Theoretical modeling of associated structures in aqueous solutions of hydrophobically modified telechelic PNIPAM based on a neutron scattering study

Koga, Tsuyoshi*; Tanaka, Fumihiko*; Motokawa, Ryuhei   ; Koizumi, Satoshi; Winnik, F. M.*

On the basis of results from a small-angle neutron scattering (SANS) study of aqueous solutions of a telechelic PNIPAM with octadecyl end groups, we developed a theoretical model of the self-assembly of this polymer in water as a function of temperature and concentration. This model leads us to the following description. In solutions of concentration 10 gL kept between 10 and 20C, telechelic PNIPAMs () 22 200 gmol) associate in the form of flower micelles, containing about 12 polymer chains, assembled in a three-layered core-shell morphology with an inner core consisting of the octadecyl units, a dense inner shell consisting of partly collapsed PNIPAM chains, and an outer shell of swollen hydrated chains. Drastic changes in the scattering profile of the solution heated above 31C are attributed to the formation of mesoglobules (diameter of 40 nm) consisting of about 1000 polymer chains. On further heating, the aggregation number of the mesoglobules increases. It reaches a value of 9000 at 34C and stays constant upon further heating. In solutions of lower concentration (1 gL), association of flower micelles and mesoglobules does not occur; however, the structure of individual flower micelles and mesoglobules is not affected by the change in concentration. In solutions of 50 gL in which flower micelles are expected to be partially connected by bridge chains, a peak attributed to correlation between flower micelles appears in the scattering profiles recorded at low temperature (10-20C). In spite of the intermicellar bridging connection, the overall temperature dependence of the scattering profile at 50 gL remains similar to that at 10 gL. Distinct features of the self-assembled structures formed in aqueous telechelic PNIPAM solutions are discussed in terms of the interactions between water and the polymer main chains.