Small angle neutron scattering study on poly(N-isopropyl acrylamide) gels near their volume-phase transition temperature

Abstract

The small angle neutron scattering experiments were conducted on N-isopropyl acrylamide (NIPA) gels in D2O and on the corresponding NIPA solutions. The NIPA gels underwent a sharp, but a continuous volume phase transition at 34.6 °C from a swollen state to a shrunken state with increasing temperature. In the case of the gels, an excess scattering due to the presence of crosslinks was observed at low q region (q≤0.02 Å−1), where q is the magnitude of the scattering vector. The scattered intensity function for the gel was well described with a combination of Gauss and Lorentz-type functions, i.e., I(q)=IG(0)exp[−Ξ2q2]+[IL(0)/(1+ξ2q2)] as proposed by Geissler et al. IG(0) and IL(0) are the intensities at q=0 for the contributions of Gaussian and Lorentzian functions, respectively. The Gaussian part results from solidlike inhomogeneity, having a characteristic size of Ξ, which is due to the introduction of crosslinks into the system. The Lorentzian part is originated from the liquid nature of the local concentration fluctuations of the gel characterized with a thermal blob of dimension ξ. Ξ decreases systematically with polymer volume fraction, φ, indicating the nature of Ξ being the solidlike inhomogeneity. On the other hand, the intensity function for solutions was well fitted with the so-called Ornstein–Zernike (OZ) equation (a Lorentzian function), i.e., I(q)=[IL(0)/(1+ξ2q2)]. It was found that both ξ and IL(0) diverged at the spinodal temperature, Ts. The critical exponents, ν and γ, for the temperature dependence of ξ and IL(0), were estimated to be ∼0.60 and 1.2 for the gel, respectively, which were larger than the values for the solution of the same polymers (ν=0.45 and γ=0.8). These critical exponents for the gels support that the volume-phase transition of gels is classified to the three dimensional Ising model reported by Li and Tanaka. The concentration dependence of ξ and IL(0) was also well described with a power law relationship, i.e., ξ∼φνφ and IL(0)∼φγ;φ. The values of νφ and γφ at 23 °C are −3/4 and ∼−1/4, respectively, for the NIPA solutions, which are in good agreement with the theoretical prediction for polymer solutions in a good solvent. In the case of the NIPA gels, however, both νφ and γφ are ∼−1. These exponents were interpreted by taking account of the effects of crosslinking on the Flory’s interaction parameter.