Crystallinity percentage

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 acetal polymers exhibit a high crystallinity." The percentage crystallinity will depend on the quench temperature and will range from about 77%, when quenched at 0°C, to about 80% when quenched at 160°C. Annealing will increase the crystallinity, this being most marked at 150°C Figure 19.5). 

The greater the percentage crystallinity the higher the yield point and tensile modulus. It has also been shown that by raising the quench temperature the spherulite size is increased and that this greatly decreases the impact toughness. 

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)... 

Cellulose is partly crystalline and partly amorphous, the percentage crystallinity varying between 50 and 90% depending upon source and also upon the method of crystallinity measurement. Numerous theoretical models have been proposed for the molecular organisa-... 

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. 

The interpretation of equation (26) is very simple. The denominator represents the maximum difference that can be observed for the polymer, namely the difference between the values for the 100% amorphous polymer and the 100% crystalline polymer. The numerator is the difference observed between the 100% amorphous polymer and the actual, partially crystalline polymer. The ratio of this observed difference to the maximum possible difference is therefore the fraction crystallinity and, when multiplied by 100, is the percentage crystallinity. 

In the calorimetric approach, it is necessary to know the heat of fusion of the totally crystalline polymer. This can be obtained from melting-point depression measurements, as described in the following section. The basic idea depends on the fact that the melting temperature is independent of the size of the system, since it is an intensive property. The extent to which it is depressed by the presence of solvent can be used to calculate a heat of fusion characteristic of the crystallites, irrespective of how many are present. This is therefore the heat of fusion of the 100% crystalline polymer. The fractional crystallinity in an actual sample is then the ratio of its calorimetrically measured heat of fusion per gram to that of the 100% crystalline polymer. For example, if the actual polymer has a heat of fusion of 7 cal per gram, and the 100% crystalline polymer a heat of fusion of 10 cal per gram, then the fractional crystallinity is 0.7, and the percentage crystallinity is 70%. 

True polymorphs Unique diffraction peaks. Useful for determination of phase purity and percentage crystallinity 0.5-2 Unique chemical shifts. Useful for determining phase purity, molecular mobility 0.5-5 Characteristic spectra, sensitive to H-bonding 5-10... 

Figure 24. Photolytic charge separation with nanocrystalline zeolite (a) correlation of percentage crystallinity of nanocrystals (shown in parentheses) with PVS" production (b) comparison of the rate of PVS " radical generation for nanocrystalline zeolite X and micronsized zeolite Y.

Table 1 presents the results of the synthesis studies carried out. The Si/Al ratio ranges for all the samples are indicated in notes 6 to 9. The number of acid sites as determined by TPD was very similar for all samples of a given Si/Al ratio (cf Table 2). All samples listed also showed percentage crystallinities of essentially 100%. Although only samples which yielded ZSM-5 are reported in this paper, it should be noted that in a few cases other phases such as ZSM-22 were obtained. In this discussion only samples where all other parameters were held essentially constant are compared. With respect to the effect...

Density measurements can be used in the calculation of the percentage crystallinity of fibers. Fiber degradation has also been monitored by both sink-float and density gradient measurements. ... 

Figure 6.4 shows the calibration curve of heat output versus amorphous content of a development compound. In this case, the technique is used to crystallize, or condition, these amorphous regions by exposure to elevated RHs. Thus, if authentic 100 percent amorphous and crystalline phases exist, it is possible to construct a calibration graph of heat output versus percentage crystallinity, so that the amount of amorphous character introduced by the milling process can be quantified. 

Commercial PVC is essentially an amorphous material, although a small amount of crystallinity is present (about 5% as measured by X-ray diffraction methods) and is attributed to the fact that that the bulky chlorine atoms do not align and pack readily. Despite this low percentage, crystallinity greatly influences the properties of PVC in solution and solid phases. 

One very important application is the determination of the percentage crystallinity of materials, fn the analysis of polymeric and fibrous materials, determin-... 

In the case of cellulosics percentage, crystallinity and crystalline index are calculated as follows [23] ... 

Figure 17. The specific enthalpy curve, /i(7) the specific enthalpy curves for the fully amorphous and crystalline states and the percentage crystallinity curve, all based on the specific heat...

The concept of percentage crystallinity implies the existence of separate crystalline and amorphous phases of constant structure, whereas there may be defects inside, or folds on the surfaces of, lamellar crystals. Crystallinity is usually measured indirectly, via measurements of density or enthalpy of fusion, although these give slightly different results than X-ray diffraction methods. For polyethylene, the density of the crystal unit cell... 

This initial molecular mechanics calculation was limited to the interaction with crystalline cellulose I of all the three possible PF dimers in which a methylene bridge links two phenol nuclei ortho-ortho, para para and ortho-para. As not much was known as to how the system would react the investigation was very extensive. As cellulose constitutes as much as 50% of wood, where its percentage crystallinity is as high as 70%, this study also inferred applicability to a wood substrate. As even dried wood always contains a certain amount of water the influence of the water was taken into account by introducing into the calculations the effect of a parameter related to the dielectric constant of water. 

FIGURE 15.8 Influence of crystallinity and chain length on the physical properties of polyethylene. (After Richards, R.B., J. Appl. Chem., 1, 370, 1951.) The percentage crystallinity c is plotted against molar mass M. 

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