Ksenija Kogej,^abc Janez Cerkovnik,^a Hugo Berghmans,^b and Sergio Paoletti^c
aFaculty of Chemistry and Chemical Technology, University of Ljubljana, SI-1000 Ljubljana, Slovenia
^b Department of Chemistry, Catholic University of Leuven, Celestijnenlaan 200F,
B-3001 Heverlee, Belgium
^cbDepartment of Biochemistry, Biophysics and Macromolecular Chemistry, University of Trieste, I-34127 Trieste, Italy

Abstract
Light scattering was used to study the properties of poly (methacrylic acid), PMA, and its sodium salt, NaPMA, in aqueous solutions. The polymer is an atactic one, composed of around 45% heterotactic and 55% syndiotactic triads. The weight-average molar mass of a single PMA chain (Mw,0 = 131 kg/mol) was estimated from experiments with NaPMA in 0.1 M NaCl. From the scattering data at zero-angle it was concluded that PMA is inter-molecularly associated in 0.1 M HCl. The data were treated according to the Open Association Model of Elias, which yielded Mw for the associated PMA form equal to 188 and 170 kg/mol at 25 °C and 55 °C, respectively. Furthermore, the second virial coefficient and a relatively high value for the apparent association constant were obtained: A2 = 1.37 ´ 10-3 and 1.24 ´ 10-3 mol ml/g2 and Kapp = 3.4 ´ 105 and 2.0 ´ 105 at 25 °C and 55 °C, respectively. The radius of gyration of NaPMA in 0.1 M NaCl is Rg = 47 nm, whereas for PMA in 0.1 M HCl it is equal to 20.4 nm at 25 °C. An increase in temperature from 25 °C to 55 °C brings about a decrease in Rg to 16.9 nm for PMA in 0.1 M HCl. The changes in thermodynamic functions point to a hydrophobically driven association process of PMA chains.