Tapping the Potential of Polymer Brushes through Synthesis

Abstract

CONSPECTUS: Polymer brushes are becoming increasing popular in the chemical literature, because scientists can control their chemical configuration, density, architecture, and thickness down to nanoscale precision with even simple laboratory setups. A polymer brush is made up of a layer of polymers attached to a substrate surface at one end with the other end dangling into a solvent. In a suitable solvent, the polymer chains stretch away from the surface due to both steric and osmotic repulsion between the chain segments. In an inadequate solvent, however, the polymer chains collapse due to enough interior free space after desolvation. This unique class of materials exhibit interesting physicochemical properties at interfaces and have numerous applications from sensing to surface/interface property control. Chemists have made recent advances in surface modification and specific application of polymer brushes, due to both profound mechanistic understanding and synthetic strategies. The commonly used synthetic strategies for generating polymer brushes are surface-initiated polymerizations (SIPs), which resemble planting rice. That is, the assembly of initiator on the surface is similar to transplanting rice seedlings, and the subsequent polymerizations are akin to rice growth. Among different SIP methods, researchers mostly use surface-initiated atom transfer radical polymerization (SI-ATRP) because it provides many advantages in the preparation of well-defined polymer brushes, including easy initiator synthesis, fair control over polymer growth, a "living" end for copolymer grafting, and polymerization in aqueous solution. However, chemists gradually realized that there still room for improvement in this method, since the conventional SI-ATRP method suffers several drawbacks. These include having limited availability on various materials surfaces, rigorous synthetic protocols, heavy consumption and waste of unreacted monomers, and limited ability to control a polymerization process. Moreover, applications of polymer brushes as model surfaces must benefit from the synergistic strategies and profound insights into the fundamental understanding of the polymerization. This is not only to optimize the SI-ATRP process but also to expand the range of monomers, simplify reaction setups, reduce the cost, and ultimately gain control of the synthesis of well-defined polymeric surfaces for material science and engineering. In this Account, we provide an overview of our and others' recent advances in the fabrication of polymer brushes by using SI-ATRP, to promote the widespread application of SI-ATRP and practical applications of the polymer brushes. We aim to provide fundamental mechanistic and synthetic features of SI-ATRP, while emphasizing the various externally applied stimuli mediated catalytic and initiation systems, including electrochemistry, chemical reducing agents, and photochemistry. In addition, we discuss how chemists can advantageously exploit these methods to synthesize functional polymeric surfaces in environ...

The Weak Interaction of Surfactants with Polymer Brushes and Its Impact on Lubricating Behavior

“…Polymer brushes were prepared by versatile surface-initiated atom transfer radical polymerization (SI-ATRP). 34 Initiator-modified Si substrates were prepared by vapor deposition of silane initiator under vacuum. The general polymerization procedure was as follows: the monomer was dissolved in a mixture of methanol/water under N 2 flow for 15 min.…”

Challenges and developments of self-assembled monolayers and polymer brushes as a green lubrication solution for tribological applications

“…The lm thicknesses of polymers produced through the 'graing to' method are limited by the molecular weights of the preformed polymer in solution. 210,211 Although this method is relatively easy to carry out as it works much like a SAM, there is steric hindrance that impedes the density of the nal lm that is formed. 71,212,213 In addition to this, the adsorption techniques are reversible so that the layers may be susceptible to high shear forces.…”

“…Free initiators are added for two reasons, i.e. to help control polymerisations 210,211,222,256,257 and allow other characterisation techniques as discussed below.…”

“…In comparison to the limit of the lms using 'graed to' methods (<100 nm thickness), the 'graing from' method can produce much thicker lms. atom transfer radical polymerisation, 3,27,181,[210][211][212]214,[221][222][223][224][225][226][227] and living ring opening polymerisations. 181,210,221 As ATRP is covered in depth its entirety is discussed in 3.2 Atom transfer radical polymerisation.…”

“…ATRP uses simple initiators that usually contain one or more halides. 211 The architecture of the polymers can be varied by the initiator, to produce linear chains of polymers that only one halide is present. Using multiple halide systems can form other shapes, such as stars or combs.…”