Imide hydrolysis, polyimides

IR spectroscopy may be used to follow two reactions occurring in polyimides exposed to high temperatures and humidities hydrolysis of the imide linkages and hydrolysis of residual anhydride end groups. The hydrolytic susceptibilities of several polyimides were measured at 90°C/95% R.H. Polymers based on benzophenone tetracarboxylic acid dianhydride (with either oxydianiline or m-phenylene diamine) appeared to undergo rather rapid hydrolysis initially, but the reaction had essentially halted by the time the measured imide content had decreased by 5-6%. Polymers based on 3,3 ,4,4 -biphenyl tetracarboxylic acid dianhydride (with p-phenylene diamine) and pyromellitic dianhydride (with oxydianiline) showed no significant imide hydrolysis. In all the polymers, the anhydride was hydrolyzed quite readily. 

Potyimides obtained by reacting pyromellitic dianhydride with aromatic amines can have ladder-like structures, and commercial materials are available which may be used to temperatures in excess of 300°C. They are, however, somewhat difficult to process and modified polymers such as the polyamide-imides are slightly more processable, but with some loss of heat resistance. One disadvantage of polyimides is their limited resistance to hydrolysis, and they may crack in aqueous environments above 100°C. 

In comparison, no structural modification of model B was seen before 120 h of aging (80 °C). However, after 120 h two small doublets appeared in the NMR spectrum and several additional peaks became noticeable in the NMR spectrum. It was determined by NMR and IR spectroscopy that the hydrolysis products were an imide/carboxylic acid and an imide/anhydride. Model B was then aged for 1200 h at 80 °C to quantitatively determine the amount of hydrolysis products as a function of time. The relative intensity of the peaks due to carboxylic acid is constant after some time. The authors suggest that an equilibrium occurs between model B and the products formed during hydrolysis, and therefore, the conversion to hydrolysis products is limited to about 12%. This critical fraction is probably enough to cause some degradation of polymeric materials, but research on six-membered polyimides has remained active. 

This class of polymers has great thermal stability and promising short-term perfor-mance. °° The six-membered ring of naphthalenic imides is preferred over the five-membered ring of phthanic imides. The latter is susceptible to acid-catalyzed hydrolysis, which leads to chain scission and membrane embrittlement. An example of six-mem-bered ring polyimides is the block sulfonated copolyimides shown below. ... 

Measurements of the changes in both the imide (4) and the anhydride (1, 2) concentrations by IR spectroscopy have already been reported. This paper will demonstrate another approach to using IR to measure anhydride contents and will then present results obtained during hydrolysis of thin films of five polyimides. The structures of the polyimides are shown in Table I. The effects of changes in the curing conditions will also be discussed. 

P(NB/MA) proved to be surprisingly resistant to hydrolysis, both in solution and as a thin film(2. Moreover, the diacid prepared by hydrolysis in concentrated TMAH reverts to the anhydride upon heating(2ri). Similarly, alcoholysis with methanol is difficult and the methyl half-ester reverts to the anhydride on heating(2ri). Reactions with amines to yield poly(amic acids) and poly(imides) proceed more readily. However polyimides are too strongly absoibing at 193 nm to make suitable resist matrices. Also, because primaiy amines are potent imidization catalysts for amic adds(P), amidization of P(1 /MA) with these also causes strong absoibance at 193 nm. Selective amidization can be achieved with buU secondary amines which yield stable poly(amic add) with aqueous base solubility and acceptable transparency (absorbance = ca 0.5 AU/pm at 193 nm). These materials... 

Spectroscopic evidence of the seven-membered rings has been found in the preparation of polyimides from pyromellitic dianhydride and methylenediphenyl-diisocyanate (MDI) [105]. The reaction is conducted in solution of aprotic solvents, with reagents addition at low temperature and a maximum reaction temperature of about 130 °C. On the other hand, polyimides of very high molecular weight have not been reported by this method. The mechanism is different when the reaction is accelerated by the action of catalysts. Catalytic quantities of water or alcohols facilitate imide formation, and intermediate ureas and carbamates seem to be formed, which then react with anhydrides to yield polyimides [106]. Water as catalyst has been used to exemplify the mechanism of reaction of phthalic anhydride and phenyl isocyanates, with the conclusion that the addition of water, until a molecular equivalent, markedly increases the formation of phthalimide [107] (Scheme 13). The first step is actually the hydrolysis of the isocyanates, and it has been claimed that ureas are present in high concentration during the intermediate steps of the reaction [107]. Other conventional catalysts have been widely used to accelerate this reaction. Thus, tertiary amines, alkali metal alcoholates, metal lactames, and even mercury organic salts have been attempted [108]. 

As described in detail elsewhere, the preparation of Pl-based PNC involved the following steps (i) synthesis of a polyamic acid with ethoxysilane end-groups (PAAS), (ii) preparation of the nanocomposite precursors by the addition of the required amount of methyl triethoxysilane (MTS), (Hi) MTS hydrolysis in the cast NCP films, and (iv) imidization of the PAAS to the corresponding polyimide by stepwise heating of the thin NCP films for 2 h at each of the following temperatures 60,100,120,150,200, and 250 C and by post-cure for 5 hat300-315°C. 

Abstract Sulfonated polyimides have been designed to be used as proton conducting membranes in fuel cells. These materials present most of the required properties for this application, including a high level of ionic conductivity, a low gas and methanol permeability, and good mechanical properties. However, they exhibit a low stability when immersed in liquid water and in hydrogen peroxide solutions at elevated temperature due to a high sensitivity of the imide functions to hydrolysis. The aim of this article is to review the different routes of synthesis, the membrane-specific properties, the structural and transport property characteristics, and finally their behavior in fuel cells in terms of performance and stability. 

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