Copolymer analysis

It is well known that most copolymers have both molecular mass and composition distributions and that copolymer properties are affected by both composition and molecular mass distributions. Therefore, we must know average values of molecular mass and composition, and their distributions. These two distributions are inherently independent of each other. However, it is not easy to determine the molecular mass distribution (MMD) independently of the composition, or inversely, to determine the chemical composition distribution (CCD) independently of the molecular mass, even by modern techniques. 

Many applications of SEC to the determination of average composition and CCD for copolymers have been reported in the literature. Two different types of detector or two different absorption wavelengths of an ultraviolet or an infrared detector are employed in SEC to obtain two chromatograms at a time, and the composition at each retention volume is calculated by measuring 

It can be said that when chemical heterogeneity of a copolymer as a function of molecular mass (or size) is observed, then the copolymer has heterogeneous composition. However, even when the SEC results show constant composition over the whole range of molecular mass (or size), it is not possible to say that the copolymer has homogeneous composition. Ogawa [3] dealt with the effects of MMD, CCD and the correlation coefficient between molecular mass and composition on the detected compositional variation for ethylene-propylene copolymers. He concluded that these three factors were equally important in evaluating the curves of the variation, and that the difference between CCDs was detectable only under limited conditions where the other factors are kept constant. 

Nevertheless, the utility of SEC in copolymer analysis is not impaired, if one applies SEC keeping the limitations described above in mind. The rapidity, simplicity, and capability of obtaining much data on copolymers by SEC overcome those drawbacks. In this chapter, the methods of determining molecular mass averages and MMD, as well as composition and chemical heterogeneity, are described. Other separation modes of HPLC used with the size-exclusion mode, such as the adsorption mode, are also explained. 

---

In analysis of homopolymers the critical interpretation problems are calibration of retention time for molecular weight and allowance for the imperfect re >lution of the GPC. In copolymer analysis these interpretation problems remain but are ven added dimensions by the simultaneous presence of molecular weight distribution, copolymer composition distribution and monomer sequence length distribution. Since, the GPC usu y separates on the basis of "molecular size" in solution and not on the basB of any one of these particular properties, this means that at any retention time there can be distributions of all three. The usual GPC chromatogram then represents a r onse to the concentration of some avera of e h of these properties at each retention time. 

In homopolymer analysis this meant a closer study of the accuracy and reproducibility of data from GPC to see how resolution correction techniques could be either circumvented or practically applied. In copolymer analysis the limitation of conventional molecular size fractionation emerged as the fundamental difficulty. An orthogonal coupling of GPCs operated so as to achieve the desired cross fractionation before detection is presented as a novel approach with considerable potential. 

From the axial dispersion viewpoint alone there is no doubt that the experimental reduction of dispersion or its conation would be preferable to assuming it negligible. Either of these options require a simple method for the assessment of axial dispersion which does not depend upon absolute molecular weight averages or assumption of distribution functions (5, Such a method will be shown in Section 3 of this report. However, first the problem of copolymer analysis which led to this method as a byproduct win be examined. 

Garcia-Rubio, L.H., MacGregor, J.F., Hamielec, A.E., "Copolymer Analysis Using GPC with Multiple Detectors , presented at the Symposium on Recent Developments in Size Exclusion Chromatography , 178th ACS National Meeting, Washington, D.C., September 9-14, 1979. 

Branching in the polymer chain affects the relationship between retention and molecular weight.83 Universal calibration has been used with some success in branched polymers, but there are also pitfalls. Viscosimetry84-91 and other instrumental methods have proved to be useful. A computer simulation of the effects of branching on hydrodynamic volume and the detailed effects observable in GPC is available in the literature.92 93 In copolymer analysis, retention may be different for block and random copolymers, so universal calibration may be difficult. However, a UV-VIS detector, followed by a low-angle light-scattering (LALLS) detector and a differential... 

Mori, S., Copolymer analysis, in Size Exclusion Chromatography, Hunt, B. J. and Holding, S. R., Eds., Blackie, Glasgow, 1989, chap. 5. 

Mori, S., Wada, A., Kaneuchi, F., Ikeda, A., Watanabe, M., and Mochizuki, K., Design of a highly sensitive infrared detector and application to a high-performance size exclusion chromatography for copolymer analysis, /. Chromatogr., 246, 215, 1982. 

A number of 2DLC applications have attempted to use liquid chromatography at critical conditions (LCCC) and are discussed in Chapter 17. This mode of operation is useful for copolymer analysis when one of the functional groups has no retention in a very narrow range of the solvent mixture. However, determining the critical solvent composition is problematic as it is very sensitive to small changes in composition. 

