Chapter - VII
VIBRATIONAL SPECTROSCOPIC STUDIES ON
TRANS-1,4-POLYCHLOROPRENE
INTRODUCTION
The role of vibrational spectroscopy in the research and development of polymer
is a prominent one. It provides useful infornlation on the chemical, conformational,
crystal and nlorphological structure of polymers. The study of elastomers by Raman
spectroscopy has for a long time been a desirable. This has allowed quantitative work
on blends and isonleric elastomers and also the direct study of the rubber vulcanization
process. Problerns associated with fluorescence, exaggerated by the presence of
impurities in the elastomer and the addition of cross linking materials and anti-
detergents, have meant that the vibrational spectra of elastomers have been limited to IR
spectroscopy. Where conventional Raman spectroscopy has been used, materials have
been specially selected to be of low fluorcsceilce and have undergone extensive
preparation before examination [ I ] . This is particularly unfortunate as the technique,
with its high sensitivity to the non-polar species (C=C and C-S), which make up
polymer chains and the products of vulcanization, has obvious potential advantages
over the complementary technique of IR spectroscopy.
Raman spectroscopy with laser source finds extensive application in analysing
the vibrational spectra of polymers in conjuction with the infrared spectroscopy.
Vibrational modes important for configurational and conformational studies often are
weak or inactive in the infrared; these bands are strong in the Raman effect. For many
polymers, bands inactive in the infrared are active in Raman and vice versa. Improved
polarisation devices and techniques made orientation studies more meaningful.
Absorption by polymers in the far infrared is due to the low frequency vibration of
heavy atoms or groups and is weak in intensity.
Recent advances, which combine the use of near infrared excitation source with
Fourier transfornm collection techniques 12.31 to produce Raman spectra, have enabled
these problen~s to be substantially reduced or overcome completely. This has triggered
a re-evaluation of the role of Raman spectroscopy in the study of elastomers.
The presence of conformational equilibriu~n for a compound normally results in
measurable changes in the physical properties of that compound, such as the vibrational,
electronic and rotational spectra, kinetic and thermodynamic properties. The mono
substituted 1.3-dienes such as isoprene and chloroprene are polymerised in different
forms. Chloroprene or 2-chloro-l,3-butadierle is a volatile liquid monomer used
extensively in the manufacture of polychloroprene (neoprene) elastomer. It is well
known that poIychloroprene have a highly regular structure primarily made up of linear
sequences of trans-2-clmloro-2-butylene units, as shown in Fig. I (a). However, other
possible strucrural isomers can also be observed in the polychloroprene.
Fig. l(a) Structure of cis- and trans- 1,4-Polychloroprene.
Polychloroprene is the one widely used polymer in many industrial applications.
In addition to butadiene and isoprene, chloroprene is the important monomer
constituting many elastomeric materials. The uses of polychloroprene are numerous
and widespread. Chloroprene rubber has excellent resistance against oil, chemicals and
deterioration, in addition to flame retarding properties. It is widely used as materials in
automobile parts and parts in variety of heavy-duty equipments. Despite of its cost, its
high durability under harsh conditions makes neoprene rubber a highly needed material.
Neoprene rubber has excellent solvent resistance potential, i.e. the ability LO withstand
conditions imposed upon it without having any substantial change in its properties over
the time of exposure. Therefore it is used in the chemical engineering industry for
lining the inside of vessels/containers which may contains solventslsolutions/reagents.
In brief, Polychloroprene is used for making hoses, gaskets, tubing for carrying
corrosive gases and oils, conveyer belts, lining of reaction vessels, adhesives, etc.
Copolymers, elastomer blends and elastomer-thermoplastic blends are studied
by FT-Raman spectroscopy. The blending of thermoplastic polymers with elastomers is
an area of great interest as the structural and flow characteristics of many thermoplastics
have been extensively studied [4]. The blending of different rubbers to make a product
of specific physical and chemical properties is a common commercial practice (for
example in the nlanufacture of tyres). The vulcanization of butadiene rubber and other
synthetic elastomers such as styrene-butadiene and nitrile rubbers have been studied [5]
in natural blends and during vulcanization.
