Microscopy hot-stage

Melting temperatures of as-polymerized powders are high, ie, 198—205°C as measured by differential thermal analysis (dta) or hot-stage microscopy (76). Two peaks are usually observed in dta curves a small lower temperature peak and the main melting peak. The small peak seems to be related to polymer crystallized by precipitation rather than during polymerization. 

Following the USP procedure for class 1A compounds, the melting range.of halcinonide is 270-272° (reference 21). This is in excellent agreement with the results of differential thermal analysis, below, as well as hot stage microscopy, section 2.12. 

Hot stage microscopy was performed using a Mettler FP52 temperature controller at a rate of 3°/minute. Melting began at 268.2°, and by 271.4° all crystals were melted. The yellow melt slowly turned orange-brown. Cooling showed no recrystallization at ambient temperature. 

Measurements of melting curves can be used to obtain very accurate evaluations of the melting point of a compound when slow heating rates are used. The phase transition can also be monitored visually, with the operator marking the onset and completion of the melting process. This is most appropriately performed in conjunction with optical microscopy, thus yielding the combined method of thermomicroscopy or hot-stage microscopy [7]. 

The solid phase in equilibrium with the saturated solution must be analyzed by techniques such as hot stage microscopy, differential scanning calorimetry, or powder x-ray diffraction, to verify if the starting material has undergone a phase transformation. 

Figure 11.5 Hot stage microscopy of a crosslinked gel in a crystalline monolayer film a) below the melting temperature, b) optical melting point at 128 °C, c) appearance of birefringence after stressing at 135 °C, and d) intact crosslinked gel after cooling to 30 °C. Photographs were provided by E. Garcia-Meitin of The Dow Chemical Company...

Applications of Hot-Stage Microscopy , 1st Proc European Therm Anal, 63-66 (1976)... 

The solid-state properties like crystallinity, polymorphism (crystal structure), shape (morphology), and particle size of drugs are important in the stability, dissolution, and processibility of drugs. Some commonly used methods in solid-state studies include microscopy, hot stage microscopy with polarized light, x-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared FTIR/Raman, and solid-state NMR. 

There are three main techniques used in characterisation of mesophases polarised optical hot-stage microscopy, differential scanning calorimetry (cf. Box 9.1) and small-angle X-ray scattering. 

Thermal property DSC, TGA Isothermal microcalorimetry Hot stage microscopy... 

Figure 12.6 Changes in granular structure of cassava starch extracted from roots (a) planted during the rainy period and harvested at 6 months (b) planted during the rainy period and harvested at 12 months (c) planted during the dry period and harvested at 6 months (d) planted during the dry period and harvested at 1 2 months, when observed by hot-stage microscopy at different temperatures.

In the previous symposium, we reviewed mesophase mechanisms involved in the formation of petroleum coke ( 2 ). Since 1975, two significant developments have been the use of hot-stage microscopy to observe the dynamic behavior of the carbonaceous mesophase in its fluid state (3-6), and the emergence of carbon fibers spun from mesophase pitch (7-9) as effective competitors in applications in which high elastic modulus or good graphiticity is important. This paper focuses on mesophase carbon fibers as an example of how the plastic mesophase can be manipulated to produce fibers with intense preferred orientations and elastic moduli that approach the theoretical limit for the graphite crystal in the a-direction. 

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