Control of polymer structure using power ultrasound G.J. Price, P.J. West and P.F. Smith School of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK 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. Keywords: polymer degradation; polymer synthesis; molecular weight Most workers regard sonochemistry as a relatively new branch of chemistry. However, applications of power ultrasound in polymer science date back to the 1930s when it was discovered that sonication of solutions of natural polymers such as agar and starch led to a reduction in viscosity 1'2. Sporadic interest has been shown since then but, with the recent development of readily available sonochemical equipment, particularly for operating on a large scale, there is currently considerable research in the area 3, alongside that in other branches of chemistry 3- 6. Polymers are among the most important materials produced commercially and are used in the huge range of applications. However, while their synthetic methods are generally straightforward, there are often several processing steps involved and these depend critically on the bulk properties such as melting temperature and viscosity. At this point, there is an extra consideration over those involved in the synthesis of small compounds, that of the molecular weight. In addition, any polymer will contain a distribution of chain lengths and this is important in determining the material properties. The purpose of this paper is to demonstrate how power ultrasound can be used to control these parameters during polymer synthesis or processing. Most effects in sonochemistry arise from cavitation. While some consequences of this, such as radical production, are used in making polymers, the exact origin of the effects, whether from thermal 'hot spots' or electrical or corona discharges is relatively unimportant to the polymer chemist. Most of the effects involved in controlling molecular weight can be attributed to the large shear gradients and shock waves generated around collapsing cavitation bubbles 7'8. This paper will begin by describing the basic effects of molecular weight reduction and how they can be controlled by varying the available experimental parameters. A specific application of its use in controlling the molecular weight distribution of polymers arising from polymerization initiated by free radicals will then be given. Experimental Irradiation of the polymer systems was performed in an apparatus shown schematically in Figure 1. The glass cell, designed and constructed in our laboratory, consisted of a pear-shaped flask, surrounded by a jacket through which thermostated water was circulated and modified by an indentation at the apex to assist the mixing caused by streaming. This obviated the need for additional stirring and allowed temperature control to _ 0.5 °C. Two versions were used; a 50 cm 3 capacity cell with a Sonics and Materials VC50 sonicator, used for the 0 Transdu Ultrasound generator Sampling N 2 inlet syringe Thermometer 1 \ l / -- /~ latero.~ jaWc~te[-~ sound horn Itr Monomer T Water in Figure 1 Schematic diagram of sonication apparatus 1350-4177/94/010051 ~)7 © 1994 Butterworth-Heinemann Ltd Ultrasonics Sonochemistry 1994 Vol 1 No 1 S51