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.

Poly(methyl methacrylate) (PMMA)/carbon black (CB) composites were fabricated using two different mixing methods: (1) mechanical mixing and (2) solution mixing of the precursors, followed by compression molding. The microstructures obtained were examined by optical and scanning electron microscopy. Electrical properties were measured using impedance spectroscopy over a wide frequency range (10(-3) to +10(9) Hz). With the mechanical mixing method, a segregated structure is produced with PMMA particles forming faceted grains with carbon black particles aligning to form a network of 3D-interconnected nanowires. This microstructure allows percolation to occur at a low volume fraction of 0.26 vol % CB. In contrast, specimens made by the solution method have a microstructure where carbon black is distributed more randomly throughout the bulk, and thus, the percolation threshold is higher (2.7 vol % CB). The electrical properties of the PMMA/CB composites fabricated by the mechanical mixing method are comparable to those obtained with single-wall nanotubes as fillers.

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The authors have used mechanical self-assembly of carbon-black nanoparticles to fabricate a three dimensional, electrically connected micronetwork of nanowires embedded within an insulating, supporting matrix of poly(methyl methacrylate). The electrical connectivity, mean wire diameter, and morphological transitions were characterized as a function of the carbon-black mass fraction. Conductive wires were produced with mean diameters as low as 24nm with lengths up to 100μm.

Composite specimens possessing polyhedral segregated network microstructures require a very small amount of nanosize filler, <1 vol %, to reach percolation because percolation occurs by accumulation of the fillers along the edges of the deformed polymer matrix particles. In this paper, electrostatic force microscopy (EFM) and conductive atomic force microscopy (C-AFM) were used to confirm the location of the nanosize fillers and the corresponding percolating paths in polymethyl methacrylate/carbon black composites. The EFM and C-AFM images revealed that the polyhedral polymer particles were coated with filler, primarily on the edges as predicted by the geometric models provided.