Ivan Krakovský

Modeling dielectric properties of composites by finite-element method

Dielectric spectra of binary composites consisting of equal-sized hard disks of one dielectric dispersed randomly in a matrix of another dielectric are modeled in two dimensions using the finite-element method. The effects of the volume fraction of the disks and difference in the component permittivities and ohmic conductivities on the effective dielectric behavior of the resulting composite are discussed. Numerical results are compared with the results predicted using a few mixture formulas chosen. For this type of composites, the comparison shows, that in the region of compositions and component properties explored, the Brugemann formula is in the best agreement with the numerical results [Ann. Phys. (Leipzig) 24, 636 (1935)].

Structure of inhomogeneous polymer networks prepared from telechelic polybutadiene

AbstractThe structure and dynamics of segmented polyurethane networks prepared from poly(butadiene) diols (PBD) of different molecularweights M n ¼ 1200; 2160, 2650, 4690 and 10,300 g mol 21 , 4,4 0 -diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT)ðM n ¼ 710 g mol 21 Þ have been investigated by dynamic mechanical analysis, dielectric relaxation spectroscopy, differential scanningcalorimetry (DSC), small- and wide-angle X-ray scattering (SAXS and WAXS). Microphase separation on nanometre scale has beenconfirmed by SAXS in all networks. Except for the highest molecular weight of PBD, the networks consist of microdomains of similar sizeand composition dispersed in the soft phase of polybutadiene segments. Dielectric spectroscopy revealed the existence of three types of non-crystalline phases, each characterised by a well-developed dynamic glass transition. Absence of crystallinity has been proven by WAXS andDSC. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polyurethane network; Microphase separation; Dielectric spectroscopy

Elastic and swelling behavior of 2‐hydroxyethyl methacrylate, diethylene glycol methacrylate, and methacrylic acid copolymers

The elastic and swelling behavior of copolymers of 2-hydroxyethyl methacrylate, diethylene glycol methacrylate, and methacrylic acid crosslinked with ethylene glycol dimethacrylate has been studied. In the range of copolymer composition studied, Youngs modulus of the swollen networks increases with the content of methacrylic acid, and its dependence on the content of diethylene glycol methacrylate passes through a maximum. The concentrations of the elastic network chains determined from Youngs moduli of swollen networks are much higher than those calculated from stoichiometry. This effect is attributed to the presence of additional physical crosslinks due to water-induced ordering of the hydrophobic backbone chains. Both the elastic and swelling behavior of the polymers mentioned above are decisive for their application in the preparation of soft contact lenses.

Structure and elasticity of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate and poly(oxypropylene) triol

Abstract Relations between structure and equilibrium mechanical properties of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared at various initial ratios of reactive groups were studied. An inhomogeneous structure of the networks was revealed by small-angle X-ray scattering. The microdomains consist of POPT molecules crosslinked with MDI and they have a diameter of about 4–5 nm. The characteristic distance of microdomains is about 9 nm. The poor mutual miscibility of the reaction components and the existence of hydroxyl group association in the reaction mixture are the probable reasons for microdomain formation. The strain dependence of the equilibrium elastic force obeyed the Mooney-Rivlin equation. The experimental moduli of dry and equilibrium swollen samples were higher than those calculated for the affine limit of the Flory junction fluctuation model on the assumption of a homogeneous network formation.

Microstructure Changes in Polyester Polyurethane upon Thermal and Humid Aging

The microstructure of compression molded Estane 5703 films exposed to 11%, 45%, and 80% relative humidity and 70 °C for 1 and 2 months has been studied by small-angle neutron scattering (SANS), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Scattering data indicated increase of the interdomain distance and domain size with a higher humidity and longer aging time. GPC data showed a progressive shortening of polyurethane chains with increasing humidity and aging time. The shortening of the polyurethane chains caused a drop of the glass transition temperature of soft segments, and promoted crystallization of the soft segments during long-time storage of the aged samples at room temperature. FTIR showed a substantial increase in the number of inter-urethane H-bonds in the aged samples. This correlates with the increase of the hard domain size and the degree of phase separation as measured by SANS. The data collected reveals that the reduced steric hindrance caused by hydrolysis of ester links in polybutylene adipate residues promotes the organization of hard segments into domains, leading to the increase of domain size and distance, as well as phase segregation in aged Estane. These findings provide insight into the effects of humidity and thermal aging on the microstructure of aged polyester urethane from molecular to nanoscale level.

Two simple approaches to sol–gel transition

We represent a theory of polymer gelation as an analogue of liquid–glass transition in which elastic fields of stress and strain shear components appear spontaneously as a consequence of the cross-linking of macromolecules. This circumstance is explained on the basis of obvious combinatoric arguments as well as a synergetic Lorenz system, where the strain acts as an order parameter, a conjugate field is reduced to the elastic stress, and the number of cross-links is a control parameter. Both the combinatoric and synergetic approaches show that an anomalous slow dependence of the shear modulus on the number of cross-links is obtained.

