Theoretical Modeling of Associated Structures in Aqueous Solutions of Hydrophobically Modified Telechelic PNIPAM Based on a Neutron Scattering Study

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Abstract

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 g L−1 kept between 10 and 20 °C, telechelic PNIPAMs (Mn = 22 200 g mol −1) 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 31 °C 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 34 °C and stays constant upon further heating. In solutions of lower concentration (1 g L−1), 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 g L−1 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 (10−20 °C). In spite of the intermicellar bridging connection, the overall temperature dependence of the scattering profile at 50 g L−1 remains similar to that at 10 g L−1. 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.

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