21 Polymers - Macromolecules

Introduction

The purpose of this chapter is help those interested in the characterisation/identification of polymers. It is not the intention of this chapter todeal with the theoretical aspects of the vibrational spectroscopy of polymers(infrared'"8-38 or Raman) nor to deal with the sampling methods for the twotechniques.'-5"8-3950 There are many good books dealing with these aspects.However, as will be appreciated, it is not possible to deal with the characterisation of polymers without some mention of these aspects but this will bekept to a minimum. For example, in dealing with sampling techniques, theaim is merely to give an idea of what commonly-available techniques may beapplied. It is also not the intention of this chapter to list recent developmentsin the field.

The vast majority of functional groups present in polymers give rise to bandsin the infrared region.1-8-38-44 Hence, vibrational spectra can be used to identify polymers through the use of group frequencies or simply by attemptingto compare the spectrum of an unknown with that of reference spectra. Thislatter approach can run into difficulties when dealing with copolymers or polymers that have been modified in one way or another - for example, by theaddition of fillers or by being chemically modified, or where there are crys-tallinity differences between samples. In addition to providing data to enablethe identification of polymers, vibrational spectroscopy can also yield valuableinformation on the microstructure of a polymer. This includes configurationaland conformational information on the structure, how successive monomerunits were added to the chain in both homo- and copolymers and the identification of end-groups and of defects.

It must be emphasised that infrared and Raman spectroscopy should not beused to the exclusion of other techniques such as 'H and 13C nuclear magneticresonance, which are particularly useful characterisation techniques. Otneruseful techniques are mass spectroscopy, ultraviolet-visible spectroscopy,chromatography, thermo-analytical techniques (such as differential scanningcalorimetry (DSC), thermal gravimetry (TG) etc.), or combined techniquessuch as GC-MS (gas chromatography combined with mass spectrometry)

or chromatography (liquid)combined with mass spectroscopy etc. Suchtechniques may either yield additional information or provide the confirmationof a group or some other aspect that is required. As an example, nuclearmagnetic resonance (NMR) and DSC may be used to distinguish between ablend or copolymer of two amorphous polymers, whereas this cannot easily bedone using either infrared or Raman spectroscopy. Simple techniques shouldalso not be ignored as they can save a great deal of time, for example, density,copper-wire flame test, etc.

A question often asked is, 'For polymer analysis, which is the better technique, infrared or Raman?' There is no simple answer since it depends onthe task in hand. Even though great improvements have been made in laserRaman spectroscopy, the technique is still considered to be inferior to infraredspectroscopy for the characterisation and analysis of polymers. Some of thereasons for this are as follows:

1. Raman spectrometers tend to be more expensive than those of infraredand so are less common and therefore not readily available to the analyst.On the other hand, infrared is generally available in most laboratories forroutine analysis and is a very versatile technique.

2. If good Raman spectra are to be obtained, more skill is required by theinstrument operator and analyst than is the case for infrared, both in theexperimental aspects and in the interpretation of the spectra obtained,although in many cases, sample preparation for Raman spectra is simplerthan for infrared.

3. Infrared spectrometers and techniques and accessories are more establishedthan those of Raman.

4. The acquisition of Raman spectral data has, in the past, been relatively slow,although in recent years great improvements have been made in this area.

5. One major advantage of the use of infrared spectra is that there is a vastbase of reference spectra which can easily be referred to. In the case ofRaman spectra, the reference libraries, although much better nowadays, stilldo not compare with those available for infrared.

6. Fluorescence has been a major source of difficulty for those using Ramanspectra. Historically, this has led to the acquisition of poor spectra or in

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spectroscopy, it is possible to examine bands which occur in relativelyinterference-free regions. For example, the alkene C=C stretching vibrationband occurs near 1640cm-' (~6.10um) where few other functional groupsabsorb. The C=C stretching vibration band is strong in Raman spectra.On the other hand, the alkene CH out-of-plane vibration bands are oftenoverlapped in the infrared spectra of polymers, making assignments usingthese bands difficult/impossible.

8. In some cases, the infrared sample preparation techniques that may berequired for the examination of a particular sample may destroy or modifythe characteristics of interest. For Raman, very little, if any. sample preparation may be required.

9. Raman spectrometers are capable of covering lower wavenumbers (downto 100cm"' or lower) than those of infrared (400 or 200cm-') and socan reveal information relating to polymer structure (see below) not easilyavailable using other techniques.

Certainly for the routine analysis of polymers, infrared is by far the morepopular but both techniques have their advantages and disadvantages.

Pretreatment of Samples

Polymers are difficult to characterise, not because their infrared or Ramanspectra are complicated, difficult to interpret or consist of broad overlappingbands, as, in general, they do not, but because so many different substancesare added to polymers for one reason or another. For example, fillers may beadded to modify the physical or chemical properties of the basic polymer or itsappearance. Fillers may be added to alter the mechanical, thermal, electricalor magnetic properties of the final product. Of course, fillers may also be usedsimply to make the product cheaper by the addition of inexpensive substancessuch as chalk, glass, wood shavings, silica, or air (gas bubbles). In addition,other substances such as lubricants (to assist in the processing of the polymer),heat stabilisers (to prevent thennal degradation during processing), pigments(for colouring), plasticisers, antioxidants, UV stabilisers, fire retardants, etc.,are added to polymers. The list is endless, basically, just about anything maybe added to polymers. In some cases, it may be difficult to believe that twosamples arc based on the same polymer, for example, polyvinylchloride (PVQplasticised and unplasticised.

It must also be remembered that, different preparative techniques may beused by different manufacturers with very different conditions. These maylead to the same polymer having similar but quite different characteristics.Different catalysts may be involved and, in some cases, the catalyst may still be

bound/remain in the polymer. Different catalysts may be used to form stereo-regular polymers. Also, the basic polymer unit may be modified chemicallyeg, polyethylene and chlorinated polyethylene.

It must also be remembered that, unlike organic or inorganic compounds,the 'molecules' are not all identical, they do not all have the same relativemass (molecular weight) (ignoring isotopic variation). Copolymers are alsocommonly encountered in many everyday products. The proportions of themonomers and/or the sequencing employed may vary from manufacturer tomanufacturer or be varied in order to obtain the properties required. The stereochemical nature of the polymer, its micro-structure, crystallinity, may have abearing on the spectrum obtained.

Polymers are very versatile materials and are used in many different products. For example, they are encountered in many different guises: fibres, paints,coatings, rubbers, adhesives, packaging, and are even used in food products,etc. Some products may appear to be a single polymer but are actually laminates or composed of mixtures of polymers. Composites may, for example,contain fibres or materials in another form and these substances may be organicor inorganic in nature.

Some polymers exist in equilibrium with water. In such polymers, theremay be as much as 2% water so that additional bands due to water willnormally be observed in their spectra. For example, for polyetherketones andpolyethersulphones, bands near 3650 cm-' (2.74 pm) and 3550 cm-' (2.82 um)may be observed. In order to avoid complications due to the presence of water,particularly if quantitative measurements are to be made, some polymers mayneed to be thoroughly dried before their spectrum is recorded.

It is seen that the spectrum of a sample may consist of the basic polymerspectrum on which are superimposed the spectra of the additives, fillers,lubricants, fire retardants, catalyst residues, contaminants, etc. in proportionsrelating to their concentrations in the sample. Hence, there are many reasonswhy difficulties may be encountered when examining polymeric samples.Obviously, there are advantages if the problem can be made simpler, perhapsby separation, eg. use of chromatography or solvent extraction etc. With certainpolymeric samples, it may be possible to use selective solvent extraction but,in some of these samples, this may result in fine particles of carbon blackremaining in suspension and being difficult to remove.

A simple technique used to determine the inorganic filler incorporated ina polymer is simply to burn the sample (place the sample in a furnace) andspectroscopically examine the residue. It may also be possible to determinethe polymer by pyrolysing the sample and examining the pyrolysate.

In Raman spectroscopy, polymer samples often exhibit fluorescence due tocontaminants on their surfaces. Wiping the sample with a solvent, e.g. acetoneor alcohol, can reduce this fluorescence. Alternatively, taking a thin slice offthe surface of the sample can also be helpful.

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Gr-—*^ Polymers - Macromolecules 265

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various spectral changes are observed, the bands become broader and oftennew bands appear. These new bands are due to the vibrational motions ofdifferent conformations and/or rotational configurations of the parts of thepolymer chains present in the disordered phases.

Heating a polymer sample will, in general, result in the broadening of bandsas the crystalline arrangement is destroyed. The opposite is also true: as apolymer is cooled, and hence crystallises, its bands become narrower. It is veryimportant to bear in mind, when examining the spectrum of a polymer, that aband should not be assigned as originating from the crystalline arrangementunless (a) it disappears on melting, (b) it is predicted by group theory andcan be shown to depend on the presence of the crystal lattice. However, it isnot always possible to ascertain that these conditions have been met Hence,vibrational spectroscopy cannot always be thought of as a good method fordetermining the crystallinity of a polymer. It should also be borne in mindthat conformational regularity may also be associated with amorphous regionsand, for example, with orientated, but not necessarily crystalline, arrangements.When in doubt, either use another technique, such as X-ray diffraction or usesuch a technique to justify the infrared (or Raman) approach to be adopted.Even when it can be shown that bands are a result of the crystallinity ofthe polymer, their intensities cannot be relied on to be a good measure ofthe degree of crystallinity and a calibration plot must be made. In addition,different crystalline arrangements of a polymer may, in fact, have commonabsorption bands. Hence, if a polymer is polymorphic, care must be taken inmaking assignments and determinations.

