Langmuir 1997, 13, 4407-4412
4407
Structure Formation in Thin Poly(E-caprolactone) Films Jo¨rg Kressler* and Chun Wang Institut fu¨ r Makromolekulare Chemie und Freiburger Materialforschungszentrum, Albert-Ludwigs-Universita¨ t, Stefan-Meier-Strasse 21, D-79104 Freiburg i. Br., Germany
Hans Werner Kammer* School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia Received September 16, 1996. In Final Form: May 5, 1997X The film formation behavior of a semicrystalline polymer, poly(-caprolactone) (PCL), during spincoating from solution in cyclohexanone and after thermal annealing, has been studied by light microscopy, light scattering, scanning electron microscopy, and atomic force microscopy. During the spin-coating process, film formation is ruled by competition of dewetting and crystallization. Depending on the concentration of the solutions, PCL films completely cover the glass substrate, open spherulites are formed, or for very dilute solutions, PCL forms dispersed droplets on the glass substrate. Films, covering the substrate, show during heating above the melting point of PCL dewetting phenomena that are influenced by both film thickness and initial crystalline morphologies.
Introduction The structure formation of thin polymer films on different substrates has been studied widely due to its fundamental theoretical and practical relevance.1,2 Spincoating of polymer solutions allows deposition of very uniform and thin films (thickness < 100 nm) on a solid substrate. These films are unstable if the spreading coefficient, S, is negative. Nevertheless, even for S < 0, continuous polymer films can be formed owing to rapid solvent evaporation. At sufficiently high temperatures, relaxation toward equilibrium advances during thermal annealing. Hence, film rupture occurs, which results in the formation of holes. This process is usually referred to as dewetting.3-6 Polystyrene films show, e.g., spontaneous dewetting on silicon wafers during thermal annealing above the glass transition temperature when their original thickness is less than 100 nm.4 In this paper, we discuss morphology formation and dewetting of thin films of a semicrystalline polymer, poly(-caprolactone) (PCL), deposited on glass. An amorphous substrate is used in order to avoid any epitaxial or nucleation effects of the substrate. Hence, morphology formation is ruled by two competing rate processes, crystallization of PCL and dewetting. These rates in turn depend on polymer concentration of the spin-coated solution, leading to different crystallization rates that arrest the dewetting process. Moreover, we present first results on the structure formation of thin PCL films after thermal annealing. Morphologies, as revealed by several microscopic techniques and light scattering, will be discussed. Experimental Section The PCL used had an Mw value of 8400 and Mw/Mn was 1.4, determined by SEC using PS standards. The glass transition temperature and the melting point of PCL were -62 and +58 °C, respectively, when measured with a heating rate of 10 °C/ min in a differential scanning calorimeter. PCL was dissolved X
Abstract published in Advance ACS Abstracts, July 15, 1997.
(1) de Gennes, P.-G. Rev. Mod. Phys. 1985, 57, 827. (2) Chan, C.-M. Polymer Surface Modification and Characterization; Hanser: Munich, 1994. (3) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. Redon, C.; Brochard-Wyart, F.; Rondelez, F. Phys. Rev. Lett. 1991, 66, 715. (4) Reiter, G. Phys. Rev. Lett. 1992, 68, 75; Langmuir 1993, 9, 1344. (5) Faldi, A.; Composto, R. J.; Winey, K. I. Langmuir 1995, 11, 4855. (6) Sheiko, S.; Lermann, E.; Mo¨ller, M. Langmuir 1996, 12, 4015.
