Ronald Hedden’s Research, Teaching & Outreach Activities
Materials Science and Engineering
Updated August 2008
Office 325C Steidle Building *** Phone 814-863-2325 *** Fax 814-865-2917
Polymer Science and Engineering: Networks, Gels, and Elastomers
Current research activities in our group span a variety of polymer science and engineering topics, but a common theme is the study of lightly crosslinked polymer networks, including gels and elastomers. Our group is well versed in organic and polymer synthesis, but our work focuses upon relating nanometer-scale structure and morphology to macroscopic mechanical behavior in crosslinked polymer networks. Characterization of structure and morphology by techniques such as NMR, FT-IR, X-ray diffraction (WAXS, SAXS) and small-angle neutron scattering (SANS, USANS) is a strength of our group.
Current & Recent Graduate Students:
Dr. Harshad Patil (Ph.D. 2008, currently at Cardinal Glass Industries)
Ms. Burcu Unal (Ph.D., anticipated 2008)
Mr. Daniel Lentz (Ph.D., anticipated 2010)
Current Undergraduate Researcher:
Ms. Morgan Iannuzzi (Soph., WISER Program participant)
Current & Past Sponsors
Mechanical Phenomena in Liquid Crystalline Elastomers
Liquid crystalline elastomers (LCE) are semi-flexible polymer networks characterized by spontaneous segment-level ordering. Spontaneous orientational (and sometimes positional) couplings between neighboring chain segments in LCE perturb the statistics of elastic network chains, which adopt oblate or prolate ellipsoidal conformations as opposed to isotropic random coils. LCE therefore exhibit strong deviations from ordinary rubber elasticity, leading to striking physical phenomena such as the polydomain-to-monodomain (P-M) transition and the spontaneous shape changes observed in globally oriented "monodomain" samples. The macroscopic mechanical response of LCE under applied stress is of primary fundamental interest, as it is the unique mechanical behavior of LCE that distinguishes them from all other soft materials. LCE have attracted interest as materials for both vibration-damping coatings and soft actuators ("artificial muscle").
Much of our work involves smectic LCE, which spontaneously form lamellar mesophases (with d spacings of 20 to 50 Å) due to short-range repulsions between hard segments (mesogens) and soft, flexible connectors. Recent work in our group revealed that smectic elastomers of the "main-chain" type, which have rigid mesogens embedded directly into the backbone of the elastic network chains. The constraints imposed by segment-level layering govern the mechanisms of stress relaxation and endow smectic elastomers with dynamic mechanical properties that distinguish them from all other elastomers, including nematic LCE. We have recently uncovered new insights regarding the influence of dangling chains and othe rimperfections on the vibration damping properties of smectic LCE. In addition, we have advanced basic understanding of the "polydomain-monodomain" transition in smectic LCE through mechanical and X-ray difraction studies.
Recent publications in this area
H.P. Patil and R.C. Hedden. "Comments on Mechanism of Polydomain-Monodomain Transition in Main-Chain Smectic Elastomers." 2008 (submitted).
H. P. Patil, J. Liao, and R. C. Hedden, "Smectic Ordering in Main-Chain Siloxane Polymers and Elastomers Containing p‑Phenyleneterephthalate Mesogens." Macromolecules 2007, 40, 6206-6216.
H.P. Patil and R.C. Hedden. "Effects of Structural Imperfections on the Dynamic Mechanical Response of Main-Chain Smectic Elastomers." J. Polym. Sci. B: Polym. Phys. 2007, 3267-3276.
Figure 1. Left: Optical birefringence micrographs showing “necking” instability during polydomain-monodomain transition in a smectic main-chain LCE. Right: observed mechanical response vs. draw ratio (lambda) in uniaxial elongation (linear strain ramp).
Ionizable Hydrogels: From Basic Physics to Bio-energy Applications
Ionizable hydrogels are loosely crosslinked, water-swollen networks that contain some fraction of chemical units which can become ionized over a range of pH and/or temperature. We are studying novel hydrogels that contain ionizable dendrimers as crosslink junctions. These hydrogels swell up to 1000 times their dry volume in neutral water, but collapse or shrink dramatically at either high or low pH, due to the presence of ionizable amine functional groups in the dendrimers. Recent work in our group explored the structure and gelation of networks prepared by end-linking neutral, linear polymer chains by reaction with the dendrimer endgroups. In addition, we have recently shown that the unusual pH-dependent swelling of these networks can be captured by the Donnan equilibrium theory.
Figure 2. Bonding architecture of a 2nd Generation dendrimer with
64 endgroups. We use ionizable dendrimers as crosslinkers
to produce hydrogels that undergo pH-dependent swelling.
Figure 3. pH-dependent swelling of a poly(ethylene glycol)
(PEG) hydrogel that contains a high concentration of polyamidoamine (PAMAM) 2nd generation (G2) dendrimers with ionizable amine endgroups.
