Infrared spectroscopy, copolymers composition determination

Thus, infrared spectroscopy permits the determination of components or group of atoms that absorb in the infrared at spiecific frequencies, pjermitting identification of the molecular structure (Bower, 1989 Koenig, 2001). These techniques are not limited to chemical analysis. In addition, the tactidty, crystallinity, and molecular strain can also be measured. Copolymer compositions can be determined as block copolymers absorb additively, and alternating copolymers deviate from this additivity due to interaction of neighboring group . 

As discussed previously (Sec. II.D.4), acquisition of a quantitative NMR spectrum presupposes careful planning. NMR offers several advantages over other techniques, such as infrared spectroscopy, for the determination of copolymer composition. First, in a truly quantitative NMR spectrum, the relative areas of all peaks are self-consistent, so that if the intensity of peak A is twice that of peak B, it is certain that the number of nuclei contributing to A is double that of B. In other words, there is no NMR equivalent of an extinction coefficient, and precharacterized standards are not needed for method development, if the experiment is properly designed and executed. Second, because of NMR s great sensitivity to subtle structural changes, even chemically similar species can be distinguished. It is, for example, possible to calculate the eoncentration of each pendant-... 

Styrene-butadiene by NIR. Polybutadiene and styrene-butadiene copolymer are used extensively in the tire and rubber industries. As mentioned earlier in this chapter, there are various stereoisomers associated with the polymerization of butadiene cis-, trans-, and vinyl, and their relative amounts appreciably affect the polymer properties. NMR and infrared spectroscopy can accurately determine the microstructure and composition of these materials. These methods usually require extensive sample preparation and usually, dissolving the polymer in a solvent or pressing the polymer into a thin film. 

Heischer et al. [172] measured the interfacial tension reductirai credited to the complexation between carboxy-terminated PBD and amine-terminated PDMS, which were added to an immiscible blend of PBD and PDMS. The changes in interfacial tensimi resembled the behavior observed for block copolymer addition to homopolymer blends there is initially a linear decrease in interfacial tension with the concentration of functional homopolymer up to a critical concentration, at which the interfacial tension becomes invariant to further increases in the concentration of functional material. However, the formation of interpolymer complexes depends on the equilibrium between associated and dissociated functional groups and, thus, the ultimate plateau value for interfacial tension reduction is dependent on the functional group stoichiometry. A reaction model for end-complexation was developed in order to reproduce the interfacial tension reduction data with Fourier transform infrared spectroscopy applied to determine the appropriate rate constants. The model provided a reasonable qualitative description of the interfacial tension results, but was not able to quantitatively predict the critical compositions observed experimentally. 

Principal Component Regression (PCR) was used by Tuchbreiter and MueUiaupt to determine the composition of a number of random ethane/propene, ethane/1-hexene, and ethane/l-octene copolymers [120]. After polymerization, the polymers were characterized by both Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FT-IR) and C NMR and multivariate calibration models using PCR were subsequently developed to estimate the co-monomer content. 

Many studies use infrared spectroscopy for quality control and quality analysis in polymer production. It is particularly used for the determination of the composition of copolymers and polymer blends and also for determination of additive and filler contents [90, 91, 92]. 

The concentration of the siloxane introduced into the copolymer is most easily determined by proton NMR for soluble systems, as briefly referred to in Sect. 3.1. This was demonstrated by Summers et al. [45] and Arnold and coworkers [46-50], as well as Rogers et al. [52]. Others have routinely conducted these experiments to establish the copolymer composition. Infrared spectroscopy has been particularly useful for demonstrating the transformation of the amic acid, which is often an intermediate for the final imide form. The assignments have been noted in many of the polyimide reviews referred to earlier [1-8]. In addition, it is useful to conduct an elemental analysis for silicon as complementary proof of copolymer composition. Solid-state NMR can be used even for intractable polyimide systems to provide a good estimate of the copolymer composition. 

