Dimitris Elias Katsoulis

A new approach to the synthesis of cage-like metallasiloxanes

Abstract A new approach to the synthesis of cage-like metallasiloxanes has been proposed. Copper-containing metallasiloxanes {Na4[(RSiO2)12Cu4](R′OH)n} (1a, R=Ph, R′=n-Bu; 1b, R=Ph, R′=Me; 2, R=Vi, R′=Me) were prepared from PhSi(OR)3·(R=n-Bu, Me) and ViSi(OMe)3, respectively. Compound 2 was characterized by X-ray single structure analysis. The structures of compounds 1a,b were deduced by their follow up reactions with Me3SiCl.

Gelation of silicone fluids using 1,3:2,4-dibenzylidene sorbitol

A variety of silicone fluids have been found to gel using low concentrations (typically < 4 wt.%) of 1,3:2,4-dibenzylidene sorbitol (DBS). DBS is known to be a chiral gelator for many organic solvents. Gels were formed when small amounts of DBS were introduced into the silicone fluids either by heating to high temperatures or at ambient temperatures with the use of a co-solvent. Optical and electron microscopy of neat silicon–DBS gels (concentration as low as 0.005 wt.%), revealed the formation of two types of fibrous network. One consisted of ribbon-like macrofibres (average width, ca. 2–3 µm) present in the opaque region of the gels and the other consisted of dense intertwined microfibres (average width, ca. 100 nm) present in the clear portion of the gels. The gels prepared in the presence of a co-solvent consisted only of the microfibrous network. N-Methylpyrrolidone was found to be a very effective co-solvent for the phenyl-containing siloxanes producing firm clear gels. Dynamic mechanical measurements indicated that the storage modulus (G′) of these gels increased with increasing DBS content over the range 1–6 wt.% DBS. Comparison with propylene glycol–DBS gels showed the silicone gels to differ in that they comprised mostly an isotropic phase with no detectable crystalline phase present.

New mesomorphic organocyclosiloxanes II. Thermal behaviour and mesophase structure of organocyclohexasiloxanes

Investigations of thermotropic phase transitions performed on organocyclosiloxanes [PhSi(O)OSiR]6, where R is Me3, Me2(CH2Cl) or Me2(CH≃CH2), have revealed that all these hexamers are mesomorphic compounds. The hexamers exhibit uncommon polymesomorphic behaviour forming two quite different mesomorphic structures. The molecular arrangement in the low temperature (LT) modification is characterized by two-dimensional (2D) long-range order with hexagonal packing. The X-ray diffraction pattern and peculiarities of molecular packing in the crystal lead us to suggest that the LT-mesophase is columnar, presumably of the Colhd type. The LT-mesophase is formed by dimeric moieties, which associate with each other in column-like substructures, the ring planes not orthogonal to the stack axis. The high temperature (HT) mesophase is a plastic crystal (3D-order), where molecules take up positions in a face-centred cubic lattice. This is a very uncommon example of thermal behaviour for plastic crystals that provides a unique opportunity to bridge the gap between plastic crystalline and liquid crystalline mesomorphic behaviour. The thermal and structural properties of the mesophases depend upon the type of side groups of the hexamers. The size of the ring also affects the phase behaviour and the mesomorphic structure. This conclusion is consistent with data obtained by us earlier for cyclotetrasiloxanes.

Gelation of silicone fluids using cholesteryl esters as gelators

A variety of silicone fluids were found to gel using low concentrations ( < 2wt.%) of cholesteryl anthraquinone-2-carboxylate (CAQ) and cholestanyl anthraquinone-2-carboxylate (CHAQ). These compounds are known to be effective gelators for a variety of organic fluids. The gelation process involved the use of a co-solvent to incorporate the CAQ and CHAQ into the silicone fluid, followed by its removal via a thermal treatment. Semi-clear to clear irreversible gels were formed in this way and optical and scanning electron microscopy revealed that their microscopic features are similar to those reported for the organic fluids. They form intertwined microscopic fibres which create a three-dimensional network that effectively immobilizes the silicone fluid. The silicone gels were found to be more stable at high temperatures than the organic gels. This is attributed to the much lower solubility of the gelator in silicone fluids at all temperatures. A number of other simple cholesteric compounds were also investigated as gelators of silicones. From those, cholesteryl phenylacetate was found to form a macrocrystalline network and to entrap the silicone fluid in a fashion analogous to CAQ. Owing to their large macrofibres, opaque paste-like gels were obtained.

Mechanical Properties and Toughening of a Polymethylsilsesquioxane Network

Polysilsesquioxanes are finding wider uses in microelectronics. However these materials are brittle and their mechanical behaviors need to be better understood. In this paper the mechanical properties of a cured methyl silsesquioxane network are studied and they are found to vary strongly with curing conditions. In the investigated curing temperature range there seems to be a correlation between the fracture toughness (K Ic ) and the viscoelastic transitions, which are affected by the network crosslink density. A higher crosslink density suppresses the cc and β transitions and reduces the K Ic . A toughening approach is designed and its applicability is demonstrated by the increased fracture toughness.

