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Adhesion
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===Diffusive=== Some materials may merge at the joint by [[Atomic diffusion|diffusion]]. This may occur when the molecules of both materials are mobile and [[soluble]] in each other. This would be particularly effective with polymer chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in [[sintering]]. When [[metal]] or [[ceramic]] powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.<ref>{{cite journal |last1=von Fraunhofer |first1=Anthony |title=Adhesion and Cohesion |journal=International Journal of Dentistry |date=21 Feb 2012 |volume=2012 |page=951324 |doi=10.1155/2012/951324 |pmid=22505913 |pmc=3296218 |doi-access=free }}</ref> [[File:Interdigitation.jpg|thumb|The interface is indicated by the dotted line. A) Non-crosslinked polymers are somewhat free to diffuse across the interface. One loop and two distal tails are seen diffusing. B) Crosslinked polymers not free enough to diffuse. C) "Scissed" polymers very free, with many tails extending across the interface.]] Diffusive forces are somewhat like mechanical tethering at the molecular level. Diffusive bonding occurs when species from one surface penetrate into an adjacent surface while still being bound to the phase of their surface of origin. One instructive example is that of polymer-on-polymer surfaces. Diffusive bonding in polymer-on-polymer surfaces is the result of sections of polymer chains from one surface interdigitating with those of an adjacent surface. The freedom of movement of the polymers has a strong effect on their ability to interdigitate, and hence, on diffusive bonding. For example, cross-linked polymers are less capable of diffusion and [[interdigitation]] because they are bonded together at many points of contact, and are not free to twist into the adjacent surface. Un[[crosslinked]] [[polymers]] ([[thermoplastics]]), on the other hand are freer to wander into the adjacent phase by extending tails and loops across the interface. Another circumstance under which diffusive bonding occurs is "scission". [[Chain scission]] is the cutting up of polymer chains, resulting in a higher concentration of [[Anatomical terms of location#Proximal and distal|distal]] tails. The heightened concentration of these chain ends gives rise to a heightened concentration of polymer tails extending across the interface. Scission is easily achieved by [[ultraviolet]] irradiation in the presence of oxygen gas, which suggests that adhesive devices employing diffusive bonding actually benefit from prolonged exposure to heat/light and air. The longer such a device is exposed to these conditions, the more tails are scissed and branch out across the interface. Once across the interface, the tails and loops form whatever bonds are favorable. In the case of polymer-on-polymer surfaces, this means more van der Waals forces. While these may be brittle, they are quite strong when a large network of these bonds is formed. The outermost layer of each surface plays a crucial role in the adhesive properties of such interfaces, as even a tiny amount of interdigitation β as little as one or two tails of 1.25 angstrom length β can increase the van der Waals bonds by an order of magnitude.<ref name=Maeda>{{cite journal|pmid=12130780|author=N. Maeda|title=Adhesion and Friction Mechanisms of Polymer-on-Polymer Surfaces|year=2002|last2=Chen|first2=N|last3=Tirrell|first3=M|last4=Israelachvili|first4=JN|volume=297|issue=5580|pages=379β82|doi=10.1126/science.1072378|journal=Science|bibcode=2002Sci...297..379M |s2cid=32153774 }}</ref>
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