Poly crystalline, 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). 

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). 

The closely related trans-I,4-poly(2 ethylbutadiene) (gutta percha) has also two crystalline polymorphs. The stable, monoclinic a-crystal form (P2jC) grows from the melt above 318 K (T = 353 K) and has an entropy of fusion of 36.4 J/(K mol), indicative of full conformational order. The s nd polymorph, the orthorhombic P-crystal form (Pnam) grows at lower crystallization temperatures, but has only a somewhat lower entropy of fusion [29.7 J/(K mol)]. Conversion from p to a is not possiUe, or at least extremely slow. Both polymorphs have been asigned distinct chain conformations , i.e. neither seems to show large stale dynamic conformational disorder. 

The structure of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fibres exhibit three crystalline structures two orthorhombic, and one pseudo-hexagonal. The orthorhombic crystals with the c-axis preferentially orientate perpendicular to the fibre axis, and pseudo-hexagonal crystals are preferentially formed in the drawing process (114). 

Assuming no conformation dependent chemical shift effects to occur and using the chemical shift of orthorhombic polyethylene (33 ppm) [6] we can now calculate the chemical shifts of the methine carbon atoms in the three triads of the solid crystalline E-VOH copolymer, respectively 000 (67 ppm), CX)E (70.4 ppm) and EOE (73.8 ppm) where 000, OOE, EOE are abbreviations for (VOH, VOH, VOH), (VOH, VOH, E) and (E, VOH, E) triads. The chemical shift values presented above are only meant to yield useful assignments of the several methine carbon NMR signals of E-VOH copolymers. These assignments are necessary because Ovenall [5] did not report dependable estimates for all three types of methines sustained by experimental results. We are aware of chemical shift differences between liquids and solids. Moreover, the choice of orthorhombic polyethylene as a basis for the shift calculations is rather arbitrary but this will only cause the same uncertainty in each of the three shifts. Of more importance is the known sensitivity of substituent-induced shifts towards different conformational equilibria. From results obtained by Cantow [7] for different poly (1,2-dimethylbutane) polymers it can be estimated that the uncertainties in our estimations amount to ca. 2 ppm. It is, however, improbable that the order of the three methine carbon signals will be misjudged. 

True crystallinity bands have been observed for only a few polymers and their origin experimentally verified by isotopic dilution studies [70]. We may quote the classical prototype case of orthorhombic polyethylene [68, 69], orthorhombic poly-oxymethylene [71], and-with caution-possibly a few others [72, 73]. 

Syndiotactic poly(styrene) displays a complex polymorphic behavior that reflects the specific role played by solvents. Four crystalline forms have been reported.(289,290) The a and p forms can be obtained from the melt (or glass), depending on the crystallization conditions.(291) Both structures comprise planar zigzag chains that have the same identity period of 5.1 A. The a form has a trigonal unit cell while the p form is orthorhombic. The P form can also be produced by crys-taflization from solution.(292,293) The y and 8 structures develop after interaction with solvent. In contrast to the all trans bond orientation of the a and p structures, the chains in the y and 8 crystals adopt a ttggttgg sequence of bond orientation. Thus a helical ordered structure evolves. This structure is similar to the crystalline chain conformation of syndiotactic poly(propylene).(294) The difference between the y and the 8 polymorphs is that in the former the sample is completely dried, while the solvent is included in the 8 form. It therefore represents a clathrate type structure. The formation of these structures is, thus, solvent specific.(292,293,295,296) The... 

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