Frontal polymerization synthesis of starch-grafted hydrogels: Effect of temperature and tube size on propagating front and properties of hydrogels

The frontal polymerization process was used to produce superabsorbent hydrogels based on acrylic acid monomers grafted onto starch. Using a simple test tube which was nonadiabatic and permitted contact with air, the effects of initial temperature and tube size on the propagating front of grafting copolymerization and the properties of hydrogels were explored. The unrestricted access of the reaction mixture to oxygen delayed the formation of self-propagating polymerization front. The ignition time was markedly lengthened with the increasing of tube size attributed to the formation of large amounts of peroxy radicals. The front velocity dependence on initial temperature could be fit to an Arrhenius function with the average apparent activation energy of 24 kJ mol(-1), and on tube size to a function of higher order. The increase of the initial temperature increased the front temperature, which lead to more soluble oligomers and higher degree of crosslinking. The interplay of two opposite effects of oligomer and crosslinking determined the sol and gel content. An increase in tube size had two effects on the propagating front. One was to reduce heat loss. The other effect was to increase the number of escaping gas bubbles. The combined action of the two effects resulted in a maximum value of front temperature, an increase in sol content and a reduction in gel content with tube size. The highest swelling capacity of hydrogels was obtained when the initial temperature or tube size favored a formation of porous microstructure of hydrogels.