Polymer crystallinity, linearity and molecular structures

For the evaluation of bistability, the angle between two relaxed states without an electric field is a useful criterion. Moreover, in practical devices with simple matrix driving, small bipolar disturbing pulses are applied during the period between signal pulses. Memory states respond to the pulses and the transmission is modulated, resulting in decrease in contrast. Stability in the pulses is also an important criterion for bistability [8]. 

In Table 5.2.1, polymers which align well on rubbing and do not align FLC materials are shown [9,10]. The good polymer materials possess linear molecular structures without crosslinking. This rule also applies for the polymers containing 

Other aspects are that linear polymers without crosslinking possess thermoplastic properties, which means they can be plastically deformed on heating and can retain the deformed situation on cooling. In the rubbing process, the plastic polymer is sheared and retains the extended state. These processes change the chain orientation and can cause partial crystallization, which can induce epitaxial growth of smectic liquid crystalline alignment. 

In Table 5.2.2, thermoplastic polymers with substantial side groups on which good alignment could not be observed, are shown. The deterioration of smectic liquid crystalline alignment due to the lack of linearity of the polymers with substantial side groups can be confirmed. 

It has been pointed out that the crystallographic class of alignment layers also affects the bistability [11]. There are two clear groups of polymers, those with monoclinic or triclinic crystal structures, and those with other crystal structures. The alignment layers with monoclinic or triclinic structures show good bistability, and other groups of alignment layers show poor bistabilities. 

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