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Multiscale Aggregation of PMMA Stereocomplexes at a Surface: An Atomic Force Microscopy Investigation Y. Grohens,*,† G. Castelein,† P. Carriere,† J. Spevacek,‡ and J. Schultz† Institut de Chimie des Surfaces et Interfaces-CNRS, 15, rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France, and Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 16206 Prague 6, Czech Republic Received June 23, 2000. In Final Form: October 17, 2000 After briefly discussing the particularity of the adsorption process of PMMA aggregates in terms of aggregative adsorption, the patterns formed by poly(methyl methacrylate) (PMMA) stereocomplexes at the surface of silicon wafers, glass, and mica were investigated by tapping mode atomic force microscopy. The effects of the solvent nature, PMMA concentration, i/s-ratio, and surface nature on the morphology of the stereocomplex layer at a surface were addressed. The aggregation phenomena are well described by the diffusion-limited cluster-cluster aggregation model (DLA) and the fractal exponent D calculated. Solvent was shown to play a major role on the structure of the polymer assembly observed on silicon. The i/s-ratio strongly influences the fractal exponent D, since slow or fast aggregation can be involved. Dilute polymer solutions were used and the concentrations were varied from 0.1 to 1 g/L in acetone to reach a 2D network of connecting aggregates. The size of the aggregates increases with the PMMA concentration and is always higher than the size of the aggregates in solution measured by light scattering. This is the result of an enhanced surface aggregation of the polymer assembly indicated by the difference in size of the aggregates observed on different substrates of varying surface energy.
Introduction Structures at surfaces at the nanometer scale are needed in many application fields such as microelectronic systems, filter technology, or biosensors. Many routes have been envisaged to fabricate nanopatterned substrates such as copolymer crystallization,1,2 LC polymers’ phase ordering,3 microphase separation,4 and other self-association in amphiphilic polymers.5 An original approach based on the properties of self-organization of stereoregular polymers in certain solvents has been envisaged recently to vary the morphology of a thin layer.6 Much efforts have been devoted over the past 40 years to the elucidation of the structure of stereocomplexes obtained from isotactic and syndiotactic poly(methyl methacrylate) (PMMA). The first tentative proposal of a structure of the stereocomplexes at the molecular level was provided by Liquori et al.,7 who claimed that only the isotactic chains were of helical conformation. These assumptions were contradicted some time ago by Spevacek et al.8 and Challa et al.,9 who suggested a double-stranded helix * To whom correspondence should be addressed. E-mail:
[email protected]. † Institut de Chimie des Surfaces et Interfaces-CNRS. ‡ Academy of Sciences of the Czech Republic. (1) Liu, Y.; Zhao, W.; Zheng, X.; King, A.; Sing, A.; Rafailovich, H.; Sokolov, J.; Dai, K. H.; Kramer, E. J.; Schwarz, S. A.; Gebizlioglu, O.; Sinha, S. K. Macromolecules 1994, 27, 4000. (2) Reiter, G.; Castelein, G.; Hoerner, P.; Riess, G.; Blumen, A.; Sommer, J. U. Phys. Rev. Lett. 1999, 83, 3844. (3) Van der Wielen, M. W. J.; Cohen Stuart, M. A.; Fleer, G. J.; de Boer, D. K. G.; Leenaers, A. J. G.; Nieuwhof, R. P.; Marcelis, A. T. M.; Sudholter, E. J. R. Langmuir 1997, 13 (17), 4762. (4) Binder, K. Adv. Polym. Sci. 1999, 138, 1. (5) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992, 20, 633. (6) Serizawa, T.; Hamada, K. I.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. J. Am. Chem. Soc. 2000, 122, 1891. (7) Liquori, A. M.; Anzuino, G.; Coiro, V. M.; D’Alagni, M.; De Santis, P.; Savino, M. Nature 1965, 206, 358. (8) Spevacek, J.; Schneider, B. Colloid Polym. Sci. 1980, 258, 621. (9) Bosscher, F.; ten Brinke, G.; Challa, G. Macromolecules 1982, 15, 1442.
