SIUC REU Projects

Polymers that are tethered to a substrate at one end are referred to as polymer brushes because the chains may stretch out away from the substrate, much like the bristles in a hairbrush. The synthesis and characterization of polymer brushes is an important field in contemporary polymer sciencei and will play an important role in microfluidics, nanofluidics, tissue engineering, and other emerging fields. We are developing photochemical initiators that may form self-assembled monolayers (SAMs) on gold, glass, and silicon.ii For instance, Figure 1 describes a thiolate initiator that may be adsorbed to gold and irradiated to initiate the growth of tethered polymers.

Figure 1. Thiol initiator (1) is based on AIBN and will form SAMs on gold substrates. Free-radical polymerization from the surface is initiated with heat or UV irradiation.

This “grafting from” technique allows us to vary the density and length of the tethered chains.

Recently, we have become intrigued with the possibility of synthesizing mixed polymer brushes, whereby two different polymers are tethered to the same substrate. While mixed brushes have been discussed from a theoretical perspective,iii it was only recently that Minko and Stamm demonstrated the synthesis and response of a mixed brush.iv They showed that a mixed brush of polystyrene (PS) and polyvinylpyridine (PVP) would respond due to changes in solvent. In particular, the PVP brush would stretch out into an aqueous solution, whereas the PS brush would collapse onto the silicon surface. Upon immersion into a nonpolar solution of toluene, the PS brush migrates to the surface and the PVP brush collapses; thus, the brush is responsive.

We propose to synthesize mixed polymer brushes with precise control of the lateral and vertical distribution of polymer chains with nanometer resolution. With photoinitiators it is possible to synthesize a brush, while leaving reactive groups at the surface. Thus, immersion into a second monomer will produce another brush that is intercalated between the first brush. These polymers may be tethered to planar substrates or micron or nanometer sized particles. The advantage of a mixed brush over a single component brush is that the surface will respond to environmental changes when the two polymers are incompatible. For instance, while changing from a polar to nonpolar solvent the hydrophilic polymer may collapse onto the substrate surface, while the hydrophobic polymer expands to the liquid interface (i.e. the polymers will exchange, see Figure 2).

Figure 2. Mixed polymer brushes can respond to their environment. (a) Immersion into a polar solvent will cause the polar polymer chains to diffuse to the liquid interface, while the nonpolar segments collapse to the surface; this process can also be reversible to yield an intermediate surface (b), or a nonpolar substrate (c).

Our preliminary results demonstrate that mixed brushes of polystyrene and poly(methylmethacrylate) may be reversibly switched by applying various organic solvents.v REU students will participate in the synthesis and characterization of the polymer brushes and initiator precursors. They will also monitor the response of the films in the presence of various solvents and solvent vapors. Several undergraduates have successfully participated in similar research projects within our group. Students will learn characterization techniques like gel permeation chromatography (GPC), multi-angle light scattering (MALS), reflection absorption infrared spectroscopy (RAIRS), ellipsometry, contact angles, NMR, and others.