Molecular chain axis

Thermoplastic polymer macromolecules usually tend to become oriented (molecular chain axis aligns along the extrusion direction) upon extrusion or injection moulding. This can have implications on the mechanical and physical properties of the polymer. By orienting the sample with respect to the coordinate system of the instrument and analysing the sample with polarised Raman (or infrared) light, we are able to get information on the preferred orientation of the polymer chains (see, for example, Chapter 8). Many polymers may also exist in either an amorphous or crystalline form (degree of crystallinity usually below 50%, which is a consequence of their thermal and stress history), see, for example, Chapter 7. 

A further complication arises with this material in that the diffraction pattern is not a true fibre pattern. The molecular chain axis in the crystallites is tilted with respect to the fibre axis (2,3) and this causes reflections to be displaced above and below the mean layer line position. Thus it is possible that some reflectionswhich would overlap in a true fibre pattern to such an extent that their intensities could not be determined separately, might be resolved with the present material. When this happens for a reflection of observable intensity, then it will be measured as a discrete reflection and the resolution of the final structure will be higher than if the material had true fibre symmetry. However, Stambaugh et al (4)... 

The chemical shift lineshape changes observed for the crystalline fraction of melt-recrystallized PTFE from /L14°C to w28°C (Figure 6) are due to reorientation about an axis essentially parallel to Ozz (which is tilted VL2° from the molecular chain axis). The temperature dependence of the line-shape changes can be simulated as an increase in the rate of rotational diffusion about this axis (Figure 6) and this process has an activation energy of 48+11 kcal/mole. In addition, the... 

In the amorphous regions of PTFE we identify d with the time average of the local chain direction and with its instantaneous direction. Since the torsional motion about the chain axis above -68°C is such that we retain effective axial symmetry of the chemical shift with respect to the molecular chain axis, we do have the angular dependence required by condition (i) ... 

Fig. 9. Schematic illustration of how the strength of 13C-13C dipolar coupling between neighbouring 13C spin pairs in polyethylene changes by virtue of the change in orientation of the 13C-,3C vector (described by 0 and Oj) with respect to the applied field, Bo, when the polyethylene chain undergoes a 180° flip about the molecular chain axis.

The ESR spectra for stretched PTFE films were also measured at room temperature and 77 K. The room temperature spectra obtained at various orientations of the stretch axis to the magnetic field are shown in Fig. 7.28. In addition to a strong signal with large anisotropy, a broad weak signal can be seen at g = 2.016. Since this peak is nearly isotropic and its g value is very close to the g value of the symmetric component of the powder samples, we can again attribute this peak to the peroxy radicals of PTFE. The value of 2.005, which was found with the field parallel to the stretch axis, is very close to g/ = 2.006 obtained for the powder spectrum, while 2.021 measured for the perpendicular direction, is close to gj. = 2.022, This means that the symmetry axis of the g tensor is parallel to the molecular chain axis at room temperature. 

An angular dependence of the spectra was also observed at 77 K. As a result, the parallel spectrum has a narrow symmetric peak at g = 2.003, while the perpendicular spectrum still extends from g = 2.005-2.038. Therefore, the direction of the maximum principal value is perpendicular to the stretch axis. This means that the direction of the O—O bond is perpendicular to the molecular chain axis. It is concluded that the COO group lies in the plane perpendicular to the molecular chain axis, as shown in Fig. 7.29. If rapid motion around the chain axis takes place, the g tensor should be axially symmetric about the molecular chain axis, which is in... 

Transition moment vector = Molecular chain axis... 

Fig, 11. Infra-red absorption, (a) Simplest situation where transition moment vector is parallel to the molecular chain axis, (b) Transition moment vector makes an angle a with the molecular chain axis. 

A TEM image and electron diffraction pattern (inset) of P(3HB) single nanofiber spun from lwt.% HFIP solution are shown in Figure 15(c). Many arc-shaped reflections were observed in the diffraction pattern. Similar diffraction patterns have been observed for the drawn fibers and stretched films of P(3HB) in which molecular chains are highly oriented. Particularly, the appearance of (002) diffraction on the meridian su ests that the crystallographic c-axis of a-form P(3HB) crystal, that is, the molecular chain axis, lies parallel to the fiber direction. 

The thermodynamic description of a nucleus as a small crystal is given in Fig. 3.59. It shows a drawing of a prismatic nucleus with a quadratic cross-section, a. Its free enthalpy, G, can be expressed as a function of dimensions a and (, where the length (is taken always along the molecular chain axis. The term - a CAgf represents the bulk... 

As will be seen later, this aspect of orientation description is important in some experimental techniques, where for example OA represents a transition moment at an angle ip to the molecular chain axis OB which has a preferred orientation with respect to the director OC. 

Enantiomorphous pairwise isoclined sequence can be generated only by repetition through a glide plane that contains the molecular chain axis. No helical repetition is possible in this case. 

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