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.

Background

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, have not been widely produced for industrial purposes.

Mesophases 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.

Classification

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 LCP 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.

Side chain LCP In side-chain LCPs, the mesogens are in the polymer side chains. The mesogens usually are linked to the backbones through flexible spacers, although for a few LCPs, the side chains directly link to the backbones. If the mesogens are directly linked to the backbones, the coil-like conformation of the backbones will impede the mesogens from forming an orientational structure. Conversely, by introducing flexible spacers between the backbones and the mesogens, the ordering of mesogens can be decoupled from the conformation of the backbones.

Mechanism

Mesogens in LCPs can self-organize to form liquid crystal regions in different conditions. LCPs can be roughly divided into two subcategories based on the mechanism of aggregation and ordering, but the distinction is not rigidly defined. LCPs can be transformed into liquid crystals with more than one method.

Lyotropic systems Lyotropic main chain LCPs have rigid mesogen cores (such as aromatic rings) in the backbones. This type of LCPs forms liquid crystals due to their rigid chain conformation but not only the aggregation of mesogen cores. Because of the rigid structure, strong solvent is needed to dissolve the lyotropic main chain polymers. When the concentration of the polymers reaches critical concentration, the mesophases begin to form and the viscosity of the polymer solution begins to decrease. Lyotropic main chain LCPs have been mainly used to generate high-strength fibers such as Kevlar. Side chain LCPs usually consist of both hydrophobic and hydrophilic segments. Usually, the side chain ends are hydrophilic. When they are dissolved in water, micelles will form due to hydrophobic force. If the volume fraction of the polymers exceeds the critical volume fraction, the micellar segregates will be packed to form a liquid crystal structure. As the concentration varies above the critical volume fraction, the liquid crystal generated may be packed in different structures. Temperature, the stiffness of the polymers, and the molecular weight of the polymers can affect the liquid crystal transformation. Lyotropic side chain LCPs such as alkyl polyoxyethylene surfactants attached to polysiloxane polymers may be used in personal care products like liquid soap.

Thermotropic systems The study of thermotropic LCPs was catalyzed by the success of lyotropic LCPs. Thermotropic LCPs can only be processed when the melting temperature is far below the decomposition temperature. When above the melting temperature but below the clearing point, the thermotropic LCPs will form liquid crystals. Above the clearing point, the melt will be isotropic and clear again. Frozen liquid crystals can be obtained by quenching liquid crystal polymers below the glass transition temperature. Copolymerization can be used to adjust the melting temperature and mesophase temperature.

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