Magnetoresponsive smart nanocomposites with highly cross‐linked polymer matrix

Programming and Reprogramming the Viscoelasticity and Magnetic Response of Magnetoactive Thermoplastic Elastomers

We present a novel type of magnetorheological material that allows one to restructure the magnetic particles inside the finished composite, tuning in situ the viscoelasticity and magnetic response of the material in a wide range using temperature and an applied magnetic field. The polymer medium is an A-g-B bottlebrush graft copolymer with side chains of two types: polydimethylsiloxane and polystyrene. At room temperature, the brush-like architecture provides the tissue mimetic softness and strain stiffening of the elastomeric matrix, which is formed through the aggregation of polystyrene side chains into aggregates that play the role of physical cross-links. The aggregates partially dissociate and the matrix softens at elevated temperatures, allowing for the effective rearrangement of magnetic particles by applying a magnetic field in the desired direction. Magnetoactive thermoplastic elastomers (MATEs) based on A-g-B bottlebrush graft copolymers with different amounts of aggregating side chains filled with different amounts of carbonyl iron microparticles were prepared. The in situ restructuring of magnetic particles in MATEs was shown to significantly alter their viscoelasticity and magnetic response. In particular, the induced anisotropy led to an order-of-magnitude enhancement of the magnetorheological properties of the composites.

Digital light processing 3D printing of dynamic magneto-responsive thiol-acrylate composites

Additive manufacturing is one of the most promising processing techniques for fabricating customized 3D objects. For the 3D printing of functional and stimuli-triggered devices, interest is steadily growing in processing materials with magnetic properties. Synthesis routes for magneto-responsive soft materials typically involve the dispersion of (nano)particles into a non-magnetic polymer matrix. Above their glass transition temperature, the shape of such composites can be conveniently adjusted by applying an external magnetic field. With their rapid response time, facile controllability, and reversible actuation, magnetically responsive soft materials can be used in the biomedical field (e.g. drug delivery, minimally invasive surgery), soft robotics or in electronic applications. Herein, we combine the magnetic response with thermo-activated healability by introducing magnetic Fe₃O₄ nanoparticles into a dynamic photopolymer network, which undergoes thermo-activated bond exchange reactions. The resin is based on a radically curable thiol-acrylate system, whose composition is optimized towards processability via digital light processing 3D printing. A mono-functional methacrylate phosphate is applied as a stabilizer to increase the resins' shelf life by preventing thiol-Michael reactions. Once photocured, the organic phosphate further acts as a transesterification catalyst and activates bond exchange reactions at elevated temperature, which render the magneto-active composites mendable and malleable. The healing performance is demonstrated by recovering magnetic and mechanical properties after the thermally triggered mending of 3D-printed structures. We further demonstrate the magnetically driven movement of 3D-printed samples, which gives rise to the potential use of these materials in healable soft devices activated by external magnetic fields.

Theoretical Modeling of Magnetoactive Elastomers on Different Scales: A State-of-the-Art Review

A review of the latest theoretical advances in the description of magnetomechanical effects and phenomena observed in magnetoactive elastomers (MAEs), i.e., polymer networks filled with magnetic micro- and/or nanoparticles, under the action of external magnetic fields is presented. Theoretical modeling of magnetomechanical coupling is considered on various spatial scales: from the behavior of individual magnetic particles constrained in an elastic medium to the mechanical properties of an MAE sample as a whole. It is demonstrated how theoretical models enable qualitative and quantitative interpretation of experimental results. The limitations and challenges of current approaches are discussed and some information about the most promising lines of research in this area is provided. The review is aimed at specialists involved in the study of not only the magnetomechanical properties of MAEs, but also a wide range of other physical phenomena occurring in magnetic polymer composites in external magnetic fields.

