Rubbery range

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

An explanation for the anomalous behavior of the most highly crosslinked DGEBA/DDS network may be that it was chemically unstable at the high test temperatures (25(K260 °C) required to reach the rubbery range for this network. Also, the threshold fracture theories may simply fail to describe the structure-fracture relationship of this very highly crosslinked network. 

Viscosity is a material s resistance to viscous deformation (flow). Its unit of measure is Pascals second (Pa s) or pounds second/in (lb s/in ). Plastic melt viscosities have a range from 2 to 3,000 Pa s (glass 10 , water 10"0-The resistance to elastic deformation is the modulus of elasticity ( ), which is measured in Pascals (Pa) or pounds per square inch (psi) its range for a plastic melt is 1,000 to 7,000 kPa (145 to 1,015 psi), which is called the rubbery range (Fig. 1-4). 

The dependence of the rubbery range on chlorine content may be shifted by two main factors ... 

Eqs. 7.20 or 7.33 can be used to confirm mathematically the Joule effect or the increase of modulus with temperature in the rubbery range (see problem 7.5). The elastic (or 30 second) modulus for the epoxy of Fig. 7.3 in the rubbery range is shown plotted vs. absolute temperature in Fig. 7.10. Obviously, the rubbery modulus does increase linearly with increasing temperature. Even though the extrapolated data does not go through the origin it does serve as confirmation of the Joule effect mentioned in Chapter 1. 

Fig. 7.10 Variation of modulus for the epoxy of Fig. 7.3 in the rubbery range. (Data from Brinson (1965), (1968).)...

For polymers in the rubbery range the volume change dV is small and if the pressure is only the atmospheric pressure the pdV term is so small as to be negligible. The negative sign for the pV term in the enthalpy definition here is due to the fact that the work of atmospheric pressure is in opposition to the positive work of a tensile force on an uniaxial specimen. 

Using the TTSP prove that modulus should increase with increasing temperature (the Joule effect) in the rubbery range. 

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