Diffusion chain entanglement, influence

In this work, we discuss briefly some aspects of the molecular theory for diffusion in entangled polymer systems proposed by de Gennes, and we review the studies on diffusion of PDMS chains in melts and in networks, not only considering the influence of free volume or polydispersity effects but also focusing attention on data analysis procedures. Since most of the data available for self-diffusion of PDMS has been obtained using NMR techniques, some considerations of the NMR methods and models used for analysis of the experimental results are included as well. 

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. 

In addition, plastics behave differently under simultaneous mechanical load [243]. The attack mechanisms function entirely differently in plastics than they do in metals. The intramolecular secondary valence bonds (van der Waals forces) are several orders of magnitude smaller (1/100 to 1/1000) in polymers than in metals. Therefore, the free volume between bulky and entangled molecular chains is so large that the comparatively small gas and liquid molecules can easily diffuse into the intermediate spaces and become embedded there. Thus, the influence on the plastic is not limited to its surface, but takes place virtually throughout its volume. In glass fiber-reinforced plastics with their heterogeneous structure, interfacial problems also develop [243]. 

Though the diffusion model predicts very well the time-dependence of the rehealing experiments, there are still many unanswered questions, such as the absolute value of D, the roles of chain-ends and of molecular weight, the influence of the relaxing fibrils derived from the fracture event, and the nature of the physical links. The relationships of D to viscoelastic and structural parameters still rely on the macroscopic formulation such as the Buche-Cashin-Debye s equation as in Eq.(1), which was derived from the free draining model for both entangled and... 

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