Article pubs.acs.org/Macromolecules
Anisotropic Mechanical Properties of Aligned Polystyrene-blockpolydimethylsiloxane Thin Films Changhuai Ye, Gurpreet Singh,† Maurice L. Wadley,‡ Alamgir Karim, Kevin A. Cavicchi, and Bryan D. Vogt* Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States ABSTRACT: Mechanical properties of multicomponent polymers such as block copolymers (BCP) are critical to their utilization in thin film applications, yet the effect of block alignment of even the simplest BCPs on thin film modulus has not been thoroughly explored. Polystyrene-block-polydimethylsiloxane (PS-b-PDMS) thin films with cylindrical PDMS domains (f PDMS = 0.23) are aligned by cold zone annealing− soft shear mode (CZA-SS). Increasing the maximum temperature of the thermal zone and/or decreasing the CZA-SS velocity improves the alignment of the PS-b-PDMS domains as determined by atomic force microscopy (AFM) and smallangle neutron scattering (SANS). The maximum Hermans orientation factor (S) for this system using the CZA-SS conditions examined is S ≈ 0.85, yielding anisotropic mechanical properties as determined by surface wrinkling. While the in-plane modulus perpendicular to PDMS cylinder axis is nearly invariant of S, the modulus parallel to the cylinder axis is increased by a maximum of 31% for S ≈ 0.85, compared to its unaligned state (S ≈ 0). These results demonstrate the highly anisotropic response of the glassy matrix strength to alignment of internal rubbery cylindrical channels. This anisotropic structure−property coupling is consistent with expectations associated with highly aligned composites, but this CZA-SS methodology enables facile examination of the influence of extent of alignment on physical properties that has not been quantitatively investigated in the past.
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INTRODUCTION Nanostructured polymeric thin films are attracting significant attention due to their potential applications in organic electronics,1,2 functional coatings,3 directed assembly for microelectronics,4 and membranes5 as examples. Block copolymer (BCP) self-assembly offers a facile route to design nanostructures;6,7 however, ordering of BCPs is commonly kinetically limited,8 especially for thin films where the kinetics of defect annihilation and grain growth are suppressed9 in comparison to the bulk.10 To address these limitations associated with obtaining long-range order of the BCP nanostructure in thin films, many processing methods have been developed with most focusing on the application of external force fields, such as electric fields,11 magnetic fields, and applied shear.12 Alternatively, substrate modification to generate a neutral surface provides a route to perpendicularly align cylindrical domains of BCPs,13 but it generally does not produce long-range lateral coherence of the ordered nanostructure. A chemically14 or physically15 nanopatterned substrate provides directionality to the alignment of the BCP domains, but these are limited to specific substrates or require advanced photolithography. As thermal gradients are utilized for the zone refining of crystals,16 a similar method based on moving thermal zones has been applied to the alignment of block copolymer films with much success.17−19 This zone annealing method is versatile and provides a facile route to control the orientation and extent of © 2013 American Chemical Society
alignment through selection of the thermal gradient and its velocity.20−22 Combining the cold zone annealing method with induced shear from a polydimethylsiloxane (PDMS) pad on the BCP thin film yields significant increases in the alignment.19 The PDMS layer confined to the BCP surface experiences thermal expansion and contraction as the thermal zone moves to generate a directional dynamic soft shear. Therefore, this methodology is termed cold zone annealing−soft shear (CZASS).19 Alignment of BCPs is known to impact their properties. Anisotropic mechanical properties of highly oriented BCPs have been studied since the 1970s23 with nearly “single crystal” polystyrene-block-polybutadiene-block-polystyrene (SBS) exhibiting a 100× enhancement in modulus perpendicular to the cylinder axis relative to the modulus parallel to the cylinder axis. Moreover, this alignment also impacts the manner of the deformation with a significant increase in the strain for affine deformation perpendicular to the cylinder axis.24 Other bulk studies of globally oriented ABA triblock copolymers at both large and small strains have demonstrated anisotropy in their mechanical properties.25−27 However, systematic variations in the extent of alignment in a quantitative manner are difficult to achieve with bulk samples. Moreover, mechanical properties in Received: August 25, 2013 Revised: October 8, 2013 Published: October 23, 2013 8608