structure of the stereocomplex which is more consistent with the possible template polymerization of stereoregular PMMA.10 The physical gelation mechanism obtained for the s-PMMA or PMMA stereocomplex suggests the physical network formed by intermolecular association of the helix structures in concentrated solutions.11 At the nanometer scale the internal structures of the aggregates and physical gels formed by conventional (atactic) PMMA are constituted of cylinder-like structures, as highlighted12 by neutron scattering. Stereocomplex association forms aggregates which consist of arrays of rigid rods connected to one other and described by the so-called fringed-micelle model.13 The microstructure of the physical gels or aggregates obtained from PMMA solutions has been investigated by optical microscopy14 and electron microscopy.14,15 Recent work has been devoted to the behavior of stereocomplexes at the gold surface,6 where the ability of assembly formation at a surface between the two PMMA stereoisomers was demonstrated by quartz-crystal microbalance analysis. The aim of this paper is to gain a better understanding of the mechanisms of the nanoscale aggregation of stereocomplexes at interacting surfaces. Factors influencing the surface patterning such as solvent nature, PMMA concentration, i/s-ratio, and surface nature were examined. This system may also serve as a model for a better understanding of the mechanisms of proteins/surfactants aggregation near a surface. Moreover, the patterning (10) Yau, H.; Stupp, S. I. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 813. (11) Berghmans, M.; Thijs, S.; Cornette, M.; Berghmans, H.; De Schryver, F. C.; Moldenaers, P.; Mewis, J. Macromolecules 1994, 27, 7669. (12) Fazel, N.; Fazel, Z.; Brulet, A.; Guenet, J. M. J. Phys. II Fr. 1992, 2, 1617. (13) Shomaker, E.; Challa, G. Macromolecules 1988, 21, 2195. (14) Takahashi, T.; Kojima, K.; Maegawa, T. Polymer 1999, 40, 3301. (15) Quitana, J. R.; Stubbersfield, R. B.; Price, C.; Katime, I. Eur. Polym. J. 1989, 25, 973.
10.1021/la000879w CCC: $20.00 © 2001 American Chemical Society Published on Web 11/30/2000
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Table 1. Characteristics of the PMMA Used in This Study PMMA
tacticitya (%) i/h/s
Mn (10-3 g/mol)
Mw/Mn
Tg (°C)
i-PMMA s-PMMA
97:03:0 0:20:80
37 33
1.21 1.05
61 131
a i represents the isotactic triads, h represents the heterotactic triads, and s represents the syndiotactic triads.
achieved from these polymer assemblies at a surface seems to be a nice route for tuning the porosity of thin polymer films for chromatographic applications, for instance. Experimental Section The characteristics of the stereoregular PMMA supplied by Polymer Source16 are listed in Table 1. The solutions of stereocomplexes were prepared by mixing the two polymers in i/s ) 1:2, 1:1, 2:1, and 4:1 weight ratios in acetone. Only the i/s ) 1:2 weight ratio was used for toluene and tetrahydrofuran (THF) solutions. Strong complexation is known to occur between stereoregular PMMA in those three solvents.17 The concentrations of the solutions were varied between 0.1 and 1 g/L, and the solutions were equilibrated for 1 h or 20 h at room temperature. The solutions were spin-cast onto silicon wafers, glass, or goldevaporated layers which had been UV-ozone treated prior to the polymer deposition. The rotating speed was 2000 rpm, and the acceleration was 2000 rpm/s. No particular thermal treatment was performed on the layer before the AFM observation. Adsorption experiments in solution were done with a spectroscopic ellipsometer Sopra operating in the UV-visible spectral domain. Silicon wafers were immersed in a quart prism containing 5 mL of the PMMA solution at a given concentration. A stabilization time of 1 h was allowed before recording the ellipsometric angles. FilmWizard software was used for curvefitting and determination of the thickness of the adsorbed layer d and the refraction index n. The Feijter formula18 was used to convert d and n in an adsorbed amount A.