If a polymer sample absorbs the exciting radiation significantly, it willbecome hotter and hence the bands will become broader. If no account istaken of the fact that radiation is strongly absorbed by the sample, it ispossible to determine incorrectly the phase transition (crystalline-amorphous)temperature.

The degree of crystallinity and the amorphous content of polyethylenecan be determined.1 The degree of crystallinity may be determined from theintegrated intensity of the band near 1415 cm"' (7.07 um) and the amorphouscontent from the intensities of the bands near 1300cm-' (7.69um) and1080 cm"1 (9.26 urn) . However, these days, the degree of crystallinity ofa polymer is often determined by correlating whole spectra or spectralregions with X-ray or differential scanning calorimetry (DSC) measurements,using such techniques as partial least squares for calibration purposes.5'-55For example, the density of polyethylene may be determined by usinga partial least-squares calibration employing micro-Raman spectroscopy-53For the determination of the amorphous content of polytetrafluoroethylene,PTFE, a univariate method based on peak heights in the infrared regioncan be used.51-52 Fourier transform Raman spectroscopy may be used tomeasure the crystallinity of polyetheretherketone in isotropic and uniaxial

samples using univariate and partial least-squares calibrations.54-55 Fouriertransform Raman spectroscopy has been used to examine the crystallinityof polyethyleneteraphthalate.55

All crystalline polymers experience low-frequency vibrations along theirchain - in effect, the chain is compressed and extended. The forces restrainingthis motion act along the axis of the chain and are very much smaller thanthose of the internal vibrations of groups. The frequencies of these vibrationsare dependent on the Young's modulus of the crystal along the axis of thechain. Since these motions occur at very low frequencies, they are referredto as longitudinal acoustic vibrations.45 It should be pointed out that thesemotions can also occur along the transverse axes as well. In general, thefrequencies of these motions are well below 200 cm"1 and the bands are notreally of any use in the identification or characterisation of polymers. However,these vibrations can yield information relating to the morphology of polymers.Normally Raman techniques are employed to observe these acoustic vibrations.

Non-crystalline PolymersAn individual, non-crystalline polymer chain may adopt a large number ofrapidly interchanging rotational conformations relative to itself and its neighbours and hence the theoretical analysis of the polymer for spectroscopicpurposes, using symmetry and fundamental vibrational modes, is impossible.The only approach which may be adopted for the analysis of such spectrais that based on the examination of the repeat unit, plus the end-groups, andtreatment of the polymer as a liquid. As mentioned earlier, the spectra ofnon-crystalline polymers tend to involve bands that are broader than might beexpected if the polymer had been crystalline.

If a vibration involves hydrogen bonding, or is affected by the conformational changes that occur, then the band may be very broad. On the otherhand, if the band is relatively insensitive to external influences then the bandmay be quite sharp. For example, the spectrum of high density polyethylene(HDPE) has relatively sharp bands when compared with that of low densitypolyethylene (LDPE). The spectrum of non-crystalline, atactic polystyrene hasbands due to the aromatic ring which are relatively sharp whereas other bandstend to be a little broader than for the crystalline, isotactic form.

It should be noted that, due to their lack of a uniform consistent structure,non-crystalline polymers do not exhibit lattice or acoustic bands.

Band Intensities

The intensities of bands are related to the concentrations of the functionalgroups producing them, allowing quantitative analysis if required. Provided

266the normal precautions are taken and calibration is feasible, good results maybe obtained. It must be borne in mind that, in general, for various reasons,using Raman for quantitative analysis is a little more difficult than usinginfrared which is quite straightforward. These days, it is fair to say that Ramanexcitation sources, lasers, are very much more stable and there is very littleproblem in making quantitative measurements provided the usual precautionsare taken.

An oriented sample, such as a drawn polymer film, exhibits different vibrational spectra when the orientation of the sample relative to the direction oflinear polarised electromagnetic radiation is altered. In other words, it shouldbe bome in mind that, in the presence of polarised radiation, the relative intensities of bands may be affected. The interaction between the polarised electricfield of the radiation and the dipole moment associated with the vibrationbecomes a maximum or minimum depending on the angle between these twovectors, 0° or 90°. Hence, in polarised light, the spectra of stretch-orientedpolymers exhibit dichroism.4 Dichroism may also be observed in the stressedareas of a polymeric sample. The dichroic behaviour of a sample can provideinformation on (a) the direction of the vibrational modes, (b) the orientation ofthe functional group in the crystalline lattice and (c) the fraction of the perfectorientation in the oriented sample. The monitoring of the dichroism can beused to monitor the production of oriented polymeric films. This is commercially important as the physical properties of drawn samples are related to thedegree of orientation.

As an example of dichroic behaviour, consider the infrared spectra ofpolyethylene. Polyethylene may be considered as a long chain of CHi unitswith its end groups, branching and any double bonds being ignored. Polyethylene molecules align themselves along the drawn axis and, as a result, theintensities of the bands due to the asymmetric and symmetric stretching vibrations reach a maximum when the electric field of the polarised radiation isperpendicular to the drawn axis, whereas the band associated with the waggingvibration reaches a maximum when these two are parallel. Other bands inthe infrared spectra of polyethylene exhibit similar behaviour with regard toorientation and polarised light. It should be noted that polyacetylenes exhibitanomalous dichroic behaviour.9

Applications - Some Examples

Introduction

There are so many polymers and copolymers and so many possible variationsthat the account given below can do no more than give selected examples in

Infrared and Raman Characteristic Group Frequencies

order to indicate the characterisation possibilities. When examining commercial products and artefacts, it must be borne in mind that, as already mentioned,the base polymers may contain numerous other products - stabilisers, fillers,etc. It should also be borne in mind that the relative intensities of bands inthe spectra of copolymers are dependent on the proportions and sometimesthe sequencing of the components present in the copolymer unless, of course,the bands are common to both units.

The difference in intensities observed for various compositions of a particular type of copolymer may be used to determine the composition of thecopolymer, that is, the relative amounts of each monomer unit present. In thesimplest case, where a particular band is due solely to one component of thecopolymer, then either the absorptivity may be determined or a calibrationgraph constructed for this purpose. For systems where a band position freefrom the absorptions of other components of the copolymer cannot be found,a slightly lengthier approach is required. The absorptivities at various suitablelocations in the spectrum must be determined for each component and then,by taking measurements for a variety of concentrations of the componentsin the copolymer at these different locations, equations can be constructedto determine the composition of an unknown copolymer. An example of thisapproach is the determination of the individual isomers of butadiene copolymers, ris-1,4-, trans-1,4- and 1,2-polybutadiene . From the solution spectra(using a suitable solvent such as carbon disulphide) of the individual components, which may be obtained separately, the absorptivities of each isomermay be determined at suitable points in the spectrum of the copolymer andhence used directly in the three equations required for the quantitative determination. In a similar manner, the isomer compositions of isoprene36-47 andchloroprene may be determined.

By making use of infrared, the pathlength of a liquid cell may be determinedby measurement of the interference pattern observed. In the same way, theinterference pattern often observed in the infrared spectra of thin polymericfilms may be used to determine the thickness of the film. A knowledge ofthe refractive index of the polymer is required for this determination. If theincidence of the radiation is not normal then the angle of incidence is alsorequired, see the equation below.

d = N2(v2-vi)(;i2-sin2t9)1/2,

where d is the thickness measured in cm,n is the refractive index of the film,/V is the number of peaks between the wavenumbers i'i and i

measured in cm-1,and 9 is the incident angle of the radiation.

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Polymers - Macroniolecules 267

The interference pattern in the infrared spectra of thin polymeric films maygive rise to difficulties when attempting to observe weak bands. This problemcan be overcome in several ways, the simplest being to place the film on aninfrared transparent (in the region of interest) plate or simply, when castinga film, to leave it adhered to the transparent plate and to examine it directlyallowing for compensation if necessary. It should be borne in mind that theinfrared spectra of laminates may also exhibit an interference as a result ofthe interaction of reflections at boundaries and radiation transmitted directly.

Depending on the nature of the sample and the information required, it maybe advantageous to use infrared, or Raman, or both techniques in a study.Remember that groups that have weak bands in Raman may have strongbands in the infrared, for example. OH, C=0, C-O, S=0, S02. P=0, P02.NO-., etc arc all strong in infrared. The C=C, C=C, C=N, C-S, S-S.N=N and 0-0 functional groups result in strong bands in Raman spectraand usually weak bands in infrared. All these groups are commonly found inmany polymers.

Stereoregttlarity, Configurations and ConformationsThese days catalysts exist for the preparation of many stereorcgular polymers.The mechanical properties and spectra of the different stereoregular isomersof a particular polymer and also its atactic form may differ significantly.In general, it is obvious that stereoregular polymers may easily formcrystals when they solidify. From the commercial point of view, sincethe different configurations of a polymer have different properties, it isessential to know the isomeric composition of a sample bearing in mindits intended application. When compared with the vibrational spectrum ofthe atactic form, the spectra of stereoregular isomers appear to have morebands and many of the bands are sharper. For example, the spectrum ofisotactic polypropylene0'41 has numerous additional sharp bands in the region1350-800cm"' (7.41-12.50um). The spectrum of isotactic polystyrenediffers significantly from that of the atactic form, which has broader bands.The same is true of the syndiotactic forms. Hence, in general, the spectra ofstereoregular isomers have additional sharp bands when compared with spectraof the atactic form.

