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Self-Assembly of an Ultrahigh‑χ Block Copolymer with Versatile Etch Selectivity Koei Azuma,∥ Jian Sun,† Youngwoo Choo,§ Yekaterina Rokhlenko,§ Jonathan H. Dwyer,‡ Beau Schweitzer,† Teruaki Hayakawa,∥ Chinedum O. Osuji,§ and Padma Gopalan*,†
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Department of Materials Science and Engineering and ‡Department of Chemical and Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States § Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States ∥ Department of Materials Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-S8-36 Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *
ABSTRACT: We report the successful synthesis of previously inaccessible poly(3-hydroxystyrene)-block-poly(dimethylsiloxane) (P3HS-b-PDMS) block copolymers (BCPs) with varying volume fractions, molecular weights, and narrow dispersities by sequential living anionic polymerization. The chemical structure and molecular weight were fully characterized by 1 H NMR and gel permeation chromatography. The BCP phase behavior was investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy. Temperature-resolved SAXS measurements from symmetric disordered sample were used to determine the interaction parameter (χ) using mean-ﬁeld theory. The results provide an estimate for interaction parameter, χHS/DMS(T) = 33.491/T + 0.3126, with an upper bound value of 0.39 at 150 °C. The calculated χ for P3HS-b-PDMS is approximately 4 times higher than that observed in a commonly studied high-χ system, PS-b-PDMS. The ultrahigh interaction parameter observed here aﬀords the formation of well-ordered materials at remarkably low molecular weight. The presence of both PDMS and P3HS provides signiﬁcant versatility in terms of etch selectivity, while the hydroxystyrene domain oﬀers additional functionality as it can be exploited for immobilizing functional organic moieties.
self-assembly into periodic nanostructures is predicted by the mean-ﬁeld theory for a χN above 10.5.11,12 In practice, the relevant product is that of N and an eﬀective or representative interaction parameter, χeff. For brevity, here we refer simply to the interaction parameter, χ. The resulting domain size of lamellar feature is predicted to scale as d ∼ χ1/6Nα where α = 2/3 and α = 1/2 for strong and weakly segregated systems, respectively.13,14 These basic principles have driven the BCP community to control domain size and to speciﬁcally to target the smallest sizes possible by reducing N while maintaining the value of χN > 10.5 by designing BCPs with high χ parameters.15,16
Macromolecules A recent review article by Sinturel et al. summarizes the advances in the design of the high-χ polymers and the outlook in the ﬁeld of lithography.10 The χ parameter depends in part simply on diﬀerences in the polarity of each polymer segment. Hence, a straightforward means of increasing χ is to increase the contrast in polarity between the two blocks. Tabulated values of the Hildebrand solubility parameters for various polymers enable a rough estimate of their relative polarities and hence their chemical incompatibilities even though they neglect the entropic contributions to χ. For example, the solubility parameters listed for common polymerspoly(dimethylsiloxane) (PDMS), polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(2-vinylpyridine) (P2VP)are 15.5, 18.5, 19.0, and 20.6 (J/cm3)1/2, respectively.17,18 The χ parameters for BCPs with polystyrene as the ﬁrst block and with increasing polarity contrast such as polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA), polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP), and polystyrene-b-poly(dimethylsiloxane) (PS-b-PDMS) calculated by using 118 Å3 as the reference volume are reported to be ∼0.03, 0.08, and 0.09, respectively, at 150 °C.19−21 Hence, the χ parameter in general does track well with the solubility parameter. Our interest is in exploring poly(4-hydroxystyrene) (PHS) as a highly functional, polar (solubility