A ) d(n - n0)/(dn/dc) where n0 is the refractive index of the pure solvent and dn/dc the refractive index increment with the increase of polymer concentration in solution as measured with an Abbe refractometer. Dynamic light scattering was used to evaluate the size of the aggregates in acetone. The photon-correlation spectrometer was a Coulter Nano-Sizer commercialized by Bekman Coulter Inc. The average particle size (hydrodynamic radius) was determined at an angle of 90°. The viscosity of the solution was measured with an Ubbelohde viscosimeter at 25 °C. 1H NMR spectroscopy has allowed the determination of an associated fraction p of monomeric units in the stereocomplex which was calculated from the integrated intensities of the OCH3 lines, as described elsewhere.17 These measurements were performed 20 h after mixing of PMMA solutions, using a Bruker DPX 300 NMR spectrometer operating at 300.1 MHz. AFM experiments were performed with a NanoScope IIIa/ Dimension 3000 (Digital Instruments). They were performed in the tapping mode at ambient conditions, using the electronic extender modulus and allowing simultaneously the phase detection and height imaging. We used Si-tips (model TESP, 125 µm length) with a resonant frequency of about 300 kHz. Scan rates were between 0.5 and 1.5 Hz, and the free oscillation amplitude A0 of the cantilever was around 50 nm, with a set point amplitude during scanning slightly lower (40-45 nm), to avoid destruction of the structures on the substrates. Image treatments were performed using the Digital Instrument software. The pictures were binarized at the same level, and the size of the aggregates was calculated using the grain size function. The volume of the aggregates was calculated from the AFM (16) Polymer Source, Inc., 771 Lajoie, Montreal, Quebec, H9P 1G7 Canada. (17) Spevacek, J.; Schneider, R. Adv. Colloid Interface Sci. 1987, 27, 81. (18) De Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759.
Figure 1. Adsorption isotherm determined from ellipsometric investigation on a silicon wafer for (9) i-PMMA, ([) s-PMMA, and (2) stereocomplex (ratio i-PMMA to s-PMMA is 1:2) in acetone after 1 h of standing. pictures using bearing analysis. The fractal coefficient D was determined by using Scion Images software on the binarized AFM pictures from the equation N ∼ RD. Concentric circles with varying radius R were drawn on the AFM pictures, and the number of particles N in each concentric circle was measured.
Results and Discussion Figure 1 shows the adsorption isotherm of i-PMMA, s-PMMA, and the stereocomplexed structures on a silicon wafer in acetone calculated from ellipsometric data. It clearly turns out from these investigations that the shape of the curve corresponds to a Langmuir isotherm for both i- and s-PMMA whereas the adsorption processes of the stereocomplexes exhibit a much more complex behavior with the existence of a maximum on the isotherm. According to Lipatov et al.,19 who first described this particular adsorption isotherm, a mechanism of adsorption called aggregative adsorption was proposed. When aggregates are present in solution together with free polymer chains, the aggregates are first preferentially transferred on the surface, because of their lower solubility, increasing rapidly the adsorbed amount with the concentration of the solution. After this first regime, when the maximum of the isotherm is reached, adsorption decreases due to two possible effects: the formation of a physical network in solution which prevents the adsorption of the aggregates and/or a modification of the equilibrium between isolated and aggregated molecules (diminishing the number and size of the aggregates with an increase in the solution concentration). Another explanation, that we propose, is that, at the maximum of the isotherm, the concentration of isolated chains is sufficient to build a monolayer at the surface, preventing the adsorption of the aggregates because of the larger fraction of bound segments per chains of the isolated coils as compared to the aggregates. In the case of the adsorption of PMMA stereocomplexes, this last assumption is preferred. It is worth mentioning that, in these first experiments, the equilibrium in the adsorption is reached after a few minutes while, in the spin-cast process, the excess of solution is removed after a few seconds, hindering the equilibrium. Figure 2 shows the morphology of the PMMA stereocomplex aggregates on silicon from acetone, toluene, or tetrahydrofuran solution as well as of the noncomplexed structures obtained from chloroform solution where aggregates of stereoregular PMMA do not exist.17 Figure 2 (19) Lipatov, Y. S.; Todosijchuk, T. T.; Chornaya, V. N.; Khramova, T. S. J. Colloid Interface Sci. 1986, 110, 1.