For some polymers, different conformers may be possible, for example,polyethylene terephthalate40 has two conformational isomers gauche andtrans. For the gauche isomer, the -0-CH-.-0- group has its oxygen atomsslightly displaced from each other, whereas in the case of the trans form, theoxygen atoms are opposite each other. The spectra of both forms are quitedifferent, additional bands being observed for each form. Two characteristicbands for the gauche form occur near 1140cm"1 (~8.77um) and 890cm"1

(~11.24 um). Two characteristic bands for the trans form occur near 970 cm '(~10.3I um) and 840cm"1 (~l l.90um).

Some polymers, in addition to having different conformers, may alsohave conligurational isomers associated with some of the conformers. Thesedifferent arrangements may all be observed in their vibrational spectra.Polyvinylchloride is an example where bands originating from differentisomers are commonly observed. In its infrared spectrum, a broad absorptionis observed in the region 750-550cm"1 (13.33-18.18urn), 'his being due toa number of overlapping bands some of which can be quite distinct.

Morphology - Lamellae and SpherulitesSome polymers, when they solidify from a melt, form crystals which havethe appearance of being composed of thin, flat platelets, lamellae which areabout 0.1 nm thick and many micrometres wide. In some cases, the polymercrystallites may be arranged in groups with their axes arranged radially. Thesegroups form features known as spherulites. Spherulites are often many timeslarger than crystallites and can sometimes be seen by the naked eye. Themorphology of a polymer has a great bearing on its mechanical strength andstability and hence is of great interest. Infrared and Raman spectroscopy maybe used to study the morphology of polymers.

The frequency bands due to longitudinal acoustic vibrations which, asmentioned previously, are not observed in infrared spectra but occur inRaman spectra, are inversely proportional to lamella thickness. These bandsare usually difficult to observe. For example, a low frequency band due toa longitudinal acoustic vibration has been found in the Raman spectra ofpolyethylene and polypropylene which is related to the chain length and thelamellar thickness.2,3 The longitudinal acoustic vibration is dependent on theforce constant (dependent on the chain's longitudinal Young's modulus), theinterlamellar forces, structure of the chain folding sequence, the proportionsof the amorphous and crystalline components and the density of the polymer.

C=C Stretching Band

An advantage of using Raman spectroscopy is that the C=C group, whichis commonly found in many polymers, has a stretching vibration resultingin a strong band (in infrared this band is. generally, weak or in inactive)and hence can often be used to determine polymer conformations in which itoccurs, determine the extent of curing or cross-linking, or follow the chemicalkinetics of polymerisation.2" For example, the different isomers of butadienemay be distinguished by using Raman and examining the band due to theC=C stretching vibration.18,1*22 Bearing in mind that in certain instances

268 Infrared and Raman Characteristic Group Frequencies

fluorescence may present problems, polymers containing aromatic groups mayalso be easily examined by the use of Raman techniques. Of course, the sameis true of polymers which are similar, such as isoprene26-48 and chloroprene.

Thermal and Photochemical Degradation

As a result of thermal degradation, both polyethylene and polypropylene formhydroperoxide groups. These groups are not easy to detect by infrared, especially as the O-O stretching vibrations result in a very weak band and, inaddition, the concentration of the hydroperoxide is low, although it should beborne in mind that peroxides give a strong Raman band. The OH stretchingband may also be difficult to observe as it only results in a medium intensity band. Fortunately for infrared analysts, hydroperoxide groups react toform a variety of carbonyl-containing compounds. It is usually possible todetect bands due to ketones which absorb near 1720cm"1 (~5.81 um), aldehydes, near 1735cm"1 (~5.76um) and carboxylic acids, near 1710cm"1(~5.85um). In a given sample, these bands are often observed to overlapone another. The carboxylic acid band near 1710cm"1 (~5.85um) may beremoved by converting the acid into a salt by treating the sample with arelatively strong alkali. The band due to the salt CO>~ group, occurs near1610cm"1 (~6.2I irm). In the case of photochemically decomposed samples,in addition to bands due to the various carbonyl groups, bands due to the vinylgroup are also observed, occurring near 910cm"1 (—10.99um) and 990cm"1(~ 10.10 urn). By making use of an infrared microscope, it is possible, forexample, to monitor the effects of weathering on a polymeric sheet by examining a cross-section of the polymer sample at positions relating to variousdepths.

Due to the sensitivity of Raman spectroscopy to the -C=C-vibrationalmotion, a strong signal being observed, the degradation of polyvinylchlo-ride, PVC, may be studied by making use of resonance Raman techniques.The degradation of PVC results in the loss of hydrogen chloride gas and theproduction of carbon-carbon double bonds, leading eventually to sequencesof conjugated polyenes.

Polyethylene and Polypropylene

Polyethylene has strong bands in its infrared spectrum near 2950 cm"1(~3.39um) and 1460cm"1 (~6.85um) and a band of medium intensity,which is often a doublet, near 725cm"1 (~13.79um). These bands are dueto the CH stretching, deformation and rocking vibrations. If the polymer hassignificant branching then additional weak bands near 1380cm"1 (~7.25um)

and 1365 cm-1 (~7.33um) are observed.42 These bands are usually observedin the spectra of samples of low density polyethylene, LDPE.

The methyl groups of chain-branched polyethylene have, as mentionedabove, a weak band near 1380 cm"1 (~7.25um). On the other hand, methylene groups have a stronger band at 1365 cm"1 (~7.33um) which overlapsthis methyl band. By making use of the spectra of linear polyethylene anddeconvolution techniques, it is possible to determine the degree of branching.

Linear low density polyethylenes (LLDPE), are low-concentration ar-olefinmodified polyethylenes. The olefines usually used are propylene, butene,hexene, octene, 4-methyl pentene-1. Due to the relatively high concentrationof methyl groups in linear low density polyethylenes, greater intensities arenormally observed for the bands associated with the CH3 group than areobserved for high density polyethylenes (HDPE) so care must be exercisedwhen making assignments.

As a result of its commercial preparation, the chemical structure of polyethylene may also contain double bonds. The percentage of double bonds may beestimated by making use, in the infrared, of the band due to the vinyl CHout-of-plane deformation vibration which occurs near 910cm"1 (~l0.99um)and, in the Raman, of the C=C stretching band near 1640cm"1 (~6.10um)and determining the ratio of the intensities of these bands compared with otherbands in the polyethylene spectrum. Some commercial low-density polyethylene, prepared using high pressures, contains vinylidene groups which, in theinfrared, absorb near 890 cm"1 (~11.24 urn) due to the CH2 out-of-planedeformation vibration. Polyethylene prepared using Ziegler catalysts oftencontains defects resulting in the presence of vinyl, vinylidene and trans-vinylene groups. The positions of the CH out-of-plane deformation vibrationbands for vinyl and vinylidene have been mentioned above, that for trans-vinylene is near 965 cm"1 (~ 10.36 urn).

It is often true that the infrared and Raman spectra of samples have fewsimilarities. In the Raman spectrum of polyethylene, the C-H stretchingvibration bands are very strong and those due to rocking vibrations, near725 cm-1 (~ 13.79 pm), are very weak or absent. In addition, in Raman spectra,the skeletal vibrations give characteristic bands near 1300cm"1 (~7.69um),1130cm"' (~8.85um) and 1070cm"1 (~9.35um).

The infrared spectrum of polypropylene31 has strong bands near 2950 cm"1(~3.39um), 1460cm-' (~6.85um) and 1380cm"1 (~7.25um). In addition,bands of medium intensity are observed near 1155 cm"1 (~8.66um) and970 cm"1 (~ 10.31pm). For isotactic polypropylene,32-41 a number of sharpbands of medium intensity are observed in the region 1250-835 cm"1(8.00-11.98 pm). In the spectra of the molten, or atactic form, ofpolypropylene, most of these sharp bands disappear, except for the bjtndsnear 1155 cm"1 (~8.66um) and 970cm"' (~10.31 pm). For some samples of

a.. . li• * • • f

c l .

93

Polymers - Macromolecules 269

polypropylene, a band near 885 cm-1 (—11.30pm) is observed which may bedue to the CH out-of-plane motions of an end group. -(CHi)C=CH2.

Some ionomers are based on polyethylene with carboxyl groups locatedalong the carbon chain. These carboxyl groups allow for the cross-linkingof chains to occur by means of ionic bonds. Metal ions, such as sodium,potassium, magnesium and zinc, form the cationic link. The infrared spectraof ionomers are composed of the bands due to polyethylene mentioned above.In addition, bands are observed for the carboxylate portion near 1640 cm"1(~6.I0pm), 1560cm"1 (~6.41pm) and 1400cm"1 (~7.l4um). Bands arealso observed in the region 1350-1100 cm"1 (7.41-9.09 um) which have theirorigin in the CHj-acid salt structure.

