Cavitation-induced Polymer Scission

The mechanism by which polymers interact with cavitation-induced shear fields informs the design and interpretation of mechanochemical experiments making use of sonochemistry. First, it is observed that scission occurs preferentially around the mid-point of the polymer chain, as the solvodynamic forces are greatest at this point... 

Ultrasound Frequency. The frequency of ultrasoimd has a significant effect on the cavitation process. At very high frequencies (>1 MHz), the cavitation effect is reduced as the inertia of a cavitation bubble becomes too high to react to fast changing pressures. Most ultrasoimd-induced reactions are therefore carried out at frequencies between 20 and 900 kHz. The optimum ultrasoimd effect as a function of frequency depends on the reaction system eg, water dissociation has an optimum frequency at approximately 500 kHz (21). For bulk pol5mierizations the maximum radical formation rate is obtained at 20 kHz. At this frequency the highest strain rates are produced, which results in a high radical formation rate by polymer scission (22). 

Mechanism. Although there is still some debate about the precise origin of scission, it has been shown that ultrasound-induced polymer breakage is a direct consequence of cavitation, because under conditions that suppress cavitation, no degradation is observed. In this nonrandom scission process the poisoner is fractured at the centre of the chain (54-56). This is clearly shown in Figure 11,... 

For the development of sustainable polymer processes, ultrasound is an interesting technology, as it allows for polymerizations without the use of initiator. The radicals are generated in situ by cavitation events [116, 117], which make possible a dean and intrinsically safe polymerization reaction. As a result of the high strain rates outside the bubble, cavitation can also induce chain scission [118,119], which provides an additional means to control the molecular weight of the polymer produced. In Sections 21.3.1 and 21.3.2 the physical background of ultrasound-induced cavitation and radical formation will be described. Subsequently (see Section 21.3.3), an overview of the several types of ultrasound-induced polymerizations will be given, namely bulk, predpitation, and emulsion polymerization. 

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