738
Langmuir 1987, 3, 738-741
Interactions in PVC-Plasticizer Dispersions Eckhard Gornitz and Helmut Zecha* Academy of Sciences of the G.D.R.,Institute of Polymer Chemistry “Erich Corrensn, Teltow-Seehof DDR-1530 Received September 18, 1986 The properties of PVC plastisols depend on the interaction of the PVC particles in the plasticizer, which can be attributed to the formation of extended border layers at the particle surfaces. It is shown that this surface layer consists of a steric and an electrostatic component and its thickness can be varied within large limits. The contributions of both components are estimated, and a model of the surface layer structure is given. 1. Introduction PVC pastes, or plastisols, are concentrated dispersions of poly(viny1 chloride) particles in plasticizers, which are widely used for preparing soft PVC products. The rheological behavior of concentrated dispersions has been shown to depend on the viscosity of dispersion medium qDM, volume concentration of disperse phase 4, size distribution of particles F ( x ) ,and interaction between particles in the dispersion medium. For better separation of these parameters of influence we carried out investigations in monodisperse dispersions free of agglomerates. Monodisperse PVC particles were prepared by discontinuous emulsion polymerization. The drying took place by freezing and further drying at room temperature, the preparation of the plastisols by mixing the PVC powder and plasticizer (in most cases dioctyl phthalate, DOP) under well-defined conditions. The viscosity of such dispersions was modeled by eq 1,’where 6 is the volume concentration of the disperse phase.
2. Definition of Surface Layer and Determination of Its Thickness In eq 1the interaction between the particles or between particles and dispersion medium, respectively, is expressed by the quotient of the effective hydrodynamic diameter x h and the hard-sphere diameter xHs. We define the thickness
of a surface layer surrounding the particles as the difference between the hydrodynamic and hard sphere radii. This thickness can be determined by viscosimetric measurements at dilute dispersions by
or by dynamic light scattering (QELS). For comparison, Table I shows the 6 values of two samples of different particle size which were determined by viscosimetric and QELS measurements. They are in the order of 6 = 70 nm and are independent of particle size, as previous experiments for a greater range of particle size have been able to prove.2 (1) Zecha, H.; Schneider, J.; Wulf, K. Kunststoffe 1986, 76, 51. ( 2 ) Zecha, H.; GBrnitz, E.; Wulf, K.; Wolf, H. J.; Schirge, H. Acta Polym. 1985,36, 114.
Table I. Determination of 6 by Viscosimetry and QELS for PVC in DOP PVC sample A
B
turbidimetric particle size xHs, nm 208 282
viscosimetry
v8Dez/# 6, nm 12.62 8.75
74 73
QELS 6, nm
nm 346 422
Xh,
69 70
Table 11. Influence of Plasticizer Type on the Thickness of the Surface Laver. 6. for 330-nm PVC Particles
plasticizer dibutyl phthalate dioctyl phthalate dioctyl adipate diundecyl phthalate diundecyl phthalate
plasticizer viscosity 7, mPa s 20 58 13 58 185
critical solvation temp, “ C 56 88 105 113 129
6, nm 90 65 54 40 30
3. Investigations on the Nature of Surface Layer 3.1. Steric Components. A number of experimental results indicate‘that the thickness of the surface layer is determined by solvation effects and by the morphology of the latex particles, thus being a steric barrier. Table I1 demonstrates the dependence of the thickness of the surface layer on the type of plasticizer. 6 decreases with increasing critical solvation temperature, Le., with decreasing solvent power, of the plasticizer.2 With increasing preparation temperature of the dispersion, the thickness of the surface layer increases, as is demonstrated in Figure 1. A change in the morphology of particles by annealing the polymerization latex above the glass transition temperature of PVC leads to a reduction of the thickness 6 in the plasticizer (Figure 2). 3.2. Electrostatic Component. On the other hand, it is known that PVC particles in plasticizers have a negative ~ h a r g e . ~This , ~ can easily be proved by the fact that a polymer layer is deposited at the anode if a dc voltage is applied to electrodes in the plastisol. By gravimetric determination of the amount of PVC deposited, the electrophoretic mobility of particles can be calculated by u=-m (4) cEtA
From eq 4 we get the [potential of particles according to the Huckel equation (5)
(3) Hoffmann, R. L. J. Colloid Interface Sci. 1974, 46, 491. (4)Merinov, Ju. A.;Guseev, V. V.; Beresov, L. V.; Krupnova, M. N. Kolloidn. Zh.1981, 43, 487.
