Ferronematic Co(II) Complex: An Active Filler for Magnetically Actuated Soft Materials

Ferronematics that are generally based on nematic liquid crystals (LCs) doped with magnetic nanoparticles, synergistically taking advantage of the anisotropic and flow characteristics of the nematic host and the magnetic susceptibility of the dopant, have powerful applications as magnetically actuated soft materials. In this work, a Co(II) complex, which alone presents both characteristics, is built with a salen-type ligand 3,5-dichlorosubstituted at the aromatic nuclei and has a tetramethyldisiloxane spacer, which makes it one of the few metallomesogens containing this structural motif. Paramagnetic crystals, through heat treatment above 110 °C, change into magnetic nematic LCs. Applying a perpendicular magnetic field of 50 mT, the nematic droplets align two by two through dipole-dipole interactions. By incorporating it into a silicone matrix consisting mainly of polydimethylsiloxane, a 3D printable ink is formulated and crosslinked under various shapes. In this environment, the cobalt complex is stabilized in an LC state at room temperature and, due to its anisotropy, facilitates the mechanical response to magnetic stimuli. The resulting objects can be easily manipulated on fluid or rough surfaces using external magnetic fields, behave like magnets by themselves, and show reversible locomotion.

From passive to emerging smart silicones

Amassing remarkable properties, silicones are practically indispensable in our everyday life. In most classic applications, they play a passive role in that they cover, seal, insulate, lubricate, water-proof, weather-proof etc. However, silicone science and engineering are highly innovative, seeking to develop new compounds and materials that meet market demands. Thus, the unusual properties of silicones, coupled with chemical group functionalization, has allowed silicones to gradually evolve from passive materials to active ones, meeting the concept of “smart materials”, which are able to respond to external stimuli. In such cases, the intrinsic properties of polysiloxanes are augmented by various chemical modifications aiming to attach reactive or functional groups, and/or by engineering through proper cross-linking pattern or loading with suitable fillers (ceramic, magnetic, highly dielectric or electrically conductive materials, biologically active, etc.), to add new capabilities and develop high value materials. The literature and own data reflecting the state-of-the art in the field of smart silicones, such as thermoplasticity, self-healing ability, surface activity, electromechanical activity and magnetostriction, thermo-, photo-, and piezoresponsivity are reviewed.

From Amorphous Silicones to Si-Containing Highly Ordered Polymers: Some Romanian Contributions in the Field

Polydimethylsiloxane (PDMS), in spite of its well-defined helical structure, is an amorphous fluid even at extremely high molecular weights. The cause of this behavior is the high flexibility of the siloxane backbone and the lack of intermolecular interactions attributed to the presence of methyl groups. These make PDMS incompatible with almost any organic or inorganic component leading to phase separation in siloxane-siloxane copolymers containing blocks with polar organic groups and in siloxane-organic copolymers, where dimethylsiloxane segments co-exist with organic ones. Self-assembly at the micro- or nanometric scale is common in certain mixed structures, including micelles, vesicles, et cetera, manifesting reversibly in response to an external stimulus. Polymers with a very high degree of ordering in the form of high-quality crystals were obtained when siloxane/silane segments co-exist with coordinated metal blocks in the polymer chain. While in the case of coordination of secondary building units (SBUs) with siloxane ligands 1D chains are formed; when coordination is achieved in the presence of a mixture of ligands, siloxane and organic, 2D structures are formed in most cases. The Romanian research group's results regarding these aspects are reviewed: from the synthesis of classic, amorphous silicone products, to their adaptation for use in emerging fields and to new self-assembled or highly ordered structures with properties that create perspectives for the use of silicones in hitherto unexpected areas.

Silicone dielectric elastomers optimized by crosslinking pattern – a simple approach to high-performance actuators

Chemical design of silicone elastomers for improving the electromechanical response of dielectric elastomer actuators.

