Identification of polymers by IR spectroscopy.
As an aid to vibrational spectroscopists and polymer scientists, this article discusses some of the most important commercial polymers and their infrared spectra. Identification of polymers is ideally suited to infrared analysis, but has become more difficult with the many blends of polymers that now exist. Spectra can be obtained as either a pressed or cast film, or by attenuated total reflectance. Other instrumental techniques would require the decomposition of the polymer and identification of its fragments.
Two dozen of the most important commercial polymers and their infrared spectra are discussed. This is followed by a flowchart which should prove useful in determination of the polymer type or identification of the source of an extra peak. The latter is usually the strongest peak in the spectrum of a minor component. The discussion that follows assumes that there are either no plasticizers or fillers present or that they have been removed from the polymer by extraction, dissolution and filtering.
The simplest polymer structure would be a chain of methylene units terminated on each end by methyl groups. This is the structure of polyethylene, or PE. Since the polymer is composed almost completely of methylene groups, its infrared spectrum would be expected to consist solely of methylene stretches and bends. Four sharp peaks dominate the spectrum: The methylene stretches at 2,920 and 2,850 [cm.sup.-1] and the methylene deformations at 1,464 and 719 [cm.sup.-1]. Due to the crystallinity of polyethylene, the 1,464 and 719 [cm.sup.-1] peaks are split, and additional peaks are seen at 1,473 and 731 [cm.sup.-1]. High density polyethylene (HDPE) is very regular and is about 70% crystalline. Low density polyethylene (LDPE), on the other hand, is more branched and is only about 50% crystalline. The crystallinity of a polyethylene sample can be determined from the ratio of the 731 to 719 [cm.sup.-1] peaks (ASTM D5576).
If all the hydrogens in polyethylene were replaced with fluorine atoms, we would have another important plastic called polytetrafluoroethylene. This is better known by its tradename, Teflon. Because fluorine atoms are more massive than hydrogen atoms, the C[F.sub.2] stretches are much lower in frequency than the C[H.sub.2] stretches. These peaks appear at 1,200 and 1,146 [cm.sup.-1]. The C[F.sub.2] deformation peaks are also lower in frequency and do not appear on the spectrum shown here. Fluorine atoms are highly electronegative, which gives the CF bond a large dipole moment. The result is an extremely strong infrared absorption. The spectrum here was collected by ATR to provide a sufficiently short pathlength of polytetrafluoroethylene so that the asymmetric and symmetric stretches could be distinguished.
The addition of a methyl side group on every other carbon atom in polyethylene gives us polypropylene and quickly complicates the infrared spectrum. In addition to the methylene, we now have methyl and methine groups present. The methyl peaks appear at 2,962/2,952 (split peak), 2,868 and 1,377 [cm.sup.-1]. A methyl deformation is also overlapped with the methylene deformation, and this peak has shifted slightly to 1,458 [cm.sup.-1]. The methine peaks are weak and of no analytical value.
An elastomer can be made from the copolymerization of ethylene and propylene. The ethylene content of EP rubber usually ranges from 50 to 75%. A diene is regularly added to provide sites for crosslinking. The diene content in EPDM is customarily less than 10%, and infrared peaks due to this component are thus very weak. The spectrum is dominated by the methylene peaks at 2,925, 2,854, 1,464 and 721 [cm.sup.-1]. The methyl peak at 1,377 [cm.sup.-1] is also significant. The methyl stretches can be seen as shoulders at 2,952 and 2,871 [cm.sup.-1].
If the group on polypropylene were a chlorine atom rather than a methyl group, the polymer would be polyvinylchloride. With one chlorine atom attached to every other carbon atom, the degree of chlorination will be 56.7%. The methyl stretches and bends have disappeared and are replaced by CCL stretches at 688 and 615 [cm.sup.-1]. Also prominent in the infrared spectrum is a strong band with a maximum at 1,255 [cm.sup.-1]. This is due to C[H.sub.2] wagging when the adjacent C atom has a chlorine atom attached. A methine wag can be seen at 1,200 [cm.sup.-1]. The methylene asymmetric stretch peaks at 2,912 [cm.sup.-1] with a methine C-H stretch peaking at 2,970 [cm.sup.-1]. The methylene scissors deformation is split at 1,435 and 1,427 [cm.sup.-1]. PVC is sometimes heavily plasticized with phthalates or phosphates, and its infrared spectrum can be obscured until the plasticizer has been extracted.
