Segmental Disparity

The two chains in the lipid bilayer are not identical, because they are positioned asymmetrically with respect to the PC head. It turns out that the tail closest to the head group is buried less deep in the bilayer than the other chain [98]. The difference is not very large it amounts to about half a segment size. From this difference, we can rationalise the disparities in order between the two tails. The chain that is pulled out will be stretched most, and the order tends to be higher than the other chain. Below we will see that the differences in the behaviour of the tails can become larger if there are differences between the tails, e.g. with respect to the degree of unsaturation. 

Brezis M, Shanley P, Silva P, Spokes K, Lear S, Epstein FH, Rosen S Disparate mechanisms for hypoxic cell injury in different nephron segments. Studies in the isolated perfused rat kidney. J Clin Invest 76 1796-806,1985... 

More than one phase border loop exists at one temperature for some mixtures of components of greater disparity. Figure 4.11 shows the isothermal gle of n-heptane + ethanol at 313° and 333°K. Two connected loops occur at each temperature. Both branches of the lower curves are dew-point states the upper curve, bubble-point states. The dew state and bubble state on the same branch of the saturation loop and at the same T and p are at phase equilibrium. To illustrate, two phases are at equilibrium at 313°K and 20 kPa and also at 333° and 50 kPa. All four pairs are shown with dashed line segments. 

Let us therefore move to the 2G SCF computations and focus on miceUar objects with two sides. Again, it is obvious that we need repulsion between the two types of segments (as in Fig. 7), or poor solvent conditions for one of the chains (as in Fig. 8) for this to occur. The exact structures that are formed may be further influenced by the adsorption strength, the chain length differences and/or the grafting density disparities. We cannot deal here with aU these degrees of freedom and therefore we choose to take the system of Fig. 8 and consider how this system behaves in a 2G analysis. 

Conformational relaxation of polymers at temperatures below their glass transition temperature is retarded by lack of segmental motions. The conformation and free volume at the glass transition temperature continues at lower temperatures since equilibrium cannot be attained over typical experimental times. Cooperative relaxation towards conformational equilibrium depends upon temperatures, relaxation time spectrum, and the disparity between the actual and equilibrium states. The approach of the vitrified polymer to equilibrium is called thermal aging. Aging is both non-linear and nonexponential and several descriptions and models have been proposed. One model is based on a concept of temporary networks where the viscoelastic... 

The chapters in this book deal with specific research situations, firom both the experimental and theoretical points of view. To conceptually unify what appears to be a diverse collection of work on disparate systems, an overview chapter by William B. Russel serves as an introduction. This chapter reviews various phenomena such as flocculation, stabliliza-tion, phase separation, and rheology of colloidal particles in polymer solutions, which are all recognized as macroscopic manifestations of the short-range forces between polymer segments and colloid surfaces. 

The location of the LOOT for symmetric P(d-S-b-nBMA) copolymers having total molecular weights of 32,000 g/mol and 78,000 g/mol and the LCST for a 40% blend of d-PS and PnBMA homopolymers at ambient pressure are shown in Table 1. Our results are in agreement with literature reports. The disparity between the LOOT for the 32K block copolymer and the LCST of homopolymer blends of d-PS and PnBMA chains having molecular weights nearly identical to the copolymer segments is a consequence of connectivity of the dissimilar blocks. 

Fig. 2.33. A comparison of the retardation spectra L of a high molecular weight PS (filled triangles), a solution of 25% PS in TCP (open squares) and PIB (filled circles). The shift factors are arranged such that the maximum of the first peak occurs at the same reduced frequency for all three samples. Downward vertical shifts by 0.869 and 1.39 of logio L have been applied to data for PS and the 25% PS solution, respectively, in order to make all data have about the same height at the first maximum. The disparity in width of the softening dispersion of bulk PS and PIB is clear. The small peak near the bottom (dashed line) is the contribution to L from the local segmental motion in bulk PS. The inset shows isothermal tan 8 data of PIB in the softening region at -66.9 °C, and tan 8 of the solution of 25% PS in TCP obtained from a reduced recoverable-compliance curve after applying time-temperature superposition to the limited isothermal data.

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