Determination of crystallinity

DSC is a technique that measures heat flow into or ont of a material as a fnnction of time or temperature. Polymer crystallinity can only be determined by DSC by quantifying the heat associated with the melting (fusion) of the polymer. The heat is reported as % crystallinity by determining the ratio against the heat of fnsion for 100% crystalline samples of the same material or more commonly by determining the ratio against a polymer of known crystallinity to obtain relative valnes. 

Crystallinity is a state of molecular structure referring to a long-range periodic geometric pattern of atomic spacings. In semi-crystalline polymers, such as PE, the degree of crystallinity (% crystallinity) influences the degree of stiffness, hardness and heat resistance. 

An understanding of the degree of crystallinity for a polymer is important since crystallinity affects physical properties such as storage modulus, permeability, density and melting point. While most of these manifestations of crystallinity can be measured, a direct measure of the degree of crystallinity provides a fundamental property from which these other physical properties can be predicted [9]. 

Crystallinity can be calculated from a DSC curve by dividing the measured heat of fusion by the heat of fusion of 100% crystalline material. The crystalline melting point (T ), which is a characteristic property, is used for quality control and for the identification of semi-crystalline polymers. 

The crystallites are destroyed upon melting and reform upon cooling. Their type and quantity depends on the sample s thermal history. Crystallinity values have been determined [10] for poly(p-biphenyl acrylate) and poly(p-cyclohexylpenhyl acrylate) from both heat of fusion and heat capacity measurements by DSC. DSC has also been used to study the degree of crystallinity of Nylon 6 [11,12] and crosslinked polyvinyl alcohol (PVOH) hydrogels submitted to a dehydration and annealing process [13]. 

The simplest way of establishing qualitatively the crystallinity of a polymer is by the observation of birefringence under a suitable microscope, taking care to exclude the possibility of orientation birefringence. Also thermotropic liquid crystalline polymers can show birefringence combined with relatively low viscosity. X-ray diffraction allows a quantitative determination of the degree of crystallinity as weU as the usual crystallographic data. 

In some cases crystalline polymers show additional absorption bands in the infrared spectrum, as in polyethylene ( crystalline band at 730 cm, amorphous band at 1300 cm ) and polystyrene (bands at 982, 1318, and 1368 cm ). By determining the intensity of these bands it is possible to follow in a simple way the changes of degree of crystallinity caused, for example, by heating or by changes in the conditions of preparation. 

The degree of crystallinity is also reflected in the density of the polymer so that the determination of density provides at least a relative measure for crystallinity. Differential scanning calorimetry (DSC) is frequently applied to determine the crystallinity from the heat of crystallization or melting (see Sect. 2.3.5.8). 

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The most simple secondary method is die determination of crystallinity from the density of the material. 

Order and polydispersity are key parameters that characterize many self-assembled systems. However, accurate measurement of particle sizes in concentrated solution-phase systems, and determination of crystallinity for thin-film systems, remain problematic. While inverse methods such as scattering and diffraction provide measures of these properties, often the physical information derived from such data is ambiguous and model dependent. Hence development of improved theory and data analysis methods for extracting real-space information from inverse methods is a priority. 

X-ray diffraction methods have been indispensable in the development of the present concept of cellulose structure, but Mark15 has offered the opinion that such methods alone are ill-suited to the quantitative determination of crystalline and non-crystalline fractions. Chemical methods which depend upon the greater reactivity or accessibility of the incompletely ordered regions have offered an alternative approach to the problem. 

Determination of the proportions of crystalline and amorphous material in partially crystalline polymers. Knowledge of the unit cell dimensions in high polymer crystals leads to a knowledge of the density of the crystalline regions. If the density of amorphous regions is also known, either by measurement of the density of an entirely amorphous specimen (if this can be obtained) or by extrapolation of the liquid density/temperature curve, it is possible to calculate, from the measured density of any partially crystalline specimen, the proportions of crystalline and amorphous material. Since the physical properties of polymer specimens are profoundly influenced by the degree of crystallinity, X-ray determinations of crystallinity are much used in such studies (see Bunn, 1957). 

The carbon atoms within each layer are arranged in almost the same manner as in graphite. The layers are nearly parallel to each other however, the relative position of these layers is random, so that there is no order as in the c direction of graphite ( turbostratic structure ) [4.3]. X-ray diffraction permits the determination of crystalline regions within the carbon black primary particle. These regions are... 

