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Liquid-Crystalline Elastomer-Nanoparticle Hybrids with Reversible Switch of Magnetic Memory

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Liquid-Crystalline Elastomer-Nanoparticle Hybrids with Reversible Switch of Magnetic Memory
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  Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013. Supporting Information for Adv. Mater. ,   DOI: 10.1002/adma.201204406 Liquid-Crystalline Elastomer-Nanoparticle Hybrids with Reversible Switch of Magnetic Memory  Johannes M. Haberl, Antoni Sánchez-Ferrer, Adriana M.  Mihut, Hervé Dietsch, Ann M. Hirt, and Raffaele Mezzenga*     Submitted to 1 Supporting Information for: Liquid-Crystalline Elastomer-Nanoparticle Hybrids with Reversible Switch of Magnetic Memory By  Johannes M. Haberl  , and  Antoni Sánchez-Ferrer  ,  Adriana M. Mihut  ,  Hervé Dietsch ,  Ann  M. Hirt  ,  Raffaele Mezzenga *   J. M. Haberl, Dr. A. Sánchez. Ferrer, Prof. R. Mezzenga [*], ETH Zürich, Department of Health Science and Technology 8092 Zürich, Switzerland E-mail: raffaele.mezzenga@hest.ethz.ch Dr. A. M. Mihut † , Dr. H. Dietsch ††  Adolphe Merkle Institute and Fribourg Center for Nanomaterials, University of Fribourg 1723 Marly, Switzerland Prof. A. M. Hirt ETH Zürich, Department of Earth Science 8092 Zürich, Switzerland   In order to obtain homogeneous liquid-crystalline elastomer nanocomposites with well-dispersed nanoparticles into the network, special care was taken from the early functionalization of the nanoparticles up to their integration in the organic matrix, devoted to  prevent sedimentation at any stage of the process. This included sonication and periodic vortexing during the process. More details of the individual steps taken to the synthesis of the hybrid nanocomposites can be summarized as follow: Synthesis of the liquid-crystalline polymer Dimethyl biphenyl-4,4-dicarboxylate (12.1 g, 44.8 mmol, 1 eq) was mixed with triethylene glycol (7.06 g, 47.0 mmol, 1.05 eq), and titanium (IV) isopropoxide (5 mg). The mixture was heated to 200 °C in a nitrogen atmosphere for 6 h, and methanol was distilled off. The volatile components were removed and the temperature was increased to 230 °C for 1 h to obtain the liquid-crystalline polymer. [1]  In two extra steps, an excess of triethylene glycol (0.025 eq) was added to the mixture repeating the same procedure. The final linear polymer was dissolved in dichloromethane and purified by three times precipitating from methanol to obtain the product as a slightly yellowish glassy material (12.8 g, 70%). 1 H NMR (400 MHz, CDCl 3 ) !  = 8.10-7.95 (m, 4H, Ar-H), 7.65-7.50 (m, 4H, Ar-H), 4.50-4.37 (m, 4H, -CO 2 CH 2 -), 3.90-3.50 (m, 8.8H, -OCH 2 -) ppm; MS (MALDI-TOF):  M  n  = 2920 g " mol -1 ,  M  w  = 3510 g " mol -1 ,  DP   = 7.8,  PDI   = 1.2 (Fig. S1). Synthesis of the spindle type maghemite nanoparticles (SCH NPs). In a first step, the bare spindle hematite, ! -Fe 2 O 3 , nanoparticles (BH NPs) were synthesized  based on the method described by Ocaña et al. [2]  The particles were coated with a layer of silica using the approach of Graf et al  . [3]  based on an initial adsorption of  polyvinylpyrrolidone (PVP) on the particles to improve their colloidal stability and the subsequent addition of tetraethylorthosilicate (TEOS) as a precursor for the growth of the silica shell. The silica-coated spindle hematite nanoparticles (SCH NPs) dispersion was dried in an air oven at 90 °C for 24 hours. The dried powder is then annealed in a furnace at 360 °C under a continuous hydrogen gas flow. After 2 hours, the hydrogen flow is turned off and the   Submitted to 2  powder exposed to air. [4]  The furnace temperature is decreased to 240 °C during 2 hours. The obtained maghemite particles (SCM NPs) have a hybrid composition consisting of 70% maghemite and 30% hematite as determined from XRD data with Rietveld method. Surface functionalization of maghemite nanoparticles Surface functionalized silica coated maghemite nanoparticles (SCM NPs) were obtained following the previously reported method. [5]  The surface modification was ensured using 3-aminopropyltriethoxysilane (APTES) coupling agent (ABCR, Germany) used without  previous purification. In a typical example, 1 g of SCM NPs was transferred in a mixture of water (330 mL), absolute ethanol (1 L) and tetramethylammonium (12 mL, 25% solution in methanol). 23.6 g of APTES were added to the mechanically stirred suspension under sonication for 2 h at 20 °C. After stirring the suspension overnight, the obtained amino-functionalized SCM NPs were centrifuged at 10000 rpm for 15 min and redispersed in ethanol, repeating this process five times. Thereafter, the mixture was dispersed in 30 mL of dichloromethane. Synthesis of the organic-inorganic nanocomposite A dispersion of MNs in dichloromethane (3.17 g, 2.365 wt-%) was ultrasonicated during 30min, and then added to a triisocyanate crosslinker solution (67.4 mg, 0.130 mmol, 1.0 eq, Basonat HI100-BASF) in freshly distilled dichloromethane (1 mL). The crosslinker and the MNs were kept for 2 h, and the liquid-crystalline polymer (571 mg, 0.391 mmol, 3 eq) was dissolved in dichloromethane (absolute, 1 mL), together with dibutyltin dilaurate (1.55 g, 6 wt-% in dichloromethane). The two mixtures were merged, vortexed, poured in a Petri dish and kept for 18 h. The resulting crosslinked film was heated to 85 °C for 2 days. In order to remove all soluble content swelling in chloroform was used to obtain the final fully crosslinked elastomer (80%).  