The thermal isomerization of polyacetylene studied by Raman scattering

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The thermal isomerization of polyacetylene studied by Raman scattering

S. Lefrant, M. Aldissi

To cite this version:

S. Lefrant, M. Aldissi. The thermal isomerization of polyacetylene studied by Raman scattering.

Journal de Physique, 1983, 44 (2), pp.235-239. �10.1051/jphys:01983004402023500�. �jpa-00209590�

The thermal isomerization of polyacetylene ^studied by ^Raman scattering

S. Lefrant

Laboratoire de Physique Cristalline (*), ^Bât. 490, Université de Paris-Sud, ⁹¹⁴⁰⁵ Orsay Cedex, ^France

and M. Aldissi

Laboratoire de Chimie Macromoléculaire, USTL, ³⁴⁰⁶⁰ Montpellier ^{Cedex, France}

(Reçu ^{le 10 mai} 1982, révisé le 8 septembre, accepte le 4 octobre 1982)

Résumé.

Nous présentons ^une étude, par diffusion Raman, de l’isomérisation thermique de films de poly- acétylène. ^Pour différents temps ^et températures d’isomérisation, ^{nous montrons} que le profil des bandes Raman induites par le trans-(CH)x ^est modifié. Dans un premier stade, ^nous observons que les séquences ^trans augmentent

en longueur jusqu’à ^{un maximum} qui correspond ^au maximum de conductivité électrique ^{mesuré sur des échan-}

tillons similaires. Ensuite, ^une dégradation ^{des films est} observée, correspondant à la formation de défauts ponctuels

altérant l’alternance des liaisons carbone-carbone.

Abstract.

A study of the thermal isomerization of cis-(CH)x films has been carried out by ^{means of Raman} scattering spectroscopy. It is found that the Raman band profiles, ^induced by trans-(CH)x films, ^are dependent

both on the temperature ^{and on} the isomerization time. The trans sequences first increase in length ^{to a maximum} length occurring ^at the maximum of the d.c. conductivity ^as measured in similar samples. ^A degradation ^{of the}

films is then observed as punctual defects alter the carbon-carbon bond conjugation.

Classification

Physics ^Abstracts

78.30 - 82.35

1. Introduction.

Polyacetylene, (CH)x, ^{is the} simplest organic polymer ^{which can exist in two}

different forms : cis or trans. Films are usually poly-

merized at low temperature ( - 78 °C), ^{where the cis}

isomer is obtained. But the trans isomer is the most

stable form. Different treatments have been used to obtain it : a polymerization ^{can be} performed ^{at a} relatively high temperature (150 °C), ^an isomerization

can be induced by doping, ^or finally ^a cis-(CH)x sample

can be thermally treated. These different methods have been described elsewhere [1]. ^A variety ^of techniques [2]

have already ^{been used to} study the thermal isome- rization from cis- to trans-(CH)x ; ^{we present in this}

paper results obtained using ^Raman spectroscopy.

Many papers ^on Raman scattering ^induced by undoped ^dr doped (CH)x ^{have been} published pre-

viously [3-6]. ^{The Raman} technique ^is complementary

to the infrared absorption technique and is therefore used first to characterize the films. The ^two isomers,

cis- and trans-(CH)x’ having ^{a different} structure, give

rise to completely ^{different Raman} spectra. ^{But this} technique ^is considerably ^limited by the fact that

(CH)x ^{films are} polymerized ^{in a} disordered form,

since they consist of randomly oriented fibrils. Only partial alignment of the fibrils can be achieved by stretching [7], ^{so that} any polarized ^Raman spectrum is inefficient in giving information about the nature of the observed vibration modes. Nevertheless, very

interesting ^{results are obtained} concerning ^{the mor-} phology ^{of the} (CH)x chains and this property ^is mainly used in this study ^{of the} thermal isomerization of polyacetylene. Moreover, it has been shown

recently [8, 9] ^that light-scattering experiments yield

fundamental information on how the two isomers,

cis and trans, behave. ^In particular, ^{a Raman} study

extended to an excitation wavelength ^{in the} deep ^red (AL

^676.4 nm) [10] ^revealed important differences in the Raman band profiles ^{observed in pure trans-} (CH)x compared ^{to those due to the} trans-part ^{of a}

mostly cis-(CH)x sample. Results obtained from a

thermally isomerized sample ^{are in this} respect very

important ^{for the} understanding of the isomerization processes.