Figure 4. Theoretical predictions for pH-dependent swelling of
the ionizable hydrogel characterized in Fig. 3. The Donnan
equilibrium theory was invoked, with the elastic free energy
change calculated by the Flory-Rehner expression for a “phantom”
network. (Green and brown symbols are two roots of the eqn.
governing acidic conditions; blue symbols are for basic conditions.)
During swelling in water, development of micro-cracks and pores in the surface of these gels generates an inhomogeneous, permeable soft solid environment that can efficiently support growth of microorganisms or other cells. These highly swelling, biocompatible polymer networks therefore have applications as tissue engineering scaffolds and growth supports for bacteria. Although this work had its roots in basic polymer science, it has now developed into an exciting new biotechnology project. We recently designed a microbial bioreactor system based upon a hydrogel-packed fluidized bed, aimed at biological production of hydrogen gas from sustainable biomass sources such as glycerol.
Figure 5. Hydrogel bioreactor schematic. Hydrogen-producing bacteria are supported within the surface of porous hydrogels that
allow exchange nutrients & wastes with the surroundings under continuous flow conditions.
Recent publications in this area
B. Unal and R. C. Hedden. “pH-Dependent Swelling of Hydrogels Containing Polyfunctional Amine Crosslinkers.’ 2008 (In Preparation).
B. Unal and R. C. Hedden. "Gelation and Swelling Behavior of End-linked Hydrogels Prepared from Linear Poly(ethylene glycol) and Poly(amidoamine) Dendrimers." Polymer 2006, 8173-8182.
MatSE 444: “Solid State Properties of Polymers”
(3 credits; offered in Fall Semester)
Among the many instrumental techniques covered in MatSE 444,
X-ray diffraction is one of the most powerful techniques available for
analyzing structure and morphology in semicrystalline and liquid crystalline polymers.
(3 credits; offered in Spring Semester)
Service & Outreach
Graduate students in our group also contributed to the WISER program (Women in Science and Engineering Research) for the first time in Spring 2008. A freshman undergraduate researcher (Ms. Morgan Iannuzzi) completed her first laboratory research project on rubber-like polymer networks and gels. She will return to our group in the fall to continue her research experience.
Dr. Hedden examines a new polymer sample with the help of several high school outreach students.
Dr. Hedden (blue shirt) and graduate student Harshad Patil (right) run a laboratory demonstration in "Polymer Processing" class.
Our group contributes to a diversity-focused summer outreach program for Pennsylvania high school students called SEEMS (Summer Experience in Earth and Mineral Sciences), which provides hands-on research experiences directed by faculty in the College of Earth and Mineral Sciences as part of the Upward Bound Math and Science program, including an intensive six-week education and research program each summer. Students' summer experiences involve interaction with Penn State faculty and graduate students, classroom instruction, hands-on training with computers and laboratory equipment, and most importantly, an introductory research project. Dr. Hedden and his graduate students served as mentors in the SEEMS program during 2004, 2005, and 2007.
MATSE 448 / Ch. E. 442 covers the interrelations between structure, processing conditions, and physical properties of industrial polymer products. Students apply engineering fundamentals and principles of polymer melt rheology to analyze industrial processing operations, learning how to optimize processing variables given a particular set of materials and conditions, and how processing conditions impact the physical properties of finished polymer products. We explore the physics governing processing operations including extrusion, mixing, calendering, blow molding, thermoforming, fiber spinning, compression molding, injection molding, plastic recycling, and nanolithography.
MatSE 444 explores relationships between structure and properties in the bulk solid state of polymers. We address the structure and characterization of glassy and semicrystalline polymers, and learn fundamentals of thermal analysis, X-ray diffraction, and FTIR spectroscopy.
** Review of States of matter; glass, crystallization and melting transitions.
** Morphology in semi-crystalline polymers from simple single crystals to shish-kebabs (row nucleated) and the procedures that produce these structures.
** Crystallization theories, with emphasis on the kinetic theory. Molecular level descriptions of crystallization processes and the annealing process.
** Rate of crystallization from the Avrami perspective; nucleation, growth, and 'secondary crystallization'.
** The influence of various parameters on rate including temperature, molecular weight, molecular structure and orientation.
** Definition of the degree of crystallinity, and measurement of how much crystallinity is present by various techniques including thermal analysis (DSC and DTA).
** Measurement of the heat of fusion and crystal melting point (Tm), and testing for reorganization/annealing effects.
** The thermodynamics of melting applied to polymer crystals. Melting points of homopolymers and copolymers.
** Orientation in semicrystalline polymers: defining, measuring and describing orientation with a variety of methods including WAXS and IR dichroism.
** The glass transition temperature (Tg) and the concept of "free volume." The influence of parameters including plasticizers, copolymerization, and molecular structure on Tg.
** Selected topics from: Temperature Rising Elution Fractionation (TREF).
Barrier properties of polymers, packaging and diffusion though thin films.