Considerably better results are obtained with copolymers of vinyl chloride and lead undecylenate. The lead salt of undecylenic acid, (CH2=CH—(CH2)g—COO)2Pb, can be copolymerized by free radicals in bulk or in methanol solution. The composition of the resulting polymer has been determined by infrared spectroscopy (4). Figure 5 shows an infrared spectrum of a film of homopolymeric PVC and of a copolymer obtained from vinyl chloride and lead undecylenate. At wavenumbers... 

Copolymer Analysis. Even though the overall copolymer composition can be determined by residual monomer analysis, it still is necessary to have reliable quantitative techniques for copolymer composition measurements on the actual copolymer, mainly because concentration detectors for SEC or HPLC are sensitive to composition and because the conversion histories are not always available. Some of the techniques used to determine copolymer composition are melt viscometry (46), chemical analysis, elemental analysis, infrared spectroscopy (IR), Nuclear Magnetic Resonance (NMR), ultra-violet spectroscopy (UV), etc. Melt viscometry, chemical and elemental analysis are general techniques that can be applied to almost any polymer. The spectroscopic techniques can be applied depending on the ability of the functional groups present to absorb at specific wavelengths. 

Very similar effects were also found upon copolymer composition, total conversion, and RMM-control in the styrene-isoprene copolymer system [168] where the analogous traces in Figure 12 shifted to slightly more anodic values, with a better total conversion at high potentials under ultrasound. Ultrasound in both systems was provided by an Ultrasonic Cleaner at 25 kHz. Copolymer compositions were determined by infrared spectroscopy. 

Infrared spectroscopy is a widely used method to characterize polymers and copolymers. However, for determination of the composition and microstructure... 

The chain microstructure has a very important influence on the properties of TPEs. As mentioned earlier, production of SBS or SIS with a high 1,4 content is necessary. TPO properties also depend quite heavily on any deviations of the microstructure from the ideal head-to-tail, pure isotactic, or syndiotactic microstructure. Properties such as tacticity, cw-trans isomerization, and copolymerization content are usually characterized using NMR. Peak positions and peak intensities are used to quantitatively ascertain microstructure to a high degree of accuracy. Copolymer composition can also be determined using NMR. Infrared spectroscopy can also be employed to determine microstructural characteristics in some polymers. 

Copolymers are comprised of chains containing two or more different types of monomers. The composition of copolymers may be quantitatively determined by using infrared spectroscopy [3, 8]. Distinctive representative modes for the polymers may be identified. For example, in the case of vinyl chloride-vinyl acetate copolymers, the ratio of the absorbance of the acetate mode at 1740 cm to that of the vinyl chloride methylene bending mode at 1430 cm can be used for quantitative analysis. Copolymers of known composition may be used for calibration. The multivariate methods described earlier in Chapter 3 may also be applied. Care must be exercised because the position and shapes of the infrared bands of the components of copolymers may be affected by the sequencing of the constituent monomers. 

McCormick, C. L., Chen, G. S. and Hutchinson, B. H., Water-Soluble Copol3rmers. V. Compositional Determination of Random Copolymers of Acrylamide with Sulfonated Comonomers by Infrared Spectroscopy and C13 Nuclear Magnetic Resonance, J. Appl. Polym. Sci., 27 3103 (1982). 

Miller and co-workers [21] used near-infrared spectroscopy to determine the microstructure and composition of polybutadiene and styrene-butadiene copolymers. The procedure was capable of distinguishing between cis-1,4, trans-1,4, and 1,2 butadiene groups. Geyer [22] has given details of a Bruker Spectrospin P/ID. 28 used for the identification of plastics using mid-infrared spectroscopy. 

The composition of resulting copolymers was determined by H NMR spectroscopy in CDCI3. NMR spectra were recorded on a Bruker Model MSL-300 spectrometer. Infrared (IR) spectra of the yUdes and copolymers synthesized were recorded on a Specord M-82 spectrophotometer. Differential scanning... 