PolystyrenesIn its vibrational spectra, polystyrene has a strong band due to the =C-Hstretching vibration between 3100 and 3000cm"1 (3.23-3.33pm). In general.this band may be observed for aromatic or olefinic components (or both). Itspresence in the polystyrene spectrum, together with that of a medium intensity band at 1600cm"1 (~6.25pm), indicates aromatic, rather than olefinic,components. This band is due to one of the aromatic ring-stretching vibrationswhich occur in the region 1600-1430 cm"1 (6.25-6.99 pm).

The very strong bands observed in the infrared spectrum near 760cm"1(~13.I6tim) and 690cm"1 (~!4.49pm) confirm the presence of a monosub-stituted aromatic group. These bands are due to the CH out-of-plane vibrationand a ring out-of-plane deformation respectively. The overtone and combination bands which occur in the region 2000-1660cm"' (5.00-6.02pm)also indicate the presence of a monosubstituted aromatic. The positions ofthese bands are approximately 1940cm"1 (~5.15um). 1870cm"1 (~5.35p.rn),1800cm"1 (~5.56um). 1740cm"1 (~5.75pm) and 1670cm"1 (~5.99um).The band due to the C-H stretching vibration of the aliphatic group occursbetween 3000-2800 cm"1 (3.33-3.57 pm). The bands due to the aliphatic CHdeformation vibrations are in their typical positions.

In addition to the bands due to polystyrene, the spectrum of styrenc-butadiene copolymer contains bands which may be associated with thebutadiene component. A band near 1640cm"1 (~6.10um). due to theC=C stretching vibration, and strong bands near 965 cm"1 (~10.36pm) and910cm-' (~l0.99pm). due to the CH out-of-plane vibrations, are observed inthe infrared spectra of this copolymer. Bands due to the different isomers ofbutadiene may also be observed. The CIS-1,4-butadiene isomer which absorbsweakly near 730 cm"1 (~ 13.70pm) is often overlooked due to the presence ofthe strong bands of styrene. The 1.2-isomer and the /rimv-I.4-isomer absorbstrongly near 965cm"1 (~10.36pm) and 910cm"1 (~10.99um). Hence, the

relative proportions of the 1.2- and the trans-1,4-isomcrs present in the sampleaffect the spectral region 1000-900cm-' (10.00-11.11 pm).

In addition to the bands mentioned in the previous paragraph, the infraredspectra of acryloniirile-butadiene-styrenc copolymers will contain bands dueto the acrylonitrile component. The additional presence of the characteristicband due to the nitrile group, which occurs near 2240cm"1 (~4.46pm), in arelatively band-free region of the infrared range, is a good indicator for thiscopolymer. It should be noted that the nitrile group gives a strong band inRaman spectra. Conformers of polyacrylonitrile57 have been studied.

Polyvinylchloridc, Polyvinylidenechloride, Polyvinylfluoride,and Polytetrafluoroethylene

The infrared spectrum of polyvinylchloride contains the bands typical ofaliphatic CH groups, except that the band due to the CH-. deformation vibration is shifted by about 30cm"1 to lower wavenumbers. to near 1430cm"1(~6.99pm). In addition to the aliphatic CH bands, the spectra of PVC containcontributions due to the C-Cl vibrations.31 For example, a broad, strong bandis observed in the region 710-590cm-' (14.08- 16.95pm) due to the C-Clstretching vibration. Since there are a very large number of additives possible,great care needs to be taken in the analysis of PVC samples. A band near1720cm"1 (~5.81pm) is often observed in the infrared spectra of commercial samples of PVC. This band may be assigned to a carbonyl group presentin the plasticiser employed and hence is assigned to the C=0 stretchingvibration.

Polyvinylidenechloride has a strong doublet in its infrared spectrum near1060 cm"1 (~9.43pm) and strong bands due the =CCI2 stretching vibrationsnear 660cm"1 (~!5.15pm) and 600cm"1 (~16.67pm). A band of mediumintensity is also observed near 1420cm"1 (~7.04pm) due to CH deformationvibrations.

Polyvinylfluoride has a strong band near 1085cm"1 (~9.22pm) due tothe C-F stretching vibration. The bands near 2940cm"1 (~3.40pm) and1430cm"1 (—6.99pm) are due to CH stretching and deformation vibrationsrespectively. Polytetrafluoroethylene31 has a strong absorption in the region1250-1100cm"1 (8.00-9.09 pm) apart from which the region .above 650cm"1(below 15.38 pm) is relatively free of absorptions. The weak band near2330cm"1 (~4.29pm) is due to the overtone of the CF2 stretching vibration.Polyviiiylidenelluoride has a relatively weak band near 2940cm"1 (~3.40pm)due to the CH stretching vibration. However, the CH2 deformation vibrationis stronger than might be expected. The spectrum of polyvinylidenefluoride isgreatly affected by the sample preparation techniques used.

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272 Infrared and Raman Characteristic Group Frequencies

|

and hence there are no changes due to crystallinity to result in problemsdetermining relative compositions. In Raman spectra, the intensity ratio ofthe components of the doublet near 1600cm-1 (~6.25um) is related tothe composition of the copolymer. The ratio of the intensities of bandsnear 1200 cm-'similar way.

(~8.33um) and 1070cm ' (~9.35um) may be used in a

Polyconjugated Molecules

Polyconjugated systems have been studied in undoped. doped (with electrondonors or acceptors) and photoexcited states.16 The spectra obtained for aparticular polymer in these three different states are quite different. Some ofthe spectral features observed are common to all polyconjugated polymersin these different states. Some of the features are related to the existence ofa network of delocalised ,t electrons which can be considered to be alonga one-dimensional lattice. An important parameter which affects the spectralfeatures observed is the conjugation length.10 As might be expected for polyconjugated polymers,9 as with most polymers, stretch-oriented samples in thedifferent states exhibit dichroism.'1 Various polyconjugated systems have beenstudied - poly acetylene, polythiophene, phenylpolyene, etc.16

The infrared spectra of undoped polymers do not show unusual features.Their spectra may be interpreted in the normal way by using group frequencycorrelations or vibrational analysis. In general, the vibrational frequencies ofgroups are independent of the number of mers in an oligomer. As the numberof mers is increased, the spectra become simpler, since the relative intensityof the bands due to the end-groups decreases compared with those due to themajority of units in the chain •core'. The chain length may be determined bymeasuring the ratio of the intensities of bands due to vibrations from thesetwo types of group - end and core. Doped and photoexcited systems exhibitdichroism.12 The Raman spectra8 of undoped polyconjugated polymers, suchas polyacetylenes, are simple - in general, only a few bands are observed.These bands are due to the C —C stretching vibration, which occurs in theregion 1500-1400cm-1 (6.67-7.14 um). and have a relatively strong intensity, and bands due to C-C and CH wagging vibrations, which are observedin the approximate region l200-900cm-1 (8.33-11.11 um). In general, thestrongest bands in the Raman spectra exhibit some degree of dispersion." thefrequency decreasing with increase in the number of conjugated units.5 Forpolyconjugated systems, the Raman scattering cross-sections of some groupshave been observed to increase rapidly with increase in the number of conjugated units.10

The infrared and Raman spectra of doped or photoexcited polyconjugatedsystems are very different from those of the equivalent undoped material.6'New bands which are extremely strong and complex are observed. These new-

bands result in a broad, poorly-defined pattern in the region 1600-700 cm-1(6.25-I4.29tim). The position (frequency) of the bands in the infrared spectraof doped samples appears to be independent of the doping species.1'4 althoughband intensities may decrease with increased doping level. The frequencies ofvibrations do not appear to alter with concentration of the doping species.13In a few oligomers, the frequency of some bands observed in the spectra ofdoped species and photoexcited species decrease with increase in the numberof conjugated units.

In general, it has been found that the infrared spectra of photoexcited anddoped materials are very similar.6 It has been found that for some samples,for example mi/i.v-polyacetylene. bands in the spectrum of the photoexcitedspecies experience a red-shift with respect to the doped material. In general,however, the spectra of the two are almost identical. Very slightly dopedmaterials usually have very similar Raman spectra to those of the equivalent undoped system. The Raman spectra of doped and photoexcited speciesusually have broad, weak bands and are often not observed unless resonanceenhancement conditions can be achieved. For example, for polyacetylene,15 atlarge doping levels, new bands appear near 1600 (~6.25um) and 1270cm-1(~7.87um). At very high doping levels, some characteristic bands of undopedpolyacetylene become weak and Raman scattering becomes weak.

The vibrational spectra of polyconjugated systems can only be interpretedby taking both the molecular structure and the electronic structure (.t electrons)into account.

Polyacetylene in the c/.v-forin undergoes a solid slate thermal isomerisation togive the /-/YH/.v-isomer. The infrared spectra of the cis- and 'ra/js-isomers are quitedifferent.6 The out-of-plane CI I vibration of the m-isomer results in a strongband which occurs near 735 cm-1 (~I3.61um) at room temperature. Whentime-dependent spectroscopy is used to examine the c/.v-isomer at an elevatedtemperature, this band is observed to decrease in intensity with time and moveto slightly higher waveiiumbers. approximately +7 cm-1. Similarly, the trans-isomer, which has its out-of-plane CH vibration near 1015cm-1 (~9.85um).when examined at elevated temperature, increases in intensity and moves tolower waveiiumbers. approximately —5 cm-1. Other bands in the spectra ofthese cis- and /ra;i.v-isomers are also observed to change position slightly.

