Molecular weight distribution chain conformations

The randomization stage refers to the equilibration of the nonequilibrium conformations of the chains near the surfaces and in the case of crack healing and processing, the restoration of the molecular weight distribution and random orientation of chain segments near the interface. The conformational relaxation is of particular importance in the strength development at incompatible interfaces and affects molecular connectivity at polymer-solid interfaces. 

Generally, the models used for simulation of living polymers can be divided roughly into two classes, focused on static or dynamic properties of the LP or GM. The static models are mainly designed to study equilibrium conformational properties of the polymer chains, critical behavior at the polymerization transition, and molecular weight distribution... 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

If the molecular weight and molecular weight distribution for a polymer are known along with a good understanding of its chain conformation, many properties of the polymer can be predicted. While polymer chain conformations were discussed in Chapter 2, polymer molecular weights are dealt with in the present chapter. 

In order to decide whether cellulose molecules have an extended or a folded-chain conformation in the native cell-wall, Muggli and co-workers (see also. Ref. 10) devised an interesting experiment based on determination of the molecular-weight distribution in ramie microfibrils before and after they had been cut into 2-ju.m sections. As shown in Fig. 14, should the molecules have an extended conformation, many of the molecular chains would be cut in a random fashion (Gaussian distribution), resulting in substantial diminution in the molecular weight. On the other hand, in the folded conformation, a few macromolecules would be cut, but the rest would retain their... 

Fig. 14.—Sectioning of the- Cellulose Fibers Having an Extended or Folded-chain Conformation and the Resulting Change in Molecular-weight Distribution Determined by Gel-permeation Chromatography, [(a) Before and (b) after sectioning. " ]...

Fig. 8.3. Schematic plot of a molecular weight distribution showing criticality of coil-stretch transition. Increasing k (elongation rate) increases the amount of stretched chain conformation. (Reprinted with permission from [2])...

If the polymer in solution has a broad molecular weight distribution, a question arises as to whether there is any preferential motion of one molecular species or another to the surface. Fleer et al. (2) examined two cases theoretically a 1000 mer polymer dissolved in its own monomer and a 1000 mer polymer dissolved in a 100 mer polymer see Figure 12.29 (1). In both cases the longer chains avoided the surface region because of loss of conformational entropy. Hence the high-molecular weight polymer is depleted from near the surface at equilibrium. The thickness of the depletion zone is proportional to extending several mer-distances into the fluid. 

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