Differential scanning calorimetry phase measurements

Levine and Slade [1.16] investigated the mechanics of cryostability by carbohydrates. Figure 1.19.1 shows an idealized phase diagram developed from differential scanning calorimetry (DSC) measurements for hydrolyzed starch (MW > 100) and for polyhydroxy combinations having a small molecular mass. With slow cooling (quasi in equilibrium conditions), no water crystallizes below the Tg curve. 

Compounds 1-3 exhibit mechano- and/or thermochromic photoluminescent properties accompanied by cubic-columnar LC phase transitions. Table 1 shows the LC properties of compounds 1-3. Compounds 1 and 3 show cubic phases and columnar phases on rapid cooling and slow cooling, respectively. A columnar phase is not observed for compound 2 on cooling. The differential scanning calorimetry (DSC) measurements of compounds 1-3 show that the cubic phases of compounds 1-3 are metastable phases, while the columnar phases are stable. The X-ray diffraction patterns indicate that compounds 2 and 3 form Pmhn cubic phases and PH a rectangular columnar phases. 

Once the probabilities are known, other physical quantities, which are function of the occupation probabilities, can be calculated from (A) — J2yPy y- or order parameters for order-disorder phase transitions. Different examples will appear in the following. For instance, the orientational contribution to the absolute polarization of the ferroelectric compound pyridinium tetrafluoroborate was estimated from 2H NMR temperature-dependent measurements on the perdeuterated pyridinium cations.116 The pyridinium cation evolves around a pseudo C6 axis, and the occupation probabilities of the different potential wells were deduced from the study of 2H NMR powder spectra at different temperatures. The same orientational probabilities can be used to estimate the thermodynamical properties, which depend on the orientational order of the cation. Using a generalized van t Hoff relationship, the orientational enthalpy changes were calculated and compared with differential scanning calorimetry (DSC) measurements.116... 

Li et al. conducted XRD and differential scanning calorimetry (DSC) measurements on cold-drawn (3-PP specimens to verify the occurrence of (3 a phase transition (26). Figure 11.8a-c shows the XRD patterns of undeformed (3-PP and the (3-PP yielded at 8% and 110% strain, respectively. The K value of undeformed (3-PP... 

Differential scanning calorimetry allows measurement of the heat exchanged between a sample (S) and a reference (R). The solidification and melting of phases induce the release and absorption of energy, respectively, and consequently are... 

Furthermore, assuming a first-order phase transition, the VPT can be detected by caloric methods. With a differential scanning calorimetry (DSC) measurement, flie heat change, which provides information about the internal free energy or latent heat, can be determined (Otake et al. 1990 Hirotsu 1993, 1994). Figure 1 shows two curves of the DSC method for PNIPAAm with different cross-linking concentrations. The cross-linker used for this case was layered siUcates (Haraguchi et al. 2002 Ferse 2007). 

Differential scanning calorimetry (DSC) measures heat flow to a polymer. This is important because, by monitoring the heat flow as a function of temperature, phase transitions such as crystalline melt temperatures and glass-transition temperatures can be characterized quite effectively. This, in turn, is quite useful to determine how a pol)oner will behave at operational temperatures. 

Figure 1.27 shows the phase transition behavior obtained from differential scanning calorimetry (DSC) measurements [102]. For the identification of the LC phase, we used optical microscopy and X-ray diffraction measurements. When comparing the FLCP with the corresponding low-molar-mass FLC compound, it became clear that the temperature range of the smectic phase was broadened. It is thought that smectic laminar structure is stabilized in FLCP because the side-chain ends are aligned forcibly by the main chain. 

X-ray single ciystal dillraction experiments at high temperatures. According to these observations, the orthorhombic modification should be named R2(Se03)3-I, and the monoclinic one R2(Se03 >3 -II. The phase transition can be also seen by means of temperature dependent X-ray powder diffraction, and for Nd2(Se03>3 it has been monitored also by differential scanning calorimetry (DSC) measurements (see section 3.4.3). 

The product must be formulated and frozen in a manner which ensures that there is no fluid phase remaining. To achieve this, it is necessary to cool the product to a temperature below which no significant Hquid—soHd phase transitions exist. This temperature can be deterrnined by differential scanning calorimetry or by measuring changes in resistivity (94,95). 

From the discussion presented of reactions in solids, it should be apparent that it is not practical in most cases to determine the concentration of some species during a kinetic study. In fact, it may be necessary to perform the analysis in a continuous way as the sample reacts with no separation necessary or even possible. Experimental methods that allow measurement of the progress of the reaction, especially as the temperature is increased, are particularly valuable. Two such techniques are thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). These techniques have become widely used to characterize solids, determine thermal stability, study phase changes, and so forth. Because they are so versatile in studies on solids, these techniques will be described briefly. 

Although there are other ways, one of the most convenient and rapid ways to measure AH is by differential scanning calorimetry. When the temperature is reached at which a phase transition occurs, heat is absorbed, so more heat must flow to the sample in order to keep the temperature equal to that of the reference. This produces a peak in the endothermic direction. If the transition is readily reversible, cooling the sample will result in heat being liberated as the sample is transformed into the original phase, and a peak in the exothermic direction will be observed. The area of the peak is proportional to the enthalpy change for transformation of the sample into the new phase. Before the sample is completely transformed into the new phase, the fraction transformed at a specific temperature can be determined by comparing the partial peak area up to that temperature to the total area. That fraction, a, determined as a function of temperature can be used as the variable for kinetic analysis of the transformation. 

Differential scanning calorimetry (DSC) compares the two different heat flows one to or from the sample to be studied, the other to or from a substance with no phase transitions in the range to be measured e. g. glassmaking sand. Figure 1.45 is the scheme of a DSC system Fig. 1.46 is a commercial apparatus for DSC measurements. 

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

Evidence for the formation of gels from aPS systems is obtained from simple mechanical, (1.4.5) viscoelastic, (7.8) thermodynamic (1.6) and spectroscopic ( ) techniques. Simple tube tilting, falling ball methods and differential scanning calorimetry have been used to determine the phase diagrams for a number of systems. Viscoelastic measurements on the aPS-carbon disulfide system show that the low frequency response indicative of a... 

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