Density Gradient Columns

The density of the polymer or its reciprocal the specific volume is also a very important parameter in many respects. It may be related to free volume, diffusivity and permeability. jMoreover, it may give information on the crystalline content. Membranes prepared from high-density polymers tend to have lower permeabilities. The density decreases and vice versa the specific volume increases, as the temperature raises, but when the glass transition temperature has been passed, the density decreases even more rapidly (see also figure II - 9). 

The overall density of a polymer can be determined via a number of techniques such as picnometry and dilatometry, and through the use of a density gradient column. A schematic drawing of such a column is given in figure IV - 39. 

The density gradient in the column is obtained by mixing two liquids, one with a high density and one with u low density, with each other in defined quantities. Often aqueous inorganic solutions such as that of sodium bromide are used for polymers with densities p 1 cmVg. The overall density (p) of a polymer sample can be obtained by measuring its flotation level. 

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Product Density. Density is measured by using a density gradient column. This method is described in ASTM D1505. 

Density. Density of LLDPE is measured by flotation in density gradient columns according to ASTM D1505-85. The most often used Hquid system is 2-propanol—water, which provides a density range of 0.79—1.00 g/cm. This technique is simple but requires over 50 hours for a precise measurement. The correlation between density (d) and crystallinity (CR) is given hy Ijd = CRj + (1 — Ci ) / d, where the density of the crystalline phase, ify, is 1.00 g/cm and the density of the amorphous phase, is 0.852—0.862 g/cm. Ultrasonic methods (Tecrad Company) and soHd-state nmr methods (Auburn International, Rheometrics) have been developed for crystallinity and density measurements of LLDPE resins both in pelletized and granular forms. 

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 densities of the polymers at 23 °C were determined using liquids in density-gradient-columns [50], The tiny samples (about 10 mg) were floated in the columns over a period of one week. The absorption of liquid by the polymer was corrected for by an extrapolation of the readings back in time to the start of the experiment [51j. 

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. 

We can use density gradient columns to measure the density of all manner of polymer samples, from fibers and films to specimens cut from molded parts. 

The experimental determination of density can be carried out by picnometry, density gradient column or by dilatometry. 

Low and Richards used a density gradient column to study the density of crystals of "mixed gramicidin fractions"56. 

The density of some materials may be found using a density gradient column. This is a cylinder containing liquid layers of varying densities, with the most dense at the bottom of the column and the least dense at the top. A sample to be determined is placed in the column and falls to the level that is similar to its density. 

In addition to lactometry (determination of the extent to which a hydrometer sinks), the density of milk can be measured by pycnometry (determination of the mass of a given volume of milk), by hydrostatic weighing of an immersed bulb (e.g. Westphal balance), by dialatometry (measurement of the volume of a known mass of milk) or by measuring the distance that a drop of milk falls through a density gradient column. 

ISO 1183-2, 2004. Methods for determining the density of non-cellular plastics - Part 2 Density gradient column. 

In dilatometry, specific volumes V or densities of materials are measured by use of a pycnometer, dilatometer, or density-gradient column. The data obtained are then interpreted through the equation... 

Density measurements were made at 23 °C on spherical neat resins of 5 mm diameter using the flotation method in accordance to ASTM D-1505. The density gradient column (Model DC-1) was supplied by Techne Incorporated, Princeton, New Jersey. Calcium nitrate column was set up which could measure density accurately ranging from 1.210 to 1.290. 

Density. The bulk density of the polymer films before and after molding were measured in a density gradient column at 23 °C. All the polyethylene samples had the same density of 0.955 gram/cc. 

The specific volumes of the pure components are normally obtained by equal density titration or by use of a density gradient column. 

The density was measured at room temperature using a density gradient column constructed from toluene and carbon tetrachloride. The column was prepared according to ASTM-D 1505 Method B. The column was calibrated with glass floats of known density. The accuracy of density measurements was estimated to be 0.001 g/cm Small pieces ( 25 mm ) were placed in the column and allowed to equilibrate before the measurements were taken. 

Density measurements were carried out at a temperature of 23 °C with a density gradient column filled with a solution of NaBr in distilled water. Weighing the samples before and after they were introduced into the solution showed that the liquid did not enter the craze pores. 

Density is measured using density gradient columns and hydrostatic (displacement) methods. Density is directly related to crystalline content and in fact can be used to estimate % crystallinity in polyethylene. 

Crystallinity of cotton cellulose was measured using a density method by means of a density gradient column (Techne Inc., Model DC-2). Xylene and carbon tetrachloride were used to make up the solution. Based on the density data, crystallinity of cellulose can be calculated from the following equation (13) ... 

The density of a polymer sample can be readily determined by allowing it to float in a density-gradient column, which is a vertical column containing a mixture of liquids with different (known) densities. The density of a small piece of polymer is determined from the position it adopts when it is dropped into the column. The density of the crystalline regions pc can be calculated from a knowledge of the crystal structure [19]. The amorphous density pa can sometimes be measured directly if the polymer can be obtained in a completely amorphous form, for example by rapid cooling of a polymer melt. Otherwise it can be determined by extrapolating either the density of the melt to the temperature of interest or that of a series of semicrystalline samples to zero crystallinity. 

For density profiling, 100-cm long density gradient columns were prepared using calcium nitrate-water solutions. The columns were calibrated with standard glass calibration balls. Polymer specimens were sectioned Into pieces whose thicknesses were measured with a micrometer, and the density of each piece determined from the equilibrium floating position In the column (7). 

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