S0743-7463(96)00899-2 CCC: $14.00
in cyclohexanone and spin-coated with 2000 rpm for 90 s on glass slides. Glass slides were used because glass is not a nucleation agent for PCL and the formation of transcrystals is avoided. Prior to the experiments, the glass slides were extensively cleaned several times. The spin-coated films were dried in a vacuum oven at room temperature and then isothermally annealed as indicated. Light microscopy was carried out with an Olympus Vanox AH2 equipped with a Linkam hot stage (THM 600) for thermal annealing experiments. Furthermore, for light scattering measurements, the light source was a 10 mW linear-polarized He-Ne laser with a wavelength of 633 nm. The incident light was attenuated by an appropriate neutral density filter. An additional rotating ground plate prevents the interference of the scattering pattern with a speckle pattern.7 The incident beam in the sample was focused to 0.8 mm. The distribution of the scattered light was determined using a twodimensional CCD detector with 512 × 512 pixels and an active area of 19 × 19 µm per pixel. The atomic force microscopy (AFM) measurements were carried out with a Nanoscope III (Digital Instruments) in the height mode. Si tips were employed with a force constant of about 0.1 N/m. The scanning electron microscopy (SEM) studies were performed on a Zeiss DSM 960 instrument in the secondary electron mode. The acceleration voltage was 5 kV. The samples were sputtered with gold.
Theoretical Background We consider the following situation: Initially, a thin film of PCL is generated by spin-coating on a nonwettable surface. The film is inherently unstable, and after annealing at sufficiently high temperatures, relaxation toward equilibrium or dewetting will occur. This process is superimposed by melting of the semicrystalline PCL or crystallization after quenching the film to crystallization temperature. First, we will focus on dewetting. Fluctuations of film thickness generate a pressure gradient that induces a flow in the film. For sufficiently thin films, the free energy, f, becomes a function of film thickness, H. Its second derivative with respect to H acts as a driving force for the relaxational process. Introducing the disjoining pressure, Π(H), according to Derjaguin,8 gives (7) Bates, F. S.; Dierker, S. B.; Wignall, G. D. Macromolecules 1986, 19, 1938. (8) Derjaguin, B. V. Theory of Stability of Colloids and Thin Films; Consultants Bureau: New York, 1989.
© 1997 American Chemical Society
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dΠ d2∆f )2 dH dH
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(1)
The disjoining pressure can be related to non-retarded van der Waals interactions9
Π)
A 6πH3
(2)
where A is the Hamaker constant. For the driving force of dewetting, one gets
d2∆f A ) 2 dH 2πH4
(3)
The Hamaker constant reflects here the interplay between adhesional and cohesional forces, adhesion between the solid substrate and liquid polymer film (Asl) and cohesion of the liquid polymer film (All)
A ) Asl - All
(4)
If one considers all substances involved as primarily nonpolar, one gets, to a good approximation,9
Asl )
2 2 2 3hν (ns - n1 ) 2 2 3/2 16x2 (ns + n1 )
(5)
and an analogous expression for All if one replaces ns2 nl2 by nl2 - 1 and ns2 + nl2 by nl2 + 2. In eq 5, Ρ is the fundamental absorption frequency (on the order of 3 × 1015 s-1) and ni represent the refractive indices of the solid and liquid polymer phases. It is easy to estimate that A becomes negative if ns - nl < nl - 1. Employing eq 5, we are able to estimate the Hamaker constant for the system under discussion. Reasonable values for the corresponding refractive indices of cyclohexanone, PCL, and glass, at a wavelength of 589 nm, are10
ncyclo ) 1.450,
nPCL ≈ 1.47,
nglass ≈ 1.65
Figure 1. AFM micrographs of PCL spin-coated with 2000 rpm for 90 s onto glass slides using different polymer concentrations in cyclohexanone: (a) 3 wt %; (b) 1 wt %.
