A New Approach to Polymer Self-assembly into Stable Nanoparticles: Poly(ethylacrylic acid) Homopolymers

Abstract

A new approach to polymer self-assembly is presented where stable polymeric nanoparticles are formed from homopolymers of one type only and without any assembly triggering additives. The mechanism of the self-assembly is based on the following idea. A thermosensitive polymer solution is heated, the solvent quality gradually worsen upon heating and polymer−polymer contacts are preferred over polymer−solvent contacts, which leads to the formation of polymer assemblies. Upon subsequent cooling to laboratory temperature, the assemblies should eventually dissolve, however, this is not the case due to the fact that polymer chains brought to a close proximity at elevated temperatures become hydrogen-bonded. In addition, hydrogen bonds strengthen upon cooling. As a result, stable nanoparticles are obtained. This mechanism is illustrated on poly(ethylacrylic acid) (PEA) homopolymers, which create a thermoresponsive system at certain degrees of ionization. Nanoparticles are resistant to agglomeration and macroscopic phase separation during heating as well as upon cooling down. The size of nanoparticles can be monitored during the growth and custom-tailored by tuning critical parameters, especially the temperature and time of heating. Nanoparticles are stable over long periods of time. They are stable in a broad range of salt concentrations, including physiological conditions, and possess a mild acceptable degree of polydispersity. Investigation of PEA solution properties, monitoring of the self-assembly process as well as a detailed characterization of nanoparticles were performed by static, dynamic, and electrophoretic light scattering. A comparison with the most related polymers (poly(methacrylic acid) and poly(propylacrylic acid)) was also performed.