Title

Water Soluble Polymers .74. pH Responsive Microdomains in Labeled n-Octylamide-Substituted Poly(sodium maleate-alt-ethyl vinyl ethers): Synthesis, Steady-State Fluorescence, and Nonradiative Energy Transfer Studies

Document Type

Article

Publication Date

6-16-1997

Department

Polymers and High Performance Materials

Abstract

n-Octoylamide-substituted copoly(sodium maleate-alt-ethyl vinyl ethers) [poly(SiM-EVEs)] have been synthesized and fluorescently labeled with naphthyl and/or dansyl chromophores attached to the polymer backbone with either a long (C8) or short (C2) carbon spacer. The chromophores serve as monitors of the local environmental character of these amphiphilic macromolecules as they undergo the ''polysoap-to-polyelectrolyte'' transition. Intramolecular (closed) associations tend to predominate over a wide range of environmental conditions in aqueous media. The extent and reversibility of these interactions have been investigated by conducting viscometric measurements, Light scattering, nonradiative energy transfer (NRET) quantum efficiency, and fluorescence quenching studies as a function of the solution pH. The results obtained for the 30% n-octylamide-modified poly(SM-EVE) in aqueous media have been contrasted with results obtained for 30% n-octylamide-modified poly(SM-EVE) in 6 M urea solution and unmodified poly(SM-EVE) in aqueous media to provide meaningful information concerning the effect of hydrophobic modification and pH responsiveness. The octylamide-modified polymer in aqueous media at low pH collapses into a compact aggregate due to the loss of polyelectrolyte character and an increase in hydrogen bonding. At moderate pH values, the modified polymer coil becomes slightly more expanded, as indicated by fluorescence and reduced viscosity measurements. At pH values between 7.0 and 8.0, the most expanded conformation is observed as determined by hydrodynamic size and nonradiative energy transfer (NRET) quantum efficiency measurements. NRET studies of both singly- and doubly-labeled, hydrophobically modified polymer solutions and fluorescence quenching in aqueous media are consistent with a globule-to-extended chain transition with increasing pH. At low pH, however, multichain complexes appear to form, even in dilute solution. At higher concentrations, above C*, multichain interactions are observed, consistent with associative thickening observed by viscometry. The onset values of C* in these systems as sensed by NRET are nearly one order of magnitude lower than those observed by viscometry. Also, the labels with long spacers are more sensitive than those with short spacers in detecting organized microdomains.

Publication Title

Macromolecules

Volume

30

Issue

12

First Page

3526

Last Page

3537