Application of polymer blending laws to composite gels of agarose and crosslinked waxy maize starch

Abstract

Composites comprising swollen granules of highly crosslinked waxy maize starch in an agarose gel matrix have been prepared in two ways: (i) by gelatinising the starch in agarose solution at 80 °C and then cooling to form the gel network, or (ii) by cooling to gel the agarose and then heating to gelatinise the starch. In the first procedure, phase volumes were derived by using centrifugation to obtain a sample of the agarose phase prior to gelation, and determining the increase in polymer concentration due to swelling of the starch granules. The storage modulus ( G') of the agarose phase was obtained by gelling the supernatant from centrifugation, under identical time-temperature conditions to those used for the unseparated composite, and the value of G' for the swollen granules was derived by the ‘model independent’ method of varying polymer concentration to determine compositions at which both phases (and hence the overall composite) have the same modulus. Measured values of G' for the composite gels agreed, to within experimental error, with values calculated by application of the Takayanagi blending laws, with no adjustable parameters, using the isostress model for systems in which the continuous matrix was weaker than the dispersed particles and the isostrain model for the converse situation (weak filler in strong matrix). For composites formed by the second procedure, the increase in modulus of the continuous phase due to swelling of starch granules within the agarose gel network was calculated by classic swelling/deswelling theory, and the resulting values of G' obtained by application of the polymer blending laws were again in close agreement with those observed experimentally.