Methods: Differential Scanning Calorimetry (DSC) experiments were performed on a DSC 1 calorimeter from Mettler Toledo equipped with a Huber TC100 cooling system, where the nanocomposite was encapsulated in a 40 " L aluminum oxide crucible under nitrogen atmosphere. The sample was analyzed in a temperature range from -20 °C   to 200 °C with heating and cooling rates of 10, 15, 20 and 25 K  " min # 1 . The first heating curves were used for removing all thermal history from the sample, and the obtained transition temperatures were extrapolated to the combined 0-heating rate transitions. Bright-field images were taken with a Canon 550D digital camera. Transmission Electron Microscopy (TEM) micrographs were obtained on a Philips CM100-Biotwin microscope operating at 80 kV. The nanocomposite sample was ultramicrotomed using a Diatome diamond knife on a Reichert-Jung UltraCut E Microtome to give 80 nm thick sections. Sections were transferred onto 600-mesh copper grids. For the MNs, the sample was prepared by placing some drops of the 0.1 wt-% silica coated MNs suspension onto a carbon-coated copper grid. Small and Wide Angle X-ray Scattering (SWAXS) experiments were performed using a Rigaku MicroMax-002+ microfocused beam (4 kW, 45 kV, 0.88 mA) with the  ! Cu K  !   = 0.15418 nm radiation in order to obtain direct information on the scattering patterns. The scattering intensities were collected by a Fujifilm BAS-MS 2025 imaging plate system (15.2 x 15.2 cm 2 , 50 µ m resolved) and a 2D Triton-200 X-ray detector (20 cm diameter). An effective scattering vector range of 0.05 nm -1  < q < 25 nm -1  was obtained, where q  is the scattering wave vector defined as q  = 4 $ ·sin !  /  ! Cu K  !   with a scattering angle of 2 !  . For the order parameter of the nanoparticles, the SAXS pattern was evaluated at low scattering angles ( q  = 0.15-0.25 nm -1 ) that have been related to the nanoparticles orientation previously [6]  and where scattering is weak in a neat liquid-crystalline elastomer reference sample (Fig. S7). Low-field magnetic susceptibility   Submitted to 3 experiments were performed on a Kappabridge MFK1-FA from Agico. High-field magnetic torque experiments were performed with a homebuilt torque magnetometer with an accuracy of 2 " 10 -3  J " kg -1 . Detailed information on the instrument and the experimental method were  previously published. [7]  The ferromagnetic tensors OFF and ON were calculated from 15 measurements, using 6 different magnetic field values from  B  = 1000 mT to 1500 mT (100 mT steps). Uniaxial deformation of the sample at a rate of 1 mm s -1  (0.05 s -1 ) up to  !  = 3.2 lead to a shrinkage in the other two directions according to Poisson’s ratio for elastomers. The stretched sample was cut into a piece of length l  ’  z  1   = 60 mm, with a width of l’   x 1  = 5.1 mm and a thickness of l’   y 1  = 0.080 mm, it was folded two times to l  ’  z   = 20 mm and l’   y  = 0.240 mm (ON). The thin film in the OFF state had a length l   z   =18.5 mm width  of l   x  = 9.1 mm and thickness of l   y  = 0.140 mm. At the operating temperature of 80 °C the sample relaxed fast to its srcinal dimensions due to entropic elasticity, as confirmed by X-ray analysis.   Submitted to 4 Calculation of anisotropic magnetic susceptibility The calculation of the anisotropy of magnetic susceptibility was done on the basis of a Stoner-Wohlfarth model: [8,9]  As the magnetization  M   of a sample can be calculated from the volume susceptibility  K  v  with  H  K  M  v ! =  (1) at the applied magnetic field  H  , the anisotropy of magnetic susceptibility  M  M  K  K   2121  = . (2) In order to calculate  M   in the nanocomposite, we assume that the magnetic properties are the result of an ensemble of non-interacting single-domain particles following the assumptions made by Stoner and Wohlfarth. [8,9]  Magnetic coupling of grains or grains and the liquid-crystal [10]  is not assessed in this calculation, because of separation and isolation by the silica shell. So the sample magnetization in the measuring direction is proportional to the mean  particle magnetization ( ) ( )( )  !  " !  " = # 2020,, sinsin $ $  % % % % % % %  d  P d  M  P  M  M  ii samplei sample  (3) with the particle population P( "  ), where !   is the angle between the major particle axis and the applied field  H  , and i is the measuring direction (parallel to  H  ),  z   or   x . In order to get the orientation distribution function P( !  ) X-ray scattering patterns were evaluated [11,12]  in azimuthal steps a m+1/2 ( )  ( ) ( ) [ ] 1sin1 21221  + !" = ++  mS mS  a N a P  mm #   , (4) where 0 #   m   #  (  N - 1), ( )  ( )  ( ) ( )  !"#$%&'''( ) = + ' = '' 212221122121  coscoscoscos nmnm  N mnn N   aaaa a I mS   (5) and 2 !   "#$%&'( =  N na n . (6) In order to calculate  M  ( !  ) of individual particles, the magnetic energy U   has to be minimized [8]  

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Apr 28, 2018
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