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Figure 2. AFM pictures (5 × 5 µm2, Z ) 50 nm) of PMMA stereocomplex (i/s ) 1:2) thin layers spin-cast on silicon from different solvents: (a) chloroform (noncomplexed PMMA); (b) toluene; (c) THF; (d) acetone at a concentration of 0.4 g/L.
evidences the significant effect of the solvent nature on the patterning of the PMMA at an interface. Large aggregated structures are generally observed from the complexing solvents (Figure 2b-d) whereas an inhomogeneous PMMA layer can be distinguished from chloroform solution (Figure 2a). The diameters of the holes observed for the sample obtained from chloroform solution depend on the concentration of the solution and are ascribed either to a partial dewetting of the polymer20 or to the condensation of water and the formation of water droplets21 during the fast evaporation of the solvent. Many more investigations will be needed to get further insights on this specific point, which is not the aim of this paper. Figure 2c shows that THF solution provides densely packed circular clusters homogeneously distributed on the silicon surface. These structures are ascribed to a phase (20) Reiter, G. Langmuir 1993, 9, 1344. (21) Wang, M.; Zhu, X.; Wang, S.; Zhang, L. Polymer 1999, 40, 7387.
separation of the aggregates at the surface of strongly adsorbed single chains. This assumption is consistent with the concept of autophobicity described by Shull,22 who claimed that the equilibrium situation can be reached for bulk droplets of a polymer in equilibrium with a continuous thin film of the same polymer. Dispersed small fibril-like aggregates appear from toluene solution, as shown in Figure 2b. The aggregates from acetone solution (Figure 2d) exhibit dendritic open structures showing a high degree of branching. We will mainly focus on these fractal aggregates in the following sections of this paper. The behavior of the gel nanoparticles at an interface, as observed in Figure 2d, is close to that of colloidal particles. This is obvious from the observation of the complex random dendritic clusters obtained by other workers with colloidal systems such as gold microparticles (22) Shull, K. R. Faraday Discuss. 1994, 98, 203.
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at a surface31 or PS particles at the water/air interface.32 The mechanism of the aggregates’ formation is discussed by introducing the concept of diffusion-limited aggregation (DLA) developed by Witten and Sander.33 This model assumes that the particles originate far away from the developing immobile structure and perform a random walk in the surrounding space. Once a particle encounters the structure, it sticks to it. The DLA model produces branching structures with a fractal dimension estimated to be around D ) 1.67. Other models such as clustercluster aggregation32 assume that there is no unique seed or growth site. Aggregation is assumed to occur at any location in the system. Moreover, the newly formed clusters can move randomly and stick to other clusters to form large aggregates. The fractal dimension for that model is D ) 1.38. The homogeneous distribution of the final aggregates and their nonradial geometry whatever the concentration of the solution spin-cast on the silicon wafer tend to orient our assumptions toward the second mechanism. This is consistent with the fractal dimension D ) 1.35 calculated for our PMMA stereocomplex on silicon. The patterning observed from acetone solutions can be discussed in terms of multiscale aggregation or association processes which can be distinguishably ordered from the molecular to the microscopic scale. At the molecular level, association occurs between the chains of opposite stereoregularity, to form the double-helix structures in THF, toluene, and acetone, as was discussed in the Introduction. At a nanoscopic level, aggregation between the doublehelix sequences in complexing solvents is responsible for the formation of gel-nanoparticles in dilute solutions and physical gelation in more concentrated solutions.11 Finally, surface aggregation takes place between the gel-nanoparticles to form the microscopic structures observed in Figure 2. In this paper, emphasis will be placed on the last stage of aggregation after discussing the first two levels of intermolecular association. From a molecular point of view, we can envisage the level of intermolecular association of the polymer chains to be a significant parameter in the patterning of surfaces. Hence, the associated fractions p determined from 1H NMR spectra for the PMMA concentration 0.6 g/L are p ) 0.67, 0.71, and 0.76 for stereocomplexes (i/s ) 1:2) obtained from acetone, THF, and toluene, respectively. These values represent the fraction of repeat units for which mobility is strongly reduced due to steric interaction in the doublehelix structures of a PMMA stereocomplex.17 Actually, these three solvents have comparable associated fractions and are all considered as strongly complexing solvents. Although the association of stereoregular sequences at a molecular level is essential to obtain an aggregated structure, the associated fraction may not be considered as a driving force for the specific patterning of the PMMA stereocomplexes at a surface according to the solvent. (23) Klein, M.; Guenet, J. M. Macromolecules 1989, 22, 3716. (24) Spevacek, J.; Saiani, A.; Guenet, J. M. Macromol. Rapid Commun. 1996, 17, 389. (25) Spevacek, J.; Suchoparek, M. Macromolecules 1997, 30, 2178. (26) Saiani, A.; Spevacek, J.; Guenet, J. M. Macromolecules 1998, 31, 703. (27) Spevacek, J.; Brus, J. Macromol. Symp. 1999, 138, 117. (28) Buyse, K.; Berghmans, H. Polymer 2000, 41, 1045. (29) Mrkvickova, L.; Porsch, B.; Sundelof, L. O. Macromolecules 1999, 32, 1189. (30) Fowkes, F. M. J. Adhesion Sci. Technol. 1987, 1, 7. (31) Matsushita, M. In The fractal approach to heterogeneous chemistry; Avnir, D., Ed.; John Wiley and Sons Ltd.: New York, 1989. (32) Von Schulthess, G. K.; Benedek, G. B.; De Blois, R. W. Macromolecules 1980, 13, 939. (33) Witten, T. A.; Sander, L. M. Phys. Rev. Lett. 1981, 47, 1400.
