Viscoelastic and Oscillatory Flow Birefringence Properties of Dilute Polymer Solutions:Experimentally Determined Polymer and Solvating Environment Contributions, and the Solvent-Controlled Polymer Relaxation Time Spectrum Cutoff

From several theoretical treatments, dilute-solution viscoelastic (VE) and oscillatory flow birefringence (OFB) properties of polymer solutions are predicted to have several different contributions such as those from long-range (global) and local polymer motions, from chain volume (VE only), and the solvating environment. From extensive experimental studies of dilute, semi-dilute, and moderately concentrated polymer solutions it has been possible to identify and separate four different contributions to VE and three to OFB since they exhibit different concentration and temperature dependences. The global motion contribution corresponds quantitatively to bead-spring model predictions provided that a finite and specific number of relaxation times N is employed in the model (N proportional to molecular weight). Thus there is an apparent short time cutoff of the polymer global motion relaxation time spectrum at surprisingly long times, but ONLY in solutions. Melts show a broader relaxation time spectrum breadth. Very recently we have been able to obtain data covering a much broader range of effective frequency, enabling us to observe the effects of the polymer as well as the solvent, and to determine the longest rotational relaxation time of the solvent and explore its role in the apparent short time cutoff of the polymer relaxation time spectrum. These results suggest that this cutoff may reflect the influence of the solvating environment on the development of the "entropic spring" force in the chain at short times.