Youngs modulus of solid polymers

One of the simplest mechanical tests that can be carried out on a solid polymer is to deform it in axial tension. For all polymers, the stress-strain curve for the initial stages of such a test is linear, so that a value for Young s modulus of the material may be obtained. Typical results for a high polymer, e.g. polystyrene, that is available in amorphous, partially crystalline and cross-linked forms is shown in Fig. 8.3, where Young s modulus E is shown as a function of temperature T. For all the conditions of the material E is high in the glassy region and practically independent of temperature. 

In the partially crystalline condition there is a small change in E as the temperature is raised through Tg. With further increase in temperature there is a gradual decrease in E until the melting temperature of the crystallites, is reached. There is then a rapid fall in E as the material becomes essentially a very viscous liquid, the condition being referred to as viscofiuid. 

For the amorphous material (typically quenched from the melt) E falls by several orders of magnitude as the temperature is raised through Tg, and above that temperature the material is rubbery or, strictly, viscoelastic. When the temperature exceeds Tf (see Fig. 8.3), known as the viscofluid transition temperature, E 

The cross-linked material shows a marked drop in as the temperature is raised through and then E is relatively constant for a wide range of temperature, only decreasing rapidly when the viscofluid region is reached. 

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