Contraction of a Polyelectrolyte upon Dilution. Light Scattering Studies on a Polycation in Basic and Acidic Water−Acetone Mixtures

Abstract

A polycation poly(methacryloylethyl trimethylammonium methylsulfate), PMETMMS, with molar mass M w ) 26 × 10 6 has been studied by viscosity and light scattering in a mixture of water and acetone. Various amounts of a base (sodium hydroxide) or acids (hydrochloric, sulfuric, or phosphoric acid) have been added to the samples. PMETMMS shows typical polyelectrolyte behavior of the reduced viscosity in water-acetone mixtures with acetone mass fraction, γ, below 0.80. When γ g 0.80, PMETMMS undergoes a reversible coil-to-globule transition. The collapse can also be induced by dilution of the polymer solution, keeping the solvent composition constant, γ ) 0.70. Correlation functions measured using dynamic light scattering show a decrease of the relaxation time as well as the narrowing of the size distribution of the polymer with dilution. It has been shown that a compact structure instead of an extended coil is formed in a very dilute solution. Base or acid added into the solvent increases the value of the critical polymer concentration at which the collapse occurs. In a medium with low dielectric constant, base or acid molecules strongly interact with the charged groups of the polycation. Addition of NaOH into the solvent (less than 6 × 10 -4 mol/L) decreases the degree of ionization of the polycation. However, the addition of acid (0.3 × 10 -4 mol/L) introduces counterions that not only screen the positive charges of the polymer chain but also form inter-and intramolecular links. The latter effect becomes more profound with increasing the counterion valency. Upon dilution of the polymer solution, the number of the contacts between the polymer and added base/acid increases, but the number of inter-and intramolecular ion pairs of the polycation decreases. The balance of inter-and intramolecular interactions changes, and at high dilution, the intramolecular interactions dominate, owing to the poor quality of the solvent.

Neutron Scattering Studies of the Structure of a Polyelectrolyte Globule in a Water−Acetone Mixture

“…These values are consistent with the extrapolated estimate M w ) 26 × 10 6 g/mol and the negative value of the second virial coefficient A 2 reported previously for this system. 11 Figure 4 shows the SANS data plotted in a Porod representation, I(q)q 4 vs q. Both samples exhibit a distinct peak at q max ≈ 0.0097 Å -1 , followed by a plateau at higher values of the wave vector q.…”

“…Samples of PMETMMS were examined by light scattering both in 1 M aqueous sodium nitrate solutions and in the collapsed state in water−acetone mixtures. − The molar masses were found to be identical in both solvents. Sedimentation−diffusion analysis gives similar results, indicating a narrow molar mass distribution of the sample .…”

“…Dynamic and static light scattering measurements have shown that poly(methacryloylethyltrimethylammonium methyl sulfate) (PMETMMS) with molecular weight in the range M w = (10−28) × 10 6 g/mol displays a coil-to-globule transition in water−acetone mixtures when the acetone mass fraction in the solvent composition (γ) increases beyond 0.80. − The conductivity of the solutions decreases drastically in the transition region. However, the remaining charges of the polycation, as well as the hydrophobic parts of the polymer soluble in acetone, prevent aggregation of PMETMMS globules in a dilute solution.…”

Conductometric properties of linear polyelectrolytes in poor-solvent condition: The necklace model

“…7,8 The most convincing of them are reported by Aseyev et al 9 who interpreted dynamic light scattering 10 and small-angle neutron scattering ͑SANS͒ data 9 of poly͑methacryloylethyltrimethylammoium methyl sulfate͒ ͑PMETMMS͒ in watermethanol mixtures as indicating the presence of compact globules connected by flexible chain segments. 7,8 The most convincing of them are reported by Aseyev et al 9 who interpreted dynamic light scattering 10 and small-angle neutron scattering ͑SANS͒ data 9 of poly͑methacryloylethyltrimethylammoium methyl sulfate͒ ͑PMETMMS͒ in watermethanol mixtures as indicating the presence of compact globules connected by flexible chain segments.…”

Phase transitions of a single polyelectrolyte in a poor solvent with explicit counterions

“…A polyelectrolyte chain in a poor solvent undergoes a series of phase transitions as the linear charge density of the chain is varied keeping temperature fixed. The rich phase diagram resulting from a competition between the long range Coulomb interaction and the short range excluded volume interaction has been studied theoretically [4][5][6][7][8][9][10][11], numerically [6][7][8][12][13][14][15] and experimentally [16][17][18][19][20][21][22]. At very low charge densities, the polyelectrolyte chain is in a collapsed phase, with the counterions uniformly distributed throughout the solution [15].…”