Rigid-rod conformation

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-... 

Figure 1.46 shows how R changes with Ljl. Parts a and b are the plots of R I L I 2) and R /(LpLj3) as a function of LjL, respectively. Figure 1.46a indicates how the chain dimension decreases from that of the rigid rod conformation of the same as the chain becomes longer or more flexible. Figure 1.46b shows how the chain approaches an ideal chain as LjL increases. Because a M, the two plots are essentially the plots of R jM and R jM, respectively, as a function of M. 

For polyaniline, the experimentally observed molecular weight dependence is assuming a rigid rod conformation in solution or... 

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. 

Fig. 8 The Haug triangle. The three extremes of conformation compact sphere, random coil and rigid rod) are placed at the apices of a triangle. The conformation of a given macromolecule is represented by a locus along the sides of the triangle between these extremes. Knowledge of the power law exponents (see text) can help to give us an idea of the conformation type. From [61]...

A simplified analysis of the effect of particle shape or molecular conformation on SEC calibration has led to the prediction that the more open structure of rigid rod shaped solutes gives a relatively flat SEC-MW calibration curve. As the solute conformation becomes more compact (random-coil to solid-sphere), the SEC-MW calibration curve becomes increasingly steep... 

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. 

In order to check the flexibility of polypeptide helix predicted by these considerations, we may utilize various conformation-dependent properties. The most tangible way will be to examine the chain-length dependence of 1/2, which ought to be linear for a series of rigid rods of constant diameter. 

Wada 109, 110) pioneered studies of polypeptide conformation by the dielectric method. He found 110) a linear dependence of (ft2)1 2 on Mw for a series of PBLG samples (ranging from 7 x 104 to 18 x 104 in Mw) in EDC at 25° C and obtained 3.5 D for gh, where D stands for debye units. He computed (ft2) by the use of an approximate equation derived by himself 109) for rigid-rod molecules, which for very dilute solutions may be written... 

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