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Phys. Low-Dim. Struct., 5/6(2002)pp.153162

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Scanning Prob e Microscopy Study of Polymer Molecules and Thin Films Dep osited from Sup ercritical Carb on Dioxide
M.O.Gallyamov1 , R.A.Vinokur2 , L.N.Nikitin2 , E.E.Said-Galiyev2 , A.R.Khokhlov2 , I.V.Yaminsky1 , and K.Schaumburg3 Physics Department, Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russia 2 Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova Str., 28, 119991 Moscow, Russia Centre for Interdisciplinary Studies of Molecular Interactions, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100, Copenhagen, Denmark
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(Received 3 March 2002, accepted for publication 6 March 2002)
We have shown that polymer solutions in supercritical carbon dioxide are convenient model systems to study the macromolecule-solvent interactions. One important advantage of such systems is that the macromoleculemacromolecule and macromolecule-solvent interactions depend strongly on the temperature and pressure of a supercritical medium, which can be easily varied during the experiment. Copolymer Teflon r AF2400 (material which is of much interest for technological applications) was chosen as an ob ject to be studied. The controlled decrease of a solvent quality (by controllable change of temperature or pressure) was applied to deposit on solid substrate surfaces both individual compacted macromolecules and thin-film coatings. Morphology of these structures was explored by scanning probe microscopy.

1. Intro duction Scanning prob e microscopy (SPM) is currently widely used in morphology examinations of thin films [1, 2], individual p olymer molecules [3], their complexes with surfactants [4, 5], proteins [6, 7], and other ligands [8]. This is achieved due to high spatial resolution of the metho d in combination with the c VSV Co.Ltd, 2002

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absence of strong restrictions on the prop erties of the ob ject and on medium of environment. While studying thin-film coatings SPM allows b oth to explore uniformity, homogeneity, defect density of a film and to determine lo cal lattice parameters (for two-dimensionally arranged films) [1, 2]. Also it is p ossible to acquire the information ab out the thickness of a coating or to create artificial structures and defects by controlled lo cal influence of a prob e [9]. Various approaches to form homogeneous and defect-free thin p olymer films were tested by SPM. The authors of Ref. [10] investigated by atomic-force microscopy (AFM) the p olystyrene-p oly(4-vinylpyridine) films prepared on a flat silica surface, dep osited by spin-coating and dip-coating metho ds. In b oth cases the excess of material was removed by washing with a solvent at the final stage, after that the samples were dried. This pro cedure resulted in the changes of film surface morphology. It was found that the degree of film inhomogeneity dep ended on the prop erties of the solvent used. Recently AFM studies [11, 12] were applied to self-assembled film with p oly (diallyldimethylammonium chloride) + sulfonated p olyaniline [11] and p oly(oethoxyaniline) + sulfonated lignin [12] (p olyanion-p olycation layers dep osited onto substrate). Created coatings were characterized by high degree of inhomogeneity and had grain structure. This inhomogeneity can b e a consequence of the film drying pro cedure. It was shown [13] using AFM and small-angle neutron scattering that the p olymer film surface b ecomes more rough and reveals cluster structure after it is dried. The drying pro cedure results in nonequilibrium and uncontrollable increase of concentrations. The ob jects to b e dep osited can b e removed from a substrate by the strong hydro dynamic flows which may arise when drying droplet. This complicates the repro ducibility of results and essentially changes the morphology of adsorb ed structures [14]. This problem can b e solved when the macromolecules visualization is carried out directly in liquid medium [15, 16]. However, there are other problems, among which we can mention the problem of macromolecule fixation on a substrate surface during scanning pro cedure. Therefore we b elieve that the use of sup ercritical (sc) media as solvents is rather promising for the dep osition of macromolecules on a substrate. The carb on dioxide (CO2 ) is a prop er choice, b ecause it can b e easily converted to a sup ercritical state (T > 31.1 C, P > 7.38 MPa). This fluid is a solvent for some p olymers and has unique prop erties: (i) the macromolecule-macromolecule and macromolecule-solvent interactions dep end strongly on temp erature and pressure, esp ecially near the critical p oint [17]; (2) CO2 has no liquid phase under atmospheric pressure, so it leaves a material completely and very rapidly after decompression. Thus, scCO2 application as a solvent for macromolecules dep os-

