Characterization of temperature-induced association in aqueous solutions of charged ABCBA-type pentablock tercopolymers

In this work, two novel linear ABCBA pentablock tercopolymers, which are composed of an anionic poly(4-styrenesulfonic acid sodium) (PSSS) block on both ends and two thermosensitive poly (N-isopropylacrylamide) (PNIPAAM) blocks that are separated by a hydrophilic poly(ethylene glycol) (PEG) block have been synthesized via a simple "one-pot'' ATRP procedure. The two studied samples have the following compositions: (P(SSS)(12)-b-P(NIPAAM)(63)-b-P(EG)(34)-b-P(NIPAAM)(63)-b-P(SSS)(12) (C-34) and P(SSS)(14)-b-P(NIPAAM)(65)-b-P(EG)(77)-b-P(NIPAAM)(65)-b-P(SSS)(14) (C-77). The main difference between the copolymers is the length of the hydrophilic block, which plays a crucial role for the physical properties of the copolymers in aqueous solution. A solution of the C-34 sample becomes turbid at elevated temperatures and compact interchain complexes are formed with a behavior that is reminiscent of a suspension of particles. A solution of C-77 becomes turbid at intermediate temperatures but it is transformed to a transparent solution at high temperatures. Depending on the polymer concentration, this pentablock copolymer forms a fragmented or interconnected network at elevated temperatures. At sufficiently high polymer concentration, the viscosity measurements indicate that a reversible hydrogel is formed. The interchain complexes or networks are readily broken up under the influence of shear flow. The results from the small angle neutron scattering experiments reveal significant structural differences between the pentablock copolymers at different temperatures.