parameter 24.55 (J/cm3)1/2), and hydrophilic block.22 The functionality comes from the phenolic OH which is amenable to a wide variety of transformations and postfunctionalization reactions.23 These chemical attributes have led to inclusion of PHS as a critical component in copolymers for chemically ampliﬁed resists employed in optical lithography.24,25 This functionality also poses a problem in terms of polymerizing the HS monomer by living methods, as the acidic −OH group needs to be protected to prevent cross-reactions and to improve solubility in nonpolar solvents. Thus, a variety of protected derivatives such as 4-acetoxystyrene,26 4-tert-butyldimethylsiloxystyrene,27 and 4-tert-butoxystyrene28 have been developed for either living anionic or living free radical polymerization. However, post polymerization their deprotection requires harsh conditions, which prevents the incorporation of an acid-sensitive second block via sequential polymerization.29 Hence, most BCPs with a PHS block reported so far have been limited to relatively inert second blocks such as polystyrene and poly(α-methylstyrene). Recently acetal protected monomers such 4-(1-ethoxyethoxy)styrene30 as well as 4-(2-tetrahydroxydropyranyloxy)styrene (4OTHPSt) and 3-(2-tetrahydroxydropyranyloxy)styrene (3OTHPSt) have been reported that overcome these limitations.31,32 The THP-based acetal protection reported by us oﬀers many advantages as it can be deprotected using catalytic amount of acid, which is tolerant to many functional groups, is easy to purify, and also shows a well-controlled living anionic polymerization. The resulting polymer has a Tg above room temperature, which makes the polymer puriﬁcation eﬃcient. This approach has been instrumental in expanding the family of PHS BCPs to previously inaccessible combination of monomers by sequential living anionic polymerizations. Silicone-based polymers such as PDMS are widely studied as the inorganic segment in high-χ BCPs as they have a quite low solubility parameter compared to other hydrophobic organic polymers.33−35 Among these, the most widely studied candidate is PS-b-PDMS due to its facile synthesis via sequential anionic polymerization, relatively large χ parameter (χHS/DMS= 0.09 at 150 °C), and high etch contrast under oxygen reactive ion etching conditions used for semiconductor fabrication.19
Other high-χ candidates with a more polar organic block such as poly(dimethylsiloxane)-block-polylactide (PDMS-b-PLA)36 and poly(dimethylsiloxane)-b-poly(2-vinylpyridine) (PDMS-bP2VP)37 have also been investigated. In our earlier work we had reported the feasibility of synthesizing P4HS-b-PDMS using 4OTHPSt monomer.31 This THP deprotection chemistry preserves the PDMS block, as these blocks are typically hydrolyzed or degraded in concentrated strong acid or base.38 Based on the solubility parameter diﬀerences, the diblock copolymer composed of hydrophilic PHS segment and hydrophobic PDMS segment is expected to exhibit higher χ than other PDMS based BCPs reported so far. In this article, we report the synthesis of a series of poly(3hydroxystyrene)-b-poly(dimethylsiloxane) (P3HS-b-PDMS) BCPs with diﬀerent molecular weights and volume fractions by sequential anionic polymerization. The ﬁrst block is composed of the 3-isomer of HS rather than the 4-isomer as it has ∼20 °C lower glass transition temperature [Tg(P3HS) = 118−163 °C (1.6−68 kg/mol), Tg(P4HS) = 166−183 °C (1.3−7.8 kg/mol)], which allows thermal annealing to develop the morphology while minimizing any possible side reactions including self-cross-linking at high temperatures.32 Bulk morphological analysis by small-angle X-ray scattering (SAXS) conﬁrms their microphase separation into lamellar and cylindrical morphologies. By analyzing the temperature-dependent correlation-hole scattering data associated with a disordered P3HS-b-PDMS using mean-ﬁeld theory,11 we estimate the value of χHS/DMS to be approximately 4 times larger than the previously reported χS/DMS of PS-b-PDMS at a reference temperature of 150 °C.
