Polymer, substrate, segmental mobility

The chains of water-insoluble swellable polymers are connected to cross-linking agents. As the amount of cross-linking is increased, the diffusion coefficient of the polymer chains is decreased with a subsequent deerease in interdiffusion between the polymer and substrate and a decrease in the polymer s bioadhesive properties [18,19]. This increase in cross-linking also lowers ehain-segment mobility and flexibility, which can reduce the... 

Diffusion. This theory proposes that adhesive macromolecules diffuse into the substrate, thereby eliminating the interface, and so can only apply to compatible polymeric substrates. It requires that the chain segments of the polymers possess sufficient mobility and are mutually soluble. The solvent welding of thermoplastics such as PVC (polyvinyl chloride), softened with a chlorinated solvent, is an example of such conditions being met. Diffusion will also take place when two pieces of the same plastic are heat-sealed. The joining of plastic service pipes for carrying gas and water makes use of the diffusion mechanism. 

Fig. 11. Master curve for thickness (t) dependence of Tg of thin films of pol3m(iers that have no specific interactions with the substrate (72). The equation for the curve is Tg = t /(l+t ) where Tg = Tg(t)/Tg(bulk), Tg is in Kelvin, t = tIL, and L is the statistical chain (Kuhn) segment length. By contrast, if strong specific interactions between the polymer and the substrate result in restricted interfacial region mobility, the behavior becomes very different from what is shown below and Tg may instead increase with decreasing thickness.

The diffusion theory explains in some cases the adhesion between pol5mers. This theory postulates that the adhesion is due to the mutoal diffusion of polymer molecules across the interface. This requires that the polymers or their chain segments are sufficiently mobile and that the two substrates are mutually soluble, e.g. they have similar solubility parameters. If the solubility parameters are very different (incompatible polymers), then there is htde chain entanglement and, thus, a very poor joint strength. 

Voyutskii [30-32] is the chief advocate of the diffusion theory of adhesion which states that the intrinsic adhesion of polymers to themselves (autohesion), and to each other, is due to mutual diffusion of polymer molecules across the interface. This requires that the macromolecules, or chain segments of the polymers (adhesive and substrate) possess sufficient mobility and are mutually soluble. This latter requirement may be restated by the condition that they... 

In summary, the interdiffusion of polymer chains across a polymer/polymer interface requires the polymers (adhesive and substrate) to be mutually soluble and the macromolecules or chain segments to have sufficient mobility. These conditions are usually met in the autohesion of elastomers and in the solvent welding of compatible, amorphous plastics. In both these examples interdiffusion does appear to contribute significantly to the intrinsic adhesion. However, where the solubility parameters of the materials are not similar, or one polymer is highly crosslinked, crystalline or below its glass transition temperature, then interdiffusion is an unlikely mechanism of adhesion. In the case of polymer/metal interfaces it appears that interdiffusion can be induced and an interphase region created. But this effect enhances the interfacial adhesion by improving the adsorption of the polymeric material rather than by a classic diffusion mechanism. 

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