Electrochemical Polymerization of a Monomer

Goals

To electrochemically polymerize poly(3-methylthiophene) from the monomer
3-methylthiophene
To understand the effects of electrolyte concentration, deposition time, and applied voltage on polymer formation

Experimental Apparatus

Gamry Instruments Interface 1000T potentiostat
Gamry Instruments Framework^™ software package installed on a host computer
Standard UV/Vis disposables cuvette
SPE cuvette adaptor (990-00421)
SPE cuvette top (972-00065)
Seventeen platinum working screen-printed electrodes (935-00122)
Ice bath at 0°C

Reagents and Chemicals

0.05 M 3-methylthiophene (monomer) and 0.05 M tetrabutylammonion tetrafluoroborate in acetonitrile solution, at 0^°C
0.05 M 3-methylthiophene (monomer) and 0.075 M tetrabutylammonion tetrafluoroborate in acetonitrile solution, at 0^°C
0.05 M 3-methylthiophene (monomer) and 0.075 M tetrabutylammonion tetrafluoroborate in acetonitrile solution, at 0^°C
0.005 M catechol in 0.01 M aqueous sulfuric acid

Background

Thin-layer films are often used in analytical chemistry as a means to separate analytes of interest from interfering materials. A film can be chosen to either take up the analyte or the interfering materials by careful consideration of several properties:

The ionic charge of the film,
The porosity of the film, and
The hydrophobicity of the film.

A film that is ionically selective is either positively or negatively charged. If it is positively charged, for example, then it repels cations, and attracts anions into the film. If the film’s selectivity is based on porosity, then it might allow smaller molecules and ions into its pores while blocking larger molecules. Films that are based on hydrophobic properties deal with the “wetting” of the film, or its affinity for water. To extract organic compounds from an aqueous solution, use a hydrophobic film because the analytes would be more attracted to the film than the water.

Some common ways to prepare a thin film are dip-coating, spin-coating, and electrochemical polymerization. In dip-coating the item that you want to coat is dipped into a solution of the polymer. As the solution dries the film is formed. The advantage to this technique is that it is very simple to perform and no special equipmentis required, but its disadvantage is that it has low reproducibility because the variables of dip time, dip extraction speed, and drying are difficult to control. In the spin-coating technique a drop of polymer solution is placed onto the surface and the surface is spun. The centripetal force exerted expels excess solution, leaving a very thin film (micrometers in thickness or less). The advantage of this technique is that it is easier to control the variables of drop size, spin duration, and spin speed, thus making the film thickness more reproducible. The disadvantage to this technique is that a special rotator must control the variables, and an enclosure for thesubstrate is required while it is spinning (if the substrate comes loose from the fitting while spinning it is now a flying projectile).

In the electrochemical polymerization technique one starts with the monomer (single unit) and forms the polymer directly onto the surface to cover. This polymerization can be activated with other chemicals, ultraviolet light, or applied potential depending upon the initial monomer. One advantage to this method is that, starting with the monomer, different monomers can be combined in controlled ratios to determine the characteristics of the film (i.e., to obtain a hydrophobic film that is also positively charged). The disadvantage of this technique is that there are more variables to take into account to form reproducible films than with the other techniques.

In this laboratory you are going to be examining just a few of the variables that can be controlled during an electrochemically controlled polymerization. You start with the monomer, 3-methylthiophene, and polymerize it to poly(3-methylthiophene) via the reaction below. To evaluate how well-formed the thin film is you will obtain CVs of catechol with each film.

MONOMER $\rightarrow$ POLYMER