Utilization of Catechol End-Functionalized PMMA as a Macromolecular Coupling Agent for Ceramic/Fluoropolymer Piezoelectric Composites

An approach based on the use of a macromolecular coupling agent and the aim to improve the interfacial adhesion between piezoelectric ceramics and piezoelectric polymer matrix in piezoelectric composites is presented. Poly(methyl methacrylate) (PMMA) bearing a catechol moiety was used as a macromolecular coupling agent, as it is known to be miscible to piezoelectric fluoropolymers and catechol groups can strongly bind to a large variety of surfaces. Thus, entanglement between the PMMA chains and the amorphous segments of the fluoropolymer would ensure the desired interfacial adhesion. Well-defined PMMA was synthesized via RAFT polymerization using 2-cyano-2-propyl dodecyl trithiocarbonate as a chain-transfer agent. The PMMA omega-chain end was then functionalized with a catechol group via a one-pot aminolysis/thia-Michael addition procedure using a dopamine acrylamide (DA) derivative as a Michael acceptor. The presence of the catechol moiety at the chain end of PMMA was controlled by 1H NMR and cyclic voltammetry measurements. The resulting PMMA-DA was then grafted onto the surface of a lead-free piezoelectric ceramic film (i.e., a thin film of H2O2-activated (Bi0.5Na0.5)TiO3 (BNT) with a large contact area). The increase of the water contact angle confirmed the efficiency of the grafting. A commercial piezoelectric copolymer P(VDF-co-TrFE) was then spin-coated onto the modified BNT surface to form a bilayer composite. The composite cross section prepared by cryofracture was examined by scanning electron microscopy and revealed that the ceramic/polymer interface of the BNT-PMMA/P(VDF-co-TrFE) bilayer composite exhibits a much better cohesion than its counterpart composite prepared from nonmodified BNT. Moreover, the grazing incidence wide-angle X-ray scattering confirmed that the copolymer crystal structure was not impacted by the presence of the PMMA-DA coupling agent. A strong piezoelectric response was locally detected by piezoresponse force microscopy. This study highlights the potential of PMMA-DA as a macromolecular coupling agent to improve the ceramic/polymer interface in piezoelectric composite materials.