Die Angewandte Makromolekulare Chemie 144 (1986) 101 - 112 (Nr. 2344)

Institute of Chemistry, N. Copernicus University, 87100 Toruli, U1. Gagarina 7, Poland

Electronic Processes in Poly(p-phenylene) and Related Compounds, 11"

Structure and Electrical Properties of Polymers Related to Poly(p-phenylene Sulfide)

Wojciech Czerwihski

(Received 24 February 1986)

SUMMARY: Poly(p-phenylene sulfide) was prepared by polymerization of benzene with elemen-

tal sulfur in the presence of aluminium chloride as catalyst. The synthesis product was exactly purified from catalyst and oligomers. IR spectra of the resulting material and the application of other analytical methods indicated a structure like PPS. The polymer is thermally resistant up to 773 K and has a small degree of crystallinity. Con- ductivity of the pristine polymer is higher than that of the original poly(p-phenylene sulfide). After doping with iodine the conductivity rises more than 6 orders in magni- tude to a level of S m-I and the spin concentration is from five to ten times larger than in the virgin polymer. The activation energy of the conductivity decreases from 0.11 eV for the original polymer to 0.03 eV for the doped one. I -V character- istics are ohmic for both the virgin and the doped materials. The barrier height of metal (Au) and polymer semiconductor junction for the doped sample is smaller than for the undoped substance.

ZUSAMMENFASSUNG: Poly(p-phenylensulfid) wurde aus Benzol und elementarem Schwefel mit Alumini-

umchlorid als Katalysator hergestellt. Das Reaktionsprodukt wurde sorgfatig von Katalysatorresten und Oligomeren befreit. IR-Spektren des erhaltenen Produkts und die Anwendung anderer analytischer Methoden lassen auf eine Struktur, wie sie in Polyphenylensulfid (PPS) vorliegt, schlieflen. Das Polymere ist bis 773 K thermisch stabil und hat einen geringen Kristallinitiitsgrad. Die Leitfiihigkeit ist hOher als die von konventionellem Polyphenylensulfid. Nach der Dotierung mit Jod stieg die Leit- fiihigkeit um mehr als das sechsfache auf eine GrORenordnung von S m-I . Die Spinkonzentration ist 5 bis IOmal gr6Rer als im undotierten Polymeren. Die Aktivie- rungsenergie der Leitfiihigkeit sinkt durch die Dotierung von 0,11 eV auf 0,03 eV. Die Strom-Spannungs-Charakteristiken sind ohmsch sowohl fiir das undotierte als auch fur das dotierte Material. Das Niveau des Metall/Polymer-Halbleiterubergangs ist im Falle der dotierten Probe kleiner als bei der undotierten Substanz.

* Part I, cf. lit.'5.

0 1986 Hiithig& Wepf Verlag, Basel OOO3-3146/86/$03.00 101

W. Czerwidski

Introduction

Extensive work has been performed on derivatives of polyphenylenes which are known as a group of thermally stable polymers and materials which can be doped with electron acceptors or donors to form electrically conducting substances. Because of this practical importance, the physico- chemical and electrical properties of these derivatives, especially those of poly(p-phenylene) I , poly(p-phenylene sulfide)*, poly(p-phenylene oxide)3 and its 2,6-dimethy14 and 2,6-dichloros derivatives, and poly(p-phenylene ~elenide)~" have been investigated by many authors during the last decade. Some of these polymers are melt or solution processible, others are not pro- cessible in a simple manner.

Poly(p-phenylene sulfide) is produced under the trade name Ryton since 1969 by Philips Petroleum Co. from dichlorobenzene and sodium sulfide according to the method of Edmonds and Hills. The virgin PPS has a glass transition temperature of 85 - 92°C and a crystalline melting point of

When the polymer is heated above its melting temperature in the presence of air, crosslinking occurs. Crosslinking is consistent with the formation of interchain or intrachain linkages. In this last case it means that sulfur atoms are incorporated probably into a thiophene ring.

The pristine poly(p-phenylene sulfide) is highly crystalline with an ortho- rhombic unit cell9. The planes of the successive phenylene rings are not flat but alternate at +45" to the 1.0.0. plane. It is possible that when the intrachain crosslinking occurs (bridging to form thiophene rings) the planes of adjacent rings become co-planar (flat) and the crystallinity decreases. In this case the geometry of the chain changes in order to facilitate delocaliza- tion of x-electrons (smaller distance between rings) and it may be that d-orbitals of the sulfur will be involved. This situation may be favourable to rise the electronic conductivity10.

