Langmuir 1986,2,508-513
508
r,
surface excess of the surfactant at saturation, gm moles cm-2 density of the liquid, g cm-3 constant = 0.816 coefficient of variation of the inlet bubble size distribution number of bubbles per unit area of the foam bed and unit time, cm-2 s-l viscosity, P surface viscosity, SP surface tension, dyn/ cm
P
6 K
9 P Pa U
=a All
t
7r
4F 0
surface tension of pure water, dyn/cm eigenvalues defined by eq 27 liquid holdup disjoining pressure quantity defined by eq 38 fraction of liquid in films residence time, s
Subscripts 0 f
inlet outlet
Wettability of Polyacetylene: Surface Energetics and Determination of Material Properties Anthony Guiseppi-Elie*t and Gary E. Wnek Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Sheldon P. Wesson Owens-Corning Fiberglas, Technical Center, Granville, Ohio 43023 Received January 6,1986. I n Final Form: April 29, 1986 The critical surface tension for wetting, yo of predominantly cis-polyacetylenehas been found from contact angle measurements to be 40.1 mN m-l. This is the largest yc found to date for a purely hydrocarbon polymeric solid. When polyacetylene is iodine-doped to (CHh.,),, ye increases modestly to 44.2 mN m-l. Changes in the limiting slope in the vicinity 71- yc of the Zisman plot suggest that the doped polymer is better wetted by polar liquids. Using ye found for cis-(CH), and empirical correlations established with the solubility parameter, 6, produced a value of 6 = 9.94 (cal ~ m - ~ ) lThis / ~ . is in good agreement with the value of 9.7 (calcm4)1/2calculated from group contributions. From established empirical correlationsbetween yc and the Lorentz-Lorentz function, the dielectric constant of cis-(CH), was found to be 3.6. This value is in excellent agreement with previously reported direct measurements. The dispersion component of the substrate surface energy,,:y has been found, by using the method of Fowkes, to be 58 mN m-l for cis-(CH), and 90 mN m-l for (CH10.20)x.
Introduction The surface chemistry of polyacetylene is implicated in many of ita suggested te~hnologies.~Battery applications, liquid junction solar cells, and fuel cells are all influenced significantly by the chemistry at the material's surface. In addition, polyacetylene is also of fundamental interest, since a polymeric system of conjugated double bonds has only rather recently been available as film for the study of surface energetics. Energetics of solid surfaces can best be approached from a study of the interaction of probe phases (liquid or gaseous) with the unkown solid. Substrate wettability can be evaluated from the contact angle made between an appropriate probe liquid and the substrate. This approach allows a comparison of the wettability of various substrates with respect to the particular probe liquid and has the advantage of requiring a minimum of instrumentation and comparative experimental facility. A second more useful approach is that credited to ZisThe Zisman method allows the evaluation of a semiempirical property of the substrate, the critical surface tension for wetting, yc. The critical surface tension for wetting is determined from a plot of the cosine of the observed contact angle made between a series of probe Present address: Molecular Electronics Corporation, Torrance,
CA 90503-2417.
liquids of known surface tensions at the unknown solid surface vs. the surface tension of the probe liquids. The critical surface tension for wetting is defined as that value of surface tension below which all liquids make zero contact angle with the substrate and is determined graphically by an extrapolation of nonzero contact angle data to cos Bobd equal unity. These generalized approaches allow a qualitative ranking of substrates as more or less wettable but do not provide fundamental information on the energetics of the solid surface. A more fundamental approach involves the application of the modified form3of the long-standing Young equation4 YS
-re =~
s l +~ l COS v Be
(1)
and the Girifalco and Good6geometric mean rule approach to interfacial tensions.
