Intermolecular slippage

FIGURE 3.4 Deformation in plastics, (a) Stretching of polymer molecule, (b) Staightening out of a coiled molecular chain, (c) Intermolecular slippage. 

In the diagram, load per unit cross section (stress) is plotted against deformation expressed as a fraction of the original dimension (strain). Even for different materials the nature of the curves will be similar, but they will differ in (1) the numerical values obtained and (2) how far the course of the typical curve is followed before failure occurs. Cellulose acetate and many other thermoplastics may follow the typical curve for almost its entire course. Thermosets like phenolics, on the other hand, have cross-linked molecules, and only a limited amount of intermolecular slippage can occur. As a result, they undergo fracture at low strains, and the stress-strain curve is followed no further than to some point below the knee, such as point 1. 

The absence of strong intermolecular forces, the presence of pendant methyl groups discouraging close association, and crankshaft action associated with the cis isomer all contribute to the flexibility of NR. The introduction of a few cross-links by vulcanization with sulfur reduces slippage of chains but still permits good flexibility. 

In a polymeric solid we find more complex conditions. Here, we have both intramolecular covalent forces and intermolecular van der Waals forces and, furthermore, a heterogeneous microscopic structure with a low degree of order. Molecular fracture mechanisms are therefore more involved and include chain slippage as well as chain scission. The latter process can only occur for fully... 

In a polymeric sohd conditions are more complex. There are both in-termolecular van der Waals forces and intramolecular covalent forces and, furthermore, the microscopic structure is amorphous or only partially crystalline. Microscopic fracture mechanisms are, therefore, more involved and also include chain slippage and chain scission. The latter process can occur in fully extended sequences that are anchored with their ends in crystallites or immobile entanglements, by a combined effect of mechanical stress and thermal fluctuations. Even if the situation is complicated, based on some rough knowledge of the bond energies and intermolecular forces one can still obtain some estimate of the theoretical limits of strength. Comparing it to measured... 

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