Charged Star Diblock Copolymers in Dilute Solutions: Synthesis, Structure, and Chain Conformations

We present a systematic investigation of a novel series of star polymers consisting of arms made up from poly­(<i>N</i>-isopropylacrylamide)-<i>b</i>-poly­(2-acrylamido-2-methylpropanesulfonate) (PNIPAAM-<i>block</i>-PAMPS) block copolymers. The polymers were synthesized as a 3-arm and 2-arm (i.e., a tetrablock copolymer) using a “core-first” method and a sequential atomic transfer radical polymerization (ATRP) protocol. Using asymmetric flow field-flow fractionation (AFFFF), Zetasizer, and small-angle X-ray scattering (SAXS), the phase behavior and nanostructure of the system in dilute solutions are studied in detail. While AFFFF equipped with a light scattering and refractive index detectors provides distribution of molecular weight and overall sizes in solution, we use SAXS combined with theoretical modeling to elucidate the inter- and intramolecular interactions of the star polymers. In particular, by employing a detailed model for a star-diblock copolymer assuming Gaussian chain statistics, we extract the chain conformation for <i>each polymer block separately</i>. We find that the radii of gyration, <i>R</i>_g, for both PNIPAAM and PAMPS are very similar to the expected dimension of free chains in solution. By adding salt, we show that the strong interstar repulsion found in water is dramatically reduced after adding as little as 0.025 M NaCl. Further increase of NaCl up to 0.2 M shows that the system essentially behaves as neutral polymers in a good solvent. Concerning the chain conformations, addition of NaCl seems to have a small effect on the <i>R</i>_g of the different blocks