Langmuir 1994,10, 1962-1970
1962
Capillary Wave Investigation of Surface Films of Diblock Copolymers on an Organic Subphase: Poly(dimethylsiloxane)-Poly( styrene) Films at the Air/ Ethyl Benzoate Interface Frank E. Runge,? Michael S. Kent,* and Hyuk Yu**t Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, and Sandia National Laboratories, Albuquerque, New Mexico 87185 Received November 29, 1993. I n Final Form: February 22,1994" The surface film viscoelastic properties of poly(dimethylsi1oxane) (PDMS) homopolymers and several poly(dimethylsi1oxane)-poly(styrene) (PDMS-PS) diblock copolymers at the aidethyl benzoate (AIEB) interface have been examined by surface quasi-elastic light scattering (SLS)as a function of surface pressure, n, determined with the Wilhelmy plate technique. All systems used in this study form stable, but to some extent, soluble monolayers at AIEB. The advantage of SLS is that it gives reliable dynamic film elasticities and viscosities even in a situation where the static elasticities cannot be determined by standard methods. Relative to the values for PDMS homopolymers at the same n, increases in both the surface dilational elasticity, Ed, and the corresponding viscosity, K , for the diblock copolymers are observed for all copolymer samples. The deviations occur at much lower surface pressures for the asymmetric copolymers (higher PS content) than for a nearly symmetric sample. This suggests that the symmetric sample interacts mainly via the surface PDMS blocks, while the highly asymmetric samples interact mainly through the submerged PS blocks. The magnitudes of Cd and K for the copolymers obtained in this study are consistent with the picture of relatively flexible, submerged PS blocks.
Introduction The configurations and physical properties of diblock copolymers at surfaces and interfaces have been the focus of much interest for several decades.lJ Besides specific technical applications, e.g., compatibilization of immiscible polymers and stabilization of particle dispersions, there is much general scientific interest in the study of block copolymers a t interfaces due to the importance of understanding polymers in confined geometries. For diblock copolymers a t the air/liquid interface, where one block is soluble in the liquid and the second block is insoluble, the concept of a "tethered chainnzLi.e., a chain fixed to a surface by one end-appears appropriate. In previous studiesM M.S.K. and co-workers performed extensive investigations of films made of poly(dimethylsi1oxane)poly(styrene) (PDMS-PS) diblock copolymers a t the air/ ethyl benzoate (A/EB) interface. Neutron reflectivity was used in conjunction with the Wilhelmy plate and the Du Nouy ring methods to determine surface pressure-area (H-A) isotherms. An important result of this work is an unexpectedly strong increase in surface pressure with surface density compared to the PDMS homopolymer. The sharp rise in surface pressure begins when the tethered PS chains are strongly overlapped and, thus, is attributed + University
of Wisconsin. Sandia National Laboratories. * Abstract published in Advance ACS Abstracts, May 15, 1994. (1) For an early series of papers comparing the adsorption of random and diblock copolymers and their effects on colloidal stabilization, see Howard, G. J.; McGrath, M. J. J. Polym. Sci. 1977, 15, 1705. (2) For recent reviews regarding the study of diblock copolymers at interfaces, see (a) Halperin, A.; Tirrell, M.; Lodge, T. P. Adu. Polym. Sci. 1992,100,31. (b) Patel, S.; Tirrell, M. Annu. Rev. Phys. Chem. 1989,40, f
597. (3) Kent, M. S.; Lee, L.-T.; Farnoux, B.; Rondelez. F. Macormolecules 1992,25, 6240. (4) Factor, B. J.; Lee, L.-T.; Kent, M. S.; Rondelez, F. Phys. Rev. E , Rapid Commun. 1993, 48 (4), 2364. (5) Kent, M. S.; Lee, L.-T.; Factor, B. J.; Rondelez, F. J. Phys. IV, Colloq. C8, 1993, 3, 49. (6) Kent. M. S.: Lee, L.-T.: Factor. B. J.: Rondelez. F.: Smith. G. S.
Submitted for publication to Macromolecules.
to the interactions of the submerged PS blocks. With increasing surface density of copolymer in the regime where the PS blocks interact, the submerged block is found to stretch along the inward normal to the surface up to 50% beyond the dimension in the isolated state.- While the stretching of the interacting tethered chains is well understood,2a the sharp rise in surface pressure is not currently well understood. In this study we used PDMS homopolymers of varying molecular weight, as well as PDMS-PS diblock copolymers in which the molecular weight of each block varied over roughly a decade. Detailed surface pressure isotherm experiments, in parallel with studies of the propagation of capillary waves by surface quasi-elastic light scattering (SLS),were performed. The SLS technique allows one to deduce the viscoelastic properties of the films through the use of theoretical modek7-l2 Very little is known about the viscoelastic properties of PDMS and the present diblock copolymers a t A/EB. If the structural picture of the tethered chain is correct, we expect the static and dynamic film properties to be sensitive to the relative molecular weights of both components. With increasing chain lengths of the submerged PS block at a constant PDMS block length, the relative contribution from the interactions among the PS blocks to the overall film properties should also increase. We expect that the viscoelastic behavior due to interactions of submerged PS blocks will differ significantly from that of interacting PDMS segments on the EB surface. In using (7) Langevin, D. J. Colloid Interface Sci. 1981,80, 412. (8)Goodrich, F. C. Proc. R. SOC.London 1961, A260,490, 503. (9) Lucassen-Reynders, E. H.; Lucassen, J. Ado. Colloid Interface Sci. 1969, 2, 347. (IO) Kramer, L. J. Chem. Phys. 1971,55, 2097. (11) Mann, J. A., Jr. In Surface and Colloid Science; Matijevic, E., Good, R. J., Eds.; Plenum: New York, 1984; Vol. 13, p 145. (12) Several authors in Light Scattering by Liquid Surfaces and
Complementary Techniques;Langevin,D.,Ed.;Surfactant ScienceSeries; Dekker: New York, 1992; Vol. 41.
