Poly(NIPAM) microgel particle de-swelling: a light scattering and small-angle neutron scattering study

Small-angle neutron scattering (SANS) has been used to investigate structural changes during the de-swelling of poly(N-isopropylacrylamide) [poly(NIPAM)] microgel particles induced by temperature variation and the addition of free polymer [poly(ethylene oxide)]. The extent of particle de-swelling was characterized by photon correlation spectroscopy (PCS). Thermally-induced de-swelling of poly(NIPAM)/deuterated water dispersions occurred on increasing the temperature in the region of the lower critical solution temperature (LCST). The latter was found to be 34 degrees C, which is 2 degrees C higher than the value reported for poly(NIPAM) particles dispersed in water. The SANS data exhibit a Pored form (Q(-4)) of scattering in regard to the size of the colloidal particles. However, in the swollen state, the scattering measured at temperatures less than the LCST, also has a contribution from poly(NIPAM) chains in a solution-like environment (Ornstein-Zernike scattering). The trends observed in the SANS data as a function of temperature were simulated using a model comprised of Pored and Ornstein-Zernike scattering terms. The SANS data confirm earlier PCS measurements showing that addition of free polymer induces particle de-swelling. The SANS data obtained using added free polymer are the first examples of their type to be reported. (C) 1999 Elsevier Science B.V. All rights reserved.