Ronald Hedden
Ronald Hedden Assistant Professor of Materials Science and Engineering, has a wide range of materials research interests including synthesis and characterization of polymer networks and gels and development of polymers for microelectronics applications. Dr. Hedden's group currently includes two graduate students and three undergraduate researchers. Hedden's group synthesizes well-defined "model" polymers and then characterizes their physical and engineering properties to better understand how molecular architecture and nanometer-scale structure impact properties. Characterization techniques used by Dr. Hedden's group include mechanical testing, neutron scattering (using facilities at the NIST Center for Neutron Research), and x-ray diffraction and reflectivity. Although Dr. Hedden's group focuses on basic principles of polymer physics, their fundamental research is key to identifying potential advancements in biomedical and microelectronic applications.
One current research project in Dr. Hedden's group involves liquid crystalline polymers (LCPs), semi-flexible polymers that can exist in one or more partially ordered states called mesophases. Crosslinked networks of LCPs, called liquid crystalline elastomers (LCE), exhibit unusual mechanical properties due to coupling between mechanical fields and the ordering of the polymer chains in the mesophase. The polymer chains in a LCE may orient spontaneously in response to changes in temperature and/or stress, leading to phase transitions, shape changes, and discontinuous mechanical properties, as illustrated in Figure 1. Three types of LCE are known, differentiated by presence and location of rigid structural units called mesogens. Main-chain LCEs have mesogens embedded in the polymeric backbone; side-chain LCEs have pendent mesogens; and polydi(n-alkyl)siloxanes have no mesogens. In each case, the driving force for spontaneous chain alignment is an orientation-dependent energetic coupling between neighboring chains that exceeds the entropic penalty for alignment, but the molecular basis for this energetic coupling is different in each case. Dr. Hedden's current research involves characterizing the properties of main-chain LCEs with well-defined molecular architecture. An understanding of molecular interaction within liquid crystalline elastomers will encourage development of applications including stress-optical devices, vibration-damping coatings, and membranes for gas separation.
Figure 1. Optical birefringence micrographs (x10) of a strain-induced amorphous to mesophase transition in a (polydiethylsiloxane) (PDES) elastomer. Like the flexible-chain PDES elastomer shown, main chain and side-chain LCE exhibit intriguing mechanical phenomena.
A second research project in Dr. Hedden's group concerns hydrogels, water-swollen polymer networks suitable for emerging biomedical applications. Current tissue engineering research uses hydrogels as scaffolds to support cell growth, and hydrogels are also being studied for drug delivery. While gels are already available for these applications, the molecular-level architecture of many useful materials is poorly defined, which obscures the relationship between structure and function. Improved control over gel molecular architecture could permit advancement of existing technology and the design of new biomedical materials. Endlinking, a reaction in which polymers with reactive endgroups form a network through reaction with a polyfunctional crosslinker, is the premier technique for preparing polymer networks of well-defined architecture. Endlinked polymer networks are the best materials available to establish connections between network architecture and physical properties. Compared to conventional gels, endlinked gels allow increased control over gel structure on the sub-micrometer length scales relevant to biological processes. By introducing biologically active macromolecules, endlinked hydrogels may be modified for applications in tissue engineering or drug delivery. Currently, Dr. Hedden studies the structure and mechanical properties of endlinked poly(ethylene glycol) (PEG) hydrogels. These hydrogels are made by crosslinking end-functionalized PEG, a biocompatible synthetic polymer, with dendrimers, a kind of layered, treelike macromolecules prepared by alternating addition of two different monomers to a branched core. Because dendrimers can have many reactive endgroups, they are well-suited for use as crosslinkers in networks and gels. These dendrimer-gels can have excellent mechanical properties and unusual crosslinking kinetics, potentially making them useful as injectable materials for drug delivery or tissue engineering applications. Because PEG hydrogels are usually brittle, their use in practical applications is limited. Hedden's group is systematic studying crosslinking architecture and mechanical properties with the goal of improving the mechanical behavior and transport properties of PEG hydrogels.
Prior to joining the Penn State faculty in August 2003, Dr. Hedden was an NRC/NIST Postdoctoral Research Associate in the Polymers Division of the National Institute of Standards and Technology, Gaithersburg, MD from 2000 to 2003. He earned his doctorate in Chemical Engineering at Cornell University in 2000 and his B. S. in Chemical Engineering at Penn State in 1995.
