Polyvinyl acetate precipitates from aqueous solution as it is formed. When potassium persulphate is used as initiator the polymer suspension is stable and constitutes the principal locus of reaction. The effect of reaction conditions on the rate of polymerization and on the number of particles formed ha5 been studied. The surface charge density appears to be the factor which determines the number of particles formed and which limits the rate of reaction.ZUSAMMENFASSUNG:Polyvinylacetat f a t bei seiner Bildung aus wainriger Losung aus. Bei Verwendung von Kaliumpersulfat als Initiator ist die Polymersuspension bestandig und stellt den hauptsachlichen Ort der Reaktion dar. Der Eidul3 der Reaktionsbedingungen auf die Polymerisationsgeschwindigkeit und auf die Zahl der sich bildenden Polymerteilchen wird untersucht. Es scheint, dal3 die Oberflachedadungsdichte die Zahl der Polymerteilchen bestimmt und auch die Reaktionsgeschwindigkeit begenzt.
SynopsisThe insolubilizatior~ of polyvirlyl alcohol by potassium dichrornate on exposure to light, has been investigated. Ultraviolet spectrometry shows that secondary hydroxyl groups are oxidized to ketone groups but insolubilization is attributed to crosslinking of polymer chains by co-ordination of alcohol groups to "nascent" chromic ions formed by reduction of the dichromate. Exposure to air saturated with water vapor eliminates the carbonyl absorption maxima from the ultraviolet spectrum of polyvinyl alcohol film but the spectrum can be restored by heating: this phenomenon is independent of the crosslinking reaction.
The problem of particle formation in aqueous solutions of vinyl acetate is treated according to the Deryaguin‐Landau‐Verwey‐Overbeek (D.L.V.O.) theory of the stability of lyophobic colloids.
In the absence of surfactants, polymer particles are stabilised by ionic end‐groups derived from persulphate initiator. Palit's dye partition method is used to determine the number of these end‐groups and the amount of adsorbed sodium dodecyl sulphate which was used as emulsifying agent in some experiments; this enables the electrical forces of repulsion between particles to be calculated. The Hamaker constant which defines the magnitude of the van der Waals' forces of attraction between particles has been estimated for polyvinyl acetate. The dependence of the stability of the polymer dispersion formed on the concentrations of initiator, emulsifier, etc. is investigated; the observations accord with expectations on the basis of D.L.V.O. theory. It is shown that small particles are unstable with respect to coalescence with larger particles. The processes of polymerisation and particle growth are essentially independent in this system.
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