Amorphous perfect crystalline structure

Plastic molecules that can be packed closer together can more easily form crystalline structures in which the molecules align themselves in some orderly pattern. During processing they tend to develop higher strength in the direction of the molecules. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs (normally up to 85% crystalline and the rest amorphous). In this book and as usually identified by the plastic industry, they are called crystalline. 

An amorphous polymer in a state of molecular alignment is not a stable structure - it is metastable. It can uansition either to a more perfectly ordered, crystalline structure, or to a more disordered, nonoriented structure In either case, the free energy of the system is reduced. Given enough time and/or thermal energy, an oriented amorphous polymer will transition in either or both of these directions. 

The crystalline plastics (basic polymers) tend to have their molecules arranged in a relatively regular repeating structure such as polyethylene (PE) and polypropylene (PP). This behavior identifies its morphology that is the study of the physical form or structure of a material. They are usually translucent or opaque and generally have higher softening points than the amorphous plastics. They can be made transparent with chemical modification. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs. The crystalline TPs normally has up to 80% crystalline structure and the rest is amorphous. 

The artificial force is estimated from the intermolecular force field experienced by each molecule. Since the molecules are not in a perfect crystalline position, there always exists a force imbalance, which may be induced by thermal excitation, defects, non-crystalline structures, or other disturbing sources. At each time step the forces acting on each molecule by neighboring molecules are averaged, and the averaged force is used as the amplitude of the external force. Another parameter to be determined is the period of the external cyclic field, which is estimated by the oscillating frequency of the lattice constant divided by the atomic velocity. Moreover, to speed up numerical calculations the external cyclic field frequency of 500 GHz are imposed on some molecules selected from the amorphous-phase, although 2.45 GHz is applied for the experimental microwave assisted crystallization [3], and much lower frequencies are currently tried for the AMFC. 

We also note that certain polymer melts crystallize partially upon cooling. The transition occurs at a well-defined temperature rf. Crystallization takes place only if the polymer has a perfect linear structure for instance, it must not contain any asymmetric carbon. However, this tacticity condition is not sufficient, since polydimethylsiloxane, which is perfectly periodical, does not crystallize under normal conditions. On the contrary, polyethylene and isotactic polyethylene crystallize easily. In general, these polymers contain a large amorphous fraction. This is why they are called semi-crystalline. In certain conditions, it is possible to prepare polymer samples that are perfect crystals, in particular by polymerization in situ of a crystal made of monomers (polydiacetylene and polyoxyethylene). 

As discussed earlier briefly, semicrystalline or amorphous nanotubes can be obtained from 3D compounds and metals by depositing a precursor on a nanotube template intermediately, and subsequently removing the template by calcination. If the template molecules are not removed and they are able to effectively passivate the dangling bonds of the compound, a perfectly crystalline nanotube composite can be obtained. However, after high-temperature calcination, the organic scaffold is removed and the inorganic oxide remains. Because a nanotube is the rolled-up structure of a 2D-molecular sheet, there is no way that all the chemical bonds of the 3D-inorganic compound will be fuUy satisfied on the nanotube inner and outer surfaces. Furthermore, the number of molecules increases with the diameter, and hence a full commensuration between the various molecular layers is not possible. Therefore, nanotubes of 3D... 

Although only two allotropic crystalline forms of elemental carbon, diamond and graphite occur in nature, carbon also occurs as a spectrum of imperfect crystalline forms that range from amorphous through mixed amorphous, graphite-like and diamond-like to the perfectly crystalline allotropes. Such imperfect crystalline structures are termed turbostatic and... 

All the aspects and processes discussed above apply also to the crystallisation of polymers [18-31]. However, in addition, the connectivity of the monomers causes several restrictions. One of the most obvious ones is represented by the quasi-two-dimensional lamellar shape of polymer crystals. A lamella is formed by crystalline segments of the chain (the stems) arranged vertically and limited (on top and below) by amorphous (fold) surfaces. Therefore, polymer crystals grow essentially only in two dimensions. Growth in the third dimension is rather difficult, in particular when the polymer contains non-crystallisable units which will segregate to the surfaces of the crystals. Growth in the third dimension necessitates deviations from the perfect lamellar structure, e.g. screw dislocations. The topic of polymer crystallisation has been the subject of a tremendous amount of studies over the last 60 years [5-8,10-65],... 

Stmctural aspects of components constituting low-density polyethylene/ ethylene-propylene-diene rubber (LDPE/EPDM) blends are studied in bulk and compared to the surface layer of materials. Solvation of a crystalline phase of LDPE by EPDM takes place. The effect is more significant for systems of amorphous matrix, despite a considerable part of crystalline phase in systems of sequenced EPDM matrix seems to be of less perfect organization. Structural data correlate perfectly with mechanical properties of the blends. Addition of LDPE to EPDM strengthens the material. The effect is higher for sequenced EPDM blended with LDPE of linear structure. 

Structure. In Chapter 5, it was identified that small distortions of the lattice can be observed. Further, measurement of any crystalline polymer will indicate that the density is less that that from the predictions for the perfect crystalline material, indicating the presence of amorphous material. It is proposed that part of the amorphous content arises from disorder irregular folding of the polymer chains (Figure 6.3). 

The specific heat of amorphous polymers increases with temperature in approximately a linear fashion below and about Tg. A step-like change occurs around the glass transition temperature as shown in Fig. 6.27(a). With semi-crystalline polymers, the step change at Tg is much less pronounced however, a very distinct maximum occurs at the crystalline melting point. At the melting point, the specific heat is theoretically infinite for a material with a perfectly uniform crystalline structure, as shown in Fig. 6.27(b). Since this is not the case in semi-crystalline polymers, these materials exhibit a melting peak of certain width as show in Fig. 6.27(c). The narrower the peak, the more uniform the crystallite morphology. 

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