0743-746319412410-1962$04.50/0 0 1994 American Chemical Society
PDMS-PS Films at AirlEthyl Benzoate
Langmuir, Vol. 10, No. 6, 1994 1963
Table 1. Characterization of the Polymers M W ,
system PDMS samples0 Me-5 Me-26
OH-87 Me-92 Me-25
PDMS-PS samples* 25-35 4-30 21-169 28-330
commentBc
kglmol MdM. 5.2 26 87 92 25
-
methyl terminated, lo00 CS methyl terminated, 500 CS hydroxy terminated, 60 OOO CS methyl terminated, 30 OOO CS methyl terminated; not blended
1.6 2.5 3.4 3 3.9
1.2 methyl terminated 1.1 methoxy terminated 1.06 methyl terminated 1.08 methyl terminated
60 34 190 358
a Shortcut nomenclature: terminal group M,, kg/mol, e.g., Me-5 means 5 kg/mol molecular weight, methyl terminated. * Shortcut nomenclature: M,,kg/molof the two blocks PDMS-PS. CSstands for a kinematic viscosity unit, centistokes.
the capillary wave technique to characterize the diblock copolymer films, we hope to be able to probe the changing intermolecular interactions among the PDMS-PS copolymers a t A/EB as a function of block asymmetry and PS block molecular weight and t o investigate the possibility of “anomalous structures” which have been mentioned with respect to the sharp rise in surface pressure.3
Experimental Section Polymer Materials. In Table 1the weight averages of the molar masses,M,,and other characteristics of the polymers used in this study are summarized. The shortcut nomenclature introduced in Table 1is kept throughout the entire paper. The following PDMS samples were purchased from commercial vendors: OH-87 from ScientificPolymer Products, Inc., and Me-5 and Me-26 from H a s Petrarch Systems. Sample Me-92 as well as the analysis of the molecular weights and distributions of the above PDMS samples (whichsometimes contradicted the stated molecular weights of the manufacturers) were kindly provided by Dr. Michael J. Owen of Dow Corning Corp., Midland, MI. Me-5 was used in a recent SLS in~estigati0n.l~Me-25 was synthesized and kindly donated by Alain Lapp (LLB, Saclay, FR). One symmetric (equivalent number of monomers) diblock copolymer sample (25-35) and three highly asymmetric diblock samples (4-30, 21-169, and 28-330) were examined. The 25-35 diblock copolymerwas a generousgift from R. Jerome (Universith de Liege, BLG). The 4-30 sample was purchased from Polymer Labs., U.K.,and the 21-169 and 28-330 samples were purchased from Polymer Standards Service, FRG. Me-25 as well as 25-35, 4-30,21-169,and 28-330 have beenused in severalrecent studies.w The 4-30, 21-169, and 28-330 samples all have fully deuterated PS blocks, while the PS block of 25-35 is protonated. Film Spreading. The subphase was ethyl benzoate (99+ % , Aldrich), which was distilled under vacuum before use. In test experiments, it was found that the use of (i) freshly distilled EB, (ii) EB as received from Aldrich, or (iii) recycled EB (distilled after being used in a surface experiment) led to identical 11-A isotherms (checked for Me-25, Me-92) and SLS results (checked for Me-25, Me-92, 25-35) within the limits of experimental precision. All PDMS films were spread from solutions in hexane (99+%,Aldrich, used as received). In some initial experiments, the diblock copolymers were spread from chloroform solutions (spectrograde, Aldrich, used as received). In most of the investigationsa 2 1 mixture of hexane and THF (certified, Fisher Scientific, refluxed for several days over Na with benzophenone and then distilled) was used as the spreading solvent for the copolymers. The PDMS mass concentrations of the spreading solutions for the homopolymers and the diblock copolymers ranged from 0.05 to 0.4 g/L. Surface Pressure Measurements. In all experiments a Teflon trough (285 X 110X 12.5mm) with a sliding barrier placed in a Plexiglas cabinet was used. Beakers filled with EB were ~
~~
~~
~
(13) Runge, F. E.; Yu, H. Langmuir 1993,9, 3191.
placed in the box to obtain conditions close to the equilibrium EB vapor pressure. The temperature of the subphase was held at 25.0 f 0.1 “C by a Lauda bath (RM6) connected to a glass coil placed at the bottom of the trough. After setting up the trough, the EB surface was cleaned by wiping with the moving bar and gently aspirating the surface. Before the experiments were started, at least 30 min was allowed to reach thermal equilibrium and to check for surface active impurities in the EB that might begin to adsorb to the freshly cleaned surface. The cleanliness of the surface was also tested by reducing the area to about one-third of its initial value which did not lead to any change in surface tension (higher compression ratios are impossible due to the geometry of the experimental setup). As an average over 14 measurements, we obtained the static surface tension 00 = 34.83 0.15 mN/m at 25 OC for EB, which is in excellent agreement with the literature.14-17 For every spreading solvent used in this work, it was ascertained that the addition of an amount of pure solvent comparable to that used in the surface studies of PDMS and PDMS-PS causes no change in UO. The surface tension was measured with a Cahn electrobalance (Model 2000) with a sandblasted platinum Wilhelmy plate (11X 26 X 0.1 mm). The plate was stored in a HNOa/H2SOd mixture and carefully rinsed with distilled THF and Millipore water and dried before use. In this study the continuous addition method to prepare the surface layers, using a Hamilton syringe, as well as the stepwise compression method were used. The waiting time after each compression step or aliquot addition was about 10-20 min and about 30min at higher surface pressures closeto the film collapse. The surface pressure, 11, was determined as ll = 00 - uswhere usis the surface tension of the monolayer-covered surface. The II-A results for Me-25 and Me-92 were reproducible to within a few percent and the isotherms obtained showed only small differences between the addition and compression methods (see later, Figure 2). On the other hand, large losses of the diblock copolymers and