Extended-Chain Induced Bulk Morphologies Occur at Surfaces of Thin

Jun 1, 2012 - (GIXRD) with a Rigaku Ultima IV type III diffractometer (Rigaku,. Tokyo, Japan), equipped with CrossBeamOptics (CBO) and a thin...
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Extended-Chain Induced Bulk Morphologies Occur at Surfaces of Thin Co-Oligomer Films Robert Schulze,† Matthias M. L. Arras,† Giovanni Li Destri,‡ Michael Gottschaldt,§,⊥ Jörg Bossert,† Ulrich S. Schubert,§,⊥ Giovanni Marletta,‡ Klaus D. Jandt,†,⊥,* and Thomas F. Keller† †

Chair of Materials Science, Faculty of Physics and Astronomy, Friedrich-Schiller-University Jena, Löbdergraben 32, 07743 Jena, Germany ‡ Laboratory for Molecular Surfaces and Nanotechnology (LAMSUN), Department of Chemical Sciences, University of Catania and CSGI, viale A. Doria 6, 95125 Catania, Italy § Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich-Schiller-University Jena, Humboldtstraße 10, 07743 Jena, Germany ⊥ Jena Center for Soft Matter (JCSM), Friedrich-Schiller-University Jena, 07743 Jena, Germany ABSTRACT: One approach to create novel amphiphilic surface nanostructures with typical lateral pattern dimensions in the order of 10 nm is to employ double crystalline cooligomers with domain sizes largely determined by the block length of extended molecular chains. The aim of the study was to test the hypothesis that the extended chain bulk morphology of an asymmetric polyethylene-block-poly(ethylene oxide) (PE-b-PEO) co-oligomer can be induced at its thin film surface. Furthermore, we explored the central role of the extended oligomer orientation and the surface affinity to achieve an amphiphilic surface nanostructure. The co-oligomer was drop-cast from dilute solution onto hydrophobic, neutral and hydrophilic substrates. Atomic-force microscopy revealed that independent of the substrate chemistry, the film thickness was quantized in integral multiples of the calculated extended chain length. At the same time, the terraces exhibited lateral domains that coarsen with increasing substrate hydrophobicity. Although subsequent annealing tends to induce dewetting, on the neutral substrate a residual polymer layer with perpendicular lamellar surface morphology remained. Grazing incidence X-ray diffraction suggested the latter to be induced by crystallization. We propose that simultaneous formation of lateral domains and discrete terraces during drop-casting are facilitated by surface-diffusion and due to a dense-packing and crystallization of vertically aligned, extended oligomer chains. Annealing permits the polymer film on the neutral surface to overcome the energy barrier for chain rotation from vertical to parallel to create amphiphilic surface nanostructures. To our best knowledge, we demonstrated for the first time that double crystalline co-oligomers can be used to pattern surfaces laterally with asymmetric lamellae with dimensions in the order of 10 nm.

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INTRODUCTION Microphase separation of block copolymers (BCP) can be used to create nanopatterns on thin film surfaces.1−4 The equilibrium morphology of these molecularly self-assembled nanostructures depends on the degree of polymerization, the block length ratio, and furthermore on the BCP film thickness and the BCP’s affinities to the substrate as well as to the environment.3,5−10 If one or more blocks of the BCP are crystallizable, the mostly entropy driven microphase separation may compete with crystallization. The final morphology depends on the location of the crystallization temperature Tc , the order− disorder-transition temperature TODT and the glass transition temperature Tg.11 Furthermore, crystallization provides a promising way to induce long-range order in surface patterns of BCP mesophases because the crystallization forces are often © 2012 American Chemical Society

significantly stronger than those encountered in microphase separation.12 If both blocks A and B of a diblock copolymer are crystallizable, the resulting morphology will depend on the relative location of the crystallization temperatures TcA and TcB with respect to TODT and Tg. Furthermore, the crystallization of one block may affect the crystallization and morphology of the second block.13−15 For short-chain double crystalline BCPs, also called block co-oligomers, a multitude of temperaturedependent bulk morphologies have recently been reported.16−24 It was suggested that the temperature-dependent morphological transitions are facilitated by the small molar Received: March 29, 2012 Revised: May 15, 2012 Published: June 1, 2012 4740

