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ScienceWeek
MATERIALS SCIENCE: ON POLYMER FLOW
The following points are made by J. Bent et al (Science 2003 301:1691):
1) The quantitative prediction of both molecular-scale structure and macroscopic flow in realistic flow fields is a central goal of research in complex fluids, the class of materials that includes polymers, emulsions, foams, and surfactants. As an example, the dynamics of entangled polymeric fluids has for the last 20 years been one of the most rapidly advancing fields of soft condensed matter science (1). This is in part because of their commercial significance but also because of their intrinsic scientific fascination: The viscosity, elasticity, and rich rheology of polymer melts depends most strongly on the topological interactions between the long-chain molecules. The effects of these noncrossing or "entanglement" interactions are universal and largely independent of chemistry, but they are influenced by overall chain architecture. This is because they arise on a length scale considerably larger (tens of nanometers rather than angstroms) than that of simple molecules or monomers.
2) The speed and stability of polymer processing depend on matching macroscopic flow conditions with the right molecular structure. Furthermore, the mechanical properties of the resulting films, fibers, or molded parts reflect the molecular configurations of the polymer chains that are frozen-in on cooling. For example, highly orientated chains possess anisotropic bulk modes of deformation and fracture. In the future, it may become possible to design new engineering polymers and processes with multiscale modeling based on molecular physics, rather than relying on empirical methods. Driving the realization of this goal with some urgency at the industrial level is the increasing ability to control aspects of chain architecture, such as the distribution of polymer chain lengths and the degree of chain branching. Purely phenomenological modeling at the macroscopic scale is powerless as a design tool linking chemistry and process to product properties.
3) In summary: Flows of complex fluids need to be understood at both macroscopic and molecular scales, because it is the macroscopic response that controls the fluid behavior, but the molecular scale that ultimately gives rise to rheological and solid-state properties. The flow field of an entangled polymer melt through an extended contraction, typical of many polymer processes is imaged by the authors optically and by small-angle neutron scattering. The dual-probe technique samples both the macroscopic stress field in the flow and the microscopic configuration of the polymer molecules at selected points. The results are compared with a recent "tube model" molecular theory of entangled melt flow that is able to calculate both the stress and the single-chain structure factor from first principles. The combined action of the three fundamental entangled processes of reptation, contour length fluctuation, and convective constraint release is essential to account quantitatively for the rich rheological behavior. The multiscale approach unearths a new feature: Orientation at the length scale of the entire chain decays considerably more slowly than at the smaller entanglement length.(2-5)
References (abridged):
1. T. C. B. McLeish, Adv. Phys. 51, 1379 (2002)
2. G. Quack, N. Hadjichristidis, L. J. Fetters, R.N. Young, Ind. Eng. Chem. Prod. Res. Dev. 19, 587 (1980)
3. C. W. Bielawski, D. Benitez, R. H. Grubbs, Science 297, 2041 (2002)
4. T. C. B. McLeish, Science 297, 2005 (2002)
5. J. Klein, D. Fletcher, Nature 304, 5926 (1983)
Science http://www.sciencemag.org
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The term "reptation" refers to the motion of a polymer in a highly entangled state, e.g., in a polymer network. The entangled state is regarded as a set of chains between crosslinks, and the chain is considered as being in a tube formed by topological constraints. The chain is longer than the tube, so that as the slack of the chain moves through the tube, the tube itself changes with time. The term is from the Latin reptare (to creep), and was introduced by the physicist P.G. de Gennes in 1971. Many experiments have indicated that reptation dominates the dynamics of polymer chains when they are entangled.
POLYMERS WITHOUT BEGINNING OR END
The following points are made by Tom McLeish (Science 2002 297:2005):
1) Natural polymer molecules dominate biology, while artificial polymers are used as plastics or emulsifiers in countless modern products. Many characteristics of their crystalline, glassy, and fluid states can be traced back to the special properties generated by the ends of the molecules. But what would happen if there were no ends? What would be the properties of polymers composed entirely of closed loops? Answers may be within reach following the discovery reported by Bielawski et al (Science 2002 297:2041) of a polymerization catalyst that releases the polymer in the form of a closed ring.
2) These are not the first ring polymers to be studied. In the 1980s, Roovers synthesized small quantities of monodisperse polystyrene rings by anionic polymerization (1). His motivation was the growing interest in the dynamics of polymer melts, in which topological entanglements between chains dominate the pattern of their motion. Experiments with linear, star-shaped, and H-shaped molecules had shown that the architecture of the molecules had a stronger influence on the viscosity and viscoelasticity than their chemistry or molecular weight. For example, the time scales for stress-relaxation in flow can increase exponentially as a function of molecular weight if the molecules are branched, but only as a power law if they are not. The promising "tube model" (2) explained these effects: The key determinant of relaxation time is the time that the locally trapped region of the melt needs to wait for a chain end to diffuse to it through a maze of tube-like constraints around the polymer contour.
3) But what would happen if there were no ends to be found? Answering this question turns out to be delicate. Roovers found (correctly) that the relaxation times of the ring melts were much lower than those of linear melts of the same molecular weight. But other researchers disagreed. There are several reasons why ring molecules are difficult to study. First, it is essential to synthesize rings that are not interlinked (although such "olympic gels" are themselves interesting as rubbery solids with no molecular cross-links at all). Second, even small amounts of linear polymer contaminants in a melt of rings alter the dynamics, bringing relaxation times rapidly up to linear melt values. Finally, polystyrene, although relatively easy to work with, is composed of very bulky molecules that diminish the effects of entanglement.
4) These experimental challenges did not prevent theoretical speculation, however. Linear chains in dense melts display the statistical properties of ideal random walks, but a melt of rings should not behave in this way. The topological constraint that the rings are not linked is permanently set from their creation. This constraint in turn biases their conformations so that they grow in size more slowly with molecular weight than do linear chains (3). Instead of the snakelike "reptation" of linear chains, theory suggested that the dynamics of rings should resemble the motion of amoebae: Unentangled loops continually thrust out and retract in the complex hedge of constraints imposed by neighboring molecules.
References (abridged):
1) J. Roovers, Macromolecules, 21, 1517 (1988)
2) M. Doi, S. F. Edwards, The Theory of Polymer Dynamics (Oxford Univ. Press, Oxford, 1986)
3) M. E. Cates, J. M. Deutsch, J. Phys. (Paris) 47, 2121 (1986)
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