Chlorinated polyethylene elastomer
When polyethylene is chlorinated, there will not be a chlorine atom on every other carbon atom. This chlorinated polyethylene elastomer, CPE, has a degree of chlorination between 25 and 47%. We would expect the infrared spectrum of CPE to be similar to that of PVC, with perhaps stronger C-H stretches and bends and weaker C-Cl stretches. The C-Cl stretches appear at 660 and 609 [cm.sup.-1]. The methylene wag appears as a broad band at 1,263 [cm.sup.-1]. The methylene stretches at 2,929 and 2,856 [cm.sup.-1] are present, and the methylene bend at 1,460 [cm.sup.-1] is also present, but shows some splitting due to the variety of geometrical structures within the polymer.
Closely related to chlorinated polyethylene is chlorosulfonated polyethylene. The latter usually contains between 24% and 43% chlorine and 1% to 1.5% sulfur. The sulfonyl chloride group gives rise to an asymmetric S[O.sub.2] stretch at 1,369 [cm.sup.-1] and a symmetric S[O.sub.2] stretch at 1,160 [cm.sup.-1].
When the side group on the methylene chain is an aromatic ring, the infrared spectrum becomes a combination of methylene and mono-substituted aromatic ring peaks. The polystyrene spectrum has dominant peaks at 2,926 and 2,851 [cm.sup.-1] from the methylene stretches. The out-of-plane C-H bends of the aromatic ring are intense at 698 and 756 [cm.sup.-1]. The aromatic ring breathing modes appear at 1,601, 1,493 and 1,452 [cm.sup.-1]. The peaks at 3,082, 3,061 and 3,027 [cm.sup.-1] are absorptions from the aromatic C-H stretches.
Polyvinylacetate has the infrared spectrum of a typical acetate ester. The dominant peaks are the carbonyl stretch at 1,739 [cm.sup.-1] and the C-O single bond stretch of the acetate group at 1,242 [cm.sup.-1]. Also significant are the methyl deformation at 1,373 [cm.sup.-1] and the C-O single bond stretch of the polymer backbone carbons at 1,022 [cm.sup.-1].
Polyvinylalcohol is produced by the reaction of polyvinylacetate with methanol. The extent of hydrolysis in commercial plastic is between 79 and 100%. The spectrum should be a fairly simple combination of methylene and hydroxyl vibrational peaks. It is, however, complicated by the electronegativity of oxygen. The resulting hydrogen bonding between hydroxyl groups produces a wide hydroxyl stretch at 3,410 [cm.sup.-1] and wide hydroxyl deformations at 1,334 and 592 [cm.sup.-1]. The methylene stretch is split at 2,910 and 2,943 [cm.sup.-1]. The methylene scissors deformation can be seen at 1,450 [cm.sup.-1]. The C-O stretch appears at 1,095 [cm.sup.-1], as is typical of secondary alcohols. Sometimes polyvinylacetate peaks are seen at 1,739 and 1,239 [cm.sup.-1] when hydrolysis is not complete. Peaks at 1,568 and 1,412 [cm.sup.-1] due to sodium acetate and/or peaks at 1,746 and 1,246 [cm.sup.-1] due to methyl acetate by-products are sometimes discernable.
EVA plastic is a combination of polyethylene and polyvinylacetate, with the latter usually contributing 7.5% to 33% of the weight of the polymer. The infrared spectrum is a combination of the peaks of the two components as listed above.
When diene monomers add to each other, the resulting polymer contains unsaturation in its backbone. Natural rubber is composed almost entirely of cis-polyisoprene, while gutta are primarily trans-polyisoprene. Besides 1,4-addition, 1,2- and 3,4-addition structures can be introduced into synthetic polyisoprene. The methylene stretches at 2,925 and 2,854 [cm.sup.-1], the methyl stretch at 2,962 [cm.sup.-1] and the deformations at 1,450 and 1,377 [cm.sup.-1] dominate the infrared spectrum. Also prominent is the =C-H out-of-plane wag at 837 [cm.sup.-1]. Smaller but distinctive peaks exist at 3,035 [cm.sup.-1] for the =C-H stretch, 2,727 [cm.sup.-1] for the overtone of the methyl deformation and 1,664 [cm.sup.-1] for the C=C stretch.
