PhD Defence Leon Smook | Electroresponsive Polymer Brushes for Molecular Adsorption

Electroresponsive Polymer Brushes for Molecular Adsorption

Leon Smook is a PhD student in the Department of Sustainable Polymer Chemistry. Promotors are dr.ir. S.J.A. de Beer and Prof. dr. ir. C.G.P.H Schroën from the Faculty of Science & Technology.

Much of how we experience the world depends on the properties of surfaces and interfaces, ranging from the color of the walls to the friction between your finger and a touch screen. In many cases, these properties come from the material from which the surface is made. Polymer brushes are an excellent way to modify the properties of surfaces on a molecular level. These arrays of densely grafted polymer chains exhibit stimulus-responsive properties. For instance, some brushes change in thickness when the temperature is increased, and others collapse when the acidity of the surrounding solution is changed. Changes in surface properties could be useful in molecular enrichment and separation. In fact, if changes can be brought about reliably with a well-controlled stimulus, responsive surfaces could become a core component in novel separation technologies.

This thesis investigates how electric fields can be used as a stimulus to modify the properties of polymer brushes such that reversible molecular enrichment can be achieved. The question is studied through a comprehensive review of the literature (Chapter 2), chapters focusing on computation work using molecular dynamics and self-consistent field theory (Chapters 3-5, 7-9), and a chapter that combines experiments and simulations (Chapter 6).

Chapter 3 explores how the electroresponse of polyelectrolyte brushes is affected by changes in the fraction of charged monomers and by differences in the chain length dispersity of the grafted polymer chains. When fully charged polyelectrolyte brushes are placed in an electric field, the structure does not change much compared to its structure in the absence of such fields. However, electric fields cause a stronger structural change when the charge fraction of the brushes is reduced. This increase in response is even more pronounced when the chain length dispersity of the grafted chains is increased. Finally, we observe a chain length dependence in the response of individual chains in brushes that contain chains of different lengths: short chains collapse first, while long chains stretch first depending on the direction of the applied field.

Chapter 4 describes the specific case of polyelectrolyte brushes with a gradually changing composition along the contour length of the grafted chains. These gradient brushes were studied under two solvent conditions for different grafting densities and charge fractions. Gradient brushes restructure in electric fields in a similar manner as the brushes described in chapter 3. In these brushes, the gradient allows for a change in local composition upon restructuring, since the chain collapse in a back-biting manner. In this chapter, I also studied the effect of grafting density, monomer ratio, and solvent quality on the electro-responsive properties of brushes. Stretching and collapse of the brushes was found to be highly affected by the physical parameters, while the quality of the solvent was much less relevant.

To see whether salt affects the electroresponse of polymer brushes, I investigated this confounding effect with coarse-grained molecular dynamics simulations and self-consistent field theory in chapter 5. When brushes are exposed to electric fields in salt solutions, one can imagine two situations: either the brush is located in a confined space where the number of ions available to influence the electroresponse is restricted (case 1) or the brush is employed in equilibrium with a bulk solution where the ions can equilibrate with this bulk solution (case 2). Case 1 was studied using coarse-grained molecular dynamics simulations under a constant amount of salt. When a field is applied, the brush both stretches and collapses under influence of external fields. If little salt is present in the simulation, most of the imposed field is screened by the redistribution of the polyions, creating a responsive system in both directions. However, if more salt is present, these free electrolyte ions contribute to the screening of the field, and the system loses its responsivity. Case 2 was studied using self-consistent field theory with a constant chemical potential of the bulk electrolyte. Under case 2 conditions, the applied field is partially screened at the counter electrode by the electrolyte in solution, even at very low concentrations. As a result, the system still collapses under an applied field, but its stretching is limited.

In chapter 6, I present a collaborative study that compares experimental observations with continuum Poisson-Nernst-Planck calculations. We synthesized highly-charged polyanionic sulphopropylmethacrylate (PSPMA) brushes and characterized their electrical properties with electrochemical impedance spectroscopy and chronoamperometry. These experiments revealed a dependence of the brush resistance and capacitance on the salt concentration: resistance decreases with increasing concentration, while capacitance increases. Interestingly, the effect of an applied bias was less pronounced: a significant asymmetry between both polarities was observed only in the salted regime. Additionally, chronoamperometry experiments revealed that the current decays via processes occurring at two different time scales. Similar time scales are reproduced in a simple model solved with a Poisson-Nernst-Planck model that only contains a fixed charged background. This indicates that the exact structure of the layer may be less important than its presence.

In Chapter 7, I study a gradient copolymer brush with coarse grained molecular dynamics simulations. In these simulations, I introduce small molecules (particles) via a grand-canonical Monte Carlo insertion/deletion procedure, which allows one to simulate under constant chemical potential conditions. The simulations revealed that partially-charged polyelectrolyte brushes may enrich in target molecules if the interaction between the target and the polymer is strong enough. In fact, if these brushes are subsequently perturbed by electric fields, this enrichment can be modulated.

In chapter 8, I performed all-atom molecular dynamics simulations of γ-butyrolactone in fully charged poly(acrylic acid) brushes at two grafting densities and temperatures. In these simulations, I observed the behavior of the lactone molecules as well as their interaction with the solvent inside and outside the brush. These simulations reveal that molecules experience reduced diffusivity inside polymer brushes. Additionally, the water solvation shell around the solute is largely unaffected upon entering the brush, even though the structure of the water is different inside compared to outside the brush.

In chapter 9, I set up a model to estimate the interaction between uncharged polymer brushes and particles or flat walls. I extend the well-known Alexander-de Gennes theory to allow one to estimate the interaction for brushes consisting of chains from and arbitrary length distribution. Based on a comparison with coarse-grained simulations, the results of the model provide excellent qualitative agreement.

In this thesis, I have presented a first step towards using polyelectrolyte brushes for molecular separation. I explored in detail how copolymer brushes respond to the application of electric fields, and specifically how this response is affected by realistic non-idealities such as dispersity in the length of the grafted chains (Chapter 3), a gradient in composition of grafted chains (Chapter 4), and the presence of electrolyte in solution (Chapter 5, 6). These studies revealed that polyelectrolyte brushes remain electroresponsive under various conditions, including those relevant to applications. I then used this knowledge to study how brushes interact with molecules, small (Chapters 7 and 8) and large (Chapter 9). The concepts presented here open up opportunities to engineer the affinity of brushes towards specific targets, paving the way for precision separation.