0 Copyright 1994 American Chemical Society
AUGUST 1994 VOLUME 10,NUMBER 8
Letters Surface Morphology of a Polymer Blend Examined by Laser Confocal Fluorescence Microscopy Lin Li,? Stanislaw Sosnowski,+Charles E. Chaffey,t Stephen T. Balke,* and Mitchell A. Winnik*>t Department of Chemistry and Department of Chemical Engineering, University of Toronto, Toronto, Ontario, Canada M5S 1Al Received December 8, 1993. In Final Form: February 22, 1994@ Films composed of a mixture of poly(methy1methacrylate) (PMMA,Mw= 306 000) and polystyrene (PS, Mw = 234 000)were prepared by solvent casting. In these films, the PMMA component,covalently labeled with a fluorescent dye, comprised 10 wt % of the mixture. These films were examined by laser confocal fluorescence microscopy (LCFM). At depth greater than 5-6 pm below the surface, the blend morphology was one of tiny PMMA spheres dispersed in a continuous PS matrix. These spheres were randomly distributed in space and were characterized by a broad size distribution. At the surface, the morphology was very different. The PMMA was present in the form of large spheres, 5-6pm in diameter, characterized by not only a very narrow size distribution but a strong periodicity in their location. The broad distribution of small particles in bulk can be attributed to phase separation by a nucleation and growth mechanism coupled with the small surfaceenergy between PS and PMMA. At the surface,two other and larger surface energies come into play, those of PSIair and PMMAfair. In addition, more rapid evaporation of solvent from the surface may lead to a spinodal decomposition mechanism for surface phase segregation. In polymer blends, the surface is normally enriched in the substance of lowest surface energy.l This factor is often taken into account in designing blends for specific uses, as for example the lubricating effects of certain silicone or fluorocarbon additives, or the use of small amounts of polyisobutylene in polyethylene “cling wrap” to render the surface tacky. The surface ofthese materials can be examined in a variety of ways. One can quantify the wettability of the surface by contact angle measurements. More sophisticated methodologies employ ultrahigh vacuum techniques such as ESCA and SIMSto detect components in the surface layer. With these techniques, the surface layer probed ranges in depth from a few angstroms to a few nanometers. Evanescent wave techniques (e.g. ATR-IR) sample depths into the micrometer + Department of Chemistry.
* Department of Chemical Engineering. @
Abstract published in Advance ACS Abstracts, June 15,1994.
(1)Koberstein, J. T. Interfacial Properties. In Encyclopedia of Polymer Science and Technology; J. Wiley & Sons: New York, 1987;
Vol. 8,pp 237-279.
range. We are interested in blends of polystyrene-poly(methylmethacrylate) (PSPMMA). Here, in spite of surface energies that are similar in magnitude,2 elegant specular neutron reflectivity measurements on PSPMMA block copolymers, which form lamellar phases, have demonstrated the strong tendency for the PS block to form the surface layer.3 To increase our understanding of the surface morphology of multicomponent polymer systems, one needs techniques that provide two distinct types of information. These must provide spatial resolution on various length scales within the surface layer and also provide sufficient depth resolution so that one can observe the transition from surface to bulk structure in the material. When the domain sizes are on the order of micrometers, they should be visible by optical microscopy. Nondestructive depth ~~~~
~
(2)Wu, S.J. Phys. Chem. 1970,74, 632. (3)(a) Anastasiadis, S.H.; Russell, T. P.; Satija, S. K.; Majkrzak, C. F. J. Chem. Phys. 1990, 92, 5677. (b) Russell, T. P.; Menelle, A.; Hamilton, W. A.; Smith, G. S.; Satija, S. K.; Majkrzak, C. F. Macromolecules 1991,24, 5721.
0743-7463/94/2410-2495$04.50/0 0 1994 American Chemical Society
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2496 Langmuir, Vol. 10, No. 8, 1994
profiling becomes possible with laser confocal fluorescence microscopy (LCFMI.4 As in other fluorescencemicroscopy technique^,^ contrast is achieved by labeling or staining one component with a fluorescent dye. The special feature of confocal optics is the presence of two pinholes. These operate to reject from the detection system all emitted light which does not originate in the focal plane. Thus one measures successive slices through a material to a depth of ca. 10 pm. LCFM has been used extensively in biology6 and to some extent in the colloid field.' We know of no previous reports of its application to the study of polymer blends. Over the past 6 months, we have been exploring the applications of LCFM to blends of PS and PMMA. Our long term interests are in the factors that affect the interface or interphase in multicomponent polymer systems. The PSPMMA system is a natural place to begin because it has been studied extensively by a variety of technique^.^^^^^ In order to obtain more global information about blend morphology as a function ofblend preparation conditions, we prepared samples of PMMA labeled with the dye NBD and examined its blends with unlabeled PS by LCFM. Our initial observations of their surface morphology were very surprising. This communication describes the surface structure in these blends and examines its transition to bulk morphology. We identify factors which we believe play a leading role in determining these structures.
