Gareth J. Price

APPLICATIONS OF ULTRASOUND TO MATERIALS CHEMISTRY

The chemical effects of ultrasound derive primarily from acoustic cavitation. Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into heating of the contents of the bubble. The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. A diverse set of applications of ultrasound to enhance chemical reactivity has been explored with important uses in synthetic materials chemistry. For example, the sonochemical decomposition of volatile organometallic precursors in low-volatility solvents produces nanostructured materials in various forms with high catalytic activities. Nanostructured metals, alloys, oxides, carbides and sulfides, nanometer colloids, and nanostructured supported catalysts can all be prepared by this general route. Another important application of sonochemistry in materials chemistry has been the preparation of biomaterials, most notably protein microspheres. Such microspheres have a wide range of biomedical applications, including their use in echo contrast agents for sonography, magnetic resonance imaging, contrast enhancement, and oxygen or drug delivery. Other applications include the modification of polymers and polymer surfaces.

Ultrasonic degradation of polymer solutions: 2. The effect of temperature, ultrasound intensity and dissolved gases on polystyrene in toluene

Abstract Solutions of polystyrene in toluene have been studied as part of a comprehensive study of the parameters affecting the degradation of polymers under irradiation with high-intensity ultrasound. Results are reported which demonstrate the molecular weight dependence of the process and the effect of solution temperature, ultrasound intensity and the nature of dissolved gases on the rate and extent of degradation over a considerably wider range than previously studied. They demonstrate that the limiting molecular weight and polydispersity of the materials can be controlled by suitable manipulation of the experimental conditions. The effects are explained in terms of the influence that each of the parameters has on the shear gradients generated around cavitation bubbles in the solution. The possibility of using the ultrasound process in the control of polymer structure and for the preparation of block copolymers is discussed.

Ultrasonic degradation of polymer solutions—III. The effect of changing solvent and solution concentration

Abstract This paper presents part of a comprehensive study of the ultrasonic degradation of polystyrene solutions. The efficiency of the process with changing concentration was studied in toluene and methyl butyrate and was found to decrease at higher concentration. In addition, degradations were performed in 13 solvents and the variations correlated in terms of the volatility and thermodynamic parameters of the solvent. The results are explained in terms of a mechanism involving shear forces generated around collapsing cavitation bubbles.

Ultrasonically enhanced polymer synthesis

Abstract Some examples of polymerization reactions with a variety of mechanisms which have been carried out under sonication are described. It is shown that they can be classified in general terms using the same reaction types proposed by Luche for low molar mass systems. The use of ultrasound allows a large degree of control over the polymer structure and hence the resulting properties, particularly in controlling the molecular weight distributions.

The use of dosimeters to measure radical production in aqueous sonochemical systems

Abstract The use of two dosimeter systems for quantifying radical production during aqueous sonochemical processes has been investigated. The Fricke (Fe 2+ /Fe 3+ ) system was found to be useful at higher concentrations but care must be taken in interpreting the results since radical production is not the only process taking place. There is some reaction even in the absence of ultrasound and this involves dissolved oxygen gas. The concentration of hydroxyl radicals formed was accurately monitored at low concentrations using the terephthalate dosimeter and the limits of its applicability were found. Both systems were used to investigate the effect of varying the ultrasound intensity.

Control of polymer structure using power ultrasound

Abstract This paper describes the use of power ultrasound to modify and control the molecular weight of polymers. The effect of experimental conditions on the degradation is illustrated using polystyrene as an example and they are interpreted in terms of a shear mechanism involving the movement of solvent molecules around collapsing cavitation bubbles. The application of the process during polymer synthesis is also illustrated.

Recent developments in sonochemical polymerisation.

High intensity ultrasound has been applied to two classes of step-growth polymerisation. The ring-opening polymerisation of cyclic lactones to polyesters was accelerated under 20 kHz ultrasound but, in the case of delta-valerolactone, sonication also promoted a depolymerisation reaction so that the molecular weight fell during later stages of the reaction. Sonication was also applied to the preparation of polyurethanes from a number of diisocyanates and diols. In all cases, the sonochemical reactions proceeded faster in the early stages and led to higher molecular weight polymers. The effect of changing the ultrasound intensity is discussed and some speculation as to the mechanisms of the reaction enhancements is given.

Sonochemical acceleration of persulfate decomposition

The decomposition kinetics of potassium persulfate in aqueous solution have been investigated using a radical trapping method. The use of ultrasound was found to markedly accelerate the decomposition so that the sonochemical process at 25°C occurs at the same rate as the purely thermal reaction at 55°C. The effect of ultrasound intensity has also been studied and can be used to control the rate of decomposition within certain limits.

Synergistic effects of combining ultrasound with the Fenton process in the degradation of Reactive Blue 19.

The decoloration of reactive dye C.I. Reactive Blue 19 (RB 19) using combined ultrasound with the Fenton process has been investigated. The effect of varying the concentrations of hydrogen peroxide and iron sulfate, initial pH, ultrasonic power, initial dye concentration and dissolved gas on the decoloration and degradation efficiencies was measured. Calibration of the ultrasound systems was performed using calorimetric measurements and oxidative species monitoring using the Fricke dosimeter and degradations were carried out with a 20 kHz probe type transducer at 2, 4, 6 and 8 W cm(-2) of acoustic intensity at 15, 25, 50 and 75 mg L(-1) initial dye concentrations. First order rate kinetics was observed. It was found that while the degradation rate due to ultrasound alone was slow, sonication significantly accelerated the Fenton reaction. While the results were similar to those reported for other dyes, the effects occurred at lower concentrations. The rate and extent of decoloration of RB 19 increased with rising hydrogen peroxide concentration, ultrasonic powers and iron sulfate concentration but decreased with increasing dye concentration. An optimum pH value of pH=3.5 was found. The rate of decoloration was higher when dissolved oxygen was present as compared with nitrogen and argon confirming the solution phase mechanism of the degradation.

Preparation and thermal properties of block copolymers of PDMS with styrene or methyl methacrylate using ATRP

Abstract This note describes the synthesis of A–B–A block copolymers with B=PDMS and A=polystyrene or PMMA. The method involves the use of chloromethyl terminated polysiloxanes as initiators for Atom Transfer Radical Polymerisation. Characterisation of the copolymers by NMR is described together with their thermal properties as measured by differential scanning calorimetery.