Raman spectroscopy has been used to investigate the dynamics of the co
il-stretch transition of dilute polymer solutions. A stagnation-point
extensional-flow field was created by an opposed jets apparatus. High
molecular weight, closely monodispersed cis-polyisoprene was studied i
n 1,1,1-trichloroethane, a goad solvent for this polymer. Raman spectr
a were obtained with a spatial resolution of 10 (mu m in the extension
al flow region. The vibrational frequency of the nu(C = C) stretch at
1664 cm(-1) was shown to be sensitive to the strain rate of the extens
ional flow field No changes were observed in the Raman spectra until t
he strain rate was larger than the critical value for the coil-stretch
transition. The Raman band was then observed to shift linearly to low
er frequency with increasing strain rate before saturating. The observ
ed changes were much smaller, however, than those predicted using a mo
del of a fully extended chain in the flow field. (C) 1998 Elsevier Sci
ence Ltd. All rights reserved.