Then, the sample was cooled from 40 ☌ to 20 ☌ without disrupting the applied field to study any hysteretic or equilibrium effects. The sample was heated from 20 ☌ to 40 ☌ to identify the LCST in the absence and presence of a 1 V applied field. This method could also be extended to other applications of DLS, such as the analysis of protein folding/unfolding, protein-protein interactions, and agglomeration of electrostatically charged particles to name a few. In this article, a protocol and methodology to study the effect of applied voltage on polymer aggregation is demonstrated. The electrochemically active polymer, p(NIPAM- b-FMMA), was also 100 mer chain length, which contains 4% ferrocenylmethyl methacrylate (FMMA) and 96% NIPAM. The non-electrochemically active polymer, pNIPAM, was synthesized as 100 mer pure pNIPAM. Both example polymers were synthesized by reversible addition fragmentation chain-transfer polymerization with controlled chain length 8, 9, 10. Specifically, ferrocenylmethyl methacrylate was used to create a poly( N-isopropylacrylamide)- block-poly(ferrocenylmethyl methacrylate) block-copolymer, or p(NIPAM- b-FMMA) 6, 7. Therefore, as a second sample polymer an electrochemically-active block was added to the polymer. pNIPAM is typically not electrochemically active. The LCST literature value of pNIPAM is around 30-35 ☌ 4. Thermal-responsive polymer materials like pNIPAM have been widely used in controlled drug release, biochemical separation, and chemical sensors in recent years 3, 4. Poly( N-isopropylacrylamide), or pNIPAM, is a thermal sensitive polymer, which contains both a hydrophilic amide group and a hydrophobic isopropyl group on the macromolecular chain 4, 5. In this protocol, two example polymers were used. The application of voltage in particle sizing measurements allows for new insights into particle behavior and subsequent applications in the fields of sensors, energy storage, drug delivery systems, soft robotics, and others. An applied voltage (i.e., applied potential or electric field) was introduced across the scattering field to observe the effects of the electric field on aggregation behavior and LCST. Below the LCST, there exists one homogeneous liquid phase above the LCST, the polymer becomes less soluble, aggregates, and condenses out of solution. 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. DLS is capable of measuring aggregation of polymers by determining particle size. Understanding the mechanisms behind such changes will be important when trying to achieve reversible polymer structures in the presence of applied voltage.ĭynamic light scattering (DLS) is a technique to determine particle size through the use of random changes in intensity of light scattered through solution 1. 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. The polymer poly( N-isopropylacrylamide) (pNIPAM) served as a test polymer for this technique, as pNIPAM's response to temperature is well-studied. Simultaneously, current data were produced, which could be compared with particle size data, to understand the relationship between current and particle behavior. Changes in polymer particle size were monitored using DLS in the presence of constant voltage. To obtain these results, a potentiostat was connected to a modified cuvette in order to apply voltage to a solution. The changes in aggregation behavior observed in these experiments were only possible through the combined application of voltage and temperature control. 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 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. 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. Dynamic light scattering (DLS) is a common method for characterizing the size distribution of polymers, proteins, and other nano- and microparticles.
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