Session W42: Focus Session: Polymer Brushes

Responsive Grafted Polymer Layers: the role of pH, temperature and surface geometry

The competition between chemical equilibrium, e.g. protonation, and physical interactions determines the molecular organization and functionality in biological and synthetic systems. Charge regulation by displacement of acid-base equilibrium induced by changes in the local environment provides for a feedback mechanism that controls the balance between electrostatic, van der Waals, steric interactions and molecular organization. What are the mechanisms that determine the interplay between all these different factors? In this talk I will describe a molecular theory to address this question. In particular, the theory will be used to study the structural and thermodynamic properties of end-grafted polyacids with hydrophobic backbones. The molecular theory explicitly includes the size, shape, conformations, charge and charge distribution of all the molecular species in the system and incorporates excluded volume, van der Walls and electrostatic interactions coupled with acid-base equilibrium. On planar surfaces, the theory predicts the formation of surface micelles with morphologies that depend upon the bulk pH, solution ionic strength, temperature and surface coverage. The self assembled aggregates, present domains of varying local pH that is very different from that of the bulk solution. We show that a qualitatively new form of local organization arises that is only found when there is explicit coupling between charge regulation and physical interactions. The different morphologies can be manipulated by changing the bulk solution conditions and they provide for local domains with controlled charge and pH with large gradients with a characteristic size of a few nanometers. Following this we will discuss how ion conductivity in nanopores functionalized with polybases changes as a function of solution pH. The predictions of the theory are in quantitative agreement with experiments and they provide a physical explanation of the interplay between molecular organization and charge in the nanopore. The last part of the talk will be devoted to a new system in which we show how to use the theory under non-equilibrium conditions to study the flux of ions in finite nanopores with grafted polybases (or polyacids) in the presence of external potentials.   

Polyelectrolyte brushes in mixed ionic medium studied via intermolecular forces

The vast uses and applications of polyelectrolyte brushes make them an attractive field of research especially with the growing interest in responsive materials. Polymers which respond via changes in temperature, pH, and ionic strength are increasingly being used for applications in drug delivery, chemical gating, etc. When polyelectrolyte brushes are found in either nature (e.g., surfaces of cartilage and mammalian lung interiors) or commercially (e.g., skin care products, shampoo, and surfaces of medical devices) they are always surrounded by mixed ionic medium. This makes the study of these brushes in varying ionic environments extremely relevant for both current and future potential applications. The polyelectrolyte brushes in this work are diblock co-polymers of poly-styrene sulfonate (N=420) and poly-t-butyl styrene (N=20) which tethers to a hydrophobic surface allowing for a purely thermodynamic study of the polyelectrolyte chains. Intermolecular forces between two brushes are measured using the SFA. As multi-valent concentrations are increased, the brushes collapse internally and form strong adhesion between one another after contact (properties not seen in a purely mono-valent environment).   

Microsecond MD Simulations of Nano-patterned Polymer Brushes on Self-Assembled Monolayers

Nano-patterned polymer brushes end-grafted onto a self-assembled monolayer interface have unique properties and application potential. However, the molecular-level interactions of these brushes with the substrate interface and the solvent are still not clear. Using a coarse-grained MD simulation approach we investigate the structure and dynamics of brushes of monomer length ranging from 25 to 75 units and an implant density $\sigma$ of 0.2 to 1.0 nm$^{-2}$ that were end-grafted onto a $5\times 5$ nm$^2$ well in a self-assembled hexadecane monolayer. The behavior of each polymer-monolayer-water complex was simulated from 3 to 12 $\mu$s.The excess width and the extended height of the polymers, the nanosecond-resolved conformational transition kinetics from a compact helical to a random coil-like structure, and the time-averaged monomer density maps were determined. The scaling behavior of these brushes differs from that of previous thermodynamic and computational studies of homogeneous brushes and nanopatterned stripes. Here, we find a weaker dependence of brush height on implant density, $\sigma^{0.29}$ rather than $\sigma^{1/3}$ and near linear scaling of the excess width on $\sigma$ rather than the $\sigma^{1/2}$. Our structural dynamics data and molecular templates are useful for future experimental and computational investigations of nano-patterned polymer brushes at the nanoscopic length and mesoscopic time scales.   

Nano-pillars created from the surface-grafted crosslinked polymer chains

Nano-patterned polymer chains with the controlled mechanical properties are widely applicable to biological and chemical studies. We synthesized linear and crosslinked polymer chains grafted onto micro/nano-patterned substrates by developing a series of unique bottom-up fabrication steps based on the iniferter-driven quasi-living polymerization. We incorporated conventional photolithography and nanosphere lithography. The AFM study provides insight into the influence of the addition of the crosslinker on the configurative, kinetic, mechanical, and wetting properties of polymer chains grafted onto micro/nano-patterns. We found that the addition of crosslinker successfully converts the mushroom-like configuration of nanopatterned polymer chains into the well-standing brush like polymer chains, i.e, soft nano-pillars. By analyzing the AFM force-distance curves obtained in the two-dimensional array and lateral force images, we found that the shear moduli of the obtained soft nano-pillar can be adjusted by varying the concentration of crosslinkers.   

Polymer brushes: Tools for surface design

Polymers brushes are ideal materials for interfacing with biological systems as they share many of the same molecular components and properties. Polymer brushes provide remarkable screening power in shielding a substrate from the environment through both steric and charge interactions. However, the majority of biomolecular species will still non-specifically bind to polymer brush surfaces unless some care is given to molecular design. Several polymer brush systems are described to control interaction of biomacromolecules and cells by design of specific and non-specific interactions in polymer brush architectures. ``Grown from'' and block copolymer brushes are described, both of which provide excellent substrates for study of brush surfaces. Examples of polymer brushes used for sensor creation and for investigation of cellular interaction are given. Brushes used in non-fouling coatings tailored for marine applications and in which amphiphilic structures play an important role are also described.   

