Performance improvement of polybenzoxazine by alloying with polyimide: effect of preparation method on the properties

Polymer alloys of polyimide and polybenzoxazine were prepared from the combination of a bifunctional benzoxazine monomer, 6,6′-(1-methylethylidene)bis(3,4-dihydro-3-2H-1,3-benzoxazine) (B-a) and a soluble polyimide (PI) or its precursor, poly(amide acid) (PAA), that was synthesized from oxydianiline (ODA) and bisphenol A di(phthalic anhydride) ether (BPADA). It was observed from DSC that the onset temperature for the curing exotherm of B-a in the blend with PAA was remarkably lowered compared with that of pristine B-a, while the curing temperature of B-a in the blend with PI was almost the same as that of pristine B-a. The cast films of PI/B-a or PAA/B-a were thermally treated at 150, 200 and 240 °C for 2 h each, affording deep wine, transparent films. The PI component in the polymer alloy films from PI/B-a dissolved thoroughly. On the other hand, the polymer alloy films from PAA/B-a were not soluble at all in NMP. The polymer alloy films from both PI/B-a and PAA/B-a showed only one glass transition temperature (T_g) from their viscoelastic analyses. The T_g values remarkably increased as the content of PI increased. The thermal stabilities of both films from PI/B-a and PAA/B-a increased as the PI component increased in the similar manner.