Liquid crystalline polymers thermal expansion

Engineering polymers sueh as polyethylene, polypropylene, polystyrene, and polyvinyl ehloride ean be reinforced with liquid crystalline polymers. The stronger, less thermally expansive Uquid... 

L. S. Rubin, K. Jayaraj, and J. M. Burnett, Liquid crystalline polymer multilayer printed circuit boards for controlled coefficient of thermal expansion. Proceedings lEPS, September (1991). 

The interest in thermotropic liquid-crystalline polymers has grown in recent years due to the their inherently high stil ess and strength, high use temperatures, excellent chemical resistance, low melt viscosity and low coefficient of thermal expansion. 

The liquid-crystalline polymer component (Vectra) is oriented in the blends and possesses an approximately invariant thermal expansion coefficient anisotropy. 

Thermotropic liquid crystalline polymers (TLCPs) have gained increased commercial attention because of their unique properties. These include their low coefficients of thermal expansion, low viscosity, and high modulus, low permeability to gases, low dielectric constants, and chemical resistance. As the demand for these characteristics increases, it is anticipated that the use of TLCPs will grow, rising at a projected annual growth rate of 25 % from an estimated use of ten million pounds in recent years. In expanding the potential uses for TLCPs, it has been found that TLCP/TLCP blends can possess characteristics which are better than those of either individual TLCP (Utracki and Favis 1989). But the better result is only possible if the LCP fibrillation is prominent in the blend system. 

Liquid crystalline polymer composites have become an increasingly important material for technological field involving photonic applications. Several attractive features such as dielectric anisotropy present in the mesophase, reasonably wide temperature range, low thermal expansion coefficient, high chemical resistance, specific thermo-mechanical response etc. make them more attractive. Of comse several limitations have to overcome in order to avail the total benefit of using such materials for opto-electronic applications. 

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

The very small residual increase (less than 0.2 nm/K) can be attributed to thermal expansion of the cured polymer. This was verified by measuring the coefficient of thermal expansion via the change of density by heating pieces of cured solid film from 20°C to 50°C. A value of A7JAT = (0.1 l+i.0.04) nm/K was calculated from this measurement, which is in good agreement with the observed wavelength shift. Preservation of the reflection band was observed down to -196°C. This sounds trivial for a polymer but should be mentioned for comparison with monomeric liquid crystalline materials, which tend to crystallize at low temperature. 

As is true for simple liquids, the vacant-volume fraction here is considerable (Table 5-6). With macromolecules, the vacant volume is not completely available for thermal motion since not all vacant sites are accessible to monomeric units on conformational grounds. The volume available for thermal expansion Xxp can be calculated from the specific volumes of the amorphous and crystalline polymer at 0 K ... 

Materials with molecular networks, such as cross-linked elastomers and crystalline polymers, do not flow and so are classified as viscoelastic solids. Shear stresses do not decay to zero with time, ie, equilibrium stresses can be supported. Above Tg, such amorphous materials are still classified as solids, but most of their physical properties such as thermal expansivity, thermal conductivity, and heat capacity are liquid-like. 

---