Fluorescence studies of a series of monodisperse telechelic α,ω-dipyrenyl poly(N-isopropylacrylamide)s in ethanol and in water

摘要

Steady-state and time-resolved fluorescence measurements were performed on solutions in ethanol and in water (23 °C) of a series of poly(N-isopropylacrylamide)s labeled with a pyrenyl group at each chain end (Py2-PNIPAM) and ranging in molecular weight (M_n) from 5900 to 44500 g·mol~(-1). Water and ethanol are of similar solvent quality towards the PNIPAM chain. The pyrene labels, in contrast, are soluble in ethanol but not in water. The efficiency of excimer formation for Py2-PNIPAM samples in ethanol decreased with increasing chain length, a trend typical of pyrene end-labeled polymers in good solvents. The ratio I_E/I_M, where I_E is the Py excimer emission intensity and I_M is the Py monomer emission intensity, scaled as M_n~(-1.4), where M_n is the number-average molecular weight of the sample. The kinetics of excimer formation were more complex for aqueous Py2-PNIPAM solutions, as a consequence of pyrene-pyrene association prior to excitation. The excimer time-dependent profiles exhibited significantly faster rise times, compared with the situation in ethanol, and they could not be fitted with the traditional Birks scheme. The results are discussed in the context of the solution properties of telechelic amphiphilic PNIPAMs and are compared with data gathered previously in studies of dipyrenyl end-labeled poly(ethylene oxides), the only other polymers soluble in water and organic solvents subjected to similar studies in the past.