Paxton and Randall [13] used Fourier transform infrared spectroscopy (FT-IR) to measure the concentration of bound ethylene in ethylene propylene copolymers in amounts down to 0.1 %. These polymers contained >95% propylene, with the ethylene units present as isolated entitles between two head-to-tail propylene units. These workers point out that most IR bands used for determining copolymer compositions are sensitive to sequences of both monomers. This IR method for compositional analysis can be calibrated if (a) known standards of similar constitution to the copolymers being analysed are available and (b) assignments and behaviour of the calibration bands are well established preferably the absorptivities of these bands should be relatively independent of the position of monomer units in the chain. Thus, quantitative IR analysis of copolymers depends primarily on the standards employed whose composition can be determined directly and reliably. Paxson and Randall [13] used C-NMR to provide such reference standards for the less time-consuming IR measurements because it is relatively inexpensive and easy to operate for copolymer analysis. They showed that an excellent correlation is obtained between C-NMR and IR results on a series of ethylene-propylene copolymers containing >95% wt% propylene. 

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. 

The difficulty results, in part, from the fact that only a small fraction of the chemical bonds, generally less than one in a thousand, are involved in me-chanochemical processes. The concentration of connecting units is therefore at the detection limit and below for traditional analytical methods such as conventional nuclear magnetic resonance and infrared spectroscopy. The sensitivity can, of course, be enhanced by techniques such as cumulative, multiple scans, Fourier transform analysis, and difference techniques for detection to one part in ten thousand and better. It may yet be difficult to determine whether polymers are linked by chemical bonds or whether they are simply intimate mixtures. For this distinction, other tests can be of value. For example, the difference between blocks and blends for ethylene-propylene polymer systems has been distinguished by thermal analysis [5]. In many cases, simple extraction tests can distinguish between copolymers and blends. For example, for rubber milled into polystyrene, the fraction of extractable rubber is a measure of mechanochemistry. Conversely, only the rubber in this system is readily cross-linked by benzoyl peroxide after which free polystyrene may be conveniently extracted [6]. In another case, homopolymers of styrene and methyl methacrylate can be separated cleanly from each other and from their copolymers by fractional precipitation [7]. The success of such processes, of course, depends on both the compositions and molecular weights involved. 

Several techniqnes are used in determining copolymer composition. These inclnde nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, MALDI mass spectrometry for low-molecnlar-weight polymers, titration for determining the degree of hydrolysis (e.g., polyvinyl alcohol) [58] and... 

Kranz and co-workers [126] have shown that acrylonitrile can he determined in styrene - butadiene - acrylonitrile terpolymers via a determination of organic nitrogen by the Kjeldahl procedure. Styrene units can be can be determined by infrared spectroscopy. Butadiene units can be determined by the iodine monochloride procedure. The compositional analysis and details of the microstructure of butadiene - acrylonitrile copolymers can be obtained by Raman spectroscopy [127]. 

The composition of ethylene oxide - propylene oxide copolymers has been determined by infrared spectroscopy [133]. 

The properties of some polymers are dependent on their microstructure for example isotactic polypropylene is crystalline whereas atactic polypropylene is amorphous. Microstructure effects are also exemplified by polybutadienes, where the mode of addition of the diene to the growing chain leads to 1,2-addition, 1,3-addition and 1,4-addition, which may be as or trans. The fraction of different addition species changes the mechanical properties of the polymer. Another example is provided by the chemical composition of a copolymer and its sequence distribution, which together determine its ultimate properties. It is thus of great importance to be able to characterize polymer micro structure. This is generally done using spectroscopic methods, specifically infrared spectroscopy and nuclear magnetic resonance spectroscopy. 

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. 

Near infrared spectroscopy has been used to determine the microstructure and composition (cis 1 4, trans 1.4 and 1.2 butadiene contents) of polybutadiene and styrene butadiene copolymers. 

If you have been working your way through this epic in a more or less linear fashion, then you might have started to ask yourself some fundamental questions such as, How do you know if a vinyl polymer is isotactic, or atactic, or whatever How do you know the composition and sequence distribution of monomers in a copolymer How do you know the molecular weight distribution of a sample This last question will have to wait until we discuss solution properties, but now is a good point to discuss the determination of chain microstructure by spectroscopic methods. The techniques we will discuss, infrared and nuclear magnetic resonance spectroscopy, can do a lot more than probe microstructure, but that would be another book and here we will focus on the basics. 

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