Tomáš Bleha - Academia.edu (original) (raw)

Papers by Tomáš Bleha

Research paper thumbnail of Origin of the pressure variation of front factor in rubber elasticity

Polymer, 1981

The origin of the non-zero value of the parameter 7 may be found in the variation of chain volume... more The origin of the non-zero value of the parameter 7 may be found in the variation of chain volume with conformation. Since r 2 is proportional to V~ '3 and is independent of the current volume Vthe 7-parameter can be expressed as 7=(Oln r2/(~ln V)u=(~ln r2/~P)dOP/Oln V)~

Research paper thumbnail of Pressure(volume) variation of the conformational terms at fusion of polymer crystals

Research paper thumbnail of 8TH Bratislava International-Conference on Polymers-Solution Properties of Modified Polymers Held in Stara-Lesna, High Tatras, Czechoslovakia June 10-14, 1991-PREFACE

Research paper thumbnail of The Uses and Impacts of Research Evaluation

Research paper thumbnail of Effect of internal rotation on surface and volume of molecules with 1, 3-dioxa moiety

The van der Waals surface envelope was calculated for molecules with О-X-О segment (where X is a ... more The van der Waals surface envelope was calculated for molecules with О-X-О segment (where X is a tetrahedral centre) as a function of the torsional angles of X-О bonds. Merely small differences were found between the surface and the volume of energetically stable conformers. The conforma tional variation of the surface was not substantially affected by an inclusion of lone electron pairs in the calculation. The implications of torsional variation of molecular surface on the microthermodynamical surface and on the Gibbs energy of solvation in Sinanoglu's solvophobic theory have been discussed. The limitations have been pointed out for the use of molecular surface as the sole criterion of the extent of solute solvation.

Research paper thumbnail of Conformational volume in statistics of chain molecules

Research paper thumbnail of Pressure dependence of polymer chain conformational parameters

Research paper thumbnail of Theoretical stereochemistry of molecules with heteroatoms linked to the tetrahedral center and the anomeric effect

Research paper thumbnail of Macromolecular conformational volume and thermodynamics of crystal melting

Polymer, 1985

Conformational parts of the thermodynamical function change on melting, in particular the conform... more Conformational parts of the thermodynamical function change on melting, in particular the conformational volume A Vconf, the conformational entropy ASconf and the pressure variation of the latter are analysed for an ideal polymer crystal. These quantities can be calculated from the data for low molecular compounds combined with the three-state rotational isomer model (with adjacent gauche states of opposite sign forbidden). The assumption of intramolecular dominance in the Gibbs energy difference between conformers in a polymethylene chain results in a negative value for AV~onr and a positive value for the quotient (t3AS¢onf/dP)T which contradicts the available experimental data for polyethylene crystal fusion. The disagreement might be reconciled by the postulation of intermolecular contribution to the Gibbs energy difference between conformers favouring the existence of trans sequences of bonds in the chain and the correlation of molecular orientations in neighbouring chains.

Research paper thumbnail of On thermal dehydrochlorination of model compounds for polyvinylchloride—I

European Polymer Journal, 1971

Research paper thumbnail of Energetics of Stretching of Conformational Defects in Extended Poly(methylene) Chains

Macromolecular Theory and Simulations, 2001

Research paper thumbnail of Elastic moduli of highly stretched tie molecules in solid polyethylene

Polymer, 2003

The elastic properties of interlamellar bridges in semicrystalline polyethylene (PE) were estimat... more The elastic properties of interlamellar bridges in semicrystalline polyethylene (PE) were estimated from the molecular-mechanics calculations on the assumption that the energy loading of a chain backbone represents the principal deformation mechanism. The calculations result in the force -length functions featuring abrupt discontinuities due to sequential annihilation of the defects by the conformational transitions. The correlation of the chain elastic moduli E with the concentration of defects in the chain and with the chain extension ratio x were established. The distribution functions zðEÞ of Young's moduli of interlamellar bridges in semicrystalline PE were calculated by using the literature data on the chain length distributions of tie molecules. The impact of the distribution function of moduli zðEÞ on the overall elastic response of solid PE materials was examined, particularly in cases of the stacked lamellae morphology involving so-called hard elastic PE. q

