Shear Degradation of Poly(Vinyl Acetate) in Toluene Solutions by High-Speed Stirring
A poly (vinyl acetate) (PVAc) of Mw 750,000 and Mw/Mn 5.10 in toluene solution was sheared in a Virtis-60 Homogenizer. The polymer concentration was 3.0 — 12.0 gm/100 ml and test temperature was 10 ± 0.5°C. The extent of degradation was measured by gel permeation chromatography (GPC). It was concluded that on shearing (i) the molecular weight decreases rapidly at the beginning of shearing, and thereafter decreases ever more slowly towards a limiting value, (ii) the molecular weight distribution is narrowed, (iii) no degradation occurs up to 5000 rpm and thereafter increases with stirring speed, (iv) degradation is more at lower concentrations but concentration is not a sensitive variable, and (v) the chain scission occurs randomly. The Mark-Houwink relationship for PVAc in THF at 25°C was derived as[η]=2.47x10-4xMv0.644.