When the methyl group of polyisoprene is replaced with a chlorine atom, the polymer is known as polychloropene. Between 85 and 92% of the polymer is formed from trans 1,4-addition of the monomer. Around 2.5% of the polymer contains vinyl groups with the chlorine atom on either the or [gamma] carbon. The remainder consists of cis 1,4-addition product. Total chlorine content is about 40% by weight. Dominant peaks include the 2,920 [cm.sup.-1] methylene stretch, the C=C stretch which is split between 1,695 and 1,660 [cm.sup.-1], and the methylene deformation which is split between 1,444. and 1,431 [cm.sup.-1]. The 1,119 [cm.sup.-1] peak, which is probably related to the methylene wag deformation, and the 825 [cm.sup.-1] peak, which is probably due to the =C-H bend, are also strong. The C-C1 stretches occur at 658 and 602 [cm.sup.-1]. Also noticeable are the 3,309 [cm.sup.-1] overtone of the C=C stretch, the 3,024 [cm.sup.-1] =C-H stretch and another methylene deformation band at 1,304 [cm.sup.-1].
Butyl rubber is produced from the polymerization of isobutylene with a small amount of isoprene. The dominant peaks are due to the methyl stretches at 2,954 and 2,897 [cm.sup.-1], the methyl/methylene deformation at 1473 [cm.sup.-1], the gem-dimethyl deformation at 1,390 and 1,367 [cm.sup.-1], and the mostly methyl rock vibration at 1,230 [cm.sup.-1]. Weaker methyl rock vibrations are also visible at 924 and 951 [cm.sup.-1]. Halogenated butyl rubber contains only 1 to 2% of chlorine or bromine by weight. Due to this very low level, it is difficult to distinguish halogenated from non-halogenated rubber by infrared spectroscopy.
1,3-butadiene can be polymerized to polybutadiene. Various proportions of different microstructures have found commercial use. This varies from almost pure cis-substitution to almost pure trans-substitution. The most significant infrared peaks are due to the methylene stretches at 2,945 and 2,854 [cm.sup.-1] for cis- and 2,918 and 2,846 [cm.sup.-1] for trans-, the =C-H stretch at 3,008 [cm.sup.-1], and the partially split methylene bend at 1,436 and 1,450 [cm.sup.-1]. Peaks due to out-of-plane bending of hydrogens on the double bonded carbons are strong when there is a high percentage of a given microstructure present. The upper spectrum has a strong and broad peak at 741 [cm.sup.-1] for the cis C-H bends, while the lower spectrum has strong peaks at 912 and 993 [cm.sup.-1] for the vinyl component and at 966 [cm.sup.-1] for the trans component.
Nitrile rubber (NBR) is produced from the copolymerization of butadiene and acrylonitrile. As such, its infrared spectrum is essentially that of trans-polybutadiene with a sharp peak at 2,238 [cm.sup.-1] from the nitrile triple bond stretch. Carboxylation of NBR with acrylic or methacrylic acid provides additional sites for crosslinks. The carbonyl band appears as a split peak at 1,699 and 1,732 [cm.sup.-1]. The C-O single bond produces a peak at 1,222 [cm.sup.-1]. Although hydrogenation is never 100% complete, HNBR can be readily distinguished by the significant reduction in the 966 [cm.sup.-1] peak upon removal of the unsaturation.
Styrene butadiene rubber
A typical emulsion SBR contains about 23% styrene and a butadiene microstructure of 18% cis, 65% trans and 17% vinyl. The microstructure of solution SBR is usually higher in cis content than emulsion SBR. The dominant infrared peaks are due to the methylene stretches at 2,925 and 2,854 [cm.sup.-1] and to the out-of-plane aromatic C-H deformation at 700 [cm.sup.-1]. Other significant peaks are related to the aromatic ring breathing mode at 1,603, 1,495 and 1,452 [cm.sup.-1], the aromatic C-H deformation at 760 [cm.sup.-1] and the trans and vinyl double bond out-of-plane C-H bending at 968 [cm.sup.-1] and 995 and 912 [cm.sup.-1], respectively.
Epichlorohydrin homopolymer contains 38% chlorine by weight, while the 1:1 copolymer of epichlorohydrin and ethylene oxide contains 26%. The additional chlorine content gives EC a better fuel resistance, while the ethylene oxide provides ECO with better low temperature flexibility. The C-O stretch produces a broad band centered at 1,110 [cm.sup.-1]. The C-Cl stretches appear at 706 and 746 [cm.sup.-1]. The intensity of the symmetric methylene stretch is enhanced relative to the asymmetric stretch. This produces a broad multi-peaked absorption in the 2,850 to 2,975 [cm.sup.-1] region. A number of methylene deformations appear as medium intensity peaks in the 1,250 to 1,475 [cm.sup.-1] region.
