Thermal history, reproducible

Figure 17 Isothermal melting of Ziegler-Natta isotactic poly(propylene). (a) Spherulites with mixed birefringence at Tc = 148°C. The top middle figure displays the melting for the same thermal history, (b) Subsequent to crystallization, the temperature was raised to 171°C spherulites acquire negative birefringence, (c), (d) and (e) Isothermal melting at 171°C for 80, 200 and 300 min, respectively. Reproduced with permission from W.T. Huang, Dissertation, Florida State University, 2005. (See Color Plate Section at the end of this book.)...

Master curves can also be constructed for crystalline polymers, but the shift factor is usually not the same as the one calculated from the WLF equation. An additional vertical shift factor is usually required. This factor is a function of temperature, partly because the modulus changes as the degree of crystallinity changes with temperature. Because crystallinity is dependent on aging and thermal history, vertical factors and crystalline polymer master curves tend to have poor reproducibility. 

The results discussed show that crystal structure transformations are considerably dependent on the thermal history of the samples to be more specific, the crystallite size, particle size and surface area have measurable effects on the transformation. It would, therefore, probably be difficult to reproduce strictly transformation data with different samples. The magnitudes of these effects are, however, not too great to result in the wide variability of temperatures of polymorphic transformations. The wide variations in transformation temperature can only be due to other factors... 

Leading models for transient heating mechanisms include shock waves, X-ray flares, the X-wind, and partially molten planetesimals. Shock waves have been shown to be able to reproduce the deduced thermal histories of chondrules, yet... 

We happened to find the countermeasure to reduce the occurrence of defective products. In all processes CET supersaturated solution should be cooled down to 5°C because CET is chemically unstable, but during the scale-up experiments the subdivided vials accidentally were kept several times at room temperature for about 15 min due to problems in the high-speed subdividing machine. Any of the above products A, B, and C could have been produced in such a tray, but only A was produced. Immediately, we examined the relationship between the thermal history and the structural change in the supersaturated solution. Fortunately, the method to reproduce the solutions having the structural state at various grades... 

Figure 5 Thermal history of a chondrule (sohd fines) passing through a 7.5 km s shock wave, over timescales of (a) hours and (b) minutes. Chondrules are heated for hours before the shock by radiation. At the shock front, chondrules must slow to the gas velocity in 1 min it takes to do so, gas drag heating causes a spike in temperature. Afterwards, chondrules are heated for hours by radiation and thermal exchange with the hot gas (dashed line) (Desch and Connolly, 2002) (reproduced by permission of Sheridan Press and Meteoritical Society from Meteorit.

Fig. 8. Influence of mechanical and thermal history on the viscosity of PVC A at 190 C (M 70000). (Reproduced with permission...

The samples for NMR spectroscopy were melted into glass tubes and allowed to cool from the melt. The observed deuterium NMR spectra are reproducible with temperature cycling, thus providing evidence that the thermal history induced by acquiring temperature-dependent spectra of the samples does not greatly affect the properties that we are measuring. 

Grove and Harrison (1999) investigated the feasibility of obtaining Th-Pb age profiles in the surface regions of natural monazites and found that ion intensities were adequate to resolve age differences of <1 Myr with better than 500 A depth resolution in late Tertiary monazites. These age gradients were then used to extract continuous thermal history information from which they constrained the displacement history of a Himalayan thrust. The sputtering of natural surfaces was found to yield inter-element calibration plots of similar reproducibility to that of polished surfaces. 

While the densities of crystalline materials are not particularly sensitive to the thermal history of the sample, densities of glasses are always dependent upon the thermal history of the particular sample measured. Although the differences in density which result from changes in thermal history are not particularly large, they can be very important in certain applications, especially those requiring highly reproducible values of the refractive index of glasses. 

Figure 3.3 shows the changes of total heat flow, heat capacity and dCp/dF with temperature for a PS sample [31]. Because of the effect of thermal history, the relaxation event appears in the total heat flow signal. It can be seen that the peak position of the dCp/dF versus temperature signal corresponds to the point of inflection of the heat capacity curve between the glassy and liquid states. If the peak position, as is often done for a melting point, is used to determine the Tg, it will be very easy and reproducible to use in subsequent analyses. 

One can obtain good reproducibility of the value of n in bulk amorphous materials, but in films n depends considerably on their preparation and thermal history. This was shown for a-Ge films by Wales et al (1967) and Theye (1970). For the edges shown in Figure 4.18 and numbered 1, 2, 3, 4, 5 she found n = 4.7, 4.6,4.5,4.5,4.1 respectively. A similar effect was reported for Si by Brodsky et al Their results are shown in Figure 4.30. One notices that even after crystallization n keeps changing with thermal treatment. 

The simulated curves obtained with the bidisperse model represents nearly well the breakthrough curve of the propane/propylene system over 13X zeolite and reproduces qualitatively the thermal history behavior of the fixed bed process. 

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