Rod-like macromolecules

The wide variety of ketomethylene and amino ketone monomers that could be synthesized, and the abiUty of the quinoline-forming reaction to generate high molar mass polymers under relatively mild conditions, allow the synthesis of a series of polyquinolines with a wide stmctural variety. Thus polyquinolines with a range of chain stiffness from a semirigid chain to rod-like macromolecules have been synthesized. Polyquinolines are most often prepared by solution polymerization of bis(i9-amino aryl ketone) and bis (ketomethylene) monomers, where R = H or C H, in y -cresol with di-y -cresyl phosphate at 135—140°C for a period of 24—48 h (92). 

The rheological behaviour of polymeric solutions is strongly influenced by the conformation of the polymer. In principle one has to deal with three different conformations, namely (1) random coil polymers (2) semi-flexible rod-like macromolecules and (2) rigid rods. It is easily understood that the hydrody-namically effective volume increases in the sequence mentioned, i.e. molecules with an equal degree of polymerisation exhibit drastically larger viscosities in a rod-like conformation than as statistical coil molecules. An experimental parameter, easily determined, for the conformation of a polymer is the exponent a of the Mark-Houwink relationship [25,26]. In the case of coiled polymers a is between 0.5 and 0.9,semi-flexible rods exhibit values between 1 and 1.3, whereas for an ideal rod the intrinsic viscosity is found to be proportional to M2. 

Dubin, P L., Kaplan, J. I., Tian, B-S., and Mehta, M., Size-exclusion chromatography dimension for rod-like macromolecules, ]. Chromatogr., 515,37,1990. 

Fig. 1. Various conformation models for macromolecules adsorbed on an interface, a) chain lying totally on the interface b) loop-train conformation c) loop-train-tail conformation d) adsorbed at one chain end e) random coil adsorbed at a single point f) rod-like macromolecules adsorbed at one end g) rod-like macromolecules adsorbed located totally on an interface...

Solutions of these substances at first show the normal increase of viscosity with concentration. Above a certain critical concentration, however, the viscosity decreases rapidly. This phenomenon is explained by orientation of the rod-like macromolecules in the flow direction. An analogous behaviour is observed, if the viscosity is plotted as a function of molecular weight at constant concentration. 

The intention of this brief survey has been to demonstrate that besides the "classical" aspects of isotropic polymer solutions and the amorphous or partially crystalline state of polymers, a broad variety of anisotropic structures exist, which can be induced by definable primary structures of the macromolecules. Rigid rod-like macromolecules give rise to nematic or smectic organization, while amphiphilic monomer units or amphiphilic and incompatible chain segments cause ordered micellar-like aggregation in solution or bulk. The outstanding features of these systems are determined by their super-molecular structure rather than by the chemistry of the macromolecules. The anisotropic phase structures or ordered incompatible microphases offer new properties and aspects for application. 

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

Auer, P. L. Thc visco-elastic properties of solutions of rod-like macromolecules. J. Chem. Phys. 19,281-283 (1951). 

Ullman, R. (1969). The viscoelastic properties of solutions of rod-like macromolecules of finite diameter. Macromolecules 2(1), 27-30. 

The dimensions of a linear macromolecule in different states have been considered [51]. The following states are known [51, 52] to he the most typical 1 - compact globule, 2 - coil at the 0-point, 3 - impermeable coil in a good solvent, 4 - permeable coil (the state typical of rigid-chain macromolecules), 5 - completely uncoiled rod-like macromolecule. 

Rod-like macromolecules or semi-flexible chains such as cellulosics with a certain rigidity may form thermotropic and, in highly concentrated solution with suitable interactions, lyotropic liquid-crystalline phases. 

As demonstrated schematically in Fig. 6.9 b, the rigid rod-like macromolecules are oriented parallel to the substrate plane and their backbones exhibit a preferential orientation along the dipping direction employed during LB processing. 

Doi and Edwards developed the most extensive theory of dynamics of rod-like macromolecules in concentrated solutions [2,12], As discussed in the previous section, the majority of commercial TLCPs are not perfect rigid rods but are semi-rigid rods having some degree of flexibility. Although several experimental results support the validity of the rigid rod approximation for both lyotropic [13-15] and thermotropic liquid crystalline polymers [16-18], there is no complete theory on the dynamics of TLCPs. To better understand the dynamics of TLCPs in the melt, the Doi-Edwards theory on the dynamics of rod-like polymers in solution is summarized here. Readers may find further details of the theory in the original reference [2,12]. 

Absent additional experimental data, it is impossible to make more precise statements about the behaviour of rod-like macromolecules in elongational flow. 

There has been considerable interest in recent years in the study of liquid crystalline order in polymeric materials. Following on from the use of small molecules in display applications, the possibility of creating polymers with similar characteristics became attractive. Onsager and Flory predicted that rigid rod-like macromolecules should form liquid crystalline phase. However, it was not until 1975 that the first observation of a thermotropic liquid crystalline polymer was reported. Several reviews have been published on polymeric liquid crystals. ... 

The phase state of lyotropic liquid crystals is mostly determined by a composition of a mixture [27]. Building elements of lyotropics are either rod-like macromolecules (like a tobacco mosaic virus or poly-7-benzyl-glutamate), or micelles formed by amphiphilic molecules in solutions. The shape of a micelle is responsible for the structure of a mesophase [28]. Figure 1.12 shows five possible shapes of building elements (a), responsible for the formation of the corresponding five mesophases (b). 

These completely rigid-rod-like macromolecules tend to be highly crystalline and intractable with melting points above the decomposition temperature of the polymers (>450 °C). 

One of the exciting concepts in the field of blend/composite materials is the reinforcement of flexible-chain polymers by highly rigid rod-like macromolecules with a persistence length Lp > lOnm. The basic principle of these blends, which are known as molecular composites , is dispersion of rigid-chain polymers in a random coil chain, ductile matrix. The basic principles, advantages, and difficulties of molecular reinforcement, and the available data on this problem were summarized... 

Lyotropics can be formed from two different kinds of molecules when mixed with suitable solvents. First, one may have large rod-like macromolecules in solution. For example, synthetic polypeptides can form meso-phases in a variety of solvents. The polypeptide PBLG adopts an a helix conformation in solution, and the mesophase is a cholesteric phase [1.30]. The second type of lyotropic consists of amphiphilic molecules in solution. Amphiphilic molecules possess two distinct parts with quite diff erent properties. The hydrophilic part of the molecule (polar head group) attracts... 

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