Identification of Polymers

Computerised identification of polymer additives on the basis of MS spectral data has been reported [1]. 

Applications Rather intractable samples, such as organic polymers, are well suited to FD, which avoids the need for volatilisation of the sample. Since molecular ions are normally the only prominent ions formed in the FD mode of analysis, FD-MS can be a very powerful tool for the characterisation of polymer chemical mixtures. Application areas in which FD-MS has played a role in the characterisation of polymer chemicals in industry include chemical identification (molecular weight and structure determination) direct detection of components in mixtures off-line identification of LC effluents characterisation of polymer blooms and extracts and identification of polymer chemical degradation products. For many of these applications, the samples to be analysed are very complex... 

Applications Identification of polymer additives by TLC-IR is labour intensive and comprises extraction, concentration of extracts, component separation by TLC on silica, drying, removal of spots, preparation of KBr pellets and IR analysis. The method was illustrated with natural rubber formulations, where N-cyclohexyl-2-benzothiazyl sulfenamide, IPPD and 6PPD antioxidants, and a naphthenic plasticiser were readily quantified [765]. An overview of polymer/additive type compounds analysed by transfer TLC-FTIR is given in Table 7.80. 

SFC-based methods still need to show their potential, in spite of past great promise. pSFC-APCI-MS is a powerful method for identification of polymer additives, provided that a library of mass spectra of polymer additives using this technique is available. SFC-MS appears less performing than originally announced nevertheless, SFE-SFC-EIMS is an interesting niche approach to additive analysis. On the other hand, we notice the lack of real breakthrough in SFE-SFC-FTIR. 

In ultrapure polymer samples, all chains are terminated in the same way. The MALDI spectrum of an ultrapure polymer resembles a comb and the spacing between the comb s teeths equals the mass, Mrepeat, of the repeat unit. This quantity is often diagnostic and it suggests an almost trivial use of MALDI is the spectral identification of polymers. The reason is that, if one computes the M,.c x.at value for common polymers, most values are different, the number of superpositions being very low [4—6]. The Mrepeat value is not an integer, due to the fact that various isotopes are present. 

MALDI-MS was developed for the analysis of nonvolatile samples and was heralded as an exciting new MS technique for the identification of materials with special use in the identification of polymers. It has fulfilled this promise to only a limited extent. While it has become a well-used and essential tool for biochemists in exploring mainly nucleic acids and proteins, it has been only sparsely employed by synthetic polymer chemists. This is because of lack of congruency between the requirements of MALDI-MS and most synthetic polymers. 

For exact identification of polymers it is important for the samples to be in the form of pure products without incorporated additives such as plasticizers, fillers, or stabilizers. One must separate additives by extraction or reprecipitation before identification. The solvents or mixtures of solvent and precipitant are substance-specific and should be chosen separately for each case. 

Polymer identification starts with a series of preliminary tests. In contrast to low molecular weight organic compounds, which are frequently satisfactorily identified simply by their melting or boiling point, molecular weight and elementary composition, precise identification of polymers is difficult by the presence of copolymers, the statistical character of the composition, macromolecular properties and, by potential polymeric-analogous reactions. Exact classification of polymers is not usually possible from a few preliminary tests. Further physical data must be measured and specific reactions must be carried out in order to make a reliable classification. The efficiency of physical methods such as IR spectroscopy and NMR spectroscopy as well as pyrolysis gas chromatography makes them particularly important. 

Method of EPR-tomography developed in the Institute of Chemical Physics of RAS [46] allows both detecting of molecular mobility and its change at thermo- or photo-destruction of polymer in various points of sample and registration of the distribution of oxidation active sites through the sample. This method allows identification of polymers parts in which destruction process proceeds. Solution of this problem is of great importance for selection of conditions of polymer materials exploitation. 

One can also use solution and solid state NMR spectroscopy in a similar manner to identify polymers (there are plenty of examples coming up shortly). But we do not want to give the impression that the identification of polymers, and especially copolymers, is as easy as we have implied in the simple infrared example shown in Figure 7-21. There are many subtleties and many industrial scientists have made a career specializing in polymer identification. However, this is enough for our purposes here and we now move on to the characterization of polymer microstructure. 

An important aspect of the use of analytical pyrolysis is its capability to provide complementary information to other analytical techniques used for polymer characterization. One such technique is IR analysis of polymers. Although IR spectra can be used as fingerprints for polymer identification, the success of this technique can be questionable when the polymer is not pure or is in a mixture with other compounds. The IR spectra are particularly difficult to use when a polymer is present only at a low level in a particular material and cannot be easily separated. The use of Py-GC/MS allows identification of polymers even in low concentration in specific mixtures because it couples pyrolysis with a chromatographic technique. On the other hand, some polymers generate by pyrolysis a low proportion of easily identifiable molecules, producing mainly char and small uncharacteristic molecules such as HF, H2O, CO2, etc. In these cases, IR is the technique of choice. Since for an unknown sample each technique can be misleading, the use of both types of information is always beneficial. 

