The cyclic polymer topology strongly alters the interfacial, physico-chemical properties of polymer brushes, when compared to the linear counterparts. In this study, we especially concentrated on poly-2-ethyl-2-oxazoline (PEOXA) cyclic and linear grafts assembled on titanium oxide surfaces by the "grafting-to" technique. The smaller hydrodynamic radius of ring PEOXAs favors the formation of denser brushes with respect to linear analogs. Denser and more compact cyclic brushes generate a steric barrier that surpasses the typical entropic shield by a linear brush. This phenomenon, translates into an improved resistance towards biological contamination from different protein mixtures. Moreover, the enhancement of steric stabilization coupled to the intrinsic absence of chain ends by cyclic brushes, produce surfaces displaying a super-lubricating character when they are sheared against each other. All these topological effects pave the way for the application of cyclic brushes for surface functionalization, enabling the modulation of physico-chemical properties that could be just marginally tuned by applying linear grafts.

The application of polymer “brushes”, with their unique physicochemical properties, has led to a radical change in the way we functionalize biomaterials or formulate hybrids; however, their attractive traits can be largely surpassed by applying different polymer topologies, beyond the simple linear chain. Cyclic and loop brushes provide enhanced steric stabilization, improved biopassivity, and lubrication compared to their linear analogues. Focusing on poly­(2-ethyl-2-oxazoline) (PEOXA), an emerging polymer in nanobiotechnology, we systematically investigate how topology effects determine the structure of PEOXA brushes and to what extent technologically relevant properties such as protein resistance, nanomechanics, and nanotribology can be tuned by varying brush topology. The highly compact structure of cyclic PEOXA brushes confers an augmented entropic barrier to the surface, efficiently hindering unspecific interactions with biomolecules. Moreover, the intrinsic absence of chain ends at the cyclic-brush interface prevents interdigitation when two identical polymer layers are sheared against each other, dramatically reducing friction. Loop PEOXA brushes present structural and interfacial characteristics that are intermediate between those of linear and cyclic brushes, which can be precisely tuned by varying the relative concentration of loops and tails within the assembly. Such topological control allows biopassivity to be progressively increased and friction to be tuned.

The availability of catalytic/reducing sites at metallic Cu0 sources during supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) is regulated by the components of the polymerization mixture, including ligand (L), alkyl halide initiator (R–X), and CuII-based deactivator. Their contributions were analyzed by quantifying the dissolution of Cu species within a quartz crystal microbalance with dissipation (QCM-D), subjecting a Cu0-coated sensor to different polymerization mixtures. The control of catalyst diffusion from Cu0 was subsequently exploited to fabricate structured polymer brushes with diverse compositions, when ATRP was performed from surface-immobilized initiators in the presence of a Cu0 plate, placed at a determined distance (d) from the substrate. Surface-initiated ATRP in the presence of Cu0 (Cu0-SI-ATRP) is compatible with a broad variety of monomers, including oligo­(ethylene glycol) acrylate (OEGA), methyl acrylate (MA), and acrylamide (AAm). The kinetics of brush growth is finely tuned by the independent variation of d, polymerization time, and concentration of added deactivator. Modulation of these parameters allowed us to generate homopolymer and multiblock copolymer brush gradients featuring a variety of morphologies and controlled interfacial properties, with unprecedented spatial resolution over the brush structure.

While topology effects by cyclic polymers in solution and melts are well-known, their translation into the interfacial properties of polymer “brushes” provides new opportunities to impart enhanced surface lubricity and biopassivity to inorganic surfaces, above and beyond that expected for linear analogues of identical composition. The impact of polymer topology on the nanotribological and protein-resistance properties of polymer brushes is revealed by studying linear and cyclic poly­(2-ethyl-2-oxazoline) (PEOXA) grafts presenting a broad range of surface densities and while shearing them alternatively against an identical brush or a bare inorganic surface. The intramolecular constraints introduced by the cyclization provide a valuable increment in both steric stabilization and load-bearing capacity for cyclic brushes. Moreover, the intrinsic absence of chain ends within cyclic adsorbates hinders interpenetration between opposing brushes, as they are slid over each other, leading to a reduction in the friction coefficient (μ) at higher pressures, a phenomenon not observed for linear grafts. The application of cyclic polymers for the modification of inorganic surfaces generates films that outperform both the nanotribological and biopassive properties of linear brushes, significantly expanding the design possibilities for synthetic biointerfaces.

Bilayer films featuring cyclic, poly(2-alkyl-2-oxazoline) brush interfaces display excellent biopassivity, lubrication and long-term stability in chemically harsh aqueous environments.