Me-5 into the bulk phase were observed during the additions: the 11-A isotherms showed a dependency upon the concentration of the spreading solution and also a clear compression-expansion hysteresis. With a change of the spreading solvent from chloroform to hexane/THF (2:1), the loss of diblock copolymer into the EB phase could be minimized, yet not prevented altogether. This problem in determining the correct surface mass density for the copolymers is known from previous worka18 and has been discussed in detail. It is possible to obtain the surface concentrations in situ from neutron reflectivity, and the 11-A isotherms of the present diblock copolymers at N E B have been published e l ~ e w h e r e In . ~this ~~~~ study, however, the surface pressure was chosen as the independent variable, and all SLS results for the copolymers will be presented as a function of II. By comparisonof the present results with the isotherms in refs 3-6, the abscissa could easily be converted from II to A, the area per PDMS monomer. For the aim of this paper, it is not required to perform this conversion. Surface Light ScatteringMeasurements. Our surfacelight scattering apparatus and the attendant instrumental details have appeared elsewhere.lg.W Figure 1shows heterodyne power spectra (Nicolet 4446 FFT spectrum analyzer) obtained for capillary wave propagation on the pure liquid surfaces of EB and water. The wave vectors, k = 263.0, 324.3, 385.5, 445.6, and 510.0 cm-l correspond to the fourth to eighth diffraction orders of our optical grating. Due to the higher bulk viscosity and smaller surfacetension, the peaks for EB are broader and shifted to lower frequencies compared to water. In the inset of Figure 1, the frequency shift (peak (14) Vogel, A. I. J. Chem. SOC.1948, 654. (15) Mumford, S. A.; Phillips, J. W. C. J. Chem. SOC.1950, 75. (16) Jasper,J. J. Phys. Chem. Ref. Data 1972, I, 841. Note the curve for recommended values for the EB surface tension in Figure 26 of this paper is mistaken. (17) Tmmermans, J. Physical-ChemicaZ Constants of Pure Organic Compounds; Eleevier: Amsterdam, London, New York 1965; Vol. 2, p 311. (18) Granick, S.; Hen, J. Macromolecules 1985,18, 460. (19) Sano, M.; Kawaguchi, M.; Chen, Y.-L.; Skarlupka,R. J.; Chang, T.; Zograf, G.;Yu, H. Rev. Sci. Instrum. 1986,57, 1158. (20) Shaya, S. A.; Han, C. C.; Yu, H. Reu. Sci. Instrum. 1973,45,280.
Runge et al.
1964 Langmuir,Vol. 10,No.6,1994
10
A / nm2/monomer Figure 1. Power spectra of the N E B (solid curves) and the A/W (dotted curves) interfaces at 25 OC for the wave vectors (from left to right) k = 263.0, 324.3,385.5,445.6, and 510.0 cm-'. For the sake of comparison, the intensities have been normalized to give identical peak values for water and EB spectra for each k. A low-pass filter set to 500 Hz has been used. The inset shows the k dependence of the frequency shift (fa); the curves (solid for A/EB, dotted for A/W) were fittedusing eq 1. The experimental uncertainty is given by the symbol size. frequency), fa, is plotted as a function of k for both surfaces. Using the relation12
Figure 2. Surface pressure-area isotherms of PDMS homopolymerswith different molar massesand end groupsat NEB, 25 O C : X,Me-5; A, Me-26; 0 , OH-87; 0,Me-92; 0, Me-25. See Table 1for the shortcut nomenclature of the systems. The open circles connected with two intersecting lines indicate two compression-expansion cycles for Me-25 started at II = 0.9 (dotted) and 5.4 mN/m (solid), respectively, where the branch with lower II represents the expansion cycles. The other data were obtained using the continuous addition method. The error of individual ll determinations is negligibly small (f0.02 mN/ m), whereas the overall error range can be gauged by the scatter in the data.
Results and Discussion I. S u r f a c e Pressure-Area Diagrams. The II-A isotherms obtained for the PDMS samples used in this study at A/EB are summarized in Figure 2. For the
isotherms of the different PDMS-PS diblock copolymers, we refer to the literature.a. Films of PDMS Homopolymer. PDMS films spread at A/EB show a II-A isotherm of the expanded type. The isotherms for low molecular weight PDMS (Me5) are noticeably different from those of samples with higher molar mass. This result for the EB subphase is in accordance with earlier findings for PDMS spread on other organic solvents with comparable p ~ l a r i t y Due . ~ ~to~ a~ ~ certain solubility of low M, PDMS in EB, the collase pressure is much lower than for the other PDMS samples used in this study. The samples with M, 1 25 kg/mol show only small differences and-as has been observed for PDMS at A/W26-probably no clear dependence on molecular weight. The slight deviations between Me-26, Me-92, and Me-25 are most likely explained by their different molecular weight distributions: the commercial products are blended to match a particular viscosity and may therefore contain very different amounts of low and high M, fractions as well as impurities. The isotherm for the only hydroxy-terminated sample, OH-87,is slightly different from the methyl-terminated PDMS of the same M, which may be due to an endgroup effect. The isotherms reported here for sample Me-25 are shifted by 5 1 0 % toward larger areas than published in an earlier paper using the same sample.3~5~6The shift is most likely attributable to differences in the experimental conditions, such as temperature, EB vapor pressure, and humidity. Also, as with most samples in surface studies, a certain minimal level of contamination cannot be excluded. For Me-25, the isotherms coincide for both methods, continuous additions and stepwise compressions, up to surface pressures of about 5 mN/m. As can be seen from Figure 2, hysteresis is observed for compression/expansion cycles
(21) H b d , S.; Hamnerius,Y.; Nilason, O.J. Appl.Phys. 1976,47,2433. (22) Yoo, K.-H.; Yu, H. Macromolecules 1989,22, 2606. (23) Kawaguchi, M.; Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 1735.
