Potentiometric study of the conformational transition in poly(acrylic acid)

Abstract

AbstractDetailed potentiometric titration curves have been obtained for poly(acrylic acid) (Mn = 2.6 × 106) in solution at four different ionic strengths at 25°C. When plotted as pH + log [(1 − α)/α] versus α (where α is the degree of dissociation), they reveal the presence of two different molecular conformations, denoted PAA(a) and PAA(b), with a transition between them. The proportion of the material present in the two conformations has been calculated for different degrees of dissociation and is found to be independent of ionic strength. PAA(a) is intact up to α = 0.22 and is completely converted into PAA(b) by α = 0.75. The free energy of transition from PAA(a) to PAA(b) when both forms are uncharged decreases numerically from −275 to −157 cal./mole monomer residues as the ionic strength increases from 0.02 to 0.20. The increase of the pH of solutions of the PAA(a) from over those of the PAA(b) form at a given value of α, is largely independent of α. This makes it possible to calculate the free energy of transition from PAA(a) to PAA(b) when both forms are charged to the same extent. This free energy is proportional to α and decreases numerically from −944α to −561α (cal.) as ionic strength increases from 0.02 to 0.20. The charge‐independent part of the free energy of transition is a constant fraction of the total, at a given value of α, independent of the ionic strength. The PAA(a) form is the more compact, less contractile, and more ordered structure. The possibility is suggested that this conformation may be partly helical in nature.