Four Regions of Viscoelastic Behavior

during the experiment the sample has undergone flow, and this area of the 10-second modulus temperature curve is therefore called the flow region. 

If temperature is increased still further, barring chemical reaction the sample will become a viscous liquid. Our simple experimental setup is no longer applicable at high temperatures where the sample will not support its own weight. The modulus will, however, continue to decrease. 

With this background, it will be easy to proceed with the analysis of the modulus-temperature behavior of more complicated systems. 

It should be emphasized that all of the crystalline polymers discussed here are not totally crystalline but partly crystalline and partly amorphous. Thus one expects to see behavior characteristic of the crystalline regions superposed upon behavior characteristic of the amorphous regions. This superposition is not necessarily a linear one but may rather be a complex coupling of the response of each region. Furthermore, the degree of crystallinity as well as the crystallite size is not a unique feature of any polymer system. Both of these properties are determined by prior thermal history. 

Once again, it should be pointed out that the exact shape of the modulus-temperature curve of a crystalline polymer depends on the thermal history of the sample, particularly on the rate of cooling from the melt and annealing treatment. Two crystalline polymers are mechanically equivalent, for practical purposes, if they have the same values of Tg, Tm chain length, percentage of crystallinity, and crystalline structure. Because this is rarely the case, semicrystalline polymers exhibit a wide spectrum of properties. 

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Dynamic mechanical properties provide information about transition temperatures, and T, phase separation and mechanical behavior of these materials. The dynamical mechanical spectra of a series of polyfether ester)s based on PBT and PMTO (1000 g/mol) are shown in Figure 10.5. All TPEEs exhibited four regions of viscoelastic behavior a glassy state in low-temperature range with... 

Dependence of relaxation modulus upon time for linear continuous line] and cross-linked broken line) polymers, showing four regions of viscoelastic behavior... 

The four variables in dynamic oscillatory tests are strain amplitude (or stress amplitude in the case of controlled stress dynamic rheometers), frequency, temperature and time (Gunasekaran and Ak, 2002). Dynamic oscillatory tests can thus take the form of a strain (or stress) amplitude sweep (frequency and temperature held constant), a frequency sweep (strain or stress amplitude and temperature held constant), a temperature sweep (strain or stress amplitude and frequency held constant), or a time sweep (strain or stress amplitude, temperature and frequency held constant). A strain or stress amplitude sweep is normally carried out first to determine the limit of linear viscoelastic behavior. In processing data from both static and dynamic tests it is always necessary to check that measurements were made in the linear region. This is done by calculating viscoelastic properties from the experimental data and determining whether or not they are independent of the magnitude of applied stresses and strains. 

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