Effect of pH and Temperature on PMPC–PDPA Copolymer Self-Assembly

Abstract

The effect of temperature on a pH-responsive amphiphilic diblock copolymer, namely poly(2-(methacryloyloxy)ethyl phosphorylcholine)–poly(2-(diisopropylamino)ethyl methacrylate) (PMPC–PDPA), has been studied using dynamic light scattering (DLS), transmission electron microscopy (TEM), and potentiometry. The dissociation constant (pKa) for the conjugate acid form of the PDPA block was determined for four PMPC–PDPA copolymers with varying volume fractions of DPA over a wide range of temperatures. The pH-modulated amphiphilic character of PMPC–PDPA drives its self-assembly in aqueous solution. Both the solution temperature and PDPA degree of polymerization have a dramatic effect on the size and morphology of the various copolymer nanostructures formed between pH 5 and pH 7.5, as judged by DLS and TEM studies. Copolymer morphologies include spherical micelles, vesicles (also known as “polymersomes”), and high genus assemblies. Interestingly, polymersomes were formed by each of the four diblock copolymers. Perhaps more surprisingly, polymersomes were obtained at 5 °C for the shortest DPA block length and at 50 °C for the longest DPA block length. Potentiometric titrations confirmed that the pKa of the PDPA block had a strong temperature dependence, with a maximum value of 7.60 at 5 °C and of a minimum value of 5.75 at 50 °C. However, no difference in pKa was observed across the four copolymers, suggesting no dependence on the mean degree of polymerization.