Introduction Because of the possible wide range of practical applica- tions, intensive theoretical and experimental studies have focused on the turbulent drag reduction (DR) phenom- enon, which arises when small amounts of polymer ad- ditives are introduced to a turbulent flow [1, 2, 3, 4, 5, 6, 7, 8, 9]. The most recent of these DR studies investigated DR in one- and two-dimensional turbulent flows [10, 11, 12]. In spite of such intense interest, however, our cur- rent understanding of the fundamental DR mechanism remains limited, and we find that various applications of polymer-induced turbulent DR are inevitably hampered by polymer degradation. In a turbulent flow, polymer additives are exposed to high elongational strain as well as high shear, which in some cases leads to scission of polymer chains over time and decreased DR effective- ness [13, 14, 15, 16]. This molecular degradation in tur- bulent flow is affected by a variety of factors, including the polymer molecular-weight distribution (MWD), temperature, polymer–solvent pairs, polymer concen- tration, turbulent intensity, and flow geometry. Although there have been numerous investigations on mechanical molecular degradation in turbulent flow conditions, previous studies on degradation mechanisms were performed under nonuniform and uncontrolled shear flows, such as high-speed mixers and pipe flows that included entrance effects. Brostow [17] related macromolecular conformations in the flow field with DR efficiency and mechanical molecular degradation. Zakin and Hunston [18] monitored DR efficiency in a capillary tube, which is very sensitive to polymer mo- lecular weight. In addition, Culter et al. [19] indicated that much of the mechanical degradation occurs at the entrance of the tube. To reduce entrance effects, Horn and Merrill [20] installed a conical funnel at the entrance of the tube from the feed solution reservoir. Several theories have postulated that degradation is caused by extremely large extensions of polymer chains [21]. Molecular degradation is related to polymer–sol- vent interactions, for example, polymer molecules de- grade more rapidly in a good solvent. In contrast to this intuitive argument, Moussa et al. [22, 23] found that polymer molecules degraded more rapidly in poor SHORT COMMUNICATION Colloid Polym Sci (2002) 280: 779–782 DOI 10.1007/s00396-002-0690-3 Kiho Lee Chul A. Kim Sung T. Lim Dae H. Kwon Hyoung J. Choi Myung S. Jhon Mechanical degradation of polyisobutylene under turbulent flow Received: 23 June 2001 Accepted: 1 March 2002 Published online: 22 May 2002 Ó Springer-Verlag 2002 K. Lee ® S.T. Lim ® D.H. Kwon H.J. Choi (&) Department of Polymer Science and Engineering, Inha University, Incheon, 402-751, Korea E-mail: hjchoi@inha.ac.kr C.A. Kim ® M.S. Jhon Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA Abstract Turbulent drag reduction (DR), induced by oil-soluble poly- isobutylene (PIB) in kerosene, and chain degradation of PIB in a tur- bulent flow field are examined via a rotating disk apparatus. The DR efficiency decreases with time owing to the mechanical degradation of the PIB molecules. The DR efficiency and the amount of degradation are fitted to an exponential function and are compared with molecular weights measured by size-exclusion chromatography. Keywords Drag reduction ® Mechanical degradation ® Polyiso- butylene ® Polymer degradation ® Kerosene