Polymeric crystallites

There remains one complication often the polymeric crystallites may occur in several polymorphic forms (e.g. hexagonal, tetragonal, ortho-rhombic, triclinic, etc.) with somewhat different densities. In those cases we have chosen an "average" crystalline density. 

A. Crystal Growth As mentioned above, crystal growth is a process in which the dimensions of stable nuclei, and later the dimensions of polymeric crystallites, increase. The most important physical quantity used to characterize crystal growth is the linear growth rate, which is constant in time, but changes with temperature. This is the rate of crystal size increase in one dimension, and can be determined from polarization optical microscopy experiments. 

There are several types of polymeric crystallites. Of these, the following need to be mentioned ... 

In most cases the polymeric crystallites are small, and the lamellae in these crystallites are very thin (their thicknesses can be as small as -lOnm). The specific surface area of these crystallites is so large that the contribution of the surface free energy cannot be ignored. Thus, the melting process of these crystallites is much more complicated than the melting of macroscopic-size equilibrium crystals. 

Reorganization and Recrystaiiization during Meiting Reorganization is a process in which the initial metastable polymeric crystallites... 

According to Hosemann-Bonart s model8), an oriented polymeric material consists of plate-like more or less curved folded lamellae extended mostly in the direction normal to that of the sample orientation so that the chain orientation in these crystalline formations coincides with the stretching direction. These lamellae are connected with each other by some amount of tie chains, but most chains emerge from the crystal bend and return to the same crystal-forming folds. If this model adequately describes the structure of oriented systems, the mechanical properties in the longitudinal direction are expected to be mainly determined by the number and properties of tie chains in the amorphous regions that are the weak spots of the oriented system (as compared to the crystallite)9). 

Taylor [648] has shown that the deceleratory decomposition of HgO is satisfactorily described by the contracting volume equation [eqn. (7), n = 3], Calculated values of E (162—201 kJ mole rise with increasing crystallite size and are somewhat greater than the enthalpy of dissociation (160 kJ mole 1). Since estimated values of A are consistent with the predictions of the Polanyi—Wigner equation, eqn. (19), it is concluded that breakdown involves the detachment of individual molecules rather than the unzipping of the long zig-zag polymeric —Hg—O— chains which constitute the reactant lattice. 

We can vary the density of very low density polyethylene from 0.90 down to 0.86 g/cm3 by varying the comonomer level from approximately 8 to 14 mole %. At the highest comonomer levels, crystallization is impeded by the branches to such an extent that only about 5% of the material crystallizes. The crystallites of very low density polyethylene are small and poorly organized. We polymerize these resins using single-site catalysts, which give us relatively narrow molecular weight and composition distributions... 

Possible morphologies of partially crystalline polymers are shown in Fig. 18. Figure 18a depicts the case of small crystallites that act as physical crosslinks between polymeric chains, thus connecting those chains into a 3-dimensional network. In the case depicted in Fig. 18b, the material forms ribbon-shaped or needle-shaped crystalline regions in which different segments of a large number of chains are incorporated. This could explain the low degree of crystallinity at the LST as detected for the iPP system [80]. 

Fig. 18a, b. Possible morphologies of partially crystalline polymers. Small crystallites act as crosslinks (a) large ribbon-shaped or needle-shaped crystalline regions connect a large number of polymeric chains (b)... 

We have adapted a commercially available x-ray diffractometer normally used for structure determinations on single crystals to operate as a very flexible device for performing x-ray pole figure determinations and related studies on polymeric materials. Descriptions of crystallite orientations, as provided by pole figures, are useful in studying many aspects of the behavior of products made from semicrystalline polymers. This paper describes the software that we have written for our pole figure facility. Except for some vendor-provided routines to drive the hardware Interface all of our software is written in FORTRAN. Menu driven operation is provided to maximize user convenience. 

X-ray powder diffractometry is widely used to determine the degree of crystallinity of pharmaceuticals. X-ray diffractometric methods were originally developed for determining the degree of crystallinity of polymers. Many polymers exhibit properties associated with both crystalline (e.g., evolution of latent heat on cooling from the melt) and noncrystalline (e.g., diffuse x-ray pattern) materials. This behavior can be explained by the two-state model, according to which polymeric materials consist of small but perfect crystalline regions (crystallites) that are embedded within a continuous matrix [25]. The x-ray methods implicitly assume the two-state model of crystallinity. 

The other process is the transformation of an organic precursor into a continuous thin ceramic fiber. In the spinning process, polycarbosilane, a high molecular weight polymer containing Si and C, is obtained by thermal decomposition and polymerization of polydimethylsilane. The fiber thus produced consists of a mixture of P-SiC, carbon crystallite and SiO. The presence of carbon crystallite suppresses the growth of SiC crystals. Yajima and coworkers (Yajima et al., 1976, 1978, 1979) were the first to produce fine (10-30 pm in diameter), continuous and flexible fibers, which are commercialized with the trade name of Nicalon (Nippon Carbon Co.). 

After Little s proposal, many researchers have pursued such an exciting system in vain. Even metallic behavior was rarely seen in doped organic polymers, gels, and actuators. As mentioned in Sect. 3.4.4, MCso with linearly polymerized Ceo" exhibited one-dimensional (M = Rb, Cs) or three-dimensional (M = K) metallic behavior [144]. Recently a doped poly aniline was reported to exhibit a metallic temperature dependence for a crystalline polymer chemical oxidation of monomers grew crystallite polyaniline [329] early doping studies on polypyrrole (PFg) and poly(3,4-ethylene-dioxythiophene)X (X = PFg, BF4, and CF3SO3) prepared by electrooxidation at low temperatures also showed a metallic temperature dependence below 10-20 K (Scheme 16) [330, 331]. 

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