Poly Unit cell

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

The equimolar copolymer of ethylene and tetrafluoroethylene is isomeric with poly(vinyhdene fluoride) but has a higher melting point (16,17) and a lower dielectric loss (18,19) (see Fluorine compounds, organic-poly(VINYLIDENE fluoride)). A copolymer with the degree of alternation of about 0.88 was used to study the stmcture (20). Its unit cell was determined by x-ray diffraction. Despite irregularities in the chain stmcture and low crystallinity, a unit cell and stmcture was derived that gave a calculated crystalline density of 1.9 g/cm. The unit cell is befleved to be orthorhombic or monoclinic (a = 0.96 nm, b = 0.925 nm, c = 0.50 nm 7 = 96%. 

Poly(vinyl fluoride) [24981-14-4] (PVF) is a semicrystaltiae polymer with a planar, zig-zag configuration (50). The degree of crystallinity can vary significantly from 20—60% (51) and is thought to be primarily a function of defect stmctures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (52). Similarity to the phase I crystal form of poly (vinytidene fluoride) suggests an orthorhombic crystal (53). 

Properties of PET Molding Resins. The fliU crystal stmcture of poly(ethylene terephthalate) has been estabhshed by x-ray diffraction (134—137). It forms triclinic crystals with one polymer chain per unit cell. The original cell parameters were estabhshed in 1954 (134) and numerous groups have re-examined it over the years. Cell parameters are a = 0.444 nm, b = 0.591 nm, and c = 1.067 nm a = 100°, (3 = 117°, and 7 = 112° and density = 1.52 g/cm. One difficulty is determining when crystallinity is fliUy developed. PET has been aimealed at up to 290°C for 2 years (137). 

An analogous case, of identical chain conformations as well as of similar unit cell dimensions, have been described for the two crystalline forms of poly-p-phenylene terephtalamide [33-36] (better known with the trade name of Kevlar). The projections along the c axis of the packing of the chains proposed for the two forms [36] has been sketched in Fig. 8, corresponding to the localization of the chain axes in (0,0, z) and (1/2,1/2, z) for the more common polymorph, in (0, 0, z) and (1/2,0, z) for the other polymorph. 

Fig. 18.—Antiparallel packing arrangement of 2-fold poly(ManA) (15) helices, (a) Stereo view of two unit cells roughly normal to the hoplane. The helix at the center (filled bonds) is antiparallel to the two in the back (open bonds). Intrachain hydrogen bonds stabilize each helix. Association of helices through direct hydrogen bonds involve the carboxylate groups for parallel chains, but involve the axial hydroxyl groups for antiparallel chains, (b) A view of the unit-cell contents down the t-axis highlights the interactions between the helices.

With lower-molecular-weight polymers unit cell parameters may also vary with the molecular mass distribution. For poly(ethylene terephthalate) the history of reported unit cell parameters reflects the progress of chemical processing technology [105]. 

To clarify the tacticity problem, trans-l,4-hexadiene and 5-methyl-l,4-hexadiene polymers were examined by X-ray diffraction. Fiber diagrams were obtained from samples stretched to four times their original lengths. Eight reflections from the poly(trans-1,4-hexadiene) fiber pattern may be interpreted on the0basis of a pseudo-orthorhombic unit cell with a = 20.81 + 0.05 A b =... 

French500 has collected together unit-cell data for maltose hydrate and some poly-O-acylsaccharides in the hope that some packing information might be obtained which could be applied to the problem of starch structure. 

The orthogonal projection of the epitaxial poly(DMDA) could not be indexed using the unit cell data for the bulk polymerized crystal (8). However, poly(DMDA) cannot usually be polymerized to completion or to high crystallinity in the bulk due to cross-linking. The use of an epitaxial substrate may have controlled the polymerization process that led to oriented single crystals. 

Similar frustration has been evidenced for the crystal structure of isotactic poly(2-vinylpyridine)150 161 (Figure 2.28). Also in this case three independent threefold helices are included in the trigonal unit cell and the frustration is related to a different azimuthal orientation of the chains due to the different interactions between the chains.160 Two chains (chains A in Figure 2.28) maximize their interactions at the expense of the third one (chain B). 

Optical compensation for polymers with chiral monomeric units may also occur when the racemic polymer consists of crystallites, each composed only of the rectus chains or only of the sinister polymer chains, and a same amount of optical antipode crystallites is present. This intercrystallite optical compensation211 has been found, for instance, in isotactic poly(propylene sulfide),212 poly ((3 -methy lpropiol ac (one),213 and poly(isopropylethylene oxide),214 where isochiral 2/1 helical chains are included in orthorhombic unit cells according to the space group P2 2 2. ... 

Figure 22. Schematic of poly vinylidene fluoride showing polarity in the unit cell when suitably oriented, so that the system exhibits macroscopic polarity and...

Fig. 5.2 The structure of crystalline poly(ethylene oxide) showing the contents of one unit cell.

As mentioned earlier, poly(thioformaldehyde) is a highly crystalline polymer, and it has been shown to have a hexagonal crystal structure (14, 24). Thus, poly-(thioformaldehyde) obtained by irradiation of trithiane followed by heating has a hexagonal unit cell with a = 5.07 A and c=36.52 A crossed by one helical chain parallel to the c axis. The chain has an identity period of 36.52 A that comprises 17 —CH2S— units in 9 turns of the helix (14). Poly(thioformaldehyde) made by other methods crystallizes similarly (24). 

In a poly crystalline mixture of solid reactants, we might expect the particle size to be of the order of 10 pm even careful and persistent grinding will only reduce the particle size to around 0.1 pm. Diffusion during a ceramic reaction is therefore taking place across anywhere between 100 and 10,000 unit cells. Various ingenious methods, some physical and some chemical, have been pioneered to bring the components of the reaction either into more intimate contact, or into contact at an atomic level, and so reduce this diffusion path in doing so, the reactions can often take place at lower temperatures. 

Fig. 10. Variation of the a axis of the tetragonal unit cell of poly-4-methylhexene/ poly-4-methylpentene mixtures as a function of the composition (from Macromolecules, in press)...

The arrangement of helices in the solid and liquid crystalline states of poly(a-phenylethyl isocyanide) were determined by X-ray and electron diffraction. Well-defined diffraction patterns were obtained from oriented films using selected area electron diffraction. Intermolecular and intramolecular patterns were calculated from the five Debye-Scherrer rings. All the reflections were indexed in terms of a pseudo-hexagonal triclinic unit cell, with... 

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