Interdiffusion of polymer chains

As mentioned earlier, adhesive bond formation is governed by interfacial processes occurring between the adhering surfaces. These interfacial processes, as summarized by Brown [13] include (1) van der Waals or other non-covalent interactions that form bonds across the interface (2) interdiffusion of polymer chains across the interface and coupling of the interfacial chains with the bulk polymer and (3) formation of primary chemical bonds between chains or molecules at or across the interface. 

The interdiffusion of polymer chains occurs by two basic processes. When the joint is first made chain loops between entanglements cross the interface but this motion is restricted by the entanglements and independent of molecular weight. Whole chains also start to cross the interface by reptation, but this is a rather slower process and requires that the diffusion of the chain across the interface is led by a chain end. The initial rate of this process is thus strongly influenced by the distribution of the chain ends close to the interface. Although these diffusion processes are fairly well understood, it is clear from the discussion above on immiscible polymers that the relationships between the failure stress of the interface and the interface structure are less understood. The most common assumptions used have been that the interface can bear a stress that is either proportional to the length of chain that has reptated across the interface or proportional to some measure of the density of cross interface entanglements or loops. Each of these criteria can be used with the micro-mechanical models but it is unclear which, if either, assumption is correct. 

Bradford and Vanderhoff (20) have also prepared films from crosslinked latex particles. These authors studied a 65 35 styrene-butadiene copolymer crosslinked with varying amounts of divinylbenzene and found that although the incorporation of divinylbenzene retarded the coalescence of latex particles, these particles did indeed coalesce, presumably due to a similar interdiffusion of polymer chain ends. 

When two blocks of the same polymer are pressed together at a temperature above the Tg for a relatively short time t, interdiffusion of polymer chains takes place (by reptation) across the interface to produce a significant number of entanglements, thereby joining the blocks together (Sperling, 1986). The-strength of the junction formed will, however, depend on time t. 

The other interesting method utilises fluorescence measurements. This approach has been mainly applied to latex film formation by Winnik and Wang [90]. In this technique, latex is prepared in two different batches. In one batch, the chains contain a donor group, while in the other, an acceptor group is attached. The interdiffusion of polymer chains between neighbouring latex particles is then studied by direct non-radiative energy transfer measurements. 

The diffusion coefficient appearing in Eq. (20.4-3) b a true measure of the molecular mobiitty of the penetrant in question. Intuitively, the free-volume theory proponents argue that a penetram can execute a diffitsive jump when a free-volume element greater than or equal to a critical size presorts itself to a penetrant. The native poiymer, totally devoid of penetrant, still possesses a certain amount of free-volume packets of distributed size which wander spontaneously and randomly through the rubbery matrix. In fact, when a packet of sufficient size presents itself to a polymer segment, the polymer may execute a self-diffiisive motion, and this is the action that causes slow interdiffusion of polymer chains. 

When a polymeric and rubbery substrate is put on contact with silicones, interdiffusion of polymer chains may occur (see Polymer diffusion reptation and interdigitation and Compatibility). The original interface becomes an interphase composed of mixtures of the two polymeric materials. Such a macromolecular interdiffusion process is limited... 

Solvent welding and solvent cementing are widely used techniques for the joining of thermoplastic polymers. A small amount of solvent along the joint promotes interdiffusion of polymer chains between the substrates. This creates a permanent weld, with no additional phases or potentially weak interfaces. A solvent cement is a solution of the polymer being joined. This also promotes interdiffusion, and leaves a single-phase joint when the solvent has evaporated. 

When two otherwise identical polymer surfaces are brought into juxtaposition at r > Tg and annealed, a healing process may take place. As a result the interface gradually disappears. Basic additional requirements are that the polymer be linear (or branched) and amorphous. The major mechanism involves the interdiffusion of polymer chain segments across the interface. For polymer chain interdiffusion, the reptation theory of de Gennes (54) and Edwards (55)... 

Polymer molding. Healing of internal weld lines. For plastics made from pellets, the sintering together of the pellets. For latex paints, the interdiffusion of polymer chains during film formation. 

Latex-type binders account for a large volume of waterbased coatings. Film formation in a latex paint involves the coalescence or fusion of discrete polymer particles of high MW polymers. When the film is applied, the polymer particles are pushed against each other under the capillary forces generated due to progressive evaporation of water. This results in coalescence at the particle boundaries, with interdiffusion of polymer chain across the boundaries (Figure 5.17). 

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. 

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. 

Numerous possibilities exist for the manufacture of polymers with latent coreactive sites °l The introduction of built in functional groups onto the surface of the polyurethane particle and proprietary reactive groups copolymerised into the backbone of the acrylic and buried deep into the core of the acrylic phase are the usual techniques for crosslinkable copolymer type systems. The crosslinking mechanism is not fully understood, but it is assumed to be made possible via interdiffusion of polymer chains across the former boundaries of adjacent particles. Evidence for the crosslinking can be found in increased solvent resistance, softening temperature and the appearance of a single Tg.. 

Molecular interdiffusion of polymer chains from one particle into another... 

The first process of interest in the cohesive strength development is the interdiffusion of polymer chains. It is well-known that the diffusion of polymer chains in a polymer matrix is strongly dependent on the molar mass of the chains. In terms of development of the cohesive strength, two opposing effects can be recognised ... 

In the drying stage at the end of water evaporation the particles adopt a hexagonal dose-packed geometry. Good subsequent film formation requires a high level of polymer particle deformability and the rapid interdiffusion of polymer chains between the particles. Emulsion polymers therefore possess a so-called minimum film formation temperature (MET), below which no compact film can be formed. The determination of the MET is discussed below. 

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