Polarized laser Raman and FTIR spectroscopies, as well
as wide angle X-ray diffraction
(WAXD), have been employed in order to investigate the distribution of
molecular orientation in uniaxially
drawn solution-cast films of well-characterized polyesters, bearing
hexyl side chains, at different draw
ratios. Both the second (〈P
2(cos
θ)〉) and fourth (〈P
4(cos θ)〉,
only with Raman) moments of the segment
orientation distribution function have been determined. Results
reveal physically meaningful trends of
both P
2 and P
4 with draw
ratio. A critical comparison among the three techniques confirms
the sensitivity
of Raman and FTIR to order at molecular level, when detecting the
orientation of a particular segment,
compared to similar WAXD results that provide information on the larger
scale liquid crystalline domain
orientation only and thus correspond to higher values of
P
2. Therefore, the corresponding
dependencies
of P
2 on draw ratio are also different. A
simplified approach for the analysis of the Raman spectra,
based on the cylindrical symmetry of the Raman tensors at a specific
vibrational normal mode,
demonstrates the effectiveness and usefulness of this technique for
accurately and fully determining the
molecular orientation in rodlike polymers.
High molecular weight liquid-crystalline polyhydroxyethers, containing functional hydroxy groups, were synthesized from an aromatic diol and the diglycidylether of an aromatic diol. The polymers showed a liquid-crystalline melt when about 70% or more of the aromatic units in the chain were biphenyl units. Tensile modulus values varied from 3 GPa for as-cast films to 6 GPa for drawn films. These polymers are potentially suitable candidates to reinforce common thermoplastics by reactive blending. For this purpose polyhydroxyethers have been prepared, having a liquidcrystalline melt in the processing range of PET, viz. 260-290ЊC.
In a more elaborate article, it was described that blends of poly(ethylene terephthalate) (PET), a liquid crystalline copolyester, and small amounts of a liquid crystalline polyhydroxyether showed an increase in tensile modulus and strength compared to the blends without polyhydroxyether. The results were obtained using a polyhydroxyether composed of 75 mol % biphenyl and 25 mol % phenyl units. In this article, the use of two other types of polyhydroxyether is described, one based on the ␣-methylstilbene unit and the other based on bisphenol A. Addition of either of these polyhydroxyethers to the PET/thermotropic liquid crystalline polymer (TLCP) blends increased the tensile modulus and strength of extruded fibers in a similar way as upon addition of the liquid crystalline polyhydroxyether. Improvement of the viscosity ratio and thereby improvement of the fibril formation, by reactions of the functional hydroxyl side groups in the polyhydroxyethers, appears to be the most important factor for the improvement of the mechanical properties.
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