Synthetic Metals, 15 ( 1 9 8 6 ) 1 8 3 - 1 9 3
183
POST-POLYMERIZATION PROCESSING OF CONDUCTIVE
POLYMERS: A WAY OF CONVERTING CONDUCTIVE POLYMERS TO
CONDUCTIVE MATERIALS?
B. W E S S L I N G a n d H. V O L K
Zipperling Kessler & Co, Kornkamp 50, D-2070 Ahrensburg (F.R.G.)
Abstract
The state of the art is to synthesize polymer directly in the form (film,
coating, sheet) in which it will be used later. More needs to be known about
methods of processing a virgin conductive polymer in 'undoped' or 'doped'
form. It will be shown that it is possible to modify the morphology after
polymerization (a) by a modification of thermoplastic processing methods
(leading to a compact formed particle) or (b) by making homogeneous blends
from conductive polymers with isolating thermoplastic polymers.
Introduction
We define 'post-polymerization processing' (PPP) as a change in the original morphology of the primary, secondary and tertiary particles of the
starting raw conductive polymers (CPs).
Our goal is to provide flexible procedures for getting many different
useful end-products out of an unshaped raw CP, instead of polymerizing
directly into the end-form (film, sheet, etc.).
We found that this is feasible with new procedures (which will be described in later publications) [1] if the CPs are polymerized [2] into very
pure and honogeneous powders, which show a clean globular primary particle structure [3] (see Figs. 1 - 3). Using these rigidly specified CPs, two different ways of PPP, leading to completely different results, are possible.
These are described below.
1. Processing of the neat conductive polymers
A modified thermomechanical process converts CP powders (which)
appear black in the case of polyacetylene (PAc) and polypyrrole (PPy), dull
blue for polyphthalocyanine (PPhc)) into a compact and homogeneous shape
These materials show a shiny metallic surface (PAc: golden, PPy: blue, PPhc:
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184
Fig. 1. Luttinger polyacetylene powder with globular morphology.
Fig. 2. Polypyrrole powder with globular morphology.
Fig. 3. Polyphthalocyanine powder.
bright red-violet). We found some evidence that the process not only leads to
a densification of the primary particle packaging, but moreover via flow to a
real basic morphological change.
1.1. Mechanical properties
The shaped particles, resulting from processed powder, show elastic
behaviour when broken under ambient temperature. Under SEM the fracture
surface shows a fibrous character.
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1.2. F l o w transition states.
We found several morphological features, which we hypothetically
believe to represent flow transition states (see Figs. 1, 4 - 7 for PAc):
(a) Starting powder is globular.
(b) First transition state: globules become aligned to make chains.
Fig. 4. Polyacetylene: first transition state during processing with globules aligned to
form chains.
Fig. 5. PAc: second transition state, fibril formation.
Fig. 6. PAc: third transition state, 'molten' fibres.
Fig. 7. PAc: end state, homogeneous mass without primary particles.
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Fig. 8. Polypyrrole during processing showing 'molten' and unmolten areas.
Fig. 9. Processed, homogeneous polypyrrole.
(c) Second transition state: chains transform to fibrils, which become
parallel.
(d) Third transition state: fibrils form 'fibres'.
(e) End state: a homogeneous c o m p a c t mass with no primary or secondary particles detectable.
Analogous transition states are found in PPy (Figs. 8, 9).
1.3. Birefringence
These materials can be made into thin films (see Fig. 10, which is of a
1 - 2 pm thin film, measuring 800 X 250 pm). These films show birefringence, possibly due to higher (crystal?) orientation after processing (see
Figs. 11 - 13).
Work is in progress to improve the processing and to characterize the
products. It is interesting to note that PAc after processing is orders of magnitude more stabile than the pristine PAc.
2. Polymer blends: CP in thermoplastic matrix
We have developed thermomechanical procedures for making homogeneous p o l y m e r blends using a suitable thermoplastic matrix and the raw CP
with the same specification as above.
The result is a two-phase system, in which the CP is the dispersed phase
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Fig. 10. Thin film (1 - 2 pm) cut from processed I-oxidized polyaeetylene under normal
light.
Fig. 11. Sample from Fig. 10, with 90 ° crossed polarizers.
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Fig. 12. S a m p l e f r o m Fig. 11 t u r n e d t h r o u g h 45 °.
Fig. 13. S a m p l e f r o m Fig. 12 t u r n e d a n o t h e r 45 ° .
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Fig. 14. Primary globular particles of polyacetylene dispersed in a thermoplastic matrix
(SEM-photo, sample prepared by solvent etching).
