Polymer complexation crystallite

If an isotropic polymer is subjected to an imposed external stress it undergoes a structural rearrangement called orientation. In amorphous polymers this is simply a rearrangement of the randomly coiled chain molecules (molecular orientation). In crystalline polymers the phenomenon is more complex. Crystallites may be reoriented or even completely rearranged and oriented recrystallisation may be induced by the stresses applied. The rearrangements in the crystalline material may be read from the X-ray diffraction patterns. 

Although the polymer matrix does not take part in charge carrier transport which, at least in the first approximation, proceeds along the conductive paths formed by CT complex crystallites, it plays an important role in obtaining the conductive systems. The polymer used must fulfill quite specific conditions for the crystallization process to occur quickly in a relatively high viscosity medium. 

Generally speaking, it has been found that the polymer matrix has a significant influence on the crystallization of the complex and the properties of the system. The chemical structure and composition of the polymer used considerably affects the structure of the CT complex crystallites and the optimum cast temperature. 

In more recent work [302-305] using propylene carbonate (PQ as a solvent. X-ray patterns showed the presence of crystallites in PAN-PC gels. These gels are particularly interesting because it has been sug ted that the junctions are not the normal crystallites of pure polymer, but crystallites of a polymer-solvent complex. 

Syndiotactic polypropylene became commercially available about ten years ago with the advent of single-site catalysts. Unlike its atactic and isotactic counterparts, its manufacture presented serious challenges to polymer scientists and engineers. Even under the best conditions, its syndiotacticity rarely exceeds 75%, based on pentad sequences. It typically has both a lower melting point (approximately 138 °C relative to approximately 155 to 160 °C) and density (0.89 g/cm3 relative to 0.93 g/cm3) than isotactic polypropylene. Syndiotactic polypropylene crystallites have a much more complex structure than the isotactic form, which impedes its crystallization. Therefore, in general, the syndiotactic form of polypropylene crystallizes very slowly. 

Most undrawn crystalline polymers possess spherulite morphology with a radial arrangement of fibrils which are complex aggregates of crystallites and amorphous regions. 

Piezoelectric polymer film is usually partially crystalline and the crystallites are embedded in the amorphous phase, which exhibits mechanical relaxations. Therefore, the strain of each crystallite, S, may differ in both amplitude and phase from that of the film as a whole, S. In this case the complex piezoelectric constant of the film is written by putting S/S — K (complex quantity) in Eq. (62) as... 

The rheological behaviour of thermotropic polymers is complex and not yet well understood. It is undoubtedly complicated in some cases by smectic phase formation and by variation in crystallinity arising from differences in thermal history. Such variations in crystallinity may be associated either with the rates of the physical processes of formation or destruction of crystallites, or with chemical redistribution of repeating units to produce non-random sequences. Since both shear history and thermal history affect the measured values of viscosity, and frequently neither is adequately defined, comparison of results between workers and between polymers is at present hazardous. 

The relaxation behavior of selected semicrystalline ESI is depicted in Figure 26.3. It can be seen that the loss peak evident in the temperature range —50 to +50 °C shows increasing breadth of the relaxation process as the styrene content in ESI decreases. The relaxation processes associated with this loss peak are complex in nature. The relaxation behavior of semicrystalline polymers is fundamentally different from that of amorphous polymers. The long-range segmental motions associated with the Tg process become hindered owing to the restrictions imposed by the crystallites. 

In type a., the separating zeolite layer is equipped with catalytic sites (Bronsted add sites, Lewis acid sites (cations, special Al-sites), metal clusters, catalytic complexes). In type b., the non-supported side of the zeolite layer serves as a support for catalytic entities, e.g. metal crystallites. In type c., zeolite crystals with catalytic power are embedded in a matrix, e.g. a polymer membrane. 

Crystallization of polymers can be carried out from the molten state or from solutions. These procedures lead to different morphologies. Starting from solution, single crystals are obtained with the shape of lamella (plates). Starting form the melt, small crystalline lamellae or crystallites are formed that are organized into complex structures known as spherulites. 

Polymers crystallized from concentrated solutions exhibit more complex crystalline morphology (7). In dilute solutions, polymer coils are isolated from each other, but if the concentration increases, association and entanglement among the chains are favored. Therefore, there is a greater probability that a chain forms part of various crystallites, thus producing aggregated lamellae in multilayer morphologies. 

Parton et al. [126] reported on the development of a synthetic system that mimics the cytochrome P-450 enzyme. They embedded zeolite Y crystallites containing encapsulated iron phthalocyanine complexes in a polymer membrane. Using tertiary-butylhydroperoxide as oxidant, this catalytic system oxidizes alkanes at room temperature with rates comparable to those of the real enzyme. 

In summarizing the results from the last three sections, one can conclude that the systematic variation of microhardness under strain performed on (a) homo-PBT (Section 6.2.1), (b) its multiblock copolymer PEE (Section 6.2.2) and (c) on blends of both of these (this section) is characterized by the ability of these systems to undergo a strain-induced polymorphic transition. The ability to accurately follow the strain-induced polymorphic transition even in complex systems such as polymer blends allows one also to draw conclusions about such basic phenomena as cocrystallization. In the present study of a PBT/PEE blend two distinct well separated (with respect to the deformation range) strain-induced polymorphic transitions arising from the two species of PBT crystallites are observed. From this observation it is concluded that (i) homo-PBT and the PBT segments from the PEE copolymer crystallize separately, i.e. no cocrystallization takes place, and (ii) the two types of crystallites are not subjected to the external load simultaneously but in a sequential manner. 

---