(24) Jarvis, N. L. J. Colloid Interface Sci. 1969, 29. (25) Ellison, A. H.; Zisman, W. A. J.Phys. Chem. 1956,60, 416. (26) Granick, S.; Clarson, S. J.; Formoy, T. R.; Semlyen, J. A. Polymer 1985, 26, 925.
y = o@/4s2k
the EB surface tension and viscosity were obtained as a0 = 34.5 f 0.3 mN/m and 7 = 2.08 f 0.20 cP, with the density fixed to the literature value of p = 1.042 g/cma. These results are consistent with literature data,le17 and a0 from SLS matches the static surface tension determined with the Wilhelmy plate technique. In the polymer film experiments three power spectra corresponding to wave vectors k = 324.3,385.5, and 445.6 cm-' were measured for each surface coverage. The power spectra were fit to a Lorentzian shape, with the frequency shift, fa, and the full width at half-height, Afa, as the spectral fitting parameters. The method by Hlvd et al.21 was used to correct the observed Afa for the Gaussian instrumental profile giving Bf,,. The accuracy for fa is about 1% ,and for AfbC5-10% due to the correctionprocedure with water, anisole, and ethanol as the standard 1iq~ids.l~ The results for fa and Afa,, were introduced into the dispersion relation? to deduce the dynamic longitudinal surface elasticity, Ed, and viscosity, K. The assumptions involved in this treatment are discussed elsewhere.7J2.22 In our calculations we used the physicalquantities for EB at 25 "C from ref 17: density p = 1.042 g/cm3, surface tension a. = 34.75 mN/m, and viscosity 7 = 1.99 cP. The values of Ed and K reported here are averages over the results for fifth to seventh diffraction orders ( k = 324.3, 385.5, and 445.6 cm-9. The analysisscheme is the same as that reported earlier.13.22.23
PDMS-PS Films at AirlEthyl Benzoate
Langmuir, Vol. 10, No. 6, 1994 1965 Table 2. Static and Dynamic Quantities for PDMS and
( A ) : A/EB I
I
PDMS-PS Films at A/EB and 25 OC
I
~
PDMSe 25-35 4-30 21-169 28-330
2 0 -"
0.0
0.1
0.2
0.3
0.4
0.2
0.1
0.0
0.3
0.4
A 1 nm'lmonomer
A / nm2/monomer 8.OVl
'
I
'
I
'
I 1
X
0
4
8
12
n I mN1m
n I "Im
Figure 3. Surface pressure isotherms for the PDMS samples Me-25, 0 (the data for the high II expansion branch are not included, cf. Figure 2), and Me-92,0, at A/EB (A) and Me-5, X, at A/W (B). Also shown as a function of II are the frequency shift cfi) and corrected full width at half maximum (Afs,J determined from fits to a Lorentzian profile for the fifth order power spectra (k = 324.3 cm-9. Long dashed curves are drawn over the data points to guide the eye. The other lines in the middle frames are the calculated f. according to eq 1(solid) and a (dotted). The Kelvin's equation, eq 1 in the limit y experimental uncertainty is 1% for fs and about 5% for Afs,c.
-
a t higher surface pressures (open circles connected by solid lines). This hysteresis may be due, a t least in part, to desorption into the EB subphase of the low molecular weight fraction of the PDMS samples having broad molecular weight distributions. A comparison of the II-A isotherms for Me-25 and Me92 a t A/EB to the M, independent isotherm for PDMS a t A/W is given in Figure 3. The results for the high II expansion cycle for Me-25 have been omitted in these plots (cf. Figure 2). At A/EB the monolayers are more expanded and the film collapse occurs a t higher surface pressure (-11.5 mN/m) than a t A/W (-9 mN/m for methylterminated PDMSl3vz6).The limiting area for full surface coverage, Ao, as obtained by extrapolation of the steepest part of the II-A isotherm to the abscissa, is about 0.22 nm2/monomer a t A/EB compared to 0.20 nm2/monomer a t A/W.I3 Also, a t A/EB no indication is obtained for the second step in the II-A diagram as is found a t A/ W (usually interpreted as a surface induced helix formation, e.g., Zisman et al.2'). The differences in polymer-subphase interaction apparently cause different molecular conformations to be preferred a t A/W and A/EB. PDMS is known to aggregate on the surface of water in the dilute (27) Fox,W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947,39,
1401.
0 0.583 0.882 0.890 0.922
11.5 12.5 9.1 11.1 11.8
9.8 10.1 10.8 11.4 11.8
~~~
4-5 5-6 4-5 7.5-8.5 5.5-6.5
Weight fraction of the PS block. Surface pressure at film collapse. Maximum dynamicsurface elasticity. d Maximum surface viscosity expressed as data intervals rather than single values because of the scatter. e Identical results were obtained for all PDMS samples with M, 1 25 kgtmol.
regime,= whereas the expanded type of isotherm for PDMS on EB suggests that the chains are well dispersed. b. Films of PDMS-PS Diblock Copolymer. Because of the solubility problem in the EB subphase, the II-A isotherms for the diblock copolymers can only be obtained when a direct method is used to determine the surface mass density in situ. As mentioned in the Introduction, it is known from previous neutron reflectivity work on these systems3*516that a steep increase in surface pressure occurs when the PS blocks strongly overlap. The rise in ll is shifted to larger areas per PDMS monomer for more asymmetric (higher PS content) copolymers. For highly asymmetric copolymers, the rise in ll appears to be related only to the molecular weight of the PS block. In this study, our focus is on the effect of the submerged PS blocks on the film properties in comparison to the pure PDMS monolayers. It is appropriate to discuss these changes either at constant PDMS surface concentration or a t constant surface pressure. Due to the difficulties involved with determining the surface mass density, we have chosen the surface pressure as the independent variable. We do not concentrate on the II-A isotherms of the diblock copolymers except to mention that the collapse pressures, II,, of the copolymers a t A/EB are comparable t o those of pure PDMS films (11.5 mN/m), while being sensitive to the molecular weight of the anchoring PDMS block. The highest surface pressures observed for each sample are collected in Table 2. These results are discussed elsewhere6 in terms of an anchoring energy of the copolymers a t the surface that increases with the molecular weight of the PDMS block. We expect that when the energy of the monolayer exceeds the anchoring energy, the copolymers desorb into the EB subphase either as isolated chains or in the form of micelles. 11. Surface Light Scattering Results. Within the tethered chain picture of PDMS-PS a t A/EB, the surface pressure can be divided into a contribution due to the PDMS blocks interacting on the surface and a second contribution arising from the interactions among the submerged PS blocks, Le., II = IIPDMS(surface) + llps (subsurface). The relative contributions will then depend on the block length asymmetry of a given copolymer. If the length of the PS block is increased, the relative contribution of PS to the surface pressure should also increase. For very highly asymmetric copolymers (high PS content), the interactions should be entirely among the PS blocks and we expect a negligible contribution from the PDMS blocks. We expect the viscoelastic behavior of interacting, submerged PS blocks to be quite different from that of surface PDMS blocks. Therefore, we expect the dynamic film elasticity and viscosity a t a given ll to be sensitive to changes in copolymer composition. In this section we show and discuss the SLS findings for the (28) Mann, E. K.; HBnon, S.; Langevin, D.; Meunier, J. J. Phys. II 1992, 2, 1683.