0743-7463/87/2403-0738$01.50/0 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 5, 1987 739
PVC-Plasticizer Dispersions
--.r__
41
' d o '
io
60
fil
'
*C
Figure 1. Thickness of the surface layer 6 for 330-nm PVC particles in DOP vs. temperature of plastisol mixing.
_,_
c+3
-1%
. - ~ -. ~ - -
?-
12
3
Table 111. Calculated Values of Debye Radius K-I and Surface Charge Density u0 for PVC Particles in DOP Depending on Particle Radius a a.
K-I, bo,
tempautun of hlsr mnabng
60 do ' Id0 ' ti0 *C Figure 2. Thickness of the surface layer 6 for 200-nm (X) and PVC particles in DOP vs. temperature of a 2-h latex 300-nm (0) annealing.
30
Figure 4. Conductivity of a dispersion of 240-nm PVC particles in DOP and conductivity of the serum vs. concentration of the PVC.
90 140 23.8
nm
rC/m2
nm 245 220 19.3
In Figure 4 the electrolytic conductivity of DOP plasticizer is given as 5 x 0-l cm- l. The conductivity of the dispersion is substantially greater and increases with the concentration. The conductivity of the clear serum, which was obtained by centrifuging the dispersion, is somewhat lower than that of the dispersion but always remains higher than that of the pure plasticizer. As suggested by Zukov? the Debye radius of the electric double layer in a nonpolar medium can be estimated with the help of conductivity by K-l
=
(D7)'/'
(6)
where D is the diffusion coefficient of the counterions and 7 is the time of relaxation of charges in the double layer. 7 results from the conductivity of the dispersion medium by 7 = t,ho/Ko (7)
1/.m
100
m
m
a
m
Figure 3. {-potential of PVC particles in DOP vs. particle diameter. where m is the amount of the polymer deposited, c is the concentration of the dispersion, E is the electric field strength, t is time, A is the area of electrodes, 77 is the viscosity of the dispersion medium,. (mop= 58 mPa s), and cr is the relative dielectric coefficient ( c r , D ~ P= 5.2). In our case the electric field strength was E = 130 V/cm. In the range of concentration from 0.1 to 15% there was no dependence of the electrophoretic mobility and thus of the f potential on the PVC concentration. However, with increasing particle size in the range from 100 to 500 nm an increase in the { potentials from ca. 20 to 50 mV was observed (Figure 3). If we want to calculate the extension of the electric double layer surrounding the particles, we encounter the difficulty that the electrolyte concentration in the dispersion medium is unknown. Measurements of the electric conductivity of the dispersion and dispersion medium reveal that the electrolyte concentration is determined by desorption of ionic components, assuming they are emulsifiers.
D was calculated for Na+ ions with a radius r = 1.0 and with the viscosity of DOP (Table 11) as D = kT/6irqr iz: 3.6 X lo-' cm2/s. The surface charge density of particles can be calculated from the particle potential and the Debye radius. For this purpose we set the {potential and surface potential equal and used the approximation Ereo+O
+
= -(1 Ka) a From eq 6, 7, and 8 with the help of l potentials and conductivity data from Figures 3 and 4 we obtained the following values for K-I and uo for a dispersion of 10% PVC in DOP, depending on particle radius a (Table 111). Thus the xa values are in the range 0.6 5 Ka 5 1.1, and the surface charge density is very low. The absolute number of elementary charges per particle is lower than 100, which shows the low degree of dissociation of ionic surface groups in the plasticizer. On the other hand, this fact is typical of nonaqueous media, where due to the low capacity of the electric double layer even small particle charges can build up distinct particle potentiah6 go
(5) iukov, A. N.; Kibirova, A. N.; Sidorova, M. P.; Fridrichsberg, D. A. Dokl. Akad. Nauk SSSR 1970,194, 130. (6) Parfitt, G. D.; Peacock, J. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum: New York, 1978; Vol. 10, pp 163-222.
740 Langmuir, Vol. 3, No. 5, 1987
Gomitz and Zecha
t L
H/nm
loa
Ea
103
150
200
Figure 7. Repulsion energy vs. particle distance: Figure 5. Compression cell: 1, capillary; 2, case; 3, sintered disk; 4, membrane filter; 5, dispersion; 6, flexible pipe; 7 , mercury reservoir.