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mass and, thus, the high degree of chain flexibility and the existence of extended chain crystals.20−24 The current work is focused on the specific requirements to induce molecularly self-assembled surface nanostructures of such a short chain, double crystalline polyethylene-blockpoly(ethylene oxide) diblock co-oligomer (PE-b-PEO) and discusses the influence of the surface affinities and annealing treatments on the lateral phase morphology. Short chain PE-b-PEO co-oligomers are well-known as surfactant agents with amphiphilic character which are used for example in blends with thermosetting epoxy resins17−20 or polyethylene homopolymers.21 Recently, the bulk phase- and crystal morphology of the PE-b-PEO16,22−25 was reported along with the characteristic temperatures, i.e., TODT > Tc,PE ≫ Tc,PEO.16 Sun et al. investigated the phase transitions and crystallization of an ExEOy diblock co-oligomer (x and y denote the numbers of ethylene and ethylene oxide mers) with x = 29 and y = 20 as a function of the temperature in bulk samples by wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM).22 At room temperature the authors observed a lamellar morphology.22,23 The crystalline PE and the adjacent crystalline PEO lamellae were formed by alternately stacked extended chains.22,23 This observation was supported by the relatively good agreement between the experimentally determined lamellae thickness of 11.9 nm and the calculated extended chain length of 12.5 nm. The deviation between observed and calculated chain length was explained by the nonuniformity of the chains and the amorphous transition zones between the two crystalline phases. For an asymmetric E17EO40 diblock co-oligomer, a similar stacked lamellar morphology was previously observed at room temperature induced by the crystallization of PE and PEO, although for amorphous BCPs with the same block length ratio a cylindrical morphology would be expected.24,25 By increasing the temperature above the melting point of the PEO, the morphology changes at 50 to 70 °C to a gyroid morphology, at 70 to 100 °C to a cylindrical morphology and above 100 °C to a spherical morphology.24,25 To our best knowledge, reports on the phase morphology of these double crystalline co-oligomers are up to now restricted exclusively to the bulk. Thus, in the current study we report for the first time the evolution of the surface morphology of a PE-b-PEO cooligomer after drop-casting and annealing. We show that this co-oligomer can be used to pattern surfaces with amphiphilic asymmetric lamellae. The role of surface affinities, film thickness, crystallization, chain orientation and conformation is discussed. PE as well as PEO homopolymers have already been established as important biomaterials.26 The combination of, i.e., both PE-b-PEO copolymers and co-oligomers, could therefore extend the range of possible biomedical applications due to their inherent biostability and amphiphilic properties as well as their mechanical properties which are adaptable by the degree of crystallinity.13 Amphiphilic PE-b-PEO surface nanostructures with lamellar dimensions of ∼10 nm may be used to induce orientation of adsorbed biomolecules27,28 and control the conformation and assembly of proteins.29−31

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octadecylmercaptan (ODM, purity 98%) were purchased from SigmaAldrich Corporation (St. Louis, MO) and used as received. The supplier specified the co-oligomer’s average molar mass as approximately 2250 g/mol with a mass content of 80 wt % PEO. All solvents were purchased from VWR (Darmstadt, Germany) and distilled before use. Substrate Preparation. To vary the surface chemistry, we used silicon and thiol modified gold surfaces for the PE-b-PEO thin films. Silicon substrates were prepared by cutting a silicon wafer into pieces of about 1 cm ×1 cm followed by ultrasonification for 5 min in acetone, ethanol and water. After each rinsing step, the samples were dried with compressed air. The gold surfaces were prepared by physical vapor deposition (Leybold Vakuum GmbH). 200 nm thick gold films were deposited onto silicon wafers via electron-beam deposition, using an evaporation rate of 0.4 nm/s. Subsequently, the gold substrates were sonicated for 5 min in ethanol and then cleaned in argon plasma for 5 min. Afterward, the gold surfaces were modified with self-assembled monolayers (SAMs) by immersing the gold substrates for 2 h into 5 mM ethanol solution of M3M or into 5 mM toluene solution of ODM followed by rinsing with ethanol and subsequent drying with compressed air. Finally, the thiol-modified gold substrates were stored for 1 h at 80 °C in vacuum. Preparation of Thin Films. Thin films were prepared by dropcasting a 0.1 wt % ethanol polymer solution onto each type of substrate. Two different types of heating treatments were accomplished to induce the formation of lamellae. In the first treatment, the samples were kept for 1 h at 60 °C in an ethanol saturated atmosphere in a glass Petri dish on a heating plate. In the second heating treatment, the samples were annealed in a vacuum oven at 120 °C. After reaching the final temperature the oven was switched-off and the samples were allowed to slowly cool down to room temperature. This cooling process takes about 3 h. A similar heating treatment was reported for the E17EO40 to induce a lamellar morphology in the bulk.24,25 Polymer Characterization. 1H NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer in deuterated chloroform. The chemical shifts were calibrated with respect to chloroform residual peaks. The molar mass was measured by size exclusion chromatography (SEC) using a Shimadzu system (Nakagyo-ku, Japan) equipped with a SCL-10A system controller, a LC-10AD pump, and a RID-10A refractive index detector using a solvent mixture containing chloroform, triethylamine, and isopropanol (94:4:2) at a flow rate of 1 mL min−1 on a PSS-SDV-linear M 5 μm column at 40 °C. The system was calibrated with poly(ethylene glycol) standards purchased from PSS Polymer Standards Service GmbH (Mainz, Germany). Characterization of Substrate Surface Affinity. A DSA10 drop shape analysis system (Krüss GmbH, Hamburg, Germany) was used to characterize the surface properties of the substrates by advancing contact angle measurements using Millipore water. The initial drop volume was 5 μL, and the dosing rate was set at 10 μL min−1 for every measurement. The entire measurement consisted of 10 individual measurements with time steps of one second between each measurement. The average advancing contact angle and standard deviation were calculated. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) of the bulk material was carried out with a PerkinElmer Pyris 1 equipped with a thermal analysis controller 7/DX (PerkinElmer Inc., Waltham, Massachusetts, USA) to study the crystallization and melting behavior of the double crystalline diblock copolymer. For the melting and crystallization studies, we used heating and cooling rates of 10 K/min. Crystallization and melting peak temperatures were taken as melting and crystallization temperatures, respectively. Optical Microscopy. The macroscopic appearance of the dropcast thin films was investigated by optical microscopy analyses using an Axio Imager.M2 from Carl Zeiss (Göttingen, Germany). Measurements were performed using a magnification of 200 times. The images

