Fluoride-responsive debond on demand adhesives: manipulating polymercrystallinity and hydrogen bonding to optimise adhesion strength at lowbonding temperatures

This paper reports the solvent-free synthesis of a series of sixfluoride responsive debond-on-demand poly-urethane (PU) adhesives that contain a silyl functionalised degradable unit (DU). To optimise the adhesionstrength and debonding nature of the adhesives, the chemical composition of the PUs was varied according tothe structure of the polyol or the diisocyanate component in the polymer mainchain.1H NMR spectroscopy wasused to study the depolymerisation behaviour in solution state. It showed thattetra-butylammoniumfluoride(TBAF) triggered the breakdown of the DU unit without fragmenting the polyol mainchain indiscriminately. Onexposure tofluoride ions, the PUs underwent depolymerisation with reductions in Mnranging from 64 to 90% asmeasured by GPC analysis. The morphology and thermal properties of the PUs were characterised by differentialscanning calorimetry (DSC), rheology and variable temperature (VT) SAXS/WAXS analysis. Each techniquedemonstrated the reversibility of the supramolecular polymer network under thermal stimuli. PUs containingpoly(butadiene) soft segments were amorphous with glass transition and viscoelastic transition temperaturesdependent on the nature of the soft segment and diisocyanate starting materials. The PU containing a polyestersoft segment exhibited a defined melting point at 49 °C. Mechanical stress-strain analysis of the series of PUsshowed each exhibited greater than 70% reduction in toughness after treatment with TBAF for 30 min as aconsequence 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 materialcould be thermally rebonded if broken by force without loss in adhesion strength over three debond-rebondcycles. Lap shear adhesion tests showed a reduction in adhesive strength of approximately 40% (from 11.4 MPato 7.3 MPa) on exposure tofluoride ions