Runge et al.
1966 Langmuir, Vol. 10, No. 6,1994 I
10.
10 -
8.
8-
zEE \
6-
u-
4-
(
2 0
0
.I
- 8 B
5p
a ci
I
I
I
I
+i
I
Y
oc 0.0
0.1
0.2
0.3
0.4
0.5
A / nm2/monomer
X
I
I
'
I
I
I
I
I
0
2
4
6
8
n I mNlm
I
1
0
I
1
2
Figure 4. Surface viscoelasticquantities for the PDMS samples Me-25 (circles) and Me-92 (squares) as a function of area per monomer. The dynamic parameters, €d and K, are averages over three wave vectors, k = 324.3, 385.5, and 445.6 cm-*. Filled symbols in the ei (i = d, a) plots are the static elasticities (eq 2), and open symbols are the dynamic elasticities as determined by SLS. The individual errors are about f0.5 mN/m for Cd and h10-8 kg/s for K.
Figure 5. Summary of the surface viscoelastic quantities as a function of surface pressure for all PDMS samples: X, Me-& A, Me-26; O,OH-87; 0 ,Me-92; 0,Me-25. The dynamic parameters €d and K are averages over three wave vectors, k = 324.3,385.5, adn 445.6 cm-'. Filled symbols in the ei (i = d, a) plots are the static elasticites (eq 2), and open symbols are the dynamic elasticity data as determined by SLS. Errors are the same as in Figure 4.
PDMS-PS copolymers after first presenting the results for the different PDMS homopolymer samples. a. Power Spectra of PDMS Films. In Figure 3, a comparison between the SLS results for PDMS a t A/EB (on the left) and A/W (on the right) is given. The experimental SLS quantities, f , and Afs,c,are shown for the wave vector k = 324.3 cm-'; since the same trends are observed for the other k-vectors they are not shown. The dashed curves through the data in Figure 3 are guides to the eye. Since the frequency shift depends on the surface pressure, we included the results for f , according to eq 1 (by replacing 00 by a,) as solid lines. We also show as dotted lines the calculated f s using Kelvin's expression for capillary wave propagation on a simple liquid,B which is the limit of eq 1 for vanishing viscosity (y m). From the finite difference between the dotted line and the data (and solid line) a t II = 0 in Figure 3, it can be seen that Kelvin's equation is only a good approximation for the water subphase. For EB (higher viscosity and smaller surface tension), eq 1must be used to obtain agreement between the curves for pure liquid surfaces and the experimental f,, which is expected in the low II and collapse regimes. The different properties of PDMS films on EB and on water lead to clear differences in the SLS data in addition to the shift in f 8 due to the subphase viscosity and surface tension. With increasing II the more expanded type PDMS monolayers a t A/EB cause only smooth deviations from the predictions of eq 1, whereas the more condensed
PDMS films a t A/W result in a sharp drop of f s a t II 1 0. The most remarkable difference in the Afs,cplots is the existence of a small but reproducible third maximum for A/W, related to the second "transition" step in the isotherm, which is missing for PDMS films a t A/EB.
-
(29) Thomson, W.(Lord Kelvin) Philos. Mag. [s. 41 1871, 42, 362.
b. Surface Viscoelastic Parameters for PDMS Films. To obtain the dynamic surface elasticity €d, and the corresponding viscosity K , the frequency shifts and corrected full widths a t half maximum together with the static surface pressures were introduced into the dispersion e q ~ a t i o n .The ~ results for Me-25 and Me-92 as obtained from the SLS data in Figure 3A are shown in Figure 4 as a function of area per monomer. Since the II-A isotherms for the other PDMS samples show certain differences, the €d and K results for all PDMS samples are plotted in Figure 5 as a function of surface pressure. In the elasticity plots, the apparent static film elasticities, eg (filled symbols), determined from the surface pressure isotherms using
are included in addition to td (open symbols). As expected from the data shown in Figures 2 and 3, close agreement between the surface properties of Me-25 and Me-92 is found in terms of e,, ed, and K , which is clearly documented in Figure 4. Both static and dynamic elasticity show a single maximum (A 0.16 nm2/ 0.09 nm2/ monomer) and a discernible shoulder (A monomer). In the dilute regime the static and dynamic elasticity agree closely. With increasing surface mass
--
PDMS-PS Films at AirlEthyl Benzoate
Langmuir, Vol. 10, No. 6, 1994 1967
5.0 -
5.2
4.8 4.6-
4.4 -
*;
4.2 4.0 -
0
4
8
n / mN/m
Figure 6. Frequency shift q8)and corrected full width at half maximum (AfB,,) from the fifth order power spectra (k = 324.3 cm-1) for the diblock copolymers: (left side) . , 25-35; A,28-330; (right side) +, 4-30; 0,21-169; at A/EB, 25 O C , as a function of n. Dashed curves are the smoothed data corresponding to the PDMS homopolymers Me-25 and Me-92, as shown in Figure 3A. The solid lines indicate fa calculated according to eq 1. The errors determinations are the same as stated for individualfa and in Figure 3. The scatter in the data is the best measure of overall reproducibility.
