Fluoride-responsive debond on demand adhesives: manipulating polymer crystallinity and hydrogen bonding to optimise adhesion strength at low bonding temperatures

This paper reports the solvent-free synthesis of a series of six fluoride responsive debond-on-demand polyurethane (PU) adhesives that contain a silyl functionalised degradable unit (DU). To optimise the adhesion strength and debonding nature of the adhesives, the chemical composition of the PUs was varied according to the structure of the polyol or the diisocyanate component in the polymer mainchain. 1H NMR spectroscopy was used to study the depolymerisation behaviour in solution state. It showed that tetra-butylammonium fluoride (TBAF) triggered the breakdown of the DU unit without fragmenting the polyol mainchain indiscriminately. On exposure to fluoride ions, the PUs underwent depolymerisation with reductions in Mn ranging from 64 to 90% as measured by GPC analysis. The morphology and thermal properties of the PUs were characterised by differential scanning calorimetry (DSC), rheology and variable temperature (VT) SAXS/WAXS analysis. Each technique demonstrated the reversibility of the supramolecular polymer network under thermal stimuli. PUs containing poly(butadiene) soft segments were amorphous with glass transition and viscoelastic transition temperatures dependent on the nature of the soft segment and diisocyanate starting materials. The PU containing a polyester soft segment exhibited a defined melting point at 49 °C. Mechanical stress-strain analysis of the series of PUs showed each exhibited greater than 70% reduction in toughness after treatment with TBAF for 30 min as a consequence of the chemo-responsive degradation of the polymer mainchain. The material featuring an ester-based polyol demonstrated excellent adhesion at bonding temperatures as low as 60 °C. Moreover, this material could be thermally rebonded if broken by force without loss in adhesion strength over three debond-rebond cycles. Lap shear adhesion tests showed a reduction in adhesive strength of approximately 40% (from 11.4 MPa to 7.3 MPa) on exposure to fluoride ions.