HAL Id: jpa-00247590
https://hal.archives-ouvertes.fr/jpa-00247590
Submitted on 1 Jan 1991
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Gelation of the conjugated polymer polydiacetylene P4BCMU
M. Adam, J. Aimé
To cite this version:
M. Adam, J. Aimé. Gelation of the conjugated polymer polydiacetylene P4BCMU. Journal de Physique II, EDP Sciences, 1991, 1 (10), pp.1277-1287. �10.1051/jp2:1991123�. �jpa-00247590�
J. Phys. II France 1 (1991) 1277-1287 OCTOBRE1991, PAGE 1277
classification Physics Abstracts
36.20 46.30C 46.60
Gelation of the conjugated polymer polydiacetylene P4BCMU
M. Adam (~) and J. P. Abnk ~, *)
(~) Service de Physique du Solide et de Rdsonance Magnktique, CEA-Saday, 91191 Gif-sur- Yvette Cedex, France
(2) Groupe de Physique du Solide, Universitk Paris VII, Tour 23, 2, place Jussieu, 75251 Paris Cedex 05, France
(Received J7 April J99J, revised 28 June J99J, accepted 2 July J99J)
Amtract. The static viscosity and the shear elastic modulus of P4BCMU in toluene in the red phase have been studied at concentrations above the concentration threshold for the gel transition. Two main results are emphasized. The elastic behaviour of P4BCMU gel gives experimental evidence for the scalar percolation theory. The color transition, yellow to red, and the gel transition appear at different temperatures the color transition is observed at a
temperature higher than the gel transition temperature.
1. Inwoducfion.
Conjugated polymers in solution show interestini features related to their optical properties.
A color transition is observed for polydiacetylene solution either by varying the temperature
or by mixing good and poor solvent [I]. Recently a large number of color phenomena have been reported for different systems: Poly-substituted thiophenes [2], polysubsfituted acetylenes [3] and polysilanes [4]. During the last decade, several attempts have been made to
explain the color transition and the relation between the statistical conformation of the macromolecule and the optical change. In most cases persistent length (the curvature fluctuation of the macromolecule) is used as the main parameter to explain the broad
absorption observed. It is reasonable to assume that the local rigidity is related to electronic
properties since we can expect an energy contribution with the electronic delocalisation larger
than the thermal energy kT, typically about one eV. Another key feature is that, in order to have soluble conjugated polymers in common organic solvents, large substituents are needed.
It has been shown recently [5, 6] that the size of the side-groups and the magnitude of the torsional fluctuations between monomer units can explain the stiffness of these systems. An
important consequence is that the persistent length, the local rigidity, cannot be easily related to the electronic properties of the conjugated backbone. That is, a red shift of the visible
optical absorption does not necessarily mean an increase of the rigidity of the macromolecule.
These claims are in agreement with another approach describing the conjugation length of the
(*) On leave to Laboratoire de Cristallographie et Physique Cristalline, Ulfiversitd Bordeaux1, 351 Cours de la Libdration, 33405 Talence Cedex, France.
chain in solution as a function of the average torsion between monomer units [7]. Besides the localisation effect due to the torsion, the side-groups screen the interaction between the w electrons and can be the origin of the therrnodynamic properties of the conjugated polyn~ers
in solution, as has already been emphasized by Schweizer [8].
Here we report a sol-gel transition study of P4BCMU in toluene. The generic fornlula of the polydiacetylene is (=RC-C-C-CR=)n, with
R : (CH~)~NHCOOCH~COOCH~~CH~.
P4BCMU in toluene has been widely studied because a sharp color transition, from yellow to
red, is observed as the temperature is decreased. One may expect a strong coupling between
electronic properties to conformational change of the backbone macromolecule. The
interpretation of the physical origin of the red phase is controversial. It was first proposed that
a single chain process, a coil to rod transition, is responsible of the yellow to red change [9- Iii, while the opposing view explained the color change as a result of an aggregation process (12-14). Later, small angle neutron scattering studies showed unambiguously that aggregation
occurs even below the overlap concentration [15]. This implies tbat a single chain process
should correspond to a good to poor solvent change, that is at infinite dilution to a chain
collapse rather than a boil to rod transition. Therefore we address the question of the chain
packing, with the aim to elucidate the influence of the w-w electron interaction between chain and parts of the chain.
