Abstract
The main goal of this project is the creation of a new generation of magnetically active polymer materials (MPMs) capable of changing their physical properties in the presence of external magnetic fields in a controllable fashion. The interplay between the polymer viscoelasticity of the matrix and the magnetic properties of individual filler particles leads to the emergence of a wide variety of unique phenomena in these composites and opens up the possibility of controlling their physical properties using external magnetic fields. The composition of MPMs, with the main components being a polymer dispersion medium and magnetic filler, provides large variability in the properties of the composite, which allows it to be “adjusted” for use in a particular field. MPMs include both magnetoactive polymeric fluids based on polymer melts in a liquid state that belong to a larger class of magnetic or magnetorheological fluids (MRFs), and magnetoactive elastomers (MAEs) based on crosslinked polymer systems.
Research done during the first year of the project
For this stage of the project, the main attention was paid to the creation of fundamentally new “active” dispersive media, allowing in situ control of the properties of the MPMs in the integral temperature-magnetic field space. The new media are based on comb polymers with a high side chain grafting density (molecular brushes) with a low fraction of segregating side chains. Due to the incompatibility of side chains of different chemical nature with each other, microphase separation occurs in the system. Micellar aggregates formed by segregating chains play the role of physical crosslinks, facilitating the formation of brush elastomers at room temperature. Thus, the use of brush copolymers makes it possible, on one hand, to create low-modulus elastomers without the use of a low molecular weight solvent, which, as was shown at the previous stage of the project, are effective media for obtaining magnetopolymer materials with a high response to magnetic fields. On the other hand, the presence of physical crosslinks rather than chemical ones makes it possible to switch the system between the elastomeric and viscous states using the temperature field. In the viscous state, which corresponds to elevated temperatures, it is possible to realize the free rearrangement of magnetic particles when a magnetic field is applied. New structures are fixed in the material after cooling.
To obtain “active media”, brush copolymers of various compositions were synthesized: with side chains of polyisobutylene and polydimethylsiloxane, polystyrene was chosen as segregating blocks. The concentration of polystyrene blocks in the copolymers varied from 2.5% to 5% by weight. The viscoelastic properties of the resulting polymer matrices of different chemical nature were studied depending on the composition and temperature. To study the structure of nanoaggregates formed in brush polymers and to determine the region of their stability, we performed computer simulations of brush copolymers of various structure and obtained the phase diagram of the system depending on the composition and block incompatibility parameter.
Based on brush copolymers, a wide range of MPMs with different concentrations and types of magnetic particles has been created. It was shown that, at room temperature, the resulting MPMs behave like a solid material. An increase in temperature leads to softening of the composite and the transition to a liquid state. The transition temperatures of the material to a viscous-flowing state are determined based on the composition; they lie in the range of 70-110 degrees C. It was shown that the process of the physical network breakdown is reversible, and as the temperature decreases, the network returns to its original state. When a magnetic field is applied at an elevated temperature, a significant (more than two orders of magnitude) increase in the elastic modulus of the MPM is observed due to the restructuring of the magnetic filler along the magnetic field lines and strong magnetic interactions between the particles. Upon cooling in a magnetic field, the magnetic structures are fixed by a physical network of micellar aggregates, and because of that when the field is turned off the elastic modulus of the sample turns out to be several times higher than in the initial state. The ordering of magnetic particles into oriented chain aggregates was observed using scanning electron microscopy. The degree of particle ordering can be controlled by changing the temperature at which the magnetic field is applied, thereby programming the resulting properties of the composite.
For the first time, siloxane comb-shaped polymers with a siloxane backbone and side chains were synthesized. The rheological properties of these materials were studied, MRFs with different concentrations of carbonyl iron were obtained. The magnetorheological effect exhibited by the obtained materials is significant and exceeds three orders of magnitude. The yield strength of MRFs with maximum magnetic filling is about 60 kPa. The yield strength values of MRFs based on siloxane brushes are higher than those of star-shaped polydimethylsiloxane.
Another important area of work has been the study of the viscoelastic and dielectric properties of elastomeric MPMs based on a hybrid filler consisting of a mixture of magnetically hard (neodymium-iron-boron) and magnetically soft (carbonyl iron) particles. A distinctive feature of such mixed fillers is their ability to create their own magnetic field after pre-magnetization. The internal magnetic field changes the properties of the material in the absence of an external field, just as an external magnetic field changes the properties of a composite with a soft magnetic filler. This simplifies the design of devices that use MPMs, but may not contain permanent magnets or electromagnets. A series of samples with a total magnetic filler concentration of 80% by weight, but different filler compositions (the iron content in the mixture varied from 0 to 100%) was obtained. It was shown that an increase in the content of carbonyl iron in the filler significantly increases the dielectric constant and conductivity of the samples. The influence of the composition of the magnetic filler and the magnetization field on the dielectric properties of MPMs is important for the practical application of MPMs as a device element with a tuned dielectric response.