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Actually, this statement should be somewhat tempered in view of the observations made in Figure 3. This figure depicts the effect of the ratio of isotactic to syndiotactic PMMA on the structure of the aggregates at the bare silicon surface. Changing the ratio of isotactic to syndiotactic i-isomer from the 1:2 to the 4:1 ratio leads to a decrease in the size and the degree of branching of the aggregates at the surface as well as to an increase in the size dispersion of the clusters. Moreover, one can clearly distinguish small particles for the 2:1 and 4:1 i/s-ratios, which are ascribed to isolated adsorbed coils. The average diameter of the clusters is 204, 244, 118, and 87 nm for 1:2, 1:1, 2:1, and 4:1 i/s-ratios, respectively. The largest size of the aggregates was observed for the 1:1 ratio. The stoichiometry of PMMA stereocomplex is often cited as being equal to 1:2 or 1:1, or between these values, depending on the stereoregularity of the PMMA and the technique of investigation.17 The depression in the size of the aggregates is correlated to a decrease of the associated fraction p from 0.67 for the i/s ) 1:2 ratio to 0.23 for the 4:1 ratio. Finally, we can notice that the density of the clusters regarded individually strongly decreases for the i/s ) 2:1 and 4:1 ratios. Moreover, coalescence or compact aggregation of the stereocomplex particles occurs at surfaces from the 2:1 and 4:1 ratios. The fractal exponent D equal to 1.35 for the 1:2 ratio is lower than that for the other i/s ratios, which are 1.46, 1.61, and 1.82 for the 1:1, 2:1, and 4:1 ratios, respectively. These calculated values are consistent with those reported by other workers31-33 for 2D cluster-cluster DLA. The low values of the fractal dimension D are indicative of a fast aggregation process where the probability of cluster-cluster sticking on collision is very high.31 The shape of the aggregates is rather open in that case. Higher values of D correspond to a slow aggregation process where the probability of sticking is low and provides a more compact structure of the aggregates. These observation can be correlated with the structure of the aggregates in solution. Indeed, the fraction of associated segments per chain is likely to influence the self-association of the helix sequences and the swelling of the resulting gel nanoparticles in the solvent. Thus, it is stated that the association of the helix rigid sequences acts as physical cross-links between macromolecules. At high values of p, the particles are rather compact while, at low p, the particles are surrounded by segments dangling in solution. The compaction of the particles in solution influences the surface-particle interactions as well as the interparticle interactions. Indeed, the steric repulsion between particles is increased by the dangling segments while the specific interactions with the substrate are favored. It turns out that diffusion and particle sticking at the surface are restricted for less compact particles (2:1and 4:1 ratios) inducing a slow aggregation process indicated by high fractal dimension (D < 1.5). In contrast, the compact stereocomplex particles with i/s ratio close to the stoichiometry aggregate faster at a surface, as suggested by lower D values (D > 1.5). The driving forces for the aggregation of colloidal particles on a surface are known to be mainly capillary forces.35 Nevertheless, we believe that these forces are not prevalent in the case of surface aggregation of stereocomplex particles. The first reason is that the spincoating technique of deposition induces a very rapid evaporation of the solvent and generates hydrodynamic forces which are likely to dominate the capillary forces. (34) De Boer, A.; Challa, G. Polymer 1976, 17, 633. (35) Denkov, N. D.; Velev, O. D.; Kralchevesky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183.
