Rigid rod polymers with flexible side chains, 4Part 3: cf.3.. Synthesis, structure and phase behaviour of polyimides prepared from pyromellitic anhydride and 2,5-di-n-alkoxy-1,4-phenylene diisocyantes
Polymer and Materials
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
A series of rigid rod polyimides 1a-f derived from pyromellitic anhydride (PMDA) and 2,5-di-n-alkoxy-1,4-phenylenediamine containing flexible alkoxy side chains with 4 to 16 carbon atoms was prepared by solution polycondensation. As demonstrated by model experiments full conversion to the imide structure is achieved by reaction of PMDA with 2,5-dialkoxyphenylene diisocyanate, the latter compound being blocked by imidazole. The best solvent for the polycondensation, found in the course of the model reaction, is 1,3-dimethyl-2-oxo-perhydro-pyrimidine together with 4-dimethylaminopyridine as an accelerator. Wide-angle X-ray diffractograms showed that all polyimides under consideration here are highly crystalline at room temperature. The occurrence of a sharp reflection at the region of low angles together with its higher orders demonstrates the presence of a layered structure. The layer spacing was found to increase linearly with the length of the alkyl side chains. Polyimides bearing long side chains (n ≥ 10) exhibit two broad endotherms in the DSC analysis. As is revealed by wide-angle X-ray analysis the first transition is related to a disordering process of the side chains. The second transition leads to a layered high-temperature phase akin to the modification (B) found in a recent study of a similar rigid polyester bearing flexible side chains.
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