Semi-rigid polymers

Fortunately such systems exhibit the unusual property to be able to form liquid-crystalline liquids under certain conditions (of molecular mass, temperature, nature of solvent, concentration, etc. the liquid-crystalline liquid obtained is mostly of the nematic type. As already described in Chap. 2, two types of semi-rigid polymers can be distinguished ... 

One of the semi-rigid polymer fibres, which is now produced on a big scale is Poly (p-Phenylene Terephtal Amide) we shall discuss this product in some more detail. 

A modification of the lattice model to account for semi-rigid polymers replaces the contour length, L, in the axial ratio with the Kuhn segmental length (34,35) i.e., x is replaced with xk = 2q/d... 

The anisotropy of the phases rich in the semi-rigid polymer (CTA or CELLOH) was demonstrated by optical microscopy and opticd rotations. Typical optical rotations for CTA (ot CELLOH), PET and solvent ternary mixtures are shown in Table XI. 

In this section, we review three representative cases of flow-induced mesophases. PET serves as an example of a semi-rigid polymer with intrinsic rigid building blocks. The shear-induced smectic ordering in the flexible-chain polymer iPP provides a special case of induced rigidity, which is still far from fully understood. PDES has been chosen as an example of a stretch-induced condis crystal. Various experimental techniques have been employed to characterize flow-induced mesophases. While scattering methods probe the ordering, a direct technique to measure the molecular mobility is nuclear... 

A solution is found in an Expansion in eigenfunctions". The sphe-rical symmetry suggest use of spherical harmonics. This leads finally to a matrix equation which can be solved numerically. In another approach"l8 which has proven successful in calculation of phase diagrams, a Landau de Gennes expansion for semi-rigid polymers has been derived. The partition function Z = J Dr P (r) and free energy F = - KT In Z are expanded in the order parameter and... 

K is related to the polymer- solvent inteiation and is an indication of chain stiffiiess (e.g. for cellulose is ca. 1.5, indicating cellulose is a semi-rigid polymer). The value of K depends of the solvent in which the intrinsic viscosity is measured. K values for different solvents may be found in the Polymer Handbook (38). Some rate data for the acid hydrolysis of different cellulose samples are in Table I. 

In this section the development of the orientation in the spinning process is discussed, mainly based on the results of experiments performed in our laboratory (see Fig. 7, 8, 9 and 10). The actual fiber formation by coagulation will be given attention in Sect. 3.4. A useful survey of the various factors involved in the solution spinning of rigid and semi-rigid polymers has been given by Ciferri and Valenti [83]. 

Better theoretical models need to be developed that explicitly incorporate the conformational features of the polymers in a mixture. Such models need to deal with behaviors that range from the limits of rigid rods to random coils in a systematic way. In particular, semi-rigid polymers, that are common to high temperature polymers, need to be studied. In addition to the conformation of the two polymers, an explicit way of incorporating specific enthalpic interactions needs to be defined. The development of the proposed model will allow for a theoretical examination of the relative importance of entropic and enthalpic effects to the phase behavior and miscibility. 

It must be noted that such hydrodynamic retention is less likely for semi-rigid polymers like xanthan since their conformation makes more probable their hydrodynamic orientation parallel to the opening axis. 

The long-chain semi-rigid polymers described in the previous sections are thermoplastic materials with glass transition temperatures ranging from 220 to 350°C. This means that they must be heated at temperatures at least 100°C higher than the... 

Table 2 Dilute solution parameters for rigid and semi-rigid polymers... 

Flory et al. considered only the shape anisotropy of the rod-like molecules, they derived expressions for the phase equilibrium in mixtures of low molar mass mesogens with rod-like polymers, semi-rigid polymers, and flexible coil polymers ... 

Lattice theories have been most successful in treating the role of molecular shape, as manifested in the axial ratios of highly elongated molecules or of rigid and semi-rigid polymer chains. The Maier-Saupe theory, put forward in 1959-60, addresses feature (ii) exclusively. The lattice theory has been extended recently to accommodate orientation-dependent interactions of the kind comprehended in the Maier-Saupe theory. 

Specimens prepared in this way show a characteristic microstructure due to the process of shear. Thus the structures observed are not identical to those obtained by other techniques such as ultramicrotomy of a bulk sample. Nevertheless, examination of these specimens at the resolution made possible by TEM has yielded much information and provided additional insight into the organization of these semi-rigid polymer molecules. 

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