(at low concentrations). This proves (Fig. 14) that the secondary structures
of a CP are built up from globular primary particles, which are linked
together only by cohesion forces. These particles have diameters between 50
and 200 nm.
2.1. Optical properties
The dispersion of a CP in a thermoplastic matrix allows the visible transmission spectra to be recorded. PAc is deep blue (uncomplexed) and remains
blue (but less intense) after homogeneous oxidation with I2 to CHI0.] (see
Fig. 15). PPy is violet, PPhc blue (Figs. 16, 17).
660
~o
o
co
Q
o
400
500
600
7 O0
800
nm
Fig. 15. Visible absorption spectra of uncomplexed and I-oxidized (dotted line) polyacetylene.
190
Oo
c~
1
400
500
600
700
800
|
nm
Fig. 16. Visible absorption spectrum of polypyrrole.
g
<=
!
400
500
!
600
700
Fig. 17. Visible absorption spectrum of polyphthaloeyanine.
800
nm
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2.2. Stability of PAc
The m a x i m u m absorption wavelength can be used for measuring the
stability of PAc in terms of conjugation length. We did this for uncomplexed
PAc and followed the absorption properties at different temperatures as a
function of time.
The results, shown in Figs. 18 - 20, demonstrate a dramatic increase of
PAc's oxidation stability.
66O
rlm
58O
|
P,
150 days
Fig. 18. Polyacetylene-PVC blend showing decrease of maximum absorption wavelength
with time at 20 °C. The cross represents the time of maximum absorption wavelength
decrease without a thermoplastic coating dispersed in DMF.
rl m
'2
'
|
'
~'~
'8 Q h" o u r s
Fig. 19. Polyacetylene-PVC blend showing decrease of maximum absorption wavelength
with time at 65 °C. The cross represents the time of maximum absorption wavelength
decrease without a thermoplastic coating dispersed in DMF.
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nm
660-
500
X
I
I
.5
I
I
I
I
12
w.
hours
Fig. 20. Polyacetylene-PVC blend showing decrease of maximum absorption wavelength
with time at 120 °C. The cross represents the time of maximum absorption wavelength
decrease without a thermoplastic coating dispersed in DMF.
2.3. Electrical properties
Using 2 - 3% CP with a pristine conductivity of 5 S/cm, we get antistatic properties (10 -9 to 10 -7 S/cm).
Work to improve the processing and to increase the conductivity o f the
blends is in progress.
3. Conclusions
Our observations, which have still to be confirmed and analyzed, lead
to some basic questions:
(1) Are the present structure and conductivity models of CPs consistent
with the results of our morphology studies?
(2) Are the structural models consistent with the variable morphology
we found?
(3) Could an important part of the instability of a CP, especially PAc,
be due to intrinsic structural rearrangement after synthesis, especially after
(inhomogeneous) doping, which could lead to migration effects?
(4) Is any kind of flow in conductive polymers consistent with an understanding o f CPs as solids?
(5) Would it be helpful to consider conductive polymers to be liquid
crystals?
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(6) We now propose to distinguish between polymeric 'conductive
c o m p o u n d s ' (which are more or less pure and homogeneous chemical substances) and 'conductive materials' (which have to have a defined purity,
homogeneity and morphology); electrical, mechanical and other properties
can only be measured on the 'conductive materials', because these depend
strongly on the long-range structure. We predict a dramatic difference
between the properties measured on the 'conductive c o m p o u n d ' (e.g., a PAc
Shirakawa-film) and on a 'conductive material', e.g., a processed PAc.
Acknowledgements
We are very grateful to the Bundesministerium f'tir Forschung und
Technologie for financial support to part of this work.
We thank Professor M. Hanack for introducing us to the field of polyphthalocyanines b y means of some samples and many synthetic recommendations and we thank Professor G. Wegner, Professor A. J. McDiarmid, and
Professor A. J. Epstein for many fruitful discussions concerning the whole
field of conductive polymers.
References
1 Eur. Pat. Appl. 85107027.6; Eur. Pat. Appl. 85107028.4.
2 Polymerization details to be published later; as basic procedures we used (a) for polyacetylene (J-oxidized): Luttinger catalyst, see ref. 3; (b) for PPy: synthesis in aqueous
medium using docedylsulphonate as counter-ion, catalytic amounts o f FeC12 and
H202 as oxidizing agent; (c) M. Hanack, A. Datz, R. Fay, J. Metz and O. Schneider,
DE-OS 3245750; J. F. Myers, G. W. Rayner Canham and A. B. P. Lever, Inorg.
Chem., 14 (1975) 461.
3 See also preliminary studies on morphology o f PAc: B. Wessling, Makromol. Chem.,
185 (1984) 1265 - 1275.