Oriented samples, macromolecule

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

Up to now we have considered the CP film as an amorphous, homogeneous and isotropic medium. However, CPs are intrinsically anisotropic since the 7r-electrons are delocalized along the macromolecule backbone. An anisotropic optical response, typical of oriented samples, is extremely important both for fundamental science (e.g., comparison with theoretical predictions) and for technological reasons (polarized emission is recommended in displays [55,56]). The orientation process for conjugated polymers is very difficult and several approaches have been used to remove the typical random-coil conformation of CP and to induce the extended aligned conformation. It is not the aim of this article to review all orientational techniques used with CPs. However, it should be mentioned that very high degrees of orientation have been achieved... 

Objects for investigation are highly oriented flexible- and rigid-chain polymers in the form of films and fibers. In such samples, macromolecules are packed quite perfectly along the draw axis, and the most extended conformations are realized. 

Figure 8. Models of PChMAA-II (a) and PChMA(a ) macromolecules and packing of PChMAA-n macromolecules in oriented samples (b,h c,cf) at n 5 (h,c) and n < 5 (b, c ) b, b —solid lines indicate macromolecules lying in the drawing plane, broken lines indicate macromolecules lying in a plane parallel to that of the drawing c,c —projection along the main chain of macromolecules.

Hedden, R. C. Tachibana, H. Duncan, T. M. Cohen, C., Effects of Molecular Structure on Segment Orientation in Siloxane Elastomers. 2. NMR Measurements from Uniaxially Stretched Samples. Macromolecules 2001, 34, 5540-5546. 

The amide A and B bands eirise fi om the NH stretching vibration in resonance with the first overtone of amide II (essentially NH bending) vibration. These bands are very sensitive to hydrogen bonding and have been extensively utilized for hydrogen bonding in biological macromolecules and pertinent model compounds. In oriented samples these bands show marked dlchroism and can be utilized to stucty the direction of N—H bonds. 

In advancing possible explanations of the observed appearance of birefringence in a homeotropically oriented sample, the probability of only partial perturbation of the homogeneous orientation of the film and slope of the optical axis due to a change in the ccmformational state of the main chain of the macromolecule at the linkages of the side groups at the transition point is indicated in [62]. Nevertheless, the formation of an unusual biaxial nematic phase is also possible in comb-sh )ed polymers, but additional expmmental evidence is required. 

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. 

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