I. Synthesis and Swelling Behavior of Pnipam Co Amp Micro- Gel Colloids

Ionic PNIPAm microgels were synthesized by free-radical precipitation polymerization using 2-acrylamido-2-methyl-1-propanesulfonic acid as an ionic co-monomer (references 7 and 9 of the paper), and fluorescently labelled with the covalently linked dye methacryloxyethyl thiocarbamoyl rhodamine B. N, N-methylene-bis-acrylamide was used as a cross-linker (with a 2.74 mol% cross-link density) and potassium per sulfate (KPS) as a free-radical initiator. After the synthesis, the suspensions were purified using multiple centrifugation and redispersion steps, followed by dialysis using deionized (< 1μS/cm) Millipore water for 2 weeks. The ionic comonomer 2-acrylamido-2-methyl-1-propanesulfonic (AMPS) is a strong acid , which accordingly ionizes completely in an aqueous medium. Hence in MilliQ water, it is expected that PNIPAM co AMPS microgels will be maximally charged. Therefore, for the purpose of the current study we made no attempt to characterize and vary the pH. [1] The temperature dependence of the hydrodynamic radius of these particles, as obtained by dynamic light scattering at very low concentrations, is shown in Supplementary Figure 1. With increasing temperature, the radius decreases, reflecting the typical thermoresponsive nature of PNIPAM based microgels. However, in our case, the volume phase transition (VPT) is slightly shifted to higher temperatures (39C) as compared to pure neutral PNIPAM microgels. This is due to the fact that swelling of the microgels is now due to a combined effect of two contributions, a strong Coulomb repulsion between neighbouring ionic groups in the polymer chain and the osmotic pressure of counterions. Moreover, a shift of the VPT to higher temperatures has also been seen in the case of other types of ionic microgels. The estimated swelling ratio (= [Rh(20 C)/Rh(45 oC)]) of our microgel is 17.