2019, Pötschke; Petra, Krause, Beate, Luo, Jinji

Composites were prepared with polypropylene (PP) as the matrix and singlewalled CNTs (SWCNTs) of the type TUBALL from OCSiAl Ltd. as the conducting component by melt processing in a small-scale twin-screw compounder. In order to switch the typical p-type behavior of such composites from positive Seebeck coefficients (S) into n-type behavior with negative Seebeck coefficients, a non-ionic surfactant polyoxyethylene 20 cetyl ether (Brij58) was used and compared with a PEG additive, which was shown previously to be able to induce such switching. For PP-2 wt% SWCNT composites Brij58 is shown to result in n-type composites. The negative S values (up to −48.2 µV/K) are not as high as in the case of previous results using PEG (−56.6 µV/K). However, due to the more pronounced effect of Brij58 on the electrical conductivity, the achieved power factors are higher and reach a maximum of 0.144 µW/(m·K2) compared to previous 0.078 µW/(m·K2) with PEG. Dispersion improvement depends on the type of SWCNTs obtained by using varied synthesis/treatment conditions. Solution prepared composites of PEG with SWCNTs also have negative S values, indicating the donation of electrons from PEG to the SWCNTs. However, such composites are brittle and not suitable as thermoelectric materials.

2021, Alghamdi, Majed, Gumbleton, Mark, Newland, Ben

Glioblastoma (GBM) is the most aggressive malignant brain tumor and is associated with a very poor prognosis. The standard treatment for newly diagnosed patients involves total tumor surgical resection (if possible), plus irradiation and adjuvant chemotherapy. Despite treatment, the prognosis is still poor, and the tumor often recurs within two centimeters of the original tumor. A promising approach to improving the efficacy of GBM therapeutics is to utilize biomaterials to deliver them locally at the tumor site. Local delivery to GBM offers several advantages over systemic administration, such as bypassing the blood-brain barrier and increasing the bioavailability of the therapeutic at the tumor site without causing systemic toxicity. Local delivery may also combat tumor recurrence by maintaining sufficient drug concentrations at and surrounding the original tumor area. Herein, we critically appraised the literature on local delivery systems based within the following categories: polymer-based implantable devices, polymeric injectable systems, and hydrogel drug delivery systems. We also discussed the negative effect of hypoxia on treatment strategies and how one might utilize local implantation of oxygen-generating biomaterials as an adjuvant to enhance current therapeutic strategies. © 2021 The Royal Society of Chemistry.

2016, Hopmann, C., Windeck, C., Cohnen, A., Onken, J., Krause, Beate, Pötschke, Petra, Hickmann, T.

Bipolar plates constitute the most important structural component in fuel cell stacks. Highly filled thermoplastic composites with high electrical conductivity obtain an increasing importance in the design of bipolar plates as alternative to conventional metallic systems. Thermoplastics (e.g. PP) have suitable properties such as a good processability, chemical resistance, light weight and low production costs. As thermoplastics have low electrical conductivities, conductive fillers have to be included in the matrix. A high content of such fillers (e.g. graphite) in excess of 80 wt.-% is necessary to achieve the desired electrical properties. However, materials with such high filler contents embrittle readily. The workability in injection and compression molding is difficult and the mechanical stability is insufficient in case of strain deformation. As consequence, material failure and an inacceptable amount of damaged goods can be observed during the processing. As no suitable thermoplastic system is available for better mechanical properties, the induction and dispersion of a rubber phase in the thermoplastic matrix can be used to increase the impact strength of the conductive composite. In this research work a ternary composite, based on PP as matrix, EPDM as impact modifier and synthetic graphite as conductive filler, was developed. The material was produced using a 26 mm co-rotating, intermeshing twin-screw extruder. The amounts of PP, EPDM and graphite were varied systematically and a process window was defined that enables improved impact strength and high electrical conductivity of the new material. The results indicate that impact strength can be enhanced by about 99 % with an EPDM content of 30 wt.-% in the PP matrix. The electrical conductivity decreases in a small range with increasing content of EPDM, but the conductivity is still excellent for producing bipolar plates.

2013, Nedelcu, S., Sommer, J.-U.

By molecular dynamics simulations, we investigated the transport of charged polymers in applied electric fields in confining environments, which were straight cylinders of uniform or non-uniform diameter. In the simulations, the solvent was modeled explicitly and, also, the counterions and coions of added salt. The electrophoretic velocities of charged chains in relation to electrolyte friction, hydrodynamic effects due to the solvent, and surface friction were calculated. We found that the velocities were higher if counterions were moved away from the polymeric domain, which led to a decrease in hydrodynamic friction. The topology of the surface played a key role in retarding the motion of the polyelectrolyte and, even more so, in the presence of transverse electric fields. The present study showed that a possible way of improving separation resolution is by controlling the motion of counterions or electrolyte friction effects.

2020, Fredi, Giulia, Simon, Frank, Sychev, Dmitrii, Melnyk, Inga, Janke, Andreas, Scheffler, Christina, Zimmerer, Cordelia

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.