The main goal of this study was to prepare polymers structurally related to PPS and to investigate its paramagnetic and electrical properties before and after doping with iodine.

280 - 292 "C.

Experimental

The polymer poly(p-phenylene sulfide) was prepared by a modified method of Grenvesse" . All the detailed synthesis data (denote as a second synthesis) are given in a

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Electronic Processes in Po&@-phenylene) and Related Compounds, II

previous work1,. The light-grey powder was obtained as a crude product. This mate- rial was carefully purified from the catalyst with hot HCl and from oligomers by ex- traction with tetrahydrofurane for 24 h.

The insoluble polymer fraction (about 58%) called PPS-1 and oligomeric fractions (after precipitation with ethanol) called PPS-2 were examined to establish the struc- ture by IR, UV, WAXS, mass spectroscopy, and elemental analysis and the physico- chemical properties of these compounds were determined by thermal analysis, electri- cal measurements, and EPR methods. The samples were characterized in pristine and in doped states. Spectroscopic pure iodine was used as dopant. The iodine is a mild acceptor in comparison with AsF, and SbC4 (strong acceptors) which may cause the halogenation of the aryl rings on the polymer backbone by substitution.

The CT complex PPS-I, was formed by the interaction of the solid polymer with iodine vapour in a vacuum line at Torr after removal of air and moisture at 298 K.

The amount of I, introduced was conveniently monitored by weighing the samples. The loose powder of PPS undoped and doped with iodine was pressed using a

stainless steel press inside a dry box to give pellets for electrical measurements. The spherical contact electrodes on both sides of the pellets were made by gold

plating, because Au has a high work function. The dark conductivity measurements were performed with undoped and I, doped samples at different temperatures. The I - V characteristics measured between - 10 and + 10 V were recorded automatically with a usual dc method and were independent of time. The current flow through the sample was recorded using an X - Y plotter.

The EPR spectra were detected using a Bruker Physics-model 4184 spectrometer. The g-value was obtained at the cross point of the signal with the base line.

The spin concentration was obtained from the integral signal intensity. Diphenyl- picrylhydrazyl (DPPH) was used as the standard. The surface and cross-section tex- ture of the pellets was examined by scanning electron microscopy.

Results

I . Structural

From elemental analysis formula CizI&,2S1,6 it is seen that the polymer synthesized directly from benzene and elemental sulfur has more than one sulfur atom per two aryl rings. This suggests the formation of a mixed struc- ture consisting of intramolecular cyclic sulfide groups of thiantrene13 type and linear structures of poly(p-phenylene sulfide) or dibenzothiophene link- ages'O. Moreover, the formation of chains with fragments of these three structures is possible.

IR-spectra of the resulting polymer showed that the intensity ratios between the peaks near 880 (four substituted benzene) and 805 cm-* (para

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W. Czerwifiski

substituted benzene), which give the possibility of a qualitative measure of the cyclization, are higher than in original poly(p-phenylene sulfide) after removal of the dopant which caused the irreversible m~dification'~.

An example of the mass spectrum of the obtained products and ions corresponding to molar masses of 290, 322, and 354 g/mol are shown in Fig. l a and Fig. lb. They are evident for a complex composition of the tested p o l p e r .

LO 20 'lh+dL 0 0 100 200 m/e 300 LOO 500

(a)

mle = 290

Fig. 1. Mass spectrum of PPS-I (a) and structures of the molecular ions (b).

The presented results of the analysis of the product by mass spectrometry correspond with the results of IR and elemental analysis.

The X-ray (WAXS) measurements indicate that the material described above is a polycrystalline substance with a small degree of crystallinity which decreases as a result of annealing the polymer six hours in nitrogen atmo- sphere (Fig. 2).

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Electronic Processes in Poly(p-phenylene) and Related Compounds, I1

I I A

I I I 1 I I I I I 2a 2L 20 16 12

28

Fig. 2. Wide-angle X-ray diffraction patterns of virgin PPS-I and polymer annealed at different temperatures; (- ) unannealed, ( - - -) annealed at 873 K, ( - - -) annealed at 973 K.

The results of thermogravimetry under dynamic conditions indicate that PPS-1 is practically thermally resistant up to about 750 K. The resulting polymer is infusible and insoluble.

On the basis of the obtained structural results one can state that PPS produced directly from benzene and elemental sulfur has a complex chemical structure and a considerable high thermal and environmental stability.