Baah and Baah [6] reported the results from the chemical attacks of organic
reagents on neoprene, nitrile and natural rubbers, and found that the mass of the rubber
specimens increased for the duration o f the experiment with the exception of the
neoprene rubbers.
The existence of different degrees of reinforcement with different types of
carbon black has been established for rubber materials. The infrared reflection
spectroscopy has been used as primary technique to analyse the polymer structural
changes in the carbon blacldrubber con~posites when coated on a metallic substrates
[7,8]. Tribiology is the science of friction, lubrication and wear of materials. Stuart
used the FT-Raman spectroscopy for investigations on polymer tribiology and reviewed
a number of studies dealing with the problem of surface plasticisation in engineering
polymers [9].
Compton et al. [ 101 analysed the vibrational spectra of 2-chlorobuta-l,3-diene in
the gaseous, liquid, solid. solution and a n argon matrix phases. They determined the
conformational properties and calculated the thermodynamic functions of 2-chlorobuta-
1,3-diene. The analysis of vibrational spectra and thermodynamic data revealed the
presence of a sillall but significant concentratio~l of high energy s-cis conformer at room
temperature in the fluid phases of this compound other than the more stable s-trans
conformer. They also compared the vibrational spectra and the thermodynamic data of
propeony1 chloride with that of chloropre~le to aid in assignment of the spectral features
of 2-chlorobuta- 1 ,3-diene. Radiation induced cross-linking of polychloroprene
containing nine different polyfunctional monomers have been investigated by Yunshu et
al. [ I 11 and found that the tensile strength of the cross-linked polychloroprene.
Tabb et al. [I23 analysed the infrared spectra of polychloroprene polymerised at
different temperatures and interpreted the spectral differences among the
polychloroprens in terms of the increase in structural irregularities that occur as the
polymerisatlon temperature is increased. They concluded that crystallisation occurs
with random inclusion of the structural irregularities into the crystalline domains and
was supported by the spectra of the crystalline regions of a chloroprene copolymer
containing known impurities in the form of 2,3-dichloro-2-butylene units in addition to
the normal 2-chloro-2-butylene units. Wallan et al. [13] coinpared the spectral changes
in both sets of Raman data of semi-crystalline and amorphous polychloroprene to the
calculated vibrations for the crystalline trans-polychloroprene unit and then related to
the molecular model for the structure of the crystalline trans- l,4-polychloroprene unit.
Coleman et al. [14] obtained the vibrational spectrum of the crystalline regions
of a semi-crystalline polychloroprene by employing an absorption subtraction routine to
remove the absorbance contribution of the amorphous regions of the polymer. Tabb
and Koenig 1151 reported the absorbance subtracted crystalline spectrum and carried out
the normal coordinate analysis of polychloroprene and compared the calculated
frequencies to the experimental values obtained.
Until now, a complete vibrational spectroscopic studies of trans-1,4-
polychloroprene have not received much attention from either the experimental or
theoretical viewpoint. Our work represents the first complete experimental and
theoretical investigation of trans-l,4-polychloroprene. The spectra have been
qualitatively analysed in terms of peak positions and intensities. With the help of the
added data and the utilisation of group theory and normal coordinate analysis, an
attempt has been made to achieve a more satisfactory interpretation of the spectra of
trans- l,4-polychloroprene.
