No abstract

Colloids of TiO2, where rutile was the only crystal modification which could be detected, with ca. 2.5 nm average particle diameter were synthesized by hydrolysis of TiCl4 in acidic solutions. The as‐prepared particles were incorporated in polymers such as poly(vinyl alcohol) (PVAL), partially hydrolyzed poly(vinyl acetate) (PVAC88), polyvinylpyrrolidone, and poly(4‐vinylpyridine). Nanocomposites transparent in the visible range were obtained. The highest TiO2 contents in such materials were achieved with PVAL and PVAC88, with TiO2 contents of ca. 35 wt.‐% (i.e. 10.5 vol.‐%). In particular, the nanocomposites with TiO2 contents above 24 wt.‐% acted as efficient UV filters for radiation up to ca. 360 nm. At very low TiO2 contents, an absorption maximum of the embedded TiO2 particles was observed at 225 nm with an extinction coefficient of 140 000 cm−1 and a full width at half maximum of 45 nm, i.e. not only the absorption at the maximum at 225 nm but also at the flank of this band contributed significantly to the broadband UV absorption in the nanocomposites at higher TiO2 fractions. The incorporation of TiO2 enhanced the refractive index of the nanocomposites: for instance a refractive index of 1.609 was measured for a nanocomposite comprising 10.5 vol.‐% TiO2 in PVAL, compared with 1.521 for the pristine polymer. TEM image of a section of a nanocomposite of poly(vinyl alcohol) and 11 wt.‐% TiO2 (appearing dark).magnified imageTEM image of a section of a nanocomposite of poly(vinyl alcohol) and 11 wt.‐% TiO2 (appearing dark).

We present a facile, high-yield synthetic route to linear poly(dibutylstannane) of relatively high molecular weight (M w = 20 6 10 3 g mol 21 ) through dehydropolymerization of dibutylstannane with the catalyst [RhCl(PPh 3 ) 3 ]. Unlike previously synthesized polystannanes, the present materials were entirely free of cyclic oligomers. A reversible phase transition was observed at a temperature of y1 uC, which was found to be associated with a marked change in crystalline order. The material appeared to be in a liquid-crystalline-like state at room temperature and could be readily oriented by simple shearing processes, which resulted in highly ordered films.

A comprehensive study was made of the synthesis of a spectrum of poly(dialkylstannane)s by catalytic dehydropolymerization of dialkylstannanes (dialkyltin dihydrides, R 2 SnH 2 , prepared by reduction of R 2 SnCl 2 ), with R ) ethyl, propyl, butyl, pentyl, hexyl, octyl, and dodecyl. The polymerization reactions were followed by 1 H and 119 Sn NMR spectroscopy, IR spectroscopy (disappearance of the Sn-H vibration), and quantitative measurement of H 2 which evolved during the reaction. Among the numerous metal complexes employed as catalyst, [RhCl(PPh 3 ) 3 ] was found to be particularly suited for the preparation of these inorganic polymers. The reaction parameters temperature, solvent, R 2 SnH 2 concentration, and [RhCl(PPh 3 ) 3 ]/R 2 SnH 2 ratio were varied, with the most prominent influence on the monomer conversion being the temperature. The numberaverage molar masses of the polystannanes were in the range of 1 × 10 4 to 1 × 10 5 g/mol, depending on the reaction conditions. For the generic case of the polymerization of Bu 2 SnH 2 with [RhCl(PPh 3 ) 3 ] as catalyst, it was demonstrated that poly(dibutylstannane) did not form by a random polycondensation, but by growth at a rhodium complex, whereby only a minor part of [RhCl(PPh 3 ) 3 ] was converted to catalytically active species by reaction with tin hydrides. The polymers featured phase transitions into liquid-crystalline states, on occasion at remarkably low temperatures. A particularly high phase transition temperature was observed for poly(dipropylstannane), which also was characterized by a high density, indicative of a particularly favorable packing of the propyl groups.

