Bioinspired Polydopamine Coating as an Adhesion Enhancer Between Paraffin Microcapsules and an Epoxy Matrix

Abstract

Microencapsulated phase change materials (PCMs) are attracting increasing attention as functional fillers in polymer matrices, to produce smart thermoregulating composites for applications in thermal energy storage (TES) and thermal management. In a polymer composite, the filler–matrix interfacial adhesion plays a fundamental role in the thermomechanical properties. Hence, this work aims to modify the surface of commercial PCM microcapsules through the formation of a layer of polydopamine (PDA), a bioinspired polymer that is emerging as a powerful tool to functionalize chemically inert surfaces due to its versatility and great adhesive potential in many different materials. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) evidenced that after PDA coating, the surface roughness increased from 9 to 86 nm, which is beneficial, as it allows a further increase in the interfacial interaction by mechanical interlocking. Spectroscopic techniques allowed investigating the surface chemistry and identifying reactive functional groups of the PDA layer and highlighted that, unlike the uncoated microcapsules, the PDA layer is able to react with oxirane groups, thereby forming a covalent bond with the epoxy matrix. Hot-stage optical microscopy and differential scanning calorimetry (DSC) highlighted that the PDA modification does not hinder the melting/crystallization process of the paraffinic core. Finally, SEM micrographs of the cryofracture surface of epoxy composites containing neat or PDA-modified microcapsules clearly evidenced improved adhesion between the capsule shell and the epoxy matrix. These results showed that PDA is a suitable coating material with considerable potential for increasing the interfacial adhesion between an epoxy matrix and polymer microcapsules with low surface reactivity. This is remarkably important not only for this specific application but also for other classes of composite materials. Future studies will investigate how the deposition parameters affect the morphology, roughness, and thickness of the PDA layer and how the layer properties influence the capsule–matrix adhesion.

Polydopamine-Coated Paraffin Microcapsules as a Multifunctional Filler Enhancing Thermal and Mechanical Performance of a Flexible Epoxy Resin

“…The coating of the microcapsules with a PDA layer was carried out in a manner similar to the one described in a previous paper by our group [52] and briefly reported here. 1.0 g dopamine hydrochloride was added to 600 mL of TRIS solution (0.1 mol/L) and dissolved in an ultrasonic bath at room temperature.…”

“…PDA has been successfully deposited onto reinforcing fibers with chemically inert surfaces, such as ultrahigh molecular weight polyethylene (UHMWPE) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers [50,51], with consequent improvement of the interfacial interaction with the surrounding polymer matrix. PDA has also been used by our group to functionalize PCM microcapsules with a MF shell [52], and the modification remarkably improved the adhesion with a rigid epoxy resin (glass transition temperature = 95 • C). An analysis using X-ray photoelectron spectroscopy (XPS) highlighted that, unlike MF, PDA is able to react with oxirane groups of the epoxy resin, thereby forming covalent groups and increasing the adhesion by chemical interaction.…”

“…An analysis using X-ray photoelectron spectroscopy (XPS) highlighted that, unlike MF, PDA is able to react with oxirane groups of the epoxy resin, thereby forming covalent groups and increasing the adhesion by chemical interaction. The deposited PDA layer had a thickness of a few hundred nanometers, featured a noticeable surface roughness that increased the interaction with the matrix by mechanical interlocking, and represented 14.3 wt % of the total PDA-coated microcapsule sample [52].…”

“…The present work aims, for the first time, to produce and characterize flexible epoxy-based composites containing different amounts of neat and PDA-modified PCM microcapsules. Taking off from the results of [52], PDA deposition parameters have been tuned to obtain a thinner PDA layer, which would be advantageous since a thick layer would unnecessarily decrease the core-to-shell mass ratio of the microcapsules and, therefore, the total melting enthalpy per unit mass, since the only contribution to the phase change enthalpy comes from the paraffinic core. Moreover, previous results [52] suggest that the main adhesion mechanism is chemical interaction between the surface functional groups of PDA and the oxirane groups of the epoxy resin, and this mechanism would also be active with a thinner PDA layer.…”

“…Taking off from the results of [52], PDA deposition parameters have been tuned to obtain a thinner PDA layer, which would be advantageous since a thick layer would unnecessarily decrease the core-to-shell mass ratio of the microcapsules and, therefore, the total melting enthalpy per unit mass, since the only contribution to the phase change enthalpy comes from the paraffinic core. Moreover, previous results [52] suggest that the main adhesion mechanism is chemical interaction between the surface functional groups of PDA and the oxirane groups of the epoxy resin, and this mechanism would also be active with a thinner PDA layer. Moreover, electron microscopy techniques were employed to characterize the surface state of the microcapsules before and after PDA deposition, as well as the fracture surface of the composites and shell/epoxy interfacial adhesion.…”