Determination of Phase Transition Temperatures

Pyrene has been used to investigate the extent of water penetration into micelles and to accurately determine critical micellar concentrations (Kalyanasundaram, 1987). Polarity studies of silica or alumina surfaces have also been reported. In lipid vesicles, measurement of the ratio Ii/Iui provides a simple tool for determination of phase transition temperatures and also the effect of cholesterol addition. 

Millisecond experiments can be used to determine thermal and electrical properties of metals and alloys in the solid phase up to the melting point at around 3000 K. This moderately fast heating rate allows, e.g., the determination of phase transition temperatures. Nevertheless, millisecond e eriments are limited to the solid state as the rate of heating is still slow compared to the gravitational collapse of a liquid sample under gravitational forces once it becomes molten. Another limitation from the moderate heating rate is the need for heat loss corrections at elevated temperatures. 

The basic methods for the determination of phase transition temperatures (7 ) are DSC (differential scanning calorimetry), DTA (differential thermal analysis), and polarization microscopy. Every method has its advantages and restrictions. DSC allows one to determine the enthalpies of phase transitions (A// ). Microscopy allows one both to determine the phase transition temperatures and to identify the t)fpe of mesophase. DTA gives reliable results for melting temperatures of LCs that show solid-state polymorphism, and of LC mixtures. The differences in that can be found in publications by different authors arise from different measurement techniques and the presence of impurities. We have selected the data with the higher 7 values in such cases. 

The error in the determination of phase transition temperatures in a capsule apparatus depends on the rates of phase reaction and variations in temperature as well as on thermal gradients along the capsule and between the capsule and die wall of furnace. The reproducibility of the temperature of such phase transitions as liquid immiscibility or critical phenomena is in the range of several tenflis of a Kelvin for the best measurements. However, an error can reach 1-2 K in most available publications on salt solubility measurements. 

Determination of Phase Transition Temperature In the previous pioneering work on PNiPAM, Guillet et al.7) used their... 

Figure 3 shows the results of the x-ray patterns taken at various temperatures from samples in the composition range 0 x 0.07. The phase boundaries at room temperature are represented as accurately as can be determined by preparing samples whose nominal compositions differ by 0.01 or 0.005 (in x). The points shown represent determinations of the transition temperature ( 5° to 10°) by taking x-ray patterns above and below that temperature. 

Besides total energy consumption for mechanochemical reactions, the level of local parameters of temperature and pressure arising in the mill is also important. It is very difficult to measure these parameters directly during activation. Therefore, an indirect method based on P and T determination of phase transitions under mechanical activation of the... 

DSC was used to determine the phase-transition temperature (Tm) of the phospholipids. represents the peak temperature of the endotherm for the... 

All the determined volnme phase transition temperatures of hydrogel or core-shell bearing thermally sensitive polymer particles are in a broad range compared to the LCSTs. The volume phase transition temperatnre of thermally sensitive particles is also dependent on the internal strnctnre (i.e., polymer composition, cross-linker density, and distribution in the particle). 

These quantities are used to determine the glass transition. A schematic comparison between a first order phase transition and the glass transition is depicted in Figure 9. Figure 10 brings an example for determination of glass transition temperature and melting temperature from isobaric V-T data. 

It follows from Eq. (3.20), that the second term in the brackets represents the contribution of stress Um and is independent on film thickness, while the thickness dependence of transition temperature is described by the third term originating from surface contribution, polarization gradient and depolarization field. Keeping in mind that for perovskite structure, which is characteristic for SrTiOs and KTaOs, the electrostriction coefficient Qu < 0, one can see, that 7/(/) increases for compressive misfit strain < 0 as well as for Xs < 0. The dependence of 7 (/) calculated on the basis ofEq. (3.20) for KTaOs is reported in Fig. 3.2. It follows from the Fig. 3.2, that Tf depends essentially on Xs value. The second term in the brackets of Eq. (3.20) is independent of the film thickness and determines the shift of phase transition temperature (approximately 50 K) at / -> 00. Note that only this contribution has been taken into account in Ref. [17]. With a film thickness decrease the third term contribution prevails so that in sufficiently thin films (/ < 50 nm) the appearance of ferroelectric phase at T < 100 K can be expected. The appearance of ferroelectricity in the films of several nm thicknesses even at room temperature (see Fig. 3.2) cannot also be excluded. 

In order to investigate miscibility and phase behavior of polymer blends, differential scanning calorimetry (DSC) has been frequently used for determination of glass transition temperature, crystalline melting temperature and other thermal properties. 

CB was purchased from Synthon Chemicals GmbH Co. KG, Germany, and used as received. With differential scanning calorimetry we determined the phase transition temperatures to be isotropic (40.5 °C), nematic (33.4 °C), smectic (22.5 C) crystalline. The silica surfaces and some of the glass surfaces were modified by N,N-di-methyl-A-octadecyl-3-amino-propyltrimethoxysilyl chloride (DMOAP), obtained from Sigma-Aldrich, Germany. 

A further point in the investigation of crystalline order and orientation is the determination of phase transitions in the crystallite lattice. Changes in temperature or pressure can cause a phase transition and thus change the symmetry of the lattice. For example, a pressure-dependent transition from the wurtzite to rocksalt phase causes the disappearance... 

A convenient method for determining the phase transition temperatures is to measure the relative permittivity k as a function of temperature, using a capacitance bridge. The ratio of the capacitance of an electrical condenser with and without the dielectric between the condenser plates gives the permittivity. The value of k approaches a maximum at the transition temperature, where it changes discontinu-ously. Hysteresis loops, spontaneous polarization, and coercive fields are obtained with a Sawyer-Tower circuit and displayed with an oscilloscope. 

---