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Figure 3. AFM pictures (5 × 5 µm2, Z ) 200 nm) of PMMA stereocomplex of varying isotactic/syndiotactic ratios [(a) 1:2; (b) 1:1; (c) 2:1; (d) 4:1] spin-cast on silicon from acetone solution at a concentration of 0.2 g/L.
The second reason is that the stereocomplex particles are very small (50-300 nm), not spherical in shape, and swollen by the solvent. It is therefore difficult to envisage any meniscus to be generated in the solvent film around the particles during the drying process. We suggest that the main driving forces for the surface aggregation are van der Waals particle/particle interactions which become effective when the concentration of particles increases, that is, interparticle distance decreases, due to the solvent evaporation. Of course, another parameter which is important to address is the aggregate/surface interaction. The influences of glass, mica, bare silicon, and grafted polystyrene (PS) surfaces were studied after adsorption of PMMA stereocomplexes as shown in Figure 4. The average diameter of the aggregates is 38, 61, 325, and 1550 nm for PS brush, glass, silicon, and mica, respectively. The surface energy of the substrates, obtained by contact angle measurements, is 38, 63, 73, and 90 mJ/m2 for PS, glass, silicon, and mica, respectively. An increase of the surface energy, therefore, leads to an increase in the size
of the aggregates at a surface. Unexpectedly, polar polymers such as PMMA which are known to favorably interact with the high-energy surfaces tend to form larger aggregates on these substrates. Unfavorable interactions between the gel nanoparticles and a high-energy surface are assumed to be responsible for the surface-enhanced aggregation on attractive surfaces. The polar side groups are likely to be buried in the double-helix structure of the PMMA stereocomplex,17 preventing the development of specific interactions with the substrate. The presence of these strong interactions is expected to stabilize the nonaggregated polymer in homogeneous thin layers. This result highlights the influence of the polymer conformation in the dewetting-aggregation processes of polar polymers at a surface. The fractal dimension D of the aggregates calculated for the different surfaces is 1.5, 1.8, 1.35, and 1.65 for PS, glass, silicon, and mica, respectively. No correlation of the calculated fractal exponent with physicochemical properties of the surfaces has been suggested up to now.
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Figure 4. AFM pictures (5 × 5 µm2, Z ) 200 nm) of PMMA stereocomplex (i/s ) 1:2) spin-cast from acetone solution at a concentration of 0.6 g/L on varying substrates: (a) PS brush; (b) glass; (c) silicon wafer; (d) mica.
Another hypothesis is that high-energy surfaces strongly favor the adsorption of the acetone molecules, increasing, therefore, the competition between the solvent and the PMMA clusters for the surface sites. Hence, the mobility of the PMMA gel nanoparticles is increased at an acidic surface such as a Si-Ox-covered silicon wafer where the acetone basic solvent molecules can be strongly bonded. The acid-base concept was used by Fowkes et al.30 to explain the features of the PMMA adsorption in certain solvents. Cluster-cluster aggregation is likely to be favored by a stronger affinity of the surface for the solvent than for the aggregates. Concomitantly, a dewetting process of the clusters on the surface can also be assumed from the large aggregated structures formed on the mica surface. The concentration of the gel particles in solution is expected to play a major role in the AFM pattern on a silicon substrate. Figure 5 shows the effect of the concentration of stereoregular PMMA in the acetone
solution on the patterning of the stereocomplexes at the silicon surface. The main feature of these pictures is that the size of the aggregates calculated from the AFM pictures is growing with the PMMA concentration as listed in Table 2. The size of the objects present in the acetone solution can be measured by light scattering for the different concentrations. The discrepancies in the size of the objects, in the solution and after solvent evaporation, are significant and cannot only be explained by the change from 3D to 2D geometry of the aggregates or by the specificity of each technique. It clearly turns out, from these experiments, that aggregation of the stereocomplex particles present in solution still occurs during the spin-coating process at the silicon surface. Thus, the particle size of the PMMA stereocomplex in the very dilute solution, namely, 0.1 and 0.2 g/L, is 20 nm, which is close to twice the radius of gyration of isolated PMMA coils in a Θ solvent. The aggregation number can be considered as equal to 2 in these dilute solutions and increases to 25 for
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Figure 5. AFM pictures (5 × 5 µm2, Z ) 200 nm) of PMMA stereocomplex (i/s ) 1:2) spin-cast on silicon from acetone solution at varying concentration: (a) 0.2 g/L; (b) 0.4 g/L; (c) 0.6 g/L; (d) 0.8 g/L; (e) 1 g/L.
