Potentiometric titrations of maleic acid copolymers in dilute aqueous solution: Experimental results and theoretical interpretation

The paper reports a study on the flexibility of a family of 1:1 hydrolysed maleic anhydride (maleic acid, MA)- olefin copolymers in dilute aqueous solution. The copolymers were MA-ethene, MA-propene and MA-isobutene. The study was carried out in the absence and in the presence of monomonovalent salts and at different polymer concentrations. Experimental data showing the negative logarithm of the 'apparent' dissociation constant, pK(a) versus the degree of dissociation, alpha, experimental data were obtained from potentiometric titrations using NaOH, KOH or tetramethylammonium hydroxide as the base. The pK(a) data were fully described with calculated curves obtained using an extension of the counterion condensation theory of linear polyelectrolytes, in which a semiflexible model for the polymers was introduced. Under the present experimental condition, no relevant specificity of the monovalent counterions. The calculated pK(a) versus alpha curves allowed the derivation of both the intrinsic dissociation constants of the first and the second dissociation steps for the different copolymers and the corresponding stiffness parameters built into the model. The agreement between the experimental and calculated data shows an appreciable success of the model. The results pointed to an increase of stiffness parallel to the increase of size of the olefin comonomer, in qualitive agreement with already published findings. Furthermore, for all copolymers the chain rigidity was larger in the alpha range of the first dissociation than in that of the second one. The former rigidity was attributed to the formation of intramolecular hydrogen bonds upon the first ionisation of the MA repeating units, followed by an increase of rotational freedom upon breaking of the H-bond in the second dissociation step. Comparison of the rigidity parameters of the MA copolymers with the data obtained for other polyelectrolytes, both natural-poly(L-glutamic acid) and pectic acid-and synthetic-poly(acrylic acid) and poly(methacrylic acid)-was also performed.