Morphology Control of Poly(styrene-block-dimethylsiloxane) by Simple

Sep 27, 2016 - Trimethylsilylated silicate nanoparticle (termed MQ resin, combining M Me3SiO1/2 and Q SiO4/2 units)/poly(styrene-block-polydimethylsil...
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Morphology Control of Poly(styrene-block-dimethylsiloxane) by Simple Blending with Trimethylsilylated Silicate Nanoparticles Ginam Kim,† Steven Swier,*,† Hae-Jeong Lee,‡ and Chengqing Wang‡ †

Product Development, Dow Corning, 2200 W Salzburg Rd., Midland, Michigan 48640, United States Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States

‡

ABSTRACT: Trimethylsilylated silicate nanoparticle (termed MQ resin, combining M Me3SiO1/2 and Q SiO4/2 units)/ poly(styrene-block-polydimethylsiloxane) (PS−PDMS, 31K− 15K, polydispersity PD = 1.15, weight-average molecular weight Mw = 45.5K) blends behave similarly to block copolymers with different PS/PDMS ratios. MQ is localized in the PDMS phase virtually extending the volume fraction in the block copolymer. This allows for microdomain morphology control beyond what can be achieved with the starting block copolymer. Synthesizing siloxane-containing block copolymers targeted at certain equilibrium morphologies can be time-consuming and in some cases technically challenging. This work shows that MQ is a robust morphology modifier, not limited by the occurrence of surface segregation and the high diffusion rates typically associated with homopolymer modification, as confirmed by looking at the PDMS/PS−PDMS reference. The convenient and robust structure control MQ nanoparticle modification of PS−PDMS provides could overcome one of the hurdles to adoption of block copolymer lithography.

1. INTRODUCTION The ability of block copolymers to achieve periodic nanostructures like spherical, cylindrical, gyroid, or lamellar morphologies with high fidelity opens up various applications.1,2 Ordered structures can be controlled by the polymer species involved, the degree of polymerization, the average composition, and the thermotropic/lyotropic processing characteristics.3,4 Poly(styrene)-b-polydimethylsiloxane (PS− PDMS) is of particular interest due to its strong phase separation tendency as evidenced by the high Flory−Huggins interaction parameter χ = 0.26.5 This in principle enables sub12 nm feature spacing, sharp interphase regions, and etch resistance contrast.6 Pushing the boundary even further below 10 nm requires both high χ and low N block polymers, with N the overall degree of polymerization.7 PS−PDMS has been explored as a precursor for nanoporous materials,8 orientationcontrolled self-assembled nanolithography,9 and nanoimprint lithography.10 Dow Chemical recently disclosed the use of PS− PDMS as an ideal material in this respect.11 The excellent thermal and oxidative stability and good etch selectivity of PDMS contribute to the attractiveness for microelectronics.8,9,12−15 IBM announced the development of air gap dielectrics using block copolymers for the next generation of computer chips.16−18 Recent popular journals indicate a broad interest in using directed self-assembly (DSA) “block copolymer (BCP) lithography” as a complementary technique to photolithography,6,19 particularly for high-density and addressable storage media. The idea is to combine bottom-up BCP self-assembly © XXXX American Chemical Society

with top-down photolithography. Chemical patterns (epitaxial self-assembly) or topographical features (graphoepitaxy) are used to direct the orientation and ordering of block copolymers to ensure long-range order. Patent filings by Seagate describe advancements in topographical guide patterns,20 while Toshiba claims specific patterns for cylindrical and lamellar morphologies21 and guide patterns to ensure high regularity.22 PS−PDMS can be made in controlled AB, ABA, and BAB architectures using living anionic polymerization schemes. However, challenges still remain in the synthesis of high molecular weight PS−PDMS block copolymers having monodispersity because of the occurrence of side reactions.23,24 It would therefore be worthwhile to achieve morphology control by adding a component miscible with one of the two phases in the copolymer and as such extend the volume fraction of that phase. One avenue to tailor the morphology is to blend block copolymers with one of the corresponding homopolymers. This has been extensively studied by using AB or ABA block copolymers and their blends with low molar mass homopolymers (A or B).25−33 Microdomains are selectively swollen by the compatible homopolymer inducing morphological transitions. In this case, the macrophase separation is suppressed, and the microphase separation becomes dominant. Adding a selectively miscible homopolymer C to an AB block copolymer Received: June 23, 2016 Revised: September 6, 2016

