Solid-State Nmr Studies of Molecular Composites and Thermally Cured Copolymers

Abstract

ABSTRACTProton spin diffusion experiments are conducted on two blends of nylon and poly(phenylenebenzobisthiazole) (PBZT). The blends are prepared different ways and utilize either a semicrystalline or fully amorphous nylon. The spin diffusion experiment, which was conducted using three different schemes, yields information about the shortest distances typifying the individual domains. Results suggest that in a 40/60 nylon 6,6/PBZT blend, the nylon and PBZT molecules do not mix on the molecular level, however, each domain is only about 4 nm in its minimum dimension. At the same time, some crystalline nylon regions also exist having somewhat larger dimensions. In contrast, a 50/50 amorphous nylon/PBZT sample, made via a different processing route, showed evidence for similar mixing in only a portion of the material; there was also strong evidence for compositional heterogeneity on a scale larger than 10 nm. The exact stoichiometries describing this compositional heterogeneity could not be uniquely identified on the basis of the spin diffusion studies alone; however the possibilities range from 55% of pure nylon to 35% of pure PBZT being sequestered from the more intimately mixed phase.In a second set of experiments, the 250°C thermal polymerization chemistry of a benzocyclobutene (BCB), a bismaleimide (BMI) and an equimolar mixture of BCB and BMI was studied. These materials are candidates for thermally polymerizable matrix materials in blends with rigid-rod polymers like PBZT. Polymerization of the BCB homopolymer is found to involve multiple pathways and there is reasonably strong evidence that the reactive carbons generated by thermal ring opening of the cyclobutene moiety not only attack one another; they also attack aromatic carbons adjacent to carbonyl carbons at sites far from the ring opening reaction. Spectra of the BCB homopolymer aged at 300°C in vacuum indicate that significant chemical changes occur. This result casts a shadow on the interpretation of the good apparent thermogravimetric stability of the BCB homopolymer in air at this temperature. The equimolar mixture of BCB and BMI ideally can polymerize via the Diels-Alder mechanism to produce a linear chain of alternating BCB and BMI repeat units. The spectrum of this thermally polymerized mixture suggests a maximum of 50% of the reaction following the Diels-Alder addition; the other 50% follows homopolymerization pathways.