EXPERIMENTAL SECTION

Materials. The polyethylene-block-poly(ethylene oxide) diblock co-oligomer, methyl 3-mercaptopropionate (M3M, purity 95%) and 4741

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document the optical homogeneity and the temperature induced dewetting of drop-cast thin films before and after annealing in vacuum. Atomic Force Microscopy. The substrates as well as the thin polymer films were characterized by AFM (Digital Instruments Dimension 3000 with a Nanoscope Controller IV). The cantilevers for the AFM measurements were purchased from Olympus (Tokyo, Japan) model OMCL-AC160TS-W with a resonance frequency of approximately 300 kHz, a spring constant of around 42 N/m and a typical tip radius of less than 10 nm. AFM height and phase images were measured with a ratio of set-point to driving amplitude (Asp/A0) of about 0.75. The thickness of the polymer thin films was determined by AFM topography measurements performed at holes or at carefully introduced scratches. Scanning Electron Microscopy. Scanning electron microscopy (SEM) images of the thin films were taken with an Auriga Crossbeam SEM/FIB (Carl Zeiss, Oberkochen, Germany) using an in-lense secondary electron detector with a working distance of 1.1 mm and an acceleration voltage of 240 V. Because of the small film thickness and the underlying gold layer, an electrical conductive surface coating was not necessary to perform SEM-measurements on the PE-b-PEO cooligomer thin films. Grazing-Incidence X-ray Diffraction. The crystal structure was determined by grazing incidence X-ray diffraction measurements (GIXRD) with a Rigaku Ultima IV type III diffractometer (Rigaku, Tokyo, Japan), equipped with CrossBeamOptics (CBO) and a thin film attachment with three degrees of freedom, at the wavelength of the copper Kα emission line. To avoid defocusing of the incident beam, a Soller slit with an angle of 5° was incorporated into the path of rays. For the diffracted beam, a second Soller slit with 0.5° was used which was aligned horizontally or vertically for out of plane and in plane measurements, respectively. In-plane implies that the detector moves in 2θ condition parallel to the substrate plane while it moves on a perpendicular plane containing the incident beam for the out-ofplane measurement. To suppress substrate diffraction the grazing angle was chosen to be 0.3°, i.e., smaller than the critical angle of total external reflection.

Figure 1. Heating and cooling thermographs of the E15EO47 measured by DSC using a scanning rate of 10 K/min. The extracted values represent the melting and crystallization peak temperatures of PEO. Note that the degree of crystallinity for the PE minority phase is presumably very low.

images with a lateral image size of 5 μm × 5 μm to be 1.48 ± 0.02 nm and 1.62 ± 0.06 nm, respectively. Surface affinities of the substrates were investigated by contact angle measurements. The advancing contact angles of silicon, M3M−gold and ODM−gold surfaces were determined to be 38.1 ± 0.48°, 62.4 ± 4.0° and 115.5 ± 1.1°, respectively. Because of these values we refer in the following to the silicon surface as hydrophilic and to the ODM−gold surface as hydrophobic. Indeed, since the water contact angle of PE is 100−110° while the one of PEO is 0−10°, the hydrophobic surface is more affine to PE and the hydrophilic surface to PEO, while the neutral surface with a water contact angle exactly in between those of the two polymers is not expected to give rise to any preferential wetting from one specific block. Thus, we refer to the M3M−gold surface as neutral. Morphologies of Drop-Cast Thin Films. To characterize the topography and morphology of drop-cast E15EO47 thin films from dilute solution AFM height and phase measurements were performed. The AFM height image in Figure 2A shows a typical terraced topography of these drop-cast thin films, shown here exemplarily for the hydrophilic substrate. The dark areas on the left side represent the substrate surface. The film thicknesses at the first, second and third terrace were

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RESULTS Polymer Characterization. The PE-b-PEO’s numberaverage molar mass (Mn) and its polydispersity index (PDI) value were determined by SEC using poly(ethylene glycol) standards to be 2540 g/mol and 1.25, respectively. On the basis of the 1H NMR measurement (data not shown), the block lengths were calculated to be 15 PE and 47 PEO units, taking the end group into account. Accordingly, we refer to this diblock co-oligomer in the following as E15EO47. Thermal Behavior and Crystallization. DSC measurements were performed to characterize the thermal behavior, especially the melting and crystallization temperatures of the E15EO47 co-oligomer. Figure 1 shows the DSC thermographs for the E15EO47 during heating and cooling. Upon cooling from the melt, a small exothermic offset was observed in the interval of 80 to 90 °C and is attributed to a phase transition in the PE minority phase. A sharp exothermic peak was observed in the DSC cooling scan and assigned to the poly(ethylene oxide) crystallization of the PEO. The crystallization temperature of PEO was determined to be 31.0 ± 0.3 °C. The DSC heating curve shows one sharp endothermic peak and one small endothermic offset; the first peak is allocated to the melting of the crystalline PEO domains. The melting temperature of the PEO was determined to be 52.3 ± 0.1 °C. The small broad offset between 90 and 100 °C is assigned to the aforementioned PE phase transition. Characterization of Substrate Surfaces. The root mean squared roughness values (Rq) of the M3M−gold and the ODM−gold surfaces were determined from AFM height