density (as A gets smaller), they start to depart slightly, with E, being 10-15% smaller than €d a t the maximum and 30% lower a t the shoulder region. The maximum elasticity is about 10 mN/m which is only l/4 of the maximum €d found a t A/W. The surface viscosity shows two maxima ( A 0.23 and 0.08 nm2/monomer) of around (4-5) X 10-8 kg/s. The K values obtained here are very small and comparable to the surface viscosity for PDMS a t A/W.la30 When compared with the surface pressure as the independent variable, the dynamic film quantities Ed and K are identical within the experimental uncertainty for all PDMS samples used (Figure 5). This contrasts with the data for the apparent static elasticity, which shows a variation with M,. The term "apparent" is used here because the static elasticity is calculated from the surface pressure isotherms which have been obtained under the assumption that all the polymer molecules remain on the surface. The comparison in Figure 5 illustrates that this assumption is clearly not fulfiied for Me-5. The difference between Ed and the apparent e, is much larger for Me-5 than for the other molecular weights. We infer that a loss of molecules into the bulk subphase for Me-5 is responsible for the difference. We reiterate the important point here that both the surface elasticity and viscosity of PDMS a t A/EB show no molecular weight dependence, provided comparisons are made with respect to lI. Since we find identical €d-n and ~ - nfor all PDMS homopolymer samples, any deviations from these curves obtained for the diblock copolymers can be attributed to interactions between the tethered PS chains rather than to variations in PDMS block length. We expect the fraction of PS in the copolymer to be the important quantity affecting the surface behavior of the copolymers. We suggest the following interpretation for the general features of e,, € d , and K in Figure 5. The complexity of the viscoelastic parameter profiles in comparison to those reported for other polymers a t liquidlgas interfaces, such
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(30) Hird, S.; Neuman,R.D.J. Colloid Interface Sci. 1987,120,15.
as poly(viny1 acetate) a t A/W,7J3322923931 is probably due the extreme flexibility of PDMS and indicates severe changes of the surface film structure with increasing lI. The origin of the elasticity maximum is likely to be due to some sort of collapse of the monolayer structure as n increases. The first maximum in K is found a t areas close to the limiting area. This maximum may indicate the onset of segmental desorption and looping into the EB or air phases. At this point the low molecular weight PDMS fractions will desorb and dissolve into the bulk as mentioned earlier. We believe that this buckling or looping effect is related to the shoulder in the ed-A plots. The second maximum may correspond to the end of chain buckling and the beginning of bilayer formation. Overall we can say that PDMS a t A/EB shows the characteristics expected from a partially soluble (low M,) and very flexible polymer chain without larger sidegroups: thus the surface elasticity and particularly the surface viscosity reach only small values. c. Power Spectra of PDMS-PS Films. The SLS results for the diblock copolymers a t A/EB are shown in Figure 6. The results for the copolymers 25-35 and 28-330 are displayed on the left and those for 4-30 and 21-169 on the right. The plots are analogous to the f , and 4,,c diagrams in Figure 3A: the solid lines are the calculated results using eq 1 and the dashed curves represent the smoothed experimental data obtained for the PDMS homopolymers. Both f s and Afs,cfor sample 25-35 follow the PDMS curves very closely until a pressure of 9 mN/m. For the more asymmetric copolymers, the deviation becomes apparent a t surface pressures of between 1and 4 mN/m. At higher II,it is a general finding that both the frequency shift and the full width a t half maximum are smaller for the diblock copolymers than for PDMS homopolymer. The data shown in Figure 6 were obtained using continuous addition as well as the stepwise compression method. We observed greater scatter in the experimental parameters for the diblock copolymers than for pure PDMS films. Since there was no obvious dependence of fs and Af,,e on the method of preparing the films, we included all data for both methods in the following analysis of the surface viscoelastic quantities. d. Surface Viscoelastic Parameters f o r PDMSPS Films. The dynamic surface elasticities and corresponding viscosities are shown for 25-35 and 28-330 in Figure 7 and for 4-30 and 21-169 in Figure 8 as a function of surface pressure. We have chosen to present the results in two plots to avoid confusion because of the overlap in the ordinate values. The dashed curves represent the results for Me-25 and Me-92 a t A/EB as shown in Figures 4 and 5. The maximum values for the surface pressure, elasticity, and surface viscosity for the diblock copolymers are summarized and compared to the PDMS data in Table 2. Positive deviations in both the elasticity and viscosity for the diblock copolymers relative to the PDMS curves are clearly apparent. The positive shifts in €d and K exceed the scatter in the data, and, as mentioned earlier, can be assigned to an effect of the submerged PS block, since we have shown that there is no dependence of the viscoelastic parameters on the molar mass of the PDMS blocks a t a given n. For clarity, the discussion is divided into two parts. We begin the discussion of these results by drawing several conclusions which follow immediately from the comparisons in Figures 7 and 8 (1.Discussion). The results will then be discussed with respect to previously reported isotherms obtained using neutron reflectivity, which (31)Kawaguchi, M.; Sano, M.;Chen, Y.-L.;Zografi, G.; Yu, H. Macromolecules 1986,19, 2606.
