RESUMO
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.
RESUMO
Recently, a considerable amount of research has centered on uniquely structured polymers synthesized through self-propagating frontal polymerization. The obtained polymer materials have better features than those obtained by using the classical batch route. The additional advantages are short reaction times and low cost. This work describes the first frontal polymerization synthesis of a graft copolymer superabsorbent hydrogel of acrylic acid onto starch at high monomer and initiator concentration. The effects of varying the relative amounts of the reaction components on the most relevant parameters relating to frontal polymerization were explored. The front velocity dependence on initiator concentration could be fit to a power function. The temperature profiles were found to be very sharp with a maximum temperature below 150 degrees C, which was responsible for high monomer conversion. The ultimate properties of the product appear to depend on the polymerization front velocity and the temperature. The high-temperature and rapid temperature increase at the polymerization front led to products with interconnected porous structures caused by the evaporation of water. So, a fast-swelling, highly absorbing hydrogel with respect to batch polymerization was obtained.