2. Experimental ^details.

^{A classical} right-angle scattering geometry ^was used. The excitation line

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01983004402023500

236

was provided by ^a Spectra-Physics ^{cw Ar+ laser}

combined with a dye ^cell using ^{Rhodamine 6G as a} dye. The scattered light ^was analyzed ^{with a Jobin-}

Yvon double monochromator (Ramanor HG2S) equipped ^with holographic gratings. ^Detected photons

were counted by ^{a classical} photon-counting system coupled ^{to a} calculator used to store and treat expe- rimental data. All the samples ^{studied were immersed}

in liquid nitrogen. ^The good ^thermal exchange together

with a low incident laser _power avoided _any parasite isomerization of cis-(CH)x films which would occur

in room temperature experiments. (CH)x ^{films were} polymerized ^{at - 78 °C} following ^the procedure ^des-

cribed by Ito et al. [11], handled in a dry ^{box and} kept

at - 30 °C before use. The thicknesses of the films

were typically ^{from 100} ym ^to 150 _ym and scanning electron-microscopy observations revealed inter- connected fibers with a diameter of 200-300 A. As will be seen below, ^we used the 600 nm excitation in

our study ^{and we} estimate the penetration depth ^of

the laser light ^{to be a} few hundred angstroms. ^There- fore, all observations described here apply ^{to the}

surface of the sample ^{as this} technique ^is essentially

sensitive to the first fibers of the film. To minimize the parasite ^scattered light, ^the shiny ^{side of the} samples ^were chosen in our experiments. ^{All the}

films studied in Raman scattering ^{came from the}

same set of samples used for d.c. conductivity, ^EPR

and IR experiments described elsewhere [2]. ^{A direct}

correlation between the different measurements obtain- ed by ^several techniques has therefore been done.

Fig. ^1.

^Raman spectra of mostly cis4CH),,; AL

^{600 nm;}

T

78 K : a) ⁱⁿ the range 100-850 cm-’ ; b) in the range 850-1600 cm-1; c) absorption spectrum ^{of the cis isomer.}

The arrow indicates the excitation laser line.

3. General features of Raman spectra.

^{The first} experimental results in Raman scattering ^{and infrared} absorption ^were published by Shirakawa et al. [3, 12]

who performed ^a normal-mode analysis ^{for both}

cis- and trans-(CH)x. Because of the unknown _crys- talline structure of the two isomers only ^isolated

chains were considered, ^thus excluding ^interchain

vibration modes. The Raman active modes for cis-

(CH)X ^{are 4} Ag ^{+ 2} B1g ^{+ 4} B2g ^{+ 2} B3g ^{(point group} D2h) and those for trans-(CH)x ⁴ Ag ^{+ 2} Bg ^(point

group C2h). ^{Raman spectra} have since been improved

and the Raman lines have been published by ^several

authors [4-6], ^but only ^those showing ^a strong ^resonant character are intense enough ^to be followed in this

study. (For weaker features see for example [5, 13].)

The Raman spectrum ^of cis-(CH)x ^is mainly composed ^{of three} strong ^{lines at} 908, ^{1-247 and} 1541 cm-’ ¹ respectively (Fig. 1). These lines are

observed with maximum intensity ^{for an} exciting wavelength ÀL

^{600 nm, which} corresponds approxi- mately ^to the maximum of the absorption ^{of the} polymer. ^{The Raman} spectrum becomes weaker when the exciting wavelength ^{is out of the} absorption range.

