Monolithic nanocomposite hydrogels with fast dual T- and pH- stimuli responsiveness combined with high mechanical properties

Abstract

Monolithic (non-porous) nanocomposite hydrogels were synthesized, with double and ultra-fast stimuli-responsiveness to temperature (T) and to pH. Such a fast stimulus-response is exceptional in case of bulk gels, and the prepared hydrogels additionally possess extraordinarily high tensile, mechanical, and self–recovery properties. The presented materials might be of interest as actuators, drug- or reagent-release systems. They are based on poly(N-isopropylacrylamide) (PNIPAm) physically crosslinked by clay nanoplatelets. This crosslinking lends the gels their unique mechanical properties, PNIPAm a distinct T responsiveness, and the sodium methacrylate (SMA) co-monomer (just 1.5 mol%) a strong pH-responsiveness. The gels display a quasi-one-way and complex response to T- and pH-stimuli: very fast deswelling and slow re-swelling. Extremely fast, as for monolithic gels, is the T induced deswelling at pH2: ca. 10 min are needed for the whole process. This rapid kinetics is attributed to micro-phase-separation which does not destroy mechanical properties. Re-swelling of the gels is always slow: more than 2 days are needed for 35% completion of the process in case of gels shrunken in the neutral bath. In case of gels shrunken in acidic conditions, simple re-swelling is practically blocked (due to the micro-phase-separation): both cooling and pH reversal is needed for ‘un-blocking’ and re-swelling.