Molecular Weight of the Polymer

Important factors are concentration, temperature, nature of both solvent and polymer, molecular weight of the polymer, among others [16-21]. 

Thermal stability was determined by assessing the color of compression molded films of the polymer. A film sample was held in a convection oven for 4 hours at 200°C. Color and clarity of the aged film varied firom no ehange to yellow for different polymers. Molecular weight of the polymer was assessed by an indi-reet measurement of melt viscosity. A film sample was produeed by applying pressure to 0.5 g of polymer powder at 225°C in a press for 30 seconds. The area of the film formed by this technique was measured as an indieation of the viscosity of the polymer. A lower viseosity, i.e., lower molecular weight, polymer spread into a larger area film sample. Table 5.41 shows a summary of the PVDF polymerization conditions and the polymer properties. The data in Table 5.41 indieate that no reaction took place in the absenee of both iron and surfactant. Polymerization yield was anemie in the presence of only surfactant or iron. 

The degree of polymerization of a polymer is simply the number of repeat units in a molecule. The degree of polymerization n is given by the ratio of the molecular weight of the polymer to the molecular weight of the repeat unit ... 

The terminal groups of a polymer chain are different in some way from the repeat units that characterize the rest of the molecule. If some technique of analytical chemistry can be applied to determine the number of these end groups in a polymer sample, then the average molecular weight of the polymer is readily evaluated. In essence, the concept is no different than the equivalent procedure applied to low molecular weight compounds. The latter is often included as an experiment in general chemistry laboratory classes. The following steps outline the experimental and computational essence of this procedure ... 

One limitation of this method that should immediately come to mind is the restriction to relatively low molecular weight polymers. This is a corollary of the fact that chain ends are inconsequential for very long chains. Hence, the sensitivity of the method decreases as the molecular weight of the polymer increases. As a general rule, molecular weights in the neighborhood of 25,000 represent the upper limit for applicability of this method. 

One of the striking omissions from our discussion has been an explicit consideration of polymer molecular weight on the viscous behavior of the sample. This omission will be corrected in the next section. 

To the extent that the segmental friction factor f is independent of M, then Eq. (2.56) predicts a first-power dependence of viscosity on the molecular weight of the polymer in agreement with experiment. A more detailed analysis of f shows that segmental motion is easier in the neighborhood of a chain end because the wagging chain end tends to open up the structure of the melt and... 

From plots of these data, estimate the Newtonian viscosity of each of the solutions and the approximate rate of shear at which non-Newtonian behavior sets in. Are these two quantities better correlated with the molecular weight of the polymer or the molecular weight of the arms ... 

An important application of Eq. (3.39) is the evaluation of M, . Flory et al.t measured the tensile force required for 100% elongation of synthetic rubber with variable crosslinking at 25°C. The molecular weight of the un-cross-linked polymer was 225,000, its density was 0.92 g cm , and the average molecular weight of a repeat unit was 68. Use Eq. (3.39) to estimate M. for each of the following samples and compare the calculated value with that obtained from the known fraction of repeat units cross-linked ... 

Above Tjj, the material is liquid and its viscosity depends on the molecular weight of the polymer and the time scale of the observation, but it would be considered high by all standards. 

In summary, T j, gives a truer approximation to a valid equilibrium parameter, although it will be less than T owing to the finite dimensions of the crystal and the finite molecular weight of the polymer. We shall deal with these considerations in the next section. For now we assume that a value for T has been obtained and consider the simple thermodynamics of a phase transition. 

We begin by recognizing that the weight fraction w of n-mers in the polymer mixture at any value of p equals the ratio of the mass of n-mer in the mixture divided by the mass of the total mixture. The former is given by the product uN Mq, where Mq is the molecular weight of the repeat unit the latter is given by NqMq. Therefore we write... 

An equivalent way of looking at the conclusion of item (2) is to recall that Eq. (5.40) gives the (number average) number of monomers of both kinds in the polymer and multiply this quantity by the average molecular weight of the two kinds of units in the structure (88 + 112)/2 = 100. 

This mode of termination produces a negligible effect on the molecular weight of the reacting species, but it does produce a terminal unsaturation in one of the dead polymer molecules. Each polymer molecule contains one initiator fragment when termination occurs by disproportionation. 

This important equation shows that the stationary-state free-radical concentration increases with and varies directly with and inversely with. The concentration of free radicals determines the rate at which polymer forms and the eventual molecular weight of the polymer, since each radical is a growth site. We shall examine these aspects of Eq. (6.23) in the next section. We conclude this section with a numerical example which concerns the stationary-state radical concentration for a typical system. 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

We shall discuss three types of phenomena for polymer solutions thermodynamic properties in Chap. 8, frictional properties in Chap. 9, and lightscattering properties in Chap. 10. A common feature of virtually all phenomena in these areas is that they all depend on the molecular weight of the solute. Thus observations of these properties can be interpreted to yield values for M we shall use this capability as a unifying theme throughout these chapters. 

At first glance, the contents of Chap. 9 read like a catchall for unrelated topics. In it we examine the intrinsic viscosity of polymer solutions, the diffusion coefficient, the sedimentation coefficient, sedimentation equilibrium, and gel permeation chromatography. While all of these techniques can be related in one way or another to the molecular weight of the polymer, the more fundamental unifying principle which connects these topics is their common dependence on the spatial extension of the molecules. The radius of gyration is the parameter of interest in this context, and the intrinsic viscosity in particular can be interpreted to give a value for this important quantity. The experimental techniques discussed in Chap. 9 have been used extensively in the study of biopolymers. 

As in osmotic pressure experiments, polymer concentations are usually expressed in mass volume units rather than in the volume fraction units indicated by the Einstein equation. For dilute solutions, however, Eq. (8.100) shows that

partial molar volume of the polymer in solution, and M is the molecular weight of the polymer. Substituting this relationship for (pin Eq. (9.9)gives... 

To a large extent, the properties of acryUc ester polymers depend on the nature of the alcohol radical and the molecular weight of the polymer. As is typical of polymeric systems, the mechanical properties of acryUc polymers improve as molecular weight is increased however, beyond a critical molecular weight, which often is about 100,000 to 200,000 for amorphous polymers, the improvement is slight and levels off asymptotically. 

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