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Unformatted text preview: Relation between the Width of an Interface between Two Polymers and Its Toughness Hugh R. Brown BHP Institute for Steel Processing and Products, University of Wollongong, Wollongong, NSW 2522, Australia Received October 28, 1999; Revised Manuscript Received February 9, 2001 ABSTRACT: Thin layers of a series of random PS/PMMS copolymers have been used to couple two sheets of PMMA, thereby permitting a study of the effect of interface width on interface toughness. The interface width and toughness varied with the copolymer composition. The use of a thin coupling layer between two PMMA sheets ensured that the crack tip craze occurred mainly in PMMA, so the material within which the crack tip deformation occurred did not change as the interface width was changed. The interface toughness was found to remain low for narrow interfaces until the interface width became larger than the mean entanglement spacing in the bulk. The toughness then climbs rapidly to saturation at an interface width of perhaps three times the entanglement spacing. A model is proposed here for the relation between interface width and interface toughness based on a well-tested model for interface toughness and known interface profile. However the assumptions made within the model, that entanglement density is constant through the interface and that the probability of a chain passing through the interface can be described just by a volume fraction ratio, are probably incorrect. The measured toughness is significantly less than that predicted at widths equal to and below the entanglement spacing. This disparity is probably caused by the entanglement spacing increasing over the bulk value in the area of the interface. For broad interfaces, where the entanglement density is expected to be close to bulk values, the predicted toughness agrees reasonably with experiment. Introduction There are a number of situations, including polymer blends and adhesive joints, where the adhesion between a pair of immiscible polymers is of considerable practical importance. Understanding of polymer- polymer adhe- sion and polymer toughness has improved considerably in the past few years. It is now clear that, for glassy polymers, adhesion, defined as the toughness of an interface, is controlled by both the chain coupling across the interface and energy dissipation processes in the materials. For tough systems, the chain coupling is described by the number of lengths of polymer chain between entanglements, , that cross the unit area of the interface. Energy dissipation normally occurs by the growth of one or more crack tip crazes. At an interface between two materials, these crazes normally form in the material with lower crazing stress. Cohesive tough- ness is controlled in a similar way by the number of entangled chain lengths that cross any plane within the material. An important issue, then, in understanding and controlling adhesion is to understand, and perhaps alter, the areal density of such entangled chains. Placingalter, the areal density of such entangled chains....
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This note was uploaded on 07/20/2011 for the course EMA 6165 taught by Professor Brennan during the Spring '08 term at University of Florida.
- Spring '08