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Figure 1. TEM image and SAXS patterns of a PS−PDMS block copolymer having a hexagonally ordered cylindrical morphology. The inset in the 2D SAXS patterns is the image of patterns near the beam blocker.

polydimethylsiloxane. MQ resins are usually prepared through sol−gel chemistry, including acid-catalyzed hydrolysis and condensation of aqueous sodium silicate followed by treatment with trimethylchlorosilane or through the cohydrolysis and condensation of trimethylchlorosilane and tetraethoxysilane.55,56 MQ resins are glasses (glass transition: Tg > 300 °C), ranging in size from 1 to 2 nm depending on their Mw.57 These nanoparticles are known to be fully miscible with polydimethylsiloxane, altering its polymer dynamics.58,59 MQ resins raise the glass transition of PDMS60 and are chemically identical but an order of magnitude smaller than typical trimethylsilyl-treated silica nanoparticles according to the Stober method.61 This work will show that MQ is a robust morphology modifier, not limited by the occurrence of surface segregation and the high diffusion rates typically associated with homopolymer modification.

system allows for modification of domain spacing and morphological transitions.34−36 In block copolymer (A−B)/ homopolymer (C) blend systems, the phase behavior is governed by a complex interplay between the three segment−segment Flory−Huggins interaction parameters.37 However, the solubility of the homopolymer in microdomains of one of the blocks is limited depending on the degree of polymerization, composition, and temperature. Also, when a polymer component has a lower surface tension, its diffusion dynamic toward to air interface can be an important factor affecting the morphological distribution in a mixture.38−40 Apart from homopolymer addition to block copolymers, nanoparticles also have been added to control the block copolymer morphology. Two recent reviews detail the selfassembly of block copolymers with nanoparticles, both experimentally and theoretically, with particular focus on particle size, surface ligands, and future applications.41,42 Theoretical studies by Pryamitsyn et al. and Spontak et al. show that the selectivity of nanoparticles is an important factor deciding the nanoparticle location.43,44 This was experimentally confirmed by Kramer and co-workers showing that the location of Au nanoparticles in a PS-b-poly(2-vinylpyridine) (PS-P2VP) copolymer can be elegantly controlled by adjusting the ligand composition.45 The location of selective nanoparticles in lamellar domains has been discussed based on the nanoparticle size.46−48 Actually, for nanoparticles with moderate selectivity for one of the blocks, particle size determines locality. Depending on the size of the particles d and the polymer domains L, localization of the particles occurs along the interface for d/L < 0.2 to gain particle translational entropy or at the center of the polymer domain for d/L > 0.3 to reduce the polymer conformational entropy loss.49 Nanoparticles can also regulate phase behavior when the right size and block selectivity are introduced.43,50 When nanoparticle size is comparable to the lamellar period or high particle loadings are used, local deformations occur, eventually leading to particle expulsion and macrophase separation due to the polymer conformational entropy penalty.51−54 In this work, trimethylsilylated silicate nanoparticles (termed MQ resins) are evaluated as modifiers for polystyrene-block-

2. RESULTS AND DISCUSSION 2.1. Microdomain Morphology of PS−PDMS. A PS− PDMS block copolymer having 32 vol % PDMS was selected and obtained from Polymer Source (polydispersity PD = 1.15, weight-average molecular weight Mw = 45.5K) as a base material for MQ resin modification. For the detection of the microphase-separated morphology in the block copolymer small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were acquired, and the results are summarized in Figure 1. For the 32 vol % PDMS composition in a PS−PDMS block copolymer, a hexagonal array of PDMS cylinders in a PS matrix is seen in the TEM image, as expected based on strong phase segregation.1,2 The strong immiscibility between PS and PDMS blocks drives each polymer block to stretch away to minimize the interfacial energy resulting in the formation of self-assembled ordered structures on the nanometer scale. The type of microdomain structure is dependent on the block volume ratio, the chain architecture, and the persistence lengths. The formation of a hexagonally ordered cylindrical morphology is also confirmed by SAXS showing the relative positions of the multiple Bragg peaks. The arrows in Figure 1 (right, including the 2-D image) denote the positions of the high-ordered reflections at √3, √4, √7, and √13 of the B

