Effect of block sequence and block length on the stimuli-responsive behavior of polyampholyte brushes: hydrogen bonding and electrostatic interaction as the driving force for surface rearrangement

Abstract

Diblock polyampholyte brushes with different block sequences (Si/SiO₂/poly(acrylic acid)-b-poly(2-vinylpyridine) (PAA-b-P2VP) brushes and Si/SiO₂/P2VP-b-PAA brushes) and different block lengths were synthesized by sequent surface-initiated atom transfer radical polymerization (ATRP). The PAA block was obtained through hydrolysis from the corresponding poly(tert-butyl acrylate). The polyampholyte brushes demonstrated unique pH-responsive behavior. In the intermediate pH region, the brushes exhibited a less hydrophilic wetting behavior and a rougher surface morphology due to the formation of polyelectrolyte complex through electrostatic interaction between oppositely charged blocks. In the low pH and high pH regions, the rearrangement of polyampholyte brushes showed great dependence on the block sequence and block length. The polyampholyte brushes with P2VP-b-PAA sequence underwent rearrangement during alternative treatment by acidic aqueous solution (low pH value) and basic aqueous solution (high pH value). The brushes with PAA-b-P2VP sequence did not show obvious rearrangement. Nevertheless, with increasing the length of inner PAA block for PAA-b-P2VP brushes, the hydrogen-bonding interaction between blocks became strong enough to implement surface rearrangement and eventually localized the inner PAA block at the polymer–air interface in the low pH region. In the high pH region, polymer brushes had least chance of surface rearrangement. P2VP block was more localized at the interface despite of the length of inner block.