Moreover, it is generally agreed that for electrically conducting polymers a network of overlapping intramolecular orbitals is needed. This is clearly important for the creation of a large bandwidth and significant electron or hole mobility. Therefore, polyaromatic sulfides in which each aromatic ring is constrained to be coplanar by adjacent sulfur bridges give the arrangement which may be favourable for conduction along the polymer chain.

In this case, the sulfur atoms in PPS play a unique role because they provide to intra and probably intermolecular (overlapping intermolecular orbitals) crosslinking.

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2. Conductivity and EPR

The samples in form of pellets used for electrical measurements have been studied by scanning electron microscopy (SEM). As seen in Fig. 3 the sur- face of pellets made from PPS-1 doped with iodine shows an essentially uni- form morphology. The micrograph of the cross-section is different from thdt- of poly(p-phenylene) where the material consists of irregular grains separated by intergranular regions".

The electric conductivities (T for undoped samples and iodine adducts vary with the temperature according to the conventional equation:

o = qexp (-E,/kT)

Fig. 3. Scanning electron micrographs of surface (1) and cross-section (2) of a (PPS-lb,,,,,, pellet.

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Electronic Processes in Poly(p-phenylene) and Related Compounds, II

Fig. 4 shows the plot of temperature dependence of conductivity for (PPS-1)o,10312. The values of (T and the thermal activation energy of the con- ductivity EA for all samples are listed in Tab. 1. As it may be seen the room temperature values of (T for the crude product and for PPS-1 are typical for insulators (lo-" S m-') but they are higher compared with other undoped

3.2 3.3 1 - [K-'] T

Fig. 4. Temperature dependence of the conductivity of PPS-1 doped with iodine (0.103).

polymers so as poly@-phenylene), poly(p-phenylene sulfide), and polyacety- lene (1O-I2 - S m-'). It is probably caused by extensive delocalization of 7t electrons along the recurring cyclic sulfide units. The conductivity of the doped materials increases about five to seven orders of magnitude. How- ever, it is not quite clear that the heavily doped polymer has a smaller (T than lightly doped PPS-1. It is important to note that even for PPS-1 heavily doped with I2 the conductivity of the adduct is constant with the time (elec- tronically conducting material).

Moreover, PPS-1 polymer has a high affinity toward iodine analogous with p0ly(2,5-thienylene)'~ and doped with iodine it shows a high stability in air which provides an advantage in using the iodine product as semiconduc- tors or positive electrodes in galvanic cells.

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W. Czerwifiski

Tab. 1. Electrical conductivity results.

Sample Pellet Tempe- Electric Activation Saturation Barrier thick- rature conduc- energy current height & ness (d) tivity (a) (m) 0 (s-m-l) (ev) (A) (ev)

PPS-crude 0.55 * lo-’ 303 product 313

PPS-1 0.49 * lo-’ 303 313

(PPS-lb,lo3 I2 0.47 * lo-’ 303 308 313

(PPS-l)o,317 I2 0.30. lo-’ 297 323

3.5 * 10-11 0.10 - - 4.0 * lo-“

6.4. lo-‘‘ 0.11 - - 7.3 * lo-”

i .o - 10-4 0.03 1.7 10-7 0.02 1.2 - 10-4 2.0 - 10-7 1.3 - 10-4 2.3 - 1 0 - 7

3.3 * 10-6 0.12 2.4 * 0.07 4.8 * 3.5 * 10-8

IR-spectra of the heavily doped PPS-1 showed no significant evidence for modification of the chemical structure of the pristine polymer. On the other hand, doping PPS-1 with Iz induces the transition from the crystalline to the amorphous structure (Fig. 5) . It is evident, that dopant molecules penetrate the bulk of crystallites without chemical changes as in poly(p-phenylene sulfide) doped with AsFS1’.

samples measured in the dark at different temperatures. These runs are linear (non- rectifying contacts with gold) for the undoped crude product and the poly- mer fraction. For polymer, light doped with iodine (0.103), I-V character- istics are near ohmic. Further increase in the amount of iodine absorbed gives the non-linear characteristics for small values of voltage (to about 1,5 V) but above this region the I-V curves become linear, too.

The current-voltage characteristics may be plotted according to the theory of charge transport in Schottky barriers1*. The metal-semiconductor-metal structure (MSM) consists of two Schottky barriers connected back-to-back with a thick semiconductor layer (PPS-1 doped Iz). Because in this case the applied voltage is always less than “reach-through-voltage” the transport of majority charge carriers is mainly due to the thermoionic emission (TE).

