1466-entanglements at polymer surfaces and interfaces

1466-entanglements at polymer surfaces and interfaces - 798...

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Communications to the Editor Entanglements at Polymer Surfaces and Interfaces Hugh R. Brown* and Thomas P. Russell* Almaden Research Center, IBM Research Division, 650 Harry Road, San Jose, California 95120 Received July 31, 1995 Revised Manuscript Received November 6, 1995 Entanglements restrict the mobility of the polymer molecules in the bulk and in concentrated solutions and are at the basis of many of the unique and unusual dynamic properties of polymers. Qualitatively, one can envision an entanglement as a crossing of polymer chains which, when subjected to a strain, remains intact and hence mechanically active. The molecular weight of the polymer between the cross-link points, M e ,i s normally obtained from the real part of the dynamic modulus, i.e., the storage modulus. It is, in fact, M e which underlies many of the theoretical treatments of the diffusion and viscoelasticity of molten polymers. For example, the reptation model, 1,2 which has met with great success in describing the diffusion of polymer chains in the melt, describes the snakelike motion of polymer chains along their own contour and uses M e to define the tube within which the polymer chain can diffuse. For polymers with molecular weights below M e , the motion of the chain is unrestrained and the chain diffuses in a Rouse-like manner. In addition, the high strain and failure properties of solid polymers, as for example the natural draw ratio 3 and crazing, 4 are profoundly influenced by M e . Failure properties, such as toughness, depend critically upon the ratio of the molecular weight of the polymer to its M e . 5 Many more examples of the unique properties of polymers can be traced to M e . While it is simple to draw a schematic picture of entangled polymer chains, it is far more complex to define quantitatively an entanglement. A number of recent authors 6 - 9 have suggested that, in the bulk, entanglements, and hence M e , are controlled mainly by the packing of the polymer chains. They proposed that the volume of space pervaded by a single chain of molecular weight M e is a fixed constant of the order of 10 times the hard core volume occupied by the chain. The actual values of the constant depends upon the definition of the pervaded volume. Their arguments, which are supported by experimental evidence, 10 are based on the idea that packing defines the constraints placed on the motion of a single chain by the surround- ing chains. A tremendous amount of attention has focused re- cently on the surface and interfacial behavior of poly- mers. Consider, now, the situation at the surface of a polymer or at the interface between two immiscible polymers or between a polymer and another solid. It is, of course, the M e of a polymer chain at the surface or interface and not in the bulk which controls the interfacial characteristics of the polymer. Three ex- amples where M e at the interface can influence the response of a polymer are as follows: (i) the initial stages
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1466-entanglements at polymer surfaces and interfaces - 798...

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