Liquid-crystal polymer

Liquid crystal polymers (LCPs) are polymers with the property of liquid crystal, usually containing aromatic rings as mesogens. Despite uncrosslinked LCPs, polymeric materials like liquid crystal elastomers (LCEs) and liquid crystal networks (LCNs) can exhibit liquid crystallinity as well. They are both crosslinked LCPs but have different cross link density. They are widely used in the digital display market. In addition, LCPs have unique properties like thermal actuation, anisotropic swelling, and soft elasticity. Therefore, they can be good actuators and sensors. One of the most famous and classical applications for LCPs is Kevlar, a strong but light fiber with wide applications, notably bulletproof vests.   

Liquid crystallinity in polymers may occur either by dissolving a polymer in a solvent (lyotropic liquid-crystal polymers) or by heating a polymer above its glass or melting transition point (thermotropic liquid-crystal polymers). Liquid-crystal polymers are present in melted/liquid or solid form. In solid form, the main example of lyotropic LCPs is the commercial aramid known as Kevlar. The chemical structure of this aramid consists of linearly substituted aromatic rings linked by amide groups. In a similar way, several series of thermotropic LCPs have been commercially produced by several companies.

A high number of LCPs, produced in the 1980s, displayed order in the melt phase analogous to that exhibited by nonpolymeric liquid crystals. Processing of LCPs from liquid-crystal phases (or mesophases) gives rise to fibers and injected materials having high mechanical properties as a consequence of the self-reinforcing properties derived from the macromolecular orientation in the mesophase.

LCPs can be melt-processed on conventional equipment at high speeds with excellent replication of mold details. The high ease of forming of LCPs is an important competitive advantage against other plastics, as it offsets high raw material cost.

Polar and bowlic LCPs, which have unique properties and potential applications^{[clarification needed]}, have not been widely produced for industrial purposes.

Same as the small molecular liquid crystal, liquid crystal polymers also have different mesophases. The mesogen cores of the polymers will aggregate into different mesophases: nematics, cholesterics, smectics and compounds with highly polar end groups. More information about the mesophases can be found on liquid crystal page.

LCPs are categorized by the location of liquid crystal cores. Due to the creation and research of different classes of LCPs, different prefixes are used to help the classification of LCPs. Main chain liquid crystal polymers (MCLCPs) have liquid crystal cores in the main chain. By contrast, side chain liquid crystal polymers (SCLCPs) have pendant side chains containing the liquid crystal cores.

Main chain LCPs have rigid, rod-like mesogens in the polymer backbones, which indirectly leads to the high melting temperature of this kind of LCPs. To make this kind of polymer easy to process, different methods are applied to lower the transition temperature: introducing flexible sequences, introducing bends or kinks, or adding substituent groups to the aromatic mesogens.