AbstractA mechanism for ultrasonic degradation is proposed as follows. At the contracting step of a violently vibrating bubble in liquid,jstructural units of a polymer molecule (partial chainj) take a linearly extended configuration owing to the suction force of bubble, which occurs as a result of slippling and friction between solvent molecules and the extended part of polymer. This partial chain is subject to a tensile force arising from the friction which is maximum at the middle carbon bond of the partial chain. If it is greater than the critical tensile strength of a carbon–carbon bond, the bond at the middle is unable to absorb the work done by it, so the bond is torn off, i.e., the polymer degrades. Under such considerations a formula for the final value of the degree of polymerization after infinite irradiation is deduced, from which the following results are obtained. (1) Cavitation which can degradate polymer molecules effectively is not stationary but nascent. (2) The cavitation intensity is very insensitive to ultrasonic intensity. Moreover, from the proposed mechanism, several experimental facts can be explained: practically monodisperse solution is obtained after long‐term irradiation; and ultrasonic degradation occurs even in solutions of infinite dilut
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