It was also proposed that the transition exhibited a two-step process [16], which raises question on the relation between the color change and the gel forrnation. Moreover, in a
previous experiment, it was claimed that P4BCMU was an example of gel with percolation of rod [II]. This was supported by the coincidence of the gel transition together with the color transition, while the concentration dependence of the elastic behaviour of the gel was considered as additional evidence of a percolation of rods. The most convenient way to address this, is to measure the viscosity and elastic behaviour of the solution at the color
change. Moreover, this study helps to elucidate the mechanical properties of systems undergoing gelation which remain poorly understood from both theoretical and experimental points of view. Here we report several experiments corresponding to different cooling tons, the aim being to investigate the way the gel is built. The paper is organised as follow : in part II, we describe the experimental methodology, the results are given in part III and part IV is devoted to a discussion of the aggregation effect on the gel transition.
2. Experimental methodology.
2.I THEORETICAL FRAMEWORK. Percolation theory has iieen widely used to describe the
gelation transition [17]. Let us denote by p the parameters that control the degree of connectivity ~p may be the concentration of connected links, the temperature, the time...).
Let us also denote by p~ the value at which the gel phase appears, setting e
=
~ ~~
,
the Pc
relative distance to the gel point. The gel structure is characterised by the connectivity
correlation length which scales like f
~
e'~ This length diverges at p = p~ implying a non zero probability that links belong to an infinite cluster. As a direct consequence, static
viscosity and elasticity have a singular behaviour. The exponents used to describe these singularities near the gel point p~ are s and t
't " 't0 ~~~ ('&)
G = Gos~ (16)
M 10 GELATION OF P4BCMU 1279
where ~o and Go are constants dependent on the properties of the system at the local qcale.
The analogy between conductance of a percolating lattice and viscosity and shear elastic modulus [18, 19] predicts viscosity and elastic behaviours close to the gel point for a complete disordered soft network. The exponents s and t were calculated by numerical methods within the frame of percolation : s
= 0.75 ± 0.04 [20] and t
= 1.94(± 0.1 [21], -Tie exponent s was also calculated within the hypothesis of nonhydrodynamic interaqtion between infinitely polydisperse polymers : s
= 4/3 [18]. With an explicit description of the elastic behaviour at a
microscopic scale (including angular stiffness), a macroscopic behaviour has been predicted
with an exponent T (Eq.(lb)) much larger than t for the macroscopic gel network:
T« t + 2
v [22]. These have been discussed at length in different papers [23, 24] and a
question arises concerning the pertinence of the scalar percolation theory allowing the
analogy between conductive and elastic properties. It has been proposed [24] that at least near the gel point the analogy between conductance and shear modulus is relevant. We thus also
discussed P4BCMU as an example of the scalar elastic behaviour for gel.
2.2 SAMPLE DESCRIPTION. The substituted Soluble polydiacetylenes form a unique class of
conjugated polymers. These macromolecules are obtained through a solid-state polymeriza-
tion in the monomer crystal. The topochemical reaction is initiated either thermally or with X,
y, UV or electron irradiation. Polydiacetylenes are especially suitable for these studies. Their molecular weight can be larger than 106, which is rather uncomtnon for conjugated polymers.
A well controlled topochemical polymerization hinders parasitic reactions, producing an
almost perfect conjugated backbone, free from chemical defects like sp3 defects, branching,
etc. The size and the polydispersity of the macromolecules are very dependent of the y irradiation dose. Here the samples were obtained with a 90 krad y dose corresponding to 30 9b of polymer conversion in a crystal of 4BCMU monomer. The molecular weight of
extracted polymers is M~ = 5 x 10~ with a polydispersity M~/M~
= 2.8 measured by S.E.C.
techniques.
P4BCMU exhibits a competition between segregation and gelation, precipitation can be observed for concentrations lower than 0.lmgcm-3, while for the gel the so called
microsyneresis can be observed, for example a gel with a concentration of 5 mg crn-3 shows a small meniscus of toluene after one month.
In good solvent, the P4BCMU chain has a statistical length b
= 300 I [15] such that, for
a
molecular weight of 5 x10~, the radius of gyration can be estimated through the relation R
~
~~~ ~~~
=
l 555 A, (N
= M~/m is the degree of polymerization with m
= 508 g the 6
molar mass of the monomer unit, and a
=
4.91I the
monomer unit length). The overlap
concentration is :
c* ~~~ 2 mg cm~ (2)
fi7R
as the polydispersity will decrease the c* value, therefore c*<2mg cm~~ For a rod, c* is much smaller. The Onsager result gives :
c*
~
~
~ ~
10~~ mg cm~ (3)
if?(Na)
In reference [11] the authors found for a M~~1.2x10+~ a critical concentration
co above which the gel transition occurs (co = 0.5 mg crn~~). Considering the same sample quality and the same polydispersity their c* value given by equation (2) would be twice the c* of our sample.
