Abstract
Molecular properties and self-assembling behavior of thermo-responsive ionic diblock copolymers, poly(N-isopropylacrylamide)-b-poly(styrenesulfonate sodium salt) (PNIPAm-b-PSSNa), which were precisely synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, were systematically studied as a function of temperature and added salt concentrations. Cloud point with elevation aqueous solution temperature was found at 35~40 °C for all the block copolymers studied which have a different block length and block ratio. Cloud point decreased with increasing added salt concentration, which was identified to be the critical micelle temperature (CMT) since micelle formation was confirmed at a higher temperature. The critical micelle concentration (cmc) at 50 °C could be determined by the static light scattering technique and cmc was found to increase with increasing added salt concentration. This is behavior typical of “non-surface active” polymers although it does not obey the famous Corin-Harkins law for ionic surfactants. However, the surface tension of the solution decreased with increasing polymer concentration and the adsorption of block copolymer molecules at the water surface was directly confirmed by X-ray reflectivity and foam formation observation. Micelle size was found to increase with increasing polymer concentration by dynamic light scattering. Vesicle-like particles were found by transmission electron microscopy (TEM) observation for some block copolymers. PNIPAm-b-PSSNa behaves as an amphiphilic block copolymer at 50 °C and has a non-surface active nature since it has an ionic block. However, the discrepancy between the non-surface active nature of cmc behavior and surface active nature of adsorption is quite mysterious, and this might be peculiar to PNIPAm-containing block copolymers since similar behavior was not observed in other ionic amphiphilic block copolymers having a conventional hydrophobic block.
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