(6)
Comparison of eqs 6 and 4 immediately tells us that A < 0. Estimation from eqs 5 and 6 yields A ) -3 × 10-20 J. Therefore, we have a nonwettable surface. Equation 3 shows that the second derivative of free energy with respect to film thickness becomes negative. Hence, there is no energy barrier for growing thickness fluctuations in the unstable region. Moreover, the driving force (eq 3) dramatically increases with decreasing film thickness. When one applies the Cahn approach11 to the dewetting process, it results that periodic thickness fluctuations of wavelength 2π/qm will grow the fastest in the early stage (linear regime). This characteristic spacing appears throughout the system at once owing to instability of the film. In later stages, when nonlinear effects become operative, the relaxational behavior will change. Nevertheless, the relaxational process advances throughout the system, leading to a uniform probability for breakup all over the film. This should be reflected in development of holes, uniform in size over a certain period of the dewetting process. In thin films of PCL, structure formation during thermal annealing procedures is ruled not only by dewetting but also by melting and crystallization. For high values of H, (9) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, U.K., 1989. (10) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley: New York, 1989. (11) Cahn, J. W. J. Chem. Phys. 1965, 42, 93.
the driving force for dewetting slows down according to eq 3. Therefore, the rate of crystallization dominates structure formation after quenching. The opposite is true for sufficiently small values of H. Between these limits, both processes cannot be separated from each other. Coupling of them occurs at the kinetic stage and leads to formation of nonequilibrium structures that depend on the thermal history. Crystallization may arrest the dewetting process. If the film is sufficiently thin, annealing above the melting point cannot completely erase the crystalline structure before dewetting advances in the amorphous regions. The initial crystalline morphology determines in that way to a certain extent the development of dewetting phenomena. Thus, competition of the two rate processes offers the possibility for formation of various structures. Results and Discussion Figures 1-3 show selected examples of AFM micrographs of PCL on glass prepared by spin-coating of solutions with concentrations ranging from 3 to 0.1 wt %. These measurements were carried out in the height mode. Hence, bright areas are related to higher regions on the sample while dark areas belong to deeper regions. For higher PCL concentrations, ranging from 3 to 1 wt % of PCL in cyclohexanone, closed two-dimensional spherulites are formed (Figure 1). These spherulites are structured
Structure Formation in Thin PCL Films
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Figure 3. Morphology of PCL after spin-coating a 0.1 wt % solution on glass.
Figure 2. (a) Morphology of a PCL sample spin-coated from a 0.5 wt % solution obtained by AFM. (b) Higher magnification AFM micrograph of an area near the holes seen in (a). (c) Height profile taken along the line shown in the inset.
by fibrils that usually contain several lamellae.12 The fibrils can be observed in the micrometer range, whereas lamellae usually have dimensions in the nanometer range. Polygons can be observed, which are typical for impinged spherulites, indicating that PCL is able to crystallize during the spin-coating process. The thickness of the spherulites is measured by AFM height profiles after (12) Wunderlich, B. Macromolecular Physics; Academic Press: New York, 1973.
partially removing the polymer film. It is defined as the distance between the glass surface and the average of all height values taken in a line scan across the spherulite surface. Several height profiles taken for different spherulites of one sample were averaged. The surface roughness of the spherulites is small compared to their overall thickness (cf., e.g., Figure 2c). For 3 wt % solutions (Figure 1a), an average thickness of the spherulites of 105 nm results. The thickness decreases to 65 and 30 nm for spherulites obtained by spin-coating of 2 and 1 wt % solutions, respectively. Obviously, these films have a layered structure; i.e., when one layer of spherulites was formed during the spin-coating process, the next layer can crystallize. Thus, on top of the spherulites shown in Figure 1a, open aggregates are formed. These open aggregates can be considered as precursors of spherulites.12 Because of the lack of material able to crystallize from solution during the spin-coating process, it is impossible to form another layer of spherulites and the open aggregates remain. If one looks on the surface of a closed spherulite in more detail, it becomes obvious that the fibrils are relatively disordered (Figure 1b). This is caused by the interplay of crystallization and dewetting during the formation process of the film. The situation changes for concentrations of 0.5 wt % and below. Since the glass surface is not completely covered with polymer, open spherulites are formed with an average thickness of the crystalline material of 18 nm (Figure 2a). However, the polymer is still in the continuous phase and can crystallize during the spin-coating of solutions up to concentrations of 0.3 wt %. The growth of this type of spherulite seems to be initiated by a single crystal that causes the characteristic appearance of a leafshaped spherical inhomogeneity in the center of the spherulite, which is different from a heterogeneously nucleated spherulite type.12 Figure 2b shows in more detail this open spherulite. One may recognize that the polymeric material between the holes is higher than that in other areas of the spherulites. The higher areas are formed by edge-on lamellae, and the lower areas consist of flat-on lamellae. The hole formation during the spincoating process leads to a bending of the lamellae (and fibrils). Accordingly, the phenomenon somewhat resembles the occurrence of banded spherulites observed by polarized light microscopy,13-16 where internal stresses lead to a torque and cooperative bending processes of (13) Keith, H. D.; Padden, F. J., Jr.; Russell, T. P. Macromolecules 1989, 22, 666. (14) Keith, H. D.; Padden, F. J., Jr. Polymer 1984, 25, 28.
