The focus of this effort is the free-radical retrograde-precipitation polymerization (FRRPP) process that was discovered in the early 1990s. Here, polymerization occurs above the lower critical solution temperature (LCST) of the polymerization system. The local heating that occurs in exothermic chain polymerization systems drives the system into the spinodal curve, wherein diffusional mass fluxes have been found to vanish. Monomer molecules, while undergoing Brownian motion, cannot diffuse from high-concentration regions to low-concentration regions adjacent to the reactive sites.
The net result of this and the precipitation into high-polymer concentration is the drastic drop in propagation-rate coefficient relative to termination-rate coefficient, which is opposite to the runaway gel-effect situation in polymer systems. Therefore, this FRRPP mechanism is associated with an Anti-Gel Effect Phenomenon.
Other results of this mechanism are efficient trapping of polymer radicals, relatively narrow molecular weight distributions, stable molecular weight and polydispersity index, and reduced conversion rate. Radical trapping results in up to 84 percent of stable polymer radicals existing relative to all polymeric species even after almost all initiator molecules have already decomposed. This result was the basis for our work in the formation of high yields of block copolymers from a free-radical chemistry that does not use any chemical mediator or template.
Another mechanism was proposed to occur especially after the initial fast-conversion-rate phase of the polymerization process. It involves the collapse of polymer chains around the reactive sites. This is associated with the so-called coil-to-globule transition in polymer physics. The added effect of the exotherm in the chain polymerization system not only could result in chain collapse but also in densification of the collapsed chains. The net result seems to be reductions in mutual diffusion coefficients.
Current conceptual effort involves the formulation of a dimensionless quantity that represents the occurrence of a strict FRRPP process.
If the reaction rate in FRRPP is under good control, then it means that polymer-rich domain growth will also be under good control if domain interaction is minimized. If the reaction is carried out in a quiescent fluid, then we should expect a large amount of nanoscale particles. These nanosized particles are indeed obtained. Thus, if polymerization is induced by radiation with nanometer-sized wavelengths, it is possible to control the dimensional features of the polymer product within the radiation zones in micro- and nanolithographic operations. Moreover, product polymers could be homopolymers and block copolymers, such as thermoplastic elastomer, hydrophilic-hydrophobic, and silicone-organic types.
It is evident that this approach (a “bottom-up” method) is different from established “top-down” photolithographic methods; which are based on casting a polymer film, irradiating the film, and then dissolving away irradiated or unirradiated regions. Our approach could produce nanoscale polymer structure from the monomer in situ, using low-wavelength low-divergence directed radiation (such as that of a synchrotron X-ray) as the initiation source. Since propagation of the polymer chains is well-controlled, the polymer-rich phase will be confined to the radiation zones. Thus, the polymer structure can be made to coincide with the nanometer-scale radiation cross-section, provided the wavelength of the radiation source is less than about 20 nm. Since live radicals can be maintained within the polymer-rich domains, exposure of the system to another monomer mixture can result in block copolymer formation. It is therefore evident that this proposed approach is more versatile to the conventional lithographic method in terms of the types of polymer that can be produced in the nanometer scale.
We have continuing efforts in the study of the formation of thermoreversible biomaterials in the nanometer scale. For example, we have demonstrated the capability of producing nanospheres based on poly(N-isopropyl acrylamide) and poly(methacrylic acid) without the use of surfactants. These materials can be useful in controlled drug release applications. Synchrotron X-ray lithography work has yielded the possibility of forming polymer membrane materials with well-defined pores in the microscale, and work is continuing to push this into the nanoscale. These membranes can be used in structurally-targetted tissue growth, membrane reactors, biosensors, bacterial studies, etc. Membranes in the nanoscale can be used for protein separations, proteomics, genomics, viral studies, etc.
In conventional statistical free-radical copolymerization, reactivity ratios and monomer addition protocol dictate monomer distribution in the polymer chains. In a selectively precipitating medium that also involves radical trapping and molecular weight control, a more effective monomer distribution control is possible. An extreme case, i.e., block copolymer formation, has already been shown to be produced from a relatively fast self-assembled single-stage single-charged reactor operation. So far, the research group has produced vinyl acetate-acrylic acid copolymers (block, random, and segmented block) that have not been efficiently produced using conventional methods. A vinyl acetate-acrylic acid material that functions as a diblock copolymer has been found to have self-emulsifying capabilities, and it seems to be a viable and inexpensive surfactant for a wide-range of applications. Some of the unique uses include: coupling agent for plastics-natural fiber composites, foaming surfactants for enhanced oil recovery, soil remediation, water-based degreasers, biodegradable plastic foams, froth flotation for iron and copper ore processing, wood preservation, longterm water-dispersible adhesives, water-based coating primers, longer lasting silicone coatings, and water-based pesticide formulations.
At a time of uncertainties in foreign crude oil supplies, decreasing domestic oil production, and projected increases in demands for petroleum products, there is a need for new technologies to recover more of the oil-originally-in-place (OOIP) from existing oil fields. Current political and military conditions in foreign oil-producing regions compel us to exploit these new technologies in the most expedient way possible. This effort is meant to help alleviate future crude oil supply problems in ways that are energy-efficient, economically-viable, and environmentally-responsible. Specifically, we are studying the use of our newly-discovered vinyl acetate-acrylic acid-based (VA/AA) block copolymer as surfactant enhanced oil recovery. It belongs to the class of anionic surfactants, which are found to have the lowest absorption in most oil-bearing sandstone formations. The VA/AA material was first generated in the laboratory using a benchscale reactor system, and in neutralized form it was found to be much better as a foaming surfactant compared to some typical commercially-available materials. It has also shown great promise in the recovery of heavy oils (heavy crude, tar, and shale) from bitumen sources. It can be manufactured in commodity material scale, with a bulk selling price as low as $2/lb of dry surfactant.
Urban warfare applications of polymers is a new field of research, where there is a need for new multifunctional polymers. The FRRPP process is most suitable for this application, due to the possible use of a wide variety of monomers from free-radical chemistry. A Phase I 1.6 million $ project was awarded in April 2005 by DARPA to Michigan Tech, through Raytheon Missile Systems (Tucson, AZ), and the FRRPP process is the basis of this work. This area of work has provided for funds towards the purchase of a high throughput experimentation robotic system, as well as pressurized reactor and supercritical-fluid-based batch and continuous foam formation systems. Since the DARPA/Raytheon project involved 10 faculty/staff/grad student researchers at Michigan Technological University and a substantial overhead return, we were able to establish the Center for Environmentally Benign Functional Materials (CEBFM), in which I now serve as its Director.
We are also using some of the expertise we developed from the DARPA/Raytheon project to study the improvement of mechanical properties of polymer blends and composites using supercritical carbon dioxide. In this new effort, we are being assisted by Prof. Munir Tasdemir of Marmara Univeristy (Istanbul, Turkey), who was on sabbatical leave for 10 months in 2007-2008.
Based on the results of above-mentioned work with DARPA/Raytheon, we are formulating new block copolymers with halogenated species as fuel cell membranes, especially those of the anion exchange types which use methanol as fuel.
My fellowship experience with NASA-Johnson Space Center has introduced me to various applications of carbon nanotubes, especially the single-walled type (SWNT). We have been able to find a niche area in the formulation of electrically conductive and capacitive SWNT-high temperature polymer films. Another discovery made here is the efficient dispersion of SWNT in processing solvents through intermittency chaos in ultrasonic cavitation. The SWNT-polymer materials can be used as radiation shields, sensors, and high emissivity coatings.