Issue 11, 2007

Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques

Abstract

New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol−1 and 450 000 g mol−1, respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio IE/IM decreases with the increase of pH, oppositely to the increase in the IE/IM ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pKa values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small- and long-sized PAA polymers, i.e., the size of the polymer matters.

Graphical abstract: Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques

Supplementary files

Article information

Article type
Paper
Submitted
14 Sep 2006
Accepted
05 Jan 2007
First published
02 Feb 2007

Phys. Chem. Chem. Phys., 2007,9, 1370-1385

Dynamics of short as compared with long poly(acrylic acid) chains hydrophobically modified with pyrene, as followed by fluorescence techniques

J. Seixas de Melo, T. Costa, A. Francisco, A. L. Maçanita, S. Gago and I. S. Gonçalves, Phys. Chem. Chem. Phys., 2007, 9, 1370 DOI: 10.1039/B613382G

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