Ultra-small- and small-angle neutron scattering studies of self-assembly in poly(N-isopropylacrylamide)-block-poly (ethylene glycol) aqueous solution

Poly(N-isopropylacrylamide)-block-poly(ethylene glycol) copolymers (NE) having nearly symmetric (s-NE) and highly asymmetric (a-NE) compositions in terms of constituent block chains were synthesized and purified: the s-NE and a-NE had a volume fraction f_PNIPA of poly(A-isopropylacrylamide) (PNIPA) block chains equal to 0.46 and 0.83, respectively. Their phase behaviors in water and self-assembled structures have been studied in the parameter space of temperature (T) and weight percent of block copolymers (w_P, weight/volume), by means of macroscopic observations on turbidity, fluidity and volume change, and of microscopic observations with ultra-small and small-angle neutron scattering. The results revealed that various states of the solutions exist as a consequence of interplay of short-range interactions among PNIPA, poly(ethylene glycol) (PEG) block chains, and solvent (especially temperature-dependent solvent selectivity) and with long-range interactions arising from elastic energy of PNIPA and PEG in the domain structures. At low polymer concentrations (w_P < w_P,C, w _P,C≅3.5%, slightly depending on f_PNIPA), with increasing T and therefore selectivity of solvent, states of both a-NE and s-NE systems changed from (I) homogeneous solution of NE, (II) transparent solution, which contains disordered micelles, composed of PNIPA cores and PEG brush emanating from the core and dispersed in water with only short-range liquid-like order) and to (III) opaque sol comprised of macrophase-separated domains rich in NE in the water rich matrix. At higher polymer concentrations (w_P > w_P,C), and with increasing T, the a-NE system changed from state (I), state (II), as described above, (IV) an opaque gel comprised of fractal network-like domains rich in NE in the water rich matrix, and to state (V) where syneresis of the opaque gel occurs, giving rise to coexistence of macroscopic opaque gel phase and squeezed-water phase. On the other hand, in between state (II) and state (IV), the s-NE system exhibits an extra state (VI) of transparent gel, as a consequence of ordered micelles due to microphase separation of NE in water. The ordered micelle are stabilized by long coronal PEG chains in s-NE, reflecting elastic energy contribution of coronal chains to the system state. The observed syneresis is unique in that it is involved in the system comprised of the fractal network-like domain rich in PNIPA and the medium rich in water.