An analysis of modern theoretical methods for studying magneto-mechanical coupling in the MAE has been carried out. The clusterization process for spherical ferromagnetic particles in a rigid nonlinear polymer medium in the presence of an external magnetic field has been theoretically studied. The average number of particles in the resulting clusters was calculated for filler volume concentrations up to 10% and magnetic fields up to 500 mT. The surface enveloping the clusters was described as an ellipsoid of revolution. The ratio of linear dimensions of the ellipsoid, averaged over the initial particle distributions, was calculated for varying values of the filler concentration and the external magnetic field strength. Material cells of the appropriate size containing a single cluster and the influence of such clusters on the mechanical properties of the cells were considered. The dependences of the effective stretch modulus of a cell on the geometric anisotropy parameter of the cluster and the filler volume concentration were calculated. The difference between the elastic moduli of cells containing ellipsoidal and cylindrical clusters was considered. The influence of the spatial orientation of a cluster relative to the direction of an external load on the value of the elastic modulus has been studied. It was shown that the presence of clusters elongated along the direction of the external load creates the most pronounced material stiffening effect. Thus, the foundations have been laid for further theoretical study of the structure and mechanical properties of MPMs, taking into account the complex shape of magnetic clusters.
The results obtained at this stage of the work on the project were reported at six conferences (9 reports, including invited, oral and poster ones). Two articles were published in highly rated journals of the first quartile, one article was accepted for publication, one article was sent to an editor and three more articles are being prepared for publication. The research results were used in the lecture course “Fundamentals of Mechanics and Rheology of Polymers” (for PhD students of Moscow State University).
Research done during the second year of the project
The main goal of this project is the creation of a new generation of magnetically active polymer materials (MPMs) capable of changing their physical properties in the presence of external magnetic fields in a controllable fashion. The interplay between the polymer viscoelasticity of the matrix and the magnetic properties of individual filler particles leads to the emergence of a wide variety of unique phenomena in these composites and opens up the possibility of controlling their physical properties using external magnetic fields. The composition of MPMs, with the main components being a polymer dispersion medium and magnetic filler, provides large variability in the properties of the composite, which allows it to be “adjusted” for use in a particular field. MPMs include both magnetoactive polymeric fluids based on polymer melts in a liquid state that belong to a larger class of magnetic or magnetorheological fluids (MRFs), and magnetoactive elastomers (MAEs) based on crosslinked polymer systems.
One of the main goals of this stage of the project was to study the influence of the anisometry of magnetic filler particles and the anisotropy of their distribution in the polymer matrix on the viscoelastic properties of MPMs and their response to an external magnetic field. To achieve this goal, a series of anisotropic MAE samples were synthesized based on traditional linear polydimethylsiloxane (PDMS) matrices, as well as “brush” PDMS matrices containing side chains, which, not being elastically active, effectively “dilute” the system, reducing the elastic modulus of the elastomer. Carbonyl iron, needle-like gamma iron oxide, platelet iron, as well as mixtures of carbonyl iron with needle-like gamma iron oxide and platelet iron with gamma iron oxide were used as magnetic fillers.
As a result of studies of the properties of MAE of different compositions, it has been shown that the orientation of aggregates of magnetic particles by applying an external magnetic field when obtaining MAE samples is an effective way to create a material with controlled anisotropy of mechanical properties, and the use of anisometric particles can significantly enhance the anisotropy and elastic modulus of the material with low concentrations of filler. In anisotropic samples, the magnetorheological effect is enhanced when the orientation of aggregates of magnetic particles coincides with the orientation of the external magnetic field. The advantage of polymer matrices with a brush structure is the absence of low-molecular components capable of sweating, the achievement of higher fillings when using anisometric (plate-like) particles, as well as a more significant magnetic response – the increase in the elastic modulus of an anisotropic sample with maximum filling exceeds an order of magnitude in a magnetic field of 1 Tesla.
