Characterization of Ban Copolymers

Molar mass and compositional distributions in copolymers of butadiene and acrylonitrile have been analyzed by gel permeation chromatography with multiple detection. In some cases, a third component could not be separated by size-exclusion chromatography and was presumed to be an antioxidant attached to the copolymer chains. The amount of this antioxidant was measured by ultraviolet detection at 280 nm of chromatographic effluents and found to increase in amount with decreasing molar volume. Molar mass and compositional distributions were derived from corrected ultraviolet responses at 254 nm coupled with refractive index detection of BAN solutions separated by GPC. The variation of composition with molecular size depends on the composition of the monomer feed and, presumably, the conversion. For copolymers close the azeotropic composition (36% ACN), the composition was fairly uniform over the entire molecular weight distribution and showed a single Tg by differential scanning calorimetry. Some of the materials showed gel or insoluble fractions which had to be separated from the soluble moieties prior to GPC. Copolymers of composition slightly above azeotropic showed a single Tg but could be separated into sol and gel fractions which differed in composition. Infrared and calorimetric analysis showed that, in these cases, the gel was enriched in acrylonitrile. A sample containing 20% acrylonitrile showed marked deviations from compositional uniformity, and the high molecular weight components were enriched in butadiene. Infrared and calorimetric analysis of the sol and gel fractions showed that they were enriched in acrylonitrile and butadiene, respectively. In fact, the elastomer evidenced two glass transition temperatures. We suggest that gel permeation chromatography with multiple detection, specifically refractive index and ultraviolet absorption at 254 and at 280 nm, permits a fairly complete characterization of BAN elastomers. With computer analysis, a sophisticated raw material quality control is feasible and simple. We plan to reexamine our assumptions using laboratory synthesized BAN copolymers. Moreover, we hope to relate the physical properties of elastomers and rubber compounds with the molecular characterization of BAN copolymers. As an alternative to ultraviolet detection in GPC, it may be possible to get a more complete functional group analysis by infrared spectroscopy.