Rod-coil conformations

A single chain rod-coil conformational transition is analogous to the helix-coil transition the more ordered... 

Rubatat, L., Kong, X.X., Jenekhe, S.A. et al. (2008) Self-assembly of polypeptide/pi-conjugated polymer/ polypeptide triblock copolymers in rod-rod-rod and coil-rod-coil conformations. Macromolecules, 41,1846-1852. 

There are two kinds of electric birefringence techniques, the FEBS and the transient electric birefringence (TEB) method [171, 173]. The TEB method was applied to the solution of polydiacetylene to investigate the rod-coil conformational transition of the polymer chains [174]. The FEBS, on the other hand, has the advantage of giving us the mobility of the carriers along the polymer chain separately from the hydrodynamic radius of the polymer chain (usually referred to as the polymer conformation) [149]. Shimomura et al. [175] have recently applied the FEBS technique to the solutions of dilutely doped PHT to study the intrachain conduction in the conducting polymer and its relation to the main-chain conformation. 

Studies of the structure and molecular size of wheat AX [41] revealed that they are shear-thinning and exhibit two critical concentrations, which correspond to the onset of coil overlapping. The existence of three domains provided the evidence for the formerly suggested rigid, rod-Uke conformation of AX in solution. In a recent study [116], the previously reported conflicting suggestions on the conformation of AX were discussed. 

The conformations adopted by polyelectrolytes under different conditions in aqueous solution have been the subject of much study. It is known, for example, that at low charge densities or at high ionic strengths polyelectrolytes have more or less randomly coiled conformations. As neutralization proceeds, with concomitant increase in charge density, so the polyelectrolyte chain uncoils due to electrostatic repulsion. Eventually at full neutralization such molecules have conformations that are essentially rod-like (Kitano et al., 1980). This rod-like conformation for poly(acrylic acid) neutralized with sodium hydroxide in aqueous solution is not due to an increase in stiffness of the polymer, but to an increase in the so-called excluded volume, i.e. that region around an individual polymer molecule that cannot be entered by another molecule. The excluded volume itself increases due to an increase in electrostatic charge density (Kitano et al., 1980). 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

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. 

Unfractionated polyisocyanides yield viscometry data which suggest random coil conformation, whereas viscometry data from fractionated samples suggest rigid rod conformation. The first viscometry of unfractionated poly(d,/-a-phenyl-... 

Fig. Z36 Representation of the energy-minimized conformation of a rod-coil diblock prepared by Radzilowski ef al. (1993). The rod is a mesogenic moiety 60 A long, the coil is low molecular weight PI. The volume fraction of rod is about 0.2.

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

The potential of block copolymers in forming supramolecular structures on surfaces has been studied in great detail by Stupp and co-workers [36]. Their studies have concentrated on the supramolecular properties of diblock and triblock rod-coil polymers which consist of one or two components that are conformationally flexible and one component with a more rod-like character at one of the ends of the polymer chain ( c in Figure 4.25). This latter component can be crystallized and is expected to yield well-defined structural arrangements. The Stupp-type copolymers are based on diblock styrene-b-isoprene coils combined with various rod-like components. The synthesis of such materials is carried out in two stages. In the first step, the diblock coil part is produced, as shown in Figure 4.26. The... 

Water, occasionally modified by a small volume of ethanol and electrolytes, is the universal food solvent. In this medium, a polysaccharide polyanion s molar volume (vm) is maximum, because charge repulsion forces the molecules into the stiff (uncurled) conformation of a rigid rod that cannot be extended further, but can be oriented (Odell, 1989). vm is larger for extended, stiff conformations than for compact random-coil conformations (Berth, 1992). As solvent quality deteriorates, e.g., by addition of salt or... 

Mechanical properties of the chemically modified wood depend on the nature of the introduced side chain. If the introduced side chain is ionized, the interaction between wood component molecules is influenced by not only the free volume but also by electrostatic repulsion of ionized side chains. For example, the conformation of polyelectrolyte depends on the degree of ionization [23-28]. The structure is transformed from random coil to rod-like conformation with an increase in the degree of ionization. A screening effect also influences the interaction because electrostatic action is inhibited by electrolytes such as NaCl. 

It follows from Eq. (29) that at x—> 0, Px— 4/x-— oo, corresponds to an infinitely thin straight rod. In the Gaussian coil conformation x tends to infinity and it follows from Eq. (29) that pj tends to unity, corresponding to a model with a spherically symmetrical segment distribution. 

The asymmetry of the shape of a worm-like chain in the conformation of a straight rod is uniquely determined by the p = L/d ratio whereas the a mmetry of the shape of a chain molecule in the Gaussian coil conformation can be characterized in various ways. 

Another method of characterizing the asymmetry of the molecular shape is related to the choice of the main direction in the molecule. In the principle axis Z in Fig. 11 tangential to the worraJike chain at its middle point can be chosen as this direction. However, in this system of coordinates the asymmetry of the shape of the molecule is adequately expressed only in the range of conformations close to a rod whereas for the Gaussian coil conformation (x — < ), according to Eq. (29), p is equal to unity and the coil is herical (see also Pig. 13). Hence, the asymmetry of the shape of the Gaussian coil cannot be adequately expressed by a system of coordinates determined by the middle element of a worm-like chain. 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

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