Application of Voltage in Dynamic Light Scattering Particle Size Analysis

Summary

Here, a protocol to apply voltage to solution during dynamic light scattering particle size measurements with the intent to explore the effect of voltage and temperature changes on polymer aggregation is presented.

Abstract

Dynamic light scattering (DLS) is a common method for characterizing the size distribution of polymers, proteins, and other nano- and microparticles. Modern instrumentation permits measurement of particle size as a function of time and/or temperature, but currently there is no simple method for performing DLS particle size distribution measurements in the presence of applied voltage. The ability to perform such measurements would be useful in the development of electroactive, stimuli-responsive polymers for applications such as sensing, soft robotics, and energy storage. Here, a technique using applied voltage coupled with DLS and a temperature ramp to observe changes in aggregation and particle size in thermoresponsive polymers with and without electroactive monomers is presented. The changes in aggregation behavior observed in these experiments were only possible through the combined application of voltage and temperature control. To obtain these results, a potentiostat was connected to a modified cuvette in order to apply voltage to a solution. Changes in polymer particle size were monitored using DLS in the presence of constant voltage. Simultaneously, current data were produced, which could be compared with particle size data, to understand the relationship between current and particle behavior. The polymer poly(N-isopropylacrylamide) (pNIPAM) served as a test polymer for this technique, as pNIPAM's response to temperature is well-studied. Changes in the lower-critical solution temperature (LCST) aggregation behavior of pNIPAM and poly(N-isopropylacrylamide)-block-poly(ferrocenylmethyl methacrylate), an electrochemically active block-copolymer, in the presence of applied voltage are observed. Understanding the mechanisms behind such changes will be important when trying to achieve reversible polymer structures in the presence of applied voltage.

Introduction

Dynamic light scattering (DLS) is a technique to determine particle size through the use of random changes in intensity of light scattered through solution¹. DLS is capable of measuring aggregation of polymers by determining particle size. For this experiment, DLS was coupled with controlled temperature changes to observe when a polymer aggregates which is indicative of exceeding the lower critical solution temperature (LCST)^2,3. Below the LCST, there exists one homogeneous liquid phase; above the LCST, the polymer becomes less soluble, aggregates, and condenses out of solution. An appl....

Protocol

1. Example polymer preparations

1. pNIPAM polymer synthesis
   NOTE: This preparation produces 10 mL of 1 g/L solution, which is enough for 3-4 experiments.
   1. Prepare the Schlenk line apparatus. Ensure the cold trap Dewar flask is filled with a slurry of dry ice and acetone, or if a mechanical refrigeration trap is used, ensure that the trap has reached an appropriate temperature.
   2. In a 50 mL round-bottom flask, add 0.566 g of N-isopropylacrylamide (NIPAM) monomer, 0.016 g of reversible addition fragmentation chain transfer polymerization (RAFT) agent (phthalimidomethyl butyl trithiocarbonate), 0.0008 g of 2,2....

Results

The real-time file output of each run in the temperature ramp is presented as a table, as seen in Figure 3. Each record can be chosen independently to see the volume size (Figure 4) and correlation coefficient (Figure 5). Volume particle size distribution (PSD) is the most accurate data to interpret the overall distribution and LCST, but the quality of data should be assessed via correlation graph (<.......

Discussion

Applying voltage to either pNIPAM or p(NIPAM-b-FMMA) solutions changed the polymer aggregation behavior in response to temperature. With both materials, when an applied voltage was present, the polymers' volume size remained high even when the solutions were cooled below their LCST. This was an unexpected result, as the trials with no voltage showed the polymers returning to their original sizes. These experiments allow us to conclude that for our temperature range, and with an applied voltage, polymer aggre.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
N-Isopropylacrylamide	Tokyo Chemical Industry CO., LTD	I0401-500G
1,4-Dioxane	Alfa Aesar	39118
2,2"-Azobis(2-methylpropionitrile)	SIGMA-ALDRICH	441090-100G
Cuvette	Malvern	DTS0012
Dynamic Light Scattering	Malvern	Zetasizer NanoZS
Ferrocenylmethyl methacrylate	ASTATECH	FD13136-1G
Phthalimidomethyl butyl trithiocarbonate	SIGMA-ALDRICH	777072-1G
Potentiostat	Gamry	Reference 600