Reagents. All solvents and reagents were purchased from SigmaAldrich Chemical Co. (Milwaukee, WI) and used without further puriﬁcation unless speciﬁed. Tetrahydrofuran (THF) and toluene were freshly puriﬁed by using solvent towers purchased from VAC. Additionally THF was puriﬁed by sec-butyllithium (sec-BuLi) immediately prior to anionic polymerization. 3OTHPSt was synthesized and puriﬁed according to the literature procedure.32 Hexamethylcyclotrisiloxane (D3) and calcium hydride were added in dry toluene, stirred at 50 °C overnight, and then distilled under reduced pressure. D3 monomer was then added to a solution of poly(styryl)lithium in toluene and distilled into the Schlenk ﬂask and stored under an argon atmosphere (the weight ratio of D3/toluene is approximately 50/50 wt %). General Procedure for Anionic Polymerization of P3OTHPSt-b-PDMS. An oven-dried ﬂask was cooled under argon gas before the addition of 60 mL of dry THF, followed by cooling at −78 °C. sec-BuLi (1.4 M in cyclohexane) was added dropwise until a yellow color persisted in solution. The ﬂask was then removed from the chiller and kept at room temperature until the yellow color disappeared. The ﬂask was cooled again to −78 °C before adding a prescribed amount of sec-BuLi while vigorously stirring. 3OTHPSt (typically 2.5 g of monomers solution in dry THF, 50/50 wt %) was rapidly added via syringe into the center of the ﬂask, resulting immediately in a dark yellow color, followed by stirring for 30 min. After the addition of D3 monomer, the dark yellow color gradually disappeared while raising temperature from −78 °C to room temperature and then stirred for 2 h. The polymerization was quenched with the mixture of anhydrous chlorotrimethysilane (TMSCl) and anhydrous pyridine. The solution was then precipitated from 500 mL of MeOH followed by ﬁltration. The solid was collected and dried under reduced pressure overnight (typical yield >70%). P3OTHPStb-PDMS: 1H NMR (400 MHz, CDCl3) δ 7.16−5.78 (Ar−H), 5.37− 5.12 (C−H), 3.91−3.65 (C−H), 3.59−3.33 (C−H), 2.22−0.93 (C−H2 and backbone), 0.08 (C−H3). B
Macromolecules Scheme 1. Synthetic Scheme of P3HS-b-PDMS BCPs by Sequential Anionic Polymerization
Deprotection of P3OTHPSt-b-PDMS. 0.5 g of P3OTHPStb-PDMS was dissolved in the mixture of THF/EtOH (25 mL/25 mL). 1 mL of concentrated aqueous HCl was diluted to 10 mL with deionized water. Approximately 0.3 mL of HCl solution was added to the polymer solution and then stirred for 3 h. The completion of the reaction was checked by 1H NMR spectroscopy. Note that extended reaction time or higher concentration of acid leads to Si−O−Si degradation of siloxane. After deprotection was complete, the solution was poured into water for precipitation followed by collecting the powder by vacuum ﬁltration and drying at room temperature under reduced pressure (quantitative yield). 1H NMR (400 MHz, acetone-d6): δ 8.28−7.65 (O−H), 7.2−5.8 (Ar−H), 2.4−1.1(backbone), 0.08 (C−H3). Characterization. NMR spectra were recorded in CDCl3 for P3OTHPSt-b-PDMS and acetone-d6 for P3HS-b-PDMS using a Bruker Avance-400 spectrometer without TMS as internal reference. Quantitative 1H NMR was performed with a 10 s relaxation delay. Gel permeation chromatography (GPC) was performed using a Viscotek 2210 system equipped with three Waters columns (HR4, HR 4E, and HR 3) and a ﬂow rate of 1 mL/min with THF as an eluent. The calibration curve for analysis consisted of nine narrow dispersity PS standards with molecular weight of from 1 to 400 kg/mol. Thermal gravimetric analysis (TGA) was performed with TA Instruments Q500 using a heating rate of 10 °C/min for three cycles. SAXS samples were prepared by the slow evaporation of THF solutions (5 wt % polymer in THF) and followed by annealing at 160 °C for 6 h under reduced pressure.31 SAXS analysis employed either a Rigaku S-MAX 3000 instrument with Cu Kα X-ray radiation source (λ = 1.54 Å) or a Complex Materials Scattering (CMS, 11-BM) beamline at the National Synchrotron Light Source II at Brookhaven National Laboratory, using a beam energy of 13.5 keV (λ = 0.918 Å). The scattering vector was rigorously calibrated using a silver behenate standard (d001 = 5.838 nm) in both cases. Temperature-dependent SAXS studies were performed with 5 min thermal pre-equilibration delay prior to data collection at a given temperature. Absolute values of scattered intensity were determined using a glassy carbon standard as well as by direct measurement for NSLSII-collected data. Transmission electron microscopy (TEM) images were taken with a FEI Tecnai Osiris TEM. The specimens for TEM were prepared by cryoultramicrotomy at −120 °C due to the rubbery nature of PDMS chain at room temperature and observed without staining.
