Subject molecular weight dependence

There are several important reasons for wanting to know molecular weights in polymer science. From the viewpoint of inorganic polymers, the main uses are for the interpretation of molecular-weight dependent properties, and for the elucidation of polymerization mechanisms. The latter involves characterization of the molecular weight distribution, which is the subject of the following section. 

What is the minimum degree of polymerization necessary for dextran to exhibit properties characteristic of a typical polymer This question should be resolved by examining the molecular weight dependence of its solution properties, although this may be subject to the limitation that native dextran is not a linear polymer but a branched one. The optical rotation data in Figure 2 demonstrate the transition from oligomer to polymer aroxmd the... 

The zero shear viscosity scales with Nf" to contrast Af dependence for isotropic polymers [20] So far, we have examined the dynamics of rod-Uke macromolecules in isotropic semi-dilute solution. For anisotropic LCP solutions in which the rods are oriented in a certain direction, the diffusion constant increases, and the viscosity decreases, but their scaling behavior with the molecular weight is expected to be unchanged [2,17], Little experimental work has been reported on this subject. The dynamics of thermotropic liquid crystalline polymer melts may be considered as a special case of the concentrated solution with no solvent. Many experimental results [16-18] showed the strong molecular weight dependence of the melt viscosity as predicted by the Doi-Edwards theory. However, the complex rheological behaviors of TLCPs have not been well theorized. 

Thus, any deviation from first-order kinetics will indicate that the azobenzene residues in the system under study isomerize at different rates. Such a dispersion of the rate constant for hindered rotation was postulated by other investigators. Cochran et al reached this conclusion on the basis of the molecular weight dependence of the relaxation times observed with polystyrene by sound absorption studies. Valeur and MonnerieS subjected a solution of polystyrene with anthracene residues incorporated into the chain backbone to a nanosecond light flash and found that the time-dependence of the anthracene fluorescence deviated from a simple exponential decay this was ascribed to differences in the local conformational mobility of the chain. 

Fig. 8.35. Crazing and fracture of fractions of PS, subject to uniaxial stress. Molecular weight dependencies of the minimum stress for the observation of crazes and the stress at failure. Data from Fellers and Kee [101]...

This section treats the flow behavior of SPS and particularly focuses on the temperature and molecular weight dependencies. The subject of this section is limited to the flow behavior of SPS in the molten state above its melting point. The effects of the tacticity on the rheological behavior in the amorphous state are discussed by comparing SPS with atactic polystyrene (APS) and isotactic polystyrene (IPS). 

For good solvents (x < 0), the Flory-Huggins theory predicts that A2 is proportional to w and is independent of chain molecular weight. However the experimental data show that A2 Attempts to explain this molecular weight dependence of the second virial coefficient have been made by Flory and Krigbaum and others. " A complete theory will not involve a lattice model and treat the averages more carefully. It is still an active subject. 

As shown in the previous section the mechanical and thermal properties of polypropylene are dependent on the isotacticity, the molecular weight and on other structure features. The properties of five commercial materials (all made by the same manufacturer and subjected to the same test methods) which are of approximately the same isotactic content but which differ in molecular weight and in being either homopolymers or block copolymers are compared in Table 11.1. 

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