Cross-linking of starch with 1, 2, 3, 4-diepoxybutane or 1, 2, 7, 8-diepoxyoctane

Water-soluble and native potato starches were modified with 1, 2, 3, 4-diepoxybutane (DEB) or 1, 2, 7, 8-diepoxyoctane (DEO). Optimal reaction conditions and the effectiveness of the cross-linkers were determined on the basis of yields of water-soluble and insoluble material. The presence of the introduced groups was confirmed by NMR spectroscopy in solution as well as in solid state. According to multi-angle laser light-scattering method, the weight average molecular masses of the water-soluble fractions prepared from water-soluble starch were larger (Mw = 471 kDa using DEB; Mw = 931 kDa with DEO) in comparison with the starting material (Mw = 291 kDa). Due to high molecular mass only new anomeric signals were observed by 1H NMR in solution. Maximal yields of insoluble parts were 18 or 50% for DEB or DEO. On native starch the yields of insoluble products decreased with increasing concentration of cross-linker and NaOH in comparison to unmodified material. Solubilized components were dialyzed and we suppose that starch was degraded under these conditions. This material is not suitable for the modification when the goal is to increase the yield of insoluble part. With water-soluble starch the initial first order rate constants of cross-linking with E, DEB and DEO were determined with rheometer. From the relation of complex shear modulus and time by linear regression method at 20-70 °C, first-order rate constants of the same order of magnitude were calculated. The corresponding activation energies of the process are decreasing in the order: E (131 kJ/mol) >DEO (105 kJ/mol) >DEB (55 kJ/mol). We consider these results in agreement with the differences in the structure of the cross-linkers and the mechanism of cross-linking. Both cross-linkers gave spherical particles when cross-linked by the water-in-oil emulsion method with dimensions 60-610 (10% < = 192; 50% < = 294; 90% < = 425) micrometer, when cross-linked with DEB or 250-730 (10% < = 278; 50% < = 392; 90% < = 548) micrometer, when DEO was used.