analysis by GPC (Figure S1). The GPC curve of P3OTHPSt shows a symmetric peak with narrow dispersity conﬁrming the controlled polymerization. The small shoulder at the high molecular weight end in the P3OTHPSt trace is an artifact due to the dimerization of the highly reactive living anion by oxygen during the removal of the aliquot and termination by MeOH.32,39 The dark yellow color of the anion slowly faded as the reactor was warmed to room temperature after D3 monomer addition. After 2 h, the reaction was quenched with excess of TMSCl and pyridine and stirred for 10 min. Pyridine was added to prevent the deprotection of THP group in P3OTHPSt by the HCl generated from the excess TMSCl added during termination. This solution was poured into MeOH and the precipitated BCP was collected and dried overnight. The GPC curve of the resulting BCP shows a clean shift to higher molecular weight while maintaining the narrow dispersity, conﬁrming the well-controlled sequential polymerization of D3 (Figure S1). As anionic polymerization of D3 is hard to control at high conversions, the reaction was terminated at 2 h after addition of the monomer. The number-averaged molecular weight (Mn) and the volume fraction of each block (f) were determined by 1H NMR (Figure 1), and the density of P3HS was determined using density gradient column method. Each peak is distinguishable in 1H NMR [7.1−5.7 ppm (phenyl group), 5.3, 3.8, 3.5 ppm (THP group), 2.5−1.1 ppm (main chain of P3OTHPSt), 0.8−0.6 (initiator) and 0 ppm (PDMS)] and hence provides a straightforward way to determine Mn and f vol P3HS. 29Si NMR shows a sharp peak at −22 ppm attributed to the O−Si−O of PDMS (Figure S2). To synthesize P3HS-b-PDMS, the THP group in P3OTHPSt was deprotected using catalytic amount of aqueous HCl solution. The main issue in this step is the potential hydrolysis of the PDMS block;40 therefore, both the amount of HCl and the duration of reaction were optimized. The optimized catalytic amount of HCl ∼ 30 μL resulted in quantitative deprotection at room temperature in just 3 h. 1H NMR revealed the peak from the hydroxyl group around 8.0 ppm and the disappearance of peaks at 5.3, 3.8, and 3.5 ppm attributed to the THP group, indicating the completion of deprotection. The integration ratio of aromatic peaks from the P3HS and the dimethylsiloxane peaks from PDMS was compared before and after the deprotection (Figure 1). This ratio remained unchanged conﬁrming that the PDMS block remains intact during the deprotection. The changes in thermal properties due to deprotection were characterized by TGA. TGA shows a two-step degradation for the protected BCPs with the ﬁrst step at 180 °C corresponding to the loss of THP group, followed by complete degradation of polymer around 335 °C (Figure S3). In contrast, a single step
RESULTS AND DISCUSSION Syntheses of P3HS-b-PDMS. A series of P3HS-b-PDMS BCPs were synthesized by sequential anionic polymerization. The THP group in the resulting polymer was deprotected using a catalytic amount of HCl (Scheme 1), and the chemical structure of the BCP was characterized by 1H NMR, 29Si NMR, and GPC. For the ﬁrst block, the anionic polymerization of 3OTHPSt was performed using sec-BuLi as initiator in THF at −78 °C. The solution turned dark yellow immediately. Before adding D3 monomer, an aliquot of solution was taken out of the ﬂask and quenched in MeOH for molecular weight C
slowly evaporated at room temperature followed by thermal annealing at 160 °C. The scattering patterns were azimuthally integrated to give one-dimensional scattering proﬁles of intensity, I, as a function of the magnitude of the scattering vector q = 4π sin(θ/2)/λ, where 2θ is the scattering angle and λ is the wavelength of X-rays used. The microdomain periodicity was calculated with d0 = 2π/q* for lamella forming BCPs, where q* is the position of the principal (ﬁrst-order) scattering peak (Table 1). Symmetric Compositions. The lowest molecular weight P3HS-b-PDMS (0.9K−0.7K) showed a weak and broad principal scattering peak from a disordered morphology (Figure 2).
