We studied polymer self-assembly into stable polymeric nanoparticles by changing the temperature, which is not triggered by any additives. Usually, a thermosensitive polymer solution is heated, the solvent quality gradually worsens upon heating and polymer-polymer contacts become preferred over polymer-solvent contacts, which leads to the formation of polymer assemblies.
Our aim was to study the temperature dependence of self-association of poly(methacrylic acid), PMA, which is a thermo-responsive and pH-responsive polymer. Three tetraalkylammonium chlorides were added to the PMA solution to follow self-association – tetramethylammonium, tetraethylammonium and tetrapropylammonium chloride. The self-assembly of atactic poly(mehacrylic acid) chains and the formation of polymeric nanoparticles with addition of salt were monitored by the UV-Vis technique, where, after mixing, the polymeric acid and salt solution, the solution was heated and cooled, while the absorbance was measured. We have previously determined the wavelenght at which we measured the absorbance so that the measurement would not be influenced by the solvent. The stability of the polymeric nanoparticles was deduced from reversibility or irreversibility of association. The type of association was also predicted from the UV-Vis results. Later, our assumptions were comfirmed by simple visual observations.
We performed all the planned experiments. The obtained results show that self-assembly of PMA takes place at elevated temperatures. So-called critical solution temperature was determined for all three chlorides. We also demonstrated in which cases the association is reversible.