RESEARCH
Vacancies : There are a number of positions available at both the graduate and undergraduate level. Following, please find descriptions of on-going and potential research projects. As new projects become available, they will be posted here.
Interested candidates please send resume to Prof. Ortiz
                         The focus of our research is to study the precise natural engineering found in hierarchical macromolecular systems, and to determine how these design concepts produce materials with unique, improved, and highly specific mechanical properties. The general classes of systems of interest include: proteins, polysaccharides, synthetic polyelectrolytes, liquid crystal polymers, and synthetically nanostructured materials. One subset of this work includes the investigation of how defects disrupt hierarchical structure and cause mechanical malfunction in biological structures and organisms. Ultimately, we would like to employ hierarchical engineering ideas to develop new biologically-inspired materials technologies, possibly high performance adhesives, coatings, membranes, and films for applications in industries such as electronic packaging, biomaterials, ophthalmics, etc.

           
                         How are organized polymeric structures produced? Typically, nature begins with non-random coil polymer chains (e.g. a-helices, b-sheets, folded globular domains, etc.) by incorporating weak, noncovalent, intramolecular interactions (e.g. electrostatic bonds, water bridges, hydrogen bonds, van der Waals forces, etc.). Multiple intermolecular interactions of similar origin cause self-assembly to successively larger length scales (*Figure 1.). An enormous range of structure and function can be achieved through the repetitive use of similar molecular constituents, controlled orientation of structural units, complex shapes, and sensitivity to water, pH, ionic strength, and temperature.

              Why are hierarchical materials interesting? Besides specificity and versatility, organized macromolecular systems generally have superior mechanical properties compared to their amorphous, random coil counterparts for a number of reasons. For example, stresses are distributed through different levels, thus minimizing stress concentrations, individual elements fail at different levels preventing catastrophic failure, crack growth is directed and arrested by different interacting components at each length scale, and localized slippage and voiding between elements can induce a variety of energy-absorbing mechanisms leading to increased fracture toughness.

Figure 1.Structure and self-assembly
of the helix-forming polysaccharide,
k-carrageenan
           
                         Our research approach is to investigate these materials from three distinct length scales: molecular (~nm), microscopic (~mm), and macroscopic (~mm) (Figure 2.). In this way, we are provided with information on the magnitude and nature of the noncovalent interactions holding together the self-assembled polymeric structure at each level. For each system studied, the primary goal is to establish and interrelate the structure-property relationships at each length scale, so as to bridge the gap from single molecules to bulk engineering structures.

          
          

Figure 2. Summary of general research approach and corresponding subtopics
           
          
I. Molecular Elasticity (~ nm)

              Intramolecular Interactions. At the nanometer scale, the traditional concept of "structure-property relationships" takes on a whole new meaning, i.e. the controlling parameters now being chemical structure and the corresponding intramolecular interactions which determine chain conformation, chain stiffness, and chain geometry. It is well-known that subtle molecular changes can result in drastic changes in microscopic and macroscopic mechanical properties. In addition, molecular elasticity plays a fundamental role in many nanometer-scale biological processes.

          
                         Recently, it has become possible to measure the elasticity of individual polymer chains using a variety of high-resolution force spectroscopy techniques, such as atomic force microscopy (AFM). At low to moderate extensions, most polymer chains behave as ideal, entropic, random coils; i.e. molecular rubber bands. This is shown in Figure 3., which displays AFM data (retraction force, Fchain, versus chain end-to-end separation distance) for stretching and uncoiling of single polystyrene chains of different lengths. By fitting experimental data with theoretical polymer physics models of freely-jointed chains (red lines in Figure 3.) or worm-like chains, we can estimate the "statistical segment length" or local chain stiffness and use this parameter as a probe of chemical structure and local environmental effects (e.g. electrostatic interactions, solvent quality, etc.). In addition, force spectroscopy can be used to measure noncovalent, physisorption forces of single polymer chains on surfaces and covalent bond strength (chain "fracture").
Figure 3. Experimental data for numerous force
spectroscopy experiments (AFM) on
single polystyrene chains in toluene
(each peak corresponds to stretching of
a single chain) compared to the
freely-jointed chain model;
statistical segment length = 0.68,
n = number of segments
          
                        For non-random coil polymers, significant deviations from ideal behavior are frequently observed at moderate to large strains due to the intrinsic elasticity of their organized, supramolecular chain architectures. Such events are unique to each chemical structure and include, for example: elastic deformations of the backbone, strain-induced conformational transitions, and mechanical denaturation. More information on single molecule force spectroscopy can be found on our Nanomechanics Web site or in our recent paper : Macromolecules, 1999, 32, 780-787 which can be downloaded here.

          
Current interests in this area include :
           · effect of electrostatic interactions on entropic elasticity of polyelectrolytes
           · effect of complexation with small molecules and surfactants on single chain elasticity
           · non-ideal elasticity of non-random coil polymers
           · cooperativity of deformation
           · mechanical hysteresis and rate-dependence of deformation
           · fracture of single polymer chains
           · physisorption of single polymer chains to surfaces
           · development of analysis software for modeling of single molecule force spectroscopy data
           · development of high resolution force spectroscopy techniques
          

                        Intermolecular Interactions. High resolution force spectroscopy techniques can also be used to the mechanical properties of multiple polymers chains, e.g. polymer "brushes," adsorbed polymers layers, and polymer melts. Recently, we initiated a project to study the nanomechanical properties of a synthetic polyelectrolyte complex formed between brushes of poly(methacrylic acid) (PMAA) and poly(ethylene glycol) (PEG). Figure 4.(a) displays a typical force spectroscopy experiment (conducted with the AFM) at low pH, where complexation occurs via H-bonds between the -COOH groups of the PMAA and the -O- of the PEG (and is also further stabilized by hydrophobic interactions).           
          