ResinsThe infrared spectra of phenol formaldehyde resins have a broad, strongband at about 3350cm"1 (~2.99um) due to the OH stretching vibrationof the phenolic group. Another strong band is observed near 1230cm-1(~8.13 um) due to the C-O stretching vibration. A doublet is usually observedat 1600cm-1 (~6.25um) due to a stretching vibration of the aromatic ring.Strong bands are also observed in the infrared spectra of phenol formaldehyde

f r ! ' $ )Polymers - Macromolecules 273

£f fi ^

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resins at about 760cm"1 (M3.16um) and 820cm-1 (~12.20um) due to thearomatic CH out-of-plane vibrations. These last two bands indicate that thearomatic ring has formed both ortho and para bonds. Novolak resins haveonly one strong band in their infrared spectra in the region 900-730 cm-1(ll.ll-13.70um), this occurring at about 760cm"1 (~13.16um).

Uncured resole resins have a strong band near 1010cm"1 (~9.90um) dueto the CO stretching vibration of the methylol group. As curing takes place,the intensity of this band decreases. Care needs to be exercised, as hexamine,which is often used as the cross-linking agent, has a weak absorption in thesame region.

The infrared spectra of melamine-formaldehyde resins contain a band dueto the OH stretching vibration near 3350cm-' (~2.99um), a strong, broadband near 1560 cm"1 (~6.41 um), principally due to the stretching motions ofthe triazine ring, a broad, medium-to-strong band near 1040 cm"1 (~9.62um),due to the C-O stretching vibration, and a band near 820cm"1 (~6.41um),again due to the triazine ring. The infrared spectra of urea-formaldehyde havea broad, strong band at about 3350 cm-1 (~2.99um) due to the OH stretchingvibration, a broad, strong band near 1570cm"' (~6.37um), a strong bandnear 1040 cm"1 (~9.62um), due to the C-O stretching vibration and a broadband near 625 cm"' (~ 16.00urn).

C o a t i n g s a n d A l k y d R e s i n s <For environmental and other reasons, these days, many coatings/paints arewater based. This means that coatings must either be first dried or cells withwater-stable windows must be used if infrared spectra4 are required. In addition, large regions in which water absorbs strongly may obscure sample bandsthat need to be observed. Heavy water can be used to expose the regions inwhich the water bands cause concern but this, in general, is not helpful ifcommercial samples need to be examined. On the other hand, water is nota problem if Raman spectra can suffice. Polymerisation and curing reactionsmay be followed spectroscopically.48

With dried coatings, reflection techniques can be employed to obtain infraredspectra. An emulsion or latex can be examined by similar techniques to thosedescribed above. Solvent-based coatings can be examined either directly inliquid cells or as dried films. Usually the evaporation of the solvent can bemonitored spectroscopically. Alkyd resins which are solvent based still forma substantial part of the commercially available coatings. The band due to theC=C stretching vibration may be used to follow the curing process.

ElastomersElastomers30-36-56 present numerous problems with regard to the acquisitionof vibrational spectra. For infrared spectra, special techniques must be used

for sample preparation.1 In the past, using Raman, fluorescence has restrictedthe number of elastomers and the manner in which they could be studied. Itwas only for pure, unvulcanized elastomers that spectra could be obtained.Certainly, near infrared Fourier transform Raman spectrometers have helpedresolve these difficulties. Of course, samples containing high proportions ofcarbon black can still present problems, especially with regard to Ramanspectra. In the case of Raman spectroscopy, the absorption of the laser excitation source by the carbon can lead to rapid heating of the sample anddegradation of the elastomer or, when a spectrum is obtained, a high baselineis observed, resulting in a poor spectrum where weak bands are lost.

It should be borne in mind that the cooling or stretching of certain elastomerscan lead to crystallisation. Hence, the morphology of such samples can bestudied using vibrational spectroscopy.

When spectra can be obtained by Raman, the bands due to the groupsC=C and S-S, which occur in many elastomers, are strong and easy toobserve. The band due to the C=C stretching vibration occurs near 1600 cm-'(~6.25um) and that due to the S—S stretching vibration occurs near 480cm-'(~20.83 um). By making use of Raman spectroscopy, it is possible to studyvulcanisation and identify different types of sulphur linkage, for example disul-phide, polysulphide, thioalkane and thioalkene.

Polyisobutylene58 has a strong, sharp band near 1220cm-1 (~9.20um)and a characteristic absorption due to the two CHj groups, near 1385 cm-1(~9.20p.rn) and 1365cm-1 (~7.33um).

Silicone rubbers have a very strong, broad absorption in their infraredspectra at HOO-lOOOcm-1 (9.09-10.00um) due to the Si-O-Si stretchingvibration. The band may be split into two broad peaks. The symmetric CHjSideformation vibration occurs at 1265cm"1 (~7.9I um) and, because it isstrong and sharp, it is easily identified even in the presence of other functionalgroups/substances. Another useful band occurs near 810cm-' (~12.35um)and is due to the Si-C stretching vibration and CH, deformation vibration. TheOH group, which is found in some silicone rubbers, absorbs near 3340 cm"'(~2.99um) due to the O-H stretching vibration. The Si-0 stretching vibration results in a broad band in the region 900-835cm-' (11.11-11.98um).However, this band may be obscured by bands associated with the S1CH3group.

Plasticisers

Strongest Band(s) in the Infrared SpectrumIn this section, the characteristics bands are given for various common polymerplasticisers.

Ik

274 Infrared and Raman Characteristic Group Frequencies

Strongest bands near 2940cm"1 (-3.40/xm) and 1475cm-1 (~6.78uw)Substances whose strongest bands appear near 2940 cm-' (~3.40um) and1475 cm-1 (~6.78um) are primarily aliphatic hydrocarbons. Hydrocarbonoils have additional weak bands near 835cm-1 (~ll.98um) and 715cm"1(~l3.99um). In addition to the two strong bands mentioned above, pureparaffin wax has a doublet near 725 cm-' (~ 13.79 um). Similar spectra are alsoObserved for aliphatic chlorinated hydrocarbon mixtures but with an additionalbroad band near 1260cm-1 (~7.94|.im).

Strangest band near lOOOtw-1 (—10.00/im) Phosphates, which arecommonly found in many plasticisers, have their strongest bands nearIOOOcm-1 (—lO.OOum) in their infrared spectra. For aliphatic phosphates,this band is usually at slightly higher waveiiumbers whereas, for aromaticphosphates, this band is usually at lower waveiiumbers, near 970cm-'(—10.31 um). It should be borne in mind that some phosphate plasticisersare mixtures and hence their vibrational spectra may differ from sample tosample. For example, some aromatic phosphate plasticisers are mixtures ofisomers and the relative intensities of their bands in the region 835-7I5cm-1(11.98-13.99um) will vary according to the composition of the plasticisers.

Strangest band near 1100 cm-1 (-9.09 pm) Ethers and alcohol-ethersusually have the strongest band in their infrared spectra near 1100cm-'(-9.09 um). In the case of alcohol-ethers, a band due to the hydroxy I groupis also normally observed near 3340cm-1 (-2.99 uni). Numerous alcohols areused, some being long chain aliphatics, such as lauryl and stearyl alcohols,others being polyhydric in nature, such as sorbitol and sucrose.

If the long chain alcohols can be examined spectroscopically in thecrystalline phase, then the weak bands in the region 1000-910 cm-1(10.00-10.99 um) may possibly be used to identify them. With regardto polyhydric alcohols, the region 1100-1000cm-1 (9.09-10.00pm) maysometimes be used for identification purposes.

Strongest band in the region 835-715 cm~' (11.98-13.99/z/w) Theinfrared spectra of aromatic hydrocarbons and chlorinated aromatichydrocarbons normally have their strongest band in the region 835-715 cm-1(11.98-13.99um). Aromatic hydrocarbons are often encountered as resins andmay be of such low molecular weight as to be viscous liquids. Examples ofaromatic hydrocarbons are methyl styrene and styrenc/butadienc copolymerand polycumarone/indene, the latter having an intense band near 750cm-1(—13.33 pin). It should be borne in mind that, for copolymers, the intensitiesobserved throughout the spectrum will be dependent on the composition of the

copolymer. These days, the hazardous nature of certain chlorinated aromatichydrocarbons is known.

Characteristic Absorption Patterns of Functional Groups Present in Plasticisers The characteristic absorption patterns of some common functionalgroups that appear in plasticisers are discussed below.

Carbonyl groups Carbonyl groups in one chemical form or another arecommonly found in many plasticisers. Plasticisers containing carbonyl groupsare discussed below.

Carboxylic acids Ether extracts often contain carboxylic acids either asfree acids or as salts. The carboxyl group absorbs strongly near 1700 cm-1(—5.88 pm) and has a broad band due to the OH stretching vibration. In addition, as most plasticisers are long chain aliphatic acids, a band is observednear 2940cm-1 (—3.40pm), due to CH stretching. In the case of high molecular weight aliphatic acids which are obtained in a reasonably pure state, theirspectra are quite distinctive. The principal differences occur in the region1280-1 ISO cm"' (7.81-8.47 um), where they have weak absorptions, andthese differences may be used for identification purposes. For example, theinfrared spectrum of lauric acid has three weak, clear, sharp bands in thisregion whereas that of stearic acid has live weak bands in this region.

