View My Projects Development of Nanostructured Lead-free Solder In order to improve the service performance of lead-free electronic solders a sub-micron size inert and stable reinforcement, that bonds extremely well and be present in grain boundary regions of the solder matrix without any agglomeration during processing or coarsening during additional exposure to service environments, is essential. Unluckily, traditionally evaluated approaches utilizing ceramic, metallic and IMC particulate reinforcements or alloying methodologies do not satisfy all these requirements. POSS-silanols containing one to three (Si-OH) groups consist of inert strongly bonded cage structure made of Si-O with surface active groups. As a consequence, the negatively charged -OH surface active groups present in POSS may strongly bond with the metallic matrix, hopefully to the grain boundary regions of the solder. The negatively charged surface active groups present on the surface of POSS should also prevent flocculation of these reinforcements during processing. Since bonding between Si and O is very strong, they will not dissociate in service environments. This will alleviate any coarsening problems that may be encountered during service.
Durability Characterization of POSS-based Polyimides and Carbon-Fiber Composites This work builds on experience and progress made in our laboratory for the past six years, which has enabled us to utilize rational chemical approaches and sound engineering design principles to make significant performance enhancements on many difficult-to-attend and yet desirable properties of conventional organic polymers by incorporate nano-structured chemicals such as Polyhedral Oligomeric Silsesquioxane (POSS). This nano-structured chemical reinforcement approach not only retains the light-weight nature of organic polymers, but also substantially improves many other desirable performance qualities, i.e., fire and flame retardancy, high temperature mechanical properties and oxidative stability, surface durability, etc., in comparison to conventional organic polymers. The three-dimensional nature and nanoscopic size of POSS are the key contributors to the material property improvements observed through POSS incorporation. Currently in our laboratory, we have been exploring the engineering, drop-in approaches to incorporate POSS chemicals into high-performance thermosetting materials such as high-performance epoxies, bismaleimides and reactive oligoimides. Recently, in collaboration with NASA-Glenn Research Center, we were able to reformulate second-generation oligoimides with additions of economical trisilanol POSS by taking advantage of the Polymerization of Monomeric Reactants (PMR) methodology. Using the PMR approach with a well-established processing method, we fabricated carbon-fiber reinforced composites with these POSS-containing oligoimides as the matrix resin. As a part of the initial characterization of these newly formulated hybrid polyimide resins and composites, we have observed an improved thermo-oxidative stability. In the next three years, we will fully characterize the durability of these hybrid polyimide resins and carbon-fiber reinforced composites with respect to time-temperature-stress-service environments (moisture, chemicals and oxidative stability). This characterization will be conducted on the molecular, microscopic and macroscopic levels in order to develop structural-performance relationships for structural design analyses and associated materials structural optimization at all dimensional levels. Additionally, we will also explore and formulate POSS-containing polyimide resins to take advantage of the state-of-art, economical composite processing techniques such as resin infusion, resin transfer molding and vacuum-assisted resin transfer molding.
Inorganic-Organic Hybrid Polymers Linear inorganic-organic hybrid polymers are receiving increasing attention, particularly in an effort to better understand performance enhancement that enable efficient material design for specific applications. Linear hybrid polymers encompass a materials chemistry approach distinct from past efforts in sol-gel systems, which often yielded cross-linked systems. Moreover, the motivation for the study of linear hybrid polymers is the increasing level of supporting evidence for nanoscale reinforcement of material properties and alteration of chain and segmental dynamics. Specifically, we investigated polymers with chemical incorporation of polyhedral oligomeric silsesquioxane (POSS) to the backbone of linear polymers. A typical POSS macromer is a well-defined cluster with inorganic silica-like core surrounded by organic corner groups. The efficiency in property enhancement depends on complex interactions between POSS-POSS and POSS-polymers. We are also interested on how POSS macromer affect the kinetics of polymerization.
Molecular Dynamics in Polymeric Glass Transition Unlike small molecules, the glass transition in polymers is influenced by the long-chain nature of polymers. Using model thermoset systems, we are currently investigating the effect of molecular network structures on the glass-like behavior of polymers. More specifically, we are interested in the influence of molecular network connectivity on the structural recovery processes that occured during physical aging of polymer glasses.
Structure-Property Relationships of Polymer Nanocomposites The goal of this research program is to understand the effect of a new class of structurally well-defined, nanometer scale, molecular clusters on long chain polymers. Using polysiloxanes as model system, we are able to demonstrate a significant enhancement in the association of polymer chains for system containing molecularly dispersed nano-clusters. Furthermore, it is possible to control the rate of enhancement by altering the chemistry of these nanometer molecular clusters. This new concept of nano-reinforcement is to be extended for other polymers in order to examine the general applicability for all polymer systems.
View My Extended Publications Reviewed Journal Publications Lee, A.; Xiao, J; and Feher, F.J., "New Approach in the Synthesis of Hybrid Polymers Grafted with Polyhedral Oligomeric Silsesquioxane and their Physical and Viscoelastic Properties", Macromolecules, 38, 438, (2005).
Lee, A.; Subramanian, K. N. "Development of Nano-Composite Lead-Free Electronic Solders," of Electronic Materials, 34 (11) 1399-1407 (2005).
Rhee, H.; Subramanian, K. N.; Lee, A. "Role of Imposed Cyclic Straining on the Stress Relaxation Behavior of Eutectic Sn-3.5Ag Solder Joints," Journal of Materials Science: Materials in Electronics 16, 169-176, (2005).
Sonje, P.U.; Subramanian, K.N.; Lee. A. "Characterization of Polymeric Composites with Low CTE Ceramic Particulate Filler," J. of Adv. Materials 36, 22, (2004).
Sonje, P.U.; Subramanian, K.N.; and Lee, A.,"Characterization of Polymeric Composites with Low CTE Ceramic Particulate Filler", J. of Adv. Materials, 36, 22, (2004).
Fu, B.X.; Lee, A.; and Haddad, T.S., "Styrene-Butadiene-Stryene Tribolck Copolymers Modified with Polyhedral Oligomeric Silsesquioxanes", Macromolecules, 37, 5211, (2004).
Fu, B.X.; Namani, M.; Lee, A. "Influence of Phenyl-Trisilanol POSS on Properties of Epoxy Network Glasses", Polymer 44, 7739, (2003).
Rhee, H.; Subramanian, K.N.; Lee, A.; Lee, J.G. "Mechanical Characterization of Sn-3.5Ag Solder Joints at Various Temperatures", Soldering and Surf. Mount Tech. 15, 21, (2003).
Conference Proceedings Lee, A.; Feher, F.J.; Brem, G.F.; and Barber, J.D. " BREAKTHROUGH POLYMER FINISHING TECHNOLOGY:Manufacturing Productivity and Polymer Performance Enhancements",SPE-ANTEC 2003 Proceedings, (2003).
Other Works Lee, A.; Lichtenhan, J.D.; Schwab, J.J., and Phillips, S.H., "Nanostructured Chemicals as Alloying Agents in Polymers", U.S. Patent # 6,716,919 (April 2004).
Lee, A.; and Campbell, S. "Hybrid Thermoset Polyimides and their Composites", NASA-GRC Technical Report (Sept. 2004).
A. Lee, "Processing and mechanical properties of POSS-BMI carbon fiber reinforced composites", AFRL Report-2004.
Lee, A. "Processing and mechanical properties of POSS-BMI carbon fiber reinforced composites", AFRL Report-2003.