Chain axis

Figure 4.10 Crystal structure of polyethylene (a) unit cell shown in relation to chains and (b) view of unit cell perpendicular to the chain axis. [Reprinted from C. W. Bunn, Fibers from Synthetic Polymers, R. Hill (Ed.), Elsevier, Amsterdam, 1953.]...

In Parylene C, the single crystalline form observed is very similar to the d form of Parylene N. Its detailed crystal stmcture has been deterrnined (a = 596 pm, b = 1269 pm, c (chain axis) = 666 pm, /5 = 135.2°) (47). X-ray studies on the crystal stmcture of Parylene D have not been reported. 

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparaHel P-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-hehcal (bees, wasps, ants) or cross- P-sheet (many insects) stmctures. The cross-P-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary stmctures during processing from soluble protein in the glands to insoluble spun fibers. 

The repeat distance along the chain axis (0.468 nm) is significantly less than that calculated for a planar zigzag stmcture. Therefore, the polymer must be in some other conformation (65—67). Based on k and Raman studies of PVDC single crystals and normal vibration analysis, the best conformation appears to be where the skeletal angle, is 120°, and the torsional of opposite sign) is 32.5°. This conformation is in... 

It is known that elevating the crystallization temperature [42,49] or annealing above the crystallization temperature [50] of PE results in a thicker folded-chain lamella of up to —200 nm. In addition to the higher temperature, if high pressure is applied, crystals can grow as thick as several micrometers in the chain axis direction [2-4,51]. 

Miyashita et al. [102] have proposed a possible model for the hexagonal-orthorhombic phase transition (Fig. 10). They proposed that since the hexagonal crystal contains many defects along the chain axis, such as kinks and jogs, on its phase transition to the orthorhombic phase, these defects are excluded from the crystal... 

Figure 4-10. Sketch of Hie operations applied ui a colacial dimer formed by two slilbene molecules separated by 4 A when investigating the role of positional disorder. The modilicalions are induced by (I) the translation of one molecule along the chain-axis direction (II) the translation of one molecule along the in-planc transverse axis (III) the rotation of one slilbene unit around its long axis and (IV) the rotation of one slilbene molecule around the slacking axis while keeping the parallelism between the molecular planes.

Long-range positional order in three dimensional is maintained only for structural features which are not point-centered (e.g., for the chain axes, for which two periodicities only are sufficient to define a three-dimensional repetition) iii) Long-range positional order of some feature is maintained only in two or in one dimension (e.g., only along each chain axis). 

Fig. 1 a, b. Projection along the chain axis and side view of models of syndiotactic polystyrene in the a) trans-planar conformation b) s(2/l)2 helical conformation... 

Fig. 2 a, b. Side view and projection along the chain axis of models of isotactic polystyrene in the a) s(3/l) helical conformation b) nearly /raw-planar conformation, proposed for the crystalline gels [12]... 

Let us recall, for instance, the case of the crystalline form of s-PP [113], or of s-PS [114,115], which contain s(2/l)2 helices. In this kind of helix (Fig. 2) there are two non-equivalent sets of methylene carbons in the main chain, the first one on the chain axis and the second one far from the chain axis. While for the methylene on the axis there are two y-carbons (that is carbons separated by three bonds) in G conformation, for the methylene on the periphery of the helix there are two y-carbons in the T conformation. This generates the so called y-elfect, that is a shift difference between the resonances of the two methylene carbons (8.7 ppm for s-PP, 10 ppm for s-PS). 

It is also well known that different polymorphic forms can present largely different crystallite moduli along the chain axis (both observed and calculated). These differences can be large if large variations in the chain conformations are involved, and can have a significant influence on the bulk properties... 

For example, infra-red dichroism measurements will give a value for . If the direction of dipole moment change for the vibration under consideration makes an angle S, with the chain axis, the average for the chain distribution in 0 relates to by the simple relationship... 

For the case where the transition moment vector makes an angle with the chain axis, the chain axis average [Pg.89]

For a biaxially drawn polymer with orthorhombic symmetry, the chain axis is preferentially oriented with respect to the XtX2 plane of the sample, and the direction normal to the chain axis is also preferentially oriented. Infra-red measurements enable the determination of the quantities Pf20 and PL202 and P222, which define these orientation effects as discussed in case (iv) above. 

The biaxially oriented PET sheets have been extensively studied with regard to their mechanical anisotropy and all nine independent elastic constants have been determined by a variety of experimental techniques 38,39). The complete set of compliances for a one-way drawn sheet of draw ratio 5 1 is shown in Table 7. It is interesting to note that these compliances clearly reflect the two major structural features, the high chain axis orientation and the preferential orientation of the benzene rings... 

Annealing drawn PE hydrostatically at high pressure, generates a wide spectrum of crystal thicknesses varying from the common oriented chain folded to the chain-extended structures — where folds and ties tend to disappear63 —. This range of crystal thicknesses coupled with the chain axis orientation, offers a suitable model in... 

---