DIFFERENTIAL SCANNING

The crystal stmcture of glycerides may be unambiguously determined by x-ray diffraction of powdered samples. However, the dynamic crystallization may also be readily studied by differential scanning calorimetry (dsc). Crystallization, remelting, and recrystallization to a more stable form may be observed when Hquid fat is solidified at a carefully controlled rate ia the iastmment. Enthalpy values and melting poiats for the various crystal forms are shown ia Table 3 (52). 

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). 

Other PDMS—sihca-based hybrids have been reported (16,17) and related to the ceramer hybrids (10—12,17). Using differential scanning calorimetry, dynamic mechanical analysis, and saxs, the microstmcture of these PDMS hybrids was determined to be microphase-separated, in that the polysiUcate domains (of ca 3 nm in diameter) behave as network cross-link junctions dispersed within the PDMS oligomer-rich phase. The distance between these... 

The process known as transimidization has been employed to functionalize polyimide oligomers, which were subsequentiy used to produce polyimide—titania hybrids (59). This technique resulted in the successhil synthesis of transparent hybrids composed of 18, 37, and 54% titania. The effect of metal alkoxide quantity, as well as the oligomer molecular weight and cure temperature, were evaluated using differential scanning calorimetry (dsc), thermogravimetric analysis (tga) and saxs. 

Most hydrocarbon resins are composed of a mixture of monomers and are rather difficult to hiUy characterize on a molecular level. The characteristics of resins are typically defined by physical properties such as softening point, color, molecular weight, melt viscosity, and solubiHty parameter. These properties predict performance characteristics and are essential in designing resins for specific appHcations. Actual characterization techniques used to define the broad molecular properties of hydrocarbon resins are Fourier transform infrared spectroscopy (ftir), nuclear magnetic resonance spectroscopy (nmr), and differential scanning calorimetry (dsc). 

Glass-transition temperatures are commonly determined by differential scanning calorimetry or dynamic mechanical analysis. Many reported values have been measured by dilatometric methods however, methods based on the torsional pendulum, strain gauge, and refractivity also give results which are ia good agreement. Vicat temperature and britde poiat yield only approximate transition temperature values but are useful because of the simplicity of measurement. The reported T values for a large number of polymers may be found ia References 5, 6, 12, and 13. 

The compositional distribution of ethylene copolymers represents relative contributions of macromolecules with different comonomer contents to a given resin. Compositional distributions of PE resins, however, are measured either by temperature-rising elution fractionation (tref) or, semiquantitatively, by differential scanning calorimetry (dsc). Table 2 shows some correlations between the commercially used PE characterization parameters and the stmctural properties of ethylene polymers used in polymer chemistry. 

The cure of novolaks with hexa has been studied with differential scanning calorimetry (dsc) and torsional braid analysis (tba) (46) both a high ortho novolak and a conventional acid-cataly2ed system were included. The dsc showed an exothermic peak indicating a novolak—hexa reaction ca 20°C higher than the gelation peak observed in tba. Activation energies were also calculated. 

Cure kinetics of thermosets are usually deterrnined by dsc (63,64). However, for phenohc resins, the information is limited to the early stages of the cure because of the volatiles associated with the process. For pressurized dsc ceUs, the upper limit on temperature is ca 170°C. Differential scanning calorimetry is also used to measure the kinetics and reaction enthalpies of hquid resins in coatings, adhesives, laminations, and foam. Software packages that interpret dsc scans in terms of the cure kinetics are supphed by instmment manufacturers. 

Thermal analysis iavolves techniques ia which a physical property of a material is measured agaiast temperature at the same time the material is exposed to a coatroUed temperature program. A wide range of thermal analysis techniques have been developed siace the commercial development of automated thermal equipment as Hsted ia Table 1. Of these the best known and most often used for polymers are thermogravimetry (tg), differential thermal analysis (dta), differential scanning calorimetry (dsc), and dynamic mechanical analysis (dma). 

Transitions such as T and are rapidly and conveniently studied using differential scanning calorimetry (dsc). This technique monitors changes in... 

Fig. 5. Differential scanning calorimetry thermogram. Amorphous PPS is heated from room temperature to 325°C at 20°C/min.

Onset value as measured by differential scanning calorimetry. 

The glass-tiansition tempeiatuiesfoi solution-polymeiized SBR as well as ESBR aie loutinely determined by nuclear magnetic resonance (nmr), differential thermal analysis (dta), or differential scanning calorimetry (dsc). 

The thermal glass-transition temperatures of poly(vinyl acetal)s can be determined by dynamic mechanical analysis, differential scanning calorimetry, and nmr techniques (31). The thermal glass-transition temperature of poly(vinyl acetal) resins prepared from aliphatic aldehydes can be estimated from empirical relationships such as equation 1 where OH and OAc are the weight percent of vinyl alcohol and vinyl acetate units and C is the number of carbons in the chain derived from the aldehyde. The symbols with subscripts are the corresponding values for a standard (s) resin with known parameters (32). The formula accurately predicts that resin T increases as vinyl alcohol content increases, and decreases as vinyl acetate content and aldehyde carbon chain length increases. 

By differential scanning calorimetry from 30 to 100°C on dried resia. 

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