Submitted to: Carbohydrate Polymers
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/31/2003
Publication Date: 1/29/2004
Citation: Fanta, G.F., Felker, F.C., Shogren, R.L. 2004. Graft polymerization of acrylonitrile onto spherocrystals formed from jet cooked cornstarch. Carbohydrate Polymers. v. 56. p. 77-84. Interpretive Summary: When hot, jet-cooked solutions of starch are allowed to slowly cool, two types of crystalline particles are formed. The smaller-sized particles are disc- or torus-shaped and often show spiral striations on the surface. The larger-sized particles are more spherical in shape and show rough surface textures. When the monomer acrylonitrile (which is used in the manufacture of Orlon) was allowed to polymerize onto these materials, graft copolymers containing about 60% grafted polyacrylonitrile (PAN) were formed. Graft copolymers were characterized with respect to the molecular weight of grafted PAN and the spacing of the PAN grafts along the starch backbone. The overall appearance of both types of crystalline particles was retained after graft polymerization, and the particles of PAN that remained after removal of starch by acid hydrolysis were also similar in appearance. When examined microscopically under polarized light, these polymer particles showed birefringence patterns that were similar to the ungrafted starting materials. This observation suggests that starch molecules act as templates for the growing PAN polymer. Knowledge related to graft polymerization onto these crystalline starch particles will be used in our efforts to find end-use applications for these materials.
Technical Abstract: Ceric ammonium nitrate-initiated graft polymerizations of acrylonitrile (AN) onto spherocrystals formed in slowly-cooled solutions of jet-cooked cornstarch yielded graft copolymers containing higher percentages of grafted polyacrylonitrile (PAN) than comparable polymers prepared from granular cornstarch. Weight percentages of PAN in grafted spherocrystals, prior to extraction with dimethylformamide to remove ungrafted PAN, were over 60% when 2.0 g of AN per gram of polysaccharide was used, and were about 55% with 1.5 g of AN. Copolymers prepared from the small (toroidal)- and large (spherical) spherocrystals contained similar amounts of grafted PAN. Molecular weights of PAN in grafted spherocrystals were higher by about a factor of six than the PAN molecular weight in grafted granular cornstarch. The calculated number of anhydroglucose units separating each PAN graft was also higher for the spherocrystal graft copolymers, indicating that the higher percentage of grafted PAN in the spherocrystal polymers (relative to grafted granular cornstarch) was due to the higher molecular weight of grafted PAN and not to a greater number of grafts on the starch backbone. Gross morphologies of the cornstarch granules and spherocrystals used as starting materials were maintained after graft polymerization. Moreover, the PAN particles remaining after starch was removed by acid hydrolysis were similar in appearance to their respective graft copolymers. PAN-grafted cornstarch granules and PAN-grafted spherocrystals both exhibited birefringence patterns similar to those of the un-grafted starting materials, although the patterns were less clear. Birefringence patterns of PAN particles remaining after removal of the starch moiety were also similar. X-ray diffraction patterns of PAN-grafted spherocrystals were similar to those of the un-grafted starting materials; however, the major amylose reflections of the 6 V-helical pattern occured at slightly higher angles, and the amylose peak intensities were smaller. When ethanol was used to wash the PAN-grafted large (spherical) particles, the 7 V-helical pattern was converted to the 6 V-helical pattern exhibited by the PAN-grafted small (toroidal) particles.