Aggregation of PMMA Stereocomplexes at a Surface Table 2. Size of the Aggregates As a Function of the Concentration of PMMA (i-PMMA + s-PMMA; i/s ) 1:2) in Acetone As Measured in Solution by DLS (Dynamic Light Scattering) and Size and Volume of the Aggregates at the Surface, Evaluated by Image Treatment Software (Grain Size or Bearing Analysis), from the AFM Images on Silicon conc (g/L)
avg particle sizea (nm)
specific viscosityb (dL/g)
volume of aggregatesc (107 nm3)
avg aggregate diameterd (nm)
0.1 0.2 0.4 0.6 0.8 1
20 ( 2 20 ( 2 50 ( 3 110 ( 5 320 ( 4 330 ( 5
0.1 0.2 0.5 1.1 3.2 9.1
1.33 ( 0.2 2.60 ( 0.3 7.90 ( 0.5 12.3 ( 1 34.2 ( 1.6 40.3 ( 2.3
130 ( 25 204 ( 30 282 ( 33 325 ( 40 498 ( 52 1050 ( 150
a In solution, as determined by DLS (dynamic light scattering). The intrinsic viscosity was measured by capillary viscosimetry. c At the surface, as determined by image treatment software from the AFM images on silicon. d Size at the surface, as determined by image treatment software from the AFM images on silicon. The size of the objects refers to the lower diameters of circles including the clusters. b
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the 1 g/L concentration by assuming no dramatic change in the hydrodynamic radius of the chains in solution due to aggregation. Besides, the aggregates observed at the silicon surface, obtained from these low concentration solutions, are roughly 3 to 6 times bigger in size than the particles in solution. From bearing analysis, the volume of the aggregates at the surface can be calculated, as a function of the PMMA concentration in solution, as shown in Table 2. This volume ranges from 1.3 × 107 to 40 × 107 nm3 along with an increase in the percentage of area of the surface covered by the aggregates, from 3.8 to 35%. These results suggest that since the increase of both the surface coverage and the volume of the aggregates is of the same order of magnitude, the aggregation only proceeds in 2D in the surface plane and not perpendicular to the surface. During the spin-coating process several phenomena occur. First, adsorption of the aggregates at the surface occurs as depicted from the ellipsometric experiments. Consequently, an increase in the concentration of the PMMA stereocomplex particles in the solvent/silicon
Figure 6. Effect of thermal history on AFM pictures (5 × 5 µm2, Z ) 100 nm) of PMMA stereocomplex (i/s ) 1:2) spin-cast on silicon from acetone solution at a concentration of 0.6 g/L and (a) nonannealed; (b) annealed for 90 min at 178 °C; (c) annealed for 1 h at 212 °C; (d) aged for 90 days at room temperature.
