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 T_V (= 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 q_c ≅ 5 × 10⁻³ Å⁻¹ a crossover to Porod's asymptotic q⁻⁴ power law. For shallow quenches of ΔT < 1.0 K above T_V the excess scattering intensity is further increasing whereas q_c 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 T_V 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 T_V (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.