Reactions of heteropolyanions in non-polar solvents. Part 3. Activation of dioxygen by manganese(II) centres in polytungstates. Oxidation of hindered phenols

The heteropolyanions [XW11O39{MnII(OH2)}]n–[X = P(1), Si (2), Ge (3), or B (4)] and α2-[P2W17O61{MnII(OH2)}]8–(5) have been transferred into non-polar solvents (benzene, toluene) with the aid of tetraheptylammonium bromide or other phase-transfer agents. Following dehydration of their non-polar solutions, anions (2) and (3) show reactivity towards dioxygen. At temperatures below –35 [(2)] and ca. 22 °C [(3)] oxygenation produces a reversible colour change (λmax 475, 585 nm); above these temperatures oxygenation results in irreversible oxidation to the manganese(III) heteropolyanions (λmax 520 nm). The coloured oxygenated product is e.s.r.-inactive and is not formed in polar solvents nor in the presence of other polar solutes such as alcohols, pyridine, etc. Traces of water are, however, necessary for the formation of the colour, which intensifies as the temperature is lowered, and it is proposed that a weak dioxygen adduct is stabilised by a hydrogen-bonded water molecule. The rapid oxidation of anhydrous (2) by dioxygen was carried out in the presence of the spin trap 5,5-dimethyl-1-pyrroline N-oxide (dmpo). E.s.r. spectra indicate the transient formation of a polyanion–O2–dmpo complex that decomposes to oxidised polyanion and dmpo–O2H*. Solutions of anions (2), (3), and (5) catalyse the oxidation of 2,6- and 2,4,6-substituted phenols to benzoquinones or polyphenyl ethers, and e.s.r. evidence of polyanion–O2–phenoxy radical complex formation is adduced. Comparisons are drawn with the chemistry of dioxygen complexes of cobalt(II). Anion (1) is neither oxygenated nor oxidised by O2, and (4) is rapidly oxidised at –70 °C. Both behaviours are understandable in terms of the manganese(III)–manganese(II) redox potentials.

Silicone-Polyoxometalate (SiPOM) Hybrid Compounds

Oligomeric and polymeric SiPOM materials have been prepared by reacting trichloro endblocked PDMS [degree of polymerization, ∼ 14 (I) and 60 (II)] with α-K 8 SiW 11 O 39 . NMR and IR spectroscopy show that the POM units are bonded to the siloxane via Si-O-W bonds. The reaction of the short PDMS segment (I) with the POM produces powdery materials, (isolated as tetrabutylammonium salts) while the longer PDMS (II) forms elastomeric materials. For the latter the POM anions act as reinforcing fillers. Other important properties of the hybrid materials are their UV absorbency, their electrochromicity, their electrical properties (volume resistivity, ∼10 11 ohm-cm; low loss tangent, memory capability) and their film formation.

Siloxanes and Silicones (Advances in Silicone Technologies 2000–15)

Abstract Silicone materials and product technologies have steadily advanced since the turn of the 21st century with the majority of advances driven by new application needs. Established silicone manufacturers or specialty formulators have primarily been responsible for the developments of new materials. The most significant advances have been in the development of silicone materials with enhanced adhesive, optical, thermal management, and aesthetics. The impact of these new materials technologies are seen in lower assembly processing temperatures, extended lifetimes of photovoltaic systems and energy efficient light emitting diode lighting devices, and improved feel of personal care and beauty products. In addition to leveraging the inherent properties of silicones, advances have been made by introducing new chemistries, such as dual cure systems and alpha silanes that speed cure rates and chemically bond to a wider range of substrates. New delivery options such as hot melts and swollen elastomers provide rapid green strength adhesives and add a “velvety” feel to beauty care products, respectively.

Free-standing films based on silicone resins

We report here the preparation of transparent, flexible films from silicone resins by solvent casting techniques. The films exhibit no birefringence, higher than 90% transparency between 350 to 1700 nm and average surface roughness below 1 nm. Thermal analysis shows that the films are stable at temperatures greater than 200 °C (depending upon the starting resin composition). The films are suitable substrates for deposition of various coatings including indium tin oxide (ITO). Transparent conducting ITO electrodes were prepared with ion plating and RF sputtering methods and characterized by microscopy, XPS, absorption spectroscopy and electrical conductivity measurements. Potential applications for silicone resin films exist in the market areas of displays, electronics and energy.

Preparation and characterization of encapsulated solid particles, composed of partially hydrolysed aluminium and zirconium oxochlorides

Partially hydrolysed aluminium oxochlorides (ACH) and aluminium–zirconium–glycine oxochlorides (AZG) have been encapsulated using lipophilic carboxylic acids from H2O–silicone solvent systems. The encapsulated materials (E-materials) precipitate out of solution in the form of spherical beads. Their particle size varies with agitation conditions. Distributions from a few tenths to a few hundreds of micrometres are easily produced.ESCA sputtering experiments and elemental analysis indicate that the beads are composed of a water-soluble ACH or AZG inner core that is surrounded by a thin lipophilic shell a few angstroms in thickness. We believe that the encapsulant is held around the core via, hydrogen bonding and coulombic interactions. Atomic absorption spectrometry was used to measure the release rates of ACH and AZG from the E-materials in ethanol–water systems. The release rates are affected by the amount of H2O in the mixed solvents, the type of encapsulant, and the particle size of the beads.