Figure 2. AFM height image (A) of a drop-cast, unannealed E15EO47 thin film shows terraces while the phase image (B) shows the phaseseparated surface structure on top of the terraces. The thin film was prepared by drop-casting a 0.1 wt % polymer solution onto a hydrophilic silicon substrate. In the inset of part A, the height profile is plotted over the region indicated by the white solid line. The lateral image size is 2 μm. 4742

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determined to be 15.9 ± 0.5 nm, 29.8 ± 1.4 nm and 46.2 ± 1.6 nm, respectively, as can be seen in the line profile in the inset of Figure 2A. This terraced topography of the drop-cast E15EO47 thin films was observed on hydrophilic, neutral and hydrophobic surfaces (data for the latter not shown). Figure 2B shows the AFM phase image of Figure 2A and provides insight into the surface morphology of the drop-cast thin film. The black and white domains on top of the first terrace in Figure 2B suggest that during drop-casting at least a partial lateral phase separation took place. The resulting phase contrast is due to a stiffness difference between PEO and PE, as at moderate tapping conditions, i.e. an amplitude ratio of 0.3 < Asp/A0 < 0.8, stiffer phases are brighter than softer phases.32 The DSC analysis of the bulk sample, as presented in Figure 1, reveals the PEO to be crystallized whereas the PE presumably is not. Assuming a similar situation in the film on the surface, we identify the bright domains with the crystallized PEO phase, while the dark domains constitute noncrystallized phases, i.e., the PE. The AFM phase images in Figure 3, parts A (enlarged section of Figure 2B), B, and C, show the surface morphology on the

in vacuum and then slowly cooled to room temperature. The phase morphologies of the drop-cast unannealed, solvent- and vacuum-annealed films are shown in parts A−C, D−F, and G-I of Figure 3, on hydrophobic, neutral, and hydrophilic surfaces. For the solvent heating treatment, we chose a temperature of 60 °C, i.e., above the melting point of the PEO, and controlled the environment conditions by an ethanol saturated atmosphere. Both elevated temperature and solvent vapor increase the chain mobility and modulate the selective domain affinity on the free polymer surface.33 Parts B and E of Figure 3 are the AFM phase images of the E15EO47 film on a neutral M3M−gold substrate before and after heating treatment in solvent rich atmosphere, respectively. Annealing changes the domain morphology into a lamellar morphology. The lamellar structure is characterized by a lamellar size of 16.1 ± 1.2 nm as obtained by a line profile analysis. The film thickness of the layer with the lamellar morphology was determined to be 15.7 ± 1.0 nm using AFM height images (data not shown) and exhibits no significant deviation as compared to the untreated film. The AFM phase image Figure 3D presents the E15EO47 surface morphology on the first terrace on hydrophilic silicon after solvent heating treatment. The upper left part of Figure 3D represents the stiffer substrate imaged in a brighter color. Below, the phase-separated surface morphology on the first terrace of the annealed film can be seen. The film thickness of about 16.4 ± 0.7 nm was determined from the corresponding height image (data not shown) and revealed no significant deviation as compared to the untreated film. Figure 3F shows the solvent-annealed thin film surface morphology on the first terrace on the hydrophobic ODM modified substrate. The film thickness of the films after solvent annealing was determined to be 16.3 ± 1.1 nm from AFM height images (data not shown). Overall, on both, the hydrophilic and hydrophobic surfaces the applied solvent treatment reduces the area fraction of the dark phase on the film surface (Figure 3D,F) as compared to the untreated surface morphology (Figure 3A,C). E15EO47 co-oligomer thin films on all substrates were annealed under vacuum at 120 °C, i.e., above the melting points of the two crystalline phases (TPEO = 52.5 ± 0.1 °C, PE phase transition in the interval of 90−100 °C) and presumably above the order−disorder-transition temperature.22,23 Subsequently, the samples were allowed to slowly cool down to room temperature.24,25 After vacuum annealing, we observed dewetting on all substrates and the formation of macroscopic aggregates presumably driven by the minimization of surface energy facilitated by increased chain mobility at elevated temperatures. This process is exemplarily documented by the optical micrographs in Figure 4, parts A and B, that show the homogeneity of the drop-cast films and the macroscopic dewetting after vacuum annealing of an E15EO47 film on the neutral M3M−gold substrate, respectively. The AFM phase image Figure 3H was measured in close vicinity to one of these aggregates and shows a lamellar surface morphology of a residual polymer layer that remained on the neutral M3M−gold surface. The film thickness of this residual polymer layer was determined from height images to be 15.8 ± 0.6 nm (data not shown). The size of lamellae on the layer surface was determined to be 15.7 ± 1.4 nm by multiple line profiles, exemplary shown by the inset of Figure 5A. Complementary to the AFM analysis, from the SEM image

Figure 3. AFM phase images (500 nm ×500 nm, the scalebar of the phase value is 10°) of E15EO47 thin films on silicon (hydrophilic), M3M−gold (neutral) and ODM−gold (hydrophobic) surfaces as drop-cast (A, B, C), after solvent atmosphere annealing for 1 h at 60 °C (D, E, F) and after vacuum annealing at 120 °C (G, H, I). All images were taken of areas with a film thickness of about 16 nm.

