Liquid Crystallline Polymers and Elastomers doped with magnetic nanoparticles in order to obtain composite materials changing their shape under magnetic field.
The nature is full of examples of systems responsive to external, physical or chemical stimuli. Synthetic systems as organic-inorganic hybrid materials, which answer to a magnetic field, mime this behaviour and because of that they are more and more studied. New properties, such as the change of shape by application of a magnetic field, are aimed. <br />On the other hand, liquid crystal elastomers, i.e. polymer networks carrying mesomorphous groups, are capable of very important shape changes due to orientation / disorientation of the mesomorphous groups. In this project, we suggest combining the just described approaches. <br />Our final goal is to make and to characterize a composite material, obtained by dispersing magnetic nanoparticles inside a liquid crystal elastomer. This kind of composite should be able to answering in a reversible way to different external stimuli, such as the temperature change, the mechanical strain or the magnetic field. We aim then at characterizing the interactions between particles and various elements of the matrix to be able to exploit all the potentialities of such a material.
The choice of the ingredients of the composite is very important in order to target the desired properties. The polymer we selected is a polysiloxane which has a very low glassy transition temperature and is thus very flexible at room temperature; furthermore it is easy to crosslink. The chosen nanoparticles are cobalt nanorods because they combine a strong anisotropy and a strong magnetization. In order to obtain the whished final elastomer we disperse the magnetic nanoparticles in the polymer before crosslinking.
As a first step we optimized some parameters like the nanoparticle quantity and the presence of interacting groups working on the polymer composites. Then, once the mesomorphous and magnetic properties were characterized, we studied the effect of nanoparticle doping on the liquid crystal order by means of X-ray scattering under magnetic field. We then synthesized and characterized elastomer composites. The nature of the mesomorphous phase as well as the phase transition temperatures were evaluated by means of differential scanning calorimetry and polarized optical microscopy; the magnetic properties of the elastomer have been estimated by means of hysteresis cycles.
In the polymer composites containing groups interacting with Co nanorods, the liquid crystal groups orient more easily than in the liquid crystal polymer alone. The differences are remarkable at low magnetic field.
We enlightened the origin of the strong coercive fields observed in the polymer composite at low temperature. An elastomer composite were the nanorods are aligned according to the magnetic field direction have been synthesized. This composite possesses highly anisotropic magnetic properties which can be exploited in the development of magnetic storage devices.
Liquid crystalline elastomers exhibit many distinctive features from standard elastomers, such as anisotropic mechanical response, and a structure a shape dependent deformation due to an external stimulus.
Till now the used stimuli to produce the shape change are mainly temperature, light or an electric field.
The exploitation of a magnetic field is not much investigated due to the too high field values necessary to align the mesogenic groups in a polymeric matrix, which makes their practical application difficult. This is a big disadvantage considering the potential fast and wireless action of a magnetic stimulus.
In doped mesogenic elastomeric material, the nanoparticles presence could lower the magnetic field threshold to induce deformation of the polymeric network. This would mean bigger effects at lower magnetic fields thanks to cooperative effects between particles and liquid crystal groups.
These materials could find many applications as actuators for example in microfluidics or medical devices.
Riou, O. et al. Polymers 2012, 4, 448-462.
In this article we compared two different methods to prepare the polymer composites.
Riou, O. et al. J. Phys. Chem. B 2014, 118, 3218-3225.
This work deals with the structural analysis of side chain liquid crystalline polysiloxanes, doped with magnetic cobalt nanorods and their orientational properties under a magnetic field. These new materials exhibit the original combination of orientational behaviour and ferromagnetic properties at room temperature.
Organic-inorganic soft materials, responsive to a magnetic field, are increasingly studied as alternative to the older rigid magnetic networks usually characterised by low flexibility and small shape changes.
At present, the main approach to obtain such materials consists in mixing an inorganic, field sensitive component (most frequently iron particles of nano- or microsizes) and an organic deformable matrix. New properties, such as shape change (using an electromagnetic induced heating on doped shape memory polyurethanes or polyacrylates) and tunable elastic modulus (doped polysiloxanes cross-linked in the presence of a magnetic field), for example, are aimed.
In the development of these composites, the phase separation between the two incompatible building blocks needs to be avoided. It is usually done by coating the inorganic nanoparticles with organic ligands, no attention for a more intimate interaction is looked for.
The use of an elastomeric material containing groups which interact with the nanoparticles and can respond to a magnetic field could produce stronger effects in term of shape change and anisotropic elastic properties.
Liquid crystal groups have these requisites. The studies about low molecular weight liquid crystal molecules doped with magnetic particles have already proved the possibility of cooperative effects, reflected in the lower magnetic fields necessary to align the liquid crystals.
These effects, observed in small molecules, could be exploited in elastomeric liquid crystal materials. Coupling the inherent advantages of magnetic nanoparticles and liquid crystal groups could result in materials with interesting magnetic properties and orientational behaviour, while providing the mechanical properties which are mandatory for applications such as actuators.
This approach combines the advantages of liquid crystal elastomers (good elastic properties, possibility to reversibly undergo very important shape changes, anisotropic character over a temperature range) with the wireless action of a magnetic field.
Research in this sense is just starting and is still immature: three publications on doped liquid crystal polymers and elastomers in the last three years. One of them is issued from the work of the coordinator during the last year: it proves (i) the feasibility of a new liquid crystal polymeric material with quite promising magnetic properties, and that (ii) nanoparticles’ re-orientation in a mesomorphous matrix is improved after application of a magnetic field.
The material’s microscopic structure is not well characterized and the interactions between the matrix (mesogens and interacting groups, polymeric chain) and the nanoparticles are not understood yet. Besides, the role of liquid crystal groups in the ordering of magnetic nanoparticles and the consequent potential for enhancing and modulating the shape change are not investigated yet.
MaCriLiMa is then a multidisciplinary project whose accomplishment needs complementary expertises in (a) the synthesis and characterization of organized systems and liquid crystal polymers and elastomers (IMRCP participants, Toulouse); (b) the synthesis and characterization of magnetic nanoparticles (LPCNO participants, Toulouse); (c) the study of the evolution of the microscopic organisation of the sample under magnetic field (IMRCP participants, Toulouse).
Madame Barbara LONETTI (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE MIDI-PYRENEES) – firstname.lastname@example.org
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IMRCP-UMR5623 CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE MIDI-PYRENEES
Help of the ANR 220,000 euros
Beginning and duration of the scientific project: - 36 Months