Preparation, Patterning, and Electrical Characterization of Conjugated Polymer Brushes

Intimate contact at donor-acceptor interfaces and electrode-film interfaces is considered important for optoelectronic devices. This presentation will describe the formation and characterization of novel conjugated polymer brushes based on end-functionalized poly(3-hexylthiophene) and poly(para-phenylene) (PPP). In each case, end-functionalized polymers were synthesized and grafted to silicon substrates, with changes in film preparation method and polymer molecular weight used to manipulate the grafting density of the interfacial layers. Highly tunable PPP brushes having thickness ranging from 4 to 108~nm were obtained by in situ aromatization of poly(cyclohexadiene) brushes. Exceptionally smooth brush-modified interfaces were prepared, and neutron reflectometry, ellipsometry, AFM, and transmission electron microscopy were used to characterize layer structure and chain density. This presentation will also describe efforts to measure properties of these nanostructured layers using DC conductivity and AC electrical impedance measurements, as well as micro and nano-patterning of conjugated polymer brushes in the context of nanocircuit or organic solar cell applications.   

Quantifying Fluctuations/Correlations in Polymer Brushes

Fast lattice Monte Carlo (FLMC) simulations with multiple occupancy of lattice sites and Kronecker $\delta$-function interactions give orders of magnitude faster/better sampling of the configurational space of multi-chain systems than conventional lattice MC simulations with self- and mutual- avoiding walks and nearest-neighbor interactions.\footnote{Q. Wang, \textbf{Soft Matter, 5}, 4564 (2009).} Using FLMC simulations with Wang-Landau -- Transition-Matrix sampling, we have studied polymer brushes in both an implicit and explicit solvent. The various quantities obtained from simulations (including the internal energy, Helmholtz free energy, constant-volume heat capacity, segmental distribution, and chain sizes) are compared with predictions from the corresponding lattice self-consistent field theory and Gaussian fluctuation theory that are based on the same Hamiltonian as in FLMC simulations (thus without any parameter-fitting) to unambiguously and quantitatively reveal the effects of system fluctuations and correlations neglected or treated only approximately in the theories.   

Structures of One and Two Polymer Mushrooms

A polymer mushroom here is referred to as a group of chains end- grafted at the same point on a flat and impenetrable substrate. Using lattice self-consistent field (LSCF) calculations with the Kronecker $\delta$-function interactions (instead of the commonly used nearest-neighbor interactions), we have studied the structures of one and two polymer mushrooms in an explicit solvent as a function of the polymer volume fraction in the system, solvent quality characterized by the Flory-Huggins $\chi$ parameter, and distance between the two mushrooms. Since LSCF results are exact only in the limit of number of chains $n \to \infty$, we also use fast lattice Monte Carlo (FLMC) simulations\footnote{Q. Wang, \textbf{Soft Matter, 5}, 4564 (2009).} with the same Hamiltonian as in LSCF theory to examine how this limit is approached with increasing $n$. Direct comparisons between LSCF and FLMC results without any parameter-fitting quantify the fluctuation/correlation effects neglected in LSCF theory.   

Tethered chains in good and poor solvent - effects of lateral confinement on adsorption and chain collapse

The grafting density of a polymer brush affects the response of the system to changes in solvent quality and surface interactions. In this work, we focus on low (mushroom) and intermediate (semi-dilute regime) grafting densities and model a polymer chain in a brush as a single tethered chain subject to an applied force field of cylindrical symmetry that pulls the chain segments toward an axis through the tethering point and normal to the surface. The polymer chain is represented by a bond-fluctuation model with extended range attractive bead-bead interactions and variable bead surface interactions. Monte Carlo simulations with a Wang-Landau type algorithm are performed to determine the density of states in the state space of monomer- monomer contacts, monomer-surface contacts, and lateral chain extension. We present results for the effect of lateral confinement on conformational transitions such as chain adsorption in good and poor solvent.   

Semiflexible Polymer Brushes

Non-crossing flexible polymer brush configurations with $N$ polymers correspond to $N$ vicious random walkers, i.e. the system stalls when any two random walkers meet. We study a system of $N$ vicious accelerating walkers with the velocity undergoing Gaussian fluctations, as opposed to the position, to model semiflexibility. We numerically compute the survival probability exponent, $\alpha$, for this system, which characterizes the probability for any two semiflexible polymers in the brush not to cross. The data suggest that $\alpha=\frac{1}{8}N(N-1)$ . We also numerically study $N$ vicious Levy flights and find, for example, for $N=3$ and a Levy index $\gamma=1$ that $\alpha=1.26\pm0.01$.   

Interfacial properties of statistical copolymer brushes

The interfacial properties of statistical copolymers have important ramifications for the design of patterned thin films with preferred morphologies. In order to explore these properties, we study the interfacial properties of random copolymer brushes in contact with a thin film composed of a homopolymer of one of the blocks. We calculate the interfacial widths and interfacial energies between them as a function of different parameters. We find that the interfacial widths decrease (signifying expulsion of the free chains from the brush) with increasing free chain length, grafting density, and Flory interaction parameter $\chi N$ as well as with decreasing grafted chain length. The interfacial energies show inverse trends to the interfacial widths, except that results for varying grafting density depend on how chemically similar the brush is to the film. We also compare the interfacial efficacies for different types of randomness and find that, except for the case of very blocky chains, blockiness has only little effect on the properties of the interface. We discuss our findings in terms of the design of neutral surfaces and show that our results are consistent with comparable experimental results.