Natural biological surfaces exhibit interesting properties due to their inhomogeneous chemical and physical structure at the micro-and nanoscale. In the case of hair or skin, this also influences how waterborne macromolecules ingredients will adsorb and form cosmetically performing deposits (i.e., shampoos, cleansers, etc.). Here, we study the adsorption of hydrophilic flexible homopolymers on heterogeneous, chemically patterned substrates that represent the surface of the hair by employing coarse-grained molecular dynamics simulations. We develop a method in which the experimental images of the substrate are used to obtain information about the surface properties. We investigate the polymer adsorption as a function of polymer chain length and polymer concentration spanning both dilute and semidilute regimes. Adsorbed structures are quantified in terms of trains, loops, and tails. We show that upon increasing polymer concentration, the length of tails and loops increases at the cost of monomers belonging to trains. Furthermore, using an effective description, we probe the stability of the resulting adsorbed structures under a linear shear flow. Our work is a first step toward developing models of complex macromolecules interacting with realistic biological surfaces, as needed for the development of more ecofriendly industrial products.
We study the stability of polymeric nanoparticles fabricated through the rapid mixing of polymers in a good solvent with a poor solvent that is miscible with the good solvent. In previous experiments where water was used as the poor solvent, a negative surface charge was measured on the precipitated nanoparticles, which led to the long-time stability of the dispersion. It was argued that these charges originate presumably from either water or hydroxide adsorption at the hydrophobic nanoparticle surface or from impurities in the feed streams that preferentially adsorb on the precipitated nanoparticles. To elucidate the origin of this stabilization mechanism, we performed experiments wherein we replaced water with a nonpolar poor solvent. The polymers aggregated into stable nanoparticles for a range of processing parameters. We investigated theoretically three possible explanations for this stability, i.e., electrostatic stabilization, conditional thermodynamic equilibrium, and steric stabilization. Our experiments and considerations suggest that steric stabilization is the most likely candidate.
We study the coil and globule states of a single polymer chain in solution by performing molecular dynamics simulations with a united atom model. Specifically, we characterize the structural properties of atactic polystyrene chains with N = 20-150 monomers in tetrahydrofuran-water mixtures at varying mixing ratios. We find that the hydrophobic polymers form rather open coils when the mole fraction of water, X, is roughly below 0.25, whereas the chains collapse into globules when X ≳ 0.75. We confirm the theoretically expected scaling laws for the radius of gyration, R, in these regimes, i.e., R ∝ N and R ∝ N for good and poor solvent conditions, respectively. For poor solvent conditions with X = 0.75, we find a sizable fraction of residual tetrahydrofuran trapped inside the collapsed polymer chains with an excess amount located at the globule surface, acting as a protective layer between the hydrophobic polystyrene and the surrounding water-rich mixture. These findings have important implications for nanoparticle fabrication techniques where solvent exchange is exploited to drive polymer aggregation, since residual solvent can significantly influence the physical properties of the precipitated nanoparticles.
A new technology platform built on the integration of theory and experiments to enable the design of Janus colloids with precision control of surface anisotropy and amphiphilicity could lead to a disruptive transformation in the next generation of surfactants, photonic or phononic materials, and coatings. Here, we exploit molecular dynamics (MD) simulations to guide the rational design of amphiphilic polymer Janus colloids by Flash NanoPrecipitation (FNP), a method capable of the production of colloids with complex structure without the compromise of reduced scalability. Aided by in silico design, we show in experiments that amphiphilic Janus colloids can be produced using a unique blend of hydrophobic homopolymers and the addition of an amphiphilic block copolymer. The final structure of the colloids depends on the mass fraction of each homopolymer as well as the concentration and composition of the block copolymer additive. To confirm the surface activity of the colloids, we demonstrate their potential to stabilize Pickering emulsions. This hybrid approach of simulations and experiments provides a pathway to designing and manufacturing complex polymeric colloids on an industrial scale.
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