Kolos Molnár

Epoxy/Polycaprolactone Systems with Triple-Shape Memory Effect: Electrospun Nanoweb with and without Graphene Versus Co-Continuous Morphology

Triple-shape memory epoxy (EP)/polycaprolactone (PCL) systems (PCL content: 23 wt %) with different structures (PCL nanoweb embedded in EP matrix and EP/PCL with co-continuous phase structure) were produced. To set the two temporary shapes, the glass transition temperature (Tg) of the EP and the melting temperature (Tm) of PCL served during the shape memory cycle. An attempt was made to reinforce the PCL nanoweb by graphene nanoplatelets prior to infiltrating the nanoweb with EP through vacuum assisted resin transfer molding. Morphology was analyzed by scanning electron microscopy and Raman spectrometry. Triple-shape memory characteristics were determined by dynamic mechanical analysis in tension mode. Graphene was supposed to act also as spacer between the nanofibers, improving the quality of impregnation with EP. The EP phase related shape memory properties were similar for all systems, while those belonging to PCL phase depended on the structure. Shape fixity of PCL was better without than with graphene reinforcement. The best shape memory performance was shown by the EP/PCL with co-continuous structure. Based on Raman spectrometry results, the characteristic dimension of the related co-continuous network was below 900 nm.

Thermochemical stabilization and analysis of continuously electrospun nanofibers

Carbon nanotube (CNT)-loaded and neat polyacrylonitrile nanofibers were produced by a needleless continuous electrospinning method as carbon nanofiber precursors. The details of the stabilization, which is a crucial issue during carbon fiber production, were investigated as these nanofibers are especially sensitive to degradation. In order to determine the optimal parameters, the nanofibers were stabilized at different temperatures. The stabilized samples were analyzed by Fourier-transform infrared spectroscopic and differential scanning calorimetric (DSC) measurements and by the determination of the color changes. The chemical changes during the stabilization (the formation of the so-called ladder-polymer) can be followed by infrared spectrometry, while the conversion can be monitored by DSC. The formation of the ladder-polymer occurs according to the Gaussian distribution function, where the temperature of the stabilization is the statistical parameter, which was also determined. In the case of CNT-loaded samples, the range of stabilization temperature was wider, which provides better controllability of the process. Based on the established models, an appropriate multi-step heat-treatment program could be determined, which led to completely stabilized nanofibers, suitable for carbonization.

Testing and Modeling the Tensile Strength Behavior of Glass Fibers, Fiber Bundles and Fiber Mat

In this paper glass fibers, chopped roving pieces, and fiber mats were tested in case of emulsion bonded glass fiber mat samples. Relationships between the tensile strength properties of the different structural levels of the fiber mats were studied and fiber bundles as structural elements of fiber mats were modeled by idealized statistical fiber bundles developed by the Department of Polymer Engineering, BUTE. The geometrical and mechanical measurements were carried out by image processing methods and a computer aided tensile tester. The results proved that the modeling method gave a tool in understanding the failure behavior of the fiber mat samples and studying the effect of the structural parameters. The applicability of the modeling method is demonstrated by the good agreement with some measurements.

An elastic phenomenological material law for textile composites and it's fitting to experimental data

Constitutive description of deformations in technical textiles mostly requires some highly nonlinear material law because of the interaction between the orthotropic yarns and the effect of the matrix. Phenomenological models aim to capture the overall (macro level) behaviour needed in engineering applications. This paper introduces a new, two-dimensional phenomenological model for technical textiles, accompanied by a data acquiring strategy to determine the material parameters involved in the model. It handles the nonlinear stress–strain relation observed in uniaxial tests and takes the interactions in the two, orthogonal directions into account. As we aim to introduce a solution applicable at the service level of the loads, our model is inherently elastic, no time dependent or plastic behaviour are introduced and no cyclic loading is taken into consideration. The model can be solidly fit to the measured data.

Electrospinning of PVA/Carbon Nanotube Composite Nanofibers: The Effect of Processing Parameters

Poly(vinyl alcohol)/carbon nanotube (CNT) composite nanofibers were processed by both conventional and needleless electrospinning method. The effect of the processing parameters on the possibility of manufacturing was investigated. The results were evaluated by surface tension and conductivity measurements. For the investigation of surface morphology scanning electron microscopy (SEM) was used. It was concluded that surface treatment of carbon nanotubes was necessary for needleless electrospinning. Lower surface tensions were better for this process but the effect of conductivity was not so significant.