(1) Guiseppi-Elie,A.; Wnek, G. E. J . Polymn. Sci., Polym. Chem. Ed. 1985,23, 2601. ( 2 ) Zisman, W . A. Ind. Eng. Chem. 1963,55 (lo), 19. (3) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983; p 426. (4) Young, T . Philos. Trans. R. SOC.London 1805, 95, 15. (5) Girifalco, L. A.; Good, R. J. J . Phys. Chem. 1957, 61, 904. (6) Good, R.J. J. Colloid Interface Sci. 1975, 52, 308. (7) Kinlock, A. J. J. Mater. Sci. 1980, 15, 2141.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 4, 1986 509
Wettability of Polyacetylene Here, ysv,ysl,and ylvare the interfacial tensions of solidvapor, solid-liquid, and liquid-vapor, respectively, P, is the equilibrium spreading pressure, 8, is the equilibrium contact angle formed when a pure liquid, 1, is placed on a smooth, homogeneous, rigid, isotropic solid, s, and is the solid-liquid interaction parameter. Assuming P, = 069' and y1 ylv,eq 1and 2 can be combined8 to yield
(3)
A plot of cos 8, vs. yl-ll2 is thus a straight line of slope which yields ys and at cos 8, = 1 gives (4)
The application of the additivity rules for the components of the surface tensionlo reduces eq 2 to Ysl
= Ys + Ylv - 2(Ys dYlvd)1/2
(5)
Here, yd is the dispersive component of the surface tension. Combining eq 1 and 59 with the assumption of T , = 06J and y1 ylvgives cos e, = -1
+ 2 Yp/2(Yp1/2/yl)
(6)
A plot of cos 8, vs. yf1/2/ylis therefore a straight line of and a t cos 8, = 1 slope 27: Yld l/ZYsd 1/2
= yc
(7)
For an intractable material such as polyacetylene, the critical surface tension for wetting and the surface energy allow indirect access to other material properties such as the solubility parameter, which is not obtainable by conventional techniques, and the refractie index, which is obtained only with much difficulty.
Experimental Section cis-Polyacetylene was synthesized as a free-standing film by the modified method of Ito et al.11.13which employs a ZieglerNatta catalyst system based on titanium tetrabutoxide and triethylaluminum. Our procedure utilized an Al/Ti ratio of 4 1 at a titanium concentration of 0.20 M in toluene (20 mL) at -78 OC. Films produced from an initial acetylene pressure of slightly less than 1 atm were typically 100-pm thick and displayed two characteristicsurfaces.I2The surface produced at the glass-walled reactor is shiny and is visibly smooth while the other surface of the film is typically reddish brown (cis form), having a dull fiiish and appearing visibly rough. The terms smooth and rough will be used in subsequent discussions to denote these respective surfaces. Samples for contact angle analysis were cut from the midlevel section of batch-produced film where the number average molecular weight has been estimated to be around 11000.13Contact angle measurementsat polyacetylene surfaceswere done at room temperature within 2 days following synthesis. The samples contained ca. 20% of the trans isomer as shown by IR spectro~copy.'~ (8)Good, R. J. In Surface and Colloid Science; Good, R. J., Stromberg, R. R., Eds.; Plenum Press: New York, 1979;Vol. 11, Chapter 1. (9)Fowkes, F. M. Znd. Eng. Chem. 1964,56 (12),40. (10)Good, R.J. In Adsorption at Interfaces; Mittal, K. L., Ed.;ACS Symposium Series 8; American Chemical Society: Washington, DC, 1975; p 28. (11)Ito, T.; Shirakawa,H.; Ikeda, S. J. Polym. Sci., Polym. Chem. Ed. 1974,13,11. (12)Shirakawa, H.;Ikeda, S. Synth. Met. 1979/1980,I, 175. (13)Chien, J. C. W. Polyacetylew: Chemistry, Physics and Materials Science; Academic Press: New York, 1984,p 56. (14)Ito, T.;Shirakawa,H.; Ikeda, S. J. Polym. Sci.,Polym. Chem. Ed. 1975,13,1943.