Research paper thumbnail of Theoretical estimation of the effect of solvent on unperturbed dimensions: 1. Isotactic poly(vinyl alcohol)

Polymer, 1976

The effect of the quality of solvent on the unperturbed chain dimensions ~2 of perfectly isotacti... more The effect of the quality of solvent on the unperturbed chain dimensions ~2 of perfectly isotactic poly(vinyl alcohol) (iso-PVA) was studied by means of some model calculations according to the matrix method of Flory et al. These investigations were based on the assumption that the quality of solvent affected mainly the energy of the intramolecular hydrogen bond, while the magnitude of the interactions between other groups, hardly varied with the change of solvent. The individual conformation energies in a chain of iso-PVA were estimated from the analysis of structurally similar polymers, as well as the results obtained with 2,4-pentanediol. It was found that the characteristic ratio C = r-o2/nl 2 increased in non-polar solvents, with energy of the intramolecular hydrogen bond stabilizing the stretched zig-zag conformation of chain. Conversely, the characteristic ratio, monotonously decreased with increasing polarity of solvent to the value of about 6, typical of a flexible random coil. The existence of an analogous relationship, valid for other polymers, able to form an intramolecular hydrogen bond in the chain, was also discussed.

Research paper thumbnail of Confined Macromolecules in Polymer Materials and Processes

ACS Symposium Series, 2005

Research paper thumbnail of Internal rotation of paraffinic chains in an anisotropic environment

Journal of Molecular Structure: THEOCHEM, 1986

ABSTRACT The influence of neighbouring rod-like molecules on the torsional potential of the paraf... more ABSTRACT The influence of neighbouring rod-like molecules on the torsional potential of the paraffin molecule was investigated by the molecular-mechanics method. The environment-induced contribution Venv(ø) to the torsional potential was calculated for various separations of surrounding molecules and the central alkane. At small distances, the environmental constraints enhanced the rigidity and alignment of paraffin by the increase in the gauche—trans energy difference ΔE and in interconformational barriers. At intermediate separations, an unexpected stabilisation of the gauche conformation in paraffin was observed. It was classified as the anomeric effect of intermolecular origin. The calculated conformational parameters concurred with those estimated for real systems with the anisotropic mean field as are thermotropic mesophases or lyotropic monolayers and bilayers.

Research paper thumbnail of Force?extension relations in macromolecules of variable excluded volume and flexibility: energy and entropy changes on stretching

Journal of the Chemical Society, Faraday Transactions, 1995

ABSTRACT Lattice chains, chain length 200 monomers, generated by the Monte Carlo method, have bee... more ABSTRACT Lattice chains, chain length 200 monomers, generated by the Monte Carlo method, have been studied on a tetrahedral lattice. Real-chain features such as conformational restrictions, variable segment interaction and chain thickness were assumed in the model. The chain vector density distribution function W(R) and the related changes in the Helmholtz energy were calculated as a function of the displacement of the chain ends, R. The force–displacement curves derived by this approach are non-linear with the shape depending on the solvent quality. The energy changes, accompanying the chain deformation, due to (a) conformational isomerization and (b) loss of segment contacts, were computed. In the model used (a) leads to a negative and (b) to a positive energetic contribution, fu, to the total tensile force, f. The mixed energy–entropy character of the single-chain deformation was demonstrated for some representative chains differing in excluded volume and flexibility.

Research paper thumbnail of Anomeric and Exo-Anomeric Effects in Carbohydrate Chemistry

Advances in Carbohydrate Chemistry and Biochemistry, 1989

Research paper thumbnail of Free energy and confinement force of macromolecules in a slit at full equilibrium with a bulk solution

