Miklós Zrínyi

Magnetic Field-Responsive Smart Polymer Composites

The combination of polymers with nano- or microsized solid materials displays novel and often enhancedproperties compared to the traditional materials. They can open up possibilities for new technologicalapplications. Materials whose physical properties can be varied by application of magnetic fields belongto a specific class of smart materials. The broad family of magnetic field-controllable soft materialsincludes ferrofluids, magneto-rheological fluids, magnetic gels, and magnetic elastomers. The magneticgels and elastomers (magnetoelasts) represent a new type of composite and consist of small magneticparticles, usually in the nanometer to micron range, dispersed in a highly elastic polymeric matrix.The magnetic particles can be incorporated into the elastic body either randomly or in ordered structure.If a uniform magnetic field is applied to the reactive mixture during the cross-linking process, particlechains form and become locked into the elastomer. The resulting composites exhibit anisotropic properties.

Deformation of ferrogels induced by nonuniform magnetic fields

Materials producing strain in magnetic fields are known as magnetoelastic or magnetostrictive materials. A new type has been developed and the elongation of magnetic field sensitive polymer gels, called ferrogels, has been studied. The driving force of the deformation is the magnetic field gradient which was produced by an electromagnet. Unidirectional deformation measurements have been performed and the dependence of elongation on the current intensity has been reported. An equation for the uniaxial magnetoelastic properties has been derived and it was proven that at small strains the elongation is proportional to the square of steady current intensity. The elastic modulus of ferrogels was found not to be influenced by the magnetic field.

Ferrogel: A new magneto-controlled elastic medium

Abstract Magnetic-field-sensitive gels, called ferrogels, have been prepared by introducing monodomain, magnetite particles of colloidal size into chemically crosslinked poly(vinyl alcohol) hydrogels. The influence of a non-uniform external magnetic field on the shape of gel tubes and the possibility to induce elongation and contraction by means of a magnetic field gradient produced by an electromagnet have been demonstrated. It was shown that both the concentration of magnetite particles and the crosslinking density of the ferrogels play an essential role in the magneto-elastic behaviour.

Electric field sensitive neutral polymer gels

Abstract In this paper we summarise the effects induced by electric field on the motility and the shape of polymer gels. Then it is followed by a description of a new driving mechanism to induce deformation of a neutral polymer gels in a non-conducting medium. The main idea is to incorporate such finely distributed colloidal particles into a swollen network which responds to electric field. Since the particles cannot leave the gel matrix, all the forces acting on the particles are transmitted directly to the polymer chains resulting in either locomotion or deformation of the gel. Bending of weakly cross-linked poly(dimethyl siloxane) gels containing finely distributed electric field sensitive particles has been studied in silicon oil. Under an external electric field these gels underwent significant and quick bending. A theoretical approach has been developed and compared with the experimental results.

Synthesis and swelling properties of novel pH-sensitive poly(aspartic acid) gels.

Chemically cross-linked poly(aspartic acid) (PASP) gels were prepared by the hydrolysis of poly(succinimide) (PSI). The latter was prepared by thermal polycondensation of aspartic acid. The PSI chains were cross-linked by natural amines and amino acid derivatives such as putrescin, spermine, spermidine, lysine and cystamine to obtain biodegradable, biocompatible, amino acid-based hydrogels. The volume of the synthesized unhydrolyzed PSI gels changes abruptly at a well-defined pH that results in ring opening, while the hydrolyzed gels show a volume phase transition around the pK values of PASP. The unidirectional stress-strain behavior of the gels as well as the dependence of equilibrium swelling degree on the pH was carefully studied and the most important network parameters were determined by a modified version of the Brannon-Peppas-Peppas theory.

Derivation of the Matalon-Packter law for Liesegang patterns

Theoretical models of the Liesegang phenomena are studied and simple expressions for the spacing coefficients characterizing the patterns are derived. The emphasis is on displaying the explicit dependences on the concentrations of the inner and the outer electrolytes. Competing theories (ion-product supersaturation, nucleation and droplet growth, induced sol-coagulation) are treated with the aim of finding the distinguishing features of the theories. The predictions are compared with experiments and the results suggest that the induced sol-coagulation theory is the best candidate for describing the experimental observations embodied in the Matalon-Packter law.

Direct observation of abrupt shape transition in ferrogels induced by nonuniform magnetic field

Unidirectional elongation of magnetic field sensitive polymer gels, called ferrogels, have been studied. In ferrogels, finely distributed colloidal particles having superparamagnetic behavior are incorporated into a swollen network. These particles couple the shape of the gel to the nonuniform magnetic field. Shape distortion occurs instantaneously and disappears when the external field is removed. A discontinuous elongation and contraction in response to infinite-small change in the external magnetic field has been observed and a theoretical interpretation based on coupled magnetic and rubber elastic properties is provided.

MAGNETISM AND COMPRESSIVE MODULUS OF MAGNETIC FLUID CONTAINING GELS

We have investigated the magnetism and have measured the compressive modulus of magnetic fluid containing gels, called ferrogels, in the presence of magnetic field. No hysteresis was shown in the magnetization curve of the ferrogel suggesting a super paramagnetic response. The equilibrium compressive modulus of the ferrogel was measured up to 4 kOe and the modulus with field was much higher than that without field. The mean change in modulus increased with increasing magnetic field, and it saturates above 2 kOe. The mean and maximum change in modulus at 4 kOe was 31 and 71 Pa which corresponds to 19% and 46% increases of that without field, respectively. The change in modulus has been analyzed theoretically and is well explained in terms of the magnetism of ferrogels.

Kinetics of the shape change of magnetic field sensitive polymer gels

Abstract This study focuses on the dynamical behaviour of magnetic-field-sensitive polymer gels. These gels have been prepared by introducing nanosized magnetic particles into polymer gels homogeneously. We have studied the kinetics of the motion of magnetic gels. The highest working frequency of magnetic gels has been established, and a simple, completed Kelvin model was suggested to describe the dynamical behaviour.

Blood pressure distribution in a Hungarian adolescent population: comparison with normal values in the USA.

Objectives To describe the blood pressure characteristics and distribution of all adolescent high school students (aged 15–18 years) in Debrecen (total population 230 000), Hungary. To define threshold values for normal blood pressure by age-, sex- and height-specific groups. To compare our data with results from a USA meta-analysis, which forms the basis of current guidelines. Participants and methods All young people attending high school in Debrecen (final sample n = 10 359) participated in the study. After they had rested for 10 min, three blood pressure measurements were taken from the right upper arm, separated by 5 min intervals. All measurements were obtained by a validated, automated, digital Omron M4 device. Results The 50th, 90th and 95th percentile values of blood pressure were defined by dividing the adolescent population into age-, sex- and height-specific subgroups. In comparison with USA guidelines, in our sample the systolic blood pressure of boys in the different subgroups was 6–11 mmHg greater, whereas this difference was less marked for girls (1–5 mmHg). There were no marked differences in diastolic blood pressure, but our values were slightly lower. Conclusions Our findings demonstrate the influence of geographical and ethnic variations on blood pressure. Acceptance and use of non-population-specific blood pressure distributions may lead to under- or overdiagnosis of adolescent hypertension. The use of geographically more relevant data should be encouraged.