We study the rheology and structuring of complex polymeric materials under well defined experimental conditions. These include extreme conditions in time, pressure, temperature, and molecular ordering. The current research focuses on two themes.
Theme 1: Experimental Polymer Rheology
Breaking and Healing of Associative Network Polymers The growth of connected structure (called ‘gelation’) and its opposite, the decay of connectivity (called ‘reverse gelation’), are common phenomena in many applications of polymeric materials (drug lease, ordered self-assembly, sealants, mechano-chemistry). While gelation typically is a slow process, reverse gelation occures rapidly and is difficult to control. Under suitable conditions, reverse gelation is followed by gelation (healing of broken structure). This interplay of gelation, reverse gelation, and gelation again is under investigation with rheo-optical methods (using model polymer systems). The rate of healing depends on the bonding mechanisms of the physical gel, chemical composition, and on the molecular mobility.
Long-Time Relaxation and Aging We recently completed the ‘Relaxometer’ that allows relaxation measurements over years without any zero drift in the force transducer (purpose: characterization of foams and of physical gels). Long time experiments are in progress on model samples.
Physical Gelation of Semi-Crystalline Polymers The liquid-to-solid transition at early stages of crystallization of polymers may be viewed as physical gelation. The physical gel point manifests itself by slow power law relaxation dynamics. Gelation responds very sensitively to small differences in chemical composition or in molecular architecture. Processing near the gel point magnifies these differences even further. Strongly affected are the end-use properties (mechanical, barrier, optical, etc.). We study time-resolved rheology in combination with static light scattering, optical microscopy, and DSC. We recently succeeded in stopping the crystallization near the gel point of a linear low density polyethylene and began exploration of gel point properties.
Rheology of Liquid Crystalline Polymers Current interests are on shear-induced molecular rotation and its effects on rheology. Rheo-optical studies are under way. For that purpose we built a miniature shearing device for the conoscope. It allows simultaneous observation of mechanical and optical properties of transient structures.
Patterns of Polymer Dynamics Polymeric materials, depending on their molecular and supramolecular structure, exhibit very specific relaxation patterns. We are exploring the linear viscoelasticity of model materials and search for underlying patterns in their relaxation behavior. Such patterns have been found for specific classes of polymers: soft materials at the gel point (CW-spectrum) and polymer melts with long linear, flexible molecules of uniform length (BSW-spectrum). The search for patterns makes use of the IRIS software (http://rheology.tripod.com/) which is designed to analyze experimental data and to compare to theory.
Rheological Methods In addition to all the standard rheological techniques, we perform rheo-optics (polarizing microscope, small angle light scattering, conoscopy) during shear, tack testing at controllable speed, micro-rheometry, shearing in an electric field, in situ fluorescence during shear, and biaxial extension (lubricated squeezing). We have been developing methods for time-resolved mechanical spectroscopy (TRMS) which is a sensitive measure of molecular changes with time. Because of its spectroscopic nature, TRMS is ideal for characterizing transient polymeric samples. A wide range of frequencies need to be applied either simultaneously or sequentially. Each TRMS data point represents a different state of the material and interpolation is necessary to obtain the desired data as a function of frequency at distinct material states.
Theme 2: Novel Polymeric Materials Through Processing
Solvent-Free, Open-Pore Polymers from Gel Processing We discovered a fundamentally new and environmentally benign technology to create ultra-clean, open-pore morphologies in polymers as needed for applications such as chromatography, filtration, or tissue scaffolds for cell growth. A structure with open pores self-assembles naturally when crystallizing a highly swollen polymer gel. More importantly, the resulting pore structure affords a unique combination of desirable properties such as high void fraction, high purity surface, mechanical strength, and retention the shape of a polymer specimen while increasing its size (and lowering its average density). The new concept has been demonstrated with polyethylene and polypropylene, some of the least expensive commercial polymers. The technology applies to crosslinked polymers with crystalline blocks.
Field-Induced Ordering in Polymer Suspensions Suspensions of polarizable particles in a non-conducting liquid (polymer) self-assemble into anisotropic structures when subjected to an electric field. This phenomenon is used in an attempt to fabricate thin membranes for gas separations. The ordering process results in a liquid-to-solid transition. Increased connectivity reduces mobility and intercepts the ordering process. Intermediate structural states are conceivable in which the anisotropic material will appear solid when probed in one direction while still appearing liquid-like in the other direction due to its anisotropic connectivity. Such anisotropic solidification process may be called ‘directional gelation’.
Theme 3: Cyber Infrastructure Initiative for Rheology
We developed a new cyber infrastructure for rheology (CIR) that has the potential of integrating the diverse rheological knowledge of experts around the world. In a multi-disciplinary effort, experts in specialized topics of rheology began to write CIR-modules that seamlessly merge into a general code so that it can be used by a wide range of engineers and scientists. At the center of CIR is a platform operating system that connects a wide range of dedicated software modules. These CIR-modules perform calculations and return the corresponding results to a central graphics screen. The computer platform allows the detailed analysis of experimental data, the communication of data, and the prediction of rheological material functions from a wide range of theories in rheology. Rheologists can access each other’s experimental results, make predictions with each other’s theories and simulate with each other’s computer codes. Through such collaboration, seemingly disparate theories and experimental observations can be linked and taken to their limits, thereby leading to unexpected insights and new questions. Beyond the pool of experts, CIR will draw industrial users into the rheology discussion. Easy-to-use CIR-tools will allow industrial rheologists to adjust rapidly to the changing needs and the pressure to obtain short-term solutions in a competitive environment. CIR has the potential of generating ideas for novel materials and novel manufacturing methods. At the same time, it will supply the tools to examine ideas quantitatively and to push these ideas even further. User-friendly methods are essential not only for research and application, but also for the teaching of rheology. We envision tools that allow a student to move seamlessly and rapidly between experimental data and the most advanced rheological theories, simulations, and modeling of applications. In-depth data analysis and evaluation of theory should become easy enough to be performed after reasonable training and without relying on over-simplifications. This will enable the student to reach a deeper understanding of rheology and to appreciate the significance that rheology has in technical applications. Even untrained talent may get introduced quickly into advanced concepts of rheology.