No abstract

Composites of trimethylammonium-modified nanofibrillated cellulose and layered silicates (TMA-NFC/LS) were prepared by high-shear homogenization followed by pressure filtration and vacuum hot-pressing, which gave rise to particularly homogeneous dispersion of the silicate particles. Thirteen different clays and micas were employed. Water vapor barrier and mechanical properties (tensile strength, E-modulus, strain at break) of the composite films were investigated, considering the effects of layered silicate types and their concentration (in the range of 0 to 85 wt %). Good interactions between TMA-NFC and LS were obtained due to electrostatic attraction between cationic fibrils and anionic silicate layers, and even favored by high-shear homogenization process. Furthermore, oriented TMA-NFC/LS composite structure was achieved. Layered silicates exerted a pronounced influence on the water vapor barrier and mechanical properties; however, there was no common trend reflecting their types. The transport of water molecules through TMA-NFC/LS composites was studied considering both diffusion and adsorption mechanisms. As a result, diffusion pathways were proposed based on two new and one well-known models: the "native network", "covered fiber composite", and "fiber-brick composite" models. Importantly, it was found that the insertion of layered silicate particles did not improve automatically the barrier properties as indicated by the commonly used "fiber-brick composite" model. Mica R120 at a 50 wt % loading in composites with TMA-NFC matrix showed 30-fold improved water vapor permeability and 5-fold higher E-modulus compared to commercially used base paper.

Although composites with, e.g.nanosized SiO2, TiO2, carbon black or gold are known for a long time, an increasing number of polymer composites comprising inorganic nanoparticles have been described only in the last 15 years. Frequently employed inorganic materials include metals (e.g. gold, silver or copper), semiconductors (e.g. PbS or CdS) or clay minerals (e.g.montmorillonite or vermiculite). In most cases, nanocomposites with spheric or plate like particles have been prepared so far but materials with nanorods (rod like particles, including nanotubes) also attracted attention. The structure of nanocomposites is essentially established by the arrangement of the particles in the polymer matrix. The particles may be dispersed as individual primary particles or as agglomerated particles (secondary particles), and the primary or secondary particles can be arranged randomly or in an ordered or oriented state, depending on the method of nanocomposite preparation and processing (including, e.g. coprecipitation, coevaporation, spin coating or drawing). In order to avoid agglomeration, the nanoparticles are frequently synthesised in situ or applied as surface modified particles, where a spheric inorganic core is preferentially surrounded by a shell composed of organic molecules (core–shell particles). Surface modification is applied routinely for clays, which is of importance for the degree of delamination of the individual silicate layers in the matrix (intercalation or exfoliation). Materials properties of nanocomposites, such as optical transparency, colour (including dichroism), iridescence, catalytic activity or superparamagnetism, can be favourable when compared with analogous composites with larger particles, and these properties may also be influenced by the structure of nanocomposites. The properties of nanocomposites are considered to provide applications in the areas of, e.g. optics, catalysis, electronics, magnetism or mechanics.

Poly(phenylene methylene) (PPM) exhibits pronounced blue fluorescence in solutions as well as in the solid state despite its non-p-conjugated nature. Optical spectroscopy was used to explore the characteristics and the physical origin of its unexpected optical properties, namely absorption in the 350-450 nm and photoluminescence in the 400-600 nm spectral regions. It is shown that PPM possesses two discrete optically active species, and a relatively long photoluminescence lifetime (>8 ns) in the solid-state. Given the evidence reported herein, p-stacking and aggregation/crystallization, as well as the formation of anthracene-related impurities, are excluded as the probable origins of the optical properties. Instead there is sufficient evidence that PPM supports homoconjugation, that is: p-orbital overlap across adjacent repeat units enabled by particular chain conformation(s), which is confirmed by DFT calculations. Furthermore, poly(2-methylphenylene methylene) and poly(2,4,6-trimethylphenylene methylene) -two derivatives of PPM -were synthesized and found to exhibit comparable spectroscopic properties, confirming the generality of the findings reported for PPM. Cyclic voltammetry measurements revealed the HOMO-LUMO gap to be 3.2-3.3 eV for all three polymers. This study illustrates a new approach to the design of light-emitting polymers possessing hitherto unknown optical properties.