Carboxylic acid stilts In their infrared spectra, carboxylic acid salts havetwo relatively strong bands near 1590cm-' (—6.29pm) and 1410cm-1(~7.09p.rn). The position and shape of the band near 1590cm-' (—6.29pm)is dependent on the anion but there is little to recommend infrared as a meansof identifying the metal anion by this method as there are simpler and morepositive methods, such as atomic emission spectroscopy.

Care should be exercised with o-hydroxyl benzophenone, as it has a strong,broad absorption in its infrared spectrum near 1590cm-' (—6.29pm) but nostrong absorption near 1410cm-1 (—7.09pm), with a hydroxy! band near3340cm-1 (—2.99 pm) also being observed, in addition to the aromatic bands.

Ortho-Phthalates Phthalates absorb at the positions give in the tablebelow. Table 21.1. It should be borne in mind that some solvent extracts frompolymeric samples may be due to the presence of o-phthalate resins ratherthan plasticisers.

Since o-pluhalaies have a very distinctive infrared spectrum, they areeasily recognised. If. in the infrared spectrum of a sample, there are noadditional bands having a significant intensity in the region 1500-600cm-1(6.67-16.67 um) then the substance is a simple alkyl phthalatc. In a relatively

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Polymers - Macromolccules 275

Table 21.1 Phthalates

Region cm pm Intensity Comment

3090-3075 3.24-3.25 m CH str3045-3035 3.28-3.31 W CH str2000-1660 5.00-6.00 w Ortho overtone pattern1740-1705 5.75-5.87 vs C=0 str. Overtone ~3550cm-11610-1600 6.21-6.25 w-m Ring str1590-1580 6.29-6.33 w-m Ring str1500-1485 6.67-6.73 m Ring str1310-1250 7.63-8.00 vs asym COC str1170-1110 8.55-9.09 s sym COC str1070-1040 9.35-9.62 m770-735 12.99-13.61 s Out-of-plane CH.def. usually 745cm"'410-400 24.39-25.00

pure state, the lower members of the alkyl series can be distinguishedby careful examination of the weak bands in the region 1000-835 cm-1(10.00-11.98 pm). Examples of these simple alkyl phthalates are dimethyl,diethyl, di-n-butyl, ... di-irr-octyl etc. The higher alkyl esters are difficult toidentify unambiguously unless they possess some significant structural feature.For example, phthalates containing a gem-dimethyl group have a doublet dueto the CHj deformation vibrations at 1385-1365 cm-1 (7.22-7.25 um). Hence,in general, the esters of higher members of the alkyl series are more difficultto identify by infrared alone. They usually have a single broad band in theregion 950cm-' (—10.53pm).

In addition to the usual bands in the region 1000-900 cm-1(10.00-11.11 pm), di-allyl phthalate has a sharp band near 1650cm-'(~6.06pm) due to the C=C stretching vibration.

The infrared spectra of samples exhibiting the normal distinctive o-phthalate pattern but. in addition, possessing sharp weak bands in the region1110-835 cm-1 (9.01-11.98 pm) may be associated with the alicyclic esters.For example, the spectra of cyclohexyl esters possess a number of sharp bandsof medium intensity in the region 1430-715 cm-1 (6.99-13.99 pm).

Phthalates derived from aromatic alcohols or phenols, such as diphenylphthalate and dibenzyl phthalate, are relatively easy to distinguish. In additionto the fi-phthalate pattern, their infrared spectra also contain bands due to thearomatic substitution pattern in the region 835-670cm-1 (11.98- 14.93pm)and the band due to CH stretching vibration near 2940 cm-1 (—3.40 pm) isvery weak, especially when compared with the relative band intensity at thisposition for the alkyl phthalates. However, the possibility of mixed alkyl-arylesters should be borne in mind when considering the band near 2940cm-'(~3.40pm) - for example, butyl benzyl phthalate has a band of medium intensity due to the CH stretching vibration. In the case of this phthalate. the region.

1000-900cm"1 (10.00-11.11 pm) may be examined for bands characteristicof the butyl group.

The infrared spectra of complex phthalates possess the characteristic o-phthalate bands but. in addition, usually have strong bands in the region1430-IOOOcm-1 (6.99-10.00pni). For example, the spectra alkyl phthalylalkyl glycollates. such as methyl phthalyl methyl glycol late, have a strongnear 1200cm-' (— 8.33pm) due to the C-O stretching vibration. It should benoted that this band appears near 1160cm-' (—8.62pm) for simple mixturesof phthalates with aliphatic esters. In the case of some glycol phthalates.one hydroxy] group is esterilicd with phihalic acid and the other hydroxy!group is condensed with an aliphatic alcohol to form an ether, for exampledi-methoxy ethyl phthalate. The C-O-C stretching vibration results in anadditional baud of mediuni-to-strong intensity near 1125cm-' (—8.62pm).This band is. generally, narrower and at higher waveiiumbers than those ofsimple alkyl ethers.

Aliphatic esters Although the infrared spectra of aliphatic esters allowsthem to be distinguished from other carbonyl-containing compounds, it isoften difficult to differentiate between esters which are similar in chemicalstructure. It is not sufficient to compare the spectrum of an unknown withthat of a reference and thus to conclude, because the spectra are similar,even after careful attention to the positions and relative intensities of bands,that an identification has been positively made. It is often necessary to carryout hydrolysis of the sample and to examine the alcohol and acid fragmentsseparately, or. alternatively, another spectroscopic technique may be used toidentify the ester directly.

All long chain aliphatic esters have a band near 725cm"1 ( — 13.79pm)which may. in some samples, appear as doublet. Typical compounds are ethylpalmitate and glyceryl di-siearate. Both of these substances exhibit a bandnear 3340cm-' (—2.99pm) clue to the presence of hydroxy! groups. Theobservation of this band is useful in that esters derived from poly-functionalalcohols generally exhibit this absorption.

Plasticisers containing the acetyl group, such as glyceryl triacetate, have astrong band in their infrared spectra near 1230cm"1 (~8.13i.mi).

Esters containing the epoxy group have a band, often a doublet, of weak-to-medium intensity near 835cm-' (-11.98pm).

The spectra of different esters based on the same dibasic, such as alkyladipates or sebacates, are very similar. Replacing the alcohol, in the case ofsimple low molecular weight alcohols, has a greater effect on the infraredspectrum than changing the acid.

As mentioned above, o-phthalate esters also containing an ether linkagenormally exhibit a medium-to-strong band near 1125cm-1 (-8.89pm). Otherplasticisers containing the aliphatic ester group and the ether group, for

276 Infrared and Raman Characteristic Group Frequencies

example those based on di- and tri-ethylene glycol and monocarboxylic acids,have a broad absorption due to the ether C-O-C stretching vibration near1110cm"1 (—9.01pm). In this latter case, the absorption is similar to thatobserved for polyethylene oxide derivatives.

Some plasticisers based on dicarboxylic acids may not only be esters butalso salts. The infrared spectra of these plasticisers obviously contain the bandsassociated with both esters and carboxylic acid salts. This means that it isdifficult to distinguish between a compound and a mixture of an ester and salt.In a similar fashion, it is difficult to distinguish between plasticisers based ondicarboxylic acids and diols and polyesters based on similar compounds. Thereare no easily recognisable spectral features to distinguish between monomelicesters and an equivalent polyester.

Aromatic esters The most common plasticisers based on aromatic estersare benzoates of one type or another. The infrared spectra of benzoates all havea strong band near 715cm-' (—13.99pm). Another reasonably common typeof aromatic ester is that based on salicylic acid. The spectra of salicylates,in common with other aromatic compounds which have a hydroxyl groupadjacent to a carbonyl group (i.e. at the ortho position), do not have the bandnear 3340 cm-1 (-2.99 pm).

of diphenyl amine and /J-phenylene diamine. These substances have spectrawhich are similar to those of suiphonamides. They have one or two bands dueto NH or NH: stretching vibrations respectively near 3340cm-' (-2.99 pm)and a strong band near 1300 cm-' (~7.69pm). However, since thesecompounds do not have the second strong band near 1165cm-' (-8.58pm),this is a reliable way of distinguishing between them and suiphonamides.

Sulphates and sulphonates have strong absorptions in their infrared spectrabetween 1250 and 1110cm-' (8.00-9.01 pm).

A strong band near 3340cm-' (—2.99pm), with one or more strong bandsnear 1250cm"' (—8.00pm). indicates a possible phenolic constituent. Itshould be borne in mind that epoxy compounds, which are often extractedfrom resins, have spectra similar to those of phenols. Since they are usuallyof low molecular weight and have only terminal hydroxyl groups, the O-Hstretching vibration band near 3340cm"' (-2.99pin) is usually of moderateintensity. In addition, it should be noted that /'-aromatic epoxy compoundshave a prominent band near 835cm"' ( — 11.98pm).

Common Inorganic Additives and Fillers

Suiphonamides, sulphates and sulphonates Suiphonamides are easilyrecognised by the strong bands in their infrared spectra near 1315 cm"1(—7.60pm) and 1165cm-1 (—8.58pm). Different sulphonainides may bedistinguished by the number bands, positions and intensities, in the region850-650cm-' (11.76-15.38pm). In addition. /V-substituted suiphonamideshave bands associated with NH rather than NH2, the band-structure of theformer being simpler (see the chapter dealing with amines).

