Temperature Behaviour of the Viscoelastic Material Functions in Polymers
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Any processing procedure on a thermoplastic polymer involves both changes of temperature and pressure which may be quite severe. Most commonly, one starts with solid pellets at a temperature about 0.6 to 0.8 times the glass transition temperature Tg. The pellets usually feed a plasticating screw, where they are deformed and heated up to a melt (at a temperature T~ 1.3 Tg to 1.5 Tg for thermoplastics, higher for elastomers). Later the melt flows through a calender or a die, at a shear rate from 100 to 102 s1 or it is pushed to fill an injection mould at a shear rate one order of magnitude higher (~103 s 1 ). Both in calendering and in injection moulding a noticeable amount of pressure was built. Finally the melt is allowed to cool to the initial temperature, either by natural cooling or at some imposed cooling rate. The duration of the whole cycle (residence time + cooling time) is a few to several minutes. During it, the moduli changed by 5-6 orders of magnitude (from a few GPa to some kPa), the melt viscosity changed by 3-5 orders of magnitude too. Also the material may have been considerably compressed.
KeywordsFree Volume Master Curve Relaxation Modulus Creep Compliance Free Volume Fraction
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