EXPERIiMENTAL
-. I he solid trans-1.4-polychloroprene was obtained from M/S Aldrich chemicals,
USA, with stated purity of 99% and used as such without further purification to record
the spectra. The FTIR and the FT-Raman spectra of trans-1,4-polychloroprene were
recorded as films, The films were prepared by dissolving the compound in CS2,
pouring onto a glass plate and evaporating off the solvent. The films were finally dried
in an oil vacuum at slightly elevated temperature. The FTIR spectrum of this compound
has been recorded in the region 4000-400 cm-' by a Bruker IFS 66V spectrometer, with
a scanning speed of 30 crn-' min-' of spectral width 2.0 cm-'. The FT-Raman spectrum
was also recorded in the same instrument, with a FRA 106 Ranlan module equipped
with Nd:YAG laser source operating at I.064pm line with 200 mW power in the
wavenumber range 4000-100 cn1-I. The frequencies of all sharp bands are accurate
NORMAL COORDINATE ANALYSIS
The maximum number of potentially active observable fundamentals of a
polymer in which the chemically repeating unit of the polymer chain contains N atoms
is equal to (3N-4), ignoring three translational degrees of freedom and rotation of the
polymer molecule about its own axis. The normal coordinate analysis of
polychloroprene is very essential to elucidate the relationship between the molecular
structure and vibrational spectra of the polymer. The earlier work on the interpretation
of the vibrational spectra of polychloroprene [I53 was not complete. With the aim of
gaining more complete knowledge of the vibrational spectra of polychloroprene a
normal coordinate calculations is carried out using Wilson's FG matrix method [16-I 81
with the aid of the computer program developed by Fuhrer et al. [19 ] , with suitable
modification to calculate the vibrational frequencies and potential energy distribution
(PED). For the normal coordinate analysis of polymers, force constants must be
approxixr,ated from values obtained from the analysis of suitable model compounds
[15]. The theoretical calculations of the vibrational modes can be made, by an
appropriate model of the conformation of the polymer chain. The Fig. I(b) represents
the schematic model of repeating unit of trans-l,4-polychloroprene.
Fig. l(b) Model of repeating unit of trans-1,4-polychloroprene.
In such an approach it is imperative that a reliable force field available so that
the molecular motions can be acci~rately described. The validity of the force constants
chosen can be evaluated by a cornparision between the observed and calculated
frequencies. The trans-l,4-polychloroprene possesses Cs point group symmetry by
assuming CH2 groups as point masses and lie in the plane of the molecule. The
distribution of the normal modes among the irreducible representations for this Cs
symmetry is given by T,,t, = 1 7af + 9a". A11 vibrations are active in both IR and Raman.
Most of the force field generally ignores the interaction between the neighbouring
stretching vibrations and between bending vibrations. So, the average distortion
between the calculated and the observed frequency is usually large. To obtain a fair
agreement between the theoretical and observed frequencies, the simple general valance
force field (SGVFF) method was adopted for both in-plane and out of plane vibrational
modes, in which the valance force constants can also be transferred between the
structurally related n~olecules, which is found to be very useful in the normal coordinate
analysis. The force constants were refined by the damped least square technique [20]
until a satisfying agreement is established between the theoretical and observed
frequencies. The I'ED calculated using the final set of force constants are presented in
Table 1 .
RESULTS AND DISCUSSION
The FTIR and FT-Rainan spectra of trans-1,4-polychloroprene are shown in
Figs. 2 and 3. The observed spectra of the con~pound are analysed on the basis of Cs
point group synlmetry by assuming CM? groups as point masses. The observed and
calculated fi-equencies of trans-l,4-polychloroprene in the infrared and Raman along
with their relative intensities and proposed assignments are summarised in Table 1.
Assignments have been made on the basis of relative intensities, magnitude of
frequencies and mainly on the normal coordinate calculations as well as literature data
of polymers of similar structure. The purity of the normal modes are further confirmed
b j calculating the PED to each fundamental vibration.
The numerous C-H modes found in trans-1,4-polychloroprene arise from the
presence of two proton types-methylene and methine. The C-H stretching fkequency is
normally placed between 3100 and 2800 cm-'. The strong infrared and medium Raman
bands which appear at 3022 and 3033 cm" rcspcctively, have been attributed to the
=C-H stretching mode, as the corresponding force constant contribute almost 100%
PED of this mode. The =C-H out of plane bending vibration occur between 1000 and
800 cnl-I. The =C-1-1 out of plane bending vibration is assigned to 887 cm-' in the
infrared. The normal coordinate analysis shows that this mode is highly mixed with the
twisting and wagging out of plane bending vibrations of methylene group. The in-plane
bending vibration of the methine C-H is observed at 1210 cm" in the infrared and at
1216 cm-' in Raman. This vibration is also overlapped to some extent with the rocking
and deformation vibrations of the methiene group. These assignments are in good
agreement with the literature [2 1,221.
