Reversibility and Relaxation Behavior of Polyelectrolyte Complex Micelle Formation

In this study, the formation and disintegration of polyelectrolyte complex micelles is studied by dynamic light scattering titrations with the aim to assess the extent to which these complexes equilibrate. Also, the time evolution of samples at fixed (electroneutral) composition was followed to obtain information about the relaxation time of the complex formation. We find that, in 3.5 mM phosphate buffer with pH 7, polyelectrolyte complex micelles consisting of the positively charged homopolymer PDMAEMA(150), the negatively charged diblock copolymer PAA(42)-PAAm(417) (both having a pH-dependent charge), as well as the positively charged protein lysozyme slowly equilibrate with a relaxation time of about 2 days. The same structures were obtained, independent of the way the polymers and proteins had been mixed. In contrast, polyelectrolyte complex micelles (at the same pH) consisting of (pH-dependent) negatively charged homopolymer PAA(139), the pH-independent positively charged diblock copolymer P2MVP(41)-PEO(205), and the negatively charged protein alpha-lactalbumin did not equilibrate. The way in which solutions containing these macromolecules were mixed yielded different results that did not change over the period of at least a week.