Table I. Surface Tensions, y,, Their Dispersion Components, y t , and Observed Contact Angles, BDbd, of Various Probe Liquids at the "Smooth"Surface of Pristine cis-(CH), and Iodine-Doped cis-(CH), of Composition (CHIo,w), BobKl
Ylt
liquid mercury water glycerol formamide ethylene glycol diethylene glycol nitrobenzene benzaldehyde
yld,
mNm-* mNm-' 484 200.0 72.8 22.0 63.4 34.0 57.0 32.3 48.4 29.3 45.2 43.9 43.9 38.5
eobd
ck-(CHL (CHb.z&, deg deg 137.7 f 2.8 121.7 f 4.1 72.0 1.6 49.6 f 3.6 58.0 f 7.2 42.8 f 1.4 46.5 f 1.7 28.4 f 2.0 32.8 f 3.6 18.0f 3.4 26.0 f 1.3 13.7 f 1.3 18.4 f 3.2 10.1 f 1.1
Iodine doping was achieved by controlled exposure of the free-standing film to iodine vapor carried in a purified argon stream. Samples were dynamically pumped at lo4 torr for 24 h following a 12-h exposure to iodine vapor. Compositionswere determined gravimetrically and analytically and were generally in good agreement. The doped polymer compositions are presented here as mole percent of iodine relative to CH. Contact angles were determined on a Ram6 Hart NRL contact angle goniometer equipped with a Ram6 Hare environmental chamber and microsyringe attachment. Free-standing polyacetylene film strips of approximately 4.0 X 1.0 X 0.01 cm were cut within a Dri-Labglovebox (VacuumAtmospheres Corporation) from the same batch and mounted smooth surface upward onto 4.5 X 2.5 X 0.1 cm glass microscope slides with doublesided Scotch tape. All contact angles reported were measured at the smooth or shiny surface of the (CH), film. The mounted film strips were placed in the environmental chamber within the glovebox, and the chamber was sealed and then transferred to the goniometer which was kept outaide the glovebox. The chamber was fist purged with purified argon and a gentle stream of argon was maintained while a 2-cm3capacity glass vessel filled with the appropriate probe liquid was introduced into the chamber. The need for an inert atmosphere was mandated by the previously observed16relative ease of oxidation of polyacetyleneby atmospheric oxygen and the consequent introduction of surface functionalities which alter the energetics of the pristine polymer surface. This system was allowed to equilibrate for approximately 3 h prior to the addition of drops of the probe liquid. An 11.7-pL drop of the appropriate liquid was added to the horizontal smooth (CH), surface from a Teflon and glass screw microsyringe held in the microsyringe attachment. Static advancing contact angles were measured within the drop after 5 min following the addition of the probe liquid.16 Table I shows the proble liquids used and their surface tensions, the dispersion component of probe liquid surface tension, and the observed contact angles, OObd, for these liquids at the smooth surface of pristine cis-(CH), and iodine-doped (CH), of composition (CH&,& The contact angle data presented is in each case the average of 12 points and the error presented is the standard deviation.
Results and Discussion Surface Energetics. The simplest approach to the evaluation of substrate wettability would be the measurement of the equilibrium contact angle, Be, made between a common probe liquid, such as water, and the substrate of interest. This value can then be compared with similar contact angles at other substrates to provide a relative ranking of the hydrophilicity of the candidate substrate. The inability to measure equilibrium angles (the result of surface heterogeneity and roughness) limits the fundamental value of this approach. These limitations (15)Gibson, H. W.; Pochan, J. M. Macromolecules 1982, 15, 242. (16)Neuman, A. W.; Good, R. J. In Surface and Colloid Science; Good, R. J., Stromberg, R. R., Eds.;Plenum Press: New York, 1979;Vol. 11, Chapter 2.
Guiseppi-Elie et al.
510 Langmuir, Vol. 2, No. 4, I986 POLYACETVLENE, (CH)x.
a. smooth surface
(X
20,000)
b. mugh surface
Figure 1. Scanning electron micrographs of polyacetylene surfaces.
Table 11. Observed Contact Angles of Water at Various Polymer Surfaces polymer @as deg poly(tetrafluoroethy1ene)(PTFE) 11P (92-115)' 95' (88-105)' polyethylene (PE) 84' polystyrene (PSI 72 cis-polyacetylene iodine-doped cis-polyacetylene
Figure 2. Zisman plot of coa flow vs. liquid surface tension, y,, for various prohe liquids at the smooth surface of pristine cispolyacetylene.
