Polymers, analysis Crystallinity

Since the properties of a polymer can be noticeably influenced by small variations in the molecular structure, and these in turn depend on the preparation conditions, it is necessary when reporting data to indicate not only the type of measurement (e.g., molecular weight by end group analysis crystallinity by infrared measurement or by X-ray diffraction etc.), but also the type of preparation (e.g., radical polymerization in bulk at 80 °C polymerization with a particular organometallic catalyst at 20 °C). 

The line-decomposition analysis of the equilibrium spectrum of the a-meth-ylene carbon was carried out using the elementary line shapes thus obtained for the three phases. The result is shown in Fig. 24. The composite curve of the decomposed components reproduces well the experimentally observed spectrum. The mass fraction of the crystalline component was estimated as 0.60 that is described in the figure. Based on the heat of fusion of 8.13 KJ/mol of this sample and the value of 14.2 KJ/mol for the crystalline material of this polymer the crystalline fraction was estimated to be 0.57. Here the heat of fusion for the crystalline material was obtained from the effect of diluent on the melting temperature with use of the relationship developed by Flory [91 ]. The crystalline fraction estimated from the NMR analysis is in good accord with the value estimated from the heat of fusion, supporting the rationality of the NMR analysis. 

As mentioned before (Section II,E,3), the determination of tac-ticity by X-ray analysis is limited by the requirement that the polymer be crystalline. For the study of poly (methyl methacrylate), which may or may not be crystalline, nuclear magnetic resonance spectroscopy has been more useful. In order to interpret the spectra, it has been found necessary to describe the stereochemistry of a unit by the configurations on both sides. Therefore, an isotactic configuration, or isotactic triad, is one where the central unit is fianked by units of the same asymmetry, that is ddd or III. Similarly, for a syndiotactic triad, the stereochemistry is did or Idl. To overcome the disadvantages of the term atactic, a new term heterotactic was introduced. The stereochemistry for heterotactic configurations is, therefore, Idd, dll, lid, and ddl. 

The first step in a polymer analysis is to identify the specific type of polymer in a given sample. This may be complicated in a formulated sample by the presence of additives. Infrared spectroscopy will usually provide information on both the base polymer(s) and the additive(s) present. The second step, if possible, is to determine details of the chemical and physical characteristics, which define the quality and properties of the polymer. The chemical properties that can be determined are stereo specificity, any irregularities in the addition of monomer (such as 1,2- versus 1,4-addition and head-to-head versus head-to-tail addition), chain branching, any residual unsaturation, and the relative eoncentration of monomers in the case of copolymers. Other important characteristics include specific additives in a formulated product, and the physical properties, which include molecular weight, molecular-weight dispersion, crystallinity, and chain orientation. Some properties such as molecular weight and molecular-weight dispersion are not determined directly by infrared and Raman spectroscopy, except in some special cases. 

In a spectrum of a polymer of mixed morphology, some absorption bands are assigned to the crystalline parts of the polymer others to the amorphous parts, and the remainder may be common to both. The intensities of the morphologically specific absorption bands correspond to the relative contributions of crystalline and amorphous components. If the intensities are correlated with either X-ray crystallographic or thermal analysis data of the polymer, the crystalline and the amorphous contributions can be determined. An alternative approach for certain polymer systems takes advantage of the difference in density between the crystalline and amorphous phases of a polymer. Density may be expressed as... 

As mentioned above, the activation of the metallocene dichlorides 1 and 2 with MAO provides very efficient catalysts for polymerization of propylene to high molecular weight, highly crystalline s-PP polymers. Detailed propylene polymerization conditions, results, and polymer analysis with 1 and 2/MAO catalyst systems are presented in Tables 1 and 2. Inspection of the data presented in Table 1 reveals, aside from the fact that all the produced polyqjropylene polymer samples are syndiotactic in nature (apparent from the large mx pentads), other important informatirHi. 

Process monitoring using Raman spectroscopy (mainly in its NIR Fourier transform variant) is proposed for QA/QC purposes, on-line polymer analysis, in situ cure kinetics, emulsion polymerisation, non-invasive analysis of physical parameters (in situ crystallinity determination, etc.) and reactor compositions, real-time measurements, molecular interactions, and components in aqueous solutions. 

The analysis of heat capacity of a given homopolymer thus starts with the evaluation of the experimental crystalline and amorphous heat capacities over as wide a temperature range as possible. For amorphous polymers, the glassy and liquid heat capacities are directly measurable. For crystallizing polymers, the crystalline and amorphous heat capacities may have to be extrapolated, as illustrated in the polyethylene example in Fig. 5.17. Only in rare cases are almost completely crystalline polymers samples available (as for example, for polyethylene, polytetrafluoroethylene, polymeric selenium, and polyoxymethylene). 

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

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