  Mechanical degradation of dilute solutions of polyethylene oxide (PEO) via
chain scission was investigated within a turbulent pipe flow. Comparisons of
the drag reduction performance with and without degradation were made by
matching the onset of drag reduction conditions, which has been shown for PEO
to be related to the mean molecular weight. The bulk flow behavior of both the
degraded and non-degraded samples were generally consistent with trends
observed in the literature, but a subset of conditions showed significant
deviation in the slope increment (drag reduction performance) between the
degraded and non-degraded samples. When they deviated, the degraded samples
were consistently more efficient than the non-degraded samples even though they
had the same mean molecular weight. The deviations were shown to scale with the
normalized difference between the initial and final molecular weights. The
current data and analysis as well as the literature suggests that the
deviations in the polymer performance (slope increment) are related to changes
in the molecular weight distribution. More specifically, the improved
performance of the degraded samples relative to the non-degraded ones at the
mean molecular weight of the degraded sample indicates an excess of longer
polymer chains since the higher chain fractions in a degraded solution more
effectively control the flow properties when within a certain degree of
degradation and Reynolds number.
