Single polymer chains

Just as it is not necessary for polymer chains to be linear, it is also not necessary for all repeat units to be the same. We have already mentioned molecules like proteins where a wide variety of different repeat units are present. Among synthetic polymers, those in which a single kind of repeat unit are involved are called homopolymers, and those containing more than one kind of repeat unit are copolymers. Note that these definitions are based on the repeat unit, not the monomer. An ordinary polyester is not a copolymer, even though two different monomers, acids and alcohols, are its monomers. By contrast, copolymers result when different monomers bond together in the same way to produce a chain in which each kind of monomer retains its respective substituents in the polymer molecule. The unmodified term copolymer is generally used to designate the case where two different repeat units are involved. Where three kinds of repeat units are present, the system is called a terpolymer where there are more than three, the system is called a multicomponent copolymer. The copolymers we discuss in this book will be primarily two-component molecules. We shall discuss copolymers in Chap. 7, so the present remarks are simply for purposes of orientation. 

The second type of isomerism we discuss in this section is stereo isomerism. Again we consider the number of ways a singly substituted vinyl monomer can add to a growing polymer chain ... 

In the above examples the size of the chain can be measured by considering the number of automobile collisions that result from the first accident, or the number of fission reactions which follow from the first neutron capture. When we think about the number of monomers that react as a result of a single initiation step, we are led directly to the degree of polymerization of the resulting molecule. In this way the chain mechanism and the properties of the polymer chains are directly related. 

An alternative point of view assumes that each repeat unit of the polymer chain offers hydrodynamic resistance to the flow such that f-the friction factor per repeat unit-is applicable to each of the n units. This situation is called the free-draining coil. The free-draining coil is the model upon which the Debye viscosity equation is based in Chap. 2. Accordingly, we use Eq. (2.53) to give the contribution of a single polymer chain to the rate of energy dissipation ... 

CMC hydrates rapidly and forms clear solutions. Viscosity buUding is the single most important property of CMC. DUute solutions of CMC exhibit stable viscosity because each polymer chain is hydrated, extended, and independent. The sodium carboxylate groups are highly hydrated, and the ceUulose molecule itself is hydrated. The ceUulose molecule is linear, and conversion of it into a polyanion (polycarboxylate) tends to keep it in an extended form by reason of coulombic repulsion. This same coulombic repulsion between the carboxylate anions prevents aggregation of the polymer chains. Solutions of CMC are either pseudoplastic or thixotropic, depending on the type. 

The stetic constraints imposed by the bulky ligands cause the propylene to bond almost entirely with a single orientation with respect to the growing polymer chain, CH2, which leads to the stereoregular product. 

Isohutyhdene Diurea., This is the condensation product of urea and isobutyraldehyde. Unlike the condensation of urea with formaldehyde, which forms a distribution of different UF polymer chain lengths, the reaction of urea with isobutyraldehyde forms a single oligomer. Although similar in chemical stmcture to methylene diurea (MDU), its physical properties are quite different (Table 4). 

It may be noted here that it is frequently possible to polymerise two monomers together so that residues from both monomers occur together in the same polymer chain. In addition polymerisation this normally occurs in a somewhat random fashion and the product is known as a binary copolymer. It is possible to copolymerise more than two monomers together and in the case of three monomers the product is referred to as a ternary copolymer or terpolymer. The term homopolymer is sometimes used to refer to a polymer made from a single monomer. 

In systems of LP the dynamic response to a temperature quench is characterized by a different mechanism, namely monomer-mediated equilibrium polymerization (MMEP) in which only single monomers may participate in the mass exchange. For this no analytic solution, even in terms of MFA, seems to exist yet [70]. Monomer-mediated equilibrium polymerization (MMEP) is typical of systems like poly(a-methylstyrene) [5-7] in which a reaction proceeds by the addition or removal of a single monomer at the active end of a polymer chain after a radical initiator has been added to the system so as to start the polymerization. The attachment/detachment of single monomers at chain ends is believed to be the mechanism of equilibrium polymerization also for certain liquid sulphur systems [8] as well as for self-assembled aggregates of certain dyes [9] where chain ends are thermally activated radicals with no initiators needed. 

The last quantity that we discuss is the mean repulsive force / exerted on the wall. For a single chain this is defined taking the derivative of the logarithm of the chain partition function with respect to the position of the wall (in the —z direction). In the case of a semi-infinite system exposed to a dilute solution of polymer chains at polymer density one can equate the pressure on the wall to the pressure in the bulk which is simply given by the ideal gas law The conclusion then is that [74]... 

With the first successful growth of a polymer single crystal in the 1950s it was found that the polymer chains are folded back and forth many times inside the crystal [161]. 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

Larger polymers are known as proteins. Aside from the amide linkages, the polymer chain is very flexible, giving rise to the possibility of an enormous number of different conformers. It is nothing short of remarkable, therefore, that proteins rapidly fold into a single conformation. Very strong forces must be at work. 

The efficiency of PPV may also be raised by introducing disorder into the polymer chains. The crystallinity of PPV may be lowered by employing a modified Wessling method utilizing a xanthate leaving group 63J. PPV produced by this method is believed to contain a mixture of cis- and rram-alkenc units. The efficiency of the material is 0.22% when employed in a single layer device with... 

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