Physical aging method

Figure 8 Physical aging method— wire insulation thermal stability. (Courtesy of TA Instruments, TS-125.)... 

As I discuss in Time A Traveler s Guide, we now know that time travel need not be confined to science fiction or Hollywood movies. Time travel is possible. For example, an object traveling at high speeds ages more slowly than a stationary object. This means that if you were to travel into outer space and return, moving close to light-speed, you could travel thousands of years into the Earth s future. In addition to highspeed travel, researchers have proposed numerous ways in which time machines can be built that do not violate any known laws of physics. These methods allow you to travel to points in the world s past or future. 

ARCHAOMETRY. Application of chemical and physical analytical methods to archaeology, Among those used are microanalyhcal methods, spectroscopic analysis, X-ray, and other types of nondestructive tests. For age determination 14 C measurement (chemical dating) is one of the most valuable techniques. 

In conclusion, microhardness evaluation is a method capable of measuring the molecular reorganization taking place above and below Tg. On one hand it detects the contribution of chain mobility with increasing annealing time and temperature leading to a more compact structure (physical ageing). On the other hand, the hardness measures the influence of a thermal expansion. 

The study of blends by physical aging is stiU largely unexplored and requires close attention both from the experimental and theoretical standpoint. One major advance would be the ability to crosscorrelate mechanical and thermodynamic physical aging data, but while there is a theory that links the two this is imperfect and requires refinement. One problem is that the aging processes, when followed by these different methods, proceed at different rates. If conditions could be established that led to these proceeding at equal rates then this cross-correlation would be possible. 

Struik [3] originally proposed a method to model physical aging through the use of a momentary creep master curve obtained from a series of short term creep tests performed at various aging times. The momentary creep master curve was then used in conjunction with the effective time theory to predict long term creep in a polymer in the presence of physical aging. 

Despite the attractiveness of time-temperature superposition and the potential saving in time, the method has not in fact been widely used to obtain creep data for design. One good reason for doubt about the precision of the method is the existence of physical ageing (see Section 4.4.1). Nevertheless, general points well worth retaining in the mind are (i) creep deformation processes are speeded up at higher temperatures (ii) the effective time at a temperature Tg is t/a-p, where r is the time for the same mechanical effect at another temperature 7. 

Physical aging effects have practical implications and need to be considered when assessing the long-term stabihty of polymers and polymer-polymer mixtures. This chapter focuses on a discussion of the effect of blending on physical aging and gives a review of the different experimental methods that can be used to compare aging rates in blends to those of the individual components. 

As the annealing temperatures T drop further away from Tg, the aging process slows down and the time scales involved become quite long. Consequently many studies are carried out under thermally accelerated conditions. The relaxation of the enthalpy and volume of the glass are convenient parameters to follow when monitoring the physical aging process, as are the time-dependent small strain mechanical properties. Spectroscopic and scattering methods can also be employed... 

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