Figure 4. (a) Force versus distance curves for end-grafted PMAA with a PEG-coated AFM probe tip at pH=2 (b) mean and standard deviation of maximum attractive force in force spectroscopy experiments as a function of pH
          
                         Upon retraction of the AFM probe tip from the surface, large, long-range adhesive forces are observed (Fadhesion =5 nN), indicating a strong complexation interaction between the polymer brushes. The force curve allows us to follow in detail the deformation and gradual rupture of the complex as its being stretched. Figure 4.(b) shows a dramatic change in the mean maximum adhesive force as a function of pH. Here we see that at high pH there is negligible adhesion because the complex is disrupted due to ionization of the PMAA carboxylic acid groups.           
          
Current interests in this area include :
           · deformation and rupture of polymer complexes
           · effect of magnitude and type of intermolecular interactions
           · chain disentanglement forces of polymer brushes
           · rate-dependence of complexation rupture and disentanglement forces
           · comparison with biomolecular adhesion (e.g. biotin-streptavidin)
          

           II. Microdeformation (~ um)

          
                         An elegant and yet, relatively simple technique to investigate the microdeformation and fracture of thin polymeric films is shown schematically in Figure 6. Here, a free-standing polymer film (< 1.0 mm thick) is bonded to an annealed, adhesive-coated copper grid. The copper grid and hence, polymer film, is strained in a miniaturized-mechanical testing device fitted with an environmental chamber while simultaneously observing and recording the microdeformation and fracture mechanisms under a polarizing optical microscope (POM). Each individual film square in the copper grid acts as a separate sample that deforms independently of neighboring grid squares. Hence, a large amount of data can be taken with ease and analyzed statistically. The sample can be removed from the mechanical testing device at any time (i.e. the strain will be maintained on the film since the copper grid is plastically deformed) and subject to further analysis, such as higher-resolution imaging using the scanning electron microscope (SEM), transmission electron microscope (TEM), or AFM.
Figure 6. Schematic of microdeformation experiment
           
          
Figure 7. (a) TEM image of a
craze (Miller and Kramer, et al., 1990)
(b) POM image of
a microcrack and shear DZ,
(c) POM image of voiding
around microcrack in LC network
               The method described above has been successfully employed by a number of groups to study synthetic, amorphous, glassy macromolecular systems including homopolymers, blends of homopolymers, random copolymers, crosslinked networks, rubber-modified composites, and nanofoams. In general, plasticity takes place by two different strain localization mechanisms: crazing and micro-shear deformation zones (DZ's) (Figures 7.(a) and 7.(b) respectively), depending primarily on the strand density, n, where a strand is any length of polymer chain bound by two entanglements or chemical crosslinks. It has been shown that these deformation mechanisms are directly related to "strain softening" and "necking" in bulk samples and also, control macroscopic fracture toughness.

              Recently, we conducted experiments on films of a crosslinked liquid crystalline (LC) epoxy network with a unique hierarchical "polydomain" microstructure, which possessed local regions of uniform molecular orientation separated by orientational discontinuities, line defects called "disclinations." Here, we discovered a very different microdeformation and fracture process than that exhibited by non-LC, amorphous networks with the same n (Figure 7.(c)). There was no evidence for localized crazes or DZ's; instead the LC domains failed in an individual and isolated manner, leaving small voids surrounding larger microcracks. These voids relieved triaxial stresses ahead of the crack tip, enabled homogeneous plastic deformation within neighboring LC domains, suppressed brittle, catastrophic film fracture, and thereby led to an extremely high fracture toughness. 		          
           One key conclusion from this study, which further motivates our research in this area, was that n was no longer the main controlling factor in the deformation; now the organized microstructure and parameters describing it (e.g. LC domain size, defect density, etc.) came into play.           
          
Current interests in this area include :
           · stress-induced molecular orientation and birefringence
           · orientation and disruption of hierarchical structures and substructures
           · crack initiation, branching, propagation, size, shape, location, and number
           · strain localization ahead of crack tip
           · application of atomic force microscopy to study microdeformation and fracture
          

           III. Bulk Mechanical Properties (~ mm)

          
                         It has recently been shown that many self-assembled organized polymeric materials have quite remarkable bulk mechanical properties, for example: increased fracture toughness, strain-induced molecular orientational transitions, stress-induced macroscopic orientation, unique dynamic mechanical properties, and high tensile strength (see our papers : Macromolecules 1998, 31 (13), 4074-4088 and Macromolecules 1998, 31 (24), 8531-8539). By comparing bulk mechanical experiments with the microdeformation studies previously described, the s's needed for particular types of deformation can be estimated. Conversely, specific types of microdeformation and fracture mechanisms may be correlated with any unique features present in the s versus e curves.
          
          
Figure 8. (a) Typical s versus e
curve of an amorphous
polymer in uniaxial compression
Figure 8. illustrates a typical s versus e curve for an amorphous polymer. It will be interesting to note deviations from this curve and whether or not they can be correlated directly with microdeformation patterns. For example, one might expect a more pronounced nonlinear yielding regime, stress discontinuities or plateaus, corresponding to the disruption of hierarchical elements, strain-induced conformational transitions of molecular secondary structure, or complete denaturation of secondary structure. By carefully observing the fine details and shape of the s versus e curve in the high e plastic regime, it will provide clues as to whether or not the microdeformation processes are cooperative (all or none) or uncooperative (gradual).           

          
Current interests in this area include :
           · elasticity
           · homogeneous and inhomogeneous (stress softening, necking) yielding
           · strain hardening
           · fracture toughness
           · viscoelasticity, dynamic mechanical analysis
           · stress relaxation
           · hysteresis