Analytical pyrolysis has a number of characteristics that can make it a very powerful tool in the study of polymers and composite materials. The technique usually requires little sample and can be set with very low limits of detection for a number of analytes. For Py-GC/MS the identification capability of volatile pyrolysate components is exceptionally good. A range of information can be obtained using this technique, including results for polymer identification, polymer structure, thermal properties of polymers, identification of polymer additives, and for the generation of potentially harmful small molecules from polymer decomposition. In most cases of analysis of a polymer or composite material, the technique does not require any sample preparation, not even solubilization of the sample, which may be a difficult task for the type of materials analyzed. The analysis can be easily automated and does not require expensive instrumentation (beyond the cost of the instrument used for pyrolysate analysis). 

Dr. Hubball also is a consultant to two companies that specialize in polymer-related cases. In his role as consultant, he has performed hundreds of polymer analyses, and has gained extensive experience in the extraction and identification of polymer additives. He has compiled a comprehensive library of GC/MS data related to polymer additives. 

Figure 4 Bootstrap standard deviation plot exhibiting the possibility for qualitative identification of polymer film coating endpoint. Dashed line indicates the three-standard deviation limit for spectral similarity. Near-IR spectra from 10 samples obtained at the 16% theoretical applied solids level were used as a training group.

Testing procedure Volatile products of degradation were absorbed in Tenax cartridges and the ethanolic eluate of the Tenax cartridge was analyzed by GC-MS."" The furnace type pyrolyzer was connected to the gas chromatograph, and pyrolysis was conducted under helium. Volatiles were used for the identification of polymer and residue to determine the amount of carbon black. The standard error of determination of carbon black was in the range of 0.2 to 3.4%. 

Kalberer, M., D. Paulsen, M. Sax, M. Steinbacher, J. Dormnen, A. S. H. Prevot, R. Fisseha, E. Weingartner, V. Frankevich, R. Zenobi and U. Baltensperger Identification of Polymers as Major Components of Atmospheric Organic Aerosols, Science 303 (2004) 1659-1662. 

Suggested SOP for the identification of polymer films by infrared spectrometry... 

The identification of polymers is largely done by fingerprint matching. Representative spectra of the most important commercial polymers are included here for the convenience of the reader (Figures 2-10). They are grouped by structure to emphasize the features that chemically related polymers have in common and to demonstrate the distinctive identifying characteristics of each polymer class,... 

Examples of the use of TGA in the literature include monitoring the decomposition of potassium and ammonium tetraphenylborates, the characterisation of chromatographic stationary phases and the identification of polymers in forensic cases. 

The area of application of Py—GC for identification is extensive. Its use is especially recommended for substances that are either difficult to identify by other techniques (e.g., insoluble polymers) or necessitate sophisticated and expensive instrumentation. The applications of Py-GC are steadily increasing we shall consider only those areas in which Py—GC has become a traditional technique, which include the analysis of polymers, drugs, biochemical substances and microorganisms. In addition to the work discussed above in connection with applying Py—GC for the identification of polymers, we may mention other studies. The identification of acrylate, methacrylate and styrene polymers and copolymers was described by McCormick [104]. Fischer and Meuser... 

The identification of polymer blends is illustrated by the DTA curve in Figure 7.48. Chiu (154) studied a physical mixture of seven commercial polymers high-pressure polyethylene (HPEE), low-pressure polyethylene (LPPE), polypropylene (PP), polyoxymethylene (POM), Nylon 6, Nylon 66, and polytetrafluoroethylene (PTFE). Each component shows its own characteristic melting endothermic peak, at 108,127,165,174,220,257, and 340°C, respectively. Polytetrafluoroethylene also has a low-temperature crystalline transition at about 20°C. The unique ability of DTA to identify this polymer mixture is exceeded by the fact that only 8 mg of sample was employed in the determination. 

The use of IR microscopy is still fairly new but appears to be a technique that will find increasing applications in the future. Some of its current applications include identification of polymer contaminants, imperfections in polymer films, and individual layers of laminated polymer sheets identification of tiny samples of fibers, paint, and explosives in criminalistics characterization of single fibers in the textile industry and identification of contaminants on electronic components. 

Identification of Polymers, it is a fact of commercial life that there Is a frequent call for the rapid Identification of synthetic polymers, usually by industrial scientists and technologists Interested In a rival product. The growing possibility that recycling of plastic material may become economically attractive compared with disposal would also require Identification of different synthetic polymers for sorting purposes. Luminescence spectroscopy could provide a convenient method of rapid identification. 

---