Salt-dependent surface activity and micellization behaviour of zwitterionic amphiphilic diblock copolymers having carboxybetaine

Zwitterionic amphiphilic diblock copolymers having n-butylacrylate(n-BA) and carboxybetaine as hydrophobic and hydrophilic blocks, respectively, with various chain lengths were synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization method. The effects of salt and chain length on the solution behaviour of zwitterionic amphiphilic diblock copolymers were investigated using surface tension, static light scattering (SLS), dynamic light scattering (DLS) and fluorescence spectroscopic measurements. Surface tension measurements revealed that zwitterionic amphiphilic diblock copolymers undergo transition from surface active to non-surface active after addition of salt. This may be due to the transition from zwitterionic (almost zero charge) to ionic nature of the betaine block by addition of small amount of salt followed by an increase of solvent power of salt solution. Foam formation behaviour decreased with increasing chain length. The critical micelle concentration was decreased drastically by the presence of a small amount of salt, which reflected the non-surface active nature of block copolymer. Increase in values of hydrodynamic radii with increase in salt is attributed to stretching of the hydrophilic chain due to the ionic nature of the block chain and the increase of solvent power. Aggregation number (N (agg) ) and the second virial coefficient (A (2)) decreased with the increase in chain length as well as after addition of salt. Packing parameters of block copolymer micelles were less than 0.3 which implies that the micellar shape is spherical. Some unique aspects of zwitterionic polymer were elucidated, which are largely different from simple ionic (anionic and cationic) polymers.