50
"Reference 19. 'Range of literature values.
aside, such a ranking of the observed contact angles can be useful as a guide to the surface energy of the substrate. Since polyacetylene synthesized by the Shirakawa technique presents two microampically different surfam'2 as shown in Figure 1, it was necessary to first evaluate these. The observed contact angle of water a t the rough surface was found to be 138 f 2' while a t the smooth or shiny surface this angle was found to be 72 f 2'. The wide disparity in the observed contact angles suggests that the roughness contribution at the rough surface was too large to provide meaningful information on surface energetics. Johnson and Dettre have shown both theoretically'? and from experiment'* that the observed contact angle rises sharply with increased surface roughness. This negated the use of the rough surface for any analytical information on surface energetics. In Table I1 are shown the contact angles of water at various polymer surfaces including cis-rich pristine polyacetylene and iodine-doped polyacetylene of composition (CHI,,&. It can be seen that OOM for polyacetylene is, as expected, lower than that of poly(tetrafluoroethy1ene) (PTFE) but is also unexpectedly lower than that of polyethylene (PE) and polystyrene (PS).This makes polyacetylene more hydrophilic than some of its closest analogues. Contact angle hysteresis at the smooth (CH),surface was evaluated from dynamic advancing and receding angles. This approach is an effective probe of the surface chemical heterogeneity, particularly where surface roughneea is not at issue.20 The dynamic advancing and receding angles made by water were measured approximately 5 8 following expansion and contraction of the drop volume. The dynamic advancing angle at the smooth, pristine polyacetylene surface was 102.1 f 2.5' and the dynamic receding angle measured was 46.0 f 3.1'. The contact angle hysteresis, 0, - 8, = 56.1 5.6O, indicates that what
*
(17)Johnson. R. E.Jr.: Dettre, R H.In Contact Angle, Wettobility
and Adhesion; Fowkes, F. M.,Ed.; Advances in Chemistry 43;Amerim Chemical Soeiety: Washington, Dc,1964;p 112.
118)Johnson. R. E.. Jr.: Dettre. R. H. In Contact Annle. Wettabilitv and Adhesion: Fowkes:F. M.,Ed.rAdvances in ChemiatG 43: America; Chemical Society: Washington, DC, 1964;p 136. (19)Dan, J.,R. J. Colloid Interface Sei. 1970.23 (2), 302. (20)Johnson, R E.,Jr.; Dettre, R H. J. Phys. Chem. 1964, MI, 1744.
Ia4t 0.2
-0.0
0
20
40
'r
00
'
80
100
Figure 3. Zisman plot of cos Sow vs. liquid surface tension, yI, for various prohe liquids at the smooth surface of iodine-doped cis-polyacetyleneof composition (CH0.&.
appears as a macroscopically acceptable uniform surface is likely plagued by surface irregularities. These irregularities which contribute to the observed hysteresis may be chemical or physical in origin. Chemical heterogeneities are supported by XPS analysis' of the smooth, pristine (CH),surface where a small amount of polar functionalities has been inferred from the presence of a Cls shoulder at 1.5 eV upfield from the primary Cls signal. This does not discount the possible contribution of surface roughness to the observed hysteresis. Contact angles are,however, more sensitive to small patches of high-energy material at the polymer near surface than is the radiation probe technique of XPS. A second useful approach to substrate wettability The data characterization is that credited to Zisman.2"'" of Table I for pristine cis-polyacetylene are presented graphically as a Zisman plot of cos , , e vs. yIin Figure 2. The characteristic critical surface tension for wetting, yE, was found from an extrapolation to cos OoM = 1to be 40.1 mN m-'. Similarly, data from Table I for iodine-doped polyacetylene of composition (CHIo& is graphed in Figure 3. The critical surface tension for wetting for this surface was found to be 44.2 mN m-l. The modest change in the critical surface tension for wetting suggess only a modest change in the polymer wettability upon doping. This can, however, be a misleading conclusion. The limiting slope, K , of the Zisman plot in the vicinity of yI= yeis given byB (8) cos eom = 1 + K ( y , - 71) (21) Zisman, W. A. Ind. Eng. Chem. 1964,56 (12), 40. (22) Zisman, W. A. In Contact Angle, Wettobility and Adhesion; Fowkes, F. M., Ed.; Advaneas in Chemistry 43; A m e r i m Chemical Society: Washington, DC, 1964: p l. (23)Schonhom,H.;Frisch, H.L.;Gaines, G. L., Jr. Polym. En#. Sei. 1977,17 (7). 440.
