This collection of neologisms is more than buzzwords. There are very real and different phenomena that become important on size scales of 10 to 100 nanometers. The ordinary chemical potential must be augmented with a gradient term, phase boundaries depend on the size of the system, and the time-average interface between phases is a smooth function. Multiphase systems have minimum structure sizes, and the fluid mechanics of multiphase systems can be treated using a single equation of motion that contains a concentration dependent body force. Although some of these facts date to van der Waals, they are just now becoming generally recognized. They offer great potential for guiding the creation of novel structures and new commercial products. Our work has centered on nano dispersions of one polymer in another and most recently on the dispersion of proteins in polymer matrices for drug delivery, biocatalysis, and bioactive scaffolds for tissue engineering. Other applications-orientated projects include polymer nanocomposites and the manufacture of nanoparticles from organic compounds having molecular weights as low as a few hundred. Theoretical work includes studies on surface enrichment, concentration enhancement in nanopores, and fluid mechanics in small, confined spaces.
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Both practical and conceptual problems arise in standard theories of multicomponent diffusion. These include the sometimes arbitrary and even artificial selection of one component as the solvent, the lack of explicit dependence on the properties of the solvent, and the difficulty in formulating compositionally dependent diffusion coefficients that satisfy material balance constraints. Two alternative approaches based on self-diffusion coefficients for the various components are physically reasonable and overcome these difficulties. The approaches differ in the extent of cross diffusion that is predicted. Either approach can be used in systems where the component diffusivities differ greatly in magnitude and where the number of chemical species is large, as in polymerizations. We have begun testing these theories for ternary systems using molecular dynamics (MD) and dissipative particle dynamics (DPD) simulations. The MD simulations are theoretically sound but computationally demanding. Attention is restricted to relatively low molecular weight hydrocarbons such as C8 to C16 alkanes. DPD is more speculative but offers hope of going to longer chain lengths by proper adjustment of the simulation constants.
The economic potential of well-formulated polymer blends is known, but the research avenues for achieving this potential remain largely Edisonian. A better, systematic approach is becoming possible. The first step in this new approach is the accurate prediction of blend morphologies, for example, cocontinuous, particulate, two phases particulate, and core-shell structures. This step has been largely accomplished. The morphologies are usually self-similar, and control of the absolute size of the predicted structures becomes the next issue. Phase growth or ripening can be controlled through chemical pinning or by the use of random or block copolymers as interfacial agents. The latter also provides the interfacial adhesion needed in most systems for best performance. "Almost a priori" theories for predicting optimal types and concentrations of interfacial agents have recently been developed. The use of random copolymers is particularly interesting since they are often much cheaper than diblocks. The final step in designing polymer blends is relating the stabilized morphology to properties valuable in commercial applications. This is becoming possible in structural applications where performance depends on impact strength and modulus. Super-high impact polymers with properties greatly exceeding those in commercial materials have been created. Opportunities remain to be exploited in more specialized applications and markets.
>In many binary blends of immiscible polymer, a pronounced increase in the size of the dispersed droplets has been found during their annealing in the molten state. Two ripening mechanisms are recognized in a quiescent particulate system: Ostwald ripening and Brownian coalescence. Ostwald ripening, also know as the evaporation condensation mechanism, is described using the modified Cahn-Hilliard equation. The particle size distribution is found to be self-similar and broadened with increasing volume fraction of the minor phase. The coalescence of droplets has been studied extensively but the final step in the coalescence process has been a mystery. Hydrodynamics predict that the droplets slow in their approach but never quite touch, much like a Greek paradox. A semi-empirical argument based on van der Waal's forces resolves the paradox for some but provides no physical insight. Nonlinear diffusion provides the answer and suggests methods for controlling coalescence rates. The interstitial fluid has a higher chemical potential and literally diffuses out of the way. We can enhance or retard coalescence by adding components which concentrate at interfaces and alter chemical potentials. There are many and potentially cheaper possibilities than using conventional amphiphiles such as detergents and block copolymers. Our work applies directly to polymer systems but are conceptually applicable to small molecules as well.
Current efforts include polymer reaction engineering of anionic, free radical and condensation polymerizations; theory and simulation of spinodal decomposition; use of Visual Basic for user friendly process models; recycling of mixed plastics, and flash devolatilization of polymers. A new form of motionless mixer (a.k.a. static mixer) is being built and tested as part of a colorborative effort with IIT Delhi. It is designed to promote axial mixing. Most static mixers are designed to promote radial mixing.