAS) NMR techniques as a means of monitoring chemical reactions as th ese occur in solid state polyimide. The relative concentration of each nitrogen-containing functional group was calculated using standard NM R methods based on determination of the values of the cross polarizati on time constant, T-HN, the proton rotating frame time constant, T-1 r ho H, and observed spectral line intensities. The polyimides were deri ved from an oligomeric poly(amic acid) precursor [pyromellitic dianhyd ride (PMDA) and 4,4'-oxydianiline (ODA)I, a high molecular weight poly (amic acid ester) precursor (PMDA m-diacyl chloride diethyl ester and ODA), and a polyisoimide oligomer [3,3',4,4'-benzophenonetetracarboxyl ic dianhydride (BTDA) and 1,3-bis(3-aminophenoxy) benzene (APB) endcap ped with (3-aminophenyl) acetylene (APA)]. The number of ''defect site s'' or imide-precursor groups where imidization does not occur was est imated to be between 6 and 9% of the total nitrogen and varies with th e type of precursor used. The degree of imidization or cure was found to vary between 91 and 94% following a cure at 400 degrees C. Residual isoimide groups were detected after an extended 400 degrees C bake of the polyisoimide precursor. Cured films were subjected to temperature and humidity stress at 85 degrees C and 81% relative humidity for 450 h. Estimates of hydrolysis range from as little as 1% of total nitrog en for the BTDA-APB-APA derived material to approximately 13% for the PMDA-ODA poly(amic acid ester) precursor. About 30% of the amide acid groups formed during stress react with water in a second hydrolysis re action with chain cleavage to yield a terminal diacid and a terminal a mine group. Hydrolysis from temperature and humidity stress is almost completely reversed if the stressed polyimide is heated at 400 degrees C after stress. The data obtained in this study are consistent with p reviously reported macroscopic observations in which polymer propertie s degrade during temperature and humidity stress and are recovered aft er post temperature and humidity bakes.