Langmuir, Vol. 2, No. 4, 1986 511
Wettability of Polyacetylene Table 111. Critical Surface Tension for Wetting, yo, of Pristine cis -Polyacetylene, Iodine-Doped Polyacetylene of Composition (CHIo,20)x, and Various Other Polymers polymer ye mN m-l poly(tetrafluoroethylene), (-CF2CF2),18.522 polyethylene, (-CH2CH2),3lZ2 cis-polybutadiene, (-CH2CH=CH-CH2),3226 polystyrene, (-CH,CHPh),3322 cis-polyacetylene, (-CH=CH),40.1 iodine-doped polyacetylene, (CHIo,~o), 44.2
The limiting slope values of 0.020 and 0.012obtained for pristine cis-polyacetylene and iodine-doped polyacetylene, respectively, are quite low compared to the expected 0.03-0.04variation among polyorganic solids.24 The difference observed for the two surfaces is, however, quite revealing. It suggests t h a t when iodine-doped, the polyacetylene surface is better wetted by the high surface tension liquids, that is, liquids with larger polar components to their surface tensions. The natural extension of this observation is the conclusion that the polar component of the substrate surface energy has increased as a result of iodine doping. This conclusion is not at all surprising, since the generation of carbenium ions on the polyacetylene backbone (induced by the doping or chargetransfer reaction) and the presence of dopant counteranions should both serve to increase the substrate's nondispersive or polar component of the surface energy. This effect does not manifest itself near yc where the probe liquids are essentially dispersive and hence insensitive to these new forces. In Table I11 is compared the critical surface tensions for wetting of cis-polyacetylene and iodine-doped polyacetylene of composition (CH10.20)xwith various other polymers. The critical surface tension for wetting of pristine cis-polyacetylene, 40.1mN m-l, is the largest value recorded to date for a purely hydrocarbon polymer. This value compares in magnitude with the values normally obtained for highly polar polymers such as Nylon 66 (yc = 43 mN m-l). Schonhorn et alaz6have reported for cis-polyacetylene a ycvalue of 51 mN m-l. This value was unchanged following oxidative exposure to laboratory air for 4 days and is considerably larger than found for cis-polyacetylene in this work. Indeed, we report here an observable change in yc upon iodine doping which is itself an oxidative reaction. The magnitude of the yc reported by Schonhorn et al. and the fact that their ycwas unchanged following exposure to laboratory air after 4 days suggest to us that their original material may well have been already oxidized. In fact, polyethylene which has a ycof 31 mN m-l is upon oxidation altered to yield a yc of 50 mN m-1.2' The concept of a critical surface tension as originally developed by Zisman utilized a homologous series of hydrocarbon liquids. The critical surface tension for wetting obtained with different liquid types, as in this work, can vary from ycobtained with n-alkanes.2s Danlg has shown that ycobtained with a series of polar liquids on essentially dispersive solids is lower than would be obtained with purely dispersive liquids. Although many attempts have been made to relate y c to the fundamental ys,many more authors regard the former only as an indication of the latter. RosofP9 has summarized in great detail the argu(24) Adamson, A. W. Physical Chemistry of Surfaces, 4th ed.; Wiley-Interscience: New York, 1982; p 351. (25) Lee, L.-H. J.Polym. Sci., Polym. Phys. Ed. 1967,5, 1103. (26) Schonhorn, H.; Baker, G. L.; Bates, F. S. J.Polym. Sci. Polym. Phys. Ed. 1985,23, 1555. (27) Nuzzo, R. G.;Smolinski, G. Macromolecules 1984, 17, 1013. (28) Fox, H. W.; Zisman, W. A. J. Colloid Sci. 1952, 7, 109.
l.OV ' 0.00
'
0.04
'
* 0.08
'
'
0.12
'
"
0.16
Figure 4. Fowkes plot of contact angle data at the smooth surface of pristine cis-polyacetylene (m) and iodine-doped polyacetylene of composition (CH10.20)x(0).