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Figure 2. Cross-sectional TEM images of PDMS (25%) + PS−PDMS block copolymer: (a) with insets of fast Fourier transform FFTs of lamellar region near the free surface (d = 48.7 nm) and inside (d = 41.3 nm); (b) at the interface between lamellae and gyroid structure; (c) gyroid structure formed inside the bulk.

studies poly(styrene−isoprene−styrene) block copolymers with the addition of polystyrene homopolymers confirm this.30,31 As seen in the TEM results in Figure 2, the addition of PDMS provides limited morphological control. The theoretical prediction at a total PDMS loading of 46 vol % in this PS− PDMS/PDMS blend would be a lamellar morphology of alternating PS (bright domains) and PDMS (dark domains) layers. However, well-oriented in-plane lamellar structures only formed near the free surface (Figure 2a), while a cocontinuous gyroid structure formed inside the bulk (Figure 2c). Figure 2b shows the interface between lamellar region and gyroid structure inside. In Figure 2a, the distance over which parallel oriented lamellae formed near the free surface (top left) is over 7 μm with the gyroid structure emerging at the bottom right of the image. By simple comparison of the periodicities using the detected fast Fourier transformation (FFT), around 18% more PDMS enriched lamellar layers (4% more PDMS in total volume %) formed near the free surface than inside the bulk near the region having a gyroid structure. This is likely due to the combination of the low surface tension of the PDMS homopolymer and its high diffusion rate.38−40 2.4. MQ Resin as a Robust Morphology Modifier. 2.4.1. Theoretical Prediction of Locality. The solubility parameter of MQ resin was estimated based on the empirical method developed by Hansen.65 By testing a range of solvents with known solubility parameters, the solubility parameter was estimated at 16.4 (J/cm3)1/2, which is very close to the value found for PDMS (16.5) but considerably different from that of PS (20.6). It is therefore expected that MQ will be preferentially localized in the PDMS block domains when mixed with a PS−PDMS block copolymer as illustrated

hexagonal lattice. The interdomain spacing of (100) was determined as 42 nm (Q = 0.148 nm−1) according to the primary reflection. A weak signal of √3 is clearly shown in the 2-D SAXS pattern showing a partial ring pattern due to the preferential orientation of cylinders through the specimen thickness. 2.2. Strong Segregation Prediction for PS−PDMS. PS− PDMS diblock copolymers consisting of immiscible PS and PDMS blocks tend to strongly segregate into separate domains due to the nonpolar nature of PDMS. The spatial extent of the domains is limited by the covalent connectivity of the blocks and an equilibrium structure forms avoiding unfavorable contacts at the interface. The degree of phase segregation can be predicted using the Flory interaction parameter χ:4 χ = (v/RT )(δ PS − δ PDMS)2

where v is the arbitrary reference volume (conveniently selected as 100 cm3) and δ is the solubility parameter. An estimate for χ can be obtained by group contribution methods.62,63 Using δPS = 18.6 (J/cm3)1/2 and δPDMS = 15.4 (J/cm3)1/2 as the solubility parameters for PS and PDMS, respectively,64 χ equals 0.33 at 373 K near the glass transition temperature of PS. For a degree of polymerization N = 493 (PDMS 14 500: N = 195; PS 31 000: N = 298), χN = 163, well in excess of χN = 10.5, the theoretical value required for strong phase segregation in a symmetric diblock copolymer according to the self-consistent mean-field theory.27 2.3. Homopolymer Addition To Alter the Morphology. Theoretical studies predict order to order transitions upon the addition of a homopolymer selective to one of the block components in a block copolymer.25,26 Experimental C

