A Thermoplastic/Thermoset Blend Exhibiting Thermal Mending and Reversible Adhesion

Abstract

In this paper, we report on the development of a new and broadly applicable strategy to produce thermally mendable polymeric materials, demonstrated with an epoxy/poly(-caprolactone) (PCL) phase-separated blend. The initially miscible blend composed of 15.5 wt % PCL undergoes polymerization-induced phase separation during cross-linking of the epoxy, yielding a "bricks and mortar" morphology wherein the epoxy phase exists as interconnected spheres (bricks) interpenetrated with a percolating PCL matrix (mortar). The fully cured material is stiff, strong, and durable. A heating-induced "bleeding" behavior was witnessed in the form of spontaneous wetting of all free surfaces by the molten PCL phase, and this bleeding is capable of repairing damage by crack-wicking and subsequent recrystallization with only minor concomitant softening during that process. The observed bleeding is attributed to volumetric thermal expansion of PCL above its melting point in excess of epoxy brick expansion, which we term differential expansive bleeding (DEB). In controlled thermal-mending experiments, heating of a cracked specimen led to PCL extrusion from the bulk to yield a liquid layer bridging the crack gap. Upon cooling, a "scar" composed of PCL crystals formed at the site of the crack, restoring a significant portion of the mechanical strength. When a moderate force was applied to assist crack closure, thermal-mending efficiencies exceeded 100%. We further observed that the DEB phenomenon enables strong and facile adhesion of the same material to itself and to a variety of materials, without any requirement for macroscopic softening or flow.

Self-healing properties of epoxy resins with poly(ε-caprolactone) healing agent

“…It is noteworthy that PCL incorporation in EP does not necessarily reduce the Tg. Luo et al (10), working with a different EP system (especially the hardener differed from ours) did not observe any change in the Tg of the EP/PCL blends.…”

“…To consider the force ratio instead of computing the fracture toughness is widely used in the literature (10,13). Results of the healing efficiency are tabulated in Table 2.…”

“…Attempts were also made to control its reaction-induced phase separation through further additives, such as organoclay (9). Thermal mending of EP via thermoplastic PCL was first explored by Luo et al [10]. The cited authors modified an aromatic amine cured diglycidyl ether of bisphenol A-based EP (DGEBA) with PCL, added up to 34.9 wt.%.…”

“…combination of shape memory and self-healing, was proved by the group of Leng on the examples of EP/PCL blends (with up to 23.3. wt.% PCL content) (13) and with EP infiltrated electrospun PCL web (14 wt.%) specimens (14) very recently. It has to be underlined that in all works, performed on studying the self-healing of EP/PCL systems (10,(13)(14), the specimens were always only partly broken.…”

Self-healing polymers and composites

“…In most reports of microcapsule based self-healing mechanisms, increasing loadings of liquid filled capsules significantly toughens the composite matrix, relative to 29,226 Epoxy resin filled capsules/solid imidazole catalyst 37 Epoxy resin/mercaptan filled capsules 43 Phase separated pEMAA particles 117 DCPD filled capsules/solid, wax encased Tungsten catalyst 85 Shape memory alloy wires 79,80 Epoxy resin filled capsules/matrix dissolved imidazole catalyst 38,40,41 Matrix dissolved thermoplastic polymer 155 Strength Epoxy resin-filled capsules/matrix dissolved imidazole catalyst (CAI) 42 PDMS containing capsules (tear) 46 Epoxy resin filled hollow fibres: large pitch spacing (flexural) [133][134][135] Epoxy resin filled microvascular network (flexural) 153 Epoxy resin filled hollow fibres: small pitch spacing (flexural) [132][133][134] Epoxy resin/mercaptan filled capsules (flexural, tensile) Phase separated Epoxy polymer particles (tension) 115,116 Phase separated pEMAA particles (flexural) 117 Phase separated poly(caprolactone) (storage) 119 …”