Spectroscopic methods, copolymers

NMR and IR are powerful spectroscopic techniques, which provide additional information about the compositional details of a sample. However, they are often unable to differentiate between a polymer blend A + B and a copolymer consisting of A and B. For such complex polymer compositions a combination of liquid chromatography and spectroscopic methods is helpful. In his recent review Pasch [57] discusses a couple of examples. 

Quantitative analysis of copolymers is relatively simple if one of the comonomers contains a readily determinable element or functional group. However, C,H elemental analyses are only of value when the difference between the carbon or hydrogen content of the two comonomers is sufficiently large. If the composition cannot be determined by elemental analysis or chemical means, the problem can be solved usually either by spectroscopic methods, for example, by UV measurements (e.g., styrene copolymers), by IR measurements (e.g., olefin copolymers), and by NMR measurements, or by gas chromatographic methods combined with mass spectroscopy after thermal or chemical decomposition of the samples. 

The resulting styrene/maleic acid copolymer is soluble in hot water, in contrast to the starting material the aqueous solution of the product gives a distinctly acid reaction. The disappearance of the anhydride moiety can be verified by IR or C-NMR spectroscopic methods.The IR spectra of polymers should be recorded from a film of the sample prepared on a KBr pellet (freshly made from KBr powder). For this, a drop of a solution of the polymer in a low-boiling solvent (e.g.,THF, methylene chloride) is placed on the pellet.The residual solvent can often be removed directly in the IR beam.The resulting spectra are characterized by their sharp bands. 

The MALDI-TOF spectrum of [G-3] poly(benzyl ether) dendrimer-po-ly(ethylene glycol) triblock copolymer shows a broad band of peaks between 4300 and 6100 D with resolution of the individual ethyleneoxide (44 D) units. The MALDI-TOF spectrum of a [G-3] dendrimer with two polystyrene blocks (molecular peak=8073 D) shows material with 6000-11,000 D and a broad band corresponding to material with 2 M+Ag+. SEC can be used to prove that the latter species is indeed an artifact of the mass spectroscopic method. The authors claim almost exact agreement between the polydispersities derived from MALDI-TOF and SEC [40]. This does, however, not leave any room for the unavoidable column spreading in the latter method. Furthermore, anionically prepared low MW polymers have a minimum polydispersity given by (1 + 1/DP) [41]. 

In the spectroscopic method, the grafting percentages were evaluated with the aid of a calibration curve. The calibration was obtained by plotting the grafting percentages calculated on the basis of the percent in weight of polyacrylonitrile over that of bamboo versus the band intensity ratio between the nitrile absorption of polyacrylonitrile and the hydroxyl absorption of bamboo. For each calibration point, a certain given amount of polyacrylonitrile and bamboo were used and the KBr pellets were prepared quantitatively for infrared measurements. The infrared spectra were recorded on a Perkin-Elmer 580 infrared spectrophotometer in absorbance mode. Similar procedures were carried out to obtain the calibration curve for holocellulose-polyacryloni-trile copolymers. 

When only spectroscopic methods are used, they are able to identify polymer components with respect to their chemical nature. However, in many cases, they are unable to answer the question whether two chemical structures are combined to yield a copolymer or a blend or both. For example, analyzing a rubber mixture one is able to identify styrene and butadiene as the monomer units. However, using FTIR or NMR it is impossible to decide if the sample is a mixture of polystyrene (PS) and polybutadiene (PB),or a copolymer of styrene and butadiene, or a blend of a styrene-butadiene copolymer and PB. For the latter case, even the copolymer composition cannot be determined just by running a FTIR or NMR spectrum. 

Obviously, what we would really like to do is not just have a feel for tendencies, useful as this is, but also calculate copolymer composition and sequence distributions, things that can also be measured by spectroscopic methods. We will start by using kinetics to obtain an equation for the instantaneous copolymer composition (it changes as the copolymerization proceeds). Later we will use statistical methods to describe and calculate sequence distributions. In deriving the copolymer equation, we only have to consider the propagation step and apply our old friend, the steady-state assumption, to the radical species present in the polymerization, and... 

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. 

Tosi, C., F. Ciampei,l.i, and A. Valvassori A new spectroscopic method for the determination of the product of reactivity ratios corresponding to ethylene-propylene copolymers. European Polymer J. 4, 107 (1968). 

Vasilescu, M. Caragheorgheopol, A. Caldararu, H. Aggregation numbers and micro structure characterization of self-assembled aggregates of poly(ethylene oxide) surfactants and related block-copolymers, studied by spectroscopic methods. Adv. Colloid Interf Sci. 2001, 89, 169-194. 