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ition on a substrate allows to eliminate the drying pro cedure and to overcome the problem of a residual solvent. It is of no doubt, that the interest to this technique will increase. This is stimulated by environment safety of CO2 in comparison with organic solvents used in technological pro cesses of p olymer coatings creation. It was found [18] that b oth sup ercritical and liquid CO2 are go o d solvents for p erfluorop olyethers. These p olymers are widely utilized for creation of protective coatings, for example in computer hard disks pro duction. Rapid expansion of sup ercritical solutions (RESS) was applied to the dep osition of an ultrathin paraffin coating from scCO2 solution on spherical SiO2 particles [19]. During rapid expansion after escaping from a nozzle the sup ercritical p olymer solution transfers into a sup ersaturated state. If the substrate is placed into the solution flow, a thin p olymer film is adsorb ed on a substrate surface from expanding sup ersaturated solution. The authors of the pap er [20] have applied the RESS metho d to creation of sensor coating from p olydimethylsiloxane microspheres. The RESS technique was applied for creation of films from a susp ension of a p olymeric comp ound in scCO2 [21]. The susp ension of p oly (2-ethylhexyl acrylate) in scCO2 was stabilized by the Monasil surfactant. However, prepared films were usually characterized as rather non-uniform surfaces. The solubility of p olymers in scCO2 was investigated by cloud p oint observation for a numb er of comp ounds including Teflon r AF class cop olymers [22]. The Teflon r AF cop olymers are optically transparent materials in a wide sp ectral range (from IR to UV). They have extremely low values of refractive index and dielectric constant (up to GHz range) and are characterized by high thermal stability (up to 300 C). Due to unique optical prop erties and high stability the Teflon r AF thin films can find diverse applications in optics, electronics, optoelectronics, etc. These cop olymers are chemically resistant against the ma jority of solvents and aggressive media, but, at the same time, they can b e dissolved in scCO2 . This fact do es make this class of cop olymers esp ecially unique and prosp ective one for sup ercritical technologies. 2. Materials and metho ds Teflon r AF2400 cop olymer (DuPont, Mn 105 , = 1.6g/cm3 , Tg = 250 C) was used in exp eriments. Chemical structure: The high purity CO2 (> 99.997%, GOST 8050-85 Russia, 0.0002% O2 , 0.001% H2 O) was used as received. The mica (muscovite) and highly oriented pyrolytic graphite (pyrographite)