first terrace of E15EO47 thin films drop-cast on hydrophilic, neutral and hydrophobic substrates, respectively. Apparently, the lateral domain size on the surface of the first terrace is increasing with increasing hydrophobicity of the underlying substrate. Thin Film Morphologies Induced by Annealing. To facilitate the formation of a perpendicular lamellar morphology, the drop-cast thin films were annealed by two different heating treatments. In the first heating treatment, the samples were kept for 1 h at 60 °C in an ethanol saturated atmosphere, while in the second treatment the thin films were annealed to 120 °C 4743

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Figure 4. (A, B) Optical micrographs of unannealed (A) and vacuum annealed (B) E15EO47 thin films prepared by drop casting on a M3M−gold substrate. The presence of aggregates after annealing indicates a strong dewetting process. Note that the surface is still fully covered with a 16 nm thick film whose surface morphology is seen in part A. (C, D) Grazing incidence diffractograms of unannealed (C) and vacuum annealed (D) E15EO47 thin films with corresponding Miller indices for PE and PEO. The measurements were performed in-plane and out-of-plane to gain information on the crystal orientation and therefore polymer chain orientation.

represents the ODM modified surface as deduced from film thickness measurements whereas the right part shows the surface morphology of the 15.7 ± 2.0 nm thick polymer layer. The film thickness was taken from the height image (data not shown) that corresponds to the phase image Figure 3I. The surface of the layer appears predominantly bright, indicating that the stiffer PEO phase is enriched at the surface. The phase contrast on the surface of the layer arises from phase shifts induced by height differences of the underlying rough gold substrate. GIXRD Measurements at Thin Films. The crystal structure and chain orientation of thin films on M3M−gold substrates was investigated by GIXRD measurements before and after vacuum annealing. The diffraction peaks with the highest relative intensities arise from the (120) and (200) plane for PEO and PE, respectively. Since in either case the c-axis is equal to the chain direction and also parallel to PEO (120) and PE (200), in-plane and out-of-plane measurements enable the determination of the degree of chains which are oriented parallel or perpendicular to the substrate. Figure 4C displays the in-plane and out-of-plane GIXRD scans for a drop-cast E15EO47 thin film. For the in-plane scan, one small peak at about 2θ = 19° was observed and assigned to the (120) plane of crystallized PEO. This observation indicates the existence of PEO chains oriented perpendicular to the substrate surface. Additionally, the out-of-plane scan shows virtually no diffraction peak. The in-plane and out-of-plane GIXRD scans of the vacuum annealed E15EO47 thin film are shown in Figure 4D. Again, the in-plane scan exhibited a (120) PEO peak at 19°. The out-of-plane scan revealed the existence of a small peak at about 24° which we assign to the (200) plane of PE. Thus, the GIXRD measurements of the vacuum annealed films suggest the presence of perpendicularly oriented PEO chains and parallel oriented PE chains with respect to the substrate surface.

Figure 5. (A) AFM phase image of the first layer of an E15EO47 thin film on a neutral M3M−gold surface after vacuum heating treatment with a film thickness of about 16 nm and a lamellar size of about 16 nm with the profile of the phase values along the white line as inset. (B) SEM image of the E15EO47 thin film on a neutral M3M−gold substrate with the perpendicular lamellar morphology induced by vacuum annealing.

in Figure 5B a lamellae thickness of 16.2 ± 0.6 nm was determined. Despite the different nature of contrast formation for AFM phase and secondary electron imaging, both lamellar sizes are in agreement. Figure 3G shows a representative AFM phase image of a vacuum annealed film on the hydrophilic silicon surface in close vicinity to one of the aggregates. Different from the observation on the neutral surface, phase-separated surface structures cannot be observed. Nevertheless, the phase value shifts from 0° for native silicon to −5°. Both phase values were measured at an amplitude ratio Asp/A0 of 0.75. Phase values of about −5° were reported for soft PE at similar amplitude ratios of Asp/ A0.32 This again indicates the presence of a residual polymer layer. We, therefore, assume that after the vacuum annealing the silicon surface is completely covered with a monolayer of the E15EO47 with the PEO and the PE block pointing toward the substrate and the free surface, respectively. Figure 3I shows the phase image of a dewetted polymer layer that surrounds the aggregates on hydrophobic ODM modified surfaces after vacuum annealing. The left part of the image

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DISCUSSION Double Crystalline Co-Oligomers. Because of the short chains of low molar mass co-oligomers crystalline lamellae 4744

