pH-Responsive, dynamically restructuring hydrogels formed by reversible crosslinking of PVA with phenylboronic acid functionalised PPO-PEO-PPO spacers (Jeffamines)

Dynamically restructuring (“self-healing”) hydrogels were prepared by reversible formation of boronic-ester crosslinks between α,ω-phenylboronic acid terminated PPO–PEO–PPO spacers and poly(vinyl alcohol) (PVA). For this purpose two different bis-(phenylboronic acid) functionalised crosslinkers with Jeff–2AMPBA and Jeff–4AMPBA were synthesised via reductive coupling of 2- and 4-formylphenylboronic acid to Jeffamine® ED1900. Due to the different positions of the secondary amino group with respect to the boronic acid moiety (ortho or para) the pKa of the amino and boronic acid groups at the spacer endgroups differ significantly, thereby influencing the pH-dependency of boronic ester formation and gel properties. Upon mixing the boronic acid spacers with PVA (72–195 kDa) in the pH range 3–9 hydrogels were immediately formed. The gel strength and relaxation time of Jeff–2AMPBA gels increase with increasing acidity of the solution whereas those of Jeff–4AMPBA increase with increasing basicity. The pH-tunable properties could be explained in terms of pH effects on intra-molecular and inter-molecular B–N interactions in the Jeff–2AMPBA gels and Jeff–4AMPBA gels, respectively.