Figure 1. 1H NMR spectra of protected 2.5K−2.4K (top, CDCl3) and after deprotection (bottom, acetone-d6) with catalytic amount of HCl in THF/ethanol (50/50 vol %).
degradation for the deprotected BCPs was observed as reported earlier for other PHS-based BCPs. P3HS-b-PDMS BCPs with molecular weight ∼1.6K−8.1K and P3HS volume fraction ( f P3HS) ∼ 0.32−0.61 were synthesized (Table 1) using Table 1. Characterization of P3HS-b-PDMS BCPs sample 5.3K− 2.8K 4.8K− 2.6K 2.4K− 4.2K 3.8K− 2.3K 3.3K− 2.0K 2.5K− 2.4K 1.5K− 1.4K 0.9K− 0.7K
Figure 2. SAXS patterns of deprotected BCPs listed in Table 1. 5.3K−2.8K, 4.8K−2.6K, 2.4K−4.2K, 2.5K−2.4K, 1.5K−1.4K, and 0.9K−0.7K were annealed at 160 °C. 3.8K−2.3K and 3.3K−2.0K were annealed at 230 °C. Data shifted vertically for clarity. In the SAXS proﬁles, triangles indicate calculated lamellar peak positions based upon q* and ﬁlled triangles indicate hexagonally packed cylinders. Filled circle indicates the primary peak of a disordered phase.
As the molecular weight is increased to 1.5K−1.4K, a sharp principal peak at q* = 0.085 Å−1 and a secondary peak at 2q* = 0.170 Å−1 were observed, suggesting a lamellar morphology with a domain periodicity of 7.4 nm. The TEM image (Figure 3a) conﬁrms that BCP 1.5K−1.4K indeed forms a well-developed lamellar structure with a domain periodicity of 5.5 nm, which is in reasonable agreement with the SAXS data. Hence, the order−disorder transition is likely occurring between a Mn = 1.6K and 2.9K for the symmetric BCPs at room temperature. SAXS proﬁles of 2.5K−2.4K and 3.8K−2.3K also revealed higher order peaks with q:q* ratios at integer multiples. For the 3.8K−2.3K BCP, 160 °C annealing resulted in coexisting morphology, which transitions to a stable lamellar morphology upon annealed above 230 °C. This transition was conﬁrmed by SAXS and TEM as shown in Figures S4 and S5. Hence, the lamellar domain periodicity increases from ∼7.4 to 13.3 nm as the Mn of the BCP increases. The access to the
Determined by quantitative 1H NMR end-group analysis comparing the sec-butyl methyl peaks from 0.5 to 0.8 ppm with an acetal peak at 5.3 ppm. bDetermined by GPC using polystyrene standard. cDetermined using the homopolymer density: ρ(P3HS) = 1.15 g/cm3 and ρ(PDMS) = 0.96 g/cm3. dDetermined by SAXS measurement at room temperature. eTotal segment density normalized degree of polymerization based on 118 Å3 reference volume.
this standard process by changing the feed ratio of sec-BuLi, 3OTHPSt, and D3 monomer. Bulk Self-Assembly of BCPs. SAXS and TEM were used to determine the type and periodicity of nanostructures induced by self-assembly of P3HS-b-PDMS BCPs. We targeted various volume fractions going from PDMS cylinder to P3HS cylinder forming BCPs to map out the phase diagram. A typical sample was prepared from a THF solution of BCP that was D
Order−Order Transition (OOT). Out of the compositions synthesized, 3.3K−2.0K (f vol P3HS = 0.59) showed an interesting temperature-dependent behavior. Room temperature SAXS after thermal annealing at 160 °C showed multiple scattering peaks that appear to be a mixture of peaks expected for lamellar (indicated by ▽) and hexagonally packed cylinders (indicated by ▼) (Figure 2). The stability of this morphology was investigated as a function of temperature. At temperatures above 200 °C, only lamellar scattering peaks were observed (Figure S6). However, upon cooling below 150 °C mixed lamellar and hexagonal scattering peaks re-emerge (Figure 5).
