Percentage crystallinity from density

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

The heat of fusion AHf (obtained from the area under the DSC melting curve) and percentage crystallinity calculated from AHf is found to be linearly dependent on butadiene content, and independent of the polymer architecture. This is shown in Figure 3. Also, the density of the block copolymers was found to be linearly dependent on butadiene content (see Figure 4). The linear additivity of density (specific volume) has been observed by other workers for incompatible block copolymers of styrene and butadiene indicating that very little change in density from that of pure components has occurred on forming the block copolymers.(32) While the above statement is somewhat plausible, these workers have utilized the small positive deviation from the linear additivity law to estimate the thickness of the boundary in SB block copolymers.(32)... 

If the numbers and types of atoms present in the unit cell are also known, then it is possible to calculate the limiting or maximum density, that is, the density the material would have if it were 100% crystalline. In brief, the volume of the unit cell can be calculated from the geometric information mentioned above, and the weight of its contents can be obtained by summing the atomic weights of the atoms. The maximum density is then simply the ratio of this weight to the unit cell volume. This value of the density can be used to estimate the percentage crystallinity of an actual sample of the same polymer. 

Chromium Disilicide, CrSij, has been prepared by the method described for the previous compound, a higher percentage of silicon being used. It has also been obtained by heating a mixture of chromium sesquioxide and charcoal with excess of silica in an electric furnace. By the first method the silieide is obtained as a grey crystalline powder by the second, as long, grey, lustrous needles, only obtained free from silicon with difficulty. Density 4-393. It is decomposed by hydrofluoric acid. 

Table 5-1. Comparison of the Crystallinity am of a Po/j(ethylene) Sample Calculated from the Density p or the Specific Volume v = II p Using a Two-Phase Model with with the Percentage Number of Crystal Defects Calculated Using a One-Phase Model...

Short-Chain Branching. Although described as high density, commercial HDPE can actually vary in density from 0.975 down to 0.935. HDPE contains a crystalline phase and an amorphous phase, and the measured density directly reflects the percentage of each. T3T>ical homopolymer is normally about 94% crystalline and has a density of 0.960-0.965. Higher densities can be achieved by low molecular weight and slow cooling. 

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