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to the theoretical prediction developed by Balazs’ group, MQ would be localized near the PS−PDMS interfaces due to the dominance of translational entropy of selective particles and the small sacrifice in conformational entropy by polymer stretching to isolate nanoparticles in the system. This prediction is consistent with the results in the study of aliphatic coated Au and silica particles mixed together in a poly(styrene-b-ethylene propylene) PS−PEP block copolymer by Bokstaller et al.49 They demonstrated that small Au particles (d/L < 0.2, where d is particle diameter and L is lamellar spacing) are preferentially localized at the interface while large silica particle (d/L > 0.2) are preferentially localized at the center of the preferred PEP domains in lamellar structure. Other theoretical studies of the distribution of nanoparticles in AB and ABA type block copolymers also show that small particles in a strongly segregated block copolymer tend to localize at the interface to reduce interfacial tension.43,44 However, high selectivity of the particles is another important factor leading to the localization of particles into its preferred phase. For small particles having high selectivity, the simulation profile of particle density distribution evolves toward a distribution within a preferred block domain. This was qualitatively confirmed by experimental works by Kim et al.45,66,67 They controlled the position of Au nanoparticles by grafting its surface with short chains either with PS or with P2VP in PS-b-P2VP diblock copolymers, and the TEM results specifically indicated that the particle distribution in a block copolymer is defined by its selectivity rather than by its size. Surface-modified PS selective Au particles having various sizes were delocalized in the PS

schematically in Figure 3. Indeed, MQ and PDMS were found to be highly miscible over a wide composition range, including

Figure 3. Illustration of phase-segregated PS and PDMS domains after mixing with MQ resin.

the range we studied in this work.58 Also, note that the surface of both molecules is trimethyl-rich, resulting in similar surface energies (the surface tension of PDMS and MQ are 21 and 22 dyn/cm, respectively). The small size (1−2 nm) of MQ resin compared to the BCP domain size (>10 nm) should allow easy accommodation of the resin into the microdomain morphology and increase the effective total siloxane (MQ + PDMS) volume fraction resulting in order−order transitions. The natural size R0 of PDMS (R0 = aN1/2), from Balazs’ theory,46−48 was calculated to be 75.1 Å, where a is the statistical segment length (a for PDMS = 5.38 Å) and N is the degree of polymerization (NPDMS). The MQ particle radius Rp = 15 Å is small in comparison, with Rp ≈ 0.2R0,PDMS. According

Figure 4. DSC run on MQ modified PS−PDMS block copolymers with different vol % loadings. Two heating cycles were run to reveal the enthalpic relaxation effects (vol % MQ in the total MQ/PS−PDMS mixture is indicated); solid lines correspond to the first heating, and dashed lines correspond to the second heating; glass transition Tg and melting point TM are indicated in the graph. D

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Figure 5. (a) SAXS data; TEM images of BCP mixed with MQ resin; (b) for 41 vol % silicone (= MQ + PDMS) and (c) for 52 vol % silicone after thermal annealing showing the morphological transformation as volume fraction of silicone (= PDMS + MQ) increased by adding MQ resin into the PS−PDMS block copolymer.

higher than the PS Tg. A second transition right below the PS Tg was also seen in collapsed dry micelles of PS core, PDMS coronae morphologies based on PS−PDMS.68 In this study, a solvent swollen morphology was frozen by removal of the solvent at temperatures below the initial solvated PS Tg. Exothermic and endothermic processes were seen in the first heating of a DSC experiment, hypothesized to be caused by the fusion of the micelles and the release of stress and interface tension. Also, an increased Cp was seen, attributed to a detectable amount of the mobility imbued to the PS segments by the “liquid” PDMS blocks. Although preliminary, this would confirm that an interface layer between PS and PDMS chains might be the origin for the third intermediate glass transition in Figure 4, since a lower mobility PDMS/MQ phase would result in a slightly lower mobility interface region as well. In conclusion, good additional evidence was found from DSC that MQ resides in the PDMS phase. 2.4.3. Morphology Control. As shown in Figure 5, morphology transitions from cylindrical over gyroid to lamellar can be controlled by the addition of MQ resin particles to the PS−PDMS block copolymer. The sequence of phase transitions exhibited by the bulk PS−PDMS from cylindrical to gyroid to lamellar structures is consistent with theoretical expectation based on the total volume percent of siloxane (MQ and PDMS). When 15 mass fraction of MQ resin in total was added to the block copolymer to make 41 vol % of silicone (MQ + PDMS) in the mixture, the peak positions in SAXS (Figure 5a) and the TEM image (Figure 5b) indicated the formation of a cocontinuous (gyroid) structure. The reflection peaks in the SAXS profile at the Q ratios of 1, √(7/3), √(10/3), and √(11/3) correspond to the formation of a gyroid structure. After the addition of more MQ resin to make 47 vol % MQ + PDMS, a lamellar morphology was revealed in SAXS data showing peaks in a Q ratio of 1:2:3. TEM micrographs also clearly showed the formation of alternating lamellae (Figure