Research has continued on the viscoelastic and magnetorheological properties of thermoactive MPMs based on brush A-g-B copolymers of various compositions containing 5 to 20vol% of spherical particles of carbonyl iron. These systems are shown to be a promising way to create magnetically active thermoplastic elastomers with programmable and reprogrammable viscoelasticity and magnetic response. Unlike chemically cross-linked polymer networks, A-g-B brushes contain an active dispersing medium that not only exhibits thermosensitive viscoelastic properties, but also allows the restructuring of magnetic particles. The latter can be achieved by applying magnetic fields of different orientations at elevated temperatures, even below the melting point of the A-g-B matrix. Cooling to room temperature leads to the formation of a material with oriented magnetic aggregates, fixed by the formation of physical cross-links, the role of which is played by micellar aggregates of segregating blocks. It has been shown that temperature treatment makes it possible to increase the magnetic response of brush MPMs by two orders of magnitude when the magnetic field and the magnetic aggregates are aligned. The ability to reprogram the properties of already manufactured materials in real time has practical value for soft robotics and medical devices. The use of plate-like iron particles (concentration varied from 5 to 20%) made it possible to increase the modulus of the MPM in a magnetic field.
Theoretical studies of both the properties of active dispersion media based on A-g-B copolymers and MPMs in general have been continued. In the first direction, using computer modeling, stress-strain relationships were obtained for copolymers of different structures, different modes of mechanical response of the material were identified depending on the architectural parameters of the copolymer and the degree of incompatibility of A and B blocks. The modeling results are important both for the fundamental explanation of the physical processes that determine the mechanical response of A-g-B copolymers, and for the targeted synthesis of matrices with the required properties.
As part of the development of theoretical approaches to describe the MPM, the model of the movement of a system of spherical soft magnetic particles in an element of the MPM volume under the influence of a magnetic field was improved. The set of initial distributions, over which the characteristics of the resulting particle cluster are averaged, was expanded to a representative sample. The characteristics of the cluster as an ellipsoid of inertia were calculated. It was shown that at low filler concentrations, ellipsoidal clusters with anisometry parameters corresponding to the range relevant to this study (from 1 to 5) are formed. Approximation of the effective cell elastic modulus dependences on the filler concentration and the inclusion anisometry parameter was carried out. Averaging of the effective elastic moduli over different initial orientations of the inclusions and their anisometry parameters was carried out. An ensemble of material cells with single inclusions was considered. It was found that the ratio of the magnetocrystalline anisotropy constant of a ferromagnetic inclusion to the effective elasticity modulus of the cell of 1.5 is a universal critical value separating the modes of monotonic and non-monotonic inclusion rotation in an external magnetic field.
The results obtained make it possible to determine the type of ferromagnetic filler and its characteristics that provide the highest degree of restructuring in MPM under the influence of a magnetic field and the strongest strengthening effect due to filling at low filler concentrations.
The results obtained at this stage of the project were reported at five conferences (6 reports, including plenary, invited, oral and poster). 3 articles have been published in high-ranking first-quartile journals, one article has been accepted for publication, one article has been sent for publication, and three more articles are being prepared for publication. The research results were used in the development and delivery of a new course of lectures “Magnetoactive polymer materials” for 1st year masters in the fall semester of 2023.
Summary
The main goal of this project is the creation of a new generation of magnetically active polymer materials (MPMs) capable of changing their physical properties in the presence of external magnetic fields in a controllable fashion. The composition of MPMs, with the main components being a polymer dispersion medium and magnetic filler, provides large variability in the properties of the composite, which allows it to be “adjusted” for use in a particular field. MPMs include both magnetoactive polymeric fluids based on polymer melts in a liquid state that belong to a larger class of magnetic or magnetorheological fluids (MRFs), and magnetoactive elastomers (MAEs) based on crosslinked polymer systems.
During the project, the main attention was paid to the creation of fundamentally new “active” dispersion media, allowing in situ control of the properties of MPMs in the integral temperature-magnetic field space. The new media are based on comb polymers with a high side chain grafting density (molecular bottlebrushes) with a low fraction of segregating side chains. Due to the incompatibility of side chains of different chemical nature with each other, microphase separation occurs in the system. Micellar aggregates formed by segregating chains play the role of physical crosslinks, facilitating the formation of bottlebrush elastomers at room temperature. Thus, the use of bottlebrush copolymers makes it possible, on one hand, to create low-modulus elastomers without the use of a low molecular weight solvent, which, as was shown at the previous stage of the project, are effective media for obtaining magnetopolymer materials with a high response to magnetic fields. On the other hand, the presence of physical crosslinks rather than chemical ones makes it possible to switch the system between the elastomeric and viscous states using the temperature field. In the viscous state, which corresponds to elevated temperatures, it is possible to realize the free rearrangement of magnetic particles when a magnetic field is applied. New structures are fixed in the material after cooling.