(- -) from compression cell experiment (eq 9); (-) for electrostatic repulsion (eq 11);(- - -) for elastic repulsion (eq 12).
energy between the particles can be determined (Figure 7):
80
The mean distance H between the particle surfaces was calculated from the volume concentration 4, assuming a
e
k Pa
60-
I
y”;
b20 o[‘.
* 45
7
180
15
Figure 6. Pressure-volume diagram from the compression cell experiment for 520-nm PVC particles in DOA (#o = 0.143, Vo= 1.5 cm3).
3.3. Contribution of Both Components to the Interaction of Particles. The point in question is to determine the contribution of the electrostatic particle repulsion to the effective hydrodynamic surface layer. An idea of the value of the repulsion energy between particles is given by experiments that were carried out with the help of a compression cell according to Ottewill’ for the system of PVC (particle size 520 nm) and dioctyl adipate (DOA) (Figure 5). A known volume of dispersion of a known concentration is put into the cell, the lower part of which contains mercury. By raising the mercury level in a second reservoir, which is connected with the compression cell by a flexible pipe, a hydrodynamic pressure is produced, and thus a part of the dispersion medium is pressed through a membrane filter. At a given pressure the filtered volume is measured in the capillary after reaching equilibrium. Thus we get the relation between the dispersion volume and the osmotic equilibrium pressure in the dispersion (Figure 6). By integration over the volume and division by the number of interacting particle pairs the repulsion (7) Barclay, L.; Harrington, A.; Ottewill, R. H. Kolloid Z.Z.Polym. 1972,250,655.
value of maximum packing density of = 0.55, which was taken from electronmicrographsof the filter sediment. In Figure 7 one can also see the electric repulsion energy Vr,electr, which was estimated from the values for the par-
ticle potential and Debye radius, measured for the case at issue (erpoA = 4.3, a = 260 nm, $+, = 50 mV, K - ~= 200 nm). These energies are very weak compared with the experiment in the compression cell, where we get energies which are 2-3 orders higher. A more satisfactory approximation to the experimental results, at least at low distances, can be obtained by assuming an elastic repulsion potential following the formula of Hertzs
on condition that the viscometrically measured hydrodynamic thickness of surface layer is 6 = 55 nm and an elastic modulus of the steric barrier of E = 3.3 MPa is presupposed. This value is in good agreement with the elastic modulus of a soft PVC gel. 4. Conclusion In summary, we suggest the following model for the surface layer of PVC in plasticizer (Figure 8). The upper part of the picture shows schematically the surface of a virgin PVC particle, which, as is known from morphological investigations: is structured in microdomains in the order of 20 nm; i.e., the PVC particles consist of amorphous and (8) Sonntag, H. Lehrbuch der Kolloidwissenschujt; Verlag Wiss.: Berlin, 1977; p 131. (9) Zecha, H.; Zenke, I.; Purz, H.-J.; Bischof, C. Angew. Mukromol. Chen. 1982,104, 169.
Langmuir 1987, 3, 741-744
without plasticizer
+
+ +
+
FVC
plasticmr
+
+ -at150
-4
mm
Figure 8. Model for the surface layer of PVC in plasticizer.
ordered or crystalline regions, which act as physical cross-links in the amorphous polymer matrix. In the plasticizer, which is considered as a poor solvent for PVC at room temperature, we can assume a limited swelling of the amorphous regions in the upper one to two microdomain layers accessible to the plasticizer. The ability to swell can be reduced by "temperature aging" of the PVC particles, by which the influence of the latex annealing on the thickness of the surface layer (Figure 2) can be explained.
