This program targets a fundamental understanding of polymers that adsorb at liquid /solid interfaces. Adsorbing polymers are important to commercial applications such as inks, paints, and coatings and to novel technologies involving surface treatments for proteomics and DNA separations. For instance, water soluble polymers, acting as colloidal stabilizers and rheological modifiers, are the enabling component in the development of environmentally-friendly water-based coatings that replace older formulations employing organic solvents. Likewise, high molecular weight cationic flocculants lie at the heart of papermaking processes. Using polymer chemistry and architecture to tune the dynamics of these flocculants relative to process timescales will enable process improvements in the papermaking industry with substantial reduction of waste streams. Finally, adsorption-based surface treatments comprise an inexpensive means of surface modification for DNA separations and recognition chips. Surface modification strategies that accommodate molecules with a complex distribution of hydrophobicity and charge will facilitate extensions to protein separations and chips which assess protein function.
Studies in the Santore labs probe static and dynamic features of polymers chosen for their relevance to established and future technologies.
Nonionics: PEO Poly(ethylene oxide), a traditional colloidal stabilizer, has reappeared in the spot light as a surface compatibilization agent for biomedical applications. Proteins tend not to adhere to adsorbed and grafted PEO layers, making these surfaces bio-neutral. Both conventional and biomedical applications of PEO motivate its use as a model system in studies of polymer adsorption and of the dynamic features of adsorbed polymer layers. Our program with nonionic polymers such as PEO reveals how chemical and processing features can be tuned to alter various aspects of interfacial polymer dynamics.
Molecular Weight Competition
Large scale industrial applications involving substantial quantities of complex fluids such as paints, inks, and coatings employ water soluble polym
ers with a broad distribution of molecular weights. The likelihood that some fraction of the added chains impart the desired interfacial properties means that changes in molecular weight distribution from batch to batch can dramatically impact the properties of a formulation. Figure 2 shows how this might occur: Upon initial mixing of a formulation, all chains attempt to adsorb on a surface. For adsorbing homopolymers, thermodyna
mics dictates a preference for adsorption of long chains, and so short chains, originally adsorbed, are displaced form the surface at longer times.
• M. M. Santore and Z. Fu, Macromolecules 30, 8516 (1997).
• Z. Fu and M. M. Santore, Macromolecules, 31, 7014-7022 (1998).
Interfacial polymer dynamics are often history dependent, as a result of polymer chain relaxations. Students in the Santore group have quantified how these relaxations affect chain mobility and adhesion at the molecular scale. We find that once polymer chains encounter a surface, they slowly reconfigure (in Figure 3) in ways that make their removal progressively more difficult. Adhesion can increase through physical interactions over a period of days and weeks, depending on the molecular parameters.
• Z. Fu and M. M. Santore, Macromolecules, 32, 1939-1948 (1999).
• E. Mubarekyan and M. Santore, Macromolecules 34, 4978-4986 (2001).
Scaling of Self Exchange
In complex coating formulations, different species compete for surfaces (pigment and resin particles, chromatographic media, specialty channels), a behavior that parallels the potential displacement of stabilizing chains from a biomaterial surface. This process is best quantified fundamentally through a dynamic behavior called self exchange, a tracer experiment in which adsorbed chains are displaced from a surface by like chains, differing only by a tracer label. The Santore lab discovered a counter-intuitive effect of surface loading on self exchange dynamics, which was shown to scale-away a generally observed molecular weight dependence.
•E. Mubarekyan and M. Santore, Macromolecules 34, 7504-7513 (2001).
Polyelectrolytes: Polymers carrying electrostatic charge, polyelectrolytes, present an important means of controlling the stability and rheology of complex fluids. Hence, polyelectrolytes play a major role in the formulation of paints, inks, and coatings. Many biological molecules are also polyelectrolytes, including proteins and DNA.
An important feature of polyelectrolyte adsorption onto oppositely charged surfaces is a reversal of the underlying substrate surface charge, called overcompensation. Many treatments of polyelectrolyte adsorption, especially those which are mean-field approaches, tend not to be able to predict this technologically important behavior, because lateral inhomogeneities in charge are neglected. Studies in the Santore lab quantify, with model systems, exactly how charge overcompensation depends on local surface and polymer charge density and other fundamental molecular properties, facilitating a more predictive understanding of interfacial charge.
• Y. Shin, J.E. Roberts and M. Santore*, J. Colloid Interface Science 244, 190-199 (2001).
• Y. Shin, J.E. Roberts and M. Santore*, J. Colloid Interface Science, 247, 220-230 (2002).
Adsorbed layers are comprised of tails, loops, and trains. In nonionic systems, the relative proportions of these features are controlled mostly by molecular weight; however, polyelectrolytes are complicated by their charge. In a collaboration with the Roberts NMR lab at Lehigh University, solvent relaxation studies revealed the influence of charge on tails, loops, and trains in a model polyelectrolyte system, for variations of charge density along the polymer backbone, in Figure 4. With moderate charge, polyelectrolytes tend to lie flat on the surface. Polyelectrolyte layers become fluffy when their charge densities are low.
• Y. Shin, J.E. Roberts. and M. Santore*, Macromolecules 35, 4090-4095 (2002).
In adsorbing polyelectrolyte systems, added salt ions can enhance or reduce polymer adhesion to a charged surface, depending on the nature of the interfacial interactions. One remarkable recent finding from our lab is a very strong adsorption cut off in which added salt completely and abruptly displaces the adsorbed polymer. This occurs in the limit of low polymer charge for adsorption driven exclusively by electrostatic interactions. Increases in the polymer charge diminishes the sharpness of the adsorption cut-of as added ions complete less effectively for the surface.
• N. Hansupalak. and M. Santore*, Langmuir,19, 7423-7426 (2003).
The dynamics of polyelectrolyte chains on surfaces is an important consideration in complex aqueous formulations and biotechnological assays. Our ongoing work relates the dynamic behavior of adsorbed chains to key molecular properties which could be chosen through the chemical choice of the polyelectrolyte or the surface, or through processing conditions. In the limit of a few polymer-surface charge interactions, we find processes consistent with simple first order rate behavior which is well described by an Arhenius form. We have also identified a crossover behavior to more complex dynamics with an increase in charge numbers.
• N. Hansupalak. and M. Santore*, in press, Macromolecules.
• N. Hansupalak. and M. Santore*, in preparation for Macromolecules.
Ongoing Studies of Adsorbed Polymers
Work continues in the Santore lab on a variety of topics involving adsorbed polymers, their fundamental properties and potential applications of adsorption. One project particularly relevant to adhesion and lubrication, is the lateral movement of adsorbed chains on surfaces. Here we seek evidence for interfacial mechanisms of chain motion, which may parallel or differ dramatically from entangled behavior in bulk melts and concentrated solutions.