R. Byron Pipes
John L. Bray Distinguished Professor of Engineering
School of Chemical Engineering
Forney Hall of Chemical Engineering
480 Stadium Mall Drive
West Lafayette, IN 47907-2100
Functionally Gradient Polyimide Foams with in situ Carbon Nanotube Sensors
The integration of carbon nanotechnology and solid state forming of high temperature polymeric cellular materials has the potential to yield functionally gradient cellular materials with in situ sensing to meet a number of important aerospace applications. Cellular microstructures of high temperature polymers can provide both high specific strength and stiffness in thermal, acoustic and structural applications. In the former case these material systems offer excellent thermal insulation as neat polymers or conduction properties by the incorporation of a carbon nanotube second phase. Through their Raman spectra, the carbon nanotube second phase can also serve as an in situ sensor for the determination of local phenomena such as strain, pressure and temperature in the cellular structure. Functionally gradient cellular structures can be produced through the solid state synthesis of polyimide polymer foams. By controlling the particle size distribution, the powder precursor form of the polyamic acid can be deposited and foamed in place to achieve the layered microstructure and designed gradients in properties of the cellular material. First, a thorough development of the processing-structure-property relationships for these innovative material forms must be undertaken. The proposed work follows earlier work by the author in the solid state synthesis of polyimide foams wherein the kinetics and kinematics of foam formation were studied. The new efforts will focus on development of layered structures in cellular materials wherein the cell size or polymer composition is varied in a prescribed manner. In the next phase, the incorporation of single-walled carbon nanotubes (SWCN) within the cell walls of the cellular structure will be examined by adding and dispersing SWCN in the polyamic acid precursor prior to the imidization step. The production of single polyimide microspheres will provide a platform for development of the sensor technology. Polarized micro-Raman spectroscopy will be utilized to examine spherical deformations due to external loading environments by observing the Raman spectrum. Both laser scanning of cellular structure surface and subsurface sensing via fiber optic connections to microsphere sensors will be examined. These concepts will be integrated in the final step in order to demonstrate the concept of functionally gradient materials with in situ sensing capabilities.
Ultra Lightweight Skin Material Systems for High Altitude Vehicles
The performance of high altitude vehicles is enhanced as vehicle weight is reduced and since the largest fraction of vehicle structure is likely the skin, the benefit in development of ultra-light materials systems is clear. Nature often responds to similar requirements in physiological systems through cellular material structures. The present approach to development of ultra-light materials is to produce cellular polymer structures that can be combined with ultra-thin, high performance fiber composites to produce hybrid materials systems with extraordinary specific strength (strength-to-weight ratio). In a second step will be to explore the concept of encapsulation of helium within the closed cells of the cellular material with the goal of developing material systems that approach ?lighter than air materials.? In order to achieve a significant performance increase, a goal of 200 grams/square meter has been established for the skin material. The development of barrier film technology to be combined with the polymer composite for retention of the small molecule of helium within the skin will also be pursued. The ultra-light materials will be developed from a high temperature polymer with acceptable performance properties above 300 degrees Centigrade. Functionally gradient cellular structures will allow for tailoring the cellular structure to allow for optimum compatibility with the ultra-thin polymer composite, as well as, providing for enhance helium encapsulation.
Recently, polyimide foams made from solid-state solutions of poly(amic acid) have been obtained with the blowing agent used as a solvent such as THF, Glyme or Dioxane, (that complex by hydrogen bonding to the diacid-diester and later to the poly(amic acid) structure while serving as the solvent for the poly(amic acid) formation. The presence of the complexing solvent allows for a more controlled reaction until optimum foaming conditions are developed.
This method has also been used to produce hollow polymeric microspheres of diameters from 100 to 1500 ?m. Another of the advantages of the solid phase process over the earlier approaches is the potential to foam in place where a prior foam exists so in-situ repair of existing foams is possible, while another advantage is the production of hollow microspheres. A more recent modification to this process consists of the partial inflation of the powder into friable balloons by a short heating cycle, avoiding imidization and blowing agent depletion. These friable balloons with remaining blowing agent and low molecular weight serve as the starting component to make foams. Foams made from friable balloons have reached high closed cell content (i.e. approximately 80% in foams of 0.048 g/cm3).
