ABSTRACT
Liquid crystalline polymers (LCPs) are a unique group of polymers that have an ordered structure in the molten (and solution) phase comparable to that found in low molecular weight nonpolymeric liquid crystals (LCs) in the liquid phase [1-3]. LCPs exist in a state in which the molecules are ordered and mutually aligned but the material can still flow; flow is “typically” a liquid or fluid characteristic. This alignment is characterized as “self-reinforcing,” and gives LCPs a high degree of orientation and exceptional mechanical properties. In addition, LCPs exhibit very low viscosity in the melt and solution phases, and have ease of processability. The ordered nature of LCPs is attributable to their unique combination of high aspect ratio (length to diameter ratio) and rigid molecular structure. This combination favors a high strength to weight (or specific strength) property. LCPs are specialty-engineering thermoplastic resins that are characterized by a unique combination of outstanding properties and attributes such as
• Dimensional stability • High specific strength • Chemical resistance • Flammability/heat resistance • Self-reinforcing • Ease of processing
LCs, often referred to as mesophases, were first identified more than a century ago. They are low molecular weight (liquid) compounds with unique electro-optical properties. LCs exist at the inter-phase of the conventional isotropic fluid and that of the solid state and tend to exhibit both fluid and solid state characteristics. Figure 21.1 is a schematic of the molecular arrangement in LCPs compared to those of their amorphous and semicrystalline thermoplastic counterparts. LCPs, unlike their thermoplastic counterparts, exhibit
ordered molecular arrangement in the liquid state. This ordered molecular arrangement or crystallinity persists and is enhanced in the solid state. LCPs consist of linear rigid molecules that align themselves somewhat differently from the “random coil,” chain-folded lamellar crystals of their semicrystalline and amorphous thermoplastic counterparts. LCPs’ linear, rigid aromatic units exhibit spontaneous anisotropy and ordered arrangement in the melt and solution phases, and as such are referred to as “mesogens.” LCPs’ spontaneous anisotropy is exhibited especially when under flow, such as through an orifice; the stiff polymer chains orient in the direction of flow. This leads to a high degree of orientation when the material cools down and the resultant anisotropic strength in the direction of orientation.