Moreover, ^{these Raman} lines show a negligible ^dis- persion ⁱⁿ frequency ^{when the} exciting ^{line is} changed (a ^{shift of 5 cm -1 was} reported only ^{for the 1 541 cm -1 1}

line by ^Lichtmann (13), going ^{from 600 nm to}

457.9 nm). ^{This is} interpreted ^as due either to a

negligible dispersion in the chain length ^{of the} poly-

mer, i.e. cis-(CH)x ^is mainly composed ^of very long chains, ^{or more} likely ^{to a} very small dependence ^of

the frequency of the vibration modes on the chain

length ^{for the cis structure.}

The Raman spectrum given by trans-(CH)x ^{in the}

range 900-1 700 cm-’ ¹ contains mainly ^{two bands}

at 1050-1 120 cm-’ and 1450-1530 cm-1. The

profile of these bands has been extensively ^studied

and shows drastic modifications if the exciting ^wave- length ^is changed ^{as shown in} figure ^{2. When} AL

^{676.4 nm the} spectrum ^is composed ^{of two} sharp ^and strong ^{lines at} 1 065 and 1 458 cm-1 ¹ res-

pectively, ^{while a} o tail » is observed in the high- frequency ^side of each band. This modification

occurs in a gradual fashion for excitation wavelengths

of higher energy, going ^{to a} double band profile ^for AL

^{457.9 nm.} This double band structure is more or less pronounced according ^{to the «} quality >> ^of

the sample. ^{The Raman} spectra presented ⁱⁿ figure ²

are obtained for a supposedly high-quality ^{film. A}

o bad » sample would lead to a Raman spectrum with the low-frequency peak of each band much weaker.

The behaviour of the Raman bands of trans-(CH)x showing ^a strong ^resonant character has been studied in detail. It is supposed ^{to be due to a} large dispersion

in the chain length ^{of the trans} polymer, ^{or trans}

sequences ^{in a} mixed sample. In unsaturated poly-

mers we observe an absorption ^{band due to the tran-}

sition from the bonding ^{7c state to the} antibonding ^7*

Fig. ^2.

^Raman spectra ^of trans-(CH)x thermally ^isome-

rized 1 hour at 140 °C ; ^{T = 78 K.} a) A,

^676.4 nm;

b) AL

^{514.5 nm;} c) AL ^457.9 nm; d) absorption spectrum ^{of the trans} isomer. The arrows indicate the different excitation laser lines.

state of the electrons of the double carbon-carbon bonds (C =C). ^This energy transition, for molecules of different length, ^was calculated a long ^time ago by

Kuhn [14, 15] using the so-called free-electron theory

of conjugated molecules, ^{and later} improved [16, 17].

The longer ^the chains, the lower in _energy is the tran-

sition, reaching ^a limit for infinite chains. The maxi-

mum absorption ^is therefore found at - 2.2 eV for

cis-(CH)x ^{and - 2.0 eV for} trans-(CH)x. ^{On the other} hand, ^recent calculations [18,19] of vibrational modes have been performed ^for trans-(CH)x ^to give ^an assign-

ment to the main bands. This high-frequency (1 458-

1 530 cm-1) ^is mainly ^{due to the} stretching ^{mode of}

the carbon-carbon double bond (C =C) ^{while the} low-frequency ^band (1 060-1 120 cm-’) ^{is a mixture}

of the stretching modes of the C =C and C-C bonds and the bending ^mode of the C H bond. By compa- rison with experimental ^{data in} polyenes ^{of known} length, ^an empirical formula has been derived for the stretching mode of the C =C bond and a limit is found for infinite chains [18]. Because of a contri- bution of this mode to the two strong ^{Raman bands} observed in trans-(CH)x, ^the frequency of these bands will depend ^{on the chain} length, ^{and the} longer

the chains the higher ^the frequency. Therefore, ^an

excitation wavelength AL

^{676.4 nm will} mainly

reveal long chains since we are in resonance with the

absorption of such chains. The observed frequencies

at 1 060 and 1 458 cm-1 _represent the lower limit for the corresponding vibration modes. On the contrary,

an exciting laser line at 457.9 nm will be in resonance

with the absorption of short chains for which resonant Raman modes will be enhanced. This ^« short chain model », although ^it gives ^a good qualitative ^inter- pretation ^of experimental ^Raman spectra, ^{does not}

explain the behaviour of the Raman bands due to the trans part ^{of 95} % cis-(CH)x ^{as shown} recently [10].