Figure 5. Girifalco-Good plot of contact angle data at the smooth surface of pristine cis-polyacetylene (m) and iodine-doped polyacetylene of composition (CH10.20)~(@).
ments for and against these ideas. Indeed, Johnson and Dettre30 have shown that the difference between ys and yc can be as large as 5-50 mN m-l. The approach of Fowkesg suggested by eq 6 provides access to the more fundamental dispersion component of the substrate surface energy. In Figure 4 are plotted the surface tension data of Table I according to the method of Fowkes. On the basis of a linear regression analysis which constrains the line of best fit to pass through the point (0, -1) as dictated by theory, we obtain as estimates of the dispersion component of the substrate surface energy the values:y = 58 and 90 mN m-l for prisine cis(CH), and (CHb.20),,respectively. The y,d values obtained for these solid substrates are appreciably larger than the critical surface tension values obtained from the Zisman plots. As indicated, ys (of which 7 : is but a component), can be much larger than yc.30The magnitude of ysd obtained is not unusual. Graphite, which is a reasonable (29) Rosoff, M. In Physical Methods in Macromolecular Chemistry; Carroll, B., Ed.; Marcel Dekker: New York, 1969. (30) Johnson, R. E.,Jr.; Dettre, R. H. J. Colloid Interface Sci. 1966, 21, 610.
512 Langmuir, Vol. 2, No. 4 , 1986
analogue of polyacetylene from the standpoint of conjugation, also has a large :y value. Contact angle measurements on graphite yield a 72 value of 109 mN m-l and literature values of 7 : range from 108 to 132 mN m-l, depending upon the technique of m e a ~ u r e m e n t . ~ ~ The Fowkes analysis exploits only the dispersion force interaction between the wetting liquid and solid substrate. The decrease in the contact angle of mercury upon iodine-doping reflects an increase in the dispersion force contribution of the substrate. The net effect of iodine doping therefore is to increase 7 : by some 32 mN m-l and to increase the polar component by a smaller, unquantifiable amount. It is equally instructive to plot the data by using the approach of Girifalco and Good5suggested by eq 3. Girifalco-Good plots of the data from Table I are shown in Figure 5. From these slopes Ody,1/2is computed to be 6.27 for pristine cis-(CH), and 6.86 for (CH10.20)x.With the assumption of OS1= 1 these slopes yield straightforwardly the solid surface energy values of 39.4 and 47.0 mN m-l for the pristine and iodine-doped substrates. Since y, cannot be less than 7 : the assumption of a,, = 1 must be considered invalid. The 7; values estimated from the Fowkes plot can, however, be accepted as lower limits of 7,.Then combining this result with that produced by eq 3 gives for +,1 the values of 0.825 and 0.721 for these two solids, respectively. Material Properties from Wettability. The critical surface tension for wetting, yo is generally accepted as an empirical characteristic of the solid surface and as a useful parameter for predicting the wetting behavior of solids. The critical surface tension for wetting has additional value, however, as it allows an estimation of bulk materials properties32such as the solubility parameter, refractive index, and dielectric constant. This approach is particularly useful for intractable polymers such as polyacetylene for which such materials properties are not all obtainable by conventional techniques. Solubility Parameter. B ~ r r e 1 1in~his ~ extension of Hildebrand's c ~ n t r i b u t i o nprovided ~~ the following relationship between the surface tension of a liquid, yl, and its solubility parameter 6. Here V , is the molar volume of the liquid and 6 is the square root of the cohesive energy density. I t was Gordon,35however, who showed a strong empirical correlation between yc as determined from a Zisman plot and the solubility parameter for solid, noncrystalline homopolymers. Iyengar and E r i ~ k s o nused ~ ~ eq 9 to estimate the solubility parameters of crystalline polymeric solids. This they achieved by substituting for the liquid surface tension the critical surface tension for wetting obtained from a Zisman plot. This relationship, however, gives poor correlation with experimental data37on liquids and is expected to be more dubious with polymers. WuS investigated this relationship for the specific case of polymeric solids and produced the following empirical correlation. (31)Fowkes, F. M.Chemistry and Physics of Interfaces; American Chemical Society: Washington, DC, 1964;p 1. (32)Olabisi, 0.; Robeson, L. M.; Shaw, M. T. Polymer-Polymer Miscibility; Academic Press: New York, 1979. (33)Burrell, H. Interchem. Reu. 1955,I4 (I), 3. (34)Hilderbrand, J. H.; Scott, R. L. The Solubility ofllronelectrolytes; Dover: New York, 1964. (35)Gardon, J. L.J.Phys. Chem. 1963,67, 1935. (36)Iyengar, Y.; Erickson, D. E. J. Appl. Polym. Sci. 1967,11,2311. (37)Lee, L.-H. J.Paint Technol. 1970,42,365. (38)Wu,S.J . Phys. Chem. 1968,72 (9),3332.