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either consist of extended or integer-folded chains.12,22,24,25 Integer-folded chains follow an integer number of complete traverses through the crystal, i.e., 1 for extended and 2 for oncefolded chains.24 Often, crystallization dominates the evolution of the bulk morphology and, therefore, it enables new nanostructures, such as asymmetric lamellae for a BCP with a block length ratio that induces a cylindrical morphology in case both blocks are amorphous.24,25 For amorphous BCP’s, the bulk morphologies can often be transferred onto the surface of thin films by utilizing specific surface affinities.1−4 As current reports on the morphology of double crystalline co-oligomers are so far restricted to the bulk state, it is interesting to analyze how the crystallization affects the substrate induced thin film morphologies of double crystalline co-oligomers. Thermal Analysis. The bulk crystallization and melting temperatures (Figure 1) of the PEO of 31.0 ± 0.3 and 52.3 ± 0.1 °C are both comparable with those obtained for the E17EO40.24,25 Furthermore, we assign the small offsets in the cooling and heating scan in the intervals of 80 to 90 °C and 90 to 100 °C to a phase transition of the PE segments from a conformationally disordered pseudohexagonal phase to the orthorhombic phase of all-trans zigzag chains. Previously, a similar phase transition of the PE segments was reported by comparing DSC measurements with WAXD, SAXS, and IR measurements.24,25 WAXD measurements of E17EO40 during cooling showed that the (110) PE diffraction intensity emerges at about 90 °C and reaches a maximum level at about 40 °C while an exothermic peak could not be found in the corresponding DSC curve for this temperature interval.24,25 The crystallization range is thus very broad and one can conclude that the crystallization of the PE in the here investigated E15EO47 is either not resolved in the DSC cooling scan as a sharp exothermic peak or did not occur at all. Discrete Film Thickness and Chain Orientation in E15EO47 Thin Films. For drop-cast E15EO47 co-oligomer thin films on hydrophilic, hydrophobic, and neutral substrates, we observed the formation of terraces with discrete film thicknesses for the first three layers, as exemplarily shown in Figure 2A for the hydrophilic substrate, and at same time a phase-separated surface structure on the surface of the first terrace (film thickness was about 16 nm) as shown in AFM phase images in Figure 3A−C. It is well-established that the thickness of as-cast amorphous BCP thin films changes in most cases continuously with processing parameters as, e.g., the BCP concentration in the solvent.3 Because of the usually fast solvent evaporation, the resulting surface morphology is in a nonequilibrium state. During annealing, a discrete film thickness can develop which is often discussed within the framework of the mechanism of island−hole formation and is also called terracing.3−6 In this case the lamellar morphology is oriented parallel to the substrate surface induced by a preferential affinity of one of the blocks to the latter. In the following, two scenarios must be distinguished: (a) In case of symmetric wetting (same block at top and bottom) the film thickness t must be given by t = n × L0, where n is a multiple integer and L0 is the equilibrium lamellar size of the bulk morphology.5,6 (b) In case of asymmetric wetting (different block at top and bottom) t = (n + 1/2) × L0 must hold.5,6 It is evident from the literature, that in the bulk the lamellar morphology of PE-b-PEO cooligomers is formed by stacking of nearly extended chains.22,24,25 This implies that L0 is close to the extended

chain length of the PE-b-PEO co-oligomer. The extended chain length is given by22 dext = dPE,ext × cos ϕE + dPEO,ext = nE × c PE × cos 22° + nEO × c PEO/7

where nE and nEO are the numbers of E and EO mers and cPE and cPEO represent the length of the c-axes of the orthorhombic PE (0.2546 nm) and the monoclinic PEO (72 helix, 1.948 nm) crystal unit cell, respectively. Furthermore, ϕE is the average tilting angle of PE chains (22°) with respect to the lamellar normal.22 For the E15EO47 we calculated the extended chain length to be dext = 16.6 nm, with relative length contributions of the PEO and the PE of 13.1 and 3.5 nm, respectively. Taking this extended chain length into account, the observed discrete film thicknesses can be interpreted in terms of terraces composed of vertically extended chains. Thus, any realized film thickness is an integral multiple of the theoretical extended chain length of 16.6 nm. The dark and bright domains on the surface of the first terrace of drop-cast E15EO47 films (e.g., Figures 3A−C) suggest a laterally phase-separated structure. The strong phase contrast in the AFM phase images measured with amplitude ratios Asp/A0 of about 0.75 indicates the existence of stiffer crystalline and softer, presumably amorphous, domains.32 This is strengthened by the in-plane GIXRD data of the as drop-cast surface (i.e., Figure 4C), revealing the presence of PEO crystallites with PEO chains oriented perpendicular to the substrate. Thus, we assume that during drop-casting the oligomer chains orient vertically in two opposite directions (i.e., PE head up or down) and, in turn, segregate laterally into domains of similar chain chemistry, i.e., into assemblies of same vertical chain directions. Obviously, within the time of solvent evaporation after drop-casting there is a sufficient surfacediffusion and chain mobility to permit such a lateral segregation. We assume that the overall surface fraction of black and white domains is already established during the initial contact of single polymer chains with the substrate surface, as the energy barrier necessary to accomplish a vertical realignment is higher than for a lateral movement. Figure 6A schematically illustrates the chain orientation within the first terrace of the drop-cast thin films, as suggested by the AFM phase contrast and the GIXRD analysis. The top of the scheme sketches the bright and dark domains in the lateral AFM phase contrast arising from the two possible vertical chain orientations. The corresponding vertical chain orientation is sketched below, with the PE block being either directed toward the substrate or the free surface. Furthermore, the lateral dimension of the domains of similar chain orientation increases with increasing hydrophobicity (see, e.g., Figure 3A−C). We assign the domain size to be a function of the solvent’s residence times on the surface34 along with different surface mobilities35 of the perpendicularly oriented chains. With increasing hydrophobicity of the substrate the wettability between polymer solution and surface is decreased and the spreading of drops is reduced. In addition, the solvent volume per area and thus the solvent residence time are increased with increasing hydrophobicity. A discrete film thickness combined with a lateral segregated surface morphology cannot be explained within the framework of the BCP thin film formation theory known from the literature. According to the latter, a discrete film thickness of BCP thin films could only occur for a parallel lamellar 4745

