ISSN:
0022-3832
Schlagwort(e):
Chemistry
;
Polymer and Materials Science
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
,
Physik
Notizen:
Gelation velocity and reaction rate for the acid-catalyzed condensation reaction in the aqueous solution (10-22%) of partially N-methylolcarbamoylethylated polyvinyl alcohol (about 20 mole-% degree of methylolcarbamoylethylation) have been investigated by the falling ball viscosity method and the chemical analysis of methylol groups including the effects of kinds of acid, concentrations of acid and polymer, and temperatures of reaction. For the reciprocal gel point, 1/tg, and the hydrogen ion concentration, [H+], the following equation holds: \documentclass{article}\pagestyle{empty}\begin{document}$ 1/t_g = c[{\rm H}^ +]^\alpha $\end{document} where α and c are constants independent of the kind of acid; also α is independent of temperature and polymer concentration, while c increases with increasing temperature or polymer concentration. The 1/tg is approximately proportional to the concentration of polymer in the range of a concentrated polymer solution above about 15% by weight. From this fact, it is suggested that, in a concentrated polymer solution, the intermolecular crosslinking proceeds predominantly as a bimolecular reaction with respect to methylol groups. Assuming that the increase of the apparent viscosity of the reacting system is mainly due to that of the structural viscosity which should be increased by a bimolecular crosslinking reaction, the following expression was derived for the gelation velocity: \documentclass{article}\pagestyle{empty}\begin{document}$ \eta _0 /(\eta _t - \eta _0) = A(1/i - 1/t_g) $\end{document} in which, \documentclass{article}\pagestyle{empty}\begin{document}$ \begin{array}{*{20}c} {A/t_g = 1 - x_g /a} & {{\rm and}} & {k = x_g /(a^2 A)} \\ \end{array} $\end{document} where η0 and ηt are the apparent viscosities at time 0 and t, respectively, A is a measure of gelation velocity, a is the initial number of methylol groups, xg is the number of crosslinked methylol groups at the gel point, and k is the bimolecular rate constant. The experimental results of viscosity changes have shown a fairly good agreement with the above expression except in the early stage of condensation. The extent of reaction at the gel point (xg) has been estimated to be about 0.03-0.05 regardless of concentration of polymer and acid, kinds of acid, and temperature. Also, it has been observed that the rate constant of reaction is independent of the concentration of polymer and is proportional to the concentration of hydrogen ions. The above expression and estimation have been verified by the chemical analysis of methylol groups in the condensation processes. However, the extent of reaction at the gel point is about ten times that predicted by Flory's theory. This discrepancy may presumably be attributed to the fact that all of the methylol groups on the polymer chains which are coiled, entangled, and strongly solvated prior to the crosslinking are not equally reactive sterically and a certain extent of intramolecular crosslinking occurs. The activation energy of reaction has been estimated to be about 21 kcal./mole.
Zusätzliches Material:
9 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/pol.1960.1204314119
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