Cavitation-induced polymerization

Ultrasound-induced polymerizations in CO -expanded MMA at various COj fractions. A Conversion-time history. B Reaction rates. Note that in the experiment without COj present, argon has been added to saturate the cavitation bubbles. 

Process flow diagram of the ultrasound-induced polymerization of MMA in CO -expanded MMA with cavitation (A) and cooling areas (B) in a loop reactor, an extraction column (C) and a separation unit (D). 

In the following sections, the physical backgroimd of ultrasound-induced cavitation and radical formation is described. Subsequently, an overview of the various types of ultrasoimd-induced polymerizations is given, including hulk, precipitation, and emulsion pol5unerization. Finally, the breakage of polymer chains by cavitation is discussed. 

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. 

Ultrasound frequency The frequency of ultrasound 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 fastchanging pressures. Most ultrasound-induced reactions are therefore carried out at frequencies between 20 and 300 kHz. Typically a frequency of 20 kHz is used for ultrasound-induced polymerizations and polymer scission reactions [140]. 

The radicals from polymer and monomer are generated by two different mechanisms. The monomer molecules are dissociated by the high temperatures inside the hot-spot, whereas the polymer chains are fractured by the high strain rates outside the bubble [141]. The majority of the radicals in an ultrasound-induced polymerization reaction originate from the polymer chains [142] see Figure 21.12. It has to be noted that the radicals are only formed in the immediate vicinity of the ultrasound source where cavitation occurs. Subsequently, these radicals are dispersed throughout the reactor. In the literature several types of ultrasound-induced polymerizations have been reported, namely bulk, precipitation, and emulsion polymerization. 

Laser-induced cavitation in polymeric hquids was the subject of studies. In accordance with theoretical predictions, it was found that deviation from of a spherical form in the course of bubble collapse is reduced to a great extent even in diluted solutions. 

Viscosity is an important factor during ultrasound-induced bulk polymerizations as the long polymer chains formed upon reaction cause a drastic increase in the viscosity of the reaction mixture, thereby hindering cavitation and consequently reducing the production rate of radicals. Precipitation polymerization forms a potential solution to this problem, because a constant viscosity and hence a constant radical formation rate can be maintained. In this perspective, high-pressure carbon dioxide is an interesting medium as most monomers have a high solubility in CO2, whereas it exhibits an anti-solvent effect for most polymers. ... 

In Figure 2, the ultrasound-induced bulk polymerization with argon, added to saturate the cavitation bubbles, is compared to the set of polymerization reactions pressurized with CO2 (i.e. C02-fraction 0.02 versus C02-fractions 0.08, 0.14 and 0.20, respectively). In this comparison, it is obvious from both the conversion-time history as well as the polymerization rate curves that during the ultrasound-induced bulk polymerization the reaction rate is declining, whereas the polymerization rate remains constant or even increases when pressurized CO2 is used. The variation in calculated reaction rates for the CO2-fractions 0.08, 0.14 and 0.20, is simply caused by the inaccuracy of determining the derivative of the conversion-time history curves. Still, Figure 2A already shows the significant difference between the decrease in polymerization rate of the experiment with C02-fraction 0.02 versus the maintained reaction rate for the polymerizations with C02-fractions 0.08, 0.14 and 0.20, respectively. 

Typically, in ultrasound-induced bulk polymerizations a maximum conversion of approximately 15% can be achieved. At this conversion the collapse of cavitation... 

Precipitation Poiymerization. As described in the previous section, ultrasound-induced bulk polymerizations are limited to relatively low conversions, because a strong viscosity increase upon reaction hinders cavitation. To obtain higher conversions, precipitation polymerization forms a potential soln-tion. Because the produced polymer precipitates from the reaction medium, the viscosity and consequently the radical formation rate are expected to remain virtually constant. In this perspective, liquid carbon dioxide is a suitable reaction medium, because most monomers have a high solubility in CO2, whereas it exhibits an antisolvent effect for most polymers. Moreover, CO2 is regarded as an environmentally friendly compound, which is nontoxic, nonflammable, and naturally abundant. Since higher pressures are required for CO2 to act as an antisolvent (31-33), the possibility of ultrasound-induced cavitation in pressurized carbon dioxide systems has been studied (34). 

During emulsion polymerization induced by ultrasound, the redundancy of initiator is advantageous in terms of process control and safety. Moreover, initiator residues do not contaminate the product. In contrast to ultrasound-induced bulk polymerizations, high conversions can be obtained in ultrasound-induced emulsion polymerization [154, 155]. Since a heterogeneous reaction system is involved, in which the polymer is insoluble in the continuous aqueous phase, the viscosity of the water phase does not increase upon reaction. The cavitation events occur in the continuous phase, producing radicals mainly from water and surfactant molecules... 

Acoustic destabilizationofsupramolecular polymer assemblies has also been considered [145-150]. Generally, polymeric micelles are induced to dissociate or adopt loosely associated morphologies when exposed to ultrasound [148]. Low-frequency ultrasound also enhances local cellular uptake of drugs, indicating that this approach could prove useful for delivery, provided precautions are taken to prevent cavitational damage to vital structures in the body. Pitt and coworkers [145,... 

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