Modeling of the Electrotransport Process in PP-Based and PLA-Based Composite Fibers Filled with Carbon Nanofibers

Polypropylene and polylactide-based composite fibers have been produced by a melt technology. Long vapor-grown carbon fibers (CNFs) have been used as electrical conductivity fillers. It is clearly shown by experimental methods that the CNFs are evenly distributed in the polymer matrix, orienting themselves along the direction of fiber extrusion and retaining their initial dimensions. It is shown that for composites fibers based on crystallizing (polypropylene) and amorphous (polylactide acid) polymer matrix, the dependence of electrical resistance on the filler concentration is percolation character and can be described as a double Boltzmann function. Four sections are identified on the dependences of the electrical resistance on the filler concentration for composite fibers, and the reasons for this character of this dependence on the formation of electrically conductive circuits are analyzed. Investigated in this work are the PP-based and PLA-based composites filled with carbon nanofibers that can be used as antistatic, shielding materials, or as sensors.

Effect of stoichiometry on crosslinked epoxy resin characteristics: structural heterogeneities, topological defects, properties, free volume and segmental mobility

Experimental studies have shown that changes in stoichiometry (R, ratio of amine groups to epoxy groups) cause considerable variations in the properties of epoxy-amine systems. Rationales based on free volume concepts have been routinely used to address these variations in properties but have hardly been satisfactorily substantiated. Many of these rationales remain as unverified conjectures to date. Substantiating these rationales will certainly bolster our understanding of the structure-stoichiometry-property relationship, but is difficult, due to inherent challenges involved in unambiguously characterizing the structural heterogeneities induced by changes in stoichiometry (structural heterogeneities include compositional distribution in the functionality of monomers, non-uniform dispersion of elastic chains and topological defects). The aim of the present work is to gain molecular-level insights into this relationship and to verify the rationales that rely on free volume concepts used for addressing the variations in properties with stoichiometry, with the help of all-atom molecular dynamics (MD) simulations. Five epoxy-amine systems with varying R ranging from 0.4 to 3, including the stoichiometric system (R = 1), were considered for these purposes. The properties of interest namely density, glass transition temperature (T_g) and thermal expansion coefficient in the rubbery state (α_rl) of these systems were predicted. The local structure, fractional free volume and segmental mobility of these systems were then subsequently characterized as a function of stoichiometry and the results were analysed in detail. The role played by defects in properties and fractional free volume was then investigated. The results revealed significant insights into the compositional distribution of monomers with different functionalities as well as offered insights into the dispersion state and mobility of dangling chains, sols and elastic chains in the systems. Further, strong correlations were found between defect composition, fractional free volume at an elevated temperature (600 K) and thermomechanical properties (T_g and α_rl) and it was established that the key mechanism underlying these correlations was the plasticization caused by defects. Analysis based on the rule of mixture models showed that these correlations were found to be in good agreement with the interpretations based on free volume concepts. The results also revealed a strong negative correlation between fractional free volume at room temperature and defect composition, a phenomenon typically associated with the antiplasticization effect.

Effect of Nanoparticles Surface Bonding and Aspect Ratio on Mechanical Properties of Highly Cross-Linked Epoxy Nanocomposites: Mesoscopic Simulations

The paper aims to study the mechanical properties of epoxy resin filled with clay nanoparticles (NPs), depending on their shapes and content on the surface of a modifying agent capable of forming covalent bonds with a polymer. The cylindrical clay nanoparticles with equal volume and different aspects ratios (disks, barrel, and stick) are addressed. The NPs' bonding ratio with the polymer (RGC) is determined by the fraction of reactive groups and conversion time and varies from RGC = 0 (non-bonded nanoparticles) to RGC = 0.65 (more than half of the surface groups are linked with the polymer matrix). The performed simulations show the so-called load-bearing chains (LBCs) of chemically cross-linked monomers and modified nanoparticles to determine the mechanical properties of the simulated composites. The introduction of nanoparticles leads to the breaking of such chains, and the chemical cross-linking of NPs with the polymer matrix restores the LBCs and strengthens the composite. At small values of RGC, the largest value of the elastic modulus is found for systems filled with nanoparticles having the smallest surface area, and at high values of RGC, on the contrary, the systems containing disk-shaped particles with the largest surface area have a larger elastic modulus than the others. All calculations are performed within the framework of a mesoscopic model based on accurate mapping of the atomistic structures of the polymer matrix and nanoparticles into coarse-grained representations, which, if necessary, allow reverse data mapping and quantitative assessment of the state of the filled epoxy resin. On the other hand, the obtained data can be used to design the functional materials with specified mechanical properties based on other practically significant polymer matrices and nanofillers.