Diffusion of polymer chains

One of the most common rubber adhesives are the contact adhesives. These adhesives are bonded by a diffusion process in which the adhesive is applied to both surfaces to be joined. To achieve optimum diffusion of polymer chains, two requirements are necessary (1) a high wettability of the adhesive by the smooth or rough substrate surfaces (2) adequate viscosity (in general rheological properties) of the adhesive to penetrate into the voids and roughness of the substrate surfaces. Both requirements can be easily achieved in liquid adhesives. Once the adhesive solution is applied on the surface of the substrate, spontaneous or forced evaporation of the solvent or water must be produced to obtain a dry adhesive film. In most cases, the dry-contact adhesive film contains residual solvent (about 5-10 wt%), which usually acts as a plasticizer. The time necessary... 

The bond fluctuation model not only provides a good description of the diffusion of polymer chains as a whole, but also the internal dynamics of chains on length scales in between the coil size and the length of effective bonds. This is seen from an analysis of the normalized intermediate coherent scattering function S(q,t)/S(q,0) of single chains ... 

The intercept Vo and slopes B in log V against 1/AT of FCSCs were plotted against Mn in Fig. 24. This showed that Vo significantly decreased with an increase of Mn, whereas B did not, as was shown by Hoffman et al. [28] Vo and B of ECSCs showed similar Mn dependence to those of FCSCs. As Vo is related to self diffusion of polymer chains and B is related to the activation free en-... 

The overall rate of crystallization is determined by both the rate of nuclei formation and by the crystal growth rate. The maximum crystal growth rate lies at temperatures of between 170 and 190 °C [71, 72], as does the overall crystallization rate [51, 61, 75], The former is measured using hot stage optical microscopy while the latter is quantified by the half-time of crystallization. Both are influenced by the rate of nucleation on the crystal surface and the rate of diffusion of polymer chains to this surface. It has been shown that the spherulite growth rate decreases with increasing molecular weight due to the decrease in the rate of diffusion of molecules to this surface [46, 50, 55, 71, 74],... 

More rigorons treatments include hydrodynamic interaction effects and relate the diffusivity directly to the molecular weight of the polymer chain. The general result is that the diffusivity of polymer chains in solntion is related to the molecular weight of the polymer, M ... 

The diffusion of polymer chain radicals has no influence on the termination rate. This is because the frequency of encounter decreases—by decreasing diffusion constant—to the same extent as the lifetime of the permeation state increases. 

Leonov AI (1994) On a self-consistent molecular modelling of linear relaxation phenomena in polymer melts and concentrated solutions. J Rheol 38( 1) 1—11 Liu B, Diinweg B (2003) Translational diffusion of polymer chains with excluded volume and hydrodynamic interactions by Brownian dynamics simulation. J Chem Phys 118(17) 8061-8072... 

There could be two possible reasons for this apparently contradictory situation,. First, in the above discussion, we adopted a diffusion constant that was evaluated from the diffusion constant of polymer chains at the interface between two bulk polymeric layers [72]. Recent measurements have revealed the existence of heterogeneous dynamics in confined geometries such as thin films and nanopores [73,74]. For stacked thin films of polymers, the dynamics vary with an essential dependence on the distance of the layer of interest from the free surface or from the substrate [75]. If such dynamical heterogeneity is taken into account, the diffusion of polymer chains could be restricted by the existence of an immobile region. 

Second, the interface between the two polymer layers of as-stacked thin films may not be sufficiently smooth for good cmitact between the two layers. In this case, the diffusion of polymer chains between the two thin layers would be limited by the quality of the contact between the surfaces of the two thin layers. It has been reported that the area of indentation increases approximately with the logarithm of time for the contact of two solid surfaces. This observation is related to the change in the quality of contact [76, 77]. As a result, the interface between two thin layers could exhibit a very slow temporal change, as shown in the present measurements. 

This theory is exclusively concerned with polymeric adhering systems. The theory proposed by Voyutskii establishes that diffusion of polymer chains across the interface determines the adhesive strength. It must be emphasized that diffusion of macromolecules across an interface is possible to a larger extent only when polymer temperatures exceed their glass transition temperatures. 

Suppose we have added some initiator to the solution of not-yet-polymerized monomers, and the reaction has begun. At first, the growing chains appear in a kind of dilute solution, in which the monomer molecules play the part of a solvent. With time, more and more molecules of the monomer become involved in the reaction. The concentration of the chains grows, and they begin to overlap. This is when the solution becomes semi-dilute. FYom this moment on, the character of the chains motion changes — they start moving by reptation. As we have already shown, this means that diffusion of polymer chains slows down substantially. 

It is currently imderstood that the solvent promotes the diffusion of polymer chains from one particle to another. As they cross boundaries they help in the formation of a uniformly built film. Temperature also plays role here as does the polymer stracture. The mobility of polymer chains increases rapidly above the glass transition temperature. It is therefore, a combination of the ambient temperature, the glass transition temperature of... 

For the specific case of two similar polymers below their glass transition temperatures in contact, there may be an interaction via the diffusion of polymer chain ends or segments across the interface, usually in both directions. The concept is straightforward and easily grasped, although the theoretical development is both difficult and complex. However, it was largely mastered some thirty years ago, mainly by Russian workers and considerable... 

One of the major problems in modern polymer science relates to the understanding and verification of the relations governing diffusion in the very short time interval, when polymer chains just leave the de Gennes tube see Section 5.4.2. Wool and co-workers (3,42) developed the minor chain model, indicating that the diffusion of polymer chains in the melt should scale as see Sections S.4.2.2 and 11.5. 

S Diffusion of Polymer Chains in a Fixed Network Although the tube model and the reptation model were originally developed to explain the diffusion of polymer chains in concentrated solutions and melts, we can use it more naturally for the motion of polymer chains in a fixed network, for instance, a cross-linked network of polymer swollen in a good solvent. In the fixed network, the constraint release is absent. Therefore, we should be able to observe the reptation without being compromised. 

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