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

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