Comb-Shaped Polymers and Liquid Crystals

The so-called comb-shaped polymers, macromolecules which contain relatively long side branches spaced comparatively closely along the main chain, occupy a special position among the large number of polymers sometimes used in unusual areas. They differ from ordinary branched polymers due to the fact that they have many side chains: each monomeric unit can contain such a chain (Fig. 1), and this quantitative difference results in the appearance of a new set of qualitative properties which are absent in linear or weakly branched polymers.

In the past twenty years in polymer physics, it has been possible to construct relatively universal models of crystalline polymer structure with folded or straight linear chains and to postulate the basic conditions and features of the crystallization of linear polymers [1–8]. The number of studies on the structure of more complex macromolecular compounds, such as branched and comb-shaped polymers, statistical copolymers, block and graft copolymers, is significantly smaller.

The nature of the molecular mobility in polymers is determined by the presence of long-chain molecules and the possibility of internal rotation of the atoms in the main and side chains. At temperatures below the glass transition temperature, movement of the kinetic units located in small volumes (short segment or unit of the main chain, side sub-stituent or part of the side substituent) is possible. This type of motion, related to the mobility of only a few groups of atoms, is usually called group motion, and is indicated by the Greek letters β, γ, δ, etc., as a function of the character of the relaxing groups. At temperatures close to Tg and higher, a second type of motion begins to appear: segmental motion, related to the cooperative movement of the kinetic segments of the main chain, called α-relaxation [1–6]. This division is arbitrary to a significant degree, since the different types of motion which frequently take place simultaneously in a similar region of frequencies and temperatures, thereby impeding their determination and unambiguous interpretation, cannot be distinguished for all polymers.

The dual character of comb-shaped polymers, which consists of the independent behavior of the main and side chains of the macromolecules in the condensed state, is manifested to the greatest degree in dilute solutions where the intermolecular interaction of the chains is reduced to a minimum. The unusual structure of comb-shaped polymers which contain long and relatively independently behaving segments of methylene groups of varying length in association with the polar groups of the macromolecules is the basis for considering them as amphiphilic systems with a given number and length of the branches.

Investigators in the field of physical chemistry of macromolecular compounds have recently turned to problems in the creation and study of polymeric liquid-crystalline (LC) systems [1–25]. The great interest in these systems is due to the significant advances in the study and practical application of low-molecular-weight liquid-crystals in such new fields of technology as electronics, electrooptics, holography, etc., as well as in medicine, chemistry, and biology [26–32]. On the other hand, the interest in this field is also due to the possibility of creating polymer systems which successfully combine the unique properties of low-molecular-weight liquid crystals and low-molecular-weight compounds, which allows for the preparation of films, fibers, and coatings with unusual properties. The use of low-molecular-weight thermotropic liquid crystals in most cases involves the creation of special hermetic membranes (electro-optical cells, microcapsules, etc.) which form the required shape and protect the LC compound from external effects. In the case of thermotropic LC polymers, the creation of these shapes can be and is optional, since the properties of low-and high-molecular-weight liquid crystals are combined in a single polymer, and this opens up new prospects for their use by significantly extending the number of LC compounds.