A thermally responsive, rigid, and reversible adhesive

In this paper we present the development of a unique self-adhesive material that, unlike conventional adhesives, maintains a high degree of rigidity at the "adhesive" state while possessing the ability to easily de-bond upon heating. Consequently, the material is both a rigid and a reversible adhesive. The material is an initially miscible blend of poly(ε-caprolactone) (PCL) and diglycidyl ether of bisphenol-A/diaminodiphenylsulfone (DGEBA/DDS) epoxy, processed to a unique morphology via polymerization-induced phase separation (PIPS). The fully cured material features a biphasic, "bricks-and-mortar" morphology in which epoxy forms highly interconnected spheres ("bricks") that interpenetrate with a continuous PCL matrix ("mortar"). When heated to melt the PCL phase (60 °C < T < 200 °C), the epoxy bricks remain rigid due to the high epoxy T_g (>200 °C) while PCL liquefies to become a melt adhesive. Moreover, the PCL liquid undergoes microscopic dilational flow to wet the sample surfaces due to its high volumetric expansion in excess to epoxy bricks expansion, a phenomenon we term "differential expansive bleeding" (DEB). Remarkably, the samples remain rigid at this state and their surfaces become covered by a thin layer of PCL now able to wet, and subsequently bond through cooling, to a variety of substrates. We observe high bonding strengths, which we attribute to a combination of good wetting and subsequent formation of a thin layer of crystalline PCL with high cohesive strength upon cooling. This adhesive layer can be melted again by heating (T > T_m) to easily de-bond and subsequent rebonding capacity was demonstrated, indicating repeated availability of PCL melt adhesive to the surface by the DEB mechanism.