Polymer crystallization filler effect

Applications. Optical microscopy finds several important applications in filled systems, including observation of crystallization and formation of spherulites and phase morphology of polymer blends. " In the first case, important information can be obtained on the effect of filler on matrix crystallization. In polymer blends, fillers may affect phase separation or may be preferentially located in one phase, affecting many physical properties such as conductivity (both thermal and electrical) and mechanical performance. 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

In summary, it seems clear that particulate fillers can have significant nucleating effects in semi-crystalline polymers and that this may lead to effects on mechanical properties. Much work remains to be done to clarify this and provide a clear, coherent description of the effects involved, however. This would be greatly helped if simple techniques for determining polymer crystal structures in filled systems were available. It would also be highly desirable to have a better understanding of how the structure of polymer crystallinity affects composite properties. 

As mentioned previously, the addition of filler may also change the amount of crystallinity in the polymer. As polymer crystals are impermeable even to low molecular weight species, an increase in crystallinity also results in improved barrier properties, through increased tortuosity [54], This effect is expected to be especially prevalent for fillers that induce a high degree of transcrystallinity. 

Effects of One-Dimensional Nanofillers Like layered-inorganic fillers, carbon nanotubes influence polymer crystallization when incorporated as filler in the polymer matrix however, these effects do not have as wide a variety as the layered silicates discussed above. In the vast majority of reports, carbon nanotubes act simply as heterogeneous nucleating agents in crystallizable polymer... 

For systems in which the polymer and filler have an affinity for each other [strong adsorption], increases in the surface area/ volume ratio of the filler can result in large changes in the volume fraction of polymer that is henceforth considered to be "bound" to the filler interface. Many changes in physical phenomena related to the polymer chain dynamics, e.g., the glass transition temperature [Tg] and degrees and rates of polymer crystallization, could be drastically altered due to this bound layer. This has been referred to as the "nano-effect." ° In cases where Tg shifts are observed, the effect is somewhat similar to that reported for thin polymer films. The most important result of an increased bound-polymer layer is the consequent changes in mechanical properties of the final composite.i ... 

Polymer crystallization behavior near an inorganic surface has been the focus of extensive study. In most cases the inorganic surface is shown to produce a nucleating or epitaxial effect,which often stabilizes the bulk crystal phase or, in some cases, promotes growth of a different crystal phase. The polymer mechanical and thermal properties can be enhanced through this mechanism, where the surface-nucleated crystalline phase has better mechanical and thermal characteristics than the bulk crystal phases. Fillers with large surface area maximize these filler-induced enhancements of the material properties a dramatic manifestation of such a response is found in nylon-6/montmorillonite... 

On the basis of the above data it has been hypothesized that the conductivity of PFCM is due not to the contact between the filler particles but the current passes across the thin (less than 1 -2 microns) polymer interlayers. The conductivity arises when a spontaneous pressure exceeding the threshold value develops in the material. The overstresses apparently arise as a result of PP crystallization in the very narrow gaps between the filler particles [312], Since crystallization must strongly affect the macromolecular conformation whereas the narrowness of the gap and fixed position of molecules on the filler prevent it, the heat released in the process of crystallization must, in part, be spent to overcome this hindrance, whereby a local high pressure may arise in the gap. This effect is possible only where there are gaps of the size comparable with that of macromolecules. The small gap thickness will also hamper pressure relaxation, since the rate of flow from such a narrow clearance should be negligibly small. 

The refractive index is the most important optical property and its effect in determining the appearance of the polymer composite has already been referred to above. Amorphous fillers such as glass fibres and beads have only one refractive index, but most mineral fillers are crystalline and have anisotropic crystal structures resulting in a number of different indices, and this can cause complex and undesirable interference effects [27]. 

The structure of crystalline polymers may be significantly modified by the introduction of fillers. All aspects of the structure change on filling, crystallite and spherulite size, as well as crystallinity, are altered as an effect of nucleation [9]. A typical example is the extremely strong nucleation effect of talc in polypropylene [10,11], which is demonstrated also in Fig. 2. Nucleating effect is characterized by the peak temperature of crystallization, which increases significantly on the addition of the filler. Elastomer modified PP blends are shown as a comparison crystallization temperature decreases in this case. Talc also nucleates polyamides. Increasing crystallization temperature leads to an increase in lamella thickness and crystallinity, while the size of the spherulites decreases on... 

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