Copolymerization reactions Copolymerization experiments with styrene and MMA employed molar fractions of 20, 40, 60, and 80% comonomers, which were reacted in ethanol 1,2-dichIorethane 60 40 (by volume) mixtures and benzoyl peroxide as catalyst. Polymerizations were carried out at 70°C. The reactions were quenched by the addition of methanol as non-solvent, and the copolymer was isolated by centrifugation. Copolymer analysis employed UV spectroscopy for copolymers with MMA, and methoxyl content determination according to a procedure by Hodges et al. (16) in the case of styrene copolymers. Reactivity ratios were determined in accordance with the method by Kelen-Tiidos (17) and that by Yezrielev-Brokhina-Roskin (YBR) (18). Experimental details and results are presented elsewhere (15). 

There are many problems associated with copolymer analysis which require a whole variety of techniques for solution. Isotopic tracers can play their own small part by establishing the exact composition in terms of relative amounts of the monomeric constituents. Consider the following examples ... 

Table 5. Copolymer analysis using labelled monomers...

Beginning in the late forties, copolymers were fractionated by adsorption chromatography poly (butadiene-co-styrene)32 34), poly(butadiene-co-acrylonitrile)32), polystyrene- -vinyl acetate)35), poly(styrene-h-ethylene oxide)36) and poly(styrene-co-acrylonitrile) 37). HPLC adsorption chromatography was first applied to copolymer analysis by Teramachi et al. in 1979 38>. 

The same conclusion is found when the copolymer is prepared using large quantities of dichloroethane and the precipitation step occurs for higher n value. Consequently we must deduce, in such cases, that added PVC is different from PVC contained in the copolymer. One is justified in concluding that PVC is grafted by butadiene copolymer, (analysis verifies this) and that all PVC chains are grafted in the samples prepared with high dichloroethane ratios. 

The copolymer composition can be estimated usefully in many cases from the composition of unreacted monomers, as measured by gas-liquid chromatography. Analytical errors are reduced if the reaction is carried to as high a conversion as possible, since the content of a given monomer in the copolymer equals the difference between its initial and final measured contents in the feed mixture. The uncertainty in the copolymer analysis is thus a smaller proportion of the estimated quantity, the greater the magnitude of the decrease in the monomer concentration in the feed. It may seem appropriate under these circumstances to estimate reactivity ratios by fitting the data to an integrated form of the copolymer equation. 

The extensive reactivity ratio data in the literature exhibit a wide scatter for many monomer pairs. This is partly due to errors in copolymer analysis and computational methods that result in larger uncertainties in n and Z2 than was realized when the results were reported. Another factor reflects the frequent reliance on an inadequate number of data points because copolymerization experiments lend to be tedious and time consuming. 

Table I shows these different approaches and their requirements, benefits, and limitations of chromatographic copolymer analysis.

In this kind of calculation the absence of segment-segment interactions and chemically monodisperse SEC fractions has to be assumed. The main benefits of this approach is ordinary SEC equipment is used and the copolymer analysis is done with the same injection without additional sample preparation. 

Thermal analysis are widely used for polymers and copolymers analysis. Glass transitions, melting, and decomposition processes are analyzed. Since the glass transition temperature Tg is marked by changes in the thermal capacity, expansion coefficient, and rigidity, TMA technique as well as DSC may be used. Tg increases with molecular mass up to certain values. Plasticizers and water depress this temperature. Thermal stability and influence of antioxidants and fillers may be analyzed by TG or DSC, under oxygen. 

Copolymer Analysis by LC Methods, Including Two-Dimensional Chrome Peter Kilz... 

R. O. Bielsa and G. R. Meira, Linear copolymer analysis with dual-detection size exclusion chromatography Correction for instrumental broadening, /. Appl. Polym. Sci. 46 835 (1992). 

Pyrolysis gas chromatography has been widely used for copolymer analysis. This technique may offer many advantages over other detection techniques for copolymer analysis by SEC. One obvious advantage is the small sample size required. Another is the capability of application to copolymers which cannot utilize UV or IR detectors [3]. 

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. 

Spectrometric Analysis. Spectroscopy has been extensively used for polymer and copolymer analysis. (59-69). The kind of information available from different spectroscopic techniques as well as the instrumentation required depends on the region of the electromagnetic spectrum in which absorption is taking place. Recent investigations (63) on the use of spectrophotometers for copolymer analysis have shown that the response from spectrophotometers is sometimes sensitive to the microstructure of the polymer molecules and that calibration of spectrophotometers with absolute measurements on the microstructure (i.e. NMR) may be necessary in order to obtain reliable quantitative information on concentration and copolymer composition determinations. 

Although phase separation techniques have been shown to be useful in copolymer analysis, the amount of work required restricts considerably their application. 

---