Poly triclinic unit cell

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... 

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. 

Table 1 shows the monomer repeats for centrosymmetric chains with a range of values of Xj and X2, where it is seen that c/2 does not vary by more than 0.2A. We are now considering the packing of the poly(MDI-BDO) chains in the proposed triclinic unit cell, and will report on this in due course. 

PDO, and EDO preparations (12) are shown in fig. 4. The poly-(MDI-BDO) pattern has been analyzed previously and indicates a triclinic unit cell in which the base plane is tilted, i.e. it is not perpendicular to the fiber axis, such that the chains are in a staggered hydrogen bonded array. The conformational analysis above shows that the polymer chain is fully extended, with the all-trans -(CH2)ij- unit coplanar with the urethane groups. 

Poly(ethylene tephthalate) (PET) is a crystalline polymer characterized by a triclinic unit cell. The chain axis is 10,7 A and corresponds to chains in fully extended conformation, with all dihedral bonds close to the trans-planar conformation (tf symmetry) [258], As shown in Fig. 30A, in this conformation, the terephthalic residues are all coplanar. 

A problem arises with many polymers, eg, pol3q>ropylene and poly(ethylene terephthalate) (PET), which lack suitable QiOO) and (O/jO) reflections, but exhibit monoclinic and triclinic unit cells. This problem was first attacked by Wilchinsky (61-63), who pointed out that... 

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). 

For poly(butylene terephthalate) (PBT), two crystalline modifications (a- and P-forms) have been reported and their crystalline structure analyzed by X-ray diffraction [36]. The primary modification is the a-form which belongs to the triclinic system with unit cell parameters a = 4.83 A, b = 5.94 A, c (fibre axis) = 12.95 A, a = 99.7°, P = 115.2° and y = 110.8°. The transition between the two modifications takes place reversibly by mechanical deformation by elongation from the a-form to the P-form and inversely by relaxation. X-ray analysis confirmed that poly(ether ester)s based on PBT has a-unit cell structure, similar to PBT-homopolymer, but with small increase in the unit cell dimensions. The unit cell parameters are not dependent on the HS length [32]. 

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