Multidimensional Spectroscopy of Polymers Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/12/18. For personal use only.
Chapter 21
Fluorescence Studies of the Behavior of Poly(dimethylacrylamide) in Dilute Aqueous Solution and at the Solid—Liquid Interface 1
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Ian Soutar , Linda Swanson , S. J . L. Wallace , K. P. Ghiggino , D. J. Haines , and T. A. Smith 2
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School of Physics and Chemistry, Lancaster University, LancasterLA14YA,United Kingdom Department of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia
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Fluorescence techniques, including quenching and anisotropy measurements, have been used to study the dynamic behavior of poly(dimethylacrylamide), PDMAC, in dilute aqueous solutions and the interactions between PDMAC and poly(methacrylic acid), PMAA, and at a silica/water interface. These studies, involving PDMAC species bearing naphthyl and pyrenyl moieties asfluorescentlabels, indicate that the polymer exists as aflexibleand relatively open coil in aqueous media whose behavior is largely unaffected by changes in pH. The polymer is adsorbed on the surface of colloidal silica and appears to "lie flat" on the particles under high surface:polymer conditions. Complexation with PMAA produces relativelyrigidspecies in which the segmental mobility of the PDMAC is dramatically reduced. Fluorescence techniques have been shown to be very powerful, versatile and sensitive tools for the investigation of polymer behavior (see for example references 1-5 and references therein) and have proved popular, in recent years, in the study of watersoluble macromolecules. In this context, fluorescence anisotropy measurements have been used to gain information at the molecular level regarding the dynamic behavior of polymers in aqueous solution (6-13). In this paper we report upon preliminary investigations using fluorescence techniques, particularly time-resolved anisotropy measurements (TRAMS), to study the dynamic behavior of poly(dimethylacrylamide) in dilute solution and its interactions at a water/silica interface and with poly(methacrylic acid). Experimental Materials. 1-Vinylnaphthalene, V N , was synthesized from 1-acetonaphthone (Aldrich) by reduction (NaBHj) to the carbinol, followed by dehydration ( K H S 0 ) (75). The monomer was purified by fractional distillation under reduced pressure immediately prior to use. 4
0097-6156/95/0598-0363$12.00/0 © 1995 American Chemical Society
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MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS
Acenaphthylene, A C E , (Aldrich) was triply recrystallized from ethanol and triply sublimed. 1-Vinylpyrene, VPy, was prepared using a modification of the procedure described by Webber (14). Methacrylic acid, M A A , (Aldrich) and dimethylacrylamide, D M A C , (Aldrich) were prepolymerized (UV radiation) and fractionally distilled immediately prior to use. Benzene (BDH) and diethylether (May and Baker) were purified by fractional distillation. Methanol (Aldrich, spectroscopic grade) was used as supplied. Ludox A M (Du Pont) was used as supplied. Polymers of M A A or D M A C were prepared by free radical polymerization in benzene solution at 60°C under high vacuum using A I B N as initator. Fluorescently labeled samples were prepared by copolymerization of D M A C with 0.5 mole% of A C E , V N , or VPy, respectively. Polymers were purified by multiple reprecitations from methanol into ether. Instrumentation. Fluorescence spectra and lifetimes were obtained using PerkinElmer LS50 and Edinburgh Instruments 199 spectrometers, respectively. Time-resolved anisotropy measurements were made using radiation either from the SRS, Daresbury, or a picosecond synchronously-pumped dye laser as excitation. Details of the experimental configurations of the synchrotron and laser sources have been previously published (references 13 and 9, respectively). Results and Discussion Poly(dimethylacrylamide), PDMAC, in Dilute Solution. Fluorescence Intensity and Lifetime Measurements. The intensity of radiation emitted by a fluorescently labeled water-soluble polymer and its mean lifetime are often relatively simple and convenient means of detecting the occurrence of pH and/or temperature induced conformational changes in polyelectrolytes, for example. Figure 1 shows the pH dependence of the fluorescence intensity, If, and average lifetime, x , of the fluorescence from a V N labeled sample of P D M A C at 298K. At pH values of greater than ca. 2, both I and i f are, within experimental error, independent of pH. At high degrees of acidity (represented by nominal values of pH
