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Polymer Interface & Surface

      I intend to investigate the adsorption and adhesion behavior of macromolecules such as polymers, colloidal and metal-nano particles, and proteins and cells, when they interface with a solid surface.  Such fundamental knowledge is practically important in fabricating micro- and nano-devices and sensors.  Furthermore, such fundamental knowledge will enable me to attain meso- and macroscopic applications for fabricating such devices.

Solid surfaces capable of repelling, attracting, and selectively detecting molecules have attracted attention for their important application in catalysts, coatings, as well as sensors and devices. For these macromolecules especially, designing and engineering surfaces and interfaces at the molecular level are even more important, because oftentimes subtle changes in organization and composition can result in dramatic performance enhancement. The surface functionality and topography affect the conformation and the connectivity of the adsorbed molecules as well as the adsorption dynamics, where the interplay between stickers and receptors is the main controlling factor.

In my PhD work on Polymer Adhesion on Surface-Tuned Solid Surfaces at the University of Delaware.  I learned that interfacial structure and strength are closely related to each other (Wool, R.P. Polymer Interface: Structure and Strength, New York, 1995) and both can be controlled by various modification methods[1]-[2].  We examined the role of sticker groups X, on the polymer and receptor groups Y on the solid substrate.  We found that a precise number of sticker groups (ca 3 mol%) and receptor groups (ca 30 % coverage on solid) were required to optimize the strength of the interface. The optimal design of the coupling molecule requires it to stick at least two points on the surface while making an excursion into the matrix molecules by at least the radius of gyration of the entanglement network.  During this work, I gained invaluable knowledge of polymer and surface modification and characterization using FTIR, XPS, AFM, wetting as well as theoretical analysis of polymer chain on surfaces, which will be a good foundation of my future research in this area.

During my Postdoctoral work at MIT, I have learned the layer-by-layer electrostatic assembly technique, which is a rich and versatile approach to the formation of polymer and organic thin films via alternating adsorption of positively and negatively charged species (Rubner and Hammond group at MIT).  Polyelectrolyte multilayers formed using this technique may contain a number of different functional groups, including optic-electronic, electroluminescent, conducting and dielectric layers, and functional organic and inorganic nanoparticles. This versatility allows the use of the multilayer as a functional thin film as well as a template, and introduces the possibility of constructing luminescent photonic devices, chemical and biosensor arrays, and other electronic and optical systems.   Further, selective metal deposition can be combined with colloidal arrays for patterned metal particle arrays.  Polyelectrolyte multilayer films can also be used to increase adhesion between the particle and the surface template.  Surface wettability as well as adhesion characteristics of the multilayer can be tuned by varying adsorption conditions.  The experimentally measured fracture energy of polymer-solid interfaces can be very high due to viscoelastic losses in the polymer layer, even if the interface is small.  Finally, the polymer-on-polymer stamping process can enable the formation of patterned polymeric layers on various surfaces, such as glasses and plastics, thus leading to inexpensive and fast nonlithographic processing on cheap, flexible substrates.

[1]. I. Lee and R. P. Wool, "Polymer Adhesion vs. Substrate Receptor Group Density," Macromolecules, 33(7), pp2680-2687 (2000).

[2]. I. Lee and R. P. Wool, ˇ°Optimum Polymer-Solid Interface Design for Adhesion Strength: Carboxylation of Polybutadiene and Mixed Silanes Surface Modification of Aluminum Oxide,ˇ± Journal of Adhesion, 75, pp299-323 (2001).

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