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Figure 4. (a) SEM micrographs with different magnifications of a PCL film as shown in Figure 1a (3 wt % solution) but after heating the sample from room temperature to 100 °C with 10 °C/min, keeping it for 10 min at 100 °C, and then cooling it to 45 °C with 10 °C/min followed by isothermal crystallization at 45 °C for 1 day. (b) AFM micrograph of the same specimen as described in (a). The arrow indicates an area of disordered fibrils.
Figure 5. Light micrographs of a sample, prepared from a 2 wt % solution, taken at different temperatures: (a) room temperature (as-prepared); (b) 60 °C; (c) 75 °C; (d) 100 °C. The sample was heated from room temperature to 100 °C with 10 °C/min. (e) SEM micrographs of the same sample as shown in (d) after slowly cooling the sample to 45 °C and crystallization for 1 day (different magnifications).
lamellae in bulk spherulites.17 The corresponding height profile across the holes in the spherulite (Figure 2c) shows that the edge-on lamellae between the holes have a regular width that corresponds to the height of the profile.
A further decrease in concentration leads to more and more open material, but still the polymer is in the continuous phase. Eventually, “phase” inversion occurs where the polymer becomes the “disperse” phase. A
Structure Formation in Thin PCL Films
concentration of 0.1 wt % leads to single droplets of PCL on the glass surface (Figure 3). It reveals that even the final stage is not completely disordered. The droplets are arranged in polygons, a pattern that was previously also observed in dewetting of thin polystyrene films.4,18 So far, we presented film morphologies as they develop initially during the spin-coating process. Now, we are going to discuss film morphologies arising after annealing above the melting point of PCL and subsequent quenching. Resulting morphologies reflect the interplay between dewetting and crystallization. In Figure 4a, an SEM micrograph depicts, in different magnifications, a film (prepared from 3 wt % solution) that was first heated to 100 °C, then annealed there for 10 min, and afterward isothermally crystallized at 45 °C. The film is sufficiently thick; hence, melting erases the spherulitic structure and a nearly homogeneous film is reestablished before the dewetting process starts during thermal annealing. Dewetting leads to simultaneous occurrence of holes seemingly randomly distributed all over the sample. However, one also recognizes that the holes are not uniform in size as discussed above. This indicates that the film was not completely homogeneous before dewetting started. In some regions, dewetting started earlier than in others. After quenching of the sample to crystallization temperature, dewetting ceases and crystallization advances. The impingement lines of the spherulites can be clearly recognized. They are not related to the holes that also appear in the inner parts of the spherulites. Figure 4b shows that the fibrils of the spherulite grow straight in areas that are not disturbed by holes while they are relatively disordered in the regions between the holes indicated by an arrow. The situation becomes different when the film thickness is reduced. Then, melting cannot completely erase the spherulitic structure before dewetting starts. Consequently, dewetting is initiated in impingement lines and resulting structures are also determined by the initial crystalline arrangement. This leads to strings of dewetted regions owing to coalescence of holes with advancing dewetting process and results in a rather regular network of dewetted regions. The development of those structures is shown in Figure 5 for a sample with an initial thickness of the spherulites of about 65 nm. The sample was heated from room temperature with 10 °C/min to 100 °C. It is clear to see that dewetting starts at the impingement lines of the spherulites (Figure 