Synthesis of composite superabsorbents with the porous structure
Abstract
Superabsorbent polymers (SAPs) are a class of hydrogels capable of retaining water in amounts hundreds to thousands of times their own weight. Currently, the most widely used SAPs are synthetic, cross-linked polymers derived from acrylate monomers. However, the large-scale introduction of these materials into the industry imposes a significant anthropogenic burden on the environment, which is a key limiting factor for their broader application, particularly in agriculture.
A promising approach to addressing this issue is the development of composite superabsorbents – hydrogels that incorporate both acrylate and biodegradable components, most commonly polysaccharides. These materials exhibit greater environmental compatibility compared to fully synthetic counterparts. However, a major challenge in composite SAP chemistry is the inverse correlation between the swelling degree and the content of the polysaccharide component: the higher the proportion of biodegradable links – and thus the greater the ecofriendliness – the lower the water absorption capacity of the material.
This study explores one potential solution to this problem: enhancing the swelling capacity of a composite SAP based on sodium carboxymethyl cellulose by introducing a porous structure into the polymer network. This was achieved through the incorporation of a porogen, ammonium carbonate, directly into the reaction mixture during synthesis. The formation of the porous structure induced by the porogen was confirmed via scanning electron microscopy. The introduction of 5 wt.% of the pore-forming agent resulted in the formation of relatively uniformly distributed pores with diameters of 10–15 μm.
Equilibrium swelling measurements demonstrated that the introduction of porosity increased the swelling capacity by 26% compared to the non-porous control sample. Kinetic analysis of swelling behavior using mathematical models revealed that water absorption in the SAP occurs not only on the surface but also within the bulk of the polymer, involving anomalous non-Fickian diffusion. This behavior was observed for both tested samples, although hydration proceeded slightly faster in the porous SAP.
In summary, this study demonstrates that the use of porogens to create porous structures in composite superabsorbents is an effective strategy for enhancing their equilibrium swelling performance.
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