Study of temperature transitions in solid poly(oxymethylene) by thermal analysis methods

Poly(oxymethylene)s with different molecular mass and chemical structure were studied using DSC, dynamic mechanical relaxation, thermomechanical analysis, and thermogravimetry.Molecular mobilities of two types were found in the amorphous phase of poly(oxymethylene). Unconstrained chains of poly(oxymethylene) soften at –70°C and then amorphous chains with different restraints from the crystalline phase are successively activated in a wide temperature interval.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayThis work has been supported by DuPont Company     

Luminescent Method of Measuring the EXAFS Spectra of Structural Defects of Alkali‐Halide Crystals

It is shown that xray spectral methods — those of EXAFS and XEOL spectroscopy — allow determination both of the structure of inherent or impurity centers of luminescence in alkalihalide crystals and investigation of the dynamics of their change under the action of ionizing radiation. The methods are noted for high sensitivity. The possibilities of the methods are demonstrated for KBr, NaCl:Ni, and NaCl:Ni,Cu crystals taken as an example. New data on the structures of different luminescence centers investigated and dynamics of their change are obtained.     

The influence of heating on the spectral characteristics of pure alkali halide NaCl crystals

The influence of high-temperature heating on the spectral characteristics of pure natural NaCl crystals is studied. It is shown that defects changing spectral characteristics of crystals are formed in them when heated. Temperature dependences of the luminescence intensity growth for these defects are described by the hyperbolic function. The action of X-rays on heated crystals results in the formation of new defects luminescent in regions of 435 and 570–585 nm. The efficiency of formation of these defects is obtained as a function of the temperature of heat pretreatment. Regularities in variations of the location and the intensity of luminescence of the defects obtained under the action of ultraviolet radiation and on long storage are determined.     

αγ angular correlations in the inelastic scattering of α particles on 24Mg at Eα = 30.3 MeV

The results from measuring the angular dependence of differential α-scattering cross sections on ²⁴Mg with final nucleus formation in the ground and three lower excited states are presented, along with the double differential ²⁴Mg(α, αγ)²⁴Mg(2⁺, 1.369 MeV) reaction cross sections at E _α = 30.3 MeV. The spin-tensor components of the density matrix of the ²⁴Mg nucleus in the 2⁺(1.369 MeV) state are reconstructed and its orientation characteristics are determined. The experimental results are compared to theoretical results under the assumption of collective excitation and mechanisms for compound nucleus formation.     

Study of orientation characteristics of the 11B(5/2?, 4.445 MeV) nucleus in the 13C(d, αγ)11B reaction at Ed = 15.3 MeV

Double differential cross sections of the ¹³C(d, αγ)¹¹B(5/2⁻, 4.445 MeV) reaction were measured at E _d = 15.3 MeV. Angular dependences of the even components of density matrix spin tensors, magnetic sublevel populations, and components of multipole moment orientation tensors of the ¹¹B nucleus in this state were obtained. Experimental results were compared with theoretical data calculated under the assumption of different reaction mechanisms: deuteron pickup, heavy particle stripping, two-step mechanism of sequential cluster transfer, and compound nucleus formation.     

Effect of stereoregularity on the structure and thermophysical characteristics of isotactic polypropylene

The structure, thermophysical, and thermomechanical characteristics of metallocene-synthesized isotactic PPs of different molecular masses containing different amounts of stereodefects have been studied. The degree of crystallinity and the content of a and g modifications in slowly cooled and melt quenched films are estimated by three independent methods, i.e., X-ray analysis, DSC, macroscopic density measurements, and changes in their phase structure upon annealing and orientation are analyzed. As the content of stereodefects increases, the fraction of g crystallites in the films increases, while the degree of crystallinity decreases (down to 5%). The formation of the g phase is assumed to be related to the epitaxial crystallization; this process is assisted by stresses induced on chains upon slow melt crystallization and after annealing of the oriented samples. This evidence allows the analysis of structural and thermodynamic characteristics of thermoplastic and elastic samples of the isotactic PP.     

The structure of CdS nanoparticles

CdS nanoparticles in the high-pressure polyethylene (HPPE) matrix have been synthesized and investigated. Shift of the fundamental absorption edge has been shown to be absent with a variation in the nanoparticle size for the given samples and a small disordering of atoms in nanoparticles is observed.     

Dynamic Mechanical Characterization of Molecular Motion in a Wide Temperature Range of Various Polymers Containing Propylene Units

The temperature transitions for a series of flexible polymers containing propylene units were studied by dynamic mechanical spectroscopy. It was found that the gradual activation of the local motion of different structural units involved in polymers occurs with increasing temperature. Initially, the rotation of the side groups, such as side methyl groups, is activated and on further heating the main chain structural units show their local motions. It is important that the temperature interval of the appearance of the local motion of each structural unit is almost independent of the presence of other structural units. Accordingly, the polymers investigated can be divided into two groups. The activation of the local motion of the most rigid structural unit determines the glass transition in the first group of polymers. The glass transition of the polymers of the second group takes place at a higher temperature which depends on the content of side methyl groups and the intermolecular interaction. The increased influence of both these factors on the cooperative amorphous motion of polymers of the second group leads to their increased T_g values.This revised version was published online in November 2005 with corrections to the Cover Date.     

Phase behaviour of blends with mesophase components

Temperature transitions and structural transformations were studied for blends of two thermotropic mesophase cyclolinear polymethylsiloxanes with linear PDMS of various molecular weights by means of DSC, optical polarizing microscopy, and optical interferometry. Compatibility of the components which depends on the chemical structure of cyclolinear polymethylsiloxanes, molecular weight of PDMS, composition, and temperature affect formation of mesophase in cyclolinear polymethylsiloxanes. The most interesting aspect of the phase behaviour consists in the fact that it is possible to reach compatibility of the components in the mesomorphic state for the blends of two cyclolinear polymethylsiloxanens due to various annealing regimes in one‐phase molten state.