SynopsisThe course of dynamic viscosity 9' and modulus of elasticity G' curves versus frequency of oscillations f was compared for poly(dimethylvinylsi1oxanes) (PDMVS) samples of similar weight-average molecular weights containing 0.1-0.27, by weight of microgels and free of microgels. It was found that the presence of microgels considerably changes the viscoelastic properties of PDMVS, particularly in the low-frequency range. Only for samples with microgels a linear course of the above-mentioned curves is observed. The content of microgels was determined by the light scattering method, whereas the 1' and G' versus f values were obtained with the use of the Weissenberg rheogoniometer.

synopsisDynamic viscoelastic properties of a system, NCO-terminated polyurethane-aniline-cresolformaldehyde liquid resin were studied during the curing process using an R-17 Weissenberg Rheogoniometer. For gel time determinations a new method was developed, the gel time being then defined as the cure time when the relaxation time 7 = 3 s at a frequency of 0.1 Hz was achieved. The dynamic storage modulus G', the dynamic viscosity q' and the relaxation time T have been determined as functions of curing time. The effects of cure temperature and of curing agent concentration on G', q', and T were studied. The influence of cure temperature on the gel time and on the visco-elastic properties determined at 18 ks (5 h) of curing was discussed. Based on the experimental results, it was concluded that curing at higher temperature led to the more crosslinked polymer. Activation energy values for the gel time were also determined and compared with the values for polyurethanes and polyurethaneureas reported elsewhere. A relationship between the relaxation time and the effect of frequency on modulus G' was presented, and it appeared to be independent of cure temperature, curing agent concentration, and curing time, being then presumably a useful method for comparative studies of viscoelastic behavior of different materials in curing process. This relationship was compared with the approximate relationship between tan 6 and d In G ' l d In o given by Staverman and Schwarzl. The agreement at shorter relaxation times and some discrepancy at longer times were obtained.

Die sterische Lage in der Umgebung yon Cys65-Cys72 wurde untersucht und die maximale Zuglinglichkeit der vier Disulfidbriicken der Ribonuclease S aufgrund eines Aufrisses ermittelt. Cys65-Cys72 bzw. Cys40-Cys95 sind zug~inglich ftir Molekiile mit Maximaldurchmesser yon 4,2 bzw. 4,4 $_. Unter Zuhilfenahme des Aufrisses der Ribonuclease Aim nativen Zustand wird die Reduktion ihrer Disulfidbriicken diskutiert. Unterschiede in der Zugiinglichkeit yon Cys40-Cys95 in den beiden Enzymen wurden festgestellt.

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