The overarching research objective in our laboratory is to understand the relationship between the deformations imposed during most conventional polymer processing (e.g., extrusion, injection molding, film blowing), the resulting microstructure or morphology, and macroscopic material properties. As rheologists, we focus primarily on the link between deformation and microstructure, which is the critical step in imbuing a material with particular properties.
Specific research projects ongoing on our lab range from the fundamental to the more applied; regardless of the proximity to application, however, an emphasis is placed on understanding fundamental mechanics. That is, the physics inspired by industrial application are of greater interest than the application itself. Ultimately, Ph.D. students in our group possess a solid foundation in rheological theory, polymer science, and suspension mechanics.
Polymer nanocomposites have become a topic of preeminent interest in our group. This class of materials is related to filled polymeric materials, which have seen widespread use for decades (e.g., carbon black-reinforced rubbers, carbon fiber-reinforced polymers). Traditional filled polymers are characterized by:
◆ Increased stiffness, strength, and dimensional stability.
◆ Increased toughness or impact strength.
◆ Increased heat distortion temperature.
◆ Reduced gas permeability.
However, negative attributes are:
◆ High weight fractions (20 – 40 wt%) required for property enhancement.
◆ Particulate aggregation is frequently a problem.
◆ Embrittlement and poor extension properties are prevalent.
One strategy for addressing these negative features is to shrink the size of the particulate phase, since the surface area to volume ratio increases for virtually any shape imaginable as the characteristic dimension is decreased. By increasing the ratio of surface area to volume, the interaction between the particulate species and the polymer may be promoted even while the overall weight fraction of particulate is decreased. Furthermore, by reducing the weight fraction of the filler, the overall weight of the product is reduced, which is of particular relevance in the manufacture of automobile fascia. In short, a shift to nanoscale fillers (i.e., polymer nanocomposites) offers the potential to improve numerous material properties with fewer negative consequences.
One class of nanoscale filler receiving a great deal of attention is montmorillonite clay, which consists of layered planar silicates 0.98 nm thick and 300 – 1,000 nm across separated by a van der Waals interlayer. Montmorillonite possesses a large aspect ratio and hence an exceedingly high surface area to volume ratio. If the individual silicate layers are fully dispersed (or exfoliated), property enhancements may be achieved with an order of magnitude lower filler content (as low as 2 – 5 wt%) than traditional microscale fillers. Even without a detailed understanding of the physics underlying property enhancements in polymer-clay nanocomposites (PCNs), they have found widespread commercial use in applications as diverse as automobile fascia, cargo beds, and seat backs, plastic beverage bottles, plastic barrier films, and electrical storage units just to name a few.
Central to the positive and negative property changes are critical molecular considerations, specifically the interfacial area between dispersed (i.e., particulate) and matrix (i.e., polymer) phases and the thermodynamic interactions between the dispersed and matrix phases. The intermediate link between these molecular considerations to macroscopic material properties like those mentioned above is the shape, distribution, and microstructure of the dispersed phase. As we are particularly interested in these characteristics, we pose broad engineering questions such as:
◆ What factors contribute to property enhancements in PCNs?
◆ How does the surface chemistry between the clay silicate layers and the polymer matrix influence clay silicate dispersion?
◆ Since PCNs are ideally processed like traditional polymers, what role does the deformation and thermal history play?
In addition to polymer-clay nanocomposites, we are also interested in the use of carbon nanotubes (CNTs) as nanoscale fillers, and the same fundamental engineering questions can be posed for that material system. The study of CNTs is also complicated by strong van der Waals attractions between individual tubes as well as the challenges associated with synthesizing and purifying nanotubes of consistent lateral and longitudinal dimension, factors that inevitably influence morphology and thereby the rheological response.
Finally, our group also has a strong interest in polymer crystallization. Semi-crystalline polymers are found among the most common commercial polymers (e.g., polyethylene, polypropylene, polyethylene terephthalate). Macromolecular crystallization is a frustrated process, however, in that individual polymer chains must diffuse along their length in a sea of neighboring chains in order to adopt a favorable conformation state and trigger a nucleation event. As an aside, nucleation is itself a poorly understood process that may require two or more chain segments suitably aligned relative to one another and a topic of interest in its own right. In any event, in addition to the time scale associated with polymer diffusion is the time scale for solidification of the material, which depends upon the degree of subcooling relative to the equilibrium melting temperature of the polymer. For large subcoolings, a sample may be characterized by a low degree of crystallinity (and a high degree of amorphous phase), because the time scale for solidification is much faster than that for polymer diffusion. Since macroscopic material properties depend strongly on the degree of crystallinity, the ability to tune crystallinity is a critical consideration.
In our group, we are particularly interested in flow-induced crystallization, where a sample has been subjected to the high stresses characteristic of industrial processing. Here, one commonly observes crystallization kinetics three or four orders of magnitude in time faster than that seen in quiescent crystallization. Also, in some cases, oriented crystallites form in the regions of highest stress. It is important to note that the process of polymer crystallization spans a wide range of length scales, from the molecular nucleation event to nanoscale crystals and crystalline lamellae and microscale spherulitic structures. Consequently, a battery of experimental techniques is required to probe structure at those length scales, including wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), transmission electron microscopy (TEM), polarizing optical microscopy (POM), and birefringence.
Our polymer crystallization efforts include studies of polypropylene-clay nanocomposites as well as a joint study with Cornell and ExxonMobil on the role of polymer molecular architecture (e.g., stars, H-polymers, combs, long-chain branched polymers) in flow-induced crystallization. These more complex polymer architectures are subject to different relaxation mechanisms, but their dimensions may be selected such that their longest relaxation times are approximately the same. Thus, we may isolate the influence of architecture from that of relaxation dynamics. A key instrument in these studies is a custom-built mini-extrusion device, which allows us to impose well-defined shear pulses of finite duration and control the thermal history of the sample. By imposing finite, high shear stress pulses, we are able to decouple the effect of flow in fomenting polymer orientation, which contributes to the formation of crystallites, from the tendency of the flow to orient already formed crystallites.
The above is a rough overview of current activities. More information can be found in the section on “Current Research Projects.” In summary, however, we believe that exciting opportunities may be found in the study of these advanced polymeric materials and hope that you share some of our enthusiasm.
Our Research Interests
Advanced Nanostructured Fluids and Polymer Processing
Oberhauser Research Group
Our Current Research Group