Growth of Polymers Tethered to a Surface: In-situ Fabrication of Polymer Brush.

Principal Investigator

Edith Sevick

Research School of Chemistry

Chemical tethering of the end of a polymer onto a surface is a common practise amongst scientists
and industrialists. Biochemists tether biopolymers to magnetic beads for various assay or separation procedures; colloidal chemists prevent aggregation of particles by surface tethering polymer chains which act as a "bumper", and surface chemists impart properties such as wettability and biocompatibility by densely end-tethering chains onto a surface. or forming a "polymer brush". In this project, two aspects of tethered chains are being investigated: (1) the compression of a single end-tethered chain by a finite-sized obstacle, and (2) the growth of a "brush" from in-situ polymerisation.
In the first, we are interested in the force required to compress a tethered chain as a function of the size of the chain and the compressing obstacle. Equilibrium scaling theories suggest a number of surprises (Guffond, Williams, and Sevick, 1997), one of which is that there is a first-order escape transitions which occurs when the chain "spurts-out" from underneath the compressing obstacle, but is constrained by the fixed tether. These predictions have significant implications for the forces between particles which are weakly grafted with polymers. However, the compressive force will depend sensitively upon how "fast" we compress the chain. This information cannot be gleaned from equilibrium theories and dynamic simulation on the VPP is required

In the second, we are investigating a new method for fabricating a polymer brush. The current method of making surfaces with densely end-tethered polymer is to adsorb or react the ends of pre-existing chains onto the surface surface - this can be a slow process. A new method is to polymerise the chain from the surface, i.e. to grow the polymer from seeded sites on the substrate. However, little is known about polymer brushes generated in this fashion: what is the resulting chain length distribution and brush height? How can these be changed with the blocking of initiation sites?

Projects

w55 - VPP

Appendix A -

What are the results to date and the future of this work?

For the compression problem, we have simulated a tethered chain in a theta (or marginal) solvent which is compressed between infinite plates. The plates are compressed over distances between 2 and 0.1 times the size of the chain over a period of time ranging from 0.1 to 10 Rouse times. We are currently investigating hysteresis in the force profiles when compressed quickly and when the obstacle diameter is only slightly larger than the size of the tethered chain. In the future, the chain mdoel will be appended to include the hydrodynamic and excluded volume (good solvent) interactions. These profiles should be comparable with experimenetal force measurements performed using Atomic Force Microscopy (AFM).

For the growth problem, we have simulated growth of a single-seeded site on a chain and measured averaged kinetic properties. In the future, we will simulate multiple chain growth from a surface multiply seeded.

What computational techniques are used?

The coding is very similar to that of Molecular Dynamics and consequently, is highly vectorisable. The equation of motions include conservative and non-conservative forces, as well as random forces which represent solvent bombardment. Constraints are minimised by modelling the chain as an extensible series of springs connected by statistical "monomers". Polymerisation is modelled as the incorporation of new "monomers" and springs onto the free end of a chain, or onto the seeded site of the grafting plane.

- Appendix A