Jozef Tiňo

Effect of molecular mobility on the decay of free radicals in amorphous polymers: a Monte Carlo study

Abstract An algorithm has been formulated for studying the diffusion-controlled reactions in amorphous polymers. A method has been proposed based on molecular mobility and leading to migration of radical centres, their approach and decay. In a volume of dimensions 70 × 100 × 123 A , using boundary periodic conditions, an amorphous polymer system and free radicals are simulated on a diamond lattice by the Monte Carlo method. Owing to crank, kink and crankshaft motions, some of them approach one another and decay. A change in the concentration of free radicals with time and temperature has been observed by the method described.

Study of the BeH radical by UHF-type and complete configuration interaction methods

The UHF, PUHF, EHF and CCI methods have been used for calculating the isotropic and anisotropic hyperfine splitting constants and electronic energy of the BeH radical employing 6 different basis sets. In all cases, except for anisotropic h.f.s. constants for the H atom, as the quality of the basis improves (based on the energy criterion) the theoretical values approach the experimental ones.

Spin-extended hartree—fock indo calculations

Abstract The spin-extended Hartree—Fock method in INDO approximation is presented. Mestechkins EHF equations are solved iteratively by the method of steepest descent. EHF spin and charge densities, equilibrium energies and interatomic distances for some small radicals are compared with the results of the other open-shell methods.

Spin-projected SCF INDO spin density calculations

Abstract Spin densities have been calculated by the INDO method in the parametrizations of Pople and Kaufman both without and with the projection of the UHF function. This projection impairs the results in the Pople parametrization but it improves them provided the Kaufman parametrization is used.

An effective basis set with bond functions for studying the energetics of bromine compounds

Abstract A new basis set is proposed for bromine compounds with bond functions located in the centre of the bond. The basis extends our preceding basis sets with bond functions. UMP2 calculations of enthalpies of reaction of HBr with chosen radicals using this basis set are in good agreement with experimental data.

Spin Probe Study of Drawn UHMW PE Fibres

The ESR method has been used to investigate the structure and properties of highly drawn ultra-high molecular weight polyethylene fibres. The values of T 50G , rotational correlation times and the free radical decay curves for fibres of various draw ratios have been obtained from ESR spectra. The model of soft domain formation in fibres at high draw ratios is used to explain the experimental results.

Quasiclassical trajectory calculation of the B++H2→BH++H reaction

Abstract Quasiclassical trajectory calculations were done for the lowest triplet potential energy surface of the reaction B + +H 2 →BH + +H. The analytical form of the surface is based on ab initio points fitted by an extended LEPS function with modification for intersection of the two surfaces. The angular and translational energy distributions calculated are compared with the results of crossed molecular beam experiments. Vibrational and rotational excitation of BH + molecules, reaction cross sections, and mean lifetimes of collision complex BH 2 + are discussed as well.

Roothaan INDO CI spin density calculation

Abstract Spin densities have been calculated by the Roothaan INDO CI method in the parametrization of Pople and Kaufman. The dependence of the spin densities on both the type and the number of configurations in CI has been followed. Results have been compared with the UHF and spin-projected UHF methods.

Quasiclassical trajectory study of the effect of reactant rotation on the reaction B+ + HD → BH+ (BD+) + D(H)

Abstract Quasiclassical trajectories were calculated for the lowest triplet potential energy surface of the title reaction at different rotational states of the HD molecule and at two values of the relative translational energy of reactants. The opacity function, reaction cross sections, rotational and vibrational excitation of products, intramolecular isotopic effect, symmetry of backward-forward scattering and lifetime of the BHD+ collision complex are discussed in connection with the initial conditions defined. Results are compared with similar systems taken from the literature.

Theoretical aspects of free radical decay in polyethylene and comparison with experimental results

Abstract Results of the study of stability of free radicals using electron spin resonance suggest a close relationship between the glass transition temperature (Tg) and free radical decay. In a detailed study of radicals in polyethylene (PE) [J. Polym. Sci. A2 6 (1968) 1435] in the sixties, it was concluded that the decay of radicals in solid polymers is connected to molecular mobility. In this report on molecular mobility using a Monte Carlo method we show with the example of PE that it is the motion of molecular structures, which, depending on temperature, lead to a transfer of radical centers and thus to the approach of radicals and their decay. Theoretical and experimental decay curves for PE are compared and based on their close correspondence it is concluded how the individual types of motions affect the stability of free radicals.