Long-Range Polymer Chain Dynamics of Pyrene-Labeled Poly(N-isopropylacrylamide)s Studied by Fluorescence

Abstract

The long-range polymer chain dynamics (LRPCD) of poly(N-isopropylacrylamide) (PNIPAM) were studied by steady-state and time-resolved fluorescence. Two types of fluorescently labeled PNIPAM constructs were considered. One polymer set, named Py2–PNIPAM–Y, where 5.6 kDa < Y < 44 kDa is the molecular weight of the polymer, consisted of five monodisperse PNIPAMs that carried 1-pyrenyl-butyl groups at each chain end (functionalization level >75%). The second polymer set consisted of five polydisperse PNIPAMs 75 kDa < Mn < 104 kDa randomly labeled with pyrene (0.1 mol % < pyrene content < 6.0 mol %). Pyrene was selected as its ability to form an excimer from the encounter of two pyrenyl pendants covalently attached onto the polymer yields quantitative information about the LRPCD of a given backbone. The fluorescence data were analyzed according to the Birks scheme for the pyrene end-labeled PNIPAMs and the fluorescence blob model (FBM) for the randomly labeled PNIPAMs. The parameters describing the process of pyrene excimer formation were found to yield equivalent trends regardless of the PNIPAM construct as would be expected since both PNIPAM constructs are essentially pyrene-labeled PNIPAM in nature. Comparison of the parameters describing the LRPCD of PNIPAM and polystyrene obtained with either end- or randomly labeled polymers led to the conclusion that PNIPAM and polystyrene have similar LRPCD in solution as expected from the similarity in bulkiness of their side-chain. This study confirms the claim made first with polystyrene that randomly labeled polymers can be used to obtain quantitative information on the LRPCD of a given polymeric backbone and that they constitute an appealing alternative to end-labeled polymers.