Necklace-like microstructure in shallow-quenched aqueous solutions of poly(n-isopropylacrylamide), detected by advanced small-angle neutron scattering methods.

Abstract

The microstructure of aqueous poly(N-isopropyl acrylamide) (PNIPA) gel and solution was investigated by small-angle neutron scattering (SANS) in the vicinity of the gel volume phase transition at TV (= 34 °C). The SANS technique was reinforced by refractive neutron lenses and perfect single crystals in order to get access to μm length scales. At 31 °C SANS shows Ornstein-Zernike (OZ) type scattering in the swollen gel which at 32 °C starts to deviate from the OZ-formalism, exhibiting excess scattering and at the wave number qc≅ 5 × 10-3Å-1 a crossover to Porod s asymptotic q-4 power law. For shallow quenches of ΔT < 1.0 K above TV the excess scattering intensity is further increasing whereas qc is shifting toward lower values. Based on this observation and analysis of the SANS q-profiles, we propose a necklace-like microstructure consisting of PNIPA-rich globules of R≅ 100 Å size which are connected by swollen PNIPA chains and stabilized for more than a day by pinning of chain connectivity. The formation of PNIPA globules near TV is discussed in terms of partially cooperative dehydration which is crucial to explain the miscibility square phase behavior of aqueous PNIPA solutions. Globule-like structure was also found in aqueous PNIPA solution of size slightly larger than in gels. At deeper quenches of gels above TV (ΔT > 1.0 K) the globules are aggregating to larger objects of R≅ 0.24 μm size as determined from a strong intensity upturn in the small q-region of USANS.