Carbon Nanotube - Fluid Interactions in Mixing and Comminution
The potential of Raman spectroscopy as a tool to determine the state of deformation and orientation of a carbon nanotube has been proved theoretically and experimentally for solids but literature is not abundant on fluids. We have been able to obtain reproducible spectra from dispersions of CNTs in quiescent organic solvents typically used in mixing. Some issues like the minimum detectable concentration and optimum laser frequency are being investigated experimentally with promising results. Ultimately that will open a way to compare and contrast different mixing protocols and methods. On the other hand, ultrasonication is being used extensively by the research community working with carbon nanotubes as a mixing tool. However there is a lack of fundamental understanding of its effects on dispersion and our main goal is to build a consistent physical picture that explains how the cavitation events that occur in the solvent lead to breakup of the aggregates (so called bundles) or even to rupture of the nanotubes themselves. That leads us to study bubble nucleation and dynamics and ultimately the interaction of solid inclusions in a fluid. Both problems are nearly a century old and not completely solved even for simple fluids and laminar regimes. We have carried out a through literature review on the last subject focusing on the classic papers in the field in order to understand the motion of the particles as a function of the flow conditions and the interactions among them. Most recent publications in the field treat the problem numerically requiring large computational capabilities due to the interaction of particles with the fluid and also among themselves. It is interesting to note that the stresses inside the solid inclusions have been ignored in most of the analyses, and this is precisely where we want to focus. Additionally, calculations of the strength of interaction between carbon nanotubes in a bundle can be done in several ways ranging from stress analysis of nanoflexural tests to integrations of the cohesive energy density or electrostatic potential of a perfect bundle. Results from these approaches will be compared and matched to the energy delivered by any given mixing process.
Energy of Separation and Dispersion of Carbon Nanotubes
The energy density and forces required to separate nanoropes into individual nanotubes was examined by studying both the dilatation separation of arrays and the peeling of a pair of single wall carbon nanotubes. The cohesive energy per unit length was determined from the universal graphitic potential. The magnitude of the peeling force for a pair of tubes configured in a double cantilever beam was calculated over a range of peeling lengths using a cohesive zone model, and compared to predictions from linear elastic fracture mechanics. The results of the analysis reveal that a linear elastic fracture model that incorporates an inherent initial crack length yields a reasonable estimation of the peeling force-deformation response. The energy of separation for the dilatation mechanism was shown to be a strong function of the array size with twice the energy density necessary to separate an array of three CNT as compared to separation of a large array. Estimates of the energy of peeling separation of 0.30 nJ/m is in good agreement with previous work.
- Si Chen
Awards and Honors
"Prediction of the chemical and thermal shrinkage in a thermoset polymer," O.G. Kravchenko, C. Li, A. Strachan, S.G. Kravchenko, and R.B. Pipes, Composites Part A: Applied Science and Manufacturing, Volume 66, (2014), Pages 35–43.
"A Strategy for Prediction of the Elastic Properties of Epoxy-Cellulose Nanocrystal-Reinforced Fiber Networks," J.E. Goodsell, R.J. Moon, A. Huizar, and R.B. Pipes, Nordic Pulp and Paper Research Journal, (2014).
"Composite Toughness Enhancement with Interlaminar Reinforcement," S. Kravchenko, O. Kravchenko, M. Wortmann, M. Pietrek, P. Horst, and R.B. Pipes, Composites: Part A, (2013), doi: http:// dx.doi.org/10.1016/j.compositesa.2013.07.006
"Dispersion and its Relation to Carbon Nanotube Concentration in Polyimide Nanocomposites," C.R. Misiego, and R.B. Pipes, Composites Science and Technology, 85, (2013), pp. 43-49.
"Interlaminar Stresses in Composite Laminates Subjected to Anticlastic Bending Deformation," J. Goodsell, N. J. Pagano, O. Kravchenko, R. B. Pipes, Journal of Applied Mechanics, ASME J Appl Mech, (2013); 80(4): 041020-1 - 041020-7.
"Subsurface Imaging of Carbon Nanotube-polymer Composites Using Dynamic AFM Methods," M. Cadena, R. Misiego, K.C. Smith, A. Avia, R.B. Pipes, R. Reifenberger, and A. Raman, Nanotechnology, 24 (2013), 135706.