Applying Copolymerization Kinetics to Understand and Optimize Swelling Responses in Superabsorbent Hydrogels

While acrylic acid (AA)-based superabsorbent hydrogels(SAHs) havebeen widely used in multiple applications, the effects of counterioncondensation and the polyelectrolyte effect at suppressing the effectivedegree of ionization in such systems limit their superabsorbency.Herein we describe the use, and investigate the mechanism, of sulfatecomonomers containing different types of polymerizable functionalgroups for increasing the superabsorbency of acrylic acid-based SAHs.Specifically, acrylamido-2-methylpropane-sulfonic acid (AMPS),3-sulfopropyl acrylate (SPAK), and 3-sulfopropyl methacrylate (SPMK)sulfated monomers featuring similar distances between the polymerbackbone and the sulfate group but different polymerizable groupswere copolymerized with acrylic acid to fabricate SAHs. Measurementsof the effective homopolymerization rate constants and copolymerizationratios associated with each copolymerization enabled the predictionof the relative chain distributions of monomers in each copolymer,as quantified by the blockiness parameter (i.e., the instantaneousor average number of consecutive monomers of each type polymerized).While all sulfated comonomers significantly enhanced swelling relativeto an acrylic acid control, copolymers in which longer AA blocks areproduced (AA-AMPS) resulted in lower swelling than copolymers in whichshorter AA blocks are produced (AA-SPAK, AA-SPMK), a result attributedto the suppressed polyelectrolyte effect in copolymers with shorterAA blocks. The formation of limited length blocks of the sulfatedmonomer SPMK showed additional benefits for enhancing swelling, consistentwith enhanced direct charge-charge repulsion between fixedcharges in such copolymer systems. We anticipate this linkage betweencopolymerization kinetics and swelling properties offers the potentialto enable improved rational design of superabsorbent hydrogels withhigher sorbency.