Polymer cavitation effects

Viscosity of dissolved polymers drops irreversibly under acoustic treatment65 A8). The depolymerization process us rather fast during the first minutes of the treatment and then it becomes slow and ceases completely when the equilibrium molecular mass (MM) M is reached. The higher the polymer s initial molecular mass N0, the higher the rate of destruction. The majority of authors associate polymer destruction in solution with cavitation effects occurring under acoustic treatment. 

Tsai, M.-L., Bai, S.-W., and Chen, R.-H. (2008). Cavitation effects versus stretch effects resulted in different size and polydispersity of ionotropic gelation chitosan-sodium tripolyphosphate nanoparticle. Carbohydr. Polym. 71, 448-457. 

This chapter aims to describe the mechanical bias of cavitational effects and how they are related to conventional mechanochemistry and force-induced physical fields in general. This subject has been well documented over the last decade [5-9], and particular attention has been paid to scenarios such as mechanically responsive polymers [10-12], micro- and nano-structured materials [13-15] and... 

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). 

Fig. 2 shows the entrance die pressure and power consumption for various wt% loadings of CNTs as a function of ultrasonic amplitude. The measured pressure is before the ultrasonic treatment of PEI/MWNT composites. A continuous decrease in pressure with increasing ultrasonic amplitude was observed. This is from a combination of heating from dissipated energy from ultrasound, cavitational effect from ultrasonic waves leading to some thixotropic and permanent changes in polymer, reduction in friction at die walls and horn surfaces due to ultrasonic vibrations and possible shear thinning effect created by ultrasound waves. The die... 

Fig. 31. Bubble wall velocity vs time during cavitational collapse for different values of the parameter X defined as X ss 0.4 c iTl] p./fri/2 (Ph — Pv)i/2). X permits us to account for the viscous and inertia effects of the polymer solution (redrawn according to Ref. [122]) ...

The paper gives an overview of effects occurring or acoustic treatment of dissolved and molten polymers. Emphasis is made on acoustic cavitation discovered recently not only in low-viscous fluids but also in molten polymers. Major guidelines have been specified for practical utilization of acoustic treatment of flowable polymers in molding intensification of extrusion processes, reduction in thickness of produced films, directed mechanical destruction, chemical activation of melts, etc. Efficiency of overlapping high-frequency vibrations in molding of molten thermoplastics is discussed in terms of power consumption. 

The decrease in polymer degradation was less pronounced than that of the iodine oxidation. High temperatures are not required to generate effective shear forces in the vicinity of the cavitation bubbles however, the effects of the two kinds of sonication cannot be simply compared by their intensities. Polymer degradation is very much favored under sonication... 

In ABS, where particle size is much smaller than in HIPS, the particles are less effective as craze initiators and the fatigue fracture surface shows evidence of considerable localized plastic deformation of the matrix polymer as well as of cavitation and/or loss of adhesion of the rubber particles. 

Somewhat similar measurements could be based on solid disruption [18], polymer degradation [7], or accelerated dissolution. These well-known mechanical effects of ultrasound also derive from cavitation. Thus one might measure the rate of particle size reduction under sonication of some standard solid dispersed in a given fluid. Alternatively one could measure the rate of dissolution of a standard solid in a solvent, or the reduction in molecular weight of polymer chains. Here again the initial particle size and surface conditions, together with pressure and temperature, should be carefully monitored. 

O CONTENTS Introduction to Series An Editor s Foreword, Albert Padwa. Introduction, Timothy J. Mason. Historical Introduction to Sonochemistry, D. Bremner. The Nature of Sonochemical Reactions and Sonoluminescence, M.A. Mar-guli. Influence of Ultrasound on Reactions with Metals, B. Pugin and A.T. Turner. Ultrasonically Promoted Carbonyl Addition Reactions, J.L. Luche. Effect of Ultrasonically Induced Cavitation on Corrosion, W.J. Tomlinson. The Effects of Ultrasound on Surfaces and Solids, Kenneth S. Suslick and Stephen J. Doktycz. The Use of Ultrasound for the Controlled Degradation of Polymer Solutions, G. Price. 

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