Long Flexible Chains

This book discusses the statistical properties of long, flexible objects, polymer chains being the fundamental example. The following is a short list of chains which are currently used in physical studies  

The number of repeat units, N. in one chain is often called the degree of polymerization (PP) and can be amazingly large. (For example, it is possible to reach IV 10 with polystyrene.) The fabrication of such long chains without error in a sequence of 10 operations is a remarkable chemical achievement. However, there are many difficulties. Two are particularly important for physical studies potydispersity and branching. 

Most preparation schemes give chains with a very Ixoad distribution of N values. It is possible, however, to obtain relatively narrow distributions either by physical selection (via precipitations, gel permeation, chromatography, etc. ) or through special methods of synthesis, such as anionic polymerization.  

Many parasitic reactions occurring during the synthesis can lead to a chain which is not perfectly linear but which contains branch points. For example, industrial polyethylene has many three-fiinctional branch points of the type 

When the fraction of branch points ill the structure is not too small, these points can be detected by various physical methods, such as infrared spectroscopy. On the other hand, if a long chain has accidentally acquired one or two branch points, it is extremely hard to demonstrate their existence or absence (they show up mainly in certain mechanical studies on concentrated systems, discussed in Chapter VIII). 

---

Polyols. Analogous to the use of linear a,C0-dibasic acids, such as adipic and sebacic, polyols with long, flexible chains between hydroxyl groups, such as 1,4-butanediol [110-63-4] 1,6-hexanediol [629-11-8J, and diethylene glycol [111-46-6] may also be used to impart greater flexibiUty ia the resia. 

Synthetic rubber (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces. They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity. Elastomers are usually cross-linked to impart strength. 

Octane, C8H18, a hydrocarbon present in gasoline, has molecules that consist of long, flexible chains. 

The longest times of the longitudinal relaxation spectrum depend on the molecular mass of the chain. In contrast, transverse times for relatively long flexible chains are independent of molecular mass. 

Natural rubber illustrates these structural requirements of an elastomer long, flexible chains weak intermolecular forces and occasional cross-linking. Rubber is c/3-1,4-polyisoprene. With no highly polar substituents, intermolecular attraction is largely limited to van der Waals forces. But these are weak because of... 

Figure 6.7 shows that the elastic modulus and relaxation time of the polymer solution increased with polymer concentration, to was 1.351 radians per second. The HPAM 1275A and HPAM 1255 polymers were used. As polymer concentration increases, the distance between polymer molecules decreases. The entanglement of the long flexible chains will be more severe, and the van der Waals force will become larger so that it is more difficult for polymer molecules to deform. When the external force is removed, polymer molecules quickly return to their curling state. 

In a long flexible chain molecule there are various segmental motions. Eventually at one point there will be such a concentration of energy that the chain breaks. In the simplest case, that of polyethylene (PE) with only C-C and C-H bonds, the disassociation energy for these bonds is about 80 and kcal/mol, respectively. So when the energy exceeds the amount, the chain breaks, the two sigma bonding electrons are separated and two lone electrons (free radicals) are formed. 

The molecular model for the ideal gas is a collection of point masses in ceaseless, random, thermal motion, the motion of any two of the point masses being completely uncorrelated with one another. The counterpart to this in the case of the ideal elastomer is a collection of volumeless, long, flexible chains... 

The special structure of polymer molecules that distinguishes them from other species is their long, flexible chain structure. To describe this situation, let us first consider an isolated polymer chain and then extend the results to ensembles of chains, that is, to the bulk polymer. An isolated linear polymer chain is capable of assuming many different conformations. Because of... 

The entropy theory falters because it does not take into account the prodigious rate effects possible when critical distances are imposed. If two functional groups are connected by a long flexible chain, then inserting a cis double bond in the chain will indeed have only a minor effect on the rate. 

After long controversies, the hypothesis of Staudinger was finally accepted around 1930. The image of a polymer that we now have, namely that of a long flexible chain, corresponds to a reversal in scientific attitude the aim was to explain everything by starting from the chemical properties of the repetitive unit, but finally it was realized that the very existence of a chain structure was in itself fundamental. 

---