Investigation of C–N Formation Between Catechols and Chitosan for the Formation of a Strong, Novel Adhesive Mimicking Mussel Adhesion

Abstract

Abstract: Mussel-inspired catechol-containing polymers provide a promising basis for developing strong biogenic adhesives. In order to develop such an adhesive, chitosan was functionalized with protocatechuic acid (PCA) via a Michael-type addition. Kinetic models for the C–N bond formation between primary amines and catechols via Michael-type addition suggest that the reaction is promoted by a low pKa value of the primary amine used and a neutral or mildly acidic reaction pH. Since the pKa of chitosan is below 7, spontaneous reactions with catechols were expected, as was confirmed using FT-IR and UV–Vis spectroscopy where spontaneous reactions occurred at pH 5 and 6.5. FT-IR spectroscopy further validated the kinetic model by showing that the spontaneous reaction between chitosan and PCA did not occur at a pH of 1. While laccases accelerated the C–N bond formation between chitosan and catechols, the formation of either PCA-chitosan agglomerates or DHC-chitosan hydrogels was promoted. For a PCA-functionalized chitosan an adhesion strength of 4.56 ± 0.54 MPa (on sand blasted aluminum surfaces) was achieved. Since the functionalization and curing process uses naturally occurring substances exclusively and is free of toxic chemicals, a novel and sustainable bio-adhesive was developed. Graphic Abstract: [Figure not available: see fulltext.].