Design and optimization of superabsorbent hydrogels based on acrylic acid/2‐acrylamido‐2‐methylpropane sulfonic acid copolymers

Abstract

Although acrylic acid-based superabsorbent hydrogels (SAHs) are widely used in hygiene and personal care applications, the low degrees of true ionization in such SAHs under practical application places a limit on the degree of superabsorbency that can be achieved. Herein, the preparation and optimization of SAHs based on copolymers of acrylic acid (AA) with 2-acrylamido-2-methylpropane sulfonic acid (AMPS), a strong acid comonomer that remains ionized at all relevant physiological pH values, is described. AA-AMPS recipes optimized using a central composite design response surface method demonstrate significantly enhanced absorbance under load (AUL) values (>15-20% increase) and comparable centrifuge retention capacity (CRC) values compared to the corresponding AA-only controls while maintaining strong mechanics even in the swollen state (as much as twice the modulus of the commercial AA-based superabsorbent). The simplicity of this recipe, coupled with the reproducible achievement of the improved superabsorbency properties using a small adaptation of the commercial SAH synthesis protocol, makes AA-AMPS superabsorbent hydrogels potential candidates for use in commercial hygiene products.

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

“…Given that the sulfate monomer residues will retain a net charge at all relevant pH conditions at which the SAH will be used and are thus not subject to the polyelectrolyte effect like the AA monomer residues, we hypothesize that limited sulfate monomer block formation is likely beneficial for enhancing swelling responses in SAHs, at least provided that the formation of such blocks does not substantially increase the length of the AA blocks and thus effectively undo the polyelectrolyte effect-associated ionization benefits of incorporating a comonomer to interrupt AA blocks in an AA-based superabsorbent material. However, consistent with our previous work with AA-AMPS copolymers, a trade-off was observed between AUL and CRC, with higher AUL samples showing lower CRC values. As such, while our previous AA-AMPS work would lead us to expect that targeted optimization of the SPAK and SPMK copolymers will lead to formulations with still high AUL values but significantly higher CRC values than the 15 mol % formulations reported herein, the benefit of using one sulfated comonomer over another may depend on whether high AUL or CRC is more important in a given application.…”

“…To assess how these different monomer chain distributions influence the swelling properties of the corresponding lightly cross-linked superabsorbent hydrogels, the absorbance under load (AUL, using a 160 g mass and a 1 h observation time) and the centrifugation retention capacity (CRC) were measured for both an acrylic acid control SAH and copolymer SAHs formed based on copolymerization of AA with either AMPS, SPAK, or SPMK. The comonomer ratio and cross-linker content found to provide optimal swelling responses in AA-AMPS copolymer SAHs were used for all copolymerizations (i.e., 87.5 mol % AA/12.5 mol % sulfated comonomer and 0.28 mol % MBAAm, a small cross-linker loading unlikely to significantly alter the copolymer structures elucidated in the absence of cross-linker) Table shows these results together with the average blockiness parameter (i.e., a weight-averaged value of the instantaneous blockiness parameters in Figure derived by calculating the average blockiness value over each 0.01 conversion fraction and averaging those values over the full conversion range) and the maximum blockiness parameter predicted for each copolymer (corresponding to v AA at 99% total monomer conversion).…”

“…Sulfated comonomers are the most widely reported comonomers used for this purpose, with acrylamido-2-methyl­propane­sulfonic acid (AMPS) most commonly demonstrated in the literature. Multiple SAHs have been prepared using AMPS as a comonomer with acrylic acid − (including recent work by our group on simple AA-AMPS copolymers) as one of multiple comonomers with AA (e.g., including acrylamide) or as the lead monomer copolymerized with other functional monomers (e.g., vinyl sulfate, zwitterionic comonomers, etc.) Other sulfated comonomers such as 3-sulfopropyl acrylate (SPAK) and 3-sulfopropyl methacrylate (SPMK) have also been reported as comonomer components in acrylic acid-based SAHs, often in combination with neutral hydrophilic comonomers to take advantage of both swelling enhancement mechanisms available. − Nanocomposite SAHs containing sulfated fillers such as clay have also been demonstrated to enhance swelling through a similar mechanism while also mechanically reinforcing the SAH, , although such improvements come with a trade-off of higher density SAHs that may be undesirable in some applications.…”

“…The comonomer ratio and cross-linker content found to provide optimal swelling responses in AA-AMPS copolymer SAHs were used for all copolymerizations (i.e., 87.5 mol % AA/12.5 mol % sulfated comonomer and 0.28 mol % MBAAm, a small cross-linker loading unlikely to significantly alter the copolymer structures elucidated in the absence of cross-linker). 26 Table 1 shows these results together with the average blockiness parameter (i.e., a weight-averaged value of the instantaneous blockiness parameters in Figure 8 derived by calculating the average blockiness value over each 0.01 conversion fraction and averaging those values over the full conversion range) and the maximum blockiness parameter predicted for each copolymer (corresponding to v AA at 99% total monomer conversion).…”

“…The instantaneous blockiness parameters v for both acrylic acid and the sulfate comonomer in each copolymer as a function of overall monomer conversion were then calculated using eq 7, the results of which are shown in Figure 8. An 87.5% AA/12.5% sulfate comonomer ratio was used for the calculation, a recipe shown in our previous paper to provide the best swelling performance in the AA-AMPS hydrogel relative to the AA control (i.e., highest and most consistent increase in AUL, similar in CRC); 26 this recipe will be also used as the basis for the superabsorbent hydrogel synthesis in the next section to enable direct comparisons.…”