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1968 Langmuir, Vol. 10,No.6, 1994 12c
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Figure 7. Dynamic surface viscoelastic quantities as a function of surface pressure for the diblock copolymers: B, 25-35; A, 28330. The parameters td and K are averages over the wave vedors, k = 324.3, 385.5, and 445.6 cm-*. Dashed curves represent the smoothed data profiles for the PDMS homopolymers, Me-25 and Me-92 (Figure 5). The errors for individual Ed and k determinations are the same as stated in Figure 4. The scatter in the data is the best measure of overall reproducibility. involved the same 21-169 and 25-35 samples as used in the present work (2. Comparison with Neutron Reflectivity Results). 1. Discussion. Of particular importance is the fact that the Cd curves for the highly asymmetric block copolymers show a different characteristic behavior in the high II region than for pure PDMS films. After passing through the maximum, the dynamic elasticity for the copolymers stays almost constant a t a high value (>8mN/ m) until film collapse is reached. For the PDMS homopolymers on the other hand, Ed drops with increasing II after the maximum, and values close to zero are observed a t film collapse. Thus, there exists a qualitative difference in the film properties of the diblock copolymers and PDMS homopolymers, and we attribute this to interactions among the submerged PS blocks. Another important observation is that the elasticity and surface viscosity vary markedly with the asymmetry of the copolymer. For the 25-35 sample (filled squares), the elasticity and surface viscosity follow closely the dashed curves in Figure 7 for PDMS a t low and intermediate n. Only in the high lI regime, from about 9 mN/m to the collapse pressure, does the elasticity for sample 25-35 exceed that for pure PDMS. Likewise, the surface viscosity appears to exceed the values for PDMS only for II > 10 mN/m as seen in the bottom plot of Figure 7. On the other hand, €d and K for the highly asymmetric samples (4-30,21-169,and 28-330) deviate from the PDMS values a t much lower pressures. For sample 4-30, increases in the Ed values compared to those for PDMS are observed a t II > 4 mN/m, while for 21-169 and 28-330, positive
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ll I mN1m Figure 8. Analogous plots to Figure 7 for the diblock copolymers: +, 4-30; 0, 21-169. deviations in Ed are apparent a t surface pressures of 2 and 1 mM/m, respectively. An increase in K relative t o PDMS is observed over the entire range of II for all three asymmetric samples, although the difference for 4-30 is comparable to the experimental uncertainty. The increase in K is most pronounced for the high n regime (-9 mN/m and higher) for the 21-169 sample. Although sample 4-30 has a block asymmetry comparable to 21-169 and 28-330, the fact that the increase in K is less pronounced may be related to the lower PS block molar mass. The above results can be well understood in terms of a tethered chain model for the copolymer monolayer. In this model, the PS block is submerged in the solvent while the PDMS block anchors the copolymer to the surface. For the symmetric copolymer, the interaction between neighboring chains should occur initially through the surface PDMS blocks. This is because the PDMS segments are distributed in 2D and thus occupy a larger area in the plane of the surface than the PS segments, which are distributed in 3D. Only for very high surface densities will the PS blocks begin to interact and contribute to Ed and K in the symmetric case. This explains why the results for 25-35 follow the curves for PDMS until very high surface pressures, where upward deviations then occur. As the asymmetry of the copolymer increases (increasing fraction of PS),one expects an increase in the contribution of interactions among the PS blocks relative to that for interactions among the PDMS blocks. For highly asymmetric copolymers, interactions between neighboring chains will occur only through the submerged PS blocks, while the surface PDMS blocks will not interact. The quasi-3D interaction of the submerged PS blocks is likely to yield different viscoelastic behavior than the 2D interactions of PDMS segments on the surface. This apparently explains the differences in the Ed and K curves for PDMS and the asymmetric copolymer samples. Another important point is that the collapse mechanisms
PDMS-PS Films at AirlEthyl Benzoate for the two types of monolayers are much different. While monolayers of PDMS homopolymer have been shown to collapse by forming multilayer^,^^^^^ this clearly is not the case for the block copolymers. For the block copolymers the interactions of the PS blocks should increase until the energy in the layer exceeds the anchoring energy, a t which point the copolymer will desorb into the subphase. The different collapse mechanisms likely account for the fact that €d remains high until the collapse point for the asymmetric block copolymers, while Cd drops to nearly zero a t the collapse point for PDMS. The fact that the surface viscosity is increased relative to PDMS for the 21-169 and 28-330 samples over the entire range of II suggests that these samples are sufficiently asymmetric that initial contact between neighboring copolymers occurs through the PS blocks and that the contribution from the PDMS blocks, if present, is small. A final comment, which follows immediately from the results in Figures 7 and 8 and Table 2, is that the maximum dynamic elasticities, Cd, for the diblock copolymers reach values only about 20 % higher than those found for PDMS. The surface viscosities, K, of the diblock copolymers, as determined by SLS, are higher than those for the pure PDMS films, by up to a factor of 2. However, the K values are still smaller in magnitude than for most surface active polymers or common amphiphiles a t liquiagas interfaces. These findings are in accord with results obtained by one of us (M.S.K., unpublished results) for surface shear viscosity experiments of PDMS and the diblock copolymers a t A/EB using adeep channel viscous traction surface viscometer. Over the total range of surface pressures, the surface shear viscosity in both cases was found to be close to the detection limit of the instrument ( kg/s). The low viscosity seems consistent with the interactions of relatively dilute, flexible chains dangling in solution and indicates the absence of more rigid structures such as a thin glassy PS layer or surface micelles. These latter structures were mentioned previously in connection with the sharp rise in the surface pressure isotherm for this system.3 2. Comparison w i t h Neutron Reflectivity Results. To proceed further with the interpretation of these data, we refer to neutron reflectivity data published elsewhere on this same system.= There, II-A isotherms were obtained by measuring the surface pressure with the Wilhelmy plate method and determining the surface concentration by integrating the segmental density profiles for the PS block obtained from fits to the reflectivity data. Under the assumption €d = e,, the elasticity data in the present work and the isotherms determined from the reflectivity may be compared using eq 2. One such comparison is given in Figure 9 for the 21-169 sample. This figure shows the isotherm as a function of the PDMS surface mass density, rpDMS, obtained from the reflectivity data (filled circles) along with the PDMS isotherm (dashed curve). In addition, the solid curve represents the isotherm calculated from the Cd data. Here r P D M s has been adjusted to match the first part of the copolymer isotherm. Thus, the comparison is between the shape of the curves as determined by the factor ( d n / d A ) ~in eq 2. We find that the curves have the same qualitative feature in that the slope remains high until the collapse pressure. However, we do not observe quantitative agreement. The isotherm from reflectivity rises even more rapidly than is suggested by the elasticity results. We infer from this that ed = e, is not entirely correct. In contrast, the static elasticities of the PDMS samples were much closer to the dynamic (32) Heslot, F.; Fraysse, N.; Cazabat, A. M. Nature 1989, 338,640.
