Roland Kádár

From self-assembly of electrospun nanofibers to 3D cm thick hierarchical foams

Electrospinning usually results in the formation of scaffolds that are a few hundred microns in thickness with pore sizes in the micron range. However some applications, such as tissue engineering, necessitate the fabrication of cm-thick nanofibrous scaffolds with large pore sizes that allow for cell infiltration. Here, we demonstrate for the first time the production of bioresorbable poly(e-caprolactone) nanofibrous cm-thick foams using the electrospinning technique. These scaffolds were obtained through the dynamic self-assembly of electrospun nanofibers into honeycomb patterns, which resulted in a unique columnar hierarchical structure with both micropores and mesopores of up to several hundreds of microns in size. This specific morphology leads to mechanical properties of thick scaffolds, suitable for handling and implanting in vivo.

Electrical, Mechanical, and Thermal Properties of LDPE Graphene Nanoplatelets Composites Produced by Means of Melt Extrusion Process

Composites of LDPE filled with different amounts of graphene nanoplatelets (GnP) were prepared in form of films by means of precoating technique and single screw melt-extrusion using two types of screws, compression and mixing. This manufacturing process imposes strong anisotropy on the sample’s morphology, in which the nanoplatelets become oriented along the extrusion direction. Such orientation of GnP in LDPE matrix is confirmed by scanning electron microscopy observations and it yields unique electrical properties. As compared to pure LDPE, significant reductions of the through-plane conductivity are found for the composites at relatively low electric fields (<20 kV/mm) at low filler concentrations. Above the field level of 20 kV/mm, a crossover effect is observed that results in a strong field dependency of the conductivity where the non-linear behavior starts to dominate. Moreover, differential scanning calorimetry (DSC) results indicate a decrease in polymer crystallinity of the composite matrix with increasing filler content, whereas thermogravimetric (TG) analysis shows a slight increase in the material’s thermal stability. Application of GnP also leads to improvement of mechanical properties, manifested by the increase of Young’s modulus and tensile strength in both types of samples.

Gas Barrier, Thermal, Mechanical and Rheological Properties of Highly Aligned Graphene-LDPE Nanocomposites

This contribution reports on properties of low-density polyethylene-based composites filled with different amounts of graphene nanoplatelets. The studied samples were prepared in the form of films by means of the precoating technique and single screw melt-extrusion, which yields a highly ordered arrangement of graphene flakes and results in a strong anisotropy of composites morphology. The performed tests of gas permeability reveal a drastic decrease of this property with increasing filler content. A clear correlation is found between permeability and free volume fraction in the material, the latter evaluated by means of positron annihilation spectroscopy. A strong anisotropy of the thermal conductivity is also achieved and the thermal conductivity along the extrusion direction for samples filled with 7.5 wt % of GnP (graphene nanoplatelets) reached 2.2 W/m·K. At the same time, when measured through a plane, a slight decrease of thermal conductivity is found. The use of GnP filler leads also to improvements of mechanical properties. The increase of Young’s modulus and tensile strength are reached as the composites become more brittle.

Linear and Nonlinear Rheology Combined with Dielectric Spectroscopy of Hybrid Polymer Nanocomposites for Semiconductive Applications

The linear and nonlinear oscillatory shear, extensional and combined rheology-dielectric spectroscopy of hybrid polymer nanocomposites for semiconductive applications were investigated in this study. The main focus was the influence of processing conditions on percolated poly(ethylene-butyl acrylate) (EBA) nanocomposite hybrids containing graphite nanoplatelets (GnP) and carbon black (CB). The rheological response of the samples was interpreted in terms of dispersion properties, filler distortion from processing, filler percolation, as well as the filler orientation and distribution dynamics inside the matrix. Evidence of the influence of dispersion properties was found in linear viscoelastic dynamic frequency sweeps, while the percolation of the nanocomposites was detected in nonlinearities developed in dynamic strain sweeps. Using extensional rheology, hybrid samples with better dispersion properties lead to a more pronounced strain hardening behavior, while samples with a higher volume percentage of fillers caused a drastic reduction in strain hardening. The rheo-dielectric time-dependent response showed that in the case of nanocomposites containing only GnP, the orientation dynamics leads to non-conductive samples. However, in the case of hybrids, the orientation of the GnP could be offset by the dispersing of the CB to bridge the nanoplatelets. The results were interpreted in the framework of a dual PE-BA model, where the fillers would be concentrated mainly in the BA regions. Furthermore, better dispersed hybrids obtained using mixing screws at the expense of filler distortion via extrusion processing history were emphasized through the rheo-dielectric tests.

Processing window for extrusion foaming of hydroxypropyl methylcellulose

Foamed materials are gaining an increased interest due to their good mechanical properties in relation to their low densities and an increased industrial demand can be expected. A few less attractive issues can however be associated with commodity foamed products. For instance the raw-material often originates from non-renewable, fossil-based, sources. Furthermore, degradation in nature is slow, therefor the disposed product is burned or end up in landfills. One possibility to reduce the impact on nature could be to produce foams from natural polymers such as starch or cellulose. In this study the possibility to produce foams from hydroxypropyl methylcellulose (HPMC) with water as blowing agent, by continuous extrusion, was investigated. A pre-study using a capillary viscometer, batch-extruder, was conducted to evaluate the foamability of HPMC. Due to promising results further experiments were conducted with a single-screw extruder. The goal was to find an adequate processing window for foaming. It was concluded that HPMC could successfully be foamed by continuous extrusion, although a careful tailoring of the processing parameters was required. Crucial parameters were here the temperature, pressure and residence time distribution in the extruder. Regions of the extruded foams were examined using optical and scanning electron microscopy and HPMC foams with a density in the range of that of fossil-based polymeric foams could be produced.

High sensitivity measurements of normal force under large amplitude oscillatory shear

The two aims of this publication are to introduce a new and rheometer-independent rheometric tool for measuring the axial normal force in oscillatory shear rheology and to study the normal forces of polyolefin melts under large amplitude oscillatory shear (LAOS). A new plate geometry with an incorporated highly sensitive piezoelectric normal force sensor was designed for a rotational rheometer. The new geometry was used to investigate normal forces of polyethylene (PE) melts under LAOS. The resulting stress and normal force data was compared with the data from measurements in commercial high performance rotational rheometers. The stress and the normal force response were Fourier-transformed and their resulting spectra were analysed. The non-linear contributions to the FT-magnitude spectra (i.e. the intensities of the higher harmonics) were analysed using the framework of the Q-parameter, Q=I3/1/γ02

Novel electrical properties of extruded LDPE-GnP filled nanocomposites

Q=I_{3/1}/{\gamma ^{2}_{0}}

A Combined NMR Relaxometry and Surface Instability Detection System for Polymer Melt Extrusion

for both the stress spectrum and the normal force spectrum, resulting in the strain-dependent Qγ0

Polystyrene comb architectures as model systems for the optimized solution electrospinning of branched polymers

Q\left (\gamma _{0}\right )

First normal stress difference and in-situ spectral dynamics in a high sensitivity extrusion die for capillary rheometry via the ʽhole effect’

and QNFγ0