Influence of the CO₂Antisolvent Effect on Ultrasound-Induced Polymer Scission Kinetics

Abstract

Ultrasound-induced polymer scission is a nonrandom process which alters the molecular weight distribution of polymers. However, transient cavitation, and consequently polymer scission, is not possible in concentrated polymer solutions due to the high liquid viscosity. The addition of an antisolvent can be used to circumvent this problem because the antisolvent decreases the gyration radius of polymer chains, which induces a reduction in liquid viscosity. To determine the influence of carbon dioxide (CO2) as an antisolvent on the ultrasound-induced scission rate, ultrasonic scission experiments of poly(methyl methacrylate) have been performed in bulk methyl methacrylate (MMA) as well as in CO2-expanded MMA. Modeling the experimental time-dependent molecular weight distributions (MWD) has revealed the scission kinetics at different polymer concentrations and CO2 fractions. At low polymer concentrations, the scission rate is decreased upon an increased CO2 content. This is a result of the higher vapor pressure of CO2, which cushions the cavitation. However, at higher polymer concentrations, this effect is counteracted by the viscosity reduction induced by CO2. As a consequence, the scission rate in CO2-expanded MMA is higher as compared to bulk MMA for solutions with a high polymer concentration. The results show that ultrasound-induced scission in pressurized CO2 can alter and control the MWD of polymers even in concentrated polymer solutions, whereas ultrasound-induced scission in bulk solutions is limited to relatively low polymer concentrations.

A Novel Process for Ultrasound‐Induced Radical Polymerization in CO2‐Expanded Fluids

“…The predicted M lim values have experimentally been confirmed for each system studied. In this study a decrease in polydispersity is observed from 2.4 to 1.5 for all scission experiments, as described previously 29,30…”

“…From the slope of Figure 8, it can be seen that the scission rate is initially higher for the higher CO 2 fractions 0.08 and 0.14 as compared to the 0.02 fraction. This effect is explained in more detail in another paper 30. The negative influence of the CO 2 ‐fraction on the implosion velocity at low polymer concentrations (cushioning effect) is thus counteracted at higher polymer concentrations by the CO 2 anti‐solvent effect.…”

“…Concerning the predicted values of M lim as calculated by our model,29 Table 2 shows a good agreement with the experimentally obtained M lim , indicating preferred scission in the middle of the polymer chain. For more concentrated polymer solutions, however, some deviations occur at lower CO 2 fractions (Table 3), probably caused by overlapping cavitation bubbles 30…”

Comparison of pH-dependent sonodisruption of re-assembled casein micelles by 35 and 130kHz ultrasounds

“…The ability of turbulent flow and shock waves produced by acoustic cavitation to drive metal particles together at very high velocities has been reported (Doktycz and Suslick, 1990). The increased bubble wall velocity at higher frequencies may also lead to a higher strain rate upon implosion of bubbles thus enhancing the shearing effect of cavitation (Kuijpers et al, 2005). In aqueous solutions, water vapor molecules and volatiles inside the cavities are thermolysed to generate highly reactive chemical species, such as hydroxyl, hydrogen and organic radicals (Ince et al, 2001;Han et al, 2002).…”

Effect of Solvent Characteristics on the Ultrasonic Degradation of Poly(vinylpyrrolidone) Studied by On‐line Monitoring

“…Ultrasonic degradation of polymers depends on a variety of parameters, such as temperature, US intensity and frequency, concentration and initial molecular weight of the polymer 20–26. Price, Madras, Malhotra, Kuijpers and Desai have reported that polymer degradation parameters depend on the solvent properties 9, 20, 27–31. Most of the ultrasonic polymer degradation studies in the literature are performed by periodically removing samples from the reaction environment and measuring their molecular weights.…”