Conducting an Ultrasonic Degradation

For an ultrasonic degradation of a polymer, a resonator is used as shown in Fig. 6.5. The setup in Fig. 6.5 for example uses a resonator with an output of -400 W with a working frequency of -20 kHz and an inert 3/4 titan tip. The solution volume for the setup in Fig. 6.5 should not exceed 200 ml and the concentration of the polymer solution should not exceed the critical concentration c (see The critical concentration from the intrinsic viscosity in Chap. 7, as a rule of thumb the concentration should be below 0.5 wt%). The resonator tip should immerse approx. 10 mm into the solution. The temperature of the solution during the ultrasonification should be kept below 30 °C to avoid a thermal degradation. A schematic view of the setup for an ultrasonic degradation is shown in Fig. 6.5. 

Metal abrasion of the resonator tip has to be removed after the ultrasonification (for example via centrifugation). After the solvent has been removed (freeze-drying), the degraded polymer sample can be used for further investigations. 

The molar mass of the degraded sample depends on the sonification time. This is shown for a hydroxyethylsulfoethyl cellulose (HESEC) in aqueous solution in 

The intrinsic viscosity as a function of the sonification time in Table 6.3 shows a similar progression as the molar mass. The intrinsic viscosity decreases exponentially with time and reaches the minimum value of 140 ml/g for this polymer and the chosen energy input. The polydispersity MJM also decreases sUghtly with the sonification time. 

The ultrasonic degradation of polymers is an established and capable method for the production of homologues series of molar masses. Ultrasonic degradation does not lead to a broadening of the molar mass distribution and a separation of monomer units and is therefore an indispensable operation for the determination of [rj]-M-relationships (see above) from a single polymer sample. 

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