Light Scattering Study of the Self-Association Behavior of Long Chain Branched Poly(2-ethyloxazoline) in Solvents

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Abstract

Self-association behavior and conformation variation of long chain branched (LCB) poly(2-ethyloxazoline) (PEOx) with a CH3-(CH2)17 (C18) modified surface are investigated using light scattering techniques in various solvents. It is found that this LCB-PEOx self-associates in less polar solvents to form metastable nanoaggregates instead of undergoing precipitation. The critical aggregation concentration (cac) of this LCB polymer was obtained from the sharp increase of both hydrodynamic radius (Rh) and scattering light intensity measured in dynamic light scattering. The cac strongly depends on solvent polarity, decreasing as the solvent becomes more hydrophobic. As the polarity of the solvent increases, C18 chains move from the outside surface to the interior. Inward movement of C18 chains can cause coupled LCB-polymer chains to move with them, resulting in shrinkage of the particle size. The self-association process in the LCB-PEOx involves intrachain (C18 chains) collapse and interchain association. By a combination of dynamic and static light scattering, the average density of individual LCB-PEOx molecules in ethanol was estimated below the cac. The hydrodynamic radius of the LCB-PEOx in ethanol was found to shrink upon increasing temperature, indicating that the solvent becomes poorer. During the heating process, self-association did not occur.

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