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were chosen as substrates for dep osition of p olymer molecules. Exp osure and dep osition were carried out using exp erimental high-pressure setup describ ed earlier in Ref. [23]. The system p ermits to create pressures up to 50 MPa and to transfer CO2 into a sup ercritical state. The system temp erature was regulated using a circulating water thermostat. For the dep osition pro cess a p olymer comp ound was placed at the b ottom of the reaction chamb er (10 ml volume); a substrate was mounted ab ove it on the holder. At first, the cuvette with p olymer and substrate inside was intensely vented by CO2 to remove air and water. After that, the cuvette was sealed and the necessary density of CO2 was pro duced at the given temp erature and pressure. Then the cuvette temp erature was raised up to necessary value. This resulted in the increase of the pressure also (the increase was calculated knowing the density and temp erature). Primary equilibrium of a p olymeric solution was reached during several hours. Then the pressure or temp erature in the cuvette was gradually reduced which resulted in the decrease of the solvent quality and macromolecules dep osition on a substrate surface. After completion of the precipitation pro cess the cuvette was decompressed. The substrate with dep osited p olymer molecules was extracted and its morphology was explored by AFM. The AFM measurements were carried out in contact and tapping mo des in air or in a liquid using "Nanoscop e-I I Ia" prob e microscop e (Digital Instruments, USA). The device was equipp ed with "D"-scanner (dynamic range 15 в 15 в 4 µm3 ). AFM-images were collected with the information density of 512 в 512 p oints at scanning frequencies of 1 Hz in the tapping mo de in air, 13 Hz in the tapping or contact mo des in liquid medium, and 5 Hz in the contact mo de in air. We used silicon nitride NP-S cantilevers and silicon TESP cantilevers (Nanoprob e, Digital Instruments, USA). To build-up the AFM-images we applied "Femtoscan Online v.1.2" software (Advanced Technology Center, Russia). 3. Results and Discussion The typical solubility area of Teflon r AF cop olymer with 35:65 comp osition, i.e. tetrafluoro ethylene to dioxole monomer unit ratio, corresp onds to: T > 65 C, P > 50 MPa according to Ref. [22]. In our case the corresp onding ratio is 10:90, which allows to exp ect b etter solubility (it is the interaction of dioxole-group with CO2 that determines the solubility of these fluoro containing cop olymers). The dissolving capacity of sup ercritical media is highly sensitive to pressure and temp erature (which define also solvent density). The prepared p olymer solution can b e moved out of the solubility area in different ways, for example:

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Figure 1. Polymer film deposited onto pyrographite substrate. Topography AFM image obtained in contact mode in air (a). The image (a) was modified by "highlighting" procedure to depict fine structure (b). Exposition at T = 55 C, P = 75 MPa for 3 h followed by temperature lowering down to 20 C and decompression.

(i) by reducing the temp erature in the closed cuvette; (ii) by reducing the pressure with CO2 outlet from the cuvette. The decrease of solubility results in macromolecules precipitation. Figs. 1 and 2 present typical AFM images of cop olymer Teflon r AF2400 thin film coatings dep osited from sc CO2 on pyrographite substrate. Initial stage of the cop olymer dissolution and the stage of equilibration of the solution were carried out during several hours at thermo dynamic parameters corresp onding to the solubility area. The dep osition pro cedure was p erformed by slow decrease of temp erature in the closed cuvette. The dep osition pro cedure was terminated by cuvette decompression, during which the unused material was removed by stream of CO2 coming out from cuvette. Figs. 1 and 2 demonstrate that the applied pro cedure really allows to create ultra thin p olymer coatings characterized by high degree of uniformity and extremely small amount of impurities on the substrate surface. The surface of coatings has low roughness that allows to conclude ab out the close-packed arrangement of the film-forming molecules. The uniformity of the film structure is much higher compared with the coating usually prepared by traditional filmforming techniques (describ ed in Intro duction). Besides, the formed coatings are ultra thin ones characterized by nanometer-level thickness.

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Figure 2. Polymer film deposited onto pyrographite substrate. Topography AFM image obtained in tapping mode in air (a), and in ethanol (b). Exposition at T = 55 C, P = 80 MPa for 3 h followed by temperature lowering down to 20 C and decompression.

We can state that these films are characterized by quality which is comparable with the quality of films prepared by Langmuir-Blo dgett metho d. However, thin LB films (few monolayers) often app ear to b e unstable and inclined to reorganization [9]. Ours coatings maintain their structure with time and during scanning pro cedure not only on air, but also in liquid medium, see Fig. 2b. It can b e seen in the images of Fig. 1 that the prepared coating has lamellar structure, and the angle of mutual orientation of lamella is close to 30 . The domain structure of the film is clearly seen in Fig. 2a: co existence of two typ es of domains (distinguished by surface morphology) with clear b oundaries is observed. The formation of domains and lamellae in thin film from amorphous cop olymer can b e explained probably by the substrate influence; mechanism of their creation requires further examination. The isolated defects in the film structure are observed in the images of Fig. 1 and Fig. 2b which allows to measure the film thickness. We found that the thickness of our thin film coatings is equal to 58 nm on graphite surface for different parameters of the exp osure. According to our observation the material excess do es not pro duce an increase of the film thickness and is sp ent to formation of aggregates, see Fig. 1 and Fig. 2a. One can estimate the size of compact globule containing one Teflon r AF2400

