Long-term stability and salt-responsive behavior of polyzwitterionic brushes with cross-linked structure

Polyzwitterionic brushes with strong anti-polyelectrolyte effect have shown great potential as smart surfaces for tunable lubrication, protein absorption/desorption, and bacteria killing and release. However, due to the brush structure in which polymer chain is tethered on the substrate by one end, this surface is easily destroyed from the detachment of the brush, greatly limiting its real-world applications. To solve this problem, herein, cross-linked structure was introduced in poly(3-(dimethyl (4-vinylbenzyl) ammonium) propyl sulfonate) (polyDVBAPS) brushes to offer the long-term stability. The cross-link structure was fabricated by adding a cross-linker during the surface initiated atom transfer radical polymerization, and the effect of such structure on the surface morphology, salt-responsive behavior, and long-term stability were investigated. The results indicated that the crosslink structure with tunable cross-link density could be readily prepared by this method. The responsive behaviors, such as switch between friction and lubrication, bacteria release in response to salt solution, were well-retained by the cross-linked polyDVBAPS brush. More importantly, high long-term stability was endowed by the cross-link structure, from which the cross-linked brush not only showed long-time lubrication and high load resistance during friction measurements, but also exhibited a high retention ratio (more than 90%) of bacteria release capability even after 30 water/salt solution switch cycles, in contrast, the non-cross-linked brush showed a low retention ratio of (similar to)50% after the similar treatment. These results indicated that cross-linked structure could be an efficient method to enhance the stability of responsive polymer brushes and greatly strengthen the practicability of such brushes in real-world applications.