The methylene (CH2) stretching typically appears in the region 3000-2800 cm-'.
A detailed investigation of the asymmetric stretching, v,(CH2), and symmetric
stretching, vS(CH2). internal modes of the CH2 groups has been established and the
assignments for these bands in the spectra are satisfactorily made, by expecting
asymmetric stretching to be much more intense than synlmetric stretching. The
frequencies seen at 2932 and 2916 cm-I in the IR, and 2914 cm" in the Raman
corresponds to v,(CN2). The frequencies observed at 2857 ems' in the IR and 2860,
2841 cm-I in the Raman corresponds to v,(CH2). It is clear from the potential energy
calculation the frequencies above 1660 ~111~' are dominated by single vibrational modes,
i.e., the contribution of potential energy arising out of the remaining force constants to
these normal modes is negligible.
The well established deformation vibrations, 6(CH2) give rise to absorption in
the region 1500-1400 cm-'. The strong band at 1432 cm-' in the Raman, and the very
strong and medium bands at 1488 and 1433 cm-I in the IR are assigned to the
deformation. 6(CH2) modes of methylcne groups. Frorn potential energy distribution
calculation, it is clear that the methylene rocking modes and the ic-plane bending of the
neighbouring bonds also contribute to the CH2 deformational vibrations. The rocking
vibrations of the CH2 group is much more susceptible to the influence of neighboring
groups than the scissoring vibrations. It would be anticipated that methylene groups
vicinal to polar groups in such structures as -0-CHI-, -C=C-CH2- and -CO-CH2-
should show characteristic rocki~lg bands. It is moderately strong in the infrared and
weak in the Raman. The rocking mode of the CH2 group, gives rise to the medium
bands at 826 and 783 cm" in the Raman and strong bands at 826,780 cm-' in IR spectra.
This is moderately coupled with the deformational mode of the CH2 group. The
assignments of the remaining active fundamentals, the CH2 wagging and twisting, occur
over a frequency range centered about 1300 cm-l. The medium and weak bands
observed at 1285, 1250 and 1254 cnl-' are assigned to the methylene twisting modes.
The twisting modc is highly coupled with the wagging mode of the CH2 group. The
peaks seen at 1345. 1337 and 1302 cm" in the IR and in the Raman are considered to be
the CIi2 wagging modes. Similarly, the normal coordinate analysis data shows that the
wagging fundamental vibrations overlapped moderately with the twisting vibration of
the CH2 group [23]. The C=C stretching fi-equency typically appears between 1680 and
1630 crn-I. In the present spectra the very strong band observed at 1659 cm-' in the IR
and 1660 cm" in the Raman is assigned to the C=C stretching mode and is in good
agreement with the literature [24,25].
The bands appear in the region 11 00-900 cm-I are associated to C-C skeletal
stretching modes. In the 18 spectrum. the methylene vibrations are cIeariy the strongest
but the C-C modes dominate the Raman spectrum. In the present work, the strong and
medium intensity infrared bands at 1100.1004 cm" and the bands found at 1084 and
1005 cm-' in the Raman spectra are ascribed to C-C stretching vibrations. The C-CI
stretching region between 800 and 600 cm" is well linown for its resonance specific to
configurational/co~~for~~~ational structures of trans-1.4-polychloroprene. The strong
band at 669 cm-' in the infrared and the medium intensity band observed at 670 cm-' in
the Raman spectra is assigned to the C-CI stretching mode. The in-plane and out of
plane vibrations of C-Cl bond are given in the table.
The Raman spectrum is particularly rich in the lower frequency range where
there is little absorbance in the IR spectrum. In the Raman spectrum, the bands seen at
619 and 622 cm-I in the infrared are assigned to the C-C in-plane bending modes.
Similarly the medium intensity bands observed at 576 and 407 cm-' are attributed to
C-C out of plane bending vibrations. The weak band at 152 cm-' in the Raman is
assigned to the C-C torsional mode. The lower frequency bands normally overlaps
with other resonances and makes difficult to ailalyse into their individual vibrational
components.
