Behavior Amorphous Polymers

Let s start our discussion of the range of viscoelastic properties of amorphous polymers by considering the modulus of an amorphous polymer measured as a function of temperature. We know that any measurement of stress, hence modulus, we make is going to vary with time (see Equation 13-71), so to compare values at different temperatures we make all our measurement 

FIGURE 13-81 Summary of properties of amorphous polymers as a function of temperature. 

As you might expect from our discussion of dynamic mechanical properties, equiva- 

FIGURE 13-83 Schematic plot of modulus versus time. 

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The mechanical and thermal behaviors depend partly on the degree of crystallinity. For example, highly disordered (dominantly amorphous) polymers make good elastomeric materials, while highly crystalline polymers, such as polyamides, have the rigidity needed for fibers. Crystallinity of polymers correlates with their melting points. 

Synthetic and natural rubbers are amorphous polymers, typically with glass transition temperatures well below room temperature. Physical or chemical crosslinks limit chain translation and thus prevent viscous flow. The resulting products exhibit elastic behavior, which we exploit in such diverse applications as hoses, automotive tires, and bicycle suspension units. 

In Fig. 8.4 a), the glassy amorphous polymer extends only a few percent before it breaks abruptly. The extension in the sample up to the point of failure is largely reversible, that is, the material behaves elastically. Polystyrene and polycarbonate, which are used to make CD jewel cases, exhibit this type of behavior. 

Many polymers are not completely amorphous but are more or less crystalline. The degree of Crystallinity and the morphology of the crystalline material have profound effects on the mechanical behavior of polymers, and since these factors can be varied over a wide range, the mechanical properties of crystalline polymers take on a bewildering array of possibilities. 

In general, there are three kinds of moduli Young s moduli E, shear moduli G, and bulk moduli K. The simplest of all materials are isotropic and homogeneous. The distinguishing feature about isotropic elastic materials is that their properties are the same in all directions. Unoriented amorphous polymers and annealed glasses are examples of such materials. They have only one of each of the three kinds of moduli, and since the moduli are interrelated, only two moduli are enough to describe the elastic behavior of isotropic substances. For isotropic materials... 

In this section we summarize the effect of structural and compositional factors on the modulus of the simplest of amorphous polymers. Actual polymers are usually more complex in behavior than the generalized examples shown here. In later chapters we discuss more complex systems in detail. 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

The two main transitions in polymers are the glass-rubber transition (Tg) and the crystalline melting point (Tm). The Tg is the most important basic parameter of an amorphous polymer because it determines whether the material will be a hard solid or an elastomer at specific use temperature ranges and at what temperature its behavior pattern changes. 

Contents Chain Configuration in Amorphous Polymer Systems. Material Properties of Viscoelastic Liquids. Molecular Models in Polymer Rheology. Experimental Results on Linear Viscoelastic Behavior. Molecular Entan-lement Theories of Linear iscoelastic Behavior. Entanglement in Cross-linked Systems. Non-linear Viscoelastic-Properties. 

Creep behavior is similar to viscous flow. The behavior in Equation 14.17 shows that compliance and strain are linearly related and inversely related to stress. This linear behavior is typical for most amorphous polymers for small strains over short periods of time. Further, the overall effect of a number of such imposed stresses is additive. Non-creep-related recovery... 

In terms of the mechanical behavior that has already been described in Sections 5.1 and Section 5.2, stress-strain diagrams for polymers can exhibit many of the same characteristics as brittle materials (Figure 5.58, curve A) and ductile materials (Figure 5.58, curve B). In general, highly crystalline polymers (curve A) behave in a brittle manner, whereas amorphous polymers can exhibit plastic deformation, as in... 

With the exception of PC, amorphous, non-oriented polymers did not produce measurable amounts of broken segments when subjected to tension. As has been shown in previous paragraphs, large axial stresses capable of chain scission in amorphous polymers can only be transmitted into the chain by friction of slipping chains requiring strong intermolecular interactions. In addition, macroscopic fracture occurs before a widespread chain overloading and scission occurs, which is opposite to the behavior of semicrystalline polymers. 

Typical amorphous polymers can exhibit each of these types of stress-strain behavior when the temperature is changed from below to above the Tg of the polymer. 

Graessley.W.M. Molecular entanglement theory of flow behavior in amorphous polymers. J. Chem. Phys. 43, 2696-2703 (1965). 

Figure 12. Typical dynamic behavior of uncrosslinked amorphous polymers. The material is a copolymer of styrene-butadiene 25)...

In the early literature it is suggested that polycarbonates can be easily plasticized with common plasticizers. Plasticization of polycarbonate has been investigated by Kozlov et al. (12). These authors described the influence of plasticization on softening points and mechanical properties of bisphenol A polycarbonates. They conclude that the behavior of plasticized polycarbonate is similar to that encountered for most amorphous polymers. The influence of crystallization effects promoted by the plasticizer was not taken into account. 

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