Identification of Synthetic Polymers

A few reviews have dealt with the identification of synthetic polymers by Py-GC/MS [76]. In addition to compositional studies, applications of pyrolysis to synthetic polymers include sequence length characterization in copolymers [77] and tacticity measurements in stereoregular homopolymers [78]. 

Classical MALDI-MS requires that the material should be soluble in a suitable solvent. A suitable solvent means a solvent that is sufficiently volatile to allow it to be evaporated prior to the procedure. Further, such a solvent should dissolve both the polymer and the matrix material. Finally, an ideal solvent will allow a decent level of polymer solubility, preferably a solubility of several percentage and greater. For most synthetic polymers, these qualifications are only approximately attained. Thus, traditional MALDI-MS has not achieved its possible position as a general use modern characterization tool for synthetic polymers. By comparison, MALDI-MS is extremely useful for many biopolymers where the polymers are soluble in water. It is also useful in the identification of synthetic polymers, such as PEO where the solubility requirements are fulfilled. Thus, for PEO we have determined the molecular weight distribution of a series of compounds with the separations in ion fragment mass 44 Da corresponding to CH2-CH2 units. 

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. 

The intention of the author was to provide information on pyrolysis for a wide range of readers, including chemists working in the field of synthetic polymers as well as for those applying pyrolysis coupled with specific analytical instrumentation as an analytical tool. Some theoretical background for the understanding of polymer structure using analytical pyrolysis is also discussed. The book is mainly intended to be useful for practical applications of analytical pyrolysis in polymer identification and characterization. 

Polymer chemistry is an important branch of science, and polymer analysis and characterization is a common subject in scientific literature. Analytical pyrolysis is one of many tools used particularly for polymer identification and for the evaluation of polymer thermal properties. Before a more in-depth discussion on analytical pyrolysis and its application to polymer science, some basic concepts regarding the chemistry of synthetic polymers will be briefly discussed. 

Before the introduction of MALDl, mass spectrometry was not a widely applicable method for the determination of MM of synthetic polymers. Since MALDl-TOE allows desorption and ionization of very large molecules, it is now possible to perform the direct identification of mass-resolved polymer chains and the measurement of MM in numerous high polymers. MM values up to 1.5 million Daltons have been measured for PS monodisperse standards. MALDI-TOF is therefore unique for the MM and MMD estimation in synthetic polymers by MS techniques. 

Following the characterization of the human genome sequence and the identification of specific proteins coded for by the genes, this has led to a resurgence of interest in protein structure determination, or structural genomics as it is sometimes called. There are a number of important 3D NMR experiments that are used for assigning the peaks in NMR spectra of macromolecules such as proteins, which have been obtained fully labeled with and N. Additionally, 3D NMR spectroscopy has been used to investigate the structures of synthetic polymers. 

Another important application of Py-GC/MS techniques is the evaluation of contamination caused by industrial wastes consisting of usual polymers such as PVC, PS, poly(vinyl acetate) (PVA), polybutadiene (PB), poly(acrylo-nitrile-co-styrene-co-butadiene) (ABS), styrene-butadiene random (SBR) and styrene-butadiene-styrene block (SBS) copolymers. The presence of synthetic polymers in environmental samples is indicated by anomalously high levels of styrene and benzene in the pyrograms, and by the detection of selected markers (e.g., chlorobenzene for PVC, acetic acid for PVA, benzenebutanenitrile for ABS, cyclohexenylben-zene for styrene-butadiene rubbers), which are useful for better identification of individual polymers. This method was applied to a particular case of contamination in an Italian lake near an industrial area. " " ... 

Although natural fibers have been used for thousands of years, synthetic fibers have only existed since the 1930s. They make up the majority of fibers used today. Raman spectroscopy has recently been used as a rapid and accurate method to separate fiber and film types for recycling. The chemical identification of these polymers is carried out in the same manner as discussed for the natural fibers in the preceding sections. Maddams [35] and Edwards et al. [36] review the use of Raman spectroscopy for the identification of polymers and studies on the kinetics of polymerization. The qualitative identification of polymers is obtained through the observation of characteristic vibrational bands, as discussed in Sections II.B and ILC. For unoriented polymers, the relative amount of comonomers can be determined quantitatively. A series of different nylons highlights this approach in the subsection on nylon. 

R-12 Levy EJ, Wampler TP. Identification and differentiation of synthetic polymers by pyrolysis capillary gas chromatography. J Chem Educ 1986 63 A64—8. 

One of the most useful studies was the division of synthetic polymers into four categories based upon solubility water-soluble polymers, polar organic soluble polymers, nonpolar organic soluble polymers, and polymers soluble only in difficult solvents such as dimethylsulfoxide, hot 1,2-dichlorobenzene, or sulfuric acid. This subdivision facilitates identification of appropriate MALDI matrices, depending upon the characteristics of the polymer. For water-soluble polymers, matrix conditions are similar to peptides or proteins. 2,5-DHB was used to analyze PEG, PPG, and PMMA at a concentration of lOg/L in water/ ethanol (9 1 ratio). " Various metal salts were added to increase cation adduction. 

One major drawback of ESI for the analysis of synthetic polymers is the necessity of using suitable solvents, which are essential for a sufficient spray formation and ionization. The fact that most synthetic polymers are soluble in solvents such as tetrahydroftiran (THF) or chloroform rather than in typical ESI solvents (e.g., water, acetonitrile, and methanol) still limits the use of ESI for polymer analysis. Moreover, since tn/z deaeases with increasing chromatographic flow rates (which reduces the efficiency of ion formation), relatively low flow rates have to be applied. All these problems are probably the main reason for a significantly lower number of ESI-polymer publications compared to the MALDI method. Nevertheless, the nirmber of applications of MALDI and ESI-MS for polymer separation and identification increased steadily since their first reports and reached a more or less constant number in the past 5 years (see Figure 5). 

The Textile Eiber Product Identification Act (TEPIA) requires that the fiber content of textile articles be labeled (16). The Eederal Trade Commission estabhshed and periodically refines the generic fiber definitions. The current definition for a polyester fiber is "A manufactured fiber ia which the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of an ester of a substituted aromatic carboxyUc acid, including but not restricted to terephthalate units, and para substituted hydroxyben2oate units."... 

Latexes of synthetic resins are identified by ir spectrometry. Selective extraction with organic solvents is used to obtain purified fractions of the polymers for spectrometric identification. Polymeric films can be identified by the multiple internal reflectance ir technique, if the film is smooth enough to permit intimate contact with the reflectance plate. TAPPI and ASTM procedures have not been written for these instmmental methods, because the interpretation of spectra is not amenable to standardization. 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

Obviously, use of such databases often fails in case of interaction between additives. As an example we mention additive/antistat interaction in PP, as observed by Dieckmann et al. [166], In this case analysis and performance data demonstrate chemical interaction between glycerol esters and acid neutralisers. This phenomenon is pronounced when the additive is a strong base, like synthetic hydrotalcite, or a metal carboxylate. Similar problems may arise after ageing of a polymer. A common request in a technical support analytical laboratory is to analyse the additives in a sample that has prematurely failed in an exposure test, when at best an unexposed control sample is available. Under some circumstances, heat or light exposure may have transformed the additive into other products. Reaction product identification then usually requires a general library of their spectroscopic or mass spectrometric profiles. For example, Bell et al. [167] have focused attention on the degradation of light stabilisers and antioxidants... 

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. 

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