Figure 3. Representative TEM images for P3HS-b-PDMS BCPs: (a) 1.5K−1.4K, (b) 4.8K−2.6K, (c) 2.4K−4.2K, and (d) 3.3K−2.0K. Insets in TEM images show the side view of hexagonally packed cylinders. Figure 5. Reversible order−order transition in 3.3K−2.0K monitored by temperature-dependent SAXS studies. Cooling from 270 to 35 °C leads to reversible transition from lamellar to the coexistence of lamellar and perforated lamellar phase.
smallest sub-5 nm feature size with this BCP indicates the high χ nature. If this is indeed the case, then we expect a scaling law for a symmetric BCP in strongly segregated system as L0 ≈ χ1/6N2/3.13 Regression analysis of a log−log plot of L0 versus degree of polymerization for lamellae-forming BCPs listed in Table S1 reveals a scaling exponent of 0.69 (Figure 4). The
The observed transitions were reversible over multiple heating and cooling cycles, and the observed scattering remained unchanged even after prolonged annealing (>100 h) at the relevant temperatures. One possibility is that the scattering at lower temperatures occurs due to a coexistence of lamellar and hexagonally packed cylinder morphologies. Such phase coexistence has been attributed to large chain length and/or compositional dispersity.41 However, our system does not feature such dispersity. GPC analysis of the protected BCP shows narrow dispersity in molecular weights. An alternate explanation is that the scattering is due to the formation of a hexagonally perforated lamellar (HPL) morphology. Indeed, TEM data suggest that this may be the case, given the appearance of hexagonally packed rectangular “brick-like” objects (Figure 3d). HPL is not a stable morphology in diblock melts; hence, it is likely that the observed morphology may correspond to a metastable state. It is notable that this speciﬁc composition lies within the cylinder-to-lamellar boundary in which the gyroid morphology is often observed in other BCPs. The observation of what appears to be a metastable HPL phase and its OOT to a lamellar phase may be related to the fact that the gyroid morphology is destabilized in strongly segregated systems.42−44 Determination of χ. To quantitatively determine the Flory− Huggins parameter χHS/DMS of this new BCP, we used the Leibler’s mean-ﬁeld theory, modiﬁed for dispersity of molecular weight and segmental volume asymmetry.45 The degree of polymerization N was determined from quantitative 1H NMR end-group analysis (Figure S7) and was normalized by reference volume 118 Å3 to give a volume-based degree of polymerization. The Mn obtained from end-group analysis is more accurate compared with the value obtained from GPC
Figure 4. Log−log scale plot of the scaling of the periodicity with the degree of polymerization for symmetric P3HS-b-PDMS BCPs listed in Table S1.
value of the scaling exponent is consistent with the formation of a strongly segregated system, for which a scaling exponent of 2/3 is expected. Asymmetrical Compositions. BCP compositions 5.3K− 2.8K, 4.8K−2.6K, and 2.4K−4.2K showed scattering peaks with a ratio of 1:√ 3:√ 4:√7, conﬁrming a hexagonally packed cylinder structure (Figure 2). With increase in the molecular weight as expected the primary peak shifts to lower q (Å) and the domain periodicity of hexagonally packed cylinders increases. The values of L0 were calculated using a L0 = 2/√3d0 for the cylinder forming BCPs. TEM images (Figure 3b) for 4.8K−2.6K (f vol P3HS = 0.61) conﬁrm the PDMS cylinders (dark regions) with a L0 of 16.5 nm. As the f vol P3HS changes to 0.32 in 2.4K−4.2K (Figure 3c), P3HS cylinders (lighter regions) were observed. E
A catalytic amount of HCl was suﬃcient to remove the THP protecting group selectively without decomposing PDMS chains at room temperature to yield P3HS-b-PDMS. All the compositions synthesized except the lowest molecular weight showed well-ordered lamellae or hexagonally packed cylinders with characteristic dimensions ranging from 7.4 to 17.7 nm. At the boundary of these two morphologies, one of the intermediate compositions showed an interesting reversible order−order transition. This order−order transition is between an apparent HPL phase observed below 150 °C and a lamellar phase observed above 200 °C. Mean-ﬁeld theory analysis of the temperature-dependent correlation-hole scattering derived from a disordered symmetric P3HS-b-PDMS composition gave χHS/DMS(T) = 33.491/T + 0.3126, which is much larger than χS/DMS of PS-b-PDMS and exhibits relatively large entropic and enthalpic contributions to the free energy of mixing. This new BCP not only has high etch contrast between the P3HS and the PDMS blocks but also oﬀers versatility in modulating the etch contrast further as P3HS block can be potentially functionalized with inorganic and organic precursors.
