Study on Self-Assembly of Telechelic Hydrophobically Modified Poly(N-isopropylacrylamide) in Water

The microscopic structures of aqueous dispersions of hydrophobically modified poly(N-isopropylacrylamides) (HM-PNIPAM) and hydrophobically modified poly(ethylene oxide) (HM-PEO), which carry an octadecyl group at each chain end and have an average molecular weight of 20,000 g/mol, were investigated by small-angle neutron scattering (SANS). When the polymer concentration exceeds the overlap concentration, connectivity through bridging chains takes place, resulting in the formation of a network in which flower micelles act as junctions. The temperature dependence of the micelle structure is significant in the case of the HM-PNIPAM system but not significant in the case of the HM-PEO system, indicating the importance of hydrophobic interactions in the structural stability of flower micelles. Two different parameters relating to the micelle size are defined depending on the polymer concentration. One is the total micelle radius in the dilute regime and the other is the "non-overlapping radius" of the micelles in the semi-dilute regime. These two parameters exhibit qualitatively different trends with respect to temperature.