Crystallinity, measurement

It has also been inferred that differences found between crystallinities measured by density and those from heat of fusion by DSC area determination, as given for polyethylenes in the example of Figure 4 [72], may be related to baseline uncertainties, or not accounting for the temperature correction of AHc. Given that similar differences in crystallinity from density and heat of fusion were reported for isotactic poly(propylene) [43] and polyfaryl ether ether ketone ketone), PEEKK [73], other features of phase structure that deviate from the two-phase model may be involved in the crystallinity discrepancy. 

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-... 

R.R Paradkar, R.M. Patel, E. Knickerbocker and A. Doufas, Raman spectroscopy for spinline crystallinity measurements. I. Experimental studies, J. Appl. Polym. Sci., 109, 3413-3420 (2008). 

Batur C, Vhora MH, Cakmak M et al. (1999) On-line crystallinity measurement using laser Raman spectrometer and neural network. ISA Trans 38 139-148... 

The heat of fusion of the fully crystalline sample AH must be known for this determination. It can be obtained by calibration of samples with widely different crystallinities in conjunction with Eq. 2, or by parallel use of a different crystallinity measuring technique, such as density or X-ray diffraction analysis 18). Besides lowering the... 

Nevertheless, when we carry out x-ray crystallinity measurements on textile fibers, we must consider distortions that always affect crystalline material. Even in a completely crystalline material, the scattered x-ray intensity is not located exclusively in the diffraction peaks. That is because the atoms move away from their ideal positions, owing to thermal motion and distortions. Therefore, some of scattered x-rays are distributed over reciprocal space. Because of this distribution, determinations of crystallinity that separate crystalline peaks and background lead to an underestimation of the crystalline fraction of the polymer. In this paper, we attempt to calculate the real crystallinity for textile fibers from apparent values measured on the x-ray pattern. This is done by taking into account the factor of disorder following Ruland s method (3). 

The relative contributions to the intensity of a composite line shape are thus a measure of the degree of crystallinity of the PTFE sample. The crystallinities thus determined are compared to values determined by density i measurements in Figure 3. These results indicate an excellent correlation between crystallinities measured with the two techniques however, the agreement is not absolute, which is not unusual when comparing values of crystallinity measured by different techniques which use different criteria for crystallinity 2. 

On the other hand, the temperature of melting and crystallinity measurement can describe only those stereosequences that crystallize. In propylene oxide polymers which we have studied, for example, isotactic sequences are the only ones that crystallize. Therefore, in these polymers we can determine only the stereosequence length of isotactic units and the average length of the uncrystallizable units. The syndiotactic and atactic sequences in the uncrystallizable blocks are not distinguishable from the crystallinity and melting temperature measurements. 

X-ray crystallinity. This characteristic is generally indicative of the zeolite content of the catalyst. It is determined by measuring the area of one or several peaks in the X-ray diffraction pattern of the catalyst and expressing it as a percentage of the corresponding area of a reference material (usually a well-crystallized Na Y Zeolite). X-ray crystallinity measurements are often used by refiners to monitor the quantity of fresh and equilibrium catalysts. 

Table 4.1. Experimental values for the density, p, crystallinity measured by density, wdp), crystallinity measured by DSC, Wc(DSC), long period, L, crystal thickness, Ic, microhardness, H, crystal hardness. He, melting temperature, T, surface free energy, Oe, and the number of chain folds per molecule, n, for PEO samples with different molecular weights. (From Baltd Calleja Santa Cruz, 1996.)...

The effect of crystal structure may be investigated by preparing catalysts, as described above, at various temperatures which assures a set of catalysts having variable surface areas, pore size distributions, and crystallinity. Measuring the catalytic activity as a function of these physical properties will help to define the role of crystal structure for the particular transition metal sulfide. In general, the HDS is poorly correlated to N2 BET surface area. This non-correlation can be most easily seen by preparing a... 

The melting temperatures of P(3HB-co-4HB) samples deereases slightly from 178 to 150°C as the 4HB content increases from 0 to 18 mol% and are almost constant in the range from 18 to 49 mol% 4HB. On the contrary, the enthalpy of fusion decreases monotonously with an increase in the 4HB unit in the composition range of 0-49 mol%. The degree of crystallinity, measured by an X-ray method, decreases from 55 to only 14% as the 4HB content increases from 0 to 49 mol%. The X-ray data indicate the existence of the P(3HB)-type of crystalline lattice at low 4HB content. 

Figure 5. TFE content and degree of crystallinity measured by x-ray diffraction for perfluorosulfonic acid membrane.

Because of the inferior quality of data that can be obtained from the powder XRD patterns, XRD was used as a qualitative fingerprinting technique for identification purposes and for crystallinity measurements during the kinetic studies of synthesis. If care is not taken to optimize the crystallinity and prepare the sample without any residual impurities, the qualitative data may not be erraneous but the quantitaive data will differ from sample to sample and from one person s preparation to another s. Also, the various conditions like the mode of scanning (continuous or step), scan speed, value of 20 for each step etc., have to be optimised. Correction of the interplanar distance d with respect to an internal standard such as silicon is a prerequisite for determining the lattice parameters or unit cell dimensions. 

Table 9.2 Results of XRD/DSC crystallinity measurements on PK copolymer samples...

A number of polyesters made by two-stage end-grafting with trimethylene carbonate and glycolide were prepared and their respective properties are shown in Table 8.5. The data in Table 8.5 show that these polymeric high-melting materials, with a sufficiently high degree of crystallinity (measured in terms of AHf), are well-suited for use as fiber formers. 

WAXD and DSC were used to measure the amount of crystalline and amorphous phase in each of the samples. Figure 4 shows the crystaUinity of the samples as a function of draw ratio. The values of crystallinity fi-om DSC and WAXD are not in exact agreement however, the trends show no significant change in crystallinity due to solid state drawing. Similar results have been reported for semicrystalline polymers with degree of crystallinity measured by DSC, WAXD, NMR, and density (5,6,24), and the discrepancy between crystalMty values has been discussed in terms of either dilation of amorphous fi ee volume by crystal constraint (6,24) or presence of a paracrystalline intermedate phase (5,6),... 

Figure 10. Degree of crystallinity measured by DSC as a fimction of rdative draw ratio for HOPE including the data of Wang and Porter (34).

Table 8.1 shows the dependence of K and n on the mechanism of nucleation and growth. Measurements of n have shown that it decreases as crystallization proceeds. Mandelkem [28)] has considered the case of sporadic nucleation on predetermined nuclei that cause a decrease in as crystallization proceeds. However, n may also decrease when A cs is not constant but varies with time. A comparison of degree of crystallinity measurements with spherulitic growth rate measurements on the same polymer [29] shows this. The polymer continues to crystallize after the volume fills with spherulites. The value of n for this secondary crystallization is usually less than for the primary crystallization process. This difference gives rise to a change of n with time. Reference 30 treats this problem theoretically as do Hilliar [31] and Price [32]. 

Fig. 10.63 Transmission (%) determined using Haze Meter and crystallinity (%) measured using X-ray diffraction are shown for polypropylene homopolymer mixed with different concentrations (0.07 wt%, 0.14 wt%, and 0.21 wt%) of Millad-3988 (Pendyala et al. 2004)...

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