Polymer, 2003

Nondilute athermal and theta solutions of nonadsorbing flexible macromolecules in equilibrium wit... more Nondilute athermal and theta solutions of nonadsorbing flexible macromolecules in equilibrium with repulsive slit-like pores were examined by the lattice Monte Carlo simulations. The free energy of confinement DA=kT and the force f =kT exerted by polymers on the slit were computed as a function of the slit width D in a wide range of bulk concentrations f: The free energy and force profiles in nondilute solutions were found to deviate considerably from the ideal chain theory; the perturbation of chains by a presence of the slit walls were substantially reduced in nondilute solutions. The free energy and force functions appropriate for nondilute solutions were derived by fitting the simulation data. Further, the relative pressure p I =p E exerted by the nonadsorbing confined molecules on the slit walls was calculated. The depletion effect relevant to colloid stabilization was found in dilute solutions to be slightly weaker for excluded-volume chains than for ideal chains. The relative pressure equation was modified to cover semidilute solutions, by using the mean-field and scaling expressions of the osmotic pressure. Both the relative pressure p I =p E and the intra-slit concentration profiles f I ðxÞ in tandem display a suppression of the depletion effect with increasing f in simidilute solutions. q

Research paper thumbnail of Steric exclusion/adsorption compensation in partitioning of polymers into micropores in good solvents

Polymer, 2000

Partitioning equilibrium between bulk and slit-like pores in dilute solution was studied by Monte... more Partitioning equilibrium between bulk and slit-like pores in dilute solution was studied by Monte Carlo (MC) simulations on a cubic lattice in the presence of attractive, polymer-pore interaction. Athermal chains with excluded volume of variable lengths were generated in a direct simulation of the equilibrium partition coefficient K. The results show that by the variation of the polymer-pore adsorption energy, e, three modes of liquid chromatography of polymers in good solvents can be reproduced. In contrast to ideal chains, the compensation point where K 1, relevant to critical chromatography, was found to be a function of the chain length. The attraction energy in the compensation point e c is independent of the slit width and can be identified with the critical energy of adsorption as well as with the adsorption theta point in infinite chains. The counterbalance of steric exclusion (the depletion layer) and wall attraction (the enrichment layer) at the compensation point was confirmed by a flat concentration profile across the pore. The distribution functions of the chain end-to-end distances perpendicular and parallel to slit walls were calculated. It was inferred that in wide pores corresponding to size exclusion chromatography the partitioning proceeds by the coil orientation, and, additionally, that the critical chromatography operates in the regime of weakly adsorbed chains characterized by a diffuse adsorption layer. ᭧

Research paper thumbnail of Determination of preferential solvation of polymers in mixed solvents by gel-permeation chromatography

Journal of Polymer Science: Polymer Letters Edition, 1976

Research paper thumbnail of Origin of the pressure variation of front factor in rubber elasticity

Polymer, 1981

The origin of the non-zero value of the parameter 7 may be found in the variation of chain volume... more The origin of the non-zero value of the parameter 7 may be found in the variation of chain volume with conformation. Since r 2 is proportional to V~ '3 and is independent of the current volume Vthe 7-parameter can be expressed as 7=(Oln r2/(~ln V)u=(~ln r2/~P)dOP/Oln V)~

Research paper thumbnail of Pressure(volume) variation of the conformational terms at fusion of polymer crystals

Research paper thumbnail of 8TH Bratislava International-Conference on Polymers-Solution Properties of Modified Polymers Held in Stara-Lesna, High Tatras, Czechoslovakia June 10-14, 1991-PREFACE

Research paper thumbnail of The Uses and Impacts of Research Evaluation

Research paper thumbnail of Effect of internal rotation on surface and volume of molecules with 1, 3-dioxa moiety

The van der Waals surface envelope was calculated for molecules with О-X-О segment (where X is a ... more The van der Waals surface envelope was calculated for molecules with О-X-О segment (where X is a tetrahedral centre) as a function of the torsional angles of X-О bonds. Merely small differences were found between the surface and the volume of energetically stable conformers. The conforma tional variation of the surface was not substantially affected by an inclusion of lone electron pairs in the calculation. The implications of torsional variation of molecular surface on the microthermodynamical surface and on the Gibbs energy of solvation in Sinanoglu's solvophobic theory have been discussed. The limitations have been pointed out for the use of molecular surface as the sole criterion of the extent of solute solvation.

