Degradation ultrasonic

The preceding results on polycarbonate are at variance with the ultrasonic degradation of poly(vinyl pyrollidone) prepared with peroxide linkages where the rate of chain cleavage was determined to be 5000 times faster at the — 0 — 0 — than the — C —C— bonds [164]. 

Fig. 63. Scission rate constant for the ultrasonic degradation of dextran as a function of molecular weight (M), in different solvents (according to Ref. [179]) ( ) formamide (a) 10% MgSQ4 ( ) water...

Similar approaches were proposed during ultrasonic degradation ... 

Chabrecek, P, Soltes, L., Kallay, Z., and Novak, I., Gel permeation chromatographic characterization of sodium hyaluronate and its fractions prepared by ultrasonic degradation, Chromatographia, 30, 201, 1990. 

Xanthans from several different sources were used in this study Xanthan samples A, B and C were kindly provided as freeze dried powder of ultrasonic degraded xanthan by Dr. B. Tinland, CERMAV, Grenoble, France. The molecular weights of these samples were determined experimentally in dilute solution by Dr. B. Tinland. Xanthan D was kindly provided as pasteurized, ultrafiltrated fermentation broth by Dr. G. Chauveteau, Institut Francais du Petrole, France. Xanthan E was kindly provided as a freeze dried sample from Dr. I. W. Sutherland, Edinburgh, Scotland. Xanthan F was obtained as a commercial, powdered material (Kelzan, Kelco Inc., a Division of Merck, San Diego CA.). Xanthan G was obtained as a commercial concentrated suspension (Flocon 4800, Pfizer, New York, NY)... 

Kubo M, Matsuoka K, Takahashi A, Shibasaki-Kitakawa N, Yonemoto T (2005) Kinetics of ultrasonic degradation of phenol in the presence of Ti02 particles. Ultrason Sonochem 12 263-269... 

Fig. 38. A Degradation experiments with pregel polymers isolated prior to the onset of macrogelation in 1,4-DVB polymerization [209] Variation of Mw ( ) and dz (O) with the time of ultrasonic degradation. The polymer sample was prepared at 5 g/100 mL monomer concentration and its initial Mw was 2.2 X106 g/mol. The dotted horizontal line shows Mw of zero conversion polymers ( individual microgels ). B Variation of Mw with the polymerization time t and monomer conversion x in 1,4-DVB polymerization at 5 g/100 mL monomer concentration. The region 1 in the box represents the limiting Mw reached by degradation experiments. [Reprinted with permission from Ref. 209,Copyright 1995, American Chemical Society].

Fig. 5.11. Effect of irradiation time on the ultrasonic degradation of aqueous native dextran [2% w/v 60 W 30°C 20 kHz).

The ultrasonic degradation of polystyrene in benzene increases with a decrease in the applied ultrasonic frequency (see Section 2.6.1). 

The ultrasonic degradation of aqueous polyacrylic acid decreases with the addition of ether i. e. increased vapour pressure (see Section 2.6.2). 

The ultrasonic degradation of an air saturated toluene solution of polystyrene is greater than when the solution is degassed. 

All the above are dependent upon the same factors i. e. intensity, frequency, gas content and gas type etc. The current view is that ultrasonic degradation is for the main part mechanical in its origin and due to the high pressures associated with the collapse of the bubble. There may be the possibility that part of the degradation is thermal but this would only be significant for those macromolecules with a sufficiently high vapour pressure to allow entry into the cavitation bubble. 

Tab. 5.5 Percentage ultrasonic degradation and final R.M.M. of polystyrene in toluene at various temperatures.

That is not to say that degradation in the presence of cavitation is thermal in origin as work by Melville has shown. Melville carried out both ultrasonic and thermal degradation of two samples of copolymer of polymethyl methacrylate and acrylonitrile, (molar ratio of methacrylate to acrylonitrile 411 1 and 40 1) and observed that whereas the latter copolymer had the faster thermal degradation rate, in the presence of ultrasound both copolymers showed practically the same rate of degradation. Further, a sample of polymethyl methacrylate had the same ultrasonic degradation rate as both of the copolymers (Fig. 5.20). 

Figures 5.24, 5.25 and 5.26 also show that the limiting molar masses are lower the higher the intensity. Whilst Okuyama [50] and Thomas et al. [51] predicted, and several workers observed [52], that the limiting molar mass is invariant with intensity, most workers now agree that decreases with increase in ultrasonic intensity. Price [39] found that the results of the ultrasonic degradation of polystyrene in toluene fitted equation (Eq. 5.22).

Tab. S.ll Rate constants and limiting molar masses for ultrasonic degradation of polystyrene at various concentrations in toluene.

Tab. 5.12a Rate constant and limiting molar masses (after 8 h ultrasonic irradiation) for the ultrasonic degradation of aqueous dextran (a) at 30°C and variable ultrasonic power (b) at an ultrasonic power input of 60 W and variable temperature.

Tab. 5.15 Ultrasonic degradation (20 kHz, 70 W, 27°C) of various polyacrylates (PRMA) in toluene.

In principle block copolymers can also be produced by irradiating solutions containing a homopolymer (from monomer type A) and a monomer (type B) (Fig. 5.34). In such cases, polymerisation resembles that of graft polymerisation where the monomer (B) is thought to be initiated by the macroradical produced by the ultrasonic degradation of the homopolymer. In those cases where polymer was deliberately absent, polymerisation was either not observed to occur or proceeded at a similar rate to that in the presence of polymer. 

M. Garcia-Alverez, F. Lopez-Carrasquer-ro, M. Morrilo, and S. Munoz-Guerra, Ultrasonic degradation of polyasparates and polyglutamates, /. Poly. Sci., Poly Phys., 1997, 14, 2379. 

Block copolymers, with segments or domains of random length, have been produced by the mechanical or ultrasonic degradation of a mixture of two or more polymers, such as hevea rubber and PMMA (Heveaplus). 

Basedow, A. M. and Ebert, K. Ultrasonic Degradation of Polymers in Solution. 

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