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interfacial region occurs. Aggregation can proceed at this step. The aggregation is further reinforced by the evaporation of the solvent, which tends to concentrate the solution before and during the spinning of the sample. For solutions of 0.6 and 0.8 g/L, the size of the aggregates in solution, measured by light scattering, increases and becomes close to that of the clusters studied by AFM, as observed from Table 2. The viscometric measurements confirm the tendency of strong aggregation in solution above 0.6 g/L. The following point can be raised: Is the mobility of the large aggregates, formed at those concentrations, restricted as compared to that of the smaller ones, formed at lower concentrations? The fractal dimension D of the aggregates at the silicon surface is 1.35 whatever the concentration. It can be concluded that the same diffusion-limited cluster-cluster aggregation mechanism occurs in the range of concentration under investigation. The question of the conformational rearrangement upon aggregation of the particles at the surface can be raised. The experiment conducted is a two-step procedure of thin film deposition which consists of spin-coating a i-PMMA solution and, subsequently, spin-casting the s-PMMA solution or vice versa. Surprisingly, the same patterns are observed at the silicon surface as those observed from the spin-coating of the stereocomplex previously formed in solution. No differences are detected whatever the stereoisomer adsorbed at first. It turns out that the aggregation forces are stronger than the Si-OH/PMMA specific interactions and that the conformational changes11 required to form aggregates are not hindered by the substrate. Moreover, the nanoparticles of gel formed “insitu” at the surface are able to diffuse at the surface to build the same fractal aggregates as observed in Figure 5 for a one-step deposition procedure. The problem of the stability of the aggregates at the surface was addressed as shown in Figure 6. The evolution of the surface patterning was studied after heating at 178 and 212 °C for 1-2 h. No modification in the size or distribution of the aggregates was observed below 170 °C. After 90 min of annealing at 178 °C, the aggregates begin to collapse at the surface and their height decreases significantly. Heating the sample at 212 °C deeply modifies the morphology of the PMMA at the silicon surface by changing the fractal aggregates to drops of PMMA scattered at the surface. The melting point of the bulk stereocomplex is still controversial. Some workers claimed that one broad melting endotherm is observed at 150 °C8 while others observed two endotherms at 185 and 213 °C.34 In this case, the first temperature is ascribed to the melting of solvent-stabilized s-PMMA and the second to the melting of the stereocomplex. The melting of the stereoregular PMMA is still under investigation because it strongly depends on the solvent from which it has crystallized and also on the degree of drying achieved before Tm measurement. The melting point of i-PMMA (60 °C) is always lower than that of s-PMMA (180 °C) from butanone or heptanone solvents.35 With annealing at 178 °C, which is close to the first Tm of the stereocomplex, our sample in thin layers retains
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the overall shape of the aggregates, with a slight decrease of their height. This probably means that the partial crystallinity, which is developed in the clusters at the surface, melts at this temperature, which is above the Tm of i-PMMA and close to the Tm of s-PMMA. The decomposition (separation of i- and s-PMMA) or melting of the stereocomplex at 212 °C totally removes the fractal morphology of the PMMA thin layer. At that high temperature, the noncomplexed polymer chains gaining a high mobility are likely to coalesce in drops, which are the most thermodynamic stable state under these conditions. Thermal treatment has not been extensively studied; however, it is expected that a more highly organized state could be reached under controlled annealing conditions. Indeed, after aging for 3 months at room temperature a spontaneous rearrangement of the PMMA stereocomplex was observed (Figure 6e). Fibrilar crystallites are formed without any exterior help. This result illustrates the metastable character of the cluster organization of the PMMA stereocomplexes at the silicon surface. Hence, fascinating possibilities for tuning the surface morphology are provided by the PMMA aggregates’ reorganization on surfaces. Much more work is needed to get further insights on the crystallization of the stereocomplexes at interfaces; this point will be the subject of another paper. Conclusion The morphology of PMMA stereocomplex aggregates was studied on various substrates. It turns out that the nature of the solvent is the predominant factor in determining the shape of the aggregates. The size of the clusters is mainly controlled by the concentration of the PMMA solution, but the mechanism of aggregation remains the same, as evidenced by the constancy of the fractal dimension. In contrast, the ratio of isotactic to syndiotactic PMMA in the stereocomplex strongly influences the mechanism of cluster-cluster aggregation by changing the compaction of the aggregates and their fractal dimension. The nature of the substrate modifies the distribution of the clusters at the surface and the fractal exponent, but this phenomenon is not fully understood. The aggregates are only metastable structures and can evolve with temperature or aging. Thus, the organization of the PMMA assembly in thin films can evolve toward a crystalline structure. The main conclusions we draw from these observations are that the association forces at the molecular level, due to the assembly formation of a double-stranded helix, are stronger than the physisorption forces at the interface. Finally, the main interest of this system is that the subtle interplay between intermolecular or interparticles forces, on one side, and particle-surface interactions, on the other side, can be tuned by several factors (tacticity, i/s ratio, solvent, ...). Therefore, these stereocomplexes could be a nice route to achieve well-designed nanopatterned surfaces. LA000879W
