Semi-flexible coil

Semi-flexible coil cellulose derivatives, chitosans... 

At a critical molar mass (M=65,000 g mol"0 the linear [rj]-M-relationships intersect (see Fig. 6.8). In general the curves of log [q]=f log M) intersect only for different polymers in solution. For the same polymer in different solvents the curves do not intersect if the solution structure stays the same. The curves might merge at low molar masses (see The influence of the molar mass below). The intersection shown in Fig. 6.8 is therefore solely caused by changes in the -solution structure. Helical structures of the polymer in solution are stiffer than the polymer chain in a flexible coil and lead to semi-flexible coils or rigid rodlike structures. The exponent a of the [q]-M-relationship shows for these extremely expanded structures values of lpoly(glutamic acid benzyl ester)... 

The stronger binding of Ca + than Na+ by chondroitin sulphate indicates that the association cannot be ascribed solely to electrostatic forces. Sedimentation coefficient and intrinsic viscosity measurements have indicated that chondroitin sulphate in solution can be represented by a semi-flexible coil with a molecular diameter of about 1.2 nm, a value which is consistent with the results of A -ray analysis. ... 

Semi-Flexible Chains Worm-Like Coils... 

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. 

The transitions of the coil-globule type were considered not only in the usual space, but also in the space of monomeric units orientation, where such transition is equivalent to the nematic liquid crystal ordering [60,61]. Such an approach using the formalism developed by I.M. Lifshitz has led to the creation of the theory of liquid crystal ordering in the solutions of semi-flexible macromolecules [62,63]. 

For a semi-flexible tube in a dilute environment, local repulsive potentials among parts of the fiber induce a self-avoiding random walk configuration (swollen coil [151]). In a crowded environment, the depletive action may dominate and the fiber will tend to collapse on itself, forming a globular phase. We know from standard statistical physics of polymers that this latter phase... 

With Mw higher than 3xl05, the experimental points could be fitted by a smooth convex curve. The slope of the convex curve was about 1.54 for Mw between 5xl05 and lxlO6 and about 1.15 for Mw between 3xl06 and 3xl07. This change in slope implies that the molecule is rodlike at lower MW and approaches a spherical coil as Mw increases, which is the characteristic behavior of semi-flexible polymers. 

The transition from slopes a<0.8 for semi-flexible rods at low molar masses to a=0.8 for flexible coils at high molar masses is often not detected because of the narrow range of molar masses available for the determination of a [/]]-M-rela-tionship. This is another reason that many different [q]-M-relationships are pub-... 

Flory et al. considered only the shape anisotropy of the rod-like molecules, they derived expressions for the phase equilibrium in mixtures of low molar mass mesogens with rod-like polymers, semi-rigid polymers, and flexible coil polymers ... 

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