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Highly oriented polymers exhibit anisotropy in optical, electric and thermomechanical properties.
Polymerization of liquid crystal (LC) monomers has been conducted extensively to obtain polymer LCs (PLCs). LC monomers can be macroscopically aligned by external forces such as electric and magnetic fields, elongational flow, and surface alignment. The alignment of monomers may affect the polymerization kinetics, the polymer structure and the microstructure of polymers. If polymerizable ferroelectric LCs (FLCs) are used, it is expected that the polymerizable groups adjoin and are aligned in one direction in each layer of the chiral smectic C phase. In situ photopolymerization of FLC monomers, which offers an advantage that the polymerization temperature can be controlled precisely, is then expected to show specific polymerization behavior. The effect of LC alignment on the rate of polymerization of LC vinyl monomers has been discussed 1 There are at least two types of polymerization behavior reported previously on the polymerization of LC monomers. First, the polymerization does not involve any rate enhancement. For instance, some LC monomers possess mesogenic groups decoupled from reactive functional moieties with a methylene spacer. In this type of LC monomers, the polymerization is little affected by LC alignment. Second, the effect of LC alignment shows a marked enhancement of the rate of polymerization. If an alkyl spacer attached to the functional group is long, the alignment of polymerizable groups is disordered even though the core part of mesogens is highly aligned. On the other hand, in short spacers, the alignment of mesogens affects that of functional groups, improving molecular packing in an LC phase. Supramolecular assembly including LCs is a current topic. In the optoelectronic materials in device applications such as sensors and memories, the key requirement is the presence of noncentrosymmetric (polar) order. To generate the permanent electric polarization (i.e., spontaneous polarization, Ps) by employing supramolecular architecture, several approaches have been conducted so far, which include the 2 silane 3 4 and 5
techniques as well as the introduction of 6 and a liquid-crystalline 7 into a molecular system. We have demonstrated the polar order in ion-based LC thin films and their quasi-ferroelectric 8 which are solid-state multilayer organizations self-formed by phosphonium surfactant complexes as a novel class of thermotropic ionic LCs. The origin of the polar structure was assumed
to be due to a ferroelectric ordering of the ion pairs as an electric dipole in the two-dimensional ionic layers.FLCs exhibit the Ps due to the self-organizing character, so that they have been regarded as one of the most effective groups of optoelectronically active 9 However, FLCs suffer from poor resistance to thermal and mechanical shock. The thermal and mechanical stability of polar FLC materials has been improved by polymerization with simultaneous cross-linking. Several approaches have been performed so far to immobilize the molecular (dipolar) alignment by using polymerization of ° We have also shown that in situ photopolymerization of FLC monomers enables the image storage (i.e., patterning of polarization) which is due to the immobilization of the ferroelectric 11 In preparation of such polymerized and cross-linked FLC materials, it is necessary to fill FLCs in a very thin sandwiched glass cell so as to obtain a surface-stabilized FLC state. This leads to a difficulty to prepare large-scale materials with an efficient Ps.
Banana-shaped LCs, a topic of current interest, show sterically induced polar packing in the bulk due to their bis-dipolar structure of molecules and possess a unique phase structure and unusual
optoelectronic 12 More recently, we have evaluated preliminarily in situ polymerization behavior of a bis-dipolar bifunctional monomer in a banana-shaped LC solvent, and succesfully prepared polar materials with a non-centrosymmetric arrangement of 13 It is known that the banana-shaped LCs in the switchable 2 phase have a dense molecular packing resulting in the polar structure.
Therefore, the molecular and polar alignment in polymer/banana-shaped LC composites
as well as the polymerization behavior may be affected by the packing manner of the molecules
between the monomer and LC.
In this study, we synthesized calamitic FLC vinyl monomers with a methacrylate group attached to the rigid core through a short spacer and a long spacer, and investigated the effect of molecular packing of the LC monomers on photopolymerization behavior. Furthermore, we evaluated the polymerization behavior as well as the molecular aUgnment of. a bis-dipolar monomer in a banana-shaped LC to further understand the evolution of the polymer structure formed within the ordered LC media. Special attention has been paid to the effect of the ferroelectric arrangement of the monomer in a bananashaped LC on its polymerization behavior.
