Configuration of polymer chains

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

Another important application of NMR to polymer systems is the elucidation of the stereochemical configurations of Polymer chains. Poly (methyl methacrylate) was first studied by Bovey in 1960. It is now possible to analyse for the statistical frequency of occurrence of all possible combinations of up to four successive pairs of units (dyads) capable of occurring with either the same (meso) or opposite (racemic) configurations. 

T. Shimanouchi and S. Mizushima. J. Chem. Phys. 23, 707-11 (1955). Thebry helical configurations of polymer chains. 

Therefore, the construction of appropriate equilibrium configurations of polymer chains in a lattice model is always a problem. For a dilute solution under good solvent conditions the excluded volume interaction between all monomers of the chain can no longer be ignored. It leads to a swelling of the chain with respect to its size in the melt or in theta solution. Thus, Eq. (5) does not hold, but has to be replaced by... 

Until now, there has been no explicit discussion of the effect of temperature oi pressure on any of the viscoelastic functions, although it has been mentioned thai below the glass transition temperature the configurations of polymer chain backbones are largely immobilized and the tremendous changes in viscoelastic properties with time or frequency which characterize polymeric systems do not appear (curves V in the figures in Chapter 2). 

Figure 2. X ray pattern of oriented monofilament configuration of polymer chains in the unit cell schematic projection on the AB plane...

Learn how statistical physics can be used to describe the configuration of polymer chains in dilute solution. 

Application of small deformations displaces the equilibrium configuration of polymer chains, and the resulting recoil and elastic effects can easily be observed due to the slow relaxation nature of polymer liquids. Given an arbitrary strain history, corresponding stress history can be calculated using Boltzmann s constitutive equation of linear viscoelasticity [86],... 

The potential of the mean force W r, in principle is determined by equilibrium statistical mechanics. The coarse grain configuration of polymer chains in melts obeys Gaussian statistics with respect to the end-to-end distance, for instance. This suggests the following approximation of the potential of the mean force ... 

The role of excluded volume effects, above all, their influence on the configuration of polymer chains. 

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