Poly orthorhombic

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). 

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 =... 

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. ... 

Porat et al. performed TEM (zero-loss bright field) studies of very thin Nation films that were cast from ethanol/water solutions, and some of the conclusions are as follows. It was suggested that the backbone had a planar zigzag conformation in large orthorhombic crystallites as in polyethylene, in contrast with the helical conformation found in poly(tetra-fluoroethylene). This is an interesting result, although there are no other studies that support this view. Sulfur imaging indicated the presence of sulfonate clusters that are 5 nm in size. 

Pyroxenes are poly silicates ( inosilicates in the mineralogical classification), crystallizing, respectively, in the monoclinic clinopyroxenes spatial groups C2lc, Pljlc, P2ln) and orthorhombic systems orthopyroxenes spatial groups... 

The monoclinic Eu(HOCH2COO)s (T 2i) and orthorhombic Gd(HOCH2COO)s Pn2id) (Table 18) contain nonacoordinated metal ions. The coordination poly-... 

Fig. 11a—c. Raman spectra of poly- (oxymethylene). a) hexagonal, perdeuterated b) orthorhombic c) hexagonal... 

Poly(L-LA) is a semicrystalline polymer with a Tg of 61 °C and Tm of 174 °C. The crystal structure of PLLA is pseudo-orthorhombic [ 173]. It is a white fibrous material. 

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

At a relative humidity of 80%, the sodium salt of pig-mucosal heparin shows a fiber repeat of 1.65 nm, with a triclinic unit-cell containing one chain. The sodium salt of macromolecular heparin from rat skin, at 78% r.h., showed a repeat of 1.73 nm, and crystallized in an orthorhombic unit-cell. After 24 hours at 84% r.h., the repeat changed to 1.65 nm, the pattern corresponding to that of pig-mucosal heparin. The stereochemistry of the chain in terms of the chain repeat and , i/r maps was discussed. It was suggested that the chemical formula for heparin is poly[(l— 4)-/3-D-GlcpA-(l— 4)- -d-GlcNAcp], and a scheme of 5-epimerization of /3-D-glucuronic acid to a-L-iduronic acid was discussed. 

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

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