With excitation in the deep ^red (ÀL

^{676.4 nm,}

647.1 nm), the Raman bands due to the small fraction of short trans segments ^are strongly enhanced and present ^{a structure different to} that which is observed at this wavelength ⁱⁿ 100 % trans-(CH)x. ^{This beha-}

viour is still unexplained. Nevertheless, ^{the model} described above need not be excluded for the _purpose of the study presented ^{in this} paper.

4. Experimental ^results.

^Raman spectra ^{have been} recorded for different thermal treatments. In order to follow their evolution, ^{we have} kept the excitation

wavelength ^{at 600 nm. We have not measured absolute}

Raman intensities and although ^{results were} quite reproduceable ^{from one} experiment ^{to the} other, ^a normalization of our spectra ^{seemed to us somewhat} speculative. Several isomerization temperatures ^were

Fig. ^3.

^Raman spectra of (CH)x ^{at T = 78 K,} ÅL ^{= b00 nm,}

after different times of thermal treatment at 115 °C. a) ^after

6 min. ; b) ^{after 15} min. ; c) ^{after 30} min.; d) ^{after 60} min. ;

e) ^{after 120} min. ; f ) after 400 min.

238

studied, ^but only results observed for three characte- ristic temperatures ^are presented : ^{115 °C, 140 °C and}

190 °C. In agreement ^{with EPR and} conductivity results, they correspond ^to three characteristic _ranges.

4.1 ISOMERIZATION TEMPERATURE 11 S °C.

At 115 OC the minimum in resistivity ^{is obtained after a}

very long ^time [20] tmin. (a ^few months) of isomeri- zation. Figure ^{3 shows Raman spectra} recorded after 6, 15, 30, 60, 120 and 400 min. of isomerization. At this relatively ^low temperature, ^{the trans content in} the (CH)x ^film increases, ^{but the cis Raman lines are} undetectable only ^{on the last} spectrum, i.e. after 400 min. of treatment. Furthermore, ^the profile ^of

the Raman bands due to the trans-(CH)x part ^is modified as the thermal treatment progresses. After 6 min. of thermal treatment, ^for example, ^Raman

bands are broad with rather flat peaks ^{at 1 100 and}

1480 cm-’ ¹ respectively. ^{Then the} high-frequency

side of each Raman band decreases.

4.2 ISOMERIZATION TEMPERATURE 14O °C. - ISO- merization times are obviously ^{shorter at} higher

temperatures. ^{At 140 °C Raman} spectra ^{have been} studied after 1.5, 3, 7.5, 15, ³⁰ and 150 min. of thermal treatment. The Raman lines due to cis-(CH)x ^{are no} longer detectable after an isomerization time of 7.5 min. Thus the isomerization is faster than was

initially supposed; the result was also confirmed by

Fig. ^4.

^Raman spectra of (CH)x ^{at T}

^{78 K,} AL

^{600 nm,}

after different times of thermal treatment at 140 °C. a) ^after

1.5 min. ; b) ^{after 3} min. ; c) ^{after 7.5} min. ; d) ^{after 15} min. ; e) ^{after 30} min. ; J) after 150 min.

IR and NMR results [2]. Moreover, ^the profile ^{of the}

trans bands is also modified as was the case for the 115°C isomerization temperature. ^For example, ^for

t

30 min. the low-frequency part ^{of each Raman} band (peaked ^{at 1 075 and 1 465 cm-1} respectively) ^is predominant. ^For longer times this behaviour is reversed, ^{as shown} by ^an increase in the high-frequency

side of the bands (see spectrum ^{f in} Fig. ^{4 for} example).

4. 3 ISOMERIZATION TEMPERATURE 190 °C.

