Unimodal networks, short chain

Figure 7 Typical dependence of nominal stress against elongation for two unimodal networks having either all short chains or all long chains, and a bimodal network having some of both.

An additional bonus exists if the chains in the bimodal network readily undergo strain-induced crystallization.92,274,278,282 It has been observed that the extent to which the bimodal networks are superior to their unimodal counterparts is larger at lower temperatures. This indicates that the bimodal character of the chain length distribution facilitates strain-induced crystallization. Apparently the short chains increase the orientation of the long chains, and this facilitates the crystallization process. 

But what happens in the case of bimodal networks having such overwhelming numbers of short chains that they cannot be ignored There is a synergistic effect leading to mechanical properties that are better than those obtainable from the usual unimodal distribution. The following sections describe these results in detail. 

If the network consists entirely of short chains, then the material is brittle (which means that the maximum extensibility is very small). If the network consists of long chains, the ultimate strength is very low. As a result, neither unimodal material has a large area under its stress-strain curve and, thus, neither is a tough elastomer. 

Mark, J. E. Curro, J. G., A Non-Gaussian Theory of Rubberlike Elasticity Based on Rotational Isomeric State Simulations of Network Chain Configurations. I. Polyethylene and Polydimethylsiloxane Short-Chain Unimodal Networks. J. Chem. Phys. 1983, 79, 5705-5709. 

Stress-strain isotherms have also been calculated with this approach. Examples are unimodal networks of polyethylene and POMS, " polymeric sulfur and seleniirm, short n-alkane chains, natural rubber, several polyoxides, and elastin, and bimodal networks of PDMS. It is possible to include excluded volume effects, in such simulations. In the case of the partially helical polymer polyoxymethylene, the simulations were used to resolve the overall distributions into contributions from imbroken rods, once-broken rods, twice-broken rods, and so on. It was also shown how applying stresses to the ends of chains of this typ>e can be used to bias the distributions in the direction of increased helical content and increased average end-to-end distances. In this sense, imposition of a stress has the same effect on the helix-coil equilibriirm as a decrease in temperature. ... 

To have greater control of the chain topology, it was decided to use model networks. PDMS networks were chosen because of the extensive work by Mark and his coworkers on these systems. The-networks were prepared by crosslinking divinyl terminated PDMS chains with a tetra silane in the presence of a platinum catalyst. All networks were prepared in bulk. By choosing the appropriate prepolymers, it was possible to prepare unimodal and bimodal networks. For the bimodal networks, the short chains had M = 770 and the long chains had M = 22,500. In both cases M /M 1.8. 

Fig. 1.33. Representative stress-strain isotherms for unimodal and bimodal PDMS networks in uniaxial extension (left-hand side), and biaxial extension (right-hand side) [132]. Each curve is labeled with the mole percentage of the short chains present in the network. The open circles represent data measured using increasing deformations, whereas filled circles represent data obtained out of sequence in order to test for reversibility.

Figure 6.5 Typical plots of nominal stress against elongation for unimodal and bimodal networks obtained by end linking relatively long chains and very short chains. Curve a is for a unimodal network of all short chains, curve b is for a unimodal network of all long chains, and curve c is for a bimodal network of short and long chains. The area under each curve represents the rupture energy (a measure of the toughness of the elastomer).

Fig. 11. Values of the maximum extensibility (elongation at rupture) shown as a function of the molecular weight M between cross-links for unfilled) tetrafunctional PDMS networks at 25 The results pertain to networks prepared and studied in a series of investigations and are typical for the types of cross-linking techniques employed (i) selectively end-linking a mixture of relatively long and very short chains to give a bimodal network (--X--), (ii) selectively linking a (unimodal) sample of chains either through their ends or side-chains (-0-), (iii) peroxide curing (C), and radiation curing (- -)...

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