Research paper thumbnail of Conformational volume in statistics of chain molecules

Research paper thumbnail of Pressure dependence of polymer chain conformational parameters

Research paper thumbnail of Theoretical stereochemistry of molecules with heteroatoms linked to the tetrahedral center and the anomeric effect

Research paper thumbnail of Macromolecular conformational volume and thermodynamics of crystal melting

Polymer, 1985

Conformational parts of the thermodynamical function change on melting, in particular the conform... more Conformational parts of the thermodynamical function change on melting, in particular the conformational volume A Vconf, the conformational entropy ASconf and the pressure variation of the latter are analysed for an ideal polymer crystal. These quantities can be calculated from the data for low molecular compounds combined with the three-state rotational isomer model (with adjacent gauche states of opposite sign forbidden). The assumption of intramolecular dominance in the Gibbs energy difference between conformers in a polymethylene chain results in a negative value for AV~onr and a positive value for the quotient (t3AS¢onf/dP)T which contradicts the available experimental data for polyethylene crystal fusion. The disagreement might be reconciled by the postulation of intermolecular contribution to the Gibbs energy difference between conformers favouring the existence of trans sequences of bonds in the chain and the correlation of molecular orientations in neighbouring chains.

Research paper thumbnail of On thermal dehydrochlorination of model compounds for polyvinylchloride—I

European Polymer Journal, 1971

Research paper thumbnail of Energetics of Stretching of Conformational Defects in Extended Poly(methylene) Chains

Macromolecular Theory and Simulations, 2001

Research paper thumbnail of Elastic moduli of highly stretched tie molecules in solid polyethylene

Polymer, 2003

The elastic properties of interlamellar bridges in semicrystalline polyethylene (PE) were estimat... more The elastic properties of interlamellar bridges in semicrystalline polyethylene (PE) were estimated from the molecular-mechanics calculations on the assumption that the energy loading of a chain backbone represents the principal deformation mechanism. The calculations result in the force -length functions featuring abrupt discontinuities due to sequential annihilation of the defects by the conformational transitions. The correlation of the chain elastic moduli E with the concentration of defects in the chain and with the chain extension ratio x were established. The distribution functions zðEÞ of Young's moduli of interlamellar bridges in semicrystalline PE were calculated by using the literature data on the chain length distributions of tie molecules. The impact of the distribution function of moduli zðEÞ on the overall elastic response of solid PE materials was examined, particularly in cases of the stacked lamellae morphology involving so-called hard elastic PE. q

Research paper thumbnail of Theoretical estimation of the effect of solvent on unperturbed dimensions: 1. Isotactic poly(vinyl alcohol)

Polymer, 1976

The effect of the quality of solvent on the unperturbed chain dimensions ~2 of perfectly isotacti... more The effect of the quality of solvent on the unperturbed chain dimensions ~2 of perfectly isotactic poly(vinyl alcohol) (iso-PVA) was studied by means of some model calculations according to the matrix method of Flory et al. These investigations were based on the assumption that the quality of solvent affected mainly the energy of the intramolecular hydrogen bond, while the magnitude of the interactions between other groups, hardly varied with the change of solvent. The individual conformation energies in a chain of iso-PVA were estimated from the analysis of structurally similar polymers, as well as the results obtained with 2,4-pentanediol. It was found that the characteristic ratio C = r-o2/nl 2 increased in non-polar solvents, with energy of the intramolecular hydrogen bond stabilizing the stretched zig-zag conformation of chain. Conversely, the characteristic ratio, monotonously decreased with increasing polarity of solvent to the value of about 6, typical of a flexible random coil. The existence of an analogous relationship, valid for other polymers, able to form an intramolecular hydrogen bond in the chain, was also discussed.

Research paper thumbnail of Confined Macromolecules in Polymer Materials and Processes

ACS Symposium Series, 2005

Research paper thumbnail of Internal rotation of paraffinic chains in an anisotropic environment

Journal of Molecular Structure: THEOCHEM, 1986

ABSTRACT The influence of neighbouring rod-like molecules on the torsional potential of the paraf... more ABSTRACT The influence of neighbouring rod-like molecules on the torsional potential of the paraffin molecule was investigated by the molecular-mechanics method. The environment-induced contribution Venv(ø) to the torsional potential was calculated for various separations of surrounding molecules and the central alkane. At small distances, the environmental constraints enhanced the rigidity and alignment of paraffin by the increase in the gauche—trans energy difference ΔE and in interconformational barriers. At intermediate separations, an unexpected stabilisation of the gauche conformation in paraffin was observed. It was classified as the anomeric effect of intermolecular origin. The calculated conformational parameters concurred with those estimated for real systems with the anisotropic mean field as are thermotropic mesophases or lyotropic monolayers and bilayers.