Dynamic mechanical and thermal behavior of novel liquid-crystalline polybutadiene-diols with azobenzene groups in side chains

Liquid-crystalline (LC) polymers containing methoxy- or butoxy-substituted azobenzene in side chains have been prepared via radical addition of the in advance synthesized nematic thiols onto double bonds of poly(butadiene)diol [Polakova et al., Pol. Bull. 54, 315–326 (2010)]. In the present work, thermal behavior of these comblike polymers has been characterized by differential scanning calorimetry and polarizing optical microscopy. LC transitions have been also determined by rheological measurement. Time–temperature superposition of mechanical functions has been successfully applied to samples undergoing direct nematic/glassy state transition.

Effect of chemical clusters on photoelastic behaviour and small-angle X-ray scattering of epoxide networks based on poly(oxypropylene)diamines

Abstract Viscoelastic and equilibrium photoelastic behaviour and small-angle X-ray scattering of epoxide networks based on bisphenol A diglycidyl ether (DGEBA) and two poly(oxypropylene)diamines (Jeffamine® D-400 and D-2000) were investigated. Networks were prepared with the stoichiometric ratio of amine hydrogens and epoxy (E) groups (rH=2[NH2]/[E]=1) with various compositions of (D-400 + D-2000)/DGEBA; seven networks with the weight ratios of D-400/D-2000=1/0, 0.83/0.17, 0.67/0.33, 0.5/0.5, 0.33/0.67, 0.17/0.83 and 0/1 were prepared. Time-temperature superposition was applied to photoelastic data. With increasing content of long D-2000 diamine, the superimposed photoelastic functions at the reference temperature Tr=0°C are shifted to shorter reduced times t/a(T, T r ) (a(T, T r ) is the horizontal shift factor) by ∼18 decades. Although mechanical and optical shift factors, log a(T, T r ) , of two-component networks are virtually the same and their temperature dependences can be described by the WLF equation, these factors for three-component networks are different and cannot be described by the WLF equation. As expected, the equilibrium modulus decreases with increasing contents of D-2000 diamine in networks. On the contrary, the equilibrium stress-optical coefficient Ce passes through a maximum at the weight fraction of DGEBA in networks, wE≈0.50. SAXS experiments show a non-homogeneous distribution of DGEBA segments in three-component networks; the deviations from homogeneous distribution reach a maximum for networks with wE≈0.50. All data suggest that in three-component networks, chemical clusters formed by sequences with non-random participation of short and long diamines exist and that optical functions are more sensitive to these chemical heterogeneities than mechanical modulus. In addition, a modification of photoelastic apparatus for automatic measurement of the strain-birefringence was suggested.

Nanostructure of hyaluronan acyl-derivatives in the solid state

Acyl derivatives of hyaluronan (acyl-HA) are promising materials for biomedical applications. Depending on the acyl length and the degree of substitution, these derivatives range from self-assembling water-soluble polymers to materials insoluble in aqueous environments. The behaviour of acyl-HA was studied in solution, but little attention was paid to the solid state, despite its importance for applications such as medical device fabrication. We thus used X-ray scattering and electron microscopy to explore the solid-state nano-structure of acyl-HA. The set of samples included various substituents, substitution degrees and molecular weights. The obtained data showed that all studied acyl-HA materials contained structures with dimensions on the order of nanometres that were not present in unmodified HA. The size of the nanostructures increased with the acyl length, while the degree of substitution and molecular weight had negligible effects. We suggest that the observed nanostructure corresponds to a distribution of hydrophobic domains in a hydrophilic matrix of unmodified HA segments.

Microphase-Separated PE/PEO Thin Films Prepared by Plasma-Assisted Vapor Phase Deposition

Immiscible polymer blends tend to undergo phase separation with the formation of nanoscale architecture which can be used in a variety of applications. Different wet-chemistry techniques already exist to fix the resultant polymeric structure in predictable manner. In this work, an all-dry and plasma-based strategy is proposed to fabricate thin films of microphase-separated polyolefin/polyether blends. This is achieved by directing (-CH2-)100 and (-CH2-CH2-O-)25 oligomer fluxes produced by vacuum thermal decomposition of poly(ethylene) and poly(ethylene oxide) onto silicon substrates through the zone of the glow discharge. The strategy enables mixing of thermodynamically incompatible macromolecules at the molecular level, whereas electron-impact-initiated radicals serve as cross-linkers to arrest the subsequent phase separation at the nanoscale. The mechanism of the phase separation as well as the morphology of the films is found to depend on the ratio between the oligomeric fluxes. For polyolefin-rich mixtures, polyether molecules self-organize by nucleation and growth into spherical domains with average height of 22 nm and average diameter of 170 nm. For equinumerous fluxes and for mixtures with the prevalence of polyethers, spinodal decomposition is detected that results in the formation of bicontinuous structures with the characteristic domain size and spacing ranging between 5 × 10(1) -7 × 10(1) nm and 3 × 10(2)-4 × 10(2) nm, respectively. The method is shown to produce films with tunable wettability and biologically nonfouling properties.