Fig. 6 shows current-voltage characteristics for (PPS-l)o,lo7

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Electronic Processes in Poly(p-phenylene) and Related Compounds, 11

5-

L-

4 3- s 5 - 7

2-

1-

0

1 I I I I I I I I 2a 2 1 2 0 16 12

28

Fig. 5. Wide-angle X-ray diffraction patterns of original PPS-I and polymer doped with I,; (- ) original; ( - - -) 0,1034 I,; ( - * - .) 0,3178 I,.

I T-313K

I I I I I I I I

0 5 u IV1

Fig. 6. I - V characteristics for (PPS-lh,l,,3 I2 measured in the dark and at different temperatures.

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W. Czerwihski

According to TE theory the relation between current and voltage in the dark is given by the expression

I = & [exp(eV/nkT)-I]

where I,, is the saturation current and is given by the Richardson formula18:

& = Apexp(-&,/kT)

with an effective constant A and the barrier height b. Experimental data plotted as In I vs. V give almost linear curves in the voltage range 0.1 - 1 .O V. The intercept of this line at zero voltage gives the saturation currents for known temperatures and it allows to obtain the & value from Richardson formula.

The barrier height for samples doped with iodine are listed in Tab. 1. It is interesting that increasing the amount of iodine caused an increase of the potential barrier of the junction M-S for emission of charge carriers across and corresponds to the same increasing in activation energy of conductivity. However, these barriers are much less than in the other polymeric materials, e.g. p~lypyrrole'~.

It has been shownis that paramagnetic centers may exist in the product even if it is polymerized. EPR spectra (Tab. 2) of the resulting material indicate that there are paramagnetic species even in the crude synthesis pro-

Tab. 2. ESR results. _ _ _ _ ~ ~ ~ ~ ~

Sample Spin Linewidth g-value concentration* (mT)

PPS-crude 0.6 product PPS-1 1 PPS-1 873 K 38 PPS-1 973 K 23 PPS-1 1073 K - (Pps-lb,l03 12 6.8 (PPS-l)0,3,7 I* I1

0.63

0.62 0.58 0.57

0.56 0.58

-

2.0033

2.0033 2.0035 2.0032

2.0036 2.0035

-

* Relative values to PPS-1, qpin = 1 . IGs spins/g

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Electronic Processes in Poly(p-phenylene) and Related Compoundr, 11

duct. PPS-1 shows one line in the EPR spectrum of about 0.62 mT in width, the line intensity at room temperature corresponding to l O l S spins/g. The shape of lines is Lorentzian for all samples. The annealed PPS-1 produces a linewidth narrower than pristine PPS-1 and the spin concentration rises after annealing in nitrogen at 873 K by more than one order of magnitude. The annealing at 973 K in N2 atmosphere exhibits that the spin concentration is higher than in pristine polymer but smaller than in the sample annealed at 873 K. PPS-1 annealed at 1073 K shows the defect of the EPR line.

After doping with iodine spin concentration rises by one order of magnitude but the shape of the EPR line remains Lorentzian. The linewidth of doped PPS-1 is narrower than in the undoped sample and the g-value increases.

PPS-2 (soluble part of PPS) has an IR spectrum nearly identical to that of PPS-1 but it has no EPR signal.

It is seen that spin concentration in the crude product is about 60% of the value in PPS-1 what is nearly the same percentage as the amount of PPS-1 in PPS directly after preparation.

Conclusion

The polymer obtained in the direct method from benzene and elemental sulfur has a complex chemical structure but this arrangement may be favourable for conduction.

The electrical conductivity of the resulting polymeric material (PPS-1) is some orders of magnitude higher than in other classic polymeric compounds without doping.

After doping with iodine the polymer has higher conductivity and smaller E.4 .

PPS-1 doped with iodine gradually loses its crystallinity by the damage imparted to its crystallographic lattice from iodine molecules.

It is important, that there are no chemical changes in the backbone of the polymer chains after doping with I*. The iodine adduct shows high stability in air (no change in conductivity after 4 weeks in air).

PPS-1 is a paramagnetic material and after doping the spin concentration increases. The annealing of PPS-1 in N2 atmosphere up to 700 "C produces a polymer with a higher spin concentration but without crystallinity.

Generally, the paramagnetic properties of PPS-1 are dependent from the thermal history of the samples.

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W. Czerwibski

The physico-chemical properties of this material showed that it may be a good organic semiconductor.

Detailed studies which are in progress may show that this polymer after doping will be a preferable material as electrodes in galvanic cells especially in high temperature cells.

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