The presence of the nitroxide radical was confirmed through EPR and XH NMR spectroscopic methods. The copolymer GPC trace (Mn=33,100, Mw/Mn=1.37) was symmetrical with no evidence of unreacted macroinitiator or homopolymer of St resulting from either thermal initiation or from disproportionation of the pSt from the TEMPO chain end [231]. The kinetic results showed a first-order relationship between monomer conversion and time and the molecular weights increased linearly with conversion, indicating the polymerization proceeded with minimal termination or chain transfer reactions. The presence of the pEAD block produces an amphiphilic copolymer with a biodegradable block that may be useful for biomedical applications. 

EO content represents the amount of EO units (-CH2CH20-) in copolyether polyols (PO-EO), and random and block copolymers. Determination of EO content is based on two NMR spectroscopic methods (Standard Test Method ASTM D4875 [113]) Method A (1H NMR) and Method B (13C NMR) which are used for an EO content greater or equal to 6%. 

To check the accuracy of our method at low 0-pinene values, we analyzed the PMR spectrum of an isobutylene-isoprene copolymer which according to commercial specifications contained 1.S mole% unsaturation (This material, Butyl 268, has been obtained by the courtesy of the ENJAY Chemical Company). Using our spectroscopic method, we found 1.S mole% unsaturation in this material. 

Molecular weight distribution information obtained by size-exclusion chromatography on its own is insufficient to characterize the properties of complex polymers, such as copolymers and block and graft polymers [23,514,524]. For these polymers the chemical composition and functionality type distributions are equally important. A major obstacle to the characterization of these materials is that their molecular properties are present as joint distributions. Unlike the mass distribution the composition and functionality distributions can only be determined by separation methods that employ interactions with the stationary phase. To fully characterize a complex polymer it is not unusual to use manual or automated tandem techniques where the sample is fractionated according to its chemical or end group composition for subsequent further separation by size-exclusion chromatography to establish their mass distribution. Chromatographic methods may also be combined with spectroscopic methods to determine microstructural information. 

The microstructures of the polybutadienes, butadiene-styrene copolymers, and polyisoprenes were determined by infrared spectroscopic methods (1,3). The spectra of alkali metal-catalyzed polybutadienes and polyisoprenes show that other reactions occur during polymerization in addition to those involving cis- and trans 1,4, 1,2, and 3,4 additions. For sodium and potassium polybutadienes and polyisoprenes, the absorbances of the bands arising from these additional structures could be taken into account satisfactorily by the methods described. No foreign structures are found in lithium-catalyzed polyisoprenes and the additional band foimd near 14.2 microns in polybutadiene spectra does not appear to affect the cis-1,4 band at 14.7 microns. (Cesium and rubidium, as well as additives such as dimethoxy-tetraglycol, affect the polymerization of butadiene so markedly that it was not possible to obtain satisfactory analyses of such polymers. The effect of these catalysts in isoprene polymerizations does not appear to be so marked and satisfactory analyses were obtained by the method described. 

Percot, A., Zhu, X.X., and Lafleur, M. 2000. A simple FTIR spectroscopic method for the determination of the lower critical solution temperature of N-isopropylacrylamide copolymers and related hydrogels. J Polym Sci Polym Phys. 38 907-915. 

The composition of carbon-chain polymers with monomeric units having widely differing analytical composition, characteristic elements or groups, or radioactive labels can be readily determined. Chemical (microanalysis, functional group determination, etc.) and spectroscopic methods (infrared, ultraviolet, nuclear magnetic resonance, etc.), as well as the determination of radioactivity, yield the average composition of the polymer. The mean composition can also be determined from the refractive indices of solid samples. The composition can be calculated from the principle that the copolymer is considered to be a solution of one unipolymer (from one of the monomeric units) in the other. The composition can also be found by means of the refractive index increment dw/dc in solution, which gives the variation in refractive index with concentration. The mass fraction of the monomeric unit A can be calculated from... 

A sensitive Fourier transform infrared spectroscopic method for the determination of ENB content in ethylene propylene diene (EPDM) copolymers is described in detail. It is applicable for ENB contents in the 0.1 to 10% weight range. The repeatability standard deviation of the method is given as 0.020 and the reproducibility standard deviation, 0.080 at the 5% weight level. 

In order to calculate a copolymerization reactivity ratio, it is first necessary to determine the composition of the copolymer or of the unconverted monomer mixture (or both). Elemental analysis, spectroscopic methods (IR, UV, NMR), refractive index determination, or turbidimetric titration can be suitable for determining the copolymer composition. 

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