Wolfgang Pechhold

Phase behavior and elastic properties of a slightly crosslinked liquid crystalline main-chain polymer

A liquid crystalline main-chain polymer was slightly crosslinked by a reaction withα, ω-difunctionalized oligo-siloxanes. Crosslinking does not disturb the liquid crystalline phases, which are identified by x-ray measurements as smecticB and smecticA phases. Measurements of the elastic properties of the crosslinked sample show rubber-like elasticity, even in the liquid crystalline phases. A difference of 7 K was found between cooling and heating for the smecticA/isotropic transition from DSC and mechanical measurements.

Spectral analysis of viscoelasticity of the human lens.

PURPOSE In the aging lens, two different structural changes become progressively apparent: loss of accommodation and formation of cataract. These changes are due to different alterations in lens structure taking place on a molecular scale. METHODS For the characterization of liquids, gels, elastic bodies and solids, dynamic mechanical analysis (DMA) represents a non-destructive macroscopic analysis method with considerable sensitivity to microscopic structural changes. Spectra of the shear compliance (reciprocal shear modulus) of human lenses at different ages were measured in a wide frequency range (1 mHz to 1 kHz). By means of computer analysis, the parameters of the involved relaxation processes and the viscous flow were determined. RESULTS The increase in cross-linking density was correlated with the disappearance of viscous flow in the measured spectra. The accumulation of high-molecular-weight aggregates was held responsible for the shift of the lower frequency relaxation process to lower frequencies with increasing stiffness, which is likely to be correlated with a prolonged accommodation time. CONCLUSION Dynamic mechanical analysis represents a useful method for the spectroscopic characterization and quantification of the changes in the macroscopic viscoelastic properties caused by molecular changes in cataract formation and loss of accommodation.

Dielectric relaxation of specially prepared polystyrene terminated with rigid polar groups

Two polystyrenes terminated withp-cyanobenzyl andp-cyanobiphenyl groups (=labels) were prepared and their complex dielectric constants were measured in the glass transition region in the frequency range 10−2–106 Hz. The glass temperaturesTg (DSC) were considerably different, 92 and 97.5°C, resp., although their molecular weights were very similar (11 000 and 10 000 g/mol, resp.). Their relaxation behavior showed that the cyano groups relax cooperatively with the polymer segments. The cyanophenyl groups were found to relax with shorter relaxation times than the cyanobiphenyl groups. The measured relaxation strengths showed that there was no association between the dipoles. The relaxation mechanisms of the cyano groups in both labels seemed to be different although the only difference between them was an additional phenyl group in the case of the second label.

Meander model of amorphous polymers

A review of the present state of the meander cube model of amorphous polymers is presented. It is mainly concerned with appropriate methods for the determination of 3r/d, the meander cube side length (measured in lateral chain distances d). Some recent results are also presented: (a) on the magnetic birefringence of Polystyrene-solutions, which can be explained by a similar cubic superstructure of the solution (with a little smaller cube side length than in the condensed polymeric state) down to a few percent of concentration. (b) on the π/A-isotherms of some polymer thin films cast from dilute solutions, which indicate the existence of either a cube-meander monolayer (e.g. for Polystyrene) or a monolayer of 2-dimensionally superfolded molecules, with the monolayer partially folded in the third direction, forming layers of 3r in height (e.g. for IR, NBR).

Dielektrisches Verhalten polarer Labelmoleküle in Polyisobutylen (PIB) Zusammenhang mit Superstrukturen im Schmelzzustand

ZusammenfassungEs wird über das dielektrische Relaxationsverhalten der beiden Mischungsreihen Tetra- (bzw. Poly-)isobutenylbern-steinsäureanhydrid/PIB im Konzentrationsbereich 0.03 (0.3) bis 6 (30) Gewichtsprozent berichtet. Der polare Mischungspartner TIBBSA (PIBBSA) relaxiert — unabhängig von der Konzentration — um den Faktor 40 (100) langsamer als PIB im Glasprozess. Sein Relaxationsbetrag zeigt im Gebiet um 1,5 (5) Gewichtsprozent eine anomale Konzentrationsabhängigkeit. Diese Mt sich unter der Annahme bevorzugter Einlagerung von Labelmolekiilpaaren in Superstrukturfalten im Rahmen des Mäandermodells quantitativ verstehen.SummaryThe dielectric relaxation of the dipolar label tetraisobutenyl-(and polyisobutenyl-) succinic anhydride has been investigated as well as in mixtures with polyisobutene (PIB), the concentration of the label varying between 0.03 (0.3) and 6 (30) percent by weight. The admixed label TIBBSA (PIBBSA) relaxes — independent of concentration —40 (100) times slower than PIB in the glass process. Its relaxation strength shows an anomalous concentration dependence in the range near 1.5 (5) percent. This finding can be quantitatively explained by the meander model if one assumes that pairs of label molecules preferentially deposit in the meander folds.

The meander model of muscle

In this paper the meander model of muscle is proposed and treated quantitatively. The geometries of myosin and uninterrupted actin filaments as well as of theZ zone in vertebrate striated muscle are derived from thermodynamic equilibrium considerations. Crossbridges formed by the myosin projections and the actin meander folds are considered either to be loose or closed. Mg ATP action enables them to rearrange cyclically. The closed bridges give rise to a thermodynamic contractive force on the actin filament. Also passive and active stress-strain behaviour (under isometric conditions) agree with experiment.

Effect of some curatives on the properties of ethylene propylene diene rubber/polyethylene blends

A series of ethylene propylene diene rubber/polyethylene (EPDM/PE) blends has been prepared containing different weight fractions of PE up to 0.66. The blends were vulcanized with a sulphur system N-cyclohexyl-2-benzthiazol sulphenamide/sulphur (CBS/S), and a non-sulphur-system dicumyl peroxide (DCUP). The concentration of the latter has been changed from 1.5phr up to 6phr calculated on the total weight of the blend composition. It has been found that the maximum torque obtained from rheographs for blends vulcanized with the CBS/S system decreases markedly with increasing PE concentration in comparison with those vulcanized with peroxide. The E modulus obtained from the stress-strain diagram at 110°C showed the role played by the crosslinking of PE, the E modulus for blends vulcanized by peroxide being higher than for samples vulcanized with CBS/S. In contrast, the values of E modulus of both samples are practically the same at room temperature and attain more than 40MPa depending on the composition. The tensile strength at room temperature strongly increases with increasing the weight fraction ofPE. It has also been confirmed that the melting point of the crystalline phase of PE decreases with increasing crosslinking density of PE. The shear modulus obtained from dynamic mechanical measurements is in accordance with that obtained from static mechanical measurements.

Corneal Viscoelasticity Spectra as a Result of Dynamic Mechanical Analysis

In this study, dynamic mechanical analysis (DMA) was utilized for the measurement of the viscoelastic properties of human cornea. The complex shear compliance J (reciprocal shear modulus G), Young’s compliance D (reciprocal Young’s modulus E), and the “indenter compliance” C were recorded as a function of frequency (spectrum), of time, of hydration and of temperature.