Nitrogen-15 solid-state NMR studies on the cure and degradation of polyimide films under temperature and humidity stress

Abstract

An investigation of the thermal cure reactions and the hydrolysis reactions involved in the degradation of polyimide films under temperature and humidity stress using nitrogen-15 solids nuclear magnetic resonance (NMR) is herein reported. Nitrogen-15 labeling was used in combination with dipolar decoupled, cross polarization magic angle spinning (CPMAS) NMR techniques as a means of monitoring chemical reactions as these occur in solid state polyimide. The relative concentration of each nitrogen-containing functional group was calculated using standard NMR methods based on determination of the values of the cross polarization time constant, Thn. the proton rotating frame time constant, , , and observed spectral line intensities. The polyimides were derived from an oligomeric poly(amic acid) precursor [pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA)], 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'-benzophenonetetracarboxylic dianhydride (BTDA) and l,3-bis(3-aminophenoxy)benzene (APB) endcapped with (3-aminophenyl)acetylene (APA)]. The number of "defect sites" or imide-precursor groups where imidization does not occur was estimated to be between 6 and 9% of the total nitrogen and varies with the type of precursor used. The degree of imidization or cure was found to vary between 91 and 94% following a cure at 400 °C. Residual isoimide groups were detected after an extended 400 °C bake of the polyisoimide precursor. Cured films were subjected to temperature and humidity stress at 85 °C and 81% relative humidity for 450 h. Estimates of hydrolysis range from as little as 1 % of total nitrogen 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 reaction with chain cleavage to yield a terminal diacid and a terminal amine group. Hydrolysis from temperature and humidity stress is almost completely reversed if the stressed polyimide is heated at 400 °C after stress. The data obtained in this study are consistent with previously reported macroscopic observations in which polymer properties degrade during temperature and humidity stress and are recovered after post temperature and humidity bakes.

Swelling behavior of pH‐ and temperature‐sensitive copolymers containing 2‐hydroxy‐ethyl methacrylate and N‐vinyl‐2‐pyrrolidone crosslinked with new crosslinkers

“…Crosslinked copolymers of VP and HEMA have been prepared by thermal, photo‐ or irradiation polymerization using EGDMA or other dimethacrylates as crosslinkers 17–19. It is known that VP cannot polymerize well due to compositional drift during the course of polymerization 20. This effect is attributed to the differences in reactivity between VP and methacrylate comonomers.…”

“…This effect is attributed to the differences in reactivity between VP and methacrylate comonomers. The reactivity ratios for the copolymerization of HEMA and VP are r 1 = 4.5 + 0.24 and r 2 = 0.02 + 0.14 20. One problem with VP/HEMA copolymers is the presence of a considerable amount of linear water‐soluble fractions.…”

“…Crosslinking with EGDMA does not solve this problem as EGDMA too undergoes a compositional drift. Q‐e schemes have been used to demonstrate the existence of heterogeneous crosslinking in terpolymer compositions 20. In the present system, new crosslinkers were used to produce crosslinked HEMA/VP copolymers with low SF .…”

Novel acetylene‐terminated polyimide oligomers with excellent processability and high toughness of films

“…Landis and Naselow also have made lots of the early research. After that, the crosslinking and isomerization reactions of acetylene‐terminated polyisomide oligomers were studied by different methods, such as differential scanning calorimetry (DSC), Fourier transform infrared (FT‐IR), nitrogen‐15 solids Nuclear Magnetic Resonance ( 15 N‐NMR), and carbon‐13 magic‐angle spinning Nuclear Magnetic Resonance ( 13 C‐NMR) . Among all the acetylene‐terminated polyimides, thermid MC‐600 (M w =1099 g/mol), thermid IP‐600 (M w =1099 g/mol), and thermid FA‐700 (M w =1343 g/mol) which were produced by National Starch and Chemical Corp. have been as three commercially available products.…”

Novel acetylene‐terminated polyisoimides with excellent processability and properties comparison with corresponding polyimides

“…Landis41 also developed a copolymer formed from a reactive functional terminal isoimide oligomer and another compound, to provide liquid blend on heating and then to cure. Later on, lots of works were concentrated on the isomerization and cure behavior of isoimide oligomers using different methods, such as Fourier transform infrared (FTIR),39, 40 differential scanning calorimetry (DSC),42 carbon‐13 magic‐angle spinning Nuclear Magnetic Resonance ( 13 C NMR),42, 43 and nitrogen‐15 solid‐state Nuclear Magnetic Resonance ( 15 N NMR) 44. Furthermore, Kurita et al2 have studied the structural influence on the properties of model isoimides and facile synthesis of polyisoimides in details.…”