Density gradient column analysis

We can measure a polymer s density by one of two methods density gradient column analysis and densimetry. Each of these methods can measure the density of a sample to a precision of four significant figures. 

The value for the heat of fusion of PPS, extrapolated to a hypothetical 100% crystalline state, is not agreed upon in the literature. Reported values range from approximately 80 J/g (19 cal/g) (36,96,101) to 146 J/g (35 cal/g) (102), with one intermediate value of 105 J/g (25 cal/g) (20). The lower value, 80 J/g, was originally measured by thermal analysis and then correlated with a measure of crystallinity deterrnined by x-ray diffraction (36). The value of 146 J/g was deterrnined independendy on uniaxiaHy oriented PPS film samples by thermal analysis, density measurement via density-gradient column, and the use of a calculated density for 100% crystalline PPS to arrive at a heat of fusion for 100% crystalline PPS (102). The value of 105 J/g was obtained by measuring the heats of fusion of weU-characterized linear oligomers of PPS and extrapolation to infinite molecular weight. 

The relationship between observed enthalpy-volume relaxations and thermal treatment of slightly oriented industrial PVC films was investigated. Differential scanning calorimetry at 20 -C per minute and specific volume analysis (density gradient column) were used to study the effects of annealing near and below Tg. Nonlinear effects in the volume relaxation at relatively long times and temperatures close to the glass transition produce deviations in the specific heat curves at temperatures far above Tg in addition to the normal overshoot effects. 

To the right. Fig. 4.15 illustrates Type 4 instrumentation, a particularly easy method of density determination at a fixed reference temperature, the density gradient method. Figure4.16 depicts the analysis method. The sample, checked for uniformity and freedom from attached air bubbles, is placed in a density gradient column, and its... 

The SFC separation of selected estrogens (estrone, equilin, a-estradiol, /3-estradiol, and d-equilenin) was achieved on a SB-cyanopropyl-50 capillary column using a carbon dioxide density gradient at an oven temperature of 73°C [5]. A typical analysis time on the 7-m column was 21... 

On the DB5 capillary column, satisfactory separation of hydroxyl- and methoxyflavones could be obtained using either of the two gradient systems, but the retention times of the analytes for P2 were shorter than those for PL This is due to the variation in the solubilities of flavones in the supercritical mobile phase when the density gradient was employed. The polarity effect of the stationary phase (BPl phase is less polar than the DB5) is illustrated in Table 2. It can be seen that, on BPl, the flavones are less retained and the analysis time is decreased by nearly half, while conserving a satisfactory separation. 

The weight of all the atoms or ions in the unit cell can be calculated if the unit cell dimensions and the density of the crystal are known. This means that crystal density measurements are important in the early stages of a crystal structure analysis. A vertical column with a density gradient, calibrated with crystals of known density, is used to measure protein densities. These are determined by where along the column the crystal setdes and so that, by interpolation, its density can be estimated (39). This method must be done with care to ensure that the measurement is good. The density of a crystal is given by... 

Van den Akker (1978) carried out a Hnear stabflity analysis for a liquid-liquid spray column operated with a positive temperature gradient. The leading idea was that local differences in mixture density would give rise to a multiple circulation pattern in a way similar to the multicellular natural convection mode in a vertical slot heated from one side (Elder, 1965 Lee and Korpela, 1983). The only way to find in the Hquid—Hquid spray colurrm... 

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