Guiseppi-Elie et al.
Here Vmsis the molar volume of the polymer repeat segment, n, is the number of atoms in each segment, and is the dispersive component to the solubility parameter. This relationship assumes a purely dispersive substrate for which y,=:y = yc. For a film density of polyacetylene of 1.13 g cm-3,39this relationship predicts a solubility parameter for polyacetylene 6d(CH), of 9.94 (cal cm-3)1/2. The solubility parameter of pristine polyacetylene may in addition be calculated from molar attraction constants after Smal140via the relationship
where Fiis the molar attraction constant of the ith moiety of the repeat unit structure, m is the repeat unit molecular weight, and p is the polymer density. For polyacetylene there is but one moiety, -CH=, in the repeat unit structure for which Small gives the value (based on heats of evaporation) of 111. This predicts for polyacetylene a solubility parameter 6(CH), = 9.7 (cal ~ m - ~ ) l If/ ~use . is made of the Hoy4l constant of 121.5 this predicts a solubility parameter of 6(CH), = 10.6 (cal Our value based on critical surface tension measurements and the use of the Wu relationship is in quite good agreement with predictions based on physical constants. Indeed, when Wu's relationship for the prediction of the critical surface tension for wetting,
is applied to polyacetylene, it yields a value of 38.6 mN m-' which agrees quite well with the 40.1 mN m-l obtained experimentally. It should be emphasized that the Wu relationship when used in its predictive role provides an estimate of the dispersive contribution to the polymer surface energy and thus gets a lower limit on the surface energy. That this value agrees well with the critical surface tension for wetting is supportive of polyacetylene being a dispersive solid. Dielectric Constant. P a p a ~ i a ndeveloped ~~ an empirical correlation between the surface tensions of dispersive liquids and the Lorentz-Lorentz function. Holmes43in his extension of the theoretical rationale for Papazian's correlation presented no less than six functions F ( d , which are correlated with liquid surface tension. Use can be made of these correlations to predict the dielectric constant of polyacetylene. To achieve this the Iyengar and Erickson approach36can be adopted in which the liquid surface tension is replaced by the critical surface tension for wetting of the polymeric solid. This approach assumes that the solid is purely dispersive, Le., as before, ys :y = yc or O = 1 (eq 4) and that the crystallinity in polyacetylene does not affect the CED. Using the correlation presented by Holmes for the Lorentz-Lorentz function and the critical surface tension of cis-polyacetylene of 40.1 mN m-l, we obtain a dielectric constant of 3.57. This result is in excellent agreement with (39)Shimamura, K.;Karaez, F. E.; Hirsh, J. A.; Chien, J. C. W. Makromol. Chem., Rapid Commun. 1981,2,473. (40)Small, P.A. J. Appl. Chem. 1953,3, 7 1 . (41)Hoy, K.L. J. Paint Technol. 1970,42,76. (42)Papazian, H.A. J. Am. Chem. SOC.1971,93(22),5634. (43)Holmes, C. F. J . Am. Chem. SOC.1973,95 (4),1014.