A few years ago this temperature ^{was used to} perform ^iso-

merization from the cis to the trans form of polyace- tylene and isomerization times were typically ^{30 to}

60 min. and even longer. ^Raman spectra ^{have been} recorded for t equal ^to 1.5, 2.75, 5, ^{9 and 200 min.}

Complete isomerization is achieved after a very short time of thermal treatment at 190°C (see ^for example spectrum ^{a in} Fig. 5). ^Raman bands due the

trans isomer present ^{an usual} profile ^{when t}

^{90 or}

165 _s, but get broader and shift towards higher frequencies ^{when t increases} (9 ^{min. for} example).

The last spectrum in this series reveals in addition a

fluorescent background ^{which is} characteristic of

some degradation of the film.

Fig. ^5.

^Raman spectra of (CH)x ^{at T}

^{78 K,} AL

^{600 nm,}

after different times of thermal treatment at 190 °C. a) ^after

1.5 min. ; b) after 2.75 min. ; c) ^{after 5} min. ; d) ^{after 9} min. ; e) after 200 min.

5. Discussion of Raman results and conclusion.

Recently, Kuzmany et ^alp [21] performed ^similar

experiments for isomerization temperatures ^lower

than those described here and they paid ^{much attention}

to the diffusion induced by ^{short trans} sequences.

In our case we have studied the thermal isomerization of (CH)x ^films, looking essentially ^at long ^chains

because of the laser excitation wavelength ^{set at}

600 nm.

The study ^{of Raman} spectra ^of polyacetylene (CH)x obtained after different times for different isomerization temperatures ^{leads to the} following

conclusions :

i) Isomerization from cis-(CH)x ^to trans-(CH)x

is much faster than initially supposed; ^for example,

it is completely achieved after a few minutes at 190 °C.

In agreement with other experiments [2], ^{and in} particular ^{with EPR results} [22], ^the temperature

range which seems to us the most appropriate ⁱⁿ

order to perform ^a good isomerization is situated between 140 °C and 160 °C, ^{with a} typical ^isomeriza-

tion time of 60 min. at 140 °C.

ii) ^The profile ^{of the Raman bands due to trans-} (CH)x (1 060-1 ¹²⁰ and 1458-1 530 cm-1 respectively)

is modified during the thermal treatment, suggesting

that the (CH)x ^chain length distribution is changed.

At the beginning, ^{short trans} sequences exist in the film in large concentration. Then this concentration decreases in favour of long chains until tmin. ^is reached, tmin. being ^the isomerization time for which the maximum of conductivity ^is obtained. If the isome- rization time is longer ^than tmin., ^some degradation

of the films begins ^{to be} observed, ^evidenced by ^a

shortening ^{of the} trans-(CH)x sequences. This pheno-

menon occurs mainly ^at high temperature (Fig. 5).

A qualitative measurement of the relative concen-

tration of short and long chains has been made _pre-

viously [23]. ^This measurement depends strongly ^on

the starting material; nevertheless it illustrates quali- tatively ^the above conclusions.

iii) ^The quality ^of trans-(CH)x obtained after ther- mal isomerization also depends ^{on the} starting ^cis- (CH)x material, where the chemistry ^is obviously

very much involved. This is in agreement with obser- vations of Druy et ^al. [7], ^who showed that different

cis-(CH)x ^{films were not} stretchable in the same

way. For example, ^a reticulated film which is difficult to stretch will keep its defects during ^the isomerization process. Nevertheless, ^{for the} starting material used in this study, the conditions given ^above (T

140°C,

t

60 min.) gave the so-called «high-quality»

samples ^of trans-(CH)x ^{as shown} by ^{the Raman} spectra of figure 2, described in detail elsewhere [13, 24].

Degradation processes ^can only ^be clearly ^{detected in}

these high-quality ^films.

Acknowledgments.

^We are indebted to Dr.

Rzepka ^for help ⁱⁿ the experimental ^{work and to}

Dr. Bernier for valuable discussions. This work was

supported ⁱⁿ part by ^{the D.R.E.T. under contract}

no N/683.

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