domains, while decreasing their size forced their distribution toward to the PS−PVP interface, implying that the selectivity of the nanoparticles is more dominant. 2.4.2. Thermal Analysis Confirms MQ Locality. Thermal analysis was used to support the preferential segregation of MQ. Two heating cycles are shown in Figure 4 since strong enthalpic relaxation events were seen around the glass transitions. Typically, the second heating cycle is used to erase the relaxation history. However, in this case the first heating contains some valuable information around phase transitions. The thermograms show that as MQ resin is incorporated into PDMS, crystallization occurring around −50 °C is suppressed effectively, with a 32 vol % MQ addition resulting in a completely amorphous material. This provides strong confirmation for the presence of MQ resin in the PDMS block as previously assumed from solubility arguments. Also, note that the PDMS Tg increases to −119 °C (pure PDMS around −125 °C) as a result of the reduced polymer mobility. Nakatani et al. reported a similar increase in Tg and disappearance of crystallinity for pure MQ/PDMS blends.58 Note that the typical uncertainty regarding Tg determination was found to be ±1.5 °C. Rheology also provided further evidence of the reduction in mobility of the MQ/PDMS blend, with the flow transition increasing to about 0 °C when 50 vol % MQ resin was added.59 Note that 32 vol % MQ in the total MQ/PS−PDMS mixture corresponds to having 57 vol % MQ in the PDMS phase. The Tg of the PS phase remains constant around 102 °C for all compositions. A third Tg close to but below the PS Tg is seen both for the PS−PDMS diblock copolymer and for the blends with MQ resin, albeit mostly in the first heating cycle. This transition could be due to the PS/ PDMS interphase region. A slight increase is seen in this Tg with MQ content, with Tg equal to 84 °C at 32 vol % MQ and 81 °C without MQ. The second transition is absent in the 48 vol % MQ loaded sample, which brings up an interesting possibility that at these high levels the interphase Tg might be E

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Macromolecules Table 1. Summary of the SAXS Data vol % silicone / (MQ + BCP)

vol % MQ / (MQ + BCP)

vol % MQ/ silicone

32

0

0

42 47

15 22

32 47

52 55 67

30 33 51

57 61 77

peak positions (q, nm−1)

d-spacing (nm)

structure

0.148, 0.256, 0.296, 0.392, 0.209

42

0.143, 0.225, 0.259, 0.274, 0.344, 0.417, 0.559 0.096, 0.124, 0.166, 0.192, 0.258, 0.286, 0.345, 0.373 0.123, 0.247, 0.373 0.120, 0.241, 0.364 0.118, 0.24 (w), 0.35 (w)

44 48

hexagonally ordered cylinders gyroid lamellae

51 52 53

lamella lamella distorted lamella

5c). SAXS results as a function of vol % silicone in the mixture are summarized in Table 1. At low MQ loading, the phase behavior of the mixture can therefore be predicted using the effective volume fraction of the entire silicone content in the system. As discussed previously, this is expected from theoretical predictions based on the modification of the effective siloxane (MQ + PDMS) volume fraction. Within the lamellar morphology composition range, namely from 47 to 55 vol % total silicone, parallels can be drawn with how homopolymer modification of block copolymers hints at additive dispersion. For example, Winey et al. determined that PS homopolymers mix nonuniformly and centralized into the core of the PS blocks of poly(styrene-bisoprene) PS-PI with sharper concentration profiles for higher molecular weight PS.69 This was attributed to the lower entropy of mixing and constrained PS chains at the PS−PI interface favoring axial swelling as high as 13% for 10 wt % of PS loading with high Mw. In our study changes in spacing are more moderate (e.g., 10 vol % MQ increase increases spacing by only 8%), likely due to the small size of MQ as compared to PDMS (Rp ≈ 0.2R0,PDMS). It is therefore expected that MQ resin is rather uniformly dispersed within the PDMS phase. At MQ loadings beyond 50 vol %, axial swelling is not sufficient to accommodate the added MQ. A decay in the degree of the order was observed by broadening peaks and lower relative peak intensities in SAXS with slight increment in the lamellar periodicity. Further distortion of the lamellar structure was seen beyond 55 vol %, as evidenced by the peak broadening and distinct decay of peak sharpness in the SAXS patterns. Figure 6 shows TEM images of block copolymers having 67 vol % of silicone (MQ + PDMS), showing distorted lamellae and some evidence of local morphological transitions from lamellae to cylinders after high-temperature annealing. The scattering peaks, however, did not show the formation of cylinders, likely due to the microstructure distortion in locally phase-separated domains. Highly MQ loaded PS−PDMS copolymers therefore exhibit lamellae distortion, but macrophase separation to form pure MQ domains was not found. As MQ content is increased, the deformation of the lamellar structure and the loss of long-range order revealed in SAXS results can be explained by the fact that MQ nanoparticles fill up the large fraction of free space in the PDMS domains and allow only local formation of cylindrical domains. Even after annealing at higher temperature, namely 3 days at 200 °C, there was still no significant change observed in the morphology (Figure 6c,d). One may argue that the limit of order−order transition beyond lamellar is due to the reduction in mobility of the siloxane phase (MQ + PDMS) by adding more MQ to the block copolymer. Indeed, the similarities in MQ/PDMS

