Rheooptical investigation of the crystallization of poly(ethylene terephthalate) under tensile strain

The crystallization of poly(ethylene terephthalate) under uniaxial tensile strain at different extension rates was investigated with optical polarimetry in a temperature range between the glass-transition temperature and the quiescent crystallization temperature. The evolution of the optical properties of the polymer, including the turbidity, birefringence, and dichroism, were monitored simultaneously with the mechanical parameters. To complete the semicrystalline microstructure characterization of the polymer under strain, an online wide-angle X-ray diffraction (WAXD) technique was used in separate experiments, which were performed under the same thermomechanical conditions. For real-time measurements, a high-energy synchrotron radiation source was used. The optical properties provided information about both the crystalline and amorphous phases, whereas the WAXD patterns essentially gave information about the crystalline phase. The two experimental techniques were then used in a complementary way to characterize the semicrystalline microstructure. Significant deviations from the stress-optical rule were found. This was attributed to both transient effects and the appearance of crystallites, which consisted of highly oriented molecular segments that could contribute to the optical anisotropy but not necessarily to the stress. The behavior of the optical dichroism was found to be qualitatively different from that of the birefringence. The latter monotonically increased with the strain, whereas the former first increased with the strain, passed through a maximum, and then decreased to a steady-state value. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1915–1927, 2004     

Plasticized microporous poly(vinylidene fluoride) separators for lithium-ion batteries. III. Gel properties and irreversible modifications of poly(vinylidene fluoride) membranes under swelling in liquid electrolytes

Microporous poly(vinylidene fluoride) (PVdF) separators for lithium-ion batteries, used in liquid organic electrolytes, have been characterized with respect to the swelling phenomena on dense PVdF membranes (obtained through hot pressing). In the first and second parts of this study, we have described the swelling equilibria and swelling kinetics of dense PVdF. Here the thermal properties of PVdF gels and their irreversible modifications induced by swelling are characterized. Particular attention is paid to crystallinity modifications, polymer plasticization, and membrane degradation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2308–2317, 2004     

Structure,morphology, and crystallization process of isotactic polypropylene/hydrogenated hydrocarbon resin blends

The structure, morphology, and isothermal and nonisothermal crystallization of isotactic polypropylene/low‐molecular‐mass hydrocarbon resin blends (iPP/HR) (up to 20% in weight of HR) have been studied, using optical and electron microscopy, wide‐ and small‐angle X‐ray and differential scanning calorimetry. New structures and morphologies can be activated, using appropriate preparation and crystallization conditions and blend composition. For every composition and crystallization condition, iPP crystallizes in α‐form, with a spherulitic morphology. The size of iPP spherulites increases with resin content, whereas the long period decreases. In the range of crystallization temperatures investigated, HR modifies the birefringence of iPP spherulites, favoring the formation of radial lamellae and changing the ratio between tangential and radial lamellae. Spherulitic radial growth rates, overall crystallization rates, and melting temperatures are strongly affected by resin, monotonically decreasing with resin content. This confirms miscibility in the melt between the two components of the blends. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3368–3379, 2004     

Extractive spectrophotometric determination of Cobalt(II) in synthetic and pharmaceutical samples using Cyanex 923.

Cyanex 923 has been proposed as a sensitive analytical reagent for the direct extractive spectrophotometric determination of cobalt(II). Cobalt(II) forms a blue-colored complex with Cyanex 923 in the organic phase. The maximum absorbance of the complex is measured at 635 nm. Beer's law was obeyed in the range 58.9 - 589.0 microg of cobalt. The molar absorptivitiy and Sandell's sensitivity of the complex was calculated to be 6.79 x 10920 l mol(-1) cm(-1) and 0.088 microg cm(-2), respectively. The nature of the extracted species was found to be Co(SCN)2 x 2S. An excellent linearity with a correlation coefficient value of 0.999 was obtained for the Co(II)-Cyanex 923 complex. Stability and regeneration of the reagent (Cyanex 923) for reuse is the main advantage of the present method. The method was successfully applied to the determination of cobalt in synthetic mixtures and pharmaceutical samples was found to give values close to the actual ones. Standard alloy samples, such as high-speed tool BCS 484 and 485, have been tested for the determination of cobalt for the purpose of validating the present method. The results of the proposed method are comparable with atomic absorption spectrometry and were found to be in good agreement.     

Crystal structure of the 4-chromanone derivative.

The title compound was extracted from a natural product and its structure was characterized by an X-ray diffraction method. It crystallizes in the tetragonal space group P41 with cell parameters a = 15.832(10)A, c = 11.622(10)A, Z = 4; the final residual factor is R1 = 0.0769. The structure has both intra and intermolecular hydrogen bonds.     

Indexing Distributions of Data: Preservice Teachers' Notions of Representativeness

The purpose of the study was to identify strategies used by preservice elementary teachers to index distributions of data. A secondary purpose of the study was to investigate whether representational form influenced the type of representative values constructed. Two hundred eighty‐three preservice teachers were administered a selection of tasks requiring them to construct representative values for a variety of distributional shapes. Participants were asked to provide justifications for their chosen representative measures. Data were analyzed from two perspectives: nature and adequacy of representative measure. Results indicated that when presented with skewed data sets, over half of the participants constructed poorly representative values. Symmetric data sets were best indexed with 67% of participants constructing highly representative values. Representational form played a significant role in determining representative values. Data presented graphically were more likely to be represented using measures of variability than were those presented nongraphically. Conversely, measures of central tendency were more frequently used to represent nongraphical data. The information arising from this study indicates that many preservice teachers construct poorly representative values to index data sets and utilize identical measures of representativeness regardless of distributional shape. The findings of the study highlight the need for exploratory data activities at the preservice level, call for provision of experiences affording insights into construction of representative values, and thus, have implications for the design of preservice mathematics education curricula.     

Three-dimensional structure of Saccharomyces cerevisiae inorganic pyrophosphatase complexed with cobalt and phosphate ions

Crystals of Saccharomyces cerevisiae inorganic pyrophosphatase suitable for X-ray diffraction study were grown by cocrystallization of the enzyme with cobalt chloride and imidodiphosphate. Saccharomyces cerevisiae is a metal-dependent enzyme which catalyzes hydrolysis of inorganic pyrophosphate to orthophosphate. The three-dimensional structure of this enzyme was solved by the molecular-replacement method and refined at 1.8 Å resolution to an R factor of 19.5%. Cobalt and phosphate ions were revealed in the active centers of both identical subunits (A and B) of the pyrophosphatase molecule. In subunit B, a water molecule was found between two cobalt ions. It is believed that this water molecule acts as an attacking nucleophile in the enzymatic cleavage of the pyrophosphate bond. It was demonstrated that cobalt ions and a phosphate group occupy only part of the potential binding sites (two chemically identical and crystallographically independent subunits have different binding sites). The arrangement of ligands and the structure of the nucleophile-binding site are discussed in relation to the mechanism of action of the enzyme and the nature of the metal activator.     

Transition flow of rarefied gases: Current models and methods of simulation

Transition (molecular-viscous) isothermal channel flow of rarefied gases is considered. Present-day engineering physical models of transition gas flow and methods of simulation are analyzed and verified in terms of the kinetic theory on the micro-and macrolevels.