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

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 (M n = 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.

Swelling and switching kinetics of gold coated end-capped poly(N-isopropylacrylamide) thin films

“…Because of the jump above the LCST deuterated water is repelled from the nbc-PNIPAM films 66 and the SLD of the films decreases. This causes a decrease of the reflected intensity and a shift of the position of the total reflection edge toward a larger λ values.…”

Non-ionic Thermoresponsive Polymers in Water

“…Therefore, the telechelic PiPAAm/water system is an interesting example of the coexistence, without competition, of two phenomena: end-chain association and hydration. These phenomena were monitored experimentally by microcalorimetry, light scattering [579,580] and small angle neutron scattering (SANS) [581]. A theoretical study of systems featuring coexisting LCST behaviour and hydrophobic association via hydrophobic end groups was described by Okada et al for systems with "random" hydration (such as PEO) and "cooperative" hydration (such as PiPAAm) [143].…”

Polyoxazoline Thermoresponsive Micelles as Radionuclide Delivery Systems

“…A detailed review of the properties of temperature-sensitive polymers, including light scattering studies, has been published. [23] Recently, light and neutron scattering methods were exploited extensively for the investigation of these polymers, e.g., of poly(N-isopropylacrylamide) [24,25] (PNIPAM), poly(N-vinyl caprolactam) [24,26] (PVCL), poly(methyl vinyl ether) [24] (PVME), hydrophobically modified poly(N-isopropylacrylamide) (HM-PNI-PAM) [27][28][29][30][31] or of hydrophobically modified polyoxazolines. [32][33][34] Several scenarios for the structural transition in a solution during phase separation could come to life, depending on the chemical composition and concentration of the thermoresponsive polymer.…”