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thin films are critical for membrane applications,28 but these properties have not been explored for thin film BCPs, especially with respect to the impact of domain orientation. These previous studies examining the mechanical anisotropy of highly aligned BCPs initially focused on thermoplastic elastomers with the majority phase being rubbery. However, more recent work with aligned BCPs have been focused on the deformation of copolymers with polyvinylcyclohexane (PCHE) as the majority block in order to enhance the toughness of these glassy copolymers through inclusion of crystalline domains29 or both crystalline and rubbery domains with a pentablock copolymer.30 In these cases, the tensile deformation studies have focused on ultimate properties31 with many studies utilizing a metal grid29,32,33 technique to examine the fracture behavior of films. Alignment of these BCPs results in anisotropic ultimate properties, but the extent of orientation has not been quantified in these cases; although TEM micrographs can illustrate near perfect alignment locally,29 the in-plane long-range order can be more limited in these aligned systems examined.32 Despite these significant efforts in understanding the fracture behavior of oriented glassy BCPs, very little has been reported regarding their elastic properties; the dynamic elastic properties have only been reported in one direction,30,34 so the anisotropy in the elastic properties have not been determined. PDMS exhibits intriguing transport properties for membranes,35 but it is limited by its low glass transition temperature such that additional segments such as polystyrene are required for mechanical reinforcement.36−38 Additionally, PDMS containing BCPs are promising materials for pattern transfers due to its high etch contrast from the high Si density in PDMS.39−42 In these cases, long-range order is difficult to obtain from thermal annealing. Solvent vapor annealing (SVA) provides a mechanism to order these BCPs, such as polystyrene-blockpolydimethylsiloxane (PS-b-PDMS), but without any preferred orientation, beyond with respect to the substrate.42 Combining SVA with lithographic patterns provides a route to aligned PSb-PDMS domains,41 but these methods are not easily commercially extendable due to the environmental and safety limitations associated with solvent vapors. In this article, CZA-SS is used to align the minority phase PDMS cylinders of PS-b-PDMS in thin films. The orientation of the PS-b-PDMS thin films can be facilely tuned through the temperature profile and velocity associated with CZA-SS. This orientation is quantified by atomic force microscopy (AFM) and small-angle neutron scattering (SANS) in terms of the Hermans orientation factor, S. The orientation of PS-b-PDMS from CZA-SS can reach S = 0.85 in this case. The influence of extent of orientation on the mechanical properties of the BCP films is elucidated through surface wrinkling.43 Similar to rubrene single crystals,44 anisotropic in-plane moduli are observed for the highly aligned PS-b-PDMS. Furthermore, the anisotropic mechanical properties of the BCP thin films are strongly correlated with the quality of the long-range orientation of the BCP domains as quantified by S.
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from the quartz, a thin layer of poly(styrenesulfonic acid) (PSS) was first spun coat at 2500 rpm on the substrate from 0.2 wt % solution from dilution of 18 wt % PSS/H2O solution (Mw ≈ 75 kg/mol, SigmaAldrich) with isopropanol (Sigma-Aldrich). The PSS release layer was subsequently dried in vacuo at 140 °C for 12 h. After cooling of the PSS-coated quartz, PS-b-PDMS was spun coated on the top of PSS layer from 2.5 wt % solution in benzene (ACS grade, Sigma-Aldrich). Thin Film Thickness Measurement. The thicknesses of the thin films were measured by a variable angle spectroscopic ellipsometer (VASE, J.A. Woollam Co., Inc.). Measurements were performed from 60° to 75° with a step size of 5° and scanned over a wavelength range of 300 to 1689 nm. The film thicknesses of PSS layers and PS-b-PDMS thin films were determined by fitting the optical properties of the polymer layer with the Cauchy model. The thicknesses of the PSS and PS-b-PDMS were approximately 8 and 200 nm, respectively. For PS-bPDMS, this corresponds to ≈5L0 (L0 = 41 nm for the cylinder-tocylinder distance as determined by SANS from the d100 peak associated with a spacing of 36 nm). Cold Zone Annealing−Soft Shear. In order to induce soft shear for CZA, a thin PDMS (Sylgard 184, Dow Corning) slab with a thickness of approximately 400 μm was used. A mass ratio of 6:1 of base to curing agent was used for preparing the PDMS from the Sylgard kit. After spreading the mixed PDMS to approximately 400 μm and allowing the PDMS to degas, it was cured at 120 °C for 2 h. A slab of the cured PDMS was placed on the PS-b-PDMS films on the quartz substrates and allowed to wet the polymer film. The film sandwiched by the quartz and PDMS was