Despite the presence of a carbonyl group, at 1,774 [cm.sup.-1], the strongest infrared absorptions in polycarbonate are due to the C-O single bond stretches. An intense triplet appears at 1,165, 1,194 and 1,228 [cm.sup.-1]. Additional C-O related peaks continue down to 1,000 [cm.sup.-1], with the strongest of these at 1,016 [cm.sup.-1]. Among the aromatic ring breathing modes, only the 1,506 [cm.sup.-1] peak is relatively intense. While the methyl stretch at 2,970 [cm.sup.-1] is weak, it is noticeable due to the absence of other peaks in this region.
Bisphenol epoxy resin
Since both bisphenol epoxy and polycarbonate are based on Bisphenol A, there are a number of similarities in their infrared spectra. There is no carbonyl band in the bisphenol epoxy spectrum, but the aromatic ring-breathing mode at 1,510 [cm.sup.-1] is very strong. Here the 1,610 [cm.sup.-1] ring-breathing mode is also relatively strong. The C-O stretch is strong and appears as two bands, a broad band with a maximum near 1,247 [cm.sup.-1] and a narrower and slightly weaker band with a maximum near 1,182 [cm.sup.-1]. Significant intensity is also seen in the out-of-plane aromatic C-H wag at 830 [cm.sup.-1].
Polyethylene terephthalate, and to a lesser extent polybutylene terephthalate, have become synonymous with the term polyester. The carbonyl stretch is lowered to 1,717 [cm.sup.-1] by conjugation with the aromatic ring. The second strongest peak at 1,261 [cm.sup.-1] is generally described as the asymmetric C-C-O stretch that involves the carbon in the aromatic ring. The O-C[H.sub.2]-C[H.sub.2]- asymmetric stretch is split at 1,128 and 1,099 [cm.sup.-1]. The aromatic C-H wag is also affected by the carbonyl group and shifted down to 723 [cm.sup.-1].
Poly(acrylates and methacrylates)
Polymers with esters as ligand groups are usually named for the monomeric repeating unit that includes the ester group. In addition to polyvinylacetate above, two of the more important polymers of this type include the acrylates and methacrylates. These are useful where hard, clear coatings are needed.
The most intense peak is the carbonyl between 1,730 and 1,737 [cm.sup.-1]. The C-O stretch forms a broad split band between 1,150 and 1,200 [cm.sup.-1] with an additional weaker broad band with a maximum between 1,240 and 1,265 [cm.sup.-1]. This additional band has two distinct maxima in polymeth-acrylates near 1,240 and 1,270 [cm.sup.-1].
Polyamides, better known as nylons, are made from the polymerization of lactams or the condensation of diamines with dicarboxylic acids. The number following the name, e.g., nylon 6 or nylon 6,6, indicates the number of carbon atoms in the starting material(s). The strongest peaks are those of the Amide I and Amide II bands found near 1,640 and 1,545 [cm.sup.-1], respectively. Also, the N-H stretch near 3,300 [cm.sup.-1] is very intense. Different nylons can be distinguished by subtle differences in frequencies and intensities in the Amide III band between 1,260 and 1,280 cm-1, and in the symmetric methylene stretch and the methylene deformation bands.
Polydimethylsiloxane is the most common of the silicone rubbers. The Si-O-Si backbone produces a broad band with two maxima at 1,092 and 1,018 [cm.sup.-1]. The methyl deformation band at 1,261 cm-1 is strong and sharp, and the methyl rock with contribution from the Si-C stretch at 798 [cm.sup.-1] is also strong. The asymmetric C[H.sub.3] stretch at 2,964 [cm.sup.-1] is also sharp and relatively strong.
Infrared spectroscopy is the most direct means of identification of polymers. Through the use of the accompanying flowchart and reference to the spectral descriptions above, an unknown polymer can be quickly identified. When additional peaks are present in the infrared spectrum, they may be the strongest peaks from an additional polymer which has been blended with the main polymer. They may also be due to unremoved plasticizer or fillers or to residual solvent used to dissolve the polymer.