Sttlphonic acid esters Alkyl aryl sulphonic acid esters, such as ethyl p-toluene sulphonate. have two strong characteristic bands near 1350cm-1(~7.41 pm) and 1180cm-' (—8.47 pm) in their infrared spectra. Aryl esters ofalkyl sulphonic acids normally have a strong absorption near 865-650cm-1(11.56-15.38 pm) due to aromatic CH out-of-plane deformation vibrationsand in this respect their spectra are similar to those of aryl suiphonamides but,of course, suiphonamides have additional bands due to their NH stretchingvibrations.

Characteristic Bands of Other Commonly Found Substances

The solvent extracts of resins may contain antioxidants, some of which maybe based on aromatic amines, examples of antioxidants being derivatives

The Inorganic chapter of this book, and some earlier chapters, should also bestudied for relevant information concerning many of the inorganic additivesand fillers commonly found in polymers. The chapters dealing with silicon,boron and phosphorus may also contain information relevant to the inorganiccompounds found in a particular polymer of interest. As mentioned earlier andin the Inorganic chapter. Raman spectroscopy is particularly useful in the characterisation of inorganic compounds that are commonly found in commercialpolymer samples.

Carbonates

The infrared spectra of inorganic carbonates consist of a strong broad bandat 1530-1320cm-1 (6.54-7.58pm) (which in Raman is of weak-to-mediumintensity and is often found near 1450cm-1 (—6.90pm)), a b<tnd of mediumintensity near 1160cm-' (~8.62pm), a weak band at 1100-1040cm-1(9.09-9.62 pm) (which is of strong-to-medium intensity in Raman), a band ofmedium intensity at 890-S00cm-1 (11.24-12.50 pm) and a band of variableintensity at 745-670cm-' (13.42- 14.93pm) (which is of weak intensityin Raman). For calcium carbonate, a strong Raman band is observed near1085 cm-1 (-9.26 pm).

in—una

f e W W

■wsa

( fi m a

T3TB

«r—H5

«r—rid

r i - r ^

t r r s j

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Polymers - Macromolecules

Table 21.2 Calcium carbonate

277

Region IntensityFunctional Groups cm"' um IR Raman Comments

CaCO, 2530-2500 3.95-4.001815-1770 5.51-5.65 w1495-1410 6.69-7.09 vs m-s-1160 -8.62 in1090-1080 9.17-9.26 w s May be absent885-870 11.30-11.49 1 m-s860-845 11.63-11.83 m Sharp-715 -13.99 m-w w705-695 14.18-14.39 m-w w-330 -30.30 vs Broad-230 -43.48 s Sharp

Sulphates

In the infrared, sulphates have medium-intensity bands at 1200-1140cm_1(8.33-8.77pm) and 680-580cm_1 (14.71-17.42pm) (these bands are ofmedium-to-strong intensity in Raman), a strong band at 1130-1080 cm"1(8.85-9.26pm) (which is of medium-to-strong intensity in Raman)and weak bands at 1065-955 cm"1 (9.39-10.47 um) and 530-405 cm"1(18.87-24.69 pm) (in Raman these bands are both of strong intensity).The Raman spectrum of barium sulphate has a strong band near 985 cm"'

Table 213 Barium sulphate

Region IntensityI R R a m a nF u n c t i o n a l G r o u p s c m - 1 u m Comments

BaS04 -3430-2350-1650-1470-13301200-11801130-11101090-10701000-980680-580-725640-630615-605-460-415

-2.92-4.26-6.06-6.80-7.528.33-8.478.85-9.019.17-9.3510.00-10.2014.70-17.24-13.7915.62-15.8716.26-16.53-21.74-24.10

inwwwwsvsm-wm-winwm-wm-www

m-sm-s

sm-swm-sm-sww

Broad

Broad

Sharp/Doublet

Sharp BroadDoublet

(~ 10.15 pm) and weak bands near 1160 cm"1(~8.81pm). 645cm-1 (~15.50pm), 615cm-1(~21.74pm) and 450cin-1 (~22.22pm).

(~8.62pm).(~I6.26pm).

1135 cm-'460 cm-'

Talc

Talc has strong bands at 1030-1005 cm-1 (9.71-9.95 pm ), 675-665 cm"1(14.81-15.04pm), 540-530cm-1 (18.52-18.87pm ) and 455-445cm"'(21.98-22.47 pm).

Clays- iMost clays usually have strong bands in the region 3670-3600 cm

(2.72-2.78 pm), 3450-3400cm-' (2.90-2.94pm) and 500-450cm-1(20.00-22.22pm), a very strong band at 1075-1050cm_1 (9.30-9.52pm),a band of medium-to-weak intensity near 1640 cm"' (~6.10pm), aband at 945-905cm-' (10.58-11.05pm), a weak band at 885-800cm-'(11.30-I2.50pm) and a band of variable intensity at 440-420cm-1(22.73-23.81 pm).

Kaolin normally contains water of crystallisation and as a result has adistinctive absorption pattern in the infrared near 3600cm-1 (~2.78pm).

Table 21.4 Talc

Region IntensityFunctional Groups

Talc

cm"' pm IR

-3685 -2.71-3675 -2.72-3660 -2.731640-1620 6.10-6.17 w1050-1040 9.52-9.611030-1005 9.71-9.95 vs785-770 12.74-12.99 w-740 -13.51700-690 14.29-14.49675-665 14.81-15.04 s540-530 18.52-18.87 s-500 -20.00475-455 21.05-21.98 m455-445 21.98-22.47 vs-440 -22.73-425 -23.53 in

Comments

SharpBroad

Sharp

Sharp

Sharp

a n g

f c E E

Si

Table 21.5 Kaolin

Region IntensityFunctional Groups cm-' pm IR Comment;Kaolin 3710-3695 2.69-2.71 s

3670-3650 2.72-2.74 m-s3655-3645 2.74-2.74 v Usually s3630-3620 2.75-2.76 m-s1650-1640 6.06-6.10 w1120-1090 8.93-9.17 s Sharp1050-1000 9.52-10.00 s1020-995 9.80-10.05 vs960-935 10.42-10.70 m Sharp920-905 10.87-11.05 s Sharp800-780 12.50-12.82 w760-745 13.16-13.42 w700-685 14.29-14.60 in-605 -16.53 w550-515 18.18-19.42 s475-460 21.05-21.74 s435-415 22.99-24.10 s-345 -28.99 w-275 -36.36 w-200 -50.00 w-190 52.63 w

Titanium DioxideTitanium dioxide has strong absorptions at 700-660cm"1 (14.29-15.15 pm)and 525-460 (19.05-21.74 pm), a medium-to-strong intensity band at360-320cm"1 (27.78-31.25pm) and weak bands at 185-170cm""(54.05-58.82pm) and 100-80cm-1 (100.00-125.00pm). The region below200 cm • (above 50 pm), which is easily accessible in Raman, can be used todistinguish between rutile and anatase.

SilicaTable 21.6 Silica

Region IntensityIRFunctional Groups cm"1 pm Comments

Silica 1225-12001175-11501100-1075805-785

8.16-8.338.51-8.709.09-9.3012.42-12.74

m-wm-wvsm

Sharp

imiaieu aim Kaman L.naractenstic Uroup Frequencies

Table 21.6 (continued)

Functional GroupsRegion

cm" pmIntensity

IR Comments795-775725-700670-595525-500490-475465-440-435395-370-260

12.58-12.9013.95-14.2917.93-16.8119.05-20.0020.41-21.0521.50-22.73-22.9925.32-27.07-38.46

mmwm-sm-s

smw

( f e m M M

Table 21.7 Antimony trioxide

Functional GroupsRegion

cm" pmIntensity

IR CommentsSb>Q3 770-740

-685-590415-395385-355-385-345-320-265-180

12.99-13.51-14.60-16.9524.10-25.3225.97-28.17-25.97-28.99-31.25-37.74-55.56

ssVm—wsswwsw

Broad

Antimony TrioxideAntimony trioxide has strong absorption bands at 770-740 cm-1(12.99-13.51 pm) and 385-355cm"' (25.97-28.17pm), a medium-to-weakintensity band at 415-395cm-' (24.10-25.32pm) and a weak band at200-180cm"1 (50.00-54.05 pm).

Infrared Flowcharts

The flowcharts given below have been based on strong bands, bands whichoccur in relatively interference free regions, or bands that are easy to identify. However, in using the flowcharts, it should be borne in mind that thespectra of polymers may differ from those on which the flowcharts havebeen based. This is especially true where copolymers are concerned. With

•■- ' !«

Polymers - Macromolecules279

copolymers, the spectra observed are dependent on the percentages of the individual components present. For example, some styrene-butadiene copolymerscontain certain small amounts of acrylonitrile, this resulting in a band near2220cm"1 (4.50pm). The presence of the band near 2220cm"1 (4.50pm)could be misleading. It should also be borne in mind that polymers preparedby different methods, or using different catalysts, may have slightly differentspectra.

