A. Havránek

Dependence of viscoelastic spectrum width on the structure of model imperfect networks prepared by endlinking

Using the theory of branching processes, structural parameters such as the molecular weights of elastically active network chains (EANCs), including dangling chains, backbone EANCs and dangling chains of networks built up by the alternating polyaddition of a bi- and trifunctional monomer, are characterized. The theory is compared with viscoelastic data on polyurethane networks prepared from poly(oxypropylene)triols and diisocyanate at various initial ratios of functional groups; in the calculation, the distribution of functionalities of the triols used and the possible incompleteness of the reaction is taken into account. The comparison reveals that both the length of the backbone EANC and the length of dangling chains contribute to the total width of the retardation spectrum.

The relaxation and equilibrium behaviour of model polyurethane networks

SummaryThe mechanical, dielectric and rheo-optical behaviour of model networks prepared from OH-terminated poly(oxypropylene)triols and 4,4′-diphenylmethane diisocyanate with various molar ratios of hydroxyl to isocyanate groups, rH=0.95 to 1.7, was investigated. The observed independence of the stress-optical coefficient of rH indicates the homogeneity of networks. Networks prepared with rH≈1 exhibit a narrow transition zone indicating a narrow chain distribution. With increasing rh, the transition becomes distinctly broader; dielectric measurements are well correlated with the mechanical ones. The equilibrium modulus markedly decreases with increasing rH.

Mobility of rubber chains near above the glass transition temperature

SummaryDetailed viscoelastic measurements of natural and ethylene-propylene rubbers cross-linked by sulphur and dicumyl peroxide to various degrees were performed. Main attention is devoted to lightly cross-linked systems where two relaxation processes occur near above the glass transition temperature. The two peaks of the relaxation spectra corresponding to these two relaxation processes begin to overlap as the cross-linking density increases. The simple structural interpretation of the observed features using the entanglement concept is given.

Electric polarization induced by strain gradient

Electric polarization induced by a strain gradient-flexoelectricity-is studied theoretically on the system of parallel chains of harmonic oscillators. Instead of the strain gradient, the external forces are chosen as an external parameter. They are applied on the boundary of a sample the shape of which ensures nonhomogeneous deformation to be created.

Photopolymer recording material with AgBr nanoparticles for optical holography

A new self-developing recording material with silver bromide nanoparticles and photopolymerization system has been prepared and tested as a recording medium for optical holography. Through the method of the real-time measurement of a diffraction grating growth, we have shown that an efficient volume phase grating is formed during a holographic exposure. The refractive index modulation of the formed grating is several times higher than in the case of standard acrylamide-based photopolymers with a similar composition. The material response on different exposure parameters has been measured and obtained results have been discussed. The transfer of nanoparticles within the recording layer, which is the main cause of the grating growth, has been experimentally verified through the direct observation of the grating micro-structure with the scanning electron microscope.

Analysis and simulations of two-wave mixing in photopolymer holographic recording media

Abstract The process of two-wave mixing in photopolymer recording materials was investigated theoretically. The diffraction grating already forms during exposition and it may influence the original interference field distribution through diffraction of waves on the refractive index modulation. In order to show this, Kogelniks coupled wave theory was extended to demonstrate the possibility of energy transfer from one recording wave to the other. The energy transfer and the intensity distribution during the recording process were systematically analysed depending on the boundary conditions. As a next step, the first harmonic model of the transmission grating recording, based on a simple material model, was implemented and solved. The ratio of the input intensities was found to be a crucial parameter and thus extensive simulations for various ratios of intensities were carried out. Modelling implies that the interference field and the refractive index grating just coincide for equal intensities. For intensities differing from unity they do not overlap themselves during the recording process. It has also turned out that the diffraction efficiency of the recorded grating drops against the case where the effects of two-wave mixing are not considered. The results of our analysis and simulation help give a better understanding of the physics of the recording process and proper adjustment of recording parameters in such applications as optical holography and holographic memories.

New recording material with AgBr nanoparticles for optical holography

A new recording material with silver bromide nanoparticles and photopolymerization system has been prepared and tested as a recording medium for optical holography. Through the method of the real-time measurement of a diffraction grating growth, we have shown that an efficient volume phase grating is formed during a holographic exposure. The material response on different exposure parameters has been measured and obtained results have been discussed. The transfer of nanoparticles within the recording layer, which is the main cause of the grating growth, has been experimentally verified through the direct observation of the grating micro-structure with the scanning electron microscope.

Thermally stimulated depolarization currents study of structural features of polymers

Molecular mobility at the glass-transition region of the block styrene-butadiene-styrene (SBS) copolymers has been investigated by means of thermally stimulated depolarization currents (TSDC). The TSDC maxima, /spl alpha//sub 1/, have been attributed to the glass transition temperature of the butadiene phase T/sub /spl alpha//, B, according to differential thermal analysis data. The main features of the TSDC spectra are: 1) peak amplitude is a linear function of external field, 2) peak shape and its temperature position depend on the polarization temperature T/sub p/. Such results are meaningful for a dipole relaxation time distribution. The dipole distribution function of relaxation times has been calculated from the TSDC curve.

Dielectric Relaxation Phenomena in Butadiene-Acrylonitrile Copolymers

Abstract The polarization phenomena of the polybutadiene - acrylonitrile (PBAN) copolymers have been investigated in the vicinity of the glass transition region by methods of thermally stimulated depolarization currents (TSDC), thermally stimulated polarization currents (TSPC) and dc conductivity. Relaxation regions above the glass transition temperature Tg have been analyzed by thermal sampling and partial depolarization techniques in more detail. The effects associated with the cyclizing after pyrolysis in PBAN copolymers have been studied by thermally stimulated and viscoelastic measurements.