Monitoring the Effects of Illumination on the Structure of Conjugated Polymer Gels Using Neutron Scattering

The overall goal of this experiment is to quantify the structural evolution of conjugated polymers throughout the gelation process as a function of both temperature change and in situ light exposure. This method can help provide answers to key questions in the behavior of conjugated polymers. It provides insight into the importance of the presence of an exeton on their structure and assembly.

The main advantage of this technique is that it allows unobtrusive and nondestructive analysis of the polymer length scales relevant to the structure’s form during the conjugated polymer gelation process. To begin the procedure, filter five grams of high purity d4-ODCB through a 0.45 micrometer sieve. Then, place 0.34 grams of P3HT and 1.66 grams of filtered d4-ODCB in five gram glass vial containing a stir bar.

Cap the vial with a foil-lined cap and seal the vial with plastic paraffin film. Wrap the vial in aluminum foil to completely exclude light. It is critical to minimize the light exposure that the sample receives from ambient lighting, such as fluorescent room lights.

Samples should not be left on bench tops or in staging areas without sufficient shielding from illumination. Stir the solution at 70 degrees Celsius for a minimum of one to three hours. If the solution is not yet homogeneous, leave the solution stirring at 70 degrees Celsius.

Once the solution is homogeneous, heat a glass pipette to 70 degrees Celsius in an oven. Use the heated pipette to quickly transfer the viscous solution to a clean, dry, one to two millimeter thick quartz banjo cell. Cap the banjo cell and seal the cell with plastic paraffin film.

Wrap the cell in aluminum foil or store the cell in a light excluding container. Prepare a banjo cell containing only filtered d4-ODCB and an empty banjo cell as backgrounds. To begin the experiment, verify that the instrument sample stage is equipped with temperature controls capable of ranging from 70 to 20 degrees Celsius.

Place the sample and background banjo cells in an appropriately sized holding block and label the holders. Wrap the block in 0.1 millimeter thick aluminum foil to exclude light while transferring the samples to the sample stage. Unwrap the samples, load the samples into the stage, and apply aluminum foil to the sample stage.

Align and calibrate the SANS instrument. Set the detector distance close to its maximum setting. Based on count rates for the P3HT sample and the solvent blank, determine the appropriate scattering time for the experiment.

Configure the sample heating scripts to ramp from 70 to 20 degrees Celsius, holding for 15 minutes at each ramp point within the scattering time. Heat the sample stage to 70 degrees Celsius and hold that temperature for 15 minutes to equilibrate the sample. Acquire SANS data, transmission data, and a blocked bean measurement for the sample.

Acquire SANS data at room temperature for both the solvent blank and the empty cell. Upon completing the dark experiment, remove the aluminum foil from the sample stage. Position an optical illuminator with a halogen white light source so that the sample slot at the scattering collection position is fully illuminated.

Measure and record the maximum light intensity at the sample position. Adjust the illuminator, if necessary, to achieve an intensity of at least 5, 000 lux. Equilibrate the sample at 70 degrees Celsius and collect sample and background data with the exact procedure used for the dark experiment.

Process and analyze the data to determine key structural parameters. The reduced SANS data was fit to a linear combination of fitting equations representing nanofibril aggregates through the elliptical cylinder model and the free chains in solution through the polymer excluded volume model. Absolute intensity was observed to increase as temperature decreased for both the dark and light experiments.

The distinct difference between the dark and light experiments for each temperature indicates that light exposure significantly affected the P3HT aggregation. Individual structural parameters were extracted from the combined model fit. The nanofibril surface area was consistently greater for samples studied in the dark.

The elliptical cylinder model scale factor, which describes the amount of P3HT in the aggregate phase, was also greater for dark samples. The free chain radius of gyration was consistently smaller and the Porod exponent was higher for dark samples. Using ultra-small angled neutron scattering, the aggregate radius of gyration was found to be larger for dark samples at lower temperatures.

Overall, the data indicates that exposure to light hindered P3HT aggregation. Once mastered, this technique should take about three to four hours for each sample. While attempting this procedure, remember to choose an appropriate fitting model that spans the entire length scale of interest.

Literature, research, and trial and error customization will help find the most robust fitting model that fits your system with minimal residual discrepancy.