Effect of Viscosity on Long-Range Polymer Chain Dynamics in Solution Studied with a Fluorescence Blob Model

Abstract

Fluorescence dynamic quenching experiments were conducted on three series of pyrene-labeled polystyrenes in nine organic solvents to evaluate the effect that viscosity has on their long-range polymer chain dynamics (LRPCD). Two series of polystyrenes were randomly labeled with the chromophore pyrene by either a short and rigid amide linker for the CoA-PS series or a long and flexible ether linker for the CoE-PS series. The third series was obtained by end-labeling five monodisperse polystyrenes with pyrene. The monomer and excimer fluorescence decays of all pyrene-labeled polymers were acquired and analyzed with the fluorescence blob model (FBM) for the randomly labeled polymers and the Birks’ scheme for the end-labeled polymers. The FBM analysis yielded the rate of excimer formation inside a blob, kblob, and the size of a blob, Nblob. Birks’ scheme analysis yielded the rate of end-to-end cyclization, kcy, for a polystyrene chain length equal to N. After normalization, the products kblob × Nblob for the randomly labeled polystyrenes and kcy × N for the end-labeled polystyrenes were found to yield identical trends, confirming that any pyrene-labeled polystyrene construct reports the same information on the LRPCD of the polystyrene backbone. The products kblob × Nblob and kcy × N increased linearly with the inverse of viscosity, η−1, for η < 1 mPa·s as expected for a diffusion-controlled process. However, the trends obtained with kblob × Nblob and kcy × N did not pass through the origin when η−1 → 0, suggesting that excimer formation is more efficient than expected in high-viscosity solvents. Nblob was found to decrease with increasing viscosity. kblob did not change much with viscosity in all but the most viscous solvent. The product η × kblob was found to scale as (Nblob)−1.73, where the exponent of −1.73 agrees with that expected from Flory’s theoretical predictions.