Experimental Section The labeled PMMA sample was prepared by two-stage surfactant-freeemulsionpolymerizationof MMA with N-methylN-(4-(7-nitrobenzo-2-oxa-1,3-diazole))-2-aminoethyl methacrylate (1, NBD-MMA).lO Full details will be reported elsewhere. Latex seeds were first formed by using a small amount of MMA and a part of the initiator (potassium persulfate, KPS). To this dispersion was added simultaneously, by continuous feeding under monomer-starved conditions, the remaining reactants (MMA + 1,and KPS). Thus the fluorescent dye was added only in the second stage. The final latex was purified by membrane dialysis. By gel permeation chromatography (GPC, PMMA standards)this sample had M , = 306 000,M,JM,, = 2.3,with an NBD chromophore content (UV-VISanalysis)of 6.3x moVg polymer. The sample of PS employed here was prepared by surfactant-free emulsion polymerization in batch and was analzyed by GPC ( M , = 234000, M#M,, = 6.3,polystyrene standards). The films examined here were prepared by solvent casting from toluene solution. First, a small amount of the PMMA (4)(a) Wijnaendts van Resandt, R. W.; Marsman, H. J. B.; Kaplan, R.; Davoust, J.; Stelzer, E. H. R;Stricker, R. J . Microsc. 1985,138,29. (b)Wilson, T.,Ed. Confocal Microscopy; Academic Press: London, 1990. ( c ) Wampler, J. E. Ed. New Methods in Microscopy and Law Light Imaging; SPIE-International Society of Optical Engineers: Bellingham, WA, 1989. ( 5 ) See, for example, Billingham, N. C.; Calvert, P. D. Uzuner, A. Polymer 1990,31,258. (6) (a) Brakenhoff, G. J.; van der Voort, H. T. M. van Spronse, E. A.; Nanninga, N. Nature 1985,317,748; Scanning Microsc. 1988,2,3340. (b) Franksson, 0.; Liljeborg, A.; Carlsson, K.; Forsgren, P. In Scanning Imaging Technology; Hayat, M. A., Ed.; Academic Press: Orlando, FL, 1989;pp 36-43. (c) James, J.; Tanke, H. J. Biomedical Light Microscopy; Kluwer Academic Press: Dordrecht, Netherlands, 1991. (d) Boyde, A.In Modern Microscopies: Techniques and Applications; Duke, P. L., Michette, A. G., Eds.; Plenum Press: New York, 1990. (e) Miller, W. I.; Foster, B. Am. Lab. 1991,23,73. (7) (a) Sasaki, K.; Masanori, M.; Masuhara, H. J. Opt. Soc. Am. A : Opt. Image Sci. 1992,9, 932. (b) Dosho, S.;Ise, N.; et al. Langmuir 1993.9. 394. (8j Russell, T. P. Macromolecules 1990,23,890. (9)Law, W. W.Y . ;Bums, C. M.; Huang, R. Y. M. J.App1. Polym. Sci. 1986.30. , - , 1187. ~~(10)(a) smigol, V.; svec, F.; Hosoya, K.; Wang, Q.; Frbchet, J. M. J. Ang. Makromol. Chem. 1992,195,151.(b) Tuin, G.;Peters, A. C. I. A.; van Diemen, A. J. G.; Stein, H. N. J. Colloid Interface Sci. 1993,158, ~
~~
508.
TOLUENE
A
\
PMMA
I
0.50
'
0.90
PS
w t % PS
Figure 1. A phase diagram for the system polystyrene-poly(methyl methacrylate)-toluene for three different samples of PMMA, as taken from ref 9. The dashed line refers to 9 : l mixture of P S P M M A which we examined here. The numbers refer to the molar masses of the constituent polymers: 1, PS 100K-PMMA 180K, 2, PS 100K-PMMA 69K, 3, PS 37KPMMA 180K, where K implies lo3 mass units. dispersion was mixed with the PS dispersion to give 1:9 weight ratio of the two polymers, and the mixture was freeze-dried. Then the mixture (33mg) was dissolved in toluene (821 mg) to give a solution of ca. 4 wt % solids. This solution, in a stoppered flask, was shaken and then allowed to stand for more than 24 h at room temperature to ensure equilibration. The solution remained clear. Films for LCFM experiments were prepared by first placing a small,carefully cleaned quartz plate (11x 25 mm) in a petrie dish (inner diameter 50 mm). A few drops of the toluene solution were then placed upon the plate. The petrie dish was covered, and the solvent was allowed t o evaporate at room temperature (23"C). Evaporation occurred over a period of several hours. After 24 h, the film was placed in a vacuum oven at 100 "C for 24 h t o remove residual solvent. Films were examined using a laser scanning confocal microscope (Bio-Rad MRC 600),using the 488-nm line of the Ar ion laser for dye excitation.
