# ATOMISTIC SIMULATIONS OF THE POLYMER/WATER INTERFACES IN LATEX PAINT

Open Access
Author:
Li, Zifeng
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 30, 2016
Committee Members:
• Scott Thomas Milner, Dissertation Advisor
• Scott Thomas Milner, Committee Chair
• Darrell Velegol, Committee Member
• Janna Kay Maranas, Committee Member
• Ralph H Colby, Outside Member
• Kristen Ann Fichthorn, Dissertation Advisor
• Kristen Ann Fichthorn, Committee Chair
Keywords:
• atomistic molecular dynamics simulations
• acrylic latex paint
• electrical double layer
• surfactant surface binding
• nanoparticle
• polymer water interface
Abstract:
Since environmental regulations have required manufacturers to reduce volatile organic components in paint, waterborne latex paints have taken an increasing market share over traditional, organic-solvent-based paint \cite{Keddie1997101}. It is a colloid suspension, in which polymer particles are dispersed in water, along with functional additives, such as surfactants, rheology modifying polymers, buffer salts. Because of the remarkably complex compositions, an empirical approach to optimizing the formulation of water-based coatings becomes increasingly challenging as the number of variables increases. To gain a molecular level understanding of the latex binder particles in water, we built a chemically detailed model of the polymer particle/water interface, which plays a key role in the end use properties of paint, such as the particle stability and rheology. We applied atomistic molecular dynamics simulations to study the surface of the latex polymer, which consists of a random copolymer of the methyl methacrylate (MMA) and n-butyl acrylate (BA). We characterize the structure and associated energy of the latex polymer/water interface in Chapter \ref{chapter3}, which validate the model and provide insight into surface group orientations. We calculated bulk densities of MMA/BA copolymer, and PMMA and PBA homopolymers, which compare well with the experiment, and the structure factor for PMMA, which agrees with neutron scattering results. The simulated $T_g$ of PMMA is 17 K from its experimental value, despite the much shorter chains and faster cooling rates of simulations versus experiments, because these two effects tend to cancel each other. When we correct for these effects, the predicted $T_g$ is within 5K of the experimental value. We found that the carbonyl groups at the polymer/water interface orient significantly toward the water phase. We calculated the copolymer/water and the copolymer/vacuum interfacial tensions, and predict a contact angle for water on copolymer of $77^\circ$, consistent with experimental values. Informed by extensive characterization of particle synthesis and surface properties, we investigate the electrostatic properties of the interface in Chapter \ref{chapter4}, which is crucial to particle stabilization. We study the double layer that results from 1) adsorbed ionic surfactant and 2) grafted hairs, which are multivalent oligomers, at polymer-water interfaces, each with and without added salt. Particularly, we compare the interfacial width and structure, bound ion and counterion distributions, and extent of counterion condensation, for the four systems (adsorbed surfactant or grafted hairs) x (with or without salt). %We extend the traditional definitions of the inner and outer Helmholtz planes to our diffuse interfaces. We found that beyond the Stern layer, the simulated electrostatic potential is well described by the Poisson-Boltzmann equation. The potential at the outer Helmholtz plane compares well to the experimental zeta potential. Although the bare charge density of a surface bearing hairs is much higher than that of a surfactant-bearing surface at realistic coverage, greater counterion condensation leads to similar zeta potentials for the two systems. Attractive interactions between additive molecules and particle surfaces are key parameters to tune the stability and rheology. We determine the potential of mean force for a commonly used industrial surfactant sodium dodecyl sulfate (SDS) interacting with acrylate latex particles in Chapter \ref{chapter5}. SDS competes the surface area with rheology modifying polymers, of which the balance is important to paint formulation. We investigate how the potential of mean force and binding free energy depend on the amount of SDS adsorbed, solution ionic strength, and presence of other charged groups on the particle surface. We show that the potential of mean force for SDS is a sum of two independent terms, from the hydrophobic surfactant tail and charged headgroup: dragging the surfactant tail into solution contributes a linear potential of about $kT$ per CH${}_2$ group, while the headgroup is repelled by like charges on the surface with a potential of about the zeta potential. For surfaces charged by hairs, SDS binds more or less strongly depending on the local environment because of the heterogeneity of the surface charge. This is the first model that targets the surface of the latex particle using explicit atom simulations. The detailed charge distribution near binder particle surface and the free energy profile of additive binding provide guidance to the design of commercial waterborne coatings. These information also supply key interaction parameters for coarse-grained simulations, in which larger scale interactions related to the stability and rheology of paint can be explored.