Backbone bond

The rate constant is a measure of the ease at which the molecule can uncoil through rotation about the C—C or other backbone bonds. This is found to vary with temperature by the exponential rate constant law so that... 

The melt viscosity of a polymer at a given temperature is a measure of the rate at which chains can move relative to each other. This will be controlled by the ease of rotation about the backbone bonds, i.e. the chain flexibility, and on the degree of entanglement. Because of their low chain flexibility, polymers such as polytetrafluoroethylene, the aromatic polyimides, the aromatic polycarbonates and to a less extent poly(vinyl chloride) and poly(methyl methacrylate) are highly viscous in their melting range as compared with polyethylene and polystyrene. 

The backbone bonds are polar but the structure is balanced and the polymer is quite a good dielectric. Reported data on resistivity indicate only moderate values presumably because of ionic fragments, impurities and additives. 

Vincent analyzed the tensile fracture stress o, of a broad range of polymers as a function of the number of backbone bonds per cross sectional area ( 2) and found a nearly linear relation, o 2, as shown in Fig. 12. 2 is related to via the theory of entanglements for random walk chains as [74]... 

Thus, if the ratio AT//3 is constant, then the behavior shown in Fig. 12 could be described by the net solution. For many polymers, the characteristic ratio is around 7-10, the ratio Mo/j is the molecular weight per backbone bond (ca. 30-50) and will not vary extensively, b = 1.54 A and the density is about 1 g/cm such that the parameter is nearly constant. Since K is not very sensitive to the polymer properties, Eq. 6.5 is considered to describe the observed fracture behavior shown in Fig. 12. However, the data are not expected to fall on the straight line due to differences in K and j6 for each polymer. As a specific test case, consider... 

Fig. 12. Tensile strength a, vs. number of backbone bonds per monomer 1 /a, reported for a range of polymers by Vincent [75]. The solid line is the theoretical line for vector percolation analysis of strength discussed herein.

The bond p- to the double bond of the unsaturated disproportionation product 2 is also weaker than other backbone bonds.10 30,32 31 However, it is now believed that the instability of unsaturated linkages is due to a radical-induced decomposition mechanism (Scheme 8.7).30 This mechanism for initiating degradation is analogous to the addition-fragmentation chain transfer observed in polymerizations carried out in the presence of 2 at lower temperatures (see 6.2.3.4, 7.6.5 and 9.5.2). 

In a real polymer chain, rotation around backbone bonds is likely to be hindered by a potential energy barrier of height AEr. If AEr < RT, the population of the... 

Figure 7.1 Crankshaft rotation Involving three carbon-carbon backbone bonds...

L. Schafer, M. Cao, and M. J. Meadows, Predictions of Protein Backbone Bond Distances and Angles from First Principles, Biopol., 35 (1995) 603-606. 

Jiang, X., C-H. Yu, M. Cao, S. Q. Newton, E. F. Paulus, andL. Schafer. 1997. cfr/ifr Torsional Dependence of Peptide and Protein Backbone Bond-Lengths and Bond-Angles Comparison of Crystallographic and Calculated Parameters. J. Mol. Struct. 403, 83-93. 

Predictions of Protein Backbone Bond Distances and Angles from First Principles... 

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