POTENTIAL ENERGY DISTRIBUTION
The potential energy disrribution (PED) has been calculated to check whether
the chosen set of assignments contribute the most to the potential energy associated with
normal co-ordinates of the molecules. The PED contribution corresponding to each of
the observed frequencies are listed in Table 1. The potential energy distribution
confirms the reliability and accuracy of the vibrational spectral analysis.
162
- ,? K 4: + 2 LJ t-'
I= + C' t .d
3 d w
bLI C . - Gn bLI s N z C1
w CC)
C? -
E r- m rC) -
E w, -3- m - . a
b - ....- CC) b o o - *
C? m I I - * * C? I
REFERENCES
1. S.W. Cornel and J.L. Koenig, Macromoicc~~les 2 ( 1 969) 540 and 546.
2. B. Chase, Analyt. Chem. 59 (1987) 881 A.
3. V.M. I-Iallmark, C.G. Zimba, J.D. Swalen and J.F. Rabolt, Spectroscopy
2 (1 987) 40.
4. K.D.O. Jackson. M.J.R. Loadman, C.fI. Jones and G. Ellis. Spectrochim. Acta.
46A (1 990) 21 7.
5. J.L. Koenig, M.M. Coleman J.R. Sheltoll and P.H. Starnier, Rubber Chen~. Tech.
44 (1971) 71.
6. C.A. Baah and J.I. Baah. Materials and Design 22 (2001) 403.
7. S.L. Oh and J.L. Koenig, Rubber Cllen~. Tech. 72 (1999) 334.
8. J . Leisen. J. Breidet and J. Kelm. Rubber Chenl. Tech. 72 (1999) 1.
9. B.H. Stuart, Spectrochim. Acta 53.4 (1997) 11 1.
10. D.A.C. Con~pton, W.O. George. J.E. Goodfield and W.F. Maddams,
Spectrochim. Acta. 37A ( 1 98 1) 147.
1 1 X. Yunshu. F. Yibei, F. Yoshit and K. Makuuchi, Radn. Phys. Chem. 53 (1998)
669.
12. D.L. Tabb, J.L. Koenig and M.M. Coleman, J. Polym. Sci. Polym. Phys. Ed.
13 (1975) 1145.
13. P.J. Wallen, Spectrochim. Acta 47A (199 1 ) 1321.
14. M.M. Coleman, P.C. Painter, D.L. Tabb and J.L. Koenig, Polm. Lett. 12 ( I 974)
577.
D.L. Tabb and J.L. Koenig, J. Polyrn. Sci. Polym. Phys. Ed. 13 (1975) 1159.
E.B. Wilson Jr, J. Chem. Phps. 7 ( 1939) 1047. Tozeaa
RI E.B. Wilson Jr, J. Chem. Phys. 9 ( 1 94 1 ) 76. m~
E.B. Wilson Jr, J.C. Decius and P.C. Cross. Molecular Vibrations, McGraw Hill,
New York, 1955.
H. Fuhrer, V.B. Kartha, K.L. Kidd, P.J. Kruger and H.H. Mantsch, Computer
Progranl for Infrared and Spectrometry, Normal Coordinate Analysis. Vol. 5 ,
National Research Council, Ottawa, Canada, 1976.
F.J. Boerio and J.L. Koenig, Chel~z. Phis. 53 (1970) 4826.
S.J. Bunce, H.G. Edwards, A.F. Johnson, I.R. Lewis and P.H. Turner,
Spectrochim. Acta 49A (1 993) 775.
D. Nava, T. Rajma~lkina de Parada, E. Gonzalez, N. Boscan and carlos De La
Cruz. Spectrochim. Acta 52A (1 996) 1201.
R.J. PetcavicIl and M.M. Coleman. J. I'olytn. Sci. Polym. Phys. Ed. 18 (1980)
2097.
C.H. Jones. Spectrocllim. Acta 49A (1 90 1 ) 13 13.
J.A. Frankland, H.G.M. Edwards. A.F. Johnson. 1.R. Lewis and
S. Poshyachinda, Spectrochi~~~. Acta 47h (1991) 15 11.