The polymers were dissolved at 85 °C in toluene to which we added the radical scavenger
triethylamine (about 0.4flo), to prevent any degradation of the macromolecule [14, 16].
Three concentrations have been studied 1.3, 4 and 11 mgcnl-3 Therefore, the concentra- tions used are close or above the overlap concentration between coils in the yellow solution and much larger than the estimated critical concentration co.
The color transition exhibits an hysteresis of about ten degrees, the yellow to red transition is located at 66 °C in the cooling run. No concentration effects have been observed at the color
change and variations observed between different samples are generally attributed to the
sample quality and the molecular weight (see below).
2.3 DESCRIPTION OF THE EXPERIMENT. The zero shear viscosity ~~ and the shear elastic modulus G were measured using the magneto-rheometer described elsewhere [25, 26]. In this apparatus a magnetic sphere (radius r) immersed in the sample experiences a magnetic force created by the electrical current in metal wire surrounding the sample ceu. The current
intensity I, which is monitored by a feed back amplifier, is such that it maintains the sphere at a fixed position. lvhen the sample cell is displaced during the time t at the speed v the viscous
and the elastic forces which exert on the sphere are balanced by the magnetic force. In a viscous medium : I io cc ~~ v, in an elastic medium : I io cc GA (where A
= vt, and
io is the current intensity needed to balance the gravitational force proportional to the density
of the solution). We check that the viscosity is Newtonian by verifying that (I io)/u is independent of u, I,e, of the shear rate (~ u/r) and that the shear elastic modulus is hookian by verifying if (I io)/A is independent of A. Actually the shear elastic modulus is found to be slightly dependent on the magnitude of the sample displacement A, this is particularly
sensitive near the gel point where large displacements (I mm) are needed for accurate measurements. For a displacement 0.6 mm we measure an elastic modulus 10 9b higher than that measured at 0.3 mm. The range of viscosities measured is : 10-2 to 10 poises and the
range of elastic modulus is 10-2 to 102 dynes crn-2 Further, we define ~~~ the specific viscosity, ~~p
=
~~ ~',
where c is the concentration and ~~ is the toluene viscosity at the
c ~~
working temperature. The sample ceu is therrnalized at better than 0.05 °C. 5.
1/q x10~ $
5 4
4
3
2 2
0 0
0 500 1000 1500
time (m%)
Fig. I. Determination of the gel point t~, from the variation of the static viscosity 1/~ and the shear
modulus $
as function of time. The data correspond to the measurements at T= 61.4 °C and concentration c
=
4mgcm-~ (see text).
M lo GELATION OF P4BCMU 1281
The experiment cycles are perforrned as follows : the solution, placed in a sealed ceu, is heated at 80 °C, then placed on the rheometer sample holder heated at 70 °C so that the
solution remains yellow. The cell is then cooled at the temperature at which the
measurements are performed, it takes typically 10min before the density of the solution reaches an equilibrium value. In the following, this time is taken as the time origin. The
experiments are repeated for several different cooling temperatures.
We assume that the number of connected links is a linear function of the time. Thus the gel
time t~ plays the role of the percolation threshold p~. The gel time
t~ is obtained by an extrapolation to zero of a least squares fit of ~j' and $ (Fig. I). Careful checks of the
influence of the t~ value on the exponents, given by equations (la) and (16), have been
perforrned. The use of other exponents for the gel time determination, close to I for the viscosity and close to 0.5 for the shear modulus, does not change significantly the values of the exponents s and t (see Tab. I). In addition, as shown by the log-log plots of the
viscosity and shear modulus (Figs. 4 and 5) the linear behaviours observed indicate that the tg determination is adequate for the range of e investigated.
3. Experhnental results.
Before each cooling the viscosity of the yellow solution was measured as a reference state.
Between 70 °C and 66. 5 °C no significant change is observed as it is expected for a coil in good solvent(I). For the concentration c=4mgcrn~3, at 66.5 °C the specific viscosity is
~~ = 436 cn1~/g, at 63.5 °C in the red phase ~~p
= 043 cm~/g. The solution was kept one day
at this temperature without showing any change in viscosity. In other words, at this concentration, the solution cooled 2.5° below the color transition does not exhibit any gel transition. For c
= II mg cm~~, in the yellow solution (T
= 70 °C), ~~p
=
555 crn~/g, while for the red solution kept for more than 51 h at 64 °C no viscosity change has been observed, the specific viscosity remains equal to ~~~ = 3 473 crn~/g. Here again a red phase is observed
t (hours)
g
140
120 O
loo 8o
~
6o a
°
a
40 o
20
o a
0 °
57 59 61 63 65 67
T (°C)
Fig. 2. -Variation of the gel times as a function of the cooling temperature and concentration of P4BCMU 1.3 mg crn-3 (Ll), 4 mg crn-3 (Zi), I I mg cm-3 (O). The small arrows are qualitative indication of the temperature above which the gel phase does not appear. The large arrow recalls the tempemture of the color change (yellow to red).