measurements, as the initiator peaks (0.5−0.8 ppm) for the low molecular weight samples do not have any overlap with other peaks. Using the near-symmetric disordered composition (0.9K−0.7K), χ(T) was estimated directly by using the meanﬁeld theory analysis of temperature-dependent correlation-hole SAXS data. The χ parameter is temperature-dependent and consists of an enthalpic component (χH) and an excess entropic component (χS) expressed by the following equation:46 χ (T ) = χH /T + χS
Scattering data for 0.9K−0.7K were acquired above the Tg of both blocks and ﬁt using the expression for correlation hole scattering from disordered BCP melts (Figure S8). Fitting parameters used for determination of χ and ﬁtting values for χHS/DMS at corresponding temperatures were listed in Tables S2 and S3. A plot of the calculated χ values as a function of 1/T (in K) yields a χHS/DMS(T) = 33.491/T + 0.3126 (Figure 6).
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01409. GPC curves, NMR spectra, TGA, temperature-dependent SAXS curves, TEM images, mean-ﬁeld theory calculation of χHS/DMS and etch selectivity (PDF)
Figure 6. χHS/DMS vs T−1 plotted using the values obtained from ﬁtting temperature-dependent correlation-hole SAXS data.
Thus, the χHS/DMS shows not only a large temperature dependence but also a large entropic contribution to the free energy of mixing. Hence, the χ parameter of P3HS-b-PDMS is quite high over a wide range of temperatures compared to many other PDMS-based BCPs.10,47,48 At the reference temperature of 150 °C, P3HS-b-PDMS has a χ ∼ 0.39. In comparison, the more commonly used PS-b-PMMA has a χ ∼ 0.03. The interaction parameter increases to ∼0.09 where the PMMA block is replaced by a more nonpolar block, i.e., for PS-b-PDMS. The χS/DMS value of 0.09 is comparable to the values of the interaction parameter of other strongly segregated systems such as P4HS-b-PS20 and PtBS-b-P2VP.39 Still larger interaction parameters can be obtained by substituting the more polar block, PMMA, for PS, providing χMMA/DMS ∼ 0.2. In this study, we were able to achieve a considerably larger segregation strength by replacing the PMMA block by P3HS, resulting in a roughly 2-fold increase over the previously reported χMMA/DMS ∼ 0.2. Preliminary studies indicate that the etch contrast between P3HS and PDMS is 15:1 when compared to PS: PDMS which has an etch contrast of 12:1 (Figure S9). The combination of high χ, improved etch contrast, and the functionality of the phenolic group makes a compelling case for facilitating eﬀective pattern transfer at sub-10 nm features, which will be the subject of future studies.
The authors declare no competing ﬁnancial interest.
ACKNOWLEDGMENTS P.G. and J.S. acknowledge partial NSF support from DMR1507409 and ECCS-1727523. Y.C., Y.R., and C.O. acknowledge NSF support (DMR-1410568). Y.R. acknowledges fellowship support from the National Physical Science Consortium (NPSC). This research used the CMS beamline (11-BM) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Oﬃce of Science User Facility operated for the DOE Oﬃce of Science by Brookhaven National Laboratory under Contract DE-SC0012704.
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CONCLUSION In summary, we have expanded the family of PHS based BCPs to incorporate acid-sensitive PDMS blocks via sequential living anionic polymerization of 3OTHPSt and D3 monomer. A series of P3OTHPSt-b-PDMS BCPs were synthesized with excellent molecular weight and dispersity control over a broad Mn range. F
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