The effect of extensional flow on the Raman spectra of dilute polymer solutions

Raman spectroscopy has been used to investigate the dynamics of the coil-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 in 1,1,1-trichloroethane, a goad solvent for this polymer. Raman spectra were obtained with a spatial resolution of 10 (mu m in the extensional 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 extensional flow field No changes were observed in the Raman spectra until the strain rate was larger than the critical value for the coil-stretch transition. The Raman band was then observed to shift linearly to lower frequency with increasing strain rate before saturating. The observed changes were much smaller, however, than those predicted using a model of a fully extended chain in the flow field. (C) 1998 Elsevier Science Ltd. All rights reserved.