then pulled at a constant velocity through a sharp temperature gradient developed by a heated filament between two cooled blocks as described previously.19 After this CZA process, the PDMS slab was removed from the surface by peeling to reveal the aligned PS-b-PDMS film. Thin Film Structure Characterization. In order to increase the contrast between the PS and PDMS phases, the PS-b-PDMS thin films were treated with ultraviolet ozone (UVO, PSD Series Digital UV Ozone System, Novascan Technologies, Inc.) for 1 h prior to characterization. The surface morphology of the PS-b-PDMS thin films was investigated with AFM (diDimension V, Veeco) in tapping mode using scan size of 3 μm × 3 μm and a scan rate of 1 Hz. To more extensively elucidate the structure of the thin films, SANS was utilized using Beamline-6 (BL-6) at Spallation Neutron Source (SNS) with scattering measured in transmission through the PS-b-PDMS and quartz substrate (single film scattering to enable elucidation of orientation). The average neutron wavelength was 10 Å, and the beam diameter was 1 cm. The sample-to-detector distance was set to 5 m. The data collection time was 2 h for each sample. The scattering data were corrected for the background scattering associated with a blank piece of quartz. Mechanical Properties. The modulus of PS-b-PDMS films was elucidated from surface wrinkling.47 For the compliant substrate for the wrinkling measurement, cross-linked PDMS (Sylgard 184, Dow Corning) with a thickness of approximately 2 mm was prepared with a mass ratio of base to curing agent of 20:1 and allowed to degas and initially cure at ambient temperature for 4 h followed by curing at 120 °C for 3 h. After cooling, the PDMS was cut into 2.5 cm × 7.5 cm strips. The Young’s modulus of this PDMS was measured using a texture analyzer (TA-TX Plus) at a strain rate of 0.05 mm/s and determined to be 0.68 ± 0.02 MPa. For the wrinkling measurements, the PDMS slab was first prestrained to 3.5%, and the PS-b-PDMS thin films were transferred to PDMS by water immersion due to the differential adhesion of the PS-b-PDMS in water between the PDMS and quartz. After transfer, the polymer thin films were dried at ambient temperature for more than 12 h. Wrinkling was induced by release of the prestrain at 0.1 mm/s using an Universal Motion Controller (Model Esp100, Newport). The wavelength of the wrinkles was determined by optical microscopy (Olympus MX51) and transformation of the real space image by fast Fourier transform (FFT). For each sample, at least eight images were analyzed to determine the wavelength. The plane strain modulus of the PS-b-PDMS films, E̅f, is determined from the wrinkling wavelength, λ, as48
EXPERIMENTAL SECTION
Thin Film Preparation. PS-b-PDMS (Mn = 46.1 kg/mol, f PDMS = 0.23, Đ = 1.21) was synthesized using RAFT polymerization as reported previously.45 Quartz (2.5 cm × 2.5 cm × 1.5 mm, GM Associates) was used as the substrate for the BCP films. The substrates were cleaned with piranha solution (H2SO4:H2O2 (30 wt % in water) = 7:3) at 90 °C for 30 min. To enable release46 of the PS-b-PDMS 8609
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⎛ λ ⎞3 ⎟ Ef̅ = 3Es̅ ⎜ ⎝ 2πh ⎠
Surface Morphology of PS-b-PDMS Thin Films. Figure 1 illustrates representative AFM micrographs of PS-b-PDMS thin films after the different processing conditions listed in Table 1. A PDMS wetting layer with a thickness of a few nanometers on the surface of these films tends to impede surface imaging, but UVO treatment increases the contrast between the PDMS and PS phases to enable characterization of the surface by AFM. Figure 1A illustrates the nanostructure with no defined alignment direction after thermal annealing of the PS-b-PDMS in an oven (no applied thermal gradient). In these phase images, the bright domains are associated with the PS; in this case, the PDMS is not sufficiently oxidized to invert the phase differences between the domains.49 The ordering of the PS-b-PDMS does not conform to the typical fingerprint morphology associated with unoriented cylindrical BCP nanostructures for other BCP systems,49 but this morphology is consistent with the as-cast morphology of mixed parallel and perpendicular cylinders previously observed when casting from an aromatic solvent (benzene).50 Difficulties with thermal induced ordering of PS-b-PDMS films have been reported previously.51 The FFT shown in the inset of Figure 1A confirms the lack of orientation by oven thermal annealing, but there is a common length scale associated with the surface morphology as evidenced by the halo. Conversely, examination of the AFM micrographs for all samples processed with CZA-SS show some degree of orientation along CZA direction. At low annealing temperature
where h is the thickness of the BCP film and E̅ s is the modulus of the PDMS slab. The modulus of the PS-b-PDMS thin films for each direction was determined by averaging at least three samples.
