Filler crystallites

In polymer regions of PFCM bordering on the filler one typically observes the formation of crystallites as tape-like structures orientated perpendicularly to the filler surface [295, 305, 306]. In real fact these are transcrystallite structures. A morphology of this kind can appear in a mechanical mixture as well [305, 306], but there the layer thickness will be much smaller [306, 307]. 

This rule of thumb does not apply to all polymers. For certain polymers, such as poly (propylene), the relationship is complicated because the value of Tg itself is raised when some of the crystalline phase is present. This is because the morphology of poly(propylene) is such that the amorphous regions are relatively small and frequently interrupted by crystallites. In such a structure there are significant constraints on the freedom of rotation in an individual molecule which becomes effectively tied down in places by the crystalhtes. The reduction in total chain mobility as crystallisation develops has the effect of raising the of the amorphous regions. By contrast, in polymers that do not show this shift in T, the degree of freedom in the amorphous sections remains unaffected by the presence of crystallites, because they are more widely spaced. In these polymers the crystallites behave more like inert fillers in an otherwise unaffected matrix. 

As already mentioned, a network can be obtained by linking polymer chains together, and this linkage may be either physical or chemical. Physical linking can be obtained by (i) absorption of chains onto the surface of finely divided particulate fillers, (ii) formation of small crystallites, (iii) coalescence of ionic groups, or (iv) coalescence of glassy sequences in block copolymers. 

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... 

Figure 1.13—occur because of simultaneous crystallization and polymerization at 150°C. This temperature is near the maximum crystallization rate temperature ( 145°C) of nylon 6 homopolymer [66]. The presence of solid crystallites increases the complex viscosity of the polymerizing system because of a filler effect. 

When pitch binder is pyrolyzed during the carbon bake operation, it is converted from an isotropic liquid, with no structural order, to a liquid-crystal (called mesophase) having a layered structure which is finally converted to layers of carbon atoms in a hexagonal lattice of graphite crystallites. These crystallites of binder coke become more disordered and crosslinked into a more-isotropic coke as the pitch QI content increases. Such moderately-isotropic coke, in contrast to highly-anisotropic microstructure (10), is preferred binder coke because it forms both physical and chemical bonds between filler coke particles which are stronger and more oxidation-resistant (8,9). 

X-ray diffraction measurements were used to determine the orientation of talc and lead carboxylate fillers in plasticised PVC extrudates. Correlations between the extrusion conditions (draw ratio and temperature), the development of filler particle orientation and the tensile properties of the plasticised PVC were studied. The presence of fillers enhanced Young s modulus and this was predicted well by the model developed by Halpin and Tsai. The extrudates were stretched above and below the gel-liquid transition temperature of PVC (about 205C). Above this temperature, the PVC could be stretched more and the tensile results indicated that the crystallites which were surrounded by more flexible chains were more oriented. 24 refs. 

Presence of crystallites diffusing molecules normally are not soluble in crystallites, and they act as fillers, inhibiting diffusion. 

The crystalline structure of composite materials can be highly varied. The measurements of crystallinity show how the combined interference of the various components of the composite influences the structure. Filled material is composed of crystalline and amorphous regions separated by an interphase which is a diffuse boundary between these two states. The crystallinity of the binder material depends on the fraction of crystalline structures and on their size. Filler may affect both the fraction and the size of crystallites. But, those two measures of crystalline structure are often insufficient and the measurement of crystallinity may give confusing in-f ormation if the results are taken without further analysis of the fine structure of the material. Table 10.1 gives examples of the effect of fillers on material crystallinity from the current literature. ... 

Three processes of orientation occur simultaneously during the processing of filled materials. These are filler particle orientation (see Chapter 7), chain orientation (or conformation change) as related to filler particle, and the direction of crystallite growth. Often orientation is detrimental to the material produced. These processes are very difficult to study. Some information is available but more is needed. 

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