Viscoelasticity semicrystalline polymers

In contrast to the mature instrumental techniques discussed above, a hitherto nonexistent class of techniques will require substantial development effort. The new instruments will be capable of measuring the thermal (e.g., glass transition temperatures for amorphous or semicrystalline polymers and melting temperatures for materials in the crystalline phase), chemical, and mechanical (e.g., viscoelastic) properties of nanoscale films in confined geometries, and their creation will require rethinking of conventional methods that are used for bulk measurements. 

They produced high performance electrets from thin polymer films metallized so as to yield high capacitance. Both electrical and mechanical properties of these transducers have been remarkable examples of how applications of science of solids, including knowledge of electron traps, conduction processes in insulators and the viscoelastic phenomena of semicrystalline polymers, can be combined.(6) Incidentally, similar ideas have been applied to optimization of the properties of particle microphones, through assemblies of perfectly microspherical polymer carbon systems. These have shown what limits of performance... 

Fig. 3.14. The data is for a very broad range of times and temperatures. The superposition principle is based on the observation that time (rate of change of strain, or strain rate) is inversely proportional to the temperature effect in most polymers. That is, an equivalent viscoelastic response occurs at a high temperature and normal measurement times and at a lower temperature and longer times. The individual responses can be shifted using the WLF equation to produce a modulus-time master curve at a specified temperature, as shown in Fig. 3.15. The WLF equation is as shown by Eq. 3.31 for shifting the viscosity. The method works for semicrystalline polymers. It works for amorphous polymers at temperatures (T) greater than Tg + 100 °C. Shifting the stress relaxation modulus using the shift factor a, works in a similar manner.

Some examples of viscoelastic materials include amorphous polymers, semicrystalline polymers, biopolymers, and metals at very high temperatures. Cracking occurs when the strain is applied quickly and outside of the elastic limit [8],... 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

The viscoelastic properties of the crystalline zones are significantly different from those of the amorphous phase, and consequently semicrystalline polymers may be considered to be made up of two phases each with its own viscoelastic properties. The best known model to study the viscoelastic behavior of polymers was developed for copolymers as ABS (acrylonitrile-butadiene-styrene triblock copolymer). In this system, spheres of rubber are immersed in a glassy matrix. Two cases can be considered. If the stress is uniform in a polyphase, the contribution of the phases to the complex tensile compliance should be additive. However, if the strain is uniform, then the contribution of the polyphases to the complex modulus is additive. The... 

The mechanical properties are dependent on both the chemical and physical nature of the polymer and the environment in which it is used. For amorphous polymers, the principles of linear viscoelasticity apply, but these are no longer valid for a semicrystalline polymer. The mechanical response of a polymer is profoundly influenced by the degree of crystallinity in the sample. 

Amorphous polymers well above Jg behave either as liquids or, if they are cross-linked, as rubbers the properties of rubbers are discussed in the next section. In the region close to Jg the viscoelastic properties dominate even at small strains and relatively short times and these are considered in the next chapter. This means that the static small-strain properties of amorphous polymers can be discussed meaningfully only when the polymers are well below Tg. Semicrystalline polymers are really composite materials. At temperatures well below the Tg of the amorphous regions the material has small-strain elastic properties that depend on the proper-... 

The time dependent response of high-density polyethylene was represented by a one-dimensional integral including recoverable viscoelastic strain and irrecoverable viscoplastic strain. The response was represented by an effective time concept. Creep, recovery, two-step creep, and constant stress loading and unloading rates. The concept can also model preconditioning of semicrystalline polymers. 

Some transitions that are only known for macromolecules, however, will not be mentioned at all since they are covered elsewhere in this Encyclopedia (see, eg. Gel Point). Also we shall not be concerned here with the transformations from the molten state to the solid state of polymeric materials, since this is the subject of separate treatments (see Crystallization Kinetics Glass Transition Viscoelasticity). Unlike other materials, polymers in the solid state rarely reach full thermal equilibrium. Of course, all amorphous materials can be considered as frozen fluids (see Glass Transition) Rather perfect crystals exist for metals, oxides, semiconductors etc, whereas polymers typically are semicrystalline, where amorphous regions alternate with crystalline lamellae, and the detailed structure and properties are history-dependent (see Semicrystalline Polymers). Such out-of-equilibrium aspects are out of the scope of the present article, which rather emphasizes general facts of the statistical thermodynamics (qv) of phase transitions and their applications to polymers in fluid phases. 

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