Langmuir 1986,2, 513-519 the experimentally determined dielectric constant of 3.5 f 0.2 determined by Mihaly et a1.44
Summary The critical surface tension for wetting of predominantly cis-polyacetylene has been determined after the method of Zisman to be 40.1 mN m-l. This value is the largest determined to date for a purely hydrocarbon polymeric solid. When polyacetylene is iodine-doped to a conductivity of 103 s cm-' at a composition of (CHI,,m), the critical surface tension for wetting is only slightly increased to 44.2 mN m-l. The critical surface tension for wetting of polyacetylene so obtained has been used in conjunction with established semiempirical correlations to predict the solubility parameter of cis-(CH), to be 9.9 (cal This value was found to be in good agreement with that calculated from group contributions. The dielectric constant (44) Mihaly, G.; Vaneco, G.; Pekker, S.; Janossy, A. Synth. Met. 1979/1980,1,357.
513
of cis-(CH), was also predicted from the critical surface tension for wetting. Using semiempirical correlations established between the surface tensions of dispersive liquids and the Lorentz-Lorentz function, we have used the critical surface tension for wetting to yield a value o f t = 3.6 which is in excellent agreement with previously reported literature values. The dispersion component of the solid surface energy has been found to be 58 mN m-l for cis-poIyacetyleneand 90 mN m-l for its iodine-doped analogue at a composition of (CH10.zo)~.
Acknowledgment. We are grateful to Owens-Corning Fiberglas for support of this work. A. Guiseppi-Elie thanks the University of the West Indies for a postgraduate scholarship and G. E. Wnek is grateful for receipt of a Du Pont Young Faculty Award. Registry No. cis-Polyacetylene (homopolymer),25768-70-1; iodine, 7553-56-2;mercury, 7439-97-6; water, 7732-18-5;glycerol, 56-81-5;formamide, 75-12-7;ethylene glycol, 107-21-1;diethylene glycol, 111-46-6;nitrobenzene, 98-95-3;benzaldehyde, 100-52-7.
Simultaneous Electrical Conductivity and Piezoelectric Mass Measurements on Iodine-Doped Phthalocyanine Langmuir-Blodgett Films Arthur W. Snow,* William R. Barger, and Mark Klusty Naval Research Laboratory, Washington, D.C. 20375
Hank Wohltjen Microsensor Systems Inc., Fairfax, Virginia 22030
N. Lynn Jarvis Chemical Research and Development Center, Aberdeen Proving Ground, Maryland 21005 Received January 14, 1986. I n Final Form: March 18, 1986 Mixed mono- and multilayer L-B films of tetrakis(cumy1phenoxy)phthalocyaninecompounds and stearyl alcohol were transferred to a dual 52-MHz surface acoustic wave (SAW)device for simultaneousmeasurement of the electrical conductivity and mass changes caused by doping with iodine vapor. The conductivity increased by 4 orders in magnitude, and a complex formation stoichiometry of two to four iodine atoms per phthalocyanine ring was measured. Variation of the complexed central metal ion, which included cobalt, nickel, copper, zinc, palladium, and platinum as well as hydrogen, had very little effect on either the magnitude of the conductivity increase or the complex stoichiometry. The measured conductivity increased with increasing film thickness but approached a constant value when the film became thicker than the planar interdigital microelectrode. The quantity of iodine a phthalocyanine film may absorb is dependent on the film morphology, while the magnitude of the conductivity increase is nearly independent of the morphology.
Introduction Thin films of phthalocyanine compounds, in general, and those prepared by the Langmuir-Blodgett (L-B) method, in particular, display novel electrical pr0perties.l The L-B technique for depositing mono- and multilayer coatings with well-controlled thickness and morphology offers excellent compatibility with microelectronic technology. Such films have recently been reviewed for their potential (1) Baker, S. Proceedings of International Syumposium on Future Electron Deoices; Bioelectronic and Molecular Electronic Devices, Tokyo, NOV, 1985; pp 53-58.
applications.2 The combination of L-B supramolecular
films with small dimensionallycomparable microelectronic substrates affords new opportunities for generation of fundamental chemical property information and evaluation of new organic thin film semiconductorsas microelectronic components. In this work an interdigital microelectrode array and surface acoustic wave (SAW) device are used in combination to obtain electrical conductivity and piezoelectric mass measurements on the iodine doping of metal-substituted and metal-free phthalocyanine-stearyl al(2) Roberts, G. G. Sens. Actuators 1983, 4 , 131.
0743-746318612402-0513$01.50J O 0 1986 American Chemical Society
