Elaboration of novel biocomposite hydrogel polymers made of alginate and sepiolite and endowed with enhanced properties

Abstract

Nowadays growing attention is given to the design and development of novel interpenetrating polymer networks (IPN) from the combination of hydrogel polymers loaded with natural clay. In this work, we used the eco-friendly IPN strategy to develop novel hydrogel biocomposite beads, made of alginate (ALG), with improved clay dispersion, higher pH sensitivity, better stretchability and swellability, together with enhanced regenerability properties and delayed biodegradability. Fibrous clay, namely, sodium sepiolite (NaS), was loaded into alginate simple biocomposite network (SBN) beads, via manual co-grinding mixture/encapsulation method, at different sepiolite loads. Alginate double biocomposite network (DBN) beads were also prepared at different sepiolite loads, via the diffusion of acrylamide monomers (AAM) inside alginate single biocomposite network (SBN) beads, followed by external and in situ free radical polymerization of AAM into polyacrylamide (pAAM), using ammonium persulfate (APS) as polymerization initiator and N,N-methylenebisacrylamide (Bis) as covalent crosslinker agent. The as-elaborated SBN and DBN beads were then characterized by digital camera recording, XRD analysis, ATR-FTIR characterization and SEM observation. FTIR results showed that NaS and pAAM were successfully incorporated into DBN beads, while partially exfoliated morphology was obtained with XRD analysis, which revealed the enhancement of fibrous clay dispersion, even at relatively high sepiolite loads. Besides, SEM microscopy confirmed the porous spongious nature of DBN beads. The properties of the as-elaborated SBN and DBN beads were also evaluated by test touching, swelling rate measurements, adsorption/desorption experiments and biodegradability evaluation. DBN beads properties were always found enhanced in comparison with those of SBN beads. This suggests that the synergetic effect between IPN and biocomposite structure improves the stretchability of the DBN beads, their stability in water whatever the pH and their swelling behavior (up to 2000 g/g adsorbed water after 110 min, which was nevertheless found dependent on the pH and on sepiolite load). Moreover, DBN beads displayed enhanced adsorption/desorption properties toward methylene blue (MB) dye (no reduction in MB adsorption capacity after 5 cycles), very good regenerability (ethanol being used as effective eluting agent) and delayed biodegradability (complete degradation only after 8 days in the presence of sepiolite). In summary, this work showed an interesting and safe IPN/biocomposite approach to develop high-performance alginate biocomposite polymers as a promising system toward their use in eco-friendly processes.

Graphical abstract