741
As could be shown'O by determination of the plasticizer diffusion coefficient at higher temperatures and of the activation energy and by extrapolation to room temperature, a swelling of the whole particle at room temperature can be excluded. In addition, the formation of the surface layer exhibits no kinetics, as could be expected for a diffusion process. By solvation processes, free loops and tails of macromolecules extend in the plasticizer. Besides, the formation of a plasticizer structure near the polymer surface can be discussed.'l All those effects lead to the formation of an effective hydrodynamic surface layer in the form of an elastic barrier with an extension of about 50-70 nm. The low number of charges fixed at the polymer molecules leads to the formation of an extended diffuse electric double layer, which is, however, hydrodynamically (i.e., for the rheological behavior of the dispersion) not effective. But it contributes in our opinion to the stability of the dispersion. This can be seen from the fact that otherwise stable PVC-plastisols begin to sediment already by application of an electric field as low as 100 V/cm. Registry No. DOP, 117-81-7;PVC, 9002-86-2. ~
~~
~~~~~
(IO) Gornitz, E.; Schulz, E.; Schulze, G.; Zecha, H. Acta Polym.1986, 37, 232. (11) Merinov, Ju. A.; Beresov, L. V.; Guseev, V. V. 1981, 55, 51.
Zh. Fiz. Khim.
Oxidation of Formic Acid at Polyaniline-Coatedand Modified-Polyaniline- Coated Electrodes M. Gholamian, J. Sundaram, and A. Q. Contractor* Department of Chemistry, I.I.T.,Powai, Bombay 400 076, India Received July 1, 1986. In Final Form: January 5, 1987 The oxidation of formic acid at polyaniline-coated electrodes has been studied. It was found that the oxidation rates at polyaniline are comparable to the rate at a platinized platinum electrode at low overpotentials. Modification of the polymer by cycling in 0.01% H2PtC&results in a 10-fold increase in activity as compared to platinum. The dependence of oxidation rate on polymer thickness and platinum loading has been investigated. The promoter action of PtCb2-may be related to the exchange of these ions into the polymer or the formation of microparticles of Pt dispersed in the polymer matrix. This study shows that conducting polymers may have interesting applications as fuel cell electrodes.
Introduction Conducting polymers are a fascinating class of materials which are currently being extensively investigated with regard to their application in energy storage,' microelectronics,2 protective coating^,^ chemical s e n ~ o r s ,and ~ ele~trocatalysis.~We have chosen to study a classical (1) (a) Nigrey, P. J.; Mac Diarmid, A. G., Heeger, A. J. J. Chem. Soc., Chem. Commun.1979,594. (b) Nigrey, P. J.; Mac Innes, D., Jr.; Nairns, D. P.; Mac Diarmid, A. G.; Heeger, A. J. J. Electrochem. SOC.1981,128, 1651. ( c ) Farrington, G. C.; Huq, R. J. Power Sources 1985, 14, 3. (2) (a) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. SOC.1984,106,7389. (b) Hikita, M.; Niwa, 0.; Sugita, A.; Tamamura, T. Jpn. J. Appl. Phys., Part 1 1985, 24, L79. (3) (a) Noufi, R.; Tench, D.; Warren, L. F. J. Electrochem. SOC.1980, 127, 2709. (b) Skotheim, T.; Lundstrom, I.: Prejza, J. Ibid. 1981, 128, 1625.
(4)(a)Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J.Phys. Chem. 1986, 89,1441. (b) Okano,M.; Fujishima, A.; Honda, K. J. Electroanal. Chem. 1985,185, 393. (5) Oyama, N.; Ohnuki, Y.; Chiba, K.; Ohsaka, T. Chem. Lett. 1983, 1759.
system in order to evaluate the catalytic properties of polyaniline as a possible fuel cell electrode. The oxidation of formic acid at noble metals, particularly platinum, has been investigated by a number of groups, and the paper by Capon and Parsons6provides an excellent review of the earlier work. The pathway of oxidation described in this paper is generally accepted, except for details regarding the nature of the strongly chemisorbed intermediate and the mechanism of its formation.' The oxidation of formic acid is believed to occur via two parallel pathways, one of which involves the strongly chemisorbed intermediate and the other a weakly bound species. A t low overpotentials, the oxidation occurs mainly via the (6) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973,44, 239. (7) (a) Angerstein-Kozlowska, H.; Mac Dougall, B.; Conway, B. E. J. Electrochem. SOC.1973, 120,756. (b) Beden, B.; Bewick, A.; Lamy, C. J. Electroanal. Chem. 1983, 148, 147. ( c ) Wolter, 0.; Willsau, J.; Heitbaum, J. J . Electrochem. SOC.1985,132, 1635. (d) Clavilier, J.; Sun, S. G. J. Electroanal. Chem. 1986, 199, 471.
0743-7463/87/2403~0741$01.50/0 0 1987 American Chemical Society
