Understanding the microstructure and absorption rate of starch-based superabsorbent polymers prepared under high starch concentration

From a microstructural view, the focus of this work was on the water absorption rate of starch-based superabsorbent polymers (starch-SAPs) prepared under high starch concentration (0.27:1 w/w starch:water). The effects of starch amylose/amylopectin ratio were disclosed. The increase in amylopectin reduced the amount (CPAM) of polyacrylamide (PAM) in starch-SAPs but increased the ratio of starch carbons grafted with PAM, which eventually decreased the average length (LPAM) of PAM chains. The shorter PAM chains could reduce starch-SAP chain flexibility, thus inducing larger mass fractal gels in swollen starch-SAPs. In general, the increases in CPAM and LPAM were preferable for a higher water absorbent capacity (WAC), whereas the denser fractal gels reduced WAC. Interestingly, all starch-SAPs had a dual-phase absorption process with the first stage showing a higher rate than the second phase (k1 > k2). The shorter PAM chains caused increases in k1 and k2.