(~) Note that at tile smallest concentration c
= 1.3 mgcm~~, the viscosity was too small to allow
accurate measurements.
~~(g an ~)x 10'~
12 lo s
6 &
4 a
a
2 ha
~
0
60.5 61.5 62.5 63.5
T (°C) a)
G
~
(dyne cnf~)
1
0.8 °
o_~ o
o
~
0.4
0.2
~ o
0
60 61 62 63 64
T (°C) b)
Fig. 3.- Variation of prefactors no (3a) and Go (3b) as a function of the tempemture at the concentration c
= 4 mg crn"
without the appearance of the gel transition. At 63.5 °C, a significant variation of the viscosity
has been measured within one day with a corresponding gel time equal to 121.5 h.
Variations of the gel time as a function of the temperature for the three concentrations are
reported in figure 2. The time t~ at which the gel transition occurs is dependent on the cooling
temperature moreover the shape of the curve suggests an asymptotic behaviour defining a temperature above which the red phase remains viscous. The limiting temperature decreases
as the concentration decreases. We can estimate an upper temperature limit for the gel
transition : 59, 63 and 64 °C corresponding to 1.3, 4 and ii mg crn-3, respectively. This is in accordance with the observation of stationary values for the static viscosity at temperatures below the color transition. On the other hand, for temperatures lower than 57, 60 and 62 °C the viscosity behaNiour in the red phase cannot be measured since the gel time is too short.
The prefactors ~o and Go and the exponents s and t (Eqs. ( la) and (16)) are obtained with a least square fit of log (~~~) and log G as a function of log e (see Tab. I). Figure 3a shows the
variation of the specific viscosity at t = 0 ~
o as a function of temperature for the concentration
c = 4mg crn~~ Only a small change occurs between 62 and 63 °C while a large change is
M 10 GELATION OF P4BCMU 1283
observed between 62 and 61 °C. Below 60.8 °C, the gel time t~ < 40 ruin, is too short to allow accurate measurements of the viscosity. The behaviour of ~o is an evidence of the different sample preparations corresponding to the different cooling temperatures. Thus the ratio
~~/~o corresponds to a specific viscosity normalized to different structures in the red solution
disregarding the nature of the link and the number and the size of the aggregates already built at t = 0.
log(n/n~)
1.2
&
°a~
0.8
0.6
0.4
0.2
0
-1.2 -1 -0.8 -0.6 -0.4 -0.2 0
log(£)
t t~
Fig. 4.-Log-log plots of the reduced viscosity as a function of e= for different
~g
temperatures (C) and concentrations : 60.8 (O), 61.4 (D), 62.5 (O), 61.9 (Zi) at the concentration
c = 4 mg cm"~ ; 62. 5 (+) at the concentration c
= II mg crn~
In figure 4 are reported several viscosity variations for different cooling temperatures and for the two highest concentrations. At low e, the departure from the general trend is probably
due to a shear rate effect. The master curve thus obtained with log (~~p/~o) as a function of
loge indicates an exponent value independent of the concentration and the sample
preparation. Tile exponent s is found to be close to I (see Tab. I) which is different from the theoretical prediction s
= 0.75 [20].
The increase of the elastic shear modulus as a function of the magnitude of the
displacement is the inverse of what one would expect if the gel were destroyed by large
stresses. This result can be understood as a probe of the gel at a local scale, that is within a unit described as a fibrillar structure. Near the gel point, average values are obtained with
displacements lying between 0.6 and 0.075mm. Including the value of the shear modulus
corresponding to the largest displacement will change the average value by about 3 iSi. Thus the general behaviour is not modified.
Go variations are reported in figure 3b and in the table for the concentration
c = 4 mg crn~~ Again the effect of the sample preparation appears on the magnitude of the
prefactor: the higher the temperature at which the sample is cooled, the higher is Go. This is not clearly shown near the limiting temperature ~ 63 °C for c = 4 mg cn1~ ~) but becomes unambiguous for the lowest temperatures measured, such as 6o.8 and 60.3 °C. These