2019, Müller, Michael Thomas, Krause, Beate, Kretzschmar, Bernd, Pötschke, Petra

In this study commercially available multiwalled carbon nanotubes (2-8 wt.%) were incorporated in polypropylene (PP) by direct powder feeding or by a masterbatch dilution procedure using a twin-screw extruder. The influence of a supplemental, electrical non-conductive talc or electrically conductive carbon black (CB), filler on the resulting composite properties was investigated. In comparison to the direct carbon nanotube (CNT) incorporation the masterbatch dilution step resulted in improved CNT macro dispersion. The use of the supplemental fillers CB or talc does not show a significant influence on the CNT dispersion state. When compared to direct CNT incorporation, the second compounding process involved in masterbatch dilution leads to higher electrical resistivity of injection molded samples. On the other hand, the supplemental fillers talc or CB decreased the electrical resistivity values. With the addition of talc or CB an increase of the Young’s modulus due to the reinforcing effect of the second filler was achieved. However, no synergistic effect between the used supplemental fillers and the CNT on the mechanical properties was obtained.

2017, Krause, Beate, Carval, J., Pötschke, Petra

Polycarbonate composites containing multiwalled carbon nanotubes (MWCNTs, 0.2-2.0 wt%) were melt mixed in small scale at different conditions of screw speed and mixing time to vary the specific mechanical energy (SME) input between 0.4 and 4.0 kWh/kg. Next to the electrical properties of compression molded plates and the MWCNT macrodispersion also the nanotube length and shape were analyzed. For this, the matrix of the composites with 0.75 wt% MWCNT loading was dissolved and the remaining nanotubes were investigated using TEM. It was found that with increasing SME input the number of remaining CNT agglomerates decreases. The MWCNT length decreased from initially about 1.4 micrometers towards 350 nanometers at a SME of 4 kWh/kg and the mean curling values were also reduced. The electrical percolation threshold increases with SME from about 0.4 wt% to 0.6 wt%.

2016, Förster, T., Sommer, G.S., Mäder, E., Scheffler, C.

Recycling of fibre reinforced polymers is in the focus of several investigations. Chemical and thermal treatments of composites are the common ways to separate the reinforcing fibres from the polymer matrices. However, most sizings on glass and basalt fibre are not designed to resist high temperatures. Hence, a heat treatment might also lead to a sizing removal, a decrease of mechanical performance and deterioration in fibre-matrix adhesion. Different basalt fibres were investigated using surface analysis methods as well as single fibre tensile tests and single fibre pull-out tests in order to reveal the possible causes of these issues. Heat treatment in air reduced the fibre tensile strength in the same level like heat treatment in nitrogen atmosphere, but it influenced the wetting capability. Re-sizing by a coupling agent slightly increased the adhesion strength and reflected a decreased post-debonding friction.

2017, Luo, J., Krause, Beate, Pötschke, Petra

The thermoelectric (TE) performance of electrically conductive thermoplastic composites prepared by melt mixing was investigated. A cost effective widely used in industry polymer, namely polypropylene (PP), was chosen as the matrix to fabricate the composites. Singlewalled carbon nanotubes (SWCNTs), the amount (2 wt%) of which was selected to be above the electrical percolation threshold (< 0.2 wt%), were used to form an electrical conducting network. Besides as-produced SWCNTs plasma modified tubes were employed to study the influence of the functionalization on the morphology, dispersion and TE properties of the PP composites. In addition, melt processing conditions, e.g. temperature, rotation speed, and time during mixing in a small-scale compounder were varied. Furthermore, an ionic liquid (IL, 1-methyl-3-octylimidazolium tetrafluoroborate) was used as a processing additive during melt mixing, which was confirmed to improve the electrical conductivity of the composites. Simultaneous increase in the Seebeck coefficient up to a value of 64 μV/K was recorded, leading to a much better power factor of 0.26 μW/(m·K2) compared to composites without IL. This melt mixing strategy opens new avenues for solvent-free, large scale fabrication of polymer based TE materials.

2009, Krause, Beate, Petzold, Gudrun, Pegel, Sven, Pötschke, Petra

In order to assess the dispersability of carbon nanotube materials, tubes produced under different synthesis conditions were dispersed in aqueous surfactant solutions and the sedimentation behaviour under centrifugation forces was investigated using a LUMiFuge stability analyzer. The electrical percolation threshold of the nanotubes after melt mixing in polyamide 6.6 was determined and the state of dispersion was studied. As a general tendency, the nanotubes having better aqueous dispersion stability showed lower electrical percolation threshold and better nanotube dispersion in the composites. This indicates that the investigation of the stability of aqueous dispersions is also able to give information about the nanotubes inherent dispersability in polymer melts, both strongly influenced by the entanglement and agglomerate structure of the tubes within the as-produced nanotube materials. The shape of the nanotubes in the aqueous dispersions was assessed using a SYSMEX flow particle image analyzer and found to correspond to the shape observed from cryofractured surfaces of the polymer composites. © 2008 Elsevier Ltd. All rights reserved.