Substrate interface, glass transition

For this type of test, the two homopolymers are typically compression molded into 1-2 mm thick sheets. A solution of the block copolymer is then spun-cast on one of the two polymers. The solvent and the homopolymer substrate to receive the block copolymer must be chosen so that the surface of the homopolymer substrate is not dissolved during the spin-casting operation. After a drying step, the two slabs are welded with the block copolymer in between to form an interface. The welding temperature is chosen to be above the respective glass transitions of the two homopolymers and the two slabs are held under a moderate pressure for a time sufficient to allow the local organization of the block copolymer at the interface to reach a metastable equilibrium. 

Hansen (57) pointed out that evaporation of a solvent from a polymer solution faced two barriers when cast on an impermeable substrate resistance to solvent loss at the air-liquid interface and diffusion from within the film to the air interface. Evaporation of neat solvents as well as moderately dilute solutions is limited by resistance at the air interface, but as solvent concentration becomes low (5-10-15%), the rate-controlling step is diffusion through the film. Hansen pointed out that at the point when solvent loss changes to a diffusion-limited process, the concentration of solvent is sufficient to reduce the glass transition temperature, Tg, of the polymer to the film temperature. 

The surface-glass-transition temperatures foimd by, eg, SFM approaches in polymers are usually lower than the corresponding values measured in the bulk. This effect has been attributed to an increased mobility of the macromolecules near or at the pol5uner-air interface. In very thin substrate-supported thin films, interfacial energy effects stemming from the substrate-polymer interface were... 

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. 

These polydiene-, polyacrylate- or silicone-based adhesives are generally formulated products (with waxes in order to lower the glass transition temperature, and tackifying resins to provide tack), used as self-adhesives, coated to materials such as paper, films, and polymer tapes, etc. The term Pressure-Sensitive Adhesive relates to the fact that pressure from a linger on the rear of the material to be affixed is all that is required in order to allow the adhesive mass to wet the substrate sufficiently and to adhere to it more or less strongly. Depending on the visco-elastic properties and the adhesion forces developed at the interface, various behaviors can be reached from nonpermanent in the case of Post-it to semipermanent in the case of dressings. 

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