Figure 6. TEM images of PDMS−PS block copolymer mixed with MQ to make 67 vol % of silicone after annealing: (a) and (b) at 130 °C overnight; (c) and (d) at 200 °C for 3 days.

solubility and surface energies are in stark contrast to their differences in mobility. For example, while PDMS is a low viscosity liquid at 15 000 g/mol with a glass transition at −125 °C, the MQ resin is a glassy solid at 9600 g/mol with a glass transition in excess of 300 °C. It was found that in this molar mass regime MQ served as a reinforcing nanoparticle, decreasing the self-diffusion coefficient of the polymer and increasing its viscosity.57 Similar trends were found from NMR spin−spin relaxation studies, with MQ reducing chain mobility at all particle loadings as long as the particle size was above 1 nm and PDMS was below its entanglement molecular weight.60 This is especially the case when high levels of MQ are used since eventually the flow onset of the MQ/PDMS mixture will exceed 25 °C. For example, 67 vol % siloxane in MQ/PS− PDMS corresponds to about 77 vol % MQ in the MQ/PDMS phase, with rheology measuring a flow onset around 140 °C and a viscosity of about 104 Pa·s at 200 °C. The best way to resolve the high MQ loading barrier would be to raise the M/Q ratio in the original resin. This will reduce its glass transition and with it the chain mobility of the MQ/PDMS mixture.

3. CONCLUSIONS The microdomain morphology of poly(styrene-blockpolydimethylsiloxane) (PS−PDMS) copolymers can be F

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Rigaku-Molecular Metrology SAXS equipment at Polymer Division in National Institute of Standards and Technology (NIST). Scattered Xrays were collected onto a 2-D wire array detector located at a distance corresponding to a measured q range of 0.006 Å−1 < q < 0.16 Å−1, where q = (4π/λ) sin(θ) and 2θ is the scattering angle. SAXS experiments for MQ added block copolymers were also performed at the 5-ID-D beamline of the DND-CAT at the Advanced Photon Source, part of Argonne National Laboratory. An X-ray energy of 17 keV was selected using a double bounce monochromator corresponding to a wavelength of 0.0729 ± 0.0001 nm. The beam size at the sample position is about 90 mm × 90 mm. Scattering intensities of the pattern recorded on a 2-dimensional charge coupled device (CCD) detector with 2048 × 2048 pixels. The pixel size was 78.75 ± 0.05 μm, and the sample−detector distance was 704.0 ± 0.1 cm. Scattering intensities were recorded as a function of the scattering vector q. 4.3. Transmission Electron Microscopy. For the bulk morphology and cross-sectional TEM observation of block copolymer and MQ/PS−PDMS mixtures, the samples were cryo-microtomed at −140 °C to make electron transparent thin sections and loaded on a C film coated Cu TEM grid for analysis. For the cross-sectional sample, annealed thick films were epoxy embedded and cured at room temperature for over 24 h after Pd/Pt coating to indicate the free surface region of the sample disk in TEM observation before cryomicrotomy. The samples were loaded in JEOL 2100F TEM, and the morphology was investigated under bright field TEM mode at 200 keV. The digital images were taken using Gatan CCD camera attached under the TEM column and Digital Micrograph software. 4.4. DSC Experiments. A TA Instruments Q2000 differential scanning calorimeter (DSC) with a liquid nitrogen cooling system (LNCS) was used to measure the glass transition (Tg). A sample of about 10 mg was introduced in a TA Instruments hermetic pan. Indium was used as a calibration standard for heat flow and temperature. Samples were heated at 10 °C/min using helium as a purge gas (25 mL/min). Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.