Research paper thumbnail of Force?extension relations in macromolecules of variable excluded volume and flexibility: energy and entropy changes on stretching

Journal of the Chemical Society, Faraday Transactions, 1995

ABSTRACT Lattice chains, chain length 200 monomers, generated by the Monte Carlo method, have bee... more ABSTRACT Lattice chains, chain length 200 monomers, generated by the Monte Carlo method, have been studied on a tetrahedral lattice. Real-chain features such as conformational restrictions, variable segment interaction and chain thickness were assumed in the model. The chain vector density distribution function W(R) and the related changes in the Helmholtz energy were calculated as a function of the displacement of the chain ends, R. The force–displacement curves derived by this approach are non-linear with the shape depending on the solvent quality. The energy changes, accompanying the chain deformation, due to (a) conformational isomerization and (b) loss of segment contacts, were computed. In the model used (a) leads to a negative and (b) to a positive energetic contribution, fu, to the total tensile force, f. The mixed energy–entropy character of the single-chain deformation was demonstrated for some representative chains differing in excluded volume and flexibility.

Research paper thumbnail of Anomeric and Exo-Anomeric Effects in Carbohydrate Chemistry

Advances in Carbohydrate Chemistry and Biochemistry, 1989

Research paper thumbnail of Free energy and confinement force of macromolecules in a slit at full equilibrium with a bulk solution

Polymer, 2003

Nondilute athermal and theta solutions of nonadsorbing flexible macromolecules in equilibrium wit... more Nondilute athermal and theta solutions of nonadsorbing flexible macromolecules in equilibrium with repulsive slit-like pores were examined by the lattice Monte Carlo simulations. The free energy of confinement DA=kT and the force f =kT exerted by polymers on the slit were computed as a function of the slit width D in a wide range of bulk concentrations f: The free energy and force profiles in nondilute solutions were found to deviate considerably from the ideal chain theory; the perturbation of chains by a presence of the slit walls were substantially reduced in nondilute solutions. The free energy and force functions appropriate for nondilute solutions were derived by fitting the simulation data. Further, the relative pressure p I =p E exerted by the nonadsorbing confined molecules on the slit walls was calculated. The depletion effect relevant to colloid stabilization was found in dilute solutions to be slightly weaker for excluded-volume chains than for ideal chains. The relative pressure equation was modified to cover semidilute solutions, by using the mean-field and scaling expressions of the osmotic pressure. Both the relative pressure p I =p E and the intra-slit concentration profiles f I ðxÞ in tandem display a suppression of the depletion effect with increasing f in simidilute solutions. q

Research paper thumbnail of Steric exclusion/adsorption compensation in partitioning of polymers into micropores in good solvents

Polymer, 2000

Partitioning equilibrium between bulk and slit-like pores in dilute solution was studied by Monte... more Partitioning equilibrium between bulk and slit-like pores in dilute solution was studied by Monte Carlo (MC) simulations on a cubic lattice in the presence of attractive, polymer-pore interaction. Athermal chains with excluded volume of variable lengths were generated in a direct simulation of the equilibrium partition coefficient K. The results show that by the variation of the polymer-pore adsorption energy, e, three modes of liquid chromatography of polymers in good solvents can be reproduced. In contrast to ideal chains, the compensation point where K 1, relevant to critical chromatography, was found to be a function of the chain length. The attraction energy in the compensation point e c is independent of the slit width and can be identified with the critical energy of adsorption as well as with the adsorption theta point in infinite chains. The counterbalance of steric exclusion (the depletion layer) and wall attraction (the enrichment layer) at the compensation point was confirmed by a flat concentration profile across the pore. The distribution functions of the chain end-to-end distances perpendicular and parallel to slit walls were calculated. It was inferred that in wide pores corresponding to size exclusion chromatography the partitioning proceeds by the coil orientation, and, additionally, that the critical chromatography operates in the regime of weakly adsorbed chains characterized by a diffuse adsorption layer. ᭧

Research paper thumbnail of Determination of preferential solvation of polymers in mixed solvents by gel-permeation chromatography

Journal of Polymer Science: Polymer Letters Edition, 1976