Stimuli responsive polymer brushes
A polymer brush consists of end-tethered (grafted, anchored) polymer chains stretched away from the substrate due to the volume-excluded effect. In mixed brushes, two or more different polymers grafted to the same substrate constitute the brush. Unlike unmixed brush polymer, different polymers in the mixed brush segregate into nanoscopic phases. The phase segregation is a lateral segregation process in a nonselective solvent in which different polymers form spherical or elongated clusters. Both the polymers are exposed on the top of the brush. In selective solvents, the mixed brush structure may be seen as a combination of lateral and layered segregation mechanisms. In the latter case, one polymer preferentially segregates to the top of the brush, while another polymer forms clusters segregated onto the grafting surface. The most important difference of the mixed brush compared to the homopolymer brush is that not only the height and density profile but also the composition profile depends on the solvent quality. In other words, the surface composition of the brush is switched by a change in its environment.
The major objective for the application of responsive polymer brushes is to regulate, adjust, and switch interaction forces between the brush and its environment constituted of liquid, vapor, solid, another brush, particles, and so forth. The simplest formulation of the problem is switching between attraction and repulsion. For example, the polymer brush-like layer stabilizes colloidal dispersion. However, upon a change of its environment (medium), the colloid coagulates because the repulsive forces of the brush have been “switched off.”
This simple effect has numerous important applications in various technologies, and it has not yet been fully explored and engineered. The same simple problem is important if the friction coefficient, adhesion, or wetting could be rapidly changed to switch off and on for capillary flow, cell adhesion, protein adsorption, cell growth, membrane permeability, and drug release.
The complexity of the problem, however, substantially increases when the design of complex responsive systems which mimic the functions of life systems are considered. Such artificially created systems will be capable of reporting on toxins, diagnosing cancer cells, monitoring important parameters of organs, and targeting the release of drugs. Currently, research is focused on how the interactions with polymer brushes may be precisely tuned and monitored in a controlled environment. This has potential benefits for biomaterials, sensors, microfluidic technologies, adhesive materials, micro-actuators, and so forth.
- Synthesis of polymer brushes via “grafting from” and “grafting to” approaches on flat surfaces and on colloidal particles;
- Synthesis of mixed polymer brushes;
- Fabrication of photo-, electro-, and thermo-switchable polymer brushes;
- Polyelectrolyte brushes;
- Synthesis of polymer brushes with a gradient of properties;
- Patterning of the mixed brushes;
- Synthesis and study of hybrid (polymers and nanoparticles, linear polymer-cross-linked polymer) brushes.
Figure 1. Stimuli responsive polymer brushes.