modified with highly PDMS selective trimethylsilylated silicate nanoparticles (MQ). Solubility parameters and resulting Flory interaction parameters support the MQ/PDMS affinity. Lowtemperature crystallization of PDMS was also suppressed with the addition of MQ, which is another confirmation for the PDMS locality of the MQ nanoparticles. Interestingly, a third glass transition was seen right below the PS Tg, possibly attributed to an interphase region. The small size of MQ particles (1−2 nm) also assists in the efficient dispersion in the PDMS-rich block copolymer domains (>10 nm). Specifically, TEM and SAXS show that a PS−PDMS (31 kg/ mol PS and 15 kg/mol PDMS blocks) with hexagonally ordered PDMS cylinders in a PS matrix can be tuned by simple MQ blending to gyroid and lamellar morphologies, following the predicted impact of the total siloxane (MQ + PDMS) volume fraction. Axial expansion of the lamellar phases is moderate indicative for a filler type with size smaller than that of the host PDMS phase and its uniform dispersion in this phase. Macrophase separation was not found in contrast to the behavior in PDMS/PS−PDMS systems where PDMS surface migration interferes. This can be related to the reduction in mobility of the MQ added PDMS microdomain, preventing MQ from being expelled from the siloxane-rich domain. Although some evidence for the transition of lamellar to cylindrical (PS cylinders in a MQ/PDMS matrix) was found, mixed morphologies were present. This indicates the limit to morphology control in this system. Mobility limitations at the very high MQ loadings (over 70 vol % MQ in the MQ/PDMS phase) are probably responsible for this limit, requiring annealing temperatures (∼200 °C) beyond the thermal stability of the PS block. In case of the former, surface segregation and fast diffusion seem to go hand in hand to preferentially expel the PDMS to the surface. In case of the latter, diffusion limitations result in failure to anneal to form the thermodynamically stable microstructure due to the high Tg of the MQ/ PDMS blends present in the PDMS rich PS−PDMS phases. If higher MQ modification is desired to further control block copolymer microstructure, future work could include increasing the M/Q ratio to reduce the glass transition and with it increase the segmental diffusion of the MQ/PDMS mixed phase. As long as one stays away from this limit, MQ nanoparticles could be used to simplify morphology control, without the need for accurate synthetic control over block length and block volume fraction, potentially overcoming one of the barriers to adoption of BCP lithography.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.S.). Present Addresses

G.K.: Material Research Center, Samsung Advanced Institute of Technology, Youngtong-Gu, Suwon, Kyounggi-Do, South Korea. C.W.: KLA-Tencor, One Technology Drive, Milpitas, CA 95035.

4. EXPERIMENTAL SECTION 4.1. MQ-Modified Poly(styrene-block-dimethylsiloxane). Blends are based on a 45 500 g/mol (Mw) poly(styrene-blockdimethylsiloxane) copolymer (polydispersity of 1.15) containing a 31 000 g/mol styrene block and a 15 000 g/mol dimethylsiloxane block (Polymer Source) blended with an MQ resin in toluene solution (5 wt % solids). The MQ resin fraction used in this work was provided by Phasex Corporation as described previously.56 The resin has the following characteristics: M0.92Q; Mn = 8460 g/mol; Mw = 9600 g/mol. The PS−PDMS/MQ solutions were slowly evaporated to make a 0.5 mm thick dried sample disk. After slow air drying, the blends were annealed either at 130 °C for 15 h or at 200 °C for 3 days in a vacuum oven to reach the equilibrium morphology and to remove solvent effect on the ultimate morphology and then were air-cooled down to room temperature. 4.2. Small-Angle X-ray Scattering. Small-angle X-ray scattering (SAXS) observations of PS−PDMS block copolymer having 32 vol % PDMS purchased for Polymer Source was performed using the

Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01350 Macromolecules XXXX, XXX, XXX−XXX