From iron coordination compounds to metal oxide nanoparticles

Various types, shapes and sizes of iron oxide nanoparticles were obtained depending on the nature of the precursor, preparation method and reaction conditions. The mixed valence trinuclear iron acetate, [Fe₂^IIIFe^IIO(CH₃COO)₆(H₂O)₃]·2H₂O (FeAc1), μ₃-oxo trinuclear iron(III) acetate, [Fe₃O(CH₃COO)₆(H₂O)₃]NO₃∙4H₂O (FeAc2), iron furoate, [Fe₃O(C₄H₃OCOO)₆(CH₃OH)₃]NO₃∙2CH₃OH (FeF), iron chromium furoate, FeCr₂O(C₄H₃OCOO)₆(CH₃OH)₃]NO₃∙2CH₃OH (FeCrF), and an iron complex with an original macromolecular ligand (FePAZ) were used as precursors for the corresponding oxide nanoparticles. Five series of nanoparticle samples were prepared employing either a classical thermal pathway (i.e., thermal decomposition in solution, solvothermal method, dry thermal decomposition/calcination) or using a nonconventional energy source (i.e., microwave or ultrasonic treatment) to convert precursors into iron oxides. The resulting materials were structurally characterized by wide-angle X-ray diffraction and Fourier transform infrared, Raman, energy-dispersive X-ray, and X-ray fluorescence spectroscopies, as well as thermogravimetric analysis. The morphology was characterized by transmission electron microscopy, atomic force microscopy and dynamic light scattering. The parameters were varied within each route to fine tune the size and shape of the formed nanoparticles.

Changes induced in the properties of dielectric silicone elastomers by the incorporation of transition metal complexes

Three new, cheap, and easily obtained complexes of triethanolamine with Cu(II), Ni(II), and Co(II) were incorporated in a high-molecular-weight polydimethylsiloxane-α,ω-diol (number-average molecular weight = 440,000 g mol−1) together with silica nanoparticles. While silica has a well-known mechanical reinforcing role, adding the metal complexes with high dielectric permittivity aims to increase values for this characteristic of the prepared silicone composites in order to be useful for electromechanical applications. A simple procedure was employed for the incorporation of fillers by mechanical mixing in bulk without a solvent. The composites were processed as films and stabilized by curing initiated with 2,4-dichlorobenzoyl peroxide at high temperature (180°C) and pressure. The cross-linking yield was estimated on the basis of swelling in chloroform, while the cross-linking density was calculated from differential scanning calorimetry traces. The filler’s effect on mechanical and dielectric properties was assessed by appropriate measurements (tensile testing and dielectric spectroscopy). Changes in dielectric behavior as a result of the repeated deformation by stretching were also highlighted. Electromechanical sensitivity and ultimate tensile toughness values were estimated on the basis of these results as primary indices on suitability of these materials in transducers. The dielectric strength of the composite films was also evaluated by electrical breakdown field measurements.

Interpenetrating poly(urethane-urea)–polydimethylsiloxane networks designed as active elements in electromechanical transducers

Original interpenetrating polymer networks suitable for active dielectric elements in actuation devices were prepared by using a dual compatibilization pathway.

Aging behavior of the silicone dielectric elastomers in a simulated marine environment

A series of silicone–barium titanate composites, designed as dielectric elastomer films to be used as active elements in wave energy conversion devices, were immersed in artificial sea water in pseudo-dynamic conditions to asses their aging behavior.

Goethite nanorods as a cheap and effective filler for siloxane nanocomposite elastomers

Composites based on PDMS and goethite nanorods are for the first time approached from the perspective of dielectric elastomers.

Highly stretchable composites from PDMS and polyazomethine fine particles

Polyazomethine fine particles are obtained in reverse micelles of a siloxane surfactant and used as a dispersed phase in PDMS to afford materials with improved and tunable electric and mechanical properties.

Superparamagnetic amorphous iron oxide nanowires self-assembled into ordered layered structures

The thermal decomposition of μ3-oxo trinuclear iron(iii) acetate in presence of dodecylamine and oleic acid, in trichloroacetic acid at 320 °C was optimized to obtain iron oxide nanoparticles with pure nanowire morphology.