Stable morphology

Commercial thermoplastics are the engineering materials containing two or more compatibilized polymers that are chemically bounded in a way that creates a controlled and stable morphology with a unified thermodynamic profile. In view of multiplicity and contradictory requirements of various properties for most of the applications, almost all the commercial PBAs are made of two or more thermoplastics, elastomeric modifiers along with a series of compatibilizers with modifiers compounded together. A considerable number of blends have been appearing in the market regularly, some of which are listed in Table 9. 

We have previously used the term interfacial reaction to describe mixing between two reactive blend components. In reality, as we have seen in the Example 11.2, there is an interphase that is formed on the surface of the dispersed phase where molecules of both components can be found and react (66,67). If the nonfunctionalized blend components have high immiscibility, then the thickness, Si, of the interphase around the droplets, as well as the volume of the interphase, Vh will be small and, thus, the probability of the functional groups to react forming compatibilizing products will be low, giving rise to coarse and not very stable morphologies. Helfand (66) defines Si as... 

Apparently, with a very small interphase thickness the two end-cap groups are too few and not easily accessible to affect compatibilization. On the other hand, when four anhydride (An) groups are attached, randomly on each PDMS chain, then the blend of 20% PDMS/4-An and PA 6/di-amine have a very fine and stable morphology (ca 0.5 pm). Thus, the amount of interfacial reaction product, although diminished by small < / values of the unmodified polymer components, is promoted by the larger number and more accessible functional groups in either or both of the reactive components. Finally, Macosko and co-workers (62) have estimated that the minimum fraction of the interphase that has to be covered by reacted compatibilization products to achieve fine and stable morphologies is about 0.2. 

Tohill MP, Mann DJ, Mantovani CM, Wiberg M, Terenghi G. Green fluorescent protein is a stable morphological marker for Schwann cell transplants in bioengineered nerve conduits. Tissue Eng 2004 10 1359-67. 

This dependence of block polymer property on casting solvent also is seen in poly(styrene-b-diene) polymers (28). Theoretical work (23) has shown that the thermodynamically most stable morphology for a diblock polymer containing 50% of each component is a lamella morphology. For this reason MEK was chosen for the tests on all other polymers. 

Plots in Fig. 5.16 also indicate that, over certain ranges of /x, u (/x) is a multivalued function where the lowest value of u obviously corresponds to the thermodynamically stable morphology (i.e., phase) the others are only metastable. Metastability ends (i.e., the confined fluid becomes unstable) if the inequality in Eq. (1.82) can no longer be satisfied. The reader should realize that in general metastability in MC simulations is an artifact caused by the limited system size and insufficient length of the Markov chain (i.e., the finite computer time available) [184]. Metastability would not be observed in an infinite system where the evolution of the system could be followed indefinitely. In other words, metastability vanishes in the thermodynamic limit. 

The problem of physical blends is not only the formation of a stable morphology. A more important problem for the users of such materials is the changing and most often the lowering of material properties. Table 9 [13] lists the effect of the reactive compatibilisation of polypropylene PA-6 blends of different compositions ... 

PPE, PC, SEBS and either PBT or PET stable morphology, processability, high impact strength, and solvent resistance Brown et al., 1987... 

Improved technology for obtaining reproducible and stable morphologies under commercial extrusion and molding conditions. 

Thus, PA6/g-PE binary blends, whose melts are highly viscous and strong, are characterized, despite a lack of thermodynamic miscibility, by stable morphology marked mutual influence of the components on their crystallinity and relaxation behavior results from interactions between the phases. 

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