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RESULTS AND DISCUSSION Alignment of PS-b-PDMS Thin Films by CZA-SS. CZASS is an effective method to align BCP thin films through the application of dynamic thermal and shear fields.19 Selections of the thermal gradient height (maximum temperature) and the velocity are both critical to determining the degree of alignment.19,22 Additionally, higher annealing temperature also provides sharper temperature gradient along the CZA direction, which has been shown to impact the alignment. In order to tune the degree of orientation of PS-b-PDMS films, processing conditions have been selected to produce a wide range of alignment. Table 1 shows these annealing conditions used here for the PS-b-PDMS films. Table 1. Processing Conditions for PS-b-PDMS Thin Films Tmax (°C) speed (μm/s) a
A
B
C
D
190 oven annealinga
175 50
190 50
205 10
The films are annealed for 3 h in the oven under vacuum.
Figure 1. AFM images with FFT in upper corner for PS-b-PDMS samples oriented by (A) oven thermal annealing at 190 °C, (B) CZA-SS at 175 °C and 50 μm/s, (C) CZA-SS at 190 °C and 50 μm/s, (D) CZA-SS at 205 °C and 10 μm/s. 8610
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and high CZA velocity, (Figure 1B, 175 °C and 50 μm/s), the orientation along CZA direction is relatively weak with small regions aligned in the CZA-SS direction. However, most of the micrograph shows randomly oriented nanostructures, but the cylindrical structure in these poorly aligned regions appears to be better ordered than those simply thermally annealed. This improvement in ordering is unexpected as the sample annealed in the oven is at 190 °C for 3 h, while the maximum temperature for the CZA-SS process in this case is only 175 °C and the local area is only exposed to temperatures greater than Tg of PS segments (100 °C) for 2.2 min (based on the temperature profile and velocity). This result suggests that the external fields induced by CZA-SS enhance the ordering kinetics for the PS-b-PDMS thin films. The FFT of the AFM micrograph (inset in Figure 1B) also illustrates the alignment of this film with the halo transforming to crescents associated with the preferred direction for the cylindrical nanostructures. The orientation is greatly improved when the maximum temperature is increased to 190 °C without any change in the CZA-SS velocity (Figure 1C). In this case, a majority of the film surface is comprised of cylinders with their primary axis aligned in the CZA-SS direction. However, there is still a large propensity for defects in the ordered structure. The enhanced orientation of the cylinders is clear from the FFT of the micrograph with diffuse spots associated with the periodic structure. By increasing both the temperature and decreasing the velocity to increase both BCP segment mobility and rearrangement time (Figure 1D), the ordering and alignment are further improved. The peaks in the FFT inset become sharper, which is consistent with this increased alignment where the nanostructure of PS-b-PDMS thin films is mostly aligned along the CZA direction. In order to quantify the degree of orientation in PS-b-PDMS thin films, the FFTs from the AFM micrographs are analyzed as a function of azimuthal angle to calculate the Hermans orientation factor, S. The orientation function is defined as52 3⟨cos2 ϕ⟩−1 2
S=
Figure 2. Hermans orientation factor for PS-b-PDMS thin films oriented by (A) oven thermal annealing at 190 °C, (B) CZA-SS at 175 °C and 50 μm/s, (C) CZA-SS at 190 °C and 50 μm/s, (D) CZA-SS at 205 °C and 10 μm/s as determined from AFM (■) and SANS (●).
patterns for the PS-b-PDMS thin films with the four different processing protocols (Table 1). These scattering profiles are analogous to the FFTs of the AFM micrographs shown in the insets of Figure 1. The qualitative agreement between the SANS and AFM measurements suggests that the surface and the bulk of the films are aligned to a similar extentsimilar to prior observations for CZA-SS using other BCP systems.19 To further analyze the ordered structure, the azimuthal averaged scattering intensity is shown in Figure 3E as a function of the momentum transfer vector, q (= (4π)/λ) sin θ, where λ is the neutron wavelength and θ is the scattering angle). These scattering patterns are rather noisy due to the limited natural contrast for PS and PDMS (no selective deuteration) and the small scattering volume associated with
