Vibrational spectroscopy copolymer composition

IR spectroscopy was used for quantitative analysis of the composition of three ethylene-acrylate copolymers, i.e. ethylene-methyl acrylate, ethylene-butyl acrylate and ethylene-2-ethylhexyl acrylate copolymers. Based on a simple model which explicitly considered vibrational band intensities characteristic for CH and for C 0, copolymer composition could be derived from the ratio of C 0 and CH integrated absorbances with a precision of + or -3 mol %. It was not necessary to know the optical path length of the copolymer samples which were subjected to IR analysis as pressed films. 15 refs. 

A series of copolymers of D,L-lactide and CL were synthesized by ROP using zinc lactate as a catalyst and carrying out the reaction at 145""C for 8 days. The characterization of PCL and its copolymers with lactides is often done by size exclusion chromatography, DSC, NMR, and stress-strain analysis. Kister et al. used vibrational spectroscopy, particularly Raman spectroscopy, for determination of morphology, conformation, configuration, and composition of the copolymers [49]. Raman spectroscopy thus appeared to be a suitable method for the identification of P(DLA-co-CL) samples directly from solid samples without any special preparation. 

More recently, Dong and Hill [49] used FT infrared (FT-IR) spectroscopy to study copolymer composition and monomer sequence distribution in styrene-methacrylonitrile copolymers. They determined the dependence of the frequencies of the individual spectral peaks on the copolymer composition, in particular, the vibration frequencies for the nitrile group is discussed. Correlations were established to relate changes in the peak positions to changes in the copolymer composition and monomer sequence distribution. Vibration band frequencies for blends of poly(methacrylonitrile) and polystyrene were examined to compare the effects of inter- and intra-chain interactions in these bands. 

Both IR and Raman spectroscopies are vibrational spectroscopies that provide a unique identification of the substance, or a fingerprint. They are used extensively to determine the composition of materials as discussed by Koenig [3]. Lang et al. [4] showed that IR and Raman provided complementary information about the fibers. They comment that sample preparation is far easier for these methods than the traditional characterization methods based on the solubility of the fibers. In this mode, Raman is used to determine whether a film or fiber is nylon, polyester, polypropylene, cotton, wool, and so forth. Each type of material will have Raman bands specific to the type of polymer of which it is composed. If copolymers are present, the Raman spectra can be used to determine the ratio of comonomers. Many comonomers are strong Raman scatterers (aromatics, double and triple bonds, carbonyls, etc.). Others are weak Raman scatterers (NH, OH, etc.) and are better determined by IR. In either case, an appropriate calibration is required and the spectroscopist needs to make an educated selection between IR and Raman or perhaps use both. 

Infrared spectroscopy is one method used to identify polymers, as discussed in Section 1.9.4. The degree of branching of polymers can also be determined if the absorption bands of the branch groups can be identified. Similarly in copolymers, the relative composition can be obtained if the different types of repeat unit have distinct vibrational modes and thus absorption bands. To make this a quantitative measure of fractional content, the absorbance in each band is measured via the Beer-Lambert law A = eel, where s is the molar absorptivity, c is the concentration of a given species and / is the path length. For a copolymer with two different types of repeat unit the ratio of absorbances yields the ratio of concentrations if the molar absorptivities are known, for example having being measured previously for samples of known composition. 

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