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perpendicularly standing lamellae. However, we did not observe (120) PEO planes in the out-of-plane scan which would indicate crystalline PEO domains with a parallel chain orientation, although AFM suggests that both, PEO and PE chains are aligned parallel to the substrate surface. The sole crystallization of the PE component after annealing the film on the neutral surface differs from the bulk of the similar E17EO40 co-oligomer, where both, the PE and PEO phase were reported to crystallize.24,25 We explain the different behavior by a confinement of PEO chains in the partially crystallized film. Since Tc, PE > Tc, PEO, the driving force for the formation of perpendicular lamellae during cooling from 120 °C is presumably the crystallization of the PE component, although it is the minority component. The rigid crystalline PE lamellae will subsequently confine the PEO chains and reduce its crystallization temperature. The reduction of crystallization temperature due to a suppressed homogeneous nucleation in confinements is widely established.16,22 By DSC analysis, we assigned the crystallization temperature of PEO in the bulk to ∼30 °C. Therefore, even a minor decrease in crystallization temperature can prevent the PEO from being crystalline at RT. In the light of these considerations the strong phase contrast in the AFM phase images in Figures 3H and 5A must be attributed to the existence of stiff crystalline PE appearing bright and soft noncrystallized PEO appearing dark. Additionally, the in-plane GIXRD scans of the annealed film surface (see Figure 4D) suggest the presence of crystalline PEO. This apparent contradiction is resolved by considering the aggregates formed during the dewetting process. Presumably, the detected crystalline PEO with chains oriented perpendicular to the substrate surface are mainly localized within these bulk-like regions. The presence of the aggregates is documented by optical light microscopy images in Figure 4B. We estimate about 20 wt % of the polymer within the drop-cast solution is required to form a 16 nm thick film covering the entire substrate surface. Correspondingly, aggregates formed during the dewetting process accompanying the annealing contain up to 80 wt % of the polymer. Within these bulk-like aggregates we assume the PE confinement to be relaxed or not to exist due to the absence of the substrate surface, permitting both, the PE and the adjacent PEO component to crystallize. The missing peak of the adjacent parallel PE crystallites in the in-plane GIXRD measurement in Figure 4D can be explained by the tilting angle between the PE chain orientation and the lamellar normal reported for the bulk system23,24 Furthermore, one would expect Bragg-scattering in-plane for the PE (200) and out-of-plane for the PEO (120) from statistically distributed crystallites within the aggregates. However, the subset of such crystallites which are in Bragg condition are much less than for the aligned crystallites in the lamellar layer (representing 20 wt % of the co-oligomer). As the signal-tobackground ratio for the lamellar layer is already at the limit of detection, Bragg scattering from the statistically oriented crystallites cannot be resolved. Although the GIXRD measurement offered input to the model developed and based on the AFM data, a final conclusion would only be possible if the whole scattering cones for (120) PEO and (200) PE were examined, but unfortunately the software options of the Rigaku device were limited, rendering these measurements currently impossible. A further prospective improvement would be to accomplish the GIXRD measurements at homogeneous E15EO47 monolayers to avoid the Bragg scattering at

Figure 6. Schematic illustration highlighting the chain orientation in drop-cast (A) and annealed (B) E15EO47 thin films on the neutral substrate. The structure on top represents an enlarged section of the AFM phase images in Figure 3, parts B and H. Note that stiffer crystalline domains appear brighter than softer noncrystalline domains. Part A shows the model of the first terrace of the drop-cast thin film with a thickness of about 16 nm comprising domains of identically oriented extended chains standing perpendicular to the substrate surface. In-plane GIXRD measurement suggests that the PEO is crystallized while the PE is not. Part B illustrates the model of the annealed thin film with a thickness of about 16 nm and a lamellar surface morphology. Note that the out-of-plane GIXRD data suggest that the PE is crystallized in the annealed film, while the PEO is not.

orientation that in turn prevents a microphase-separated surface morphology on the top surface.3−6 Thin Film Morphologies after Annealing. Vacuum annealing at 120 °C induced a strong dewetting process on all substrates resulting in the formation of macroscopic aggregates with diameters of several micrometers, as documented by the optical microscopy image in Figure 4B, and a height of about 400 to 1,000 nm (determined by AFM, data not shown). On M3M−gold surfaces, we observed a residual E15EO47 layer with a thickness of ∼16 nm. Thus, we conclude that the dewetting occurs on top of a single monolayer.36 The phaseseparated, highly ordered lamellar morphology on top of this monolayer can be seen in the AFM phase images in Figures 3H and 5A and the SEM image in Figure 5B. The lateral lamellar size of ∼16 nm suggests the lamellae to consist of stacked, extended E15EO47 chains. A similar lamellar morphology, built by stacked extended chains, was found in the bulk of an E17EO40 co-oligomer.24,25 The nonpreferential affinity of the E15EO47 co-oligomer toward the neutral M3M−gold surface permitted the establishment of the perpendicular lamellar surface morphology in the residual E15EO47 monolayer during vacuum annealing. This implies a chain rotation from surface perpendicular to surface parallel orientation during vacuum annealing, as depicted in Figure 6B. A similar chain alignment and reorientation effect has previously been reported for crystallizing thin films of a hydrogenated poly(butadiene-bethylene oxide) diblock copolymer with a molar mass of 6800 g/mol and one amorphous component.37 As the PEO and PE chain lengths are 13.1 and 3.5 nm it seems surprising that the lamellae are symmetric. However, it is known that asymmetries of heterogeneous surface arrangements of a size smaller than the AFM tip diameter cannot be resolved and leads to an apparent symmetry.38,39 The out-of-plane GIXRD data of the vacuum annealed film on the neutral surface (e.g., Figure 4D) shows the presence of (200) PE planes which reveals the existence of PE chains oriented parallel to the substrate surface. This observation further facilitates the existence of chains aligned parallel to the substrate surface and correspondingly confirms the model of 4746

