Martin Cvek

A facile controllable coating of carbonyl iron particles with poly(glycidyl methacrylate): a tool for adjusting MR response and stability properties

This study is focused on the controllable coating of the carbonyl iron (CI) particles widely applied in magnetorheology. These particles were grafted with poly(glycidyl methacrylate) (PGMA) with narrow polydispersity via surface-initiated atom transfer radical polymerization. Two types of core–shell particles differing in molecular weights of grafted polymer chains were synthesized. The effect of shell thickness on the thermo-oxidation stability of particles as well as the sedimentation stability of their silicone oil suspensions was evaluated. The successful coating process was confirmed by Fourier transform infrared spectroscopy and energy-dispersive spectrometry. The differences in the magnetic properties of bare and coated CI particles were clarified through vibrating sample magnetometry. Due to the controllable length of the PGMA grafts, the magnetic properties remain almost the same as those for bare CI. The magnetorheological (MR) behavior of silicone oil suspensions containing 60 wt% of bare CI particles as well as PGMA-coated analogues was investigated in the absence and in the presence of various magnetic field strengths, demonstrating the negligible impact of surface modification on final MR performance. Thus, the grafting of the particles with PGMA negligibly affected magnetic properties but considerably enhanced thermo-oxidation and sedimentation stabilities. Finally, a novel tensiometric method for sedimentation stability measurements of MR suspensions was successfully implemented.

The chemical stability and cytotoxicity of carbonyl iron particles grafted with poly(glycidyl methacrylate) and the magnetorheological activity of their suspensions

Carbonyl iron (CI) particles were grafted with poly(glycidyl methacrylate) (PGMA) using atom transfer radical polymerization. Compact coating of PGMA largely improved the chemical stability of the particles in an acid environment and thus reduced the common drawback of bare CI particles. Furthermore, due to possible medical applications of CI-polymer systems for magnetic drug targeting, an in vitro cytotoxicity test was performed using an NIH/3T3 cell line. The cell viability was evaluated by spectrometric assay (MTT). The results show that the prepared particles are not cytotoxic. Moreover, bare CI particles as well as synthesized core–shell particles were suspended in silicone oil, and the rheological behavior of MR suspensions was investigated in controlled shear rate mode under various magnetic field strengths. Dynamic yield stress as a measure of the rigidity of the created internal structures of the suspensions was determined using the Herschel–Bulkley model, which provided a reasonably good fit for rheological data. MR suspensions of PGMA-coated particles exhibited only slightly decreased yield stresses due to their negligibly-affected magnetic performance.

A rheological evaluation of steady shear magnetorheological flow behavior using three-parameter viscoplastic models

Knowledge of the complicated flow characteristics of magnetorheological (MR) suspensions is necessary for simulations, calculations in engineering processes, or designing new devices utilizing these systems. In this study, we employed three constitutive equations (three-parameter models) for an evaluation of steady shear behavior of MR suspensions. The predictive/fitting capabilities of the Robertson–Stiff (R–S) model were compared with the commonly used Herschel–Bulkley (as a reference) and the Mizrahi–Berk models. The appropriateness of the models was examined using rheological data for diluted as well as concentrated MR systems. The effect of magnetic field strength on model fitting capabilities was also investigated. The suitability of the individual models was evaluated by observing correlation coefficient, sum of square errors, and root mean square errors. A statistical analysis demonstrated that the best fitting capabilities were exhibited by the R–S model, while others provided less accurate fits ...

Carbonyl iron coated with a sulfobetaine moiety as a biocompatible system and the magnetorheological performance of its silicone oil suspensions

In this study, surface modification of carbonyl iron (CI) particles with sulfobetaine moieties (SBE) was performed by the silanization of activated CI to form stable CI–SBE particles. The modification led to a significant improvement of the thermo-oxidation stability and a negligible suppression of the magnetization of the particles, as revealed by thermogravimetric analysis and vibrating sample magnetometry, respectively. The effect of a magnetic field and temperature on the magnetorheological performance of particle suspensions was investigated using a rotational rheometer in order to clarify the suitability of these systems for the local embolization of blood veins. The suspension based on CI–SBE exhibited a pseudoplastic behaviour and a tunable yield stress in a range from 0.3–4 kPa at the normal human body temperature. Moreover, cell viability for fibroblasts and macrophages was examined via MTT assay, which revealed their suitability for the intended applications for the local embolization of blood veins.

Electrorheological behavior of iron(II) oxalate micro-rods

Electrorheological (ER) fluids represent smart materials with extensive application potential due to their rheological properties which can be readily changed under an external electric field. In this study, the iron(II) oxalate particles with rod-like morphology were successfully synthesized by the co-precipitation method using sulphate heptahydrate and oxalic acid dihydrate. The characterization of particles was performed via X-ray diffractometry and scanning electron microscopy. Subsequently, the ER fluids were prepared by dispersing the synthesized particles in silicone oil. The optical microscopy demonstrated the formation of chain-like particle structures upon the application of an electric field. Rheological properties were determined by means of rotational rheometry including creep-recovery experiments. The viscoelastic behavior of systems under investigation in the presence of the electric field was confirmed by the presence of recoverable strain of the system.

Light-induced and sensing capabilities of SI-ATRP modified graphene oxide particles in elastomeric matrix

Photoactuators can concern light stimuli in appropriate wavelength into mechanical response. Such reversible changes in the material shape are highly promising in their applications as remote controllers, or safety sensors. In this work we were focused on light-induced actuation and sensing performance of the prepared materials. In this case poly(dimethyl siloxane) PDMS with various amounts of silicone oil and curing agent was used as matrix. Graphene oxide (GO) as filler in its neat form as well as its modified analogue were used in concentration of 0.1 vol. %. Modified GO particles were controllably coated with poly(methyl methacrylate) polymer chains using surface-initiated atom transfer radical polymerization (SI-ATRP) approach in order improve interactions between the filler and matrix which consequently lead to the enhanced light-induced actuation performance. Generally, the both, GO particles as well as modified ones were characterized using FTIR, Raman spectroscopy and finally conductivity measurement to confirm the controllable coating and simultaneously proceeded reduction. By studying of dielectric properties (activation energies), viscoelastic properties, which were investigated using dynamic mechanical analysis, the interactions between the filler and matrix were evaluated with connection to their light-responsive and sensing capabilities.