It has been demonstrated that main chain polymer liquid crystals often cannot show mesogenic behavior over a wide temperature range (see Main Chain Polymer Liquid Crystals). Side chain polymer liquid crystals, however, are able to expand this scale. These materials are formed when mesogenic units are attached to the polymer as side chains.
Side chain polymer liquid crystals have three major structural components: the backbone, the spacer, and the mesogen. The versatility of SC-PLCs arises because these structures can be varied in a number of ways.
The backbone of a side chain polymer liquid crystal is the element that the side chains are attached to. The structure of the backbone can be very important in determining if the polymer shows liquid crystal behavior. Polymers with rigid backbones typically have high glass transition temperatures, and thus liquid crystal behavior is often difficult to observe. In order to lower this temperature, the polymer backbone can be made more flexible.
Perhaps the most important part of a side chain polymer liquid crystal is the mesogen. It is the alignment of these groups that causes the liquid crystal behavior. Usually, the mesogen is made up of a rigid core of two or more aromatic rings joined together by a functional group. The following diagram is a typical repeating unit in a side chain polymer liquid crystal. Notice the spacer of methylene units and the mesogen of aromatic rings.
Like their main chain counterparts, mesogens attached as side groups on the backbone of side chain polymer liquid crystals are able to orient because the spacer allows for independent movement. Notice in the following diagram that even though the polymer may be in a tangled conformation, orientation of the mesogens is still possible because of the decoupling action of the spacer.
The structure of the spacer is an important determining factor in side chain polymer liquid crystals. Generally, the spacer consists of two to four methylene (CH2) groups attached together in a line. Accordingly, the spacer length has a profound effect on the temperature and type of phase transitions. Usually, the glass transition temperature decreases with increasing spacer length. Short spacers tend to lead to nematic phases, while longer spacers lead to smectic phases.
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