Materials Literature
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Two hot areas in chemistry are dendrimers—highly branched polymers that are synthesized in stages from a core, in a manner resembling a snowflake—and self-assembly, the synthesis of molecules through molecular recognition. These ideas have been combined by Zimmerman et al. (p. 1095) in a report in this week's issue of…
Hydrogen bond-mediated self-assembly is a powerful strategy for generating large structures from smaller subunits. The synthesis of molecules containing two isophthalic acid units covalently attached to a rigid aromatic spacer is described. By normal pairing of carboxylic acids into hydrogen-bonded dimers, these molec…
The melting behavior of the alternating copolymer of ethylene and tetrafluoroethylene (ETFE) was investigated by differential scanning calorimetry (DSC). A complex melting pattern is observed, strongly influenced by the thermal history. Three melting processes can be identified, whose heats of fusion and peak temperat…
Monte Carlo computations employing a pivot algorithm have been used to obtain rather precise results for a linear bead-straw chain. The results are used to discriminate between several proposals for the asymptotic long-chain universal form of the distribution of the end-to-end length. It is found that a prediction bas…
BN molecules, isoelectronic with C2, are produced by laser vaporization of boron nitride and trapped in solid neon. Through Fourier-transform absorption and laser-induced fluorescence spectra, three new electronic states are observed. Unlike C2, BN has a triplet ground state (X 3Π), but with an exceedingly low-lying s…
The homogeneous grafting of acrylonitrile onto cellulose was carried out in a dimethyl sulfoxide/paraformaldehyde solvent system. The grafted products were added to cellulose/polyacrylonitrile (PAN) blends as compatibilizers. The thermal decomposition behavior of the blends was investigated by thermogravimetry. The th…
Copolymers of ε-CL/L-LA and ε-CL/DL-LA and for comparison homopolymers PLLA, PDLLA, and PCL were allowed to age in a buffer solution of pH 7 at 23 and 37°C and studied for the changes in the mechanical properties taking place as a function of hydrolysis time. Tensile modulus measurements showed the copolymers to retai…
As part of an ongoing effort to develop processable, high-performance resins for aerospace applications, a phenylethynyl-terminated imide (PETI) oligomer designated LaRCMTPETI-1 was developed. This reactive oligomer has a number-average molecular weight of 6300 g/mol and a Tg of 218°C. Upon curing the reactive oligome…
Studies on structure and properties of natural vegetable fibers (NVF) show that composites made of NVF combine good mechanical properties with a low specific mass. The high level of moisture absorption by the fiber, its poor wettability, as well as the insufficient adhesion between untreated fibers and the polymer mat…
Fourier transform infrared measurements on the spectra of the products of the vaporization of silicon/carbon mixtures trapped in argon at ∼10 K combined with the results from a published ab initio study and from new theoretical calculations carried out in the present work, have resulted in the first observation of a s…
The interactions between rubbers and silicas were studied by means of bound rubber determination, vulcanizate swelling procedure, and tensile retraction measurement. The mobilities of rubber chain segments in mixture with silicas were investigated by thermomechanical and dynamic mechanical analyzers. The results show…
Copolymers of ε-CL/L-LA and ε-CL/DL-LA were allowed to age in a buffer solution of pH 7 at 23 and 37°C. The effects of time and temperature on the rate of hydrolysis were examined by various techniques including weighing (water absorption and weight loss), SEC (molecular weight and polydispersity), and DSC (thermal pr…
The metallic regime of the hydrogen plasma is studied by ab initio Molecular Dynamics simulations, for classical protons and fully degenerate electrons, in the strong coupling regime of the protons. The breakdown of linear screening observed for decreasing density gives rise to a surprisingly rich low-temperature phas…
Copolymers of ε-caprolactone and L-lactide (ε-CL/L-LA) and ε-caprolactone and DL-lactide (ε-CL/DL-LA) were synthesized with compositions 80/20, 60/40, and 40/60 (wt % in feed). The polymerization temperature was 140°C and Sn(II)octoate was used as a catalyst. The effect of the comonomer ratio on the thermal and mechan…
Studies on structure and properties of natural vegetable fibers (NVF) show that composites made of NVF combine good mechanical properties with a low specific mass. The high level of moisture absorption by the fiber, its poor wettability, as well as the insufficient adhesion between untreated fibers and the polymer mat…
The nature of dielectric relaxation has been studied in a number of alcohols and polyols in the frequency range 106–10-3 Hz and the temperature range 77–368 K. Differential scanning calorimetry measurements have also been carried out on all the systems. In all the cases, the main dielectric relaxation rates were found…
We report results of calculations that explain in the itinerant electron picture weak ferromagnetism in hematite, α-Fe 2 O 3 . We use the local approximation to spin-density functional theory and the ASW method incorporating spin-orbit coupling (SOC) and non-collinear moment arrangements. By a detailed symmetry analys…
Sintering of Si 3 N 4 nanoclusters is investigated with the molecular-dynamics approach. At 2000 K thermally rough nanocrystals develop an asymmetric neck in 100 picoseconds. The neck contains more fourfold than threefold coordinated Si atoms. Amorphous nanoclusters develop a symmetric neck which has nearly equal numb…
The amplitudes of electric-field-induced translational vibrations of 90\ifmmode^\circ\else\textdegree\fi{} domain walls formed in a tetragonal ferroelectric thin film grown on a cubic substrate are calculated theoretically. The domain wall contribution to the dielectric response of an epitaxial film is evaluated and s…
We apply density functional theory to study the influence of polydispersity on the stability of columnar, smectic, and solid ordering in the solutions of rodlike macromolecules. For sufficiently large length polydispersity (standard deviation $\ensuremath{\sigma}>0.25$) a direct first-order nematic-columnar transit…