S. Wartewig

Polymerizations in the presence of seeds: 3. Emulsion polymerization of vinyl acetate in the presence of quartz powder☆

Abstract The emulsion polymerization of vinyl acetate in the presence of quartz powder (‘Suprasil’) was investigated. Ultrasonic velocity measurements showed that the concentration of monomers in the vicinity of the particles is very high compared with the concentration in the bulk dispersion medium. Therefore, polymerization in a surface layer around the particles should be enhanced. Electron micrographs of samples extracted during the first step of the polymerization and the dependence of reaction rate on pH value and initiator concentration can be explained in terms of this surface coating process.

Longitudinal viscoelastic relaxation in amorphous polybutadiene by dynamic light scattering and ultrasonic absorption

Abstract We report polarized photon correlation measurements of amorphous bulk polybutadiene (Mw = 13 000, Tg = 222 K) in the temperature range 230–246 K and complementary temperature- (160–360 K) and frequency- (0.1–63 MHz) dependent ultrasonic relaxation measurements on an identical sample. The light scattering and ultrasonic data on the longitudinal density fluctuations over nine decades of time conform to a single WLF equation. The strong temperature dependence of the average relaxation time was found to be different from that of the viscosity. The coupling model of relaxation can account for this difference. At temperatures higher than Tg, photon correlation spectroscopy detects slower relaxation modes, which are active in the ultrasonic experiment. From the latter, these modes are related to subchain motions and entanglement effects.

Analysis of temperature-dependent longitudinal ultrasonic properties of amorphous polymers

Abstract The temperature dependence of the components of the longitudinal moduli M ′( T ) and M ″( T ) was derived, from ultrasonic measurements at 1 MHz. The measurements were performed in polybutadiene ( M w = 13000) and poly(phenylmethylsiloxane) ( M w = 4100). The temperature dependence of M ′ and M ″ was calculated in the framework of linear viscoelasticity using the Kohlrausch-Williams-Watts (KWW) relaxation function and including the temperature dependence of the mean relaxation time described by the Vogel-Fulcher-Tamman-Hesse equation. The only adjustable parameter was the KWW parameter β. We also considered the possibility of a temperature-dependent β parameter to describe the data.