Small-angle neutron scattering study on weakly charged temperature sensitive polymer gels

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Abstract
The static structure factor for poly(N‐isopropylacrylamide‐co‐acrylic acid) (NIPA/AAc) gels was investigated in terms of small‐angle neutron scattering (SANS). The NIPA/AAc gels underwent a discrete volume phase transition at 50.8 °C from swollen to shrunken states as temperature increased. The static structure factors for NIPA/AAc gels were well described by a Lorentz‐type scattered intensity function at temperatures below 34 °C which was near the so‐called Θ temperature of NIPA in D2O. At higher temperatures, the static structure factor had a distinct scattering maximum although the gel was still in the highly swollen state. The scattering maximum was located around q=0.02 Å and was temperature and concentration dependent, where q is the magnitude of the scattering vector. The appearance of the peak indicates the strong concentration fluctuations that created polymer rich and poor regions in the system. The contrast in the concentrations appeared to result from the competing interactions in the network, i.e., the electrostatic repulsive interaction between AAc segments on the network and the hydrophobic attractive interaction among NIPA segments. The static structure factors were analyzed quantitatively with the theory of Borue and Erukhimovich for polyelectrolyte solutions in a poor solvent. It is found that the concentration fluctuations lead to a microphase separation between polymer rich and poor domains before the system undergoes the volume‐phase transition.