Synthesis and Investigation of Double Stimuli-Responsive Behavior of N-Isopropylacrylamide and Maleic Acid Copolymer in Solutions

Abstract

Poly(N-isopropylacrylamide-co-maleic acid) [poly(NIPAAm-co-MA)] linear copolymer with comonomer molar ratio NIPAAm:MA = 94:6 was synthesized by free-radical copolymerization. The molar mass, M_sD = 28,000 Da, of poly(NIPAAm-co-MA) was determined using hydrodynamic methods. The self-assembly of poly(NIPAAm-co-MA) in aqueous solution at a concentration of 0.015 g cm⁻³ within the pH interval from 1.8 to 10.6 and the temperature interval from 22 to 60°C was investigated by static and dynamic light scattering. The copolymer showed a double temperature and pH responsiveness. Three types of particles, namely, macromolecules (or unimers), micellar-like structures, and loose aggregates existed in the poly(NIPAAm-co-MA) aqueous solutions. The fraction of dissolved entities and the hydrodynamic radii of the micellar-like structures and loose aggregates depended considerably on temperature and pH. The temperature of the phase separation and the width of the phase separation interval increased with pH. An influence of pH on kinetic processes in poly(NIPAAm-co-MA) solutions was observed.

Keywords:

• aggregation
• double thermosensitive and pH-sensitive polymers
• dynamic light scattering
• free-radical copolymerization
• poly(N-isopropylacrylamide)
• self-assembly
• static light scattering

Acknowledgments

Financial support was given by the Russian Science Foundation (project # 14-13-00231) for the Russian coauthors (who performed temperature and pH dependence studies and MM determination), and by a grant of the Romanian Ministry of Education, CNCS-UEFISCDI, project number PN-II-RU-PD-2012-3-0059 for the Romanian coauthors [synthesis and primary characterization of poly(NIPAAm-co-MA)].