Dilip Gersappe

Confinement-induced miscibility in polymer blends

The use of polymer thin films in technology is increasingly widespread—for example, as protective or lithographic surface coatings, or as active (electronic or optical) elements in device architectures. But it is difficult to generate films of polymer mixtures with homogeneous surface properties, because of the tendency of the polymers to phase-separate,. Copolymer compatibilizers can induce miscibility in polymer blends, but only with chemical components that are either close to a critical point in the phase diagram or which have an attractive interaction between them,. Instead of manipulating the chemical composition of the blend, we show here that complete mixing can be obtained in polymer blends by the physical effect of confinement in thin films. The compatibilization results from entropic inhibition of phase separation into micelles, owing to confinement. The result is an intimately mixed microemulsion with a perfectly flat surface and a two-dimensional maze-like structure with columnar domains that extend through the film.

Control of cell migration in two and three dimensions using substrate morphology

We have shown that en masse cell migration of fibroblasts on the planar surface results in a radial outward trajectory, and a spatially dependent velocity distribution that decreases exponentially in time towards the single cell value. If the cells are plated on the surface of aligned electrospun fibers above 1 microm in diameter, they become polarized along the fiber, expressing integrin receptors which follow closely the contours of the fibers. The velocity of the cells on the fibrous scaffold is lower than that on the planar surface, and does not depend on the degree of orientation. Cells on fiber smaller than 1 microm migrate more slowly than on the planar surface, since they appear to have a large concentration of receptors. True three-dimensional migration can be observed when plating the droplet on a scaffold comprises of at least three layers. The cells still continue to migrate on the fibers surfaces, as they diffuse into the lower layers of the fibrous scaffold.

Modeling the dynamics of DNA electrophoresis on a flat surface

We use molecular dynamics simulations to study the mechanism by which a flat, homogeneous surface can serve as an electrophoretic separation medium for DNA. We find that the mobility of DNA on the surface is a function of the conformation of the adsorbed DNA molecule, and that this mobility is controlled by the attraction between the DNA and the surface. Our results will provide guidelines for the fabrication of surfaces that can be used to separate DNA in a wide size range.

Aggregation in grafted polymers with attractive end groups

We used Monte Carlo computer simulations to determine the properties of grafted polymers that contain attractive functional groups (‘‘stickers’’) on the free ends of the chains. By varying the number of stickers and the grafting density, we isolated conditions that drive the chains to aggregate into clusters. We also observed that the attraction between the stickers effectively ‘‘pulls’’ the chain ends from the bulk of the brush and localizes these sites to the top of the grafted layer. The results provide guidelines for tailoring polymer brushes that display enhanced adhesive properties.

Computational studies of protein adsorption at bilayer interfaces

We used a Monte Carlo computer simulation to determine the behavior of proteins near and within a bilayer. The bilayer is represented by a hydrophobic slab, which is bounded above and below by hydrophilic regions. The protein is modeled as a neutral, self‐avoiding chain, which contains both hydrophobic and hydrophilic sites. Through these simulations, we examined the effect of sequence distribution and the strength of the interaction energies on the interfacial activity of the proteins. The findings reveal both structural and energetic conditions under which proteins will remain localized at the surface of the bilayer or, penetrate and traverse the membrane. The results provide design criteria for fabricating proteins or biopolymers that display the desired interactions with bilayers.

Patterned Polymer Films

Polymer films that contain well-defined patterns can be used in a variety of novel applications. For example such films can serve as the scaffolding in fabricating organic/inorganic composites with controlled architectures. One means of forming patterned films is to anchor the ends of homopolymers onto a substrate (so that the ends are fixed and cannot move) and immerse the system in a poor solvent. The incompatibility between the polymer and solvent drives the system to phase-separate. Since the ends are immobilized however, the polymers can only escape the unfavorable solvent by clustering with neighboring chain s into distinct aggregates or “pinned micelles.” These micelles have a uniform size and spacing, and form a regular array on the surface. In this article, we use theoretical models to extend this concept and show that, by tethering copolymers—chains that contain more than one type of monomer—we can drive the system to form more complicated surface patterns. These copolymer patterns provide a handle for engineering the interaction between surfaces and thus facilitate the fabrication of novel optical devices. If the copolymer films are composed of both hydrophilic and hydrophobic domains, the surface can also be used as a template for growing biological cells with tailored shapes and sizes.

Interfacial slip in sheared polymer blends.

We have developed a dynamic self-consistent field theory, without any adjustable parameters, for unentangled polymer blends under shear. Our model accounts for the interaction between polymers, and enables one to compute the evolution of the local rheology, microstructure, and the conformations of the polymer chains under shear self-consistently. We use this model to study the interfacial dynamics in sheared polymer blends and make a quantitative comparison between this model and molecular dynamics simulations. We find good agreement between the two methods.

A theoretical model for copolymer-bilayer interactions

We develop a theory to model the interactions between an amphiphilic copolymer and a bilayer. The copolymer is represented as a Gaussian chain, which contains an alternating arrangement of hydrophobic and hydrophilic sites along the length of the chain. The bilayer is modeled as a hydrophobic layer embedded in a hydrophilic environment. We use the transfer matrix technique to determine the polymer density profiles and the phase diagram for this system. Two distinct phases are observed. In one phase, the copolymer is localized at the surface or within the bilayer. In the second phase, the polymer is unbound or ‘‘delocalized.’’ There is a continuous transition between the two phases. We also determine the scaling behavior for the density profiles. The scaling exponents agree with our analytical arguments. We discuss the implications of our findings on designing copolymers that can act as adhesives or macromolecular surfactants.

Manipulation of cell adhesion and dynamics using RGD functionalized polymers

We have successfully synthesized an ABA tri-block co-polymer of poly(methacrylic acid)-block-poly(2-hydroxyethyl methacrylate)-block-poly(methacrylic acid), having Mw = 100k and 272k where we were able to insert RDG or RGD peptide sequences using thiol-acrylate Michael addition. A soft silicone stamp was then used to imprint a 0.4-micron wide grating of the copolymer with a period of 10 microns. The samples were then examined with atomic force microscopy after application of an external electric field and the pattern was observed to stretch by a factor of five. Cells plated onto these substrates showed clear preference for the striped patterns formed under the influence of the external field, and no preferential attachment to the patterns formed in the absence of the field. Cell migration experiments, using the agarose droplet method, performed on spun cast copolymer films showed minimal migration and adhesion on the substrates without peptides or those with only with the RDG peptide, while good adhesion and significant outward migration was observed for cells plated on the copolymers with the RGD sequence. Taken together these results confirmed our hypothesis that a smart biomimetic polymer substrate could be constructed where functional domains could be revealed selectively allowing us to mimic the natural design of engineered tissue constructs.

Compatibilization dynamics in highly immiscible polymer blends

The morphology of incompatible polymer blends are often stabilized by the addition of block copolymers that ideally will localize to the polymer-polymer interface and reduce the interfacial tension. However, the effectiveness of adding copolymer is significantly reduced by the tendency of the diblock to form micelles that become trapped within one of the phases. Recent theoretical and experimental results show that using compatibilizers in confined physical geometry will reduce the configurational entropy of the diblock and make it energetically more favorable for the diblock to locate to the interface [1]. Dynamics studies with Scanning Transmission X-ray Microscopy (STXM) show two regimes in the dynamical process, where growth regimes are characterized by growth exponents, α, where R(t)∼Rαt. The first growth regime consists of round micelle-like domain formation and relatively fast growth (α=2/3) of these structures within the PS layer. The second regime results in a relatively stable bicontinuous domai...