5a-d) and leads to thread-like arrangements of more or less coalesced holes (Figure 5e). One also recognizes that the holes are uniform in size, indicating that there was initially a uniform film thickness in the impingement areas of the spherulites. Eventually, network-like dewetted regions are formed, as shown in Figure 6a. The dark phase represents polymeric material, whereas the bright regions belong to the substrate that is not covered by the polymer film anymore. The dewetted regions form a quite regular pattern. Digital image analysis and light scattering are tools to study the regularity of the structures shown in Figure 6a. The 3D plot of the power spectrum, resulting from the two-dimensional Fourier transformation of the digitized image of Figure 6a, is shown in Figure 6b. The typical shape of a “Vulcan” shows that there is a regular spacing.19 In addition, it is possible to study directly the sample in the inverse space by light scattering since the substrate (15) Schulze, K.; Kressler, J.; Kammer, H. W. Polymer 1993, 34, 3704. (16) Kressler, J.; Kammer, H. W.; Silvestre, C.; DiPace, E.; Cimmino, S.; Martuscelli, E. Polym. Blends Netw. 1991, 1, 225. (17) Keith, H. D.; Padden, F. J., Jr. Polymer 1984, 25, 28. (18) Henn, G.; Bucknall, D. G.; Stamm, M.; Vanhoorne, P.; Jerome, R. Macromolecules 1996, 29, 4305. (19) Tanaka, H.; Nishi, T. J. Appl. Phys. 1986, 59, 3627.
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Figure 6. (a) Light micrograph of the same sample as shown in Figure 5e. (b) Two-dimensional Fourier transformation of the digitized image shown in (a).
Figure 7. Scattering intensity, I, versus scattering vector, q, for the same sample as shown in Figure 6a. The light scattering pattern is shown in the inset.
of the polymer film is glass. The inset of Figure 7 shows the typical light scattering pattern. The ring is characteristic for structures with a periodic domain spacing. In the case under discussion, this means a dominant spacing occurs between wetted and dewetted regions of the polymer film on glass. The numerical value of the dominant spacing can be calculated from a plot of the relative intensity as a function of the wave vector q ) (4π/λ) sin (Θ/2), as shown in Figure 7, where λ is the wavelength of light and Θ is the scattering angle. The characteristic spacing, Λ, itself is obtained by combining Bragg’s
4412 Langmuir, Vol. 13, No. 16, 1997
equation, λ ) 2Λ sin(Θ/2), with q. It results that Λ ) 2π/qm, which yields a value of about 12.6 µm. qm is the q value at maximum intensity. Conclusions Film formation during spin-coating of polymer solutions, comprising a polymer that is able to crystallize, and structure formation during subsequent annealing and quenching of the film have been studied by several experimental methods. Spin-coating of sufficiently concentrated solutions leads to films completely covering the glass substrate. Rapid crystallization results in spherulites with distinct impingement lines. A decrease in polymer concentration reduces the film thickness and open spherulites developed that do not completely cover the glass. At very low polymer concentrations, the driving
Kressler et al.
force for dewetting causes decay of the film in dispersed droplets. Development of film morphologies during annealing and quenching is ruled by two competing rate processes, dewetting and crystallization. For sufficiently thick films, crystal morphologies can melt during annealing to a high extent before the dewetting process starts. Hence, dewetting leads to hole formation all over the sample. In thin films, however, annealing above the melting temperature cannot erase the crystalline structure before dewetting starts. Accordingly, the approach toward dewetting equilibrium is also influenced by the initial crystalline structure. In the course of the dewetting process, regular spacing of wetted and dewetted regions is observed. LA960899R