To obtain “active media”, brush copolymers of various compositions were synthesized: with side chains of polyisobutylene and polydimethylsiloxane, polystyrene was chosen as segregating blocks. The viscoelastic properties of the resulting polymer matrices of different chemical nature were studied depending on the composition and temperature. To study the structure of nanoaggregates formed in brush polymers and to determine the region of their stability, we performed computer simulations of brush copolymers of various structure, obtained the phase diagram of the system depending on the composition and block incompatibility parameter and studied their mechanical response.
Based on brush copolymers, a wide range of MPMs with different concentrations and types of magnetic particles (spherical and anisometric) has been created, and the possibility of programming and reprogramming the distribution of magnetic particles and properties of MPMs with variations in temperature and magnetic field has been demonstrated. It is shown that when a magnetic field is turned on at an elevated temperature, a significant (more than two orders of magnitude) increase in the elastic modulus of the MPM occurs due to the structuring of the magnetic filler along the magnetic field lines and strong magnetic interactions between particles. When cooled in a magnetic field, the magnetic structures are fixed by a physical network of micellar aggregates, due to which the elastic modulus of the sample when the field is turned off is several times higher than in the initial state. The degree of particle ordering can be controlled by changing the temperature at which the magnetic field is applied, thereby programming the resulting properties of the composite.
For the first time, siloxane comb-shaped polymers with a siloxane backbone and side chains were synthesized. The rheological properties were studied and MRFs with different concentrations of carbonyl iron were obtained. The MR effect demonstrated by the resulting materials exceeds three orders of magnitude, which is significant. Based on comb-shaped PDMS, low-modulus elastomeric MPMs with a giant response to a magnetic field have been obtained.
The influence of the anisometry of magnetic particles and their distribution in the polymer matrix was studied. It has been shown that the orientation of magnetic particles by applying an external magnetic field when preparing MAE samples is an effective way to create a material with controlled anisotropy of mechanical properties, and the use of anisometric particles can significantly enhance the anisotropy and elastic modulus of the material in the region of low filler concentrations. The advantage of polymer matrices with a brush structure is the absence of low-molecular components capable of sweating, the achievement of higher fillings when using anisometric (plate-like) particles, as well as a more significant magnetic response.
Viscoelastic and dielectric properties of elastomeric MPMs based on a hybrid filler consisting of a mixture of magnetically hard (neodymium-iron-boron) and magnetically soft (carbonyl iron) particles have been studied. A distinctive feature of such mixed fillers is their ability to create their own magnetic field after pre-magnetization. This simplifies the design of devices that use MPMs, but may not contain permanent magnets or electromagnets. It was shown that an increase in the content of carbonyl iron in the filler significantly increases the dielectric constant and conductivity of the samples. After magnetization, they increase considerably.
Theoretical studies of the formation of spherical filler particle clusters in magnetopolymer materials for MPMs with a volume concentration of soft magnetic filler ranging from 1% to 10%, Young’s modulus of the polymer matrix ranging from 10 kPa to 100 kPa, and external magnetic fields with magnetic flux up to 500 mT have been performed. As a result of considering a representative sample of the initial distributions of particles for each concentration value, it was shown that at low filler concentrations, the formation of ellipsoidal clusters with anisometry parameters belonging to the interval considered in this study (from 1 to 5) takes place. As part of the study of the mechanical response of a material cell containing a single anisotropic ferromagnetic inclusion, the optimal shape of ferromagnetic particles and the structures they form was considered based on resulting the properties of the composite. It was shown that composites with elongated inclusions have the highest stiffness, while using platelet inclusions allows for greater degree of control over the process of filler restructuring in the material and over the magnetorheological effect. It was found that the ratio of the magnetocrystalline anisotropy constant of a ferromagnetic inclusion to the effective elasticity modulus of the cell of 1.5 is a universal critical value separating the modes of monotonic and non-monotonic inclusion rotation in an external magnetic field. Thus, the results of theoretical work on the project are of both fundamental and practical importance for the targeted synthesis of new MPMs with the required properties.
The results of the project were reported at 11 conferences. Seven articles were published, four of them in highly rated journals of the first quartile, one article was accepted for publication, one article was sent to an editor and three more articles are being prepared for publication. The research results were used when delivering a course of lectures “Fundamentals of mechanics and rheology of polymers” (for graduate students of Moscow State University) and in the development and delivery of a new course of lectures “Magnetoactive polymer materials” for masters of the 1st year in the fall semester of 2023.