Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

A novel shortened electrospun nanofiber modified with a 'concentrated' polymer brush

We report the fabrication of shortened electrospun polymer fibers with a well-defined concentrated polymer brush. We first prepared electrospun nanofibers from a random copolymer of styrene and 4-vinylbenzyl 2-bromopropionate, with number-average molecular weight M_n=105 200 and weight-average molecular weight M_w=296 700 (M_w/M_n=2.82). The fibers had a diameter of 593±74 nm and contained initiating sites for surface-initiated atom transfer radical polymerization (SI-ATRP). Then, SI-ATRP of hydrophilic styrene sodium sulfonate (SSNa) was carried out in the presence of a free initiator and the hydrophobic fibers. Gel permeation chromatography confirmed that M_n and M_w/M_n values were almost the same for free polymers and graft polymers. M_n agreed well with the theoretical prediction, and M_w/M_n was relatively low (<1.3) in all the examined cases, indicating that this polymerization proceeded in a living manner. Using the values of the graft amount measured by Fourier transform infrared spectroscopy, the surface area, and M_n, we calculated the graft density σ as 0.22 chains nm^-2. This value was nearly equal to the density obtained on silicon wafers (σ=0.24 chains nm^-2), which is categorized into the concentrated brush regime. Finally, we mechanically cut the fibers with a concentrated poly(SSNa) brush by a homogenizer. With increasing cutting time, the fiber length became shorter and more homogenous (11±17 μm after 3 h). The shortened fibers exhibited excellent water dispersibility owing to the hydrophilic poly(SSNa) brush layer.