Harper (1979) and Doner and Hicks (1982) have reviewed the various methods for analysis of lactose and its derivatives. More recently, Roetman (1981) has described methods for the quantitative determination of crystalline lactose in milk products. The reader should consult these reviews for information on specific procedures. 

Roetman, K. 1981. Methods for the quantitative determination of crystalline lactose in milk products. Neth. Milk Dairy J. 35, 1-52. 

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 structure determination of crystalline borates is well advanced, and it is hoped that the present enthusiasm will continue. There are many candidates for further investigation, particularly where spectroscopic or dehydration characteristics have suggested a particular boron anion. For example, the borate 2ZnO - 3B203 - 3.5H20 loses its water at temperatures in excess of 260°C, implying that its formula water is in the form of hydroxyl groups. 

Artursson T, Hagman A, Bjork S, Trygg J, Wold S, Jacobsson SP. 2000. Study of preprocessing methods for the determination of crystalline phases in binary mixtures of drug substances by X-ray powder diffraction and multivariate calibration. Appl. Spectrosc. 54(8) 1222-1230. 

Ruland, W. X-ray determination of crystallinity and diffuse disorder scattering. Acta Crystallogr., 1961 14 1180-1185. 

Weidinger, A., and P. H. Hermans On the determination of crystalline fraction of isotactic polypropylene from X-ray diffraction. Makromol. Chem. 50, 98-115 (1961). 

Another important application of wide-line NMR is the determination of crystallinity. Some polymers give spectra, which can be graphically separated into a broader and a narrower component. In this way Wilson and Pake determined the crystallinity of polyethylene and poly(tetrafluoroethylene) as early as 1953. 

It is understandable that the various methods of determination of crystallinity may lead to somewhat different figures for the same polymer. 

General Aspects of the Opportunity to Use Evolutionary Algorithms in Structure Determination of Crystalline Solids... 

Calibration of impact testing machines for metals. 1965 Determination of crystallinity. 

There are two broad applications of X-rays in the characterization of materials (i) X-ray spectrometry and (ii) X-ray diffractometry. The former technique is used for chemical analysis and has found only limited use in the characterization of pharmaceuticals. On the other hand, X-ray diffractometry, by providing a means for the study of the structure of crystalline materials, is extensively used to characterize pharmaceutical solids. There are two principal applications of X-ray diffractometry. X-ray crystallography is concerned with the structure determination of crystalline phases. Single crystals are usually used for this purpose. On the other hand, in X-ray powder diffractometry, the sample is usually in the form of a powder. X-ray powder diffractometry is recognized as a powerful technique for the identification of crystalline phases. The technique can also be used for the quantitative analyses of solids. This article will be restricted to the principles and applications of X-ray powder diffractometry (XRD) in the characterization of pharmaceutical solids. 

According to Craig, this method has been applied successfully tc the determination of cycloserine in biological fluids, such as blood, urine, cerebrospinal fluid and to the determination of crystalline cycloserine. No naturally occurring amino acids have been found to interfere with assay results. The minimum assayable level is about 100 ppm in tissue or solid samples such as animal feeds, and about 25 ppm in liquid samples. 

G. W. Brindley, The effect of grain or particle size on X-ray reflections from mixed powders and alloys, considered in relation to the quantitative determination of crystalline substances by X-ray methods, Philos. Mag., 1945, 36, 347-369. 

The melting behavior and determination of crystalline content were studied with a Perkin-Elmer DSC-1 differential scanning calorimeter. The heating rate was 8 C/min, the sample size, 12.0 mg for LDPE and 3.0 mg for HDPE, and n-C H-p. The runs were performed in a nitrogen atmosphere. Before analysis all samples were given the same thermal treatment by heating to 150 C and cooling down to room temperature in a controlled manner. 

None of these approaches can be used on a technical scale for commercial PHA production. However, in vitro synthesis of PHAs on a semipreparative scale is possible, allowing production of small amounts of novel PHAs also including constituents that cannot be provided by the metabolism or of a certain distribution of the comonomers, and sufficient amounts for determination of crystallinity, melting point etc. can be obtained. 

Keller, L. and Dollase, W.A. (2000) X-ray determination of crystalline hydroxyapatite to amorphous calcium phosphate ratio in plasma sprayed coatings. /. Biomed. Mater. 

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