Magnetic field control of structure and function of poly(N-isopropylacrylamide) gels

Abstract

The swelling and elastic behavior, hydration energy, and molecular diffusion of poly(N-isopropylacrylamide) gels (NIPA gels) prepared under magnetic fields (mag-gels) were investigated. The diameter of gels in swollen state at 298 K increased steeply with increasing magnetic field to become 40% larger at 28 T than at zero-field. The magnetic field effect on volume change depended strongly on the intensity and direction of the field. The change in length was largerin the direction perpendicular than parallel to the applied magnetic field, suggesting that polymer chains in the network should be aligned, despite very low chain density, to show structural and magnetic anisotropy. This structural anisotropy caused a difference in diffusivity of iodine in mag-gels. Mag-gels showed a more significant discontinuity in the volume phase transition than zero-field gels. Magnetic fields induced the volume phase transition over a critical magnetic field Hc even in high cross-link conditions for the preparation of continuous volume change gels. Mag-gels had a small density of cross-link, e.g. the effective degree of cross-link of less than 1/2 at 10 T, and increase inthe heat of volume phase transition or hydration energy. The magnetic-field induced volume phase transition relates to significant decrease in cross-link.