ISOIMIDE GROUP

Polyisoimides have better solubihty and improved melt flow characteristics [59, 60]. The isoimide groups rearrange to imides on heating above 300 °C. Better processability is also achieved by substituting hexafluoroisopropylidene-bis(phthalic anhydride) (6FDA) for BTDA, Thermid FA-700 [61-63]. 

AijAT-dicyclohexylcarhodiimide (DCC) also leads to essentially quantitative conversion of amic acids to isoimides, rather than imides (30,31). Combinations of trifluoroacetic anhydride—triethjlarnine and ethyl chi oroform a te—triethyl amine also result in high yields of isoimides (30). A kinetic study on model compounds has revealed that isoimides and imides are formed via a mixed anhydride intermediate (12) that is formed by the acylation of the carboxylic group of amic acid (8). 

Based on the rate data, it is possible to rank the relative importance of these reactions. Therefore, it follows that reactions 1 and 3 are of key importance, followed by reaction 2 and possibly reactions 4, 5 and 6. Unfortunately, little or no data exists for the latter set of reactions under the conditions employed for poly(amic acid) preparation. However, if isoimide formation were to occur to any significant degree, then reactions 5 and 6 are possible with reaction 6 being the more dominant pathway, since amine concentration due to end-groups or depolymerization would be quite low [21, 22],... 

Since the second solvent pair fall within the poor hydrogen bonding group of solvents, increased basicity of the organic base in these solvents would be consistent with the observed behavior. Based on the model compound studies, indications are that the base-catalyzed imidization process may involve a two-step mechanism, Jee Scheme 23. The first step corresponds to the complete or partial proton abstraction from the amide group with the formation of an iminolate anion. Since this iminolate anion has two possible tautomers, the reaction can proceed in a split reaction path to either an isoimide- or imide-type intermediate. Although isoimide model reactions indicate an extremely fast isomerization to the imide under the conditions employed for base-catalysis, all indications to date are that it is not an intermediate in the base-catalyzed imidization of amic alkyl esters. 

From all these XPS informations in order to take into account the fact that after the laser treatments several species like isoimide, C=N-,-CO,0-C-0, and carboxylic groups are present at the surface of polyimide, we propose the following surface reaction under UV laser radiation above the ablation threshold in presence of air moisture ... 

The high yield of carbon dioxide may be explained by the decomposition of non-cyclic amide links this occurs by hydrolysis of the imide ring into its isoimide isomer, with subsequent decomposition of the latter as well as by condensation of the NCO groups formed via radical decomposition of the imide ring [3, 18]. 

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