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Figure 3. Polymer film deposited onto mica substrate. Topography
AFM image obtained in contact mode in air. Exposition at: T = 65 C, P = 79 MPa for 6 h (a), and T = 35 C, P = 40 MPa for 3 h (b) followed by temperature lowering down to 20 C and decompression.

cop olymer molecule using the formula: D=2
3

3M , 4

(1)

where M is the molecular mass and is the material density. Applying the estimation for the molecular mass M Mn /NA we estimate the diameter of the compact globule as D 6 nm. The film thickness exp erimentally determined by AFM approximately coincides with the estimation of compact globular size. The AFM image in Fig. 2b is obtained in a liquid cell. When using ethanol as an investigation medium, it was p ossible to achieve b etter spatial resolution (in tapping mo de) in comparison with the exp eriments in air or in water. It can b e explained either by b etter dynamics of the prob e-sample force interaction in ethanol [24] or by more effective removal of p ossible traces of contamination from the film surface. When using mica as a substrate, the formed thin film revealed other typ e of morphology. The excess of a p olymer material in this case results in increase of coating thickness. However, the film formation pro cess was followed by structural reorganization of p olymer molecules on the substrate. This reorganization is seen in the AFM image of the film (Fig. 3a). The created film is not homogeneous in thickness (as on pyrographite substrate) but consists of a set of

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Figure 4. Polymer molecules deposited onto mica substrate. Topography (a) and phase (b) AFM images obtained in tapping mode in air. The phase image (b) is characterized by better contrast. Exposition at T = 60 C, P = 70 MPa for 4 h followed by temperature lowering down to 50 C and decompression.

structures with complex morphology. 20 to 60 nm ab ove the substrate surface. In more mild exp osure conditions (at lower p olymer solubility) it is p ossible to form the film 47 nm in thickness (Fig. 3b) on a mica surface. The dep osited film is not closely packed, that allows to see individual globules. The images of individual p olymer molecules dep osited on a mica substrate are given in Fig. 4. Two typ es of structures are seen. Higher ob jects have 2.55 nm height. We supp ose that they are either completely compacted individual globules or compacted regions of individual p olymer molecules. At the same time the ob jects with smaller height (0.51nm) can b e seen which are most probably the partially compacted macromolecules. The morphology of these structures is clearly visualized in the phase image (Fig. 4b). The corresp onding top ography image is shown in Fig. 4a. The phase image is obtained in tapping mo de and its contrast is determined by the difference of visco elastic prop erties of the substrate and adsorb ed ob ject which usually allows to achieve the b etter contrast in comparison with the top ographical image. 4. Conclusion We have explored the structure of thin-film coatings and individual molecules of Teflon r AF2400 cop olymer dep osited on different substrates from solutions in

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sup ercritical CO2 . Using AFM we have shown that the thin Teflon r AF2400 films dep osited on pyrographite substrate are characterized by p erfect homogeneity (in comparison with other typ es of p olymers films), they reveal high resistance against mechanical influence of the scanning prob e, and stability in air and in liquid medium. At the same time the coatings are ab out 6 nm in thickness (this corresp onds to thickness of one monolayer of closely packed compact globules). Films formed on a mica substrate are less homogeneous and tend to reorganization. It was also p ossible to visualize b oth completely and partially compacted individual p olymer molecules. Such examinations may contribute to b etter understanding of macromolecule conformational changes in sup ercritical solutions. Acknowledgements This work was supp orted by the Russian Foundation for Basic Research, pro ject No 01-03-32766a. References
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