Amorphous alignment

Figure Bl.24.11. The backscattering yield from an Si sample tiiat has been implanted with Si atoms to fonn an amorphous layer. Upon annealing this amorphous layer reerystallizes epitaxially leading to a shift in the amorphous/single-erystal interfaee towards the surfaee. The aligned speetra have a step between the amorphous and erystal substrate whieh shifts towards the surfaee as the amorphous layer epitaxially reerystallizes on the Si.

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

The crystallinity can be dismpted by substituents on the chains that interfere with the alignment process. Amorphous nylons are produced by deUberately engineering this effect, eg, nylon-NDT/INDT (also known as PA-6-3-T or PA-TMDT), which uses trimethyl-substituted hexamethylenediamine isomers combined with terephthaUc acid. 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

The dipole-dipole (Keesom) interaetion eomes about from the faet that on the average, two freely rotating dipoles will align themselves so as to result in an attraetive foree, similar to that eommonly observed with bar magnets. In order to ealeulate the net dipole-dipole interaetion, it is neeessary to examine all the possible orientations of the dipoles with respeet to one another. It is also neeessary to determine any jr effeets due to the field assoeiated with a point eharge, in order to determine the net effeet when amorphous solids are plaeed side by side. We also need to eonsider what happens if the dipoles ean reorient in eaeh other s fields. 

Work on the production and oxidation of SWNT samples at SRI and other laboratories has led to the observation of very long bundles of these tubes, as can be seen in Fig. 2. In the cleanup and removal of the amorphous carbon in the original sample, the SWNTs self-assemble into aligned cable structures due to van der Waals forces. These structures are akin to the SW nanotube crystals discussed by Tersoff and Ruoff they show that van der Waals forces can flatten tubes of diameter larger than 2.5 nm into a hexagonal cross-sectional lattice or honeycomb structure[17]. 

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. 

Fig. 18.2H FT spectra of the alignment echo for different waiting times t2 and different temperatures. Sample LPE, amorphous fraction...

The properties of a semicrystalline polymer are controlled by its degree of crystallinity, the alignment of crystallites relative to one another, the number and type of links between the crystallites and amorphous regions, and the overall orientation of molecules within the material. 

Re-organization of amorphous chain segments adjacent to crystallite surfaces leads to crystallite thickening. These segments align themselves to form additional crystalline unit cells that increase the c dimensions of the crystallites. 

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