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- ItemBioinspired Polydopamine Coating as an Adhesion Enhancer Between Paraffin Microcapsules and an Epoxy Matrix(Washington, DC : ACS Publications, 2020) Fredi, Giulia; Simon, Frank; Sychev, Dmitrii; Melnyk, Inga; Janke, Andreas; Scheffler, Christina; Zimmerer, CordeliaMicroencapsulated 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.
- ItemBio-inspired deposition of electrochemically exfoliated graphene layers for electrical resistance heating applications(Bristol : IOP Publishing, 2020-12-4) Utech, Toni; Pötschke, Petra; Simon, Frank; Janke, Andreas; Kettner, Hannes; Paiva, Maria; Zimmerer, CordeliaElectrochemically exfoliated graphene (eeG) layers possess a variety of potential applications, e.g. as susceptor material for contactless induction heating in dynamic electro-magnetic fields, and as flexible and transparent electrode or resistivity heating elements. Spray coating of eeG dispersions was investigated in detail as a simple and fast method to deposit both, thin conducting layers and ring structures on polycarbonate substrates. The spray coating process was examined by systematic variation of dispersion concentration and volume applied to heated substrates. Properties of the obtained layers were characterized by UV-VIS spectroscopy, SEM and Confocal Scanning Microscopy. Electrical conductivity of eeG ring structures was measured using micro-four-point measurements. Modification of eeG with poly(dopamine) and post-thermal treatment yields in the reduction of the oxidized graphene proportion, an increase in electrical conductivity, and mechanical stabilization of the deposited thin layers. The chemical composition of modified eeG layer was analyzed via x-ray photoelectron spectroscopy pointing to the reductive behavior of poly(dopamine). Application oriented experiments demonstrate the direct electric current heating (Joule-Heating) effect of spray-coated eeG layers.
- ItemPolydopamine-Coated Paraffin Microcapsules as a Multifunctional Filler Enhancing Thermal and Mechanical Performance of a Flexible Epoxy Resin(Basel : MDPI, 2020) Fredi, Giulia; Zimmerer, Cordelia; Scheffler, Christina; Pegoretti, AlessandroThis work focuses on flexible epoxy (EP) composites containing various amounts of neat and polydopamine (PDA)-coated paraffin microcapsules as a phase change material (PCM), which have potential applications as adhesives or flexible interfaces with thermal management capability for electronics or other high-value-added fields. After PDA modification, the surface of PDA-coated capsules (MC-PDA) becomes rough with a globular appearance, and the PDA layer enhances the adhesion with the surrounding epoxy matrix, as shown by scanning electron microscopy. PDA deposition parameters have been successfully tuned to obtain a PDA layer with a thickness of 53 ± 8 nm, and the total PDA mass in MC-PDA is only 2.2 wt %, considerably lower than previous results. This accounts for the fact that the phase change enthalpy of MC-PDA is only marginally lower than that of neat microcapsules (MC), being 221.1 J/g and 227.7 J/g, respectively. Differential scanning calorimetry shows that the phase change enthalpy of the prepared composites increases with the capsule content (up to 87.8 J/g) and that the enthalpy of the composites containing MC-PDA is comparable to that of the composites with MC. Dynamic mechanical analysis evidences a decreasing step in the storage modulus of all composites at the glass transition of the EP phase, but no additional signals are detected at the PCM melting. PCM addition positively contributes to the storage modulus both at room temperature and above Tg of the EP phase, and this effect is more evident for composites containing MC-PDA. As the capsule content increases, the mechanical properties of the host EP matrix also increase in terms of elastic modulus (up to +195%), tensile strength (up to +42%), Shore D hardness (up to +36%), and creep compliance (down to −54% at 60 min). These effects are more evident for composites containing MC-PDA due to the enhanced interfacial adhesion.
- ItemAnalysis of riboflavin/ultraviolet a corneal cross-linking by molecular spectroscopy(London [u.a.] : Elsevier, 2023) Melcher, Steven; Zimmerer, Cordelia; Galli, Roberta; Golde, Jonas; Herber, Robert; Raiskup, Frederik; Koch, Edmund; Steiner, GeraldCorneal cross-linking (CXL) with riboflavin and ultraviolet A light is a therapeutic procedure to restore the mechanical stability of corneal tissue. The treatment method is applied to pathological tissue, such as keratoconus and induces the formation of new cross-links. At present, the molecular mechanisms of induced cross-linking are still not known exactly. In this study, we investigated molecular alterations within porcine cornea tissue after treatment with riboflavin and ultraviolet A light by surface enhanced Raman spectroscopy (SERS). For that purpose, after CXL treatment a thin silver layer was vapor-deposited onto cornea flaps. To explore molecular alterations induced by the photochemical process hierarchical cluster analysis (HCA) was used. The detailed analysis of SERS spectra reveals that there is no general change in collagen secondary structure while modifications on amino acid side chains are the most dominant outcome. The formation of secondary and aromatic amine groups as well as methylene and carbonyl groups were observed. Even though successful cross-linking could not be registered in all treated samples, Raman signals of newly formed chemical groups are already present in riboflavin only treated corneas.