Polymers ultrasound effects

Gul, R. J., Wan, P. S., Hyimgsu, K., and Wook, L. 2004. Power ultrasound effects for in situ compat-ibilization of polymer-day nanocomp)osites. Journal of Materials Science and Engineering C 24 285. 

Jambrak AR, Herceg Z, Subaric D, et al. 2010. Ultrasound effect on physical properties of com starch. Carbohydr Polym 79 91-100. 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

The potential of sonochemistry was identified over sixty years ago in a wide ranging paper entitled The Physical and Biological Effects of High Frequency Sound-Waves of Great Intensity [13]. Over the few years which followed this paper a great deal of pioneering work in sonochemistry was carried out and, as a result of this, two reviews on the applications of ultrasound in polymer and chemical processes were published... 

We are now in a position to discuss the effects of low frequency (< 400 kHz) high intensity (> 3 W cm ) ultrasonic waves on macromolecules (polymers) and examine how the parameters such as frequency, intensity, hydrostatic pressure etc affect both the polymerisation and also the depolymerisation processes. We will also consider in this chapter the use of ultrasound in polymer processing. 

Miyata and Nakashio [77] studied the effect of frequency and intensity on the thermally initiated (AIBN) bulk polymerisation of styrene and found that whilst the mechanism of polymerisation was not affected by the presence of ultrasound, the overall rate constant, k, decreased linearly with increase in the intensity whilst the average R.M.M. increased slightly. The decrease in the overall value of k they interpreted as being caused by either an increase in the termination reaction, specifically the termination rate constant, k, or a decrease in the initiator efficiency. The increase in kj(= kj /ri is the more reasonable in that ultrasound is known to reduce the viscosity of polymer solutions. This reduction in viscosity and consequent increase in Iq could account for our observed reductions [78] in initial rate of polymerisation of N-vinyl-pyrrolidone in water. However this explanation does not account for the large rate increase observed for the pure monomer system. 

More recently Kashimura et al. [96] has investigated the effect of ultrasound on the electroreductive synthesis of polysilanes, polymer germanes and related polymers using magnesium electrodes. They found that the presence of ultrasound greatly facilitated the reactions. 

Investigations into the effect of ultrasound upon these polymerisation processes began in the mid 1980 s when Akbulut and Toppare [81] examined the potentiostatic control of a number of copolymerisations. In such copolymerisations initiation takes place once a potential in excess of the oxidation potential of either monomer has been applied. However, often potentials even higher than these are required due to the formation at the electrode of a polymer film. These films create a resistance to the passage of current in the bulk medium with consequent reductions in the possible electrochemical reactions and therefore reductions in the rate and the yield. The use of ultrasound has been rationalised in terms of its removal of this layer in a... 

Akbulut has also reported the effect of ultrasound upon the electronitiated homopolymerisation of butadiene sulfone (Fig. 6.21) in MeCN/Bu4NBF4 using platinum electrodes [84]. Interestingly ultrasound did not completely clear the electrode of a polymer film, although it did produce an improvement in the percentage conversion versus time characteristics of the polymerisation. 

Henglein, A. and Gutierrez, M., Effects of continuous and pulsed ultrasound a comparative study of polymer degradation and iodide oxidation,. Phys. Chem., 94, 5169-5172, 1990. 

The peak temperature obtained for low ultrasonic energy (25 J) is below 80 °C, whereas for high energies (125-150 J) it is above 140 °C. In mixtures of keto-profen with acrylic polymers [90], the increase in temperature was slightly lower. In this respect it must be mentioned that a recent modification of the ultrasound-assisted tableting machine that involves the suppression of Teflon isolators in contact with the powder must result in a faster decrease in temperature inside the compression chamber. Thermal effects can cause the total or partial fusion of some components of the formulation. Nevertheless, in the assayed controlled-release formulations, the components are usually below its melting points. 

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