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

Abstract

This paper reports the solvent-free synthesis of a series of six fluoride responsive debond-ondemand 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. 1 H 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 64-90 % 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 minutes 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.

A fluoride degradable crosslinker for debond-on-demand polyurethane based crosslinked adhesives

“…In our previous work, [38,39] the hydrogenated polybutadiene (HPB) based PU showed butt-tensile adhesion (ca. 7 MPa) at 140 °C.…”

“…We have recently reported the synthesis of fluoride degradable linear PU based adhesives. [38,39] The uncrosslinked, linear structure of these materials results in adhesives that can be thermally rebonded through multiple cycles without loss of adhesive qualities, but that also degrade on the addition of fluoride ions which results in a permanent reduction in adhesive performance. The materials are comprised of diisocyanate linkers with hydrogenated polybutadienes or polyester soft segments.…”

Triggerable Super Absorbent Polymers for Coating Debonding Applications

“…Especially regarding plastics and textiles, several additives can give them new life by aiding with the debonding of coatings that are difficult to recycle [ 18 ]. Additionally, by modifying certain properties of the polymeric coatings, such as crystallinity and hydrogen bonding, these adhesives can help optimize adhesion strength at low bonding temperatures [ 19 ]. Core–shell structures can be fabricated using a wide variety of methods, such as electrospinning (Shao, et al) [ 20 ], electrospraying (Wang, et al) [ 21 ], molecular self-assembly (Simonova, et al) [ 22 ], and the usage of micelles as templates (Kiss, et al) [ 23 ].…”

Supramolecular oligourethane gel as a highly selective fluorescent “on–off–on” sensor for ions

“…33 (ii) Inserting linear 1,6-diisocyanatohexane offers strong van der Waals interactions among alkyl chains, limiting internal rotation of the molecular chains, thereby blocking the non-radiative pathways and favoring AIE. Taking advantage of the rich hydrogen bond acceptors/donors (CQO/N-H) among the oligourethane skeleton, [34][35][36][37][38][39][40][41] we introduced solvents with hydrogen-bonding acceptor units (CQO or SQO) as external crosslinking agents, to self-assemble a supramolecular oligourethane gel (OUG) relying on multiple hydrogen bonds.…”