Influence of the Surface Chemistry of Graphene Oxide on the Structure–Property Relationship of Waterborne Poly(urethane urea) Adhesive

Abstract

Small amounts—0.04 wt.%—graphene oxide derivatives with different surface chemistry (graphene oxide—GO-, amine-functionalized GO—A-GO-, reduced GO—r-GO) were added during prepolymer formation in the synthesis of waterborne poly(urethane urea) dispersions (PUDs). Covalent interactions between the surface groups on the graphene oxide derivatives and the end NCO groups of the prepolymer were created, these interactions differently altered the degree of micro-phase separation of the PUDs and their structure–properties relationships. The amine functional groups on the A-GO surface reacted preferentially with the prepolymer, producing new urea hard domains and higher percentage of soft segments than in the PUD without GO derivative. All GO derivatives were well dispersed into the PU matrix. The PUD without GO derivative showed the most noticeable shear thinning and the addition of the GO derivative reduced the extent of shear thinning differently depending on its functional chemistry. The free urethane groups were dominant in all PUs and the addition of the GO derivative increased the percentage of the associated by hydrogen bond urethane groups. As a consequence, the addition of GO derivative caused a lower degree of micro-phase separation. All PUs containing GO derivatives exhibited an additional thermal decomposition at 190–206 °C which was ascribed to the GO derivative-poly(urethane urea) interactions, the lowest temperature corresponded to PU+A-GO. The PUs exhibited two structural relaxations, their temperatures decreased by adding the GO derivative, and the values of the maximum of tan delta in PU+r-GO and PU+A-GO were significantly higher than in the rest. The addition of the GO derivative increased the elongation-at-break, imparted some toughening, and increased the adhesion of the PUD. The highest T-peel strength values corresponded to the joints made with PUD+GO and PUD+r-GO, and a rupture of the substrate was obtained.