Strength of LCPs

Thermotropic polyesters are melt-spun from the nematic phase and orient easily in an elongational flow field (moderate drawdowns/forces are sufficient). In the fiber case, highly oriented fibers form easily with an initial modulus close to theory—typical values range from about 70 to 150 GPa. Ward [46] has shown that the tensile modulus may be described by an aggregate model, i.e., the modulus is a function of the inherent chain modulus, the molecular chain orientation, and the shear modulus (which described the stress transfer between chains). The tensile strength of LCP fibers follows the prediction of the lag-shear model [47]. Both the aggregate model and the lag shear model treat the LCP as though it... 

The electrical insulation properties of LCP are excellent, and because the LCP is homogeneous material, it is not prone to void formation as in fiberglass epoxy composite. The electrical breakdown strength of LCP is higher than polycarbonate—a plastic that is well-known for its good electrical properties. 

Impact strength of pure LCP was 346 kJ/m after ultrasonic treatment at an amplitude of 7.5 ocm. The impact strength of as-received sample was 244 kJ/m, while that of sample extruded without ultrasonic treatment was 261 kJ/m. The improvements in the tensile and impact strengths of LCP indicate a possibility of homopolymerization under the effect of ultrasound. Other researchers identified solid state polymerization as the cause of increased tenacity of LCP fibers after prolonged heat treatment [2, 3, 13]. As opposed to heat treatment, the ultrasonically induced homo-polymerization reactions in LCP take place in the melt state at very short residence times as also shown for other polymers [7, 8]. 

Figure 20.11 The tensile strength of the LCP/PEN blends as a function of the LCP content at draw ratios of 10 ( ) and 20 ( ) [13]. From Kim, S. H., Hong, S. M., Hwang, S. S. and Yoo, H. O., J. Appl. Polym. Sci., 74, 2448-2456 (1999), Copyright (1999, John Wiley Sons, Inc.). This material is used by permission of John Wiley Sons, Inc...

It has been the merit of Picken (1989, 1990) having modified the Maier-Saupe mean field theory successfully for application to LCPs. He derived the stability of the nematic mesophase from an anisotropic potential, thereby making use of a coupling constant that determines the strength of the orientation potential. He also incorporated influences of concentration and molecular weight in the Maier-Saupe model. Moreover, he used Ciferri s equation to take into account the temperature dependence of the persistence length. In this way he found a relationship between clearing temperature (i.e. the temperature of transition from the nematic to the isotropic phase) and concentration ... 

Although liquid crystalline polyesters (LCP s) are interesting polymers exhibiting high mechanical strength and modulus due to a high degree of selforientation, no commercial blends of LCP (except with PPS) are available. LCP blends have been... 

The dielectric constant of LCP films is typically 2.7 to 3.0, making them attractive for high speed electronic circuit substrates. Tlie dielectric strength of over 7,000V per mil combined with very low moisture absorption (less than 0.2 percent by weight) meets requirements for electric wire and cable insulation. 

The rigid-rod nature of LCPs provides the superior properties, but can result in problems if not properly controlled. This is shown in Rgure 5 where the film on tlie left. splits easily because it has little transverse orientation. The film on the right, made from the same LCP, is biaxially oriented and has high strength in all directions. Orientation is the key to making high performance LCP films. 

The unifying feature of all fibers spun from LCPs is the very high axial molecular orientation, which leads to extreme anisotropy of microstructure and mechanical properties. In the transverse direction, the strength is only about 20% of the axial strength and the modulus is typically less than 10% of the axial value. The microstructure of LCP fibers reflects the very... 

---