If polymers are examined spectroscopically without removing additives suchas fillers, plasticisers, stabilisers, lubricants, etc. |hen their infrared spectra maybe affected drastically by the presence of these substances. .Also, if care has notbeen taken during the preparation of a sample, bands due to contaminants suchas water, silicate, phthalates, polypropylene (from laboratory ware), etc, mayappear in the spectra and so result in some confusion. Hence, the flowchartsgiven below should be used with some degree of caution. In order to confirman assignment made by use of the flow chart, it is important finally to makeuse of known infrared reference spectra. However, it should be borne in mindthat stereoregular polymers may have spectra which differ from their atacticform and that sample preparative techniques may also affect the spectrumobtained for a particular polymeric sample.

Table 21.8 List of polymers used in flowcharts

Name Abbreviation

Acryloniuile-butadiene-styreneAlkyd resin - aliphaticAlkyd resin - aromaticAramideButadiene acrylonitrile (Nitrile rubber)Butyl rubberCellolose filmCellulose acetateCellulose elher modified ArCellulose etherCellulose nitrateEpoxyEpoxy - AliphaticEpoxy - AromaticEpoxy - Aromatic (Modified)Ethyl celluloseEthylacrylate acrylonitrileEthylene vinylacetateEthylene polysulphonelonomerMelamine-forrnaldehydeMethyl cellulose

ABSALK-AALK-ARARNBRBUTYLCFCACE-MCECNEPEP-AEP-AREP-MECEAANEVAEPSIONMFMC

Chart 21.1 Infrared - polymer flowchart I

Yes-

START HEREStrong band 1810-1700 cm"

-No

Absorption al -3010 & bands at -1600.-1590 and 1495 cm"1?

Ye s _ J

GO TO FLOW CHART II

-No+Strong broad band -830 cm"1?

_ I N oYes-♦

Strong band-1155 cm-1?

Y e s J N o+ +

Medium intensity, sharp band -I4J0 cm" ?

A N o+Yes.+Band

-3490-3330 cm"1?

Yes Jt

-No

PC-AR Suongband Strong band Board band-1050cm"1? -1250cm-'? 710-590cm-'?

V - T N n Y e s — I N o Y e s c * N o♦ + I * + +

Board band710-590 cm"1?

Y e s J N o+ +PVC-PVA Sharp band

-2220 cm"1?

Ye«

Band-3490-3330 cm"1?

I N o

EP-AR.EP-M

Pttl. ALK-AR CE-M PPVCUP-AR

PCT Yes.*

EVA

ALK-A. PUR-A.CA

No

PCA.EAAN.PMCA I

Band-3490-3330 cm"1?

Y e s 1 N o

UP-A Strong band-1800 cm"1?

Yes. -No*

PC-A PMMA. PEA. PVA

Table 21.8 (continued)

Name Abbreviation

Neoprene NPNitrated polystyrene NPSNylon-11 N i lNylon-6, 10 N6I0O.O-Novolac OONOVPhenol -formaldehyde PFPhenolic resin PHRPlasticised polyvinylchloride/vinylidenechloride PVC-PVDCPlasticised polyvinylchloride PPVCPoly(4-methyl penten-1) TPX

(continued overleaf)

Chart 21.2 Infrared - polymer flowchart II

Nei llrong band 1810-1700 cm"1

TAhsorpnon al 3010cm and bands al -1600. -1590 and 1495 cm"'?

V P * N o♦ *Band 3490-3330 cm"*?

Ycs_i+Slrong Kinds al

-7604-690 cm"1?Ye s L .T

- N oY

Ycs-

Band 3490-3330 cm"1?1 N o

-Not YesT

Strong band Strong hand1115-IOOOcm'1? -1665 cm"1?

— I N o Y « s I _♦ +

PUR. Slrong bands I'DI'S. Strong hands Medium IniensiijPF ul-1640.-1560 SI-MI- at-760&-690ein"'? sharp band

& - 1 4 0 0 c m " 1 ? 1 - 1 4 3 0 c m " 1 ?I Y e s — I N o 1

_ I N n 1 1 Y e s 1T

Yest

-Not PPMS.

PVT CF.PUCN

Strong bandION Strong band 1155 cm"1?

- 1 6 6 0 c m " 1 ? Y c s _ J N o

Y e s — I N o I T| | P f O S h a r p b a n d

A R O O N O V . E P - 2 2 2 0 c m " ' ?Y e s — I N o♦ T

ABS.SBR PS .PMS. Yes .S l t K - A N P S T

Strong baud-1540 cm"1?

Yes-I NoT

♦Sheup band

-2220 cm"1?

- N o Y e s T N o

j MP, UPPA, eg. N66,

N6, N610PAM. CN

EPS, PSN

YesiT

EC MC, CD

Slrong bandsal 1360-1300

tt11701120 cm"1?

1 N o

♦Strong band-1055 cm"1')

1 N oT

Five strong bands1250-1000 cm"1?

1 N o♦

YesI

TIIIOK EP-A, POM, PVAL

YesT

SARAN

Suong bond -1055 cm"1?

1 N o

Yes.t

Slrong band 970-925 cm"1?Y e s — I N o

NBR PAN Medium Eniensity,sharp band -1425 cm"1?

1 N o

Slrong broad bands SI -1030. -790at.il slrong band -390 cm"1? Strong bands al -1155. -625. -555 & 500cm"'?Y " ~ i N " Y e s i n o♦ f

POMS Slrong band -845 cm'' ?Y e s — l N o

Sharp band-1610Cm"'? Strong bands -835 ft 780cm"Y e s — I N o Y e s — i N o

tpth-:

Yes.

T* F 7 FI K P V F P C R F i v e s t r o n g b a n d

1250-IOOOcm"1?

Y e s — i N o♦ T

PSS Slrong band710-590 cnr1?

Y e s — I N oT t

Snong band PVEE1335-1215 cm"'?

I N o

♦

Slrong bantls-835 &-780 cm"1?

Y e s — I N oT T

N P S t r o n g b a n d970-925 em"1?

Y e s — I N o

Yes.T

TStrong board band1115-1000cm"1?

1 N o

Snong bantls1150 cm"1?

i N oT

POP siii-s

Yes-I

PVC

Yes-T

pm

PPO-A Strong band970-925 cm"1?

Y e s 1 N o♦ T

PUR S t rong band765 cm"1?

1 N o+PF.. PP. Pill. PI'l. IPX

1'VDC, BUTYL

neccr .

o3c

a1SsoOO

. . . I . . . . . , . , . , . , . , . , . , . , . , . , - ,

Polymers - Macromolecules 281

Table 21.8 (continued) Table 21.8 (continued)

Name Abbreviation Name Abbreviation

Poly-/>-isopropylstyre liePoly-/>methylstyrenePolyacrylamiilcPolyacrylonitrilePolyamide - aromaticPolyamide - aliphaticPolyanhydridePolybutadicnePolybutene-1Polycarbonate - aliphaticPolycarbonate - aromaticPolycarpolactam-Nylon-6PolychloroprenePolycyanoacrylateI'olyilimelhylsiloxanePolydiphenylsiloxanePolyester - aromaticPolyester-aliphatic aminePolyester - aliphaticPolyether - aliphaticPolyelhylacrylatePolyethylene tcrcphthalatePolyethylenePolylicxanielliylcnc adipamidc-Nylon 66PolyiinidePolyisobutylenePolyisoprenePolyisoprene 1,4 cisPolynicthylcyanoacrylatePolymethylmethacrylatePolymethylphenylsiloxanePolymethylstyieiieI'olyoxyiiiethylene-PolyacetalPolyoxypropylenePolypentene-1Polyphenylene oxidePolypliospinc oxide - aliphatic aromaticPoiyphospine oxide - aliphaticPolyphospine oxide - aromaticPolypropylenePolystyrenePolysulphidePolysulphidc-forinalPolysulphonePolytetrafluoroethylenePolyureaPolyurelhane

PPIPSPPMSRAMPANPA-ARPA-APAHPBRPB1PC-APC-ARN6PCRPCAPDMSPDPSUP-ARUP-NHUP-APOEPEAPETPEN66PIMPIBIRIRCPMCAPMMASI-MPPMSPOMPOPPPIPPOPPO-AARPPO-APPO-ARPPPSPSSTHIOKPSNFIFEPUPUR-AR

Polyurelhane - aliphaticPolyvinyformalPolyvinyl fluoridePolyvinylacetalePolyvinylalcoholPolyvinylbutyralPolyvinylchloridePolyvinylchloride vinylacctatc copolymerPolyvinylethylellierPolyvinylidene chloridePolyvinylidene chloride.acrylonitrile.vinylacetatePolyvinylidene fluoridePolyvinylpyrrolidonePolyvinyltoluene-butadienePolyvinyltoluene,p65%.o339eSiec-butylpolysilicnteStyrene- acrylonitrileSty rene -butadieneTri(chloroethyl iphosphateUrea formaldehvde

PUR-APVFLPVFPVAPVALPVBPVCPVC-PVAPVEEPVDCSARANPVDFPVPPVTBPVTSBPSSANSBRTCEPUF

References

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282 Infrared and Raman Characteristic Group Frequencies

9

15.

16.17.18.19.20.21.22.23!24.25.26.27.28.29.30.31.32.33.34.35.36.37.38.

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39.

40.41.42.43.

44.

45.46.47.48.49.50.

51.52.53.54.55.56.57.58.

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