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CONCLUSION We present a novel approach to create amphiphilic surface nanostructures with lateral dimensions of the order of ∼10 nm by employing a perpendicular lamellar thin film morphology of an asymmetric double crystalline polyethylene-block-poly(ethylene oxide) co-oligomer. These amphiphilic nanopatterned surfaces could be interesting as templates for various applications as, e.g., in the biomaterials field as well as for nanotechnology applications. After drop-casting, we observed terraced thin films with a thickness corresponding to multiple integers of the extended chain length, and at the same time laterally phase-separated domains. Our model suggests that the polymer chains are extended and oriented perpendicular to the substrate surface. The time for solvent evaporation facilitates lateral segregation into domains of chains with identical orientation of the blocks. Annealing of films on neutral surfaces induces a chain rotation from surface perpendicular to surface parallel, enabling surface perpendicular lamellae. Independent of the substrate chemistry, annealing at higher temperatures induces strong dewetting that results in the formation of aggregates. Annealing at lower temperatures in ethanol atmosphere impedes the dewetting and facilitates an enrichment of PEO on the film surface. On the basis of these findings, we conclude that chain realignment, morphology development and macroscopic dewetting are induced by a temperature promoted chain flexibility and take place on different time scales.

crystallites within the aggregates to solely achieve the crystallite orientation from the monolayer. Aggregates formed on hydrophilic and hydrophobic substrates by dewetting during vacuum annealing at 120 °C were surrounded by co-oligomer monolayers as shown in Figure 3, parts G and I. These monolayers did not show a phase-separated surface morphology, suggesting that they consist of vertically aligned co-oligomer chains with identical orientation, i.e., any block with a preferential affinity to the substrate is facing toward the latter. On the hydrophilic substrate, this monolayer completely covers the whole substrate, presumably with the PE block located on the free monolayer surface. On the hydrophobic substrate this monolayer covers the surface only partially which could be due to weaker van der Waals interactions between PE and the ODM as compared to interactions between PEO and the silicon oxide layer.40 The macroscopic dewetting observed on polymer films vacuum annealed at 120 °C could be reduced by annealing the drop-cast polymer films at 60 °C in an ethanol saturated atmosphere. In contrast to the vacuum annealed polymer films we observed on all substrates (hydrophilic, neutral and hydrophobic) monolayers (Figure 3, parts D, E, and F) with laterally phase-separated surface morphologies. Thus, the reduced chain mobility during solvent annealing as compared to the vacuum annealing impeded a complete dewetting of the surface. However, the chain mobility was still sufficiently high to facilitate chain rearrangement processes. Parts D, E, and F of Figure 3 suggest that the shape and size of these phaseseparated structures are different. The solvent-annealed thin film on the neutral surface exhibited a similar lamellar morphology as observed after vacuum annealing at 120 °C with the same lamellar period. This observation indicates that the chain mobility at decreased temperatures in ethanol vapor is sufficient to promote chain rotations from vertical to parallel alignment. On the other hand the solvent-annealed thin films on silicon and on ODM−gold did not exhibit a lamellar morphology. Furthermore, on both the hydrophilic and hydrophobic film surfaces a decreased fraction of the dark phase appeared, which could be due to the ethanol atmosphere with a preferential affinity to PEO that facilitates an enrichment of PEO at the film surface. Summary. Overall, this study shows that the extended chain induced asymmetric lamellar bulk morphology of an asymmetric double crystalline polyethylene-block-poly(ethylene oxide) co-oligomer can be transferred toward the surface of a thin film. After drop-casting from dilute solution we observed terraced thin films with a discrete film thickness, whereby the latter corresponds to multiple integers of the extended chain length. At the same time we found on the surface of these thin films a domain structure. Vacuum annealing at 120 °C induces dewetting and the formation of aggregates. On neutral surfaces, a residual polymer monolayer remains that exhibits a lamellar surface morphology where the lamellae size corresponds to the extended chain length. In contrast, after vacuum annealing on hydrophilic and hydrophobic surfaces polymer layers with lamellar surface morphologies cannot be observed. Annealing the thin films in ethanol saturated atmosphere at 60 °C reduces the dewetting. On neutral surfaces, we observed the same lamellar morphology as after vacuum annealing, while on hydrophilic and hydrophobic surfaces we deduced an enrichment of the PEO-phase at the thin film surface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +49 (0) 3641 94 77 30. Fax: +49 (0) 3641 94 77 32. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.D.J. gratefully acknowledges the partial financial support of the Deutsche Forschungsgemeinschaft (DFG), grant reference INST 275/241-1 FUGG, and the Thüringer Ministerium für Bildung, Wissenschaft und Kultur (TMBWK), grant reference 62-4264 925/1/10/1/01.

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