Langmuir, Vol. 10, No. 6,1994 1969
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Figure 9. Three-way comparison of surfacepressure isotherms. The filled circles correspond to the isotherm for the 15-175sample determined previously using neutron reflectivity.6 The dashed curve represents the isotherm of the PDMS homopolymer. The solid curve is a calculatedisotherm based on the surface elasticity results for 21-169in Figure 8. The surfacemass density has been adjusted to coincide with the beginning of the isotherm determined by the neutron reflectivity method. The error bars are due to changes of II during the neutron reflectivity experiment and the statisticalerror in the fitting procedure of the reflectivity data.
values, with the static values falling only slightly below, as shown in Figure 4. We offer two possible interpretations for the lack of quantitative agreement in the copolymer case. It is entirely possible that the static elasticity may indeed differ from the dynamic one as obtained from SLS. (i) Although the dimensions of the submerged PS blocks are much smaller than the capillary wavelength, it is an open question whether PS segments interacting a t depths of several hundred angstroms contribute significantly to capillary wave motion in comparison to segments a t shallower depths. The apparent dynamic elasticity obtained from SLS may therefore be smaller than the “true” film elasticity. This effect plays no role in the PDMS homopolymer films. (ii) If the sharp rise in the surface pressure isotherm for the asymmetric copolymers is due to limited interpenetration of the PS blocks resulting from real chain excluded volume and uncrossibility as suggested elsewhere,6**then it is possible that the amount of interpenetration or the effects of the constraints imposed by neighboring chains may vary in the static and dynamic cases. It should be kept in mind that the segmental concentration in the submerged PS layer is always 10 mN/m from the PDMS homopolymer viscosities, while the values for 21-169 show positive deviations over the entire range of II. While quantitative agreement is not obtained for the
1970 Langmuir, Vol. 10, No.6, 1994 surface pressures at which the deviations become apparent in €d and K and for the isotherms reported previously, the qualitative trends are quite consistent between these two experiments. For the three asymmetricsamples, the molecular weight of the PS block, Mps, varies roughly an order of magnitude. One might initially expect larger increases in the surface viscoelastic parameters with increasing molecular weight of the PS block. However, the previous reflectivity work has shown that the average concentration in the PS layer decreases strongly with Mps. The maximum volume fractions in the segmental profiles of the PS blocks were found to range up to 0.16,0.08, and 0.05 for the 4-30,21169, and 28-330 samples, respectively.3-6 We believe that the offsetting effects of increasing molecular weight and decreasing concentration lead to the relative insensitivity of the viscoelastic parameters to Mps which we observe here. However, in Table I1 there does appear to be a small increase in the value of € d w with Mps. Finally, from the surface density obtained from reflectivity and the estimated radii of gyration of the dangling PS blocks, the location on the isotherm corresponding to initial contact of the PS blocks has been determined for each sample. This has been estimated to occur at a pressure around 4 mN/m for 25-35 and at a pressure near zero (i.e., within the uncertainty in the measurement, k0.2 mN/m) for 4-30,21-.169, and 28-330. Thus, the pressures at which the deviations of €d and K from the PDMS values become apparent in the present work are indeed consistent with strongly interacting PS blocks.
Conclusions We close this report by stating that clear evidence is obtained for interactions among the submerged PS blocks of PDMS-PS diblock copolymers a t A/EB. Since the PDMS homopolymers show finite solubility in EB and some fraction of all PDMS-PS copolymers may be lost to the EB bulk phase during spreading or compression, we choose surface pressure as the independentvariable rather than area per PDMS monomer. SLS has the advantage of allowing determination of the dynamic film elasticities and viscosities even in a situation where the static elasticities cannot be precisely obtained by standard methods. When plotted as a function of n, all PDMS homopolymers show identical elasticity and viscosity curves, independent of molecular weight. This independence of Ed and K constitutes a crucial element in the interpretation of the diblock copolymer results, because the PDMS block length was not uniform for the PDMSPS samples. Any deviations from the PDMS homopolymer results are attributed to interactions among the submerged PS blocks. The dynamic elasticities and viscosities for the diblock copolymers both exhibit positive deviations from those of PDMS homopolymer. The results for the diblock copolymers are consistent with the model of tethered chains. The elasticity and surface viscosity curves for the symmetric sample (25-35) follow the curves for the PDMS homopolymers up to very high pressures, where positive deviations occur. Hence, we infer that these copolymers interact initially through the PDMS surface blocks and
Runge et al.
Figure 10. Schematic illustrations of the PDMS-PS f i b at NEB consistent with the conclusions from the SLS investigation: (A) symmetric diblock copolymer; (B) asymmetric diblock copolymer (negligibleinteractionsbetween the anchoringPDMS blocks). that the submerged PS blocks interact only a t very high pressures (surface densities). This is illustrated in Figure 1OA. The elasticity curves for the more asymmetric copolymers (4-30, 21-169, and 28-330) are qualitatively different from those for the PDMS homopolymers, in that the elasticity remains high up to the collapse pressure. We interpret this result as due to the interaction of the PS blocks, and as evidence of a different collapse mechanism for the copolymer monolayers than for the PDMS films. The fact that the surfaceviscosities for allthree asymmetric copolymers are higher than that of PDMS homopolymer over the entire range of n is taken as an indication that these copolymers interact mainly through the submerged PS blocks, as illustrated in Figure 10B. Both the form of the elasticity curves for the asymmetric copolymer samples and the variation of the elasticity and viscosity curves with the block length asymmetry agree qualitatively with the copolymer surface pressure isotherms reported previ~usly.~ However, a precise quantitative agreement is not obtained, and this is attributed to a greater sensitivity of the static surface pressure to the interactions of the submerged blocks than the dynamic viscoelasticparameters as determined by the capillary wave method, SLS. The contributions of the submerged PS chains a t a given to the surface film viscoelastic parameters are of similar magnitude as those obtained for PDMS films. The observed effects are significant but small. The maximum dynamic surface elasticity found for the copolymers is 20% higher, whereas the maximum viscosity is about a factor of 2 higher than for the PDMS homopolymers (but still quite low). Even a t very high surface coverage, where the strongest interactions between the PS blocks are expected, the film viscosity is relatively small. Therefore, we conclude that the submerged PS blocks in the good solvent EB have to be considered relatively flexible, and more rigid structures such assurface micelles or a thin glassy layer of PS a t the surface may be ruled out.
Acknowledgment. We are most grateful to Alain Lapp for providing sample Me-25 and Dr. Michael Owen for providing sample Me-92 and the molecular weight analysis of the commercially available PDMS samples used in this study. We should also thank our colleague, Ms. Kaoru Tamada, for many helpful discussions. This work is supported in part by the Polymers Program of the NSF (Grant DMR-9203289) and by the U.S. Department of Energy under Contract DE-AC04-94AL85000.
