Plateau modulus Thermoplastics

Measurement of modulus over an extensive temperature range offers more information than T alone (16). Typical modulus—temperature curves are shown in Figure 1. Assuming that the reference temperature is the transition temperature of the copolymer, then curve A of Figure 1 is that of a softer polymer and curve B is that of a harder polymer. Cross-linking of the polymer elevates and extends the mbbery plateau Htde effect on T is noted until extensive cross-linking has been introduced. In practice, cross-linking of methacryhc polymers is used to decrease thermoplasticity and solubihty and to increase residence. 

In nearly aU cases, thermoplastic elastomers will be a copolymer (i.e., there will be at least two monomers in the polymer chain). A thermoplastic elastomer will generally have the modulus vs. temperature curve shown in part c of Figure 13.1. The plateau region must include the service temperature of the material. Typically through changes in comonomer composition or identity, the plateau can be shifted upward or downward, giving the manufacturer a great deal of flexibility. 

The behavior of a thermoplastic material above its glass transition temperature depends on its level of crystallinity. As a noncrystalline (amorphous) polymer is slowly heated from a temperature below its Tg, it displays a large decrease in modulus as the glass transition temperature is reached. As one heats a semicrystalline plastic from a temperature below its Tg, it displays a relatively small modulus change at the glass transition temperature, followed by a plateau and then a decreasing modulus as the temperature increases and approaches the crystalline melting point. 

Eceiza et al investigated a series of thermoplastic PUs synthesized in bulk by two-step polymerization, obtained with the isocyanate-chain extender couple MDI-BG and various SS macrodiols. The chemical structure, the SS molecular weight and the HS content were varied. The effects on the thermal and mechanical properties were investigated [77]. The changes in the macrodiols in the case of PUs based on MDl-BG, resulted in a modulus curve that showed a plateau indicating the existence of physical crosslinks because of the increase in the size and inter-connectivity... 

At very low temperatures, both phases are hard and so the material is stiff and brittle. At a somewhat higher temperature, the elastomer phase becomes soft and the thermoplastic elastomer now resembles a conventional vulcanizate. As the temperature is further increased, the modulus stays relatively constant (a region often described as the rubbery plateau ) until finally the hard phase softens. At this point, the thermoplastic elastomer becomes fluid. Thus, thermoplastic elastomers have two service temperatures. The lower service temperature depends on the T of the elastomer phase, while the upper service temperature depends on the T or of the hard phase. Values of T and T for the various phases in some commercially important thermoplastic elastomers... 

Scheme 1 The elastic modulus (log E) versus temperature in a amorphous and semi-crystalhne thermoplastics b their corresponding fiber-reinforced composites (domains A—glass plateau, B— glass transition, C—semi-crystalhne plateau, D— flow http //www.pluscomposites.eu/publications)...

There is however an aspect which is qualitatively common to all filler-thermoplastic systems the linear viscoelastic behavior exhibited by most pure polymers at sufficiently low strain or low rate of deformation disappear above a sufficient filler level. For instance, the so-called Newtonian plateau on the shear viscosity function is no longer observed, the d5mamic modulus is strongly strain dependent and the terminal region in the elastic modulus function disappears and is replaced by a low frequency plateau. As we have seen, such typical effects are also observed with filled rubber compounds. 

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