Results and Discussion The initial solution from which the film is cast contains three components: PS, PMMA, and toluene. Several research groups have examined this system, and a phase diagram taken from Law et aL9is presented in Figure 1. Their samples were of narrow molecular weight distribution, but the essential features of the figure pertain to our system. When sufficient toluene is present, a single phase exists. As the solvent evaporates, the system passes into a two-phase region and begins to demix. A dashed line indicates the trajectory for a 9:lPSPMMA mixture. The final morphology obtained in bulk will be determined by the rate of solvent evaporation compared to that of demixing. An important feature of the mechanism of the process is the amount of time the system remains within the metastable binodal region and when it crosses into the spinodal domain. Once the system approaches the glassy state, further evolution of morphology will become very slow. Annealing the system at or near the glass transition temperature (ca. 110 "C) will drive residual solvent from the film but not lead to extensive polymer diffusion. Within the binodal, nucleation and growth normally give rise to a dispersion of spheres ofthe minor component in the major component matrix. In contrast, spinodal decomposition leads initially to a bicontinuous network which eventually breaks up into droplets. Both mechanisms would lead to the formation PMMA spheres in a PS matrix. In Figure 2 we present images of the final blend
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Langmuir, Vol. 10, No.8, 1994 2497 tension between PS and PMMA (3.2dyn/cm at 23 0C)2but the much smaller surface tension between PS and PMMA in the presence of toluene (ca. dyn/cm).ll The most likely origin of the broad distribution of droplet sizes is phase separation by nucleation and growth,12a-cwhich seems consistent with the trajectory for the evolution of this system as indicated in Figure 1. The real surprise is shown in the uppermost image in Figure 2. This image is of the surface layer, a slice ca. 0.5 pm thick at the air-film interface. Two features are striking. The first is the absence of small domains and the nearly uniform size of the PMMA spheres, 5-6 pm in diameter. The second is their remarkable periodicity. The middle image in Figure 2 is of a slice 3 pm below the surface. Here one sees the lower part of the large PMMA spheres located at the surface, as well as the much smaller droplets characteristic of the PMMA in bulk. On a spatial scale of 1-2 pm, we find a sharp transition between surface morphology and the bulk morphology of the system. There are two possible explanations for the morphology differencesobserved at the surface. First the aidpolymer surface tensions are both very much larger than that in bulk ( y w a i r = 40.5 dyn/cm and Y P w a i r = 40.9 dyn/cm at 23 0C).2 While these values from the literature are remarkably similar, the finding of the strong surface preference of PS over PMMA in PS-PMMA block copolymers3suggests that surface tension differences could drive the coalescence of PMMA-rich droplets at the air surface following phase separation. This would explain the large droplet size but not the regularity of droplet spacing. A more attractive explanation is that during film preparation, solvent evaporation from the surface is rapid. Phase separation begins at the surface after the surface composition passes into the spinodal region of the phase diagram. A skin formed in this way would slow down evaporation of solvent from the bulk contributing to the crossover in phase separation mechanism. We are as yet unable to monitor the phase-separation process in real time. This is an experiment worth doing which should allow this issue to be resolved. Nevertheless, it is well recognized that spinodal decomposition leads to periodicity in domain formation.12*13Thus the crossover in phase separation mechanism provides a convenient explanation for both the surface and bulk morphologies of the films.
film surface, t h e middle image is of a slice 3 p m beneath t h e surface, and t h e lowermost image is of a slice 6 p m beneath t h e surface. I n each image, t h e bar represents 25 pm.
Acknowledgment. This research was supported by the Ontario Centre for Materials Research with additional assistance from NSERC Canada. We thank Dr. X. Gu of the Ontario Laser and LightwaveResearch Centre for his help in using the laser scanning confocal microscope.
film as a function of depth of the focal plane from the surface. The PMMA domains are prominent against the dark PS background. In the lowermost image, correspondingto the bulk phase of the film, one observes a broad distribution of sizes for these spheres, ranging a few micrometers in diameter to sizes ( ~0.5 pm) beyond the resolution of the microscope. The spherical shape is a consequence of surface energy minimization within each droplet, and the presence of small droplet sizes reflects not only the small surface
(11) Venugopal, G.; Krause, S. Macromolecules 1992,25,4626. (12) (a) Nishi, T. CRC Crit.Rev. Solid State Mater. Sei. 1985, 12, 329. (b)Tanaka,H.;Nishi, T. Phys.Reu.Lett. 1985,55,102. (c) Tanaka, H.; Nishi, T. Phys. Reu. A. 1989, A39,783. (d) Tanaka, H.; Hayashi, T.; Nishi, T. J . Appl. Phys. 1986, 59, 3627 and 1989, 65, 4480. (e) Tanaka, H.; Suzuki, T.; Hayashi, T.; Nishi, T. Macromolecules 1992, 25,4453. (13) (a) Hashimoto, T.; Sasaki, IC;Kawai, H. Macromolecules 1984, 17,2812. (b)Sasaki, IC;Hashimoto,T. Macromolecules 1984,17,2818. (c) Hashimoto, T.; Takenaka, M.; Jinnai, H. J . Appl. Crystallogr. 1991, 24, 457.
Figure 2. Laser confocal microscopy images of solvent cast film (from toluene) comprised of 10 wt % PMMA labeled with t h e dye NBD and 90 wt % polystyrene. The top image is of t h e
