Molecular interdiffusion, polymer adhesion

In the theory of diffusion it is postulated that high molecular weight polymer molecules interdiffuse w ith each other across the inter ce. The term autohesion is often applied to this process in the adhering of portions of the same plastic material together. Since molecules of the same material diffuse across the inteiface, makhtg the two layers one, the original joint disappears. Once the joint is completely healed, there is little chance that adhesive failure will occur at the original inteiface. 

Much of Voyutskii s original work was done on the self-adhesion (called autohesion) of unvulcanized rubbers. It was subsequently extended to polymer adhesion, more generally. The theory postulates that the molecules of the two parts of the specimen interdiffuse, so that the interface becomes diffuse and eventually disappears. For polymers in contact, Voyutskii studied the effects on adhesion of such variables as time, temperature, contact pressure, molecular weight, polarity, and crosslinking. He argued that the results proved that the adhesion was associated with the interdiffusion of polymer chains. 

In numerous applications of polymeric materials multilayers of films are used. This practice is found in microelectronic, aeronautical, and biomedical applications to name a few. Developing good adhesion between these layers requires interdiffusion of the molecules at the interfaces between the layers over size scales comparable to the molecular diameter (tens of nm). In addition, these interfaces are buried within the specimen. Aside from this practical aspect, interdififlision over short distances holds the key for critically evaluating current theories of polymer difllision. Theories of polymer interdiffusion predict specific shapes for the concentration profile of segments across the interface as a function of time. Interdiffiision studies on bilayered specimen comprised of a layer of polystyrene (PS) on a layer of perdeuterated (PS) d-PS, can be used as a model system that will capture the fundamental physics of the problem. Initially, the bilayer will have a sharp interface, which upon annealing will broaden with time. 

Interdiffusion between a pair of polymers is a demonstration of their thermodynamic miscibility. The adhesion between contacted rubber sheets parallels the extent of any interdiffusion of the polymer chains (Roland and Bohm, 1985). If the contacted sheets are comprised of immiscible rubbers, no interdiffusion occurs. Natural rubber (NR) and 1,2-polybutadiene (1,2-BR) are miscible even at high molecular weights (Roland, 1988a Roland, 1987). When NR is brought into contact with 1,2-BR, they interdiffuse spontaneously. When some form of scattering contrast exists between the materials, interdiffusion will enhance the scattering intensity (either X-ray or neutron) measured from the plied sheets. A variety of spectroscopic methods (Klein, 1981 ... 

Figure 4J3 Schematic events following bioadhesion of a matrix tablet at a mucosal surface (a) initial contact of the dry polymeric matrix with the mucosal surface (b) polymeric chains are progressively hydrated at the matrix surface and in contact with the mucous layer lining the mucosa (c) progressive chain interdiffusion between bioadhesive polymer chains and mucous glycoproteins encourages intimate contact and favours development of adhesive interactions, schematically depicted as black spots at the molecular level.

Tack refers to the adhesion of two surfaces of the same rubbery polymer. When two such surfaces are pressed together and subsequently pulled apart, the maximum force necessary to break the junction depends on the initial time of contact and the normal force applied, as well as the rate of separation and the temperature and other variables. " From the dependence on temperature and polymer molecular weight, it can be inferred that the effectiveness of the bond depends partly on the interdiffusion of molecules across the interface and hence on molecular motions which are reflected in viscoelastic properties in the terminal zone. - However, the effectiveness depends also on the ultimate properties of the polymer itself as discussed in Section E below, and the phenomenon is still not fully understood. 

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