Ultrasonic Degradation of Polymers in Solution

Ultrasonic Degradation of Polymers in Solution

Arno Max Basedow and Klaus Heinrich Ebert Institut fiir Angewandte Physikalische Chemie, Universitat Heidelberg, D-6900 Heidelberg

Table of Contents 1.

Introduction

84

2. 2.1. 2.2. 2.3. 2.4.

Effects of High Intensity Ultrasonics in Liquids . Elementary Physics of Ultrasonic Waves Generation of Ultrasound in Liquids Cavitation . . . . . . . . . . Shock Waves Produced by Cavitation .

84 84

3. 3.1. 3.2. 3.3."

Physicochemical Actions of Ultrasound on Macromolecules . Chemical Effects . . . . Depolymerization Formation ofMacroradicals

4. Parameters Affecting the Degradation of Polymers in Solution . 4.1. Quantities Characterizing Ultrasound . . 4.2. Properties of the Solution . . . . . . .

86 88 95 97 97 99 101 l 03 l 03 l 06

5. 5 .1. 5.2. 5.3. 5 .4.

Experimental Investigation of Degradation . Kinetic Analysis of the Degradation Reaction Evaluation of Molecular-Weight Averages . . Determination of Molecular-Weight Distributions Degradation Models . . . . . . . . .

6. 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. 6.7.

Mechanisms of Ultrasonic Degradation . . . Fundamentals of Mechanochemical Reactions Force Necessary to Rupture Covalent Bonds. Direct Action of Ultrasonic Waves on Macromolecules Shear Degradation . . . . . . Pulsating Resonant Bubbles . . . Flow Fields Produced by Cavitation Shock Waves Produced by Cavitation

110 110 112 114 120 125 125 127 129 132 134 136 140

7.

Concluding Remarks.

142

8.

References . .

145

. .

A. M. Basedow and K. H. Ebert

118

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60

80

100

Fig. 23. Differential molecularweight distribution curves of ultrasonically degraded dextran (M w = 39700,Mw!Mn = 1.05 in water at 24 watts/cm 2 and 20 kc/sec). Times of irradiation (min): e: 0 •: I 0 o: 20 •: 40 +: 80•: 160 !Ref. (2, J)l dm/dllf =differential molecular weight distribution; dm = mass of polymer having a molecular weight between Mand M + dM

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Fig. 24. First-order degradation rate plot of dectran (molecular-weight distribution curves from Fig. 23). Ordinate: weight fraction (m); abscissa: time of irradiation(!). Parameter of lines is molecular weight x 10-J (Ref. (2, J)]

is represented in Fig. 24. In dilute solutions the reaction order of ultrasonic degradation is found to be pseudo first order in most cases. The true reaction order is certainly more complex, since the rate constants decrease with increasing concentration in a nonlinear fashion (24). This is attributed by some investigators to entanglements and the building up of polymer networks, which slow down the degradation process.


119

From the slopes of the lines in Figure 24 the rate constants of degradation of dextran, having a definite molecular weight, can be calculated. A typical set of degradation constants is shown in Fig. 25, which demonstrates that the rate constants

•

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Fig. 25. Dependence of degradation rate constants (k) on molecular weight (M) of dextran in different

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10

50

30

70

M 70-

90 1

solvents. Ultrasonic intensity 24 watts/cm2 at 20 kc/sec. Solvents are: .A: water e: solution of 10% MgS04 in water+: formamide (Ref. (2)1

increase linearly with increasing molecular weight. An extrapolation of the lines, which represent the dependence of the rate constants on the molecular weight, to the rate constant zero should give in principle the limiting molecular weight, below which degradation ceases. For dextran the values thus obtained for several different solvents are of the order of 20000. Long-time investigations of degradation, however, showed that the limiting molecular weight of dextran in water was only approxi-

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Fig. 26. Plot of slopes of straight lines, which give dependence of degradation rate constants on molecular weights (Fig. 25), versus degradation constant of dextran with molecular weight of 50.000 (k5 0 ). Numbers refer to different solvents: 2 1: water (ultrasonic intensity 24 watts/cm } 2: 2 water (ultrasonic intensity 5 watts/cm ) 3: water (different initial molecular-weight distribution) 4: deuterium oxide 5: dimethylsulfoxide 6: formamide 7: ethylene glycol 8: glycerin 9: ethanolamine JO: 20% methanol in. water 11: water saturated with diethyl ether 12: water with 0.1% potassium palmitate 13: 1% MgS04 in water (20 °C) 14: 1% MgS04 in water (50 °C) 15: 10% MgS04inwater16: 10% glucose in water 17: 10% urea in water 18: 10% urea in dimethylsulfoxide. If not indicated, ultrasonic intensity was 24 watts/cm 2 at 20 kc/sec (Ref. (2, 3)1

I


124

Fig. 29. Calculated molecular-weight distributions of degraded dcxtran in water, for f3 = 1 and assuming variable fragmentation ratios. Initial molecular weight Mw = 39 700, Mw!Mn = 1.05; degradation time 80 min at 24 watts/cm 2 and 20 kc/sec. Numbers indicate different fragmentation ratios: I= 1: 12=2:13=3:14=9:1 .... : experimental curve (Ref. (100)1

dm.7(] dM I.

2

0

20

1.0

60

Fig. 30. Calculated molecular-weight distributions of degraded dextran in water, for fragmentation ratio of 1: 1 and assuming variable values of IJ. Initial molecular weight Mwf = 5 3 200, Mw!M n = 1.08; degradation time 40 min at 24 watts/cm 2 and 20 kc/sec. Numbers indicate different values of {3: I: f3 = 0.5 2: f3 = 1 3: f3 = 1.5 4: f3 = 2.5 .... : experimental curve (Ref. (100)1


145

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31. Harrington, R. E., Zimm, B. H.: Degradation of polymers by controlled hydrodynamic shear. J. Phys. Chem. 69 (1), 161 (1965). 32. Harrington, R. E.: Degradation of polymers in high speed rotary homogenizers. J. Polym. Sci. 4, 489 ( 1966). 33. Henglein, A.: Die Auslosung und der Verlauf der Polymerisation des Acrylamids unter dem Einflull. van Ultraschallwellen. Makromol Chem. 14, 15 (1954). 34. Henglein, A.: Die Reaktion des DPPH mit langkettigen freien Radikalen. Makromol. Chem. 15, 188 (1955). 35. Henglein, A.: Die Kombination van freien makromolekularen Radikalen, die