Evaluation of the Interaction Parameter for Poly(solketal methacrylate

Jan 23, 2018 - Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States. § Mater...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Evaluation of the Interaction Parameter for Poly(solketal methacrylate)-block-polystyrene Copolymers Duk Man Yu,† Jose Kenneth D. Mapas,‡ Hyeyoung Kim,† Jaewon Choi,† Alexander E. Ribbe,† Javid Rzayev,*,‡ and Thomas P. Russell*,†,§,∥ †

Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States ‡ Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States § Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ∥ Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: A series of symmetric poly(solketal methacrylate-bstyrene) (PSM-b-PS) copolymers with varying molecular weights that can transform a hydrophobic PSM block to a hydrophilic poly(glycerol monomethacrylate) (PGM) block through an acid hydrolysis were investigated. This simple chemical transformation significantly enhances the segmental interaction parameter (χ), enabling a phase-mixed block copolymer (BCP) to microphase separate without any additives. Temperature-dependent small-angle X-ray scattering (SAXS) measurements as a function of the degree of polymerization (16 ≤ N ≤ 316) and PSM hydrolysis conversion were conducted to characterize the order-to-disorder transition (ODT) behavior as well as the lamellar microdomain features. Using a mean-field correlation-hole analysis of the scattering, the χ value for PSM and PS was determined as a function of the conversion of PSM to PGM. For 100% conversion of PSM to PGM, the χ with PS was found to be given by χ = 0.3144 + 36.91/T, with χ = 0.438 at 25 °C, which is ∼13 times larger in magnitude than χ parameter for PSM-b-PS copolymer (∼0.035 at 25 °C) calculated using a 118 Å3 reference volume. With this large increase in χ, even the smallest synthesized PGM-b-PS copolymers underwent microphase separation, allowing us to achieve a center-to-center lamellar microdomain spacing (commonly referred to as the full pitch) of 5.4 nm, obtained for the lowest molecular weight sample (Mn = 2200 g/mol, N = 16).

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lamellar microdomain morphology occurs when χN > 10.5.13−16 Therefore, a phase-mixed or disordered morphology is observed at χN < 10.5. Since χ is typically inversely proportional to temperature,17,18 BCPs generally undergo an order-to-disorder transition (ODT) upon heating. In the strong segregation limit, the periodicity of the microphase-separated domains (L0) varies as L0 ∼ χ1/6N2/3. Since L0 is a stronger function of N than χ, decreasing N is the easier route to minimize L0. However, since the χN must be larger than a critical value for the microphase separation to occur, reducing N requires χ to be increased.16,19−22 Various strategies have been used for the preparation of high χ−low N BCP by exploiting the inherent chemical differences between the blocks or by using external additives.23 For example, salt doping has been demonstrated by Epps and coworkers24 as a route to increase χ, where the phase behavior of poly(styrene-b-ethylene oxide) (PS-b-PEO) was investigated as

INTRODUCTION The self-assembly of block copolymers (BCPs) has received substantial attention as a bottom-up platform for the semiconductor industry because BCP lithography has the potential to surpass the spatial resolution achievable by top-down photolithographic processes.1−5 Top-down approaches are reaching intrinsic limitations, including limits of optical diffraction and light wavelength, to achieve sub-30 nm features.6−8 The microphase separation and self-assembly of BCPs, using BCPs of different molecular weights, can lead to arrays of highly ordered nanostructures having feature sizes from several to hundreds of nanometers due to chemical immiscibility between two blocks. Consequently, the use of BCPs as nanopatterning templates with smaller pitch and microdomain sizes is a promising method for functional nanostructures and microelectronics.9−12 The microphase separation of BCPs is dictated by the segregation strength parameter χN as a function of the volume fraction, where χ is the Flory−Huggins segmental interaction parameter and N is the total number of BCP segments. For a symmetric diblock copolymer, microphase separation to form a © XXXX American Chemical Society

Received: October 18, 2017 Revised: January 3, 2018

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Macromolecules Table 1. Sample Codes and Characteristics of PSM-b-PS Copolymers sample code P(SM21-S22) P(SM13-S14) P(SM5-S5) P(SM2-S2) P(SM1-S1)

total Mn (g/mol) 43000 26800 10500 4180 2200

ĐPSM−PSa 1.13 1.22 1.08 1.08 1.02

PSM Mn (g/mol) b

21000 13200b 5400b 2180b 1200c

ĐPSMa

NPSM

PS Mn (g/mol)

NPS

f PSMd

1.15 1.09 1.05 1.02 1.01

105 66 27 11 6

22000 13600 5100 2000 1000

211 131 49 19 10

0.46 0.47 0.49 0.50 0.52

Determined by SEC in THF using PS calibration. bDetermined by SEC with a light scattering detector (T = 30 °C; λ = 630 nm) using a refractive index increment (dn/dc) of 0.067 for PSM. cCalculated from 1H NMR end group analysis. dVolume fraction of PSM a

attain a 5.9 nm lamellar domain pitch. Pattern transfer and etching processes involving BCPs that form sub-10 nm pitch dimensions have great potential for nanoelectronic applications; however, the limited number of BCPs with large enough χ values to enable access to such small domain spacings remains a challenge. Recently, we reported the formation of 5.4 nm full pitch lamellar microdomains from an initially phase-mixed poly(solketal methacrylate-b-styrene) (PSM-b-PS) copolymer.34 Acid hydrolysis of the hydrophobic PSM block into a hydrophilic poly(glycerol monomethacrylate) (PGM) segment resulted to a significant increase in χ, causing the BCP with N = 16 to microphase separate into sub-3 nm microdomains. The remarkable success in reducing the pitch to the single nanometer range using PGM-b-PS warrants a quantitative investigation of its χ value, since it points to a strategy through which the pitch of the BCP can be further reduced. In addition, the chemical transformation occurs in the solid state, which offers interesting processing opportunities for generating uniform films. In this study, exact χ values between the two blocks are determined as a function of the degree of conversion of PSM into PGM using the correlation-hole scattering analysis of the copolymer in the phase-mixed (disordered) state. This is perhaps the most quantitative route by which χ can be determined. Conversion of PSM into PGM occurs randomly along the PSM block; in the early stages of the hydrolysis reaction, the copolymer is composed of a PS block and a segment containing a mix of solketal methacrylate (SM) and glycerol monomethacrylate (GM), essentially a random copolymer block (PSM-r-PGM). As the fraction of GM increases in the poly(methacrylate) block over the course of the reaction, this segment becomes progressively more immiscible with the PS block. Using a reference volume of 118 Å3, the completely hydrolyzed copolymer displays a χ value that is over an order of magnitude higher than the pristine copolymer.

a function of lithium salt doping concentration and annealing temperature. They showed that the salt concentration in the BCP led to a morphological change from hexagonally packed cylinder to lamellar phase, and χ parameter for the salt-doped BCPs varied linearly with the salt concentration. Russell and coworkers25 achieved cylindrical microdomains of 3 nm in diameter with areal densities of 10 terabit/in.2 on the faceted sapphire surfaces gold salt-complexed PS-b-PEO. Recently, they also reported long-range sub-15 nm pitch line patterns using poly(2-vinylpyridine-b-styrene-b-2-vinylpyridine) (P2VP-b-PSb-P2VP) complexed with 10% copper chloride.26 This low N BCP was driven into the ordered state by selectively doping P2VP blocks with copper ions, which effectively increased the χ parameter; moreover, enhanced lateral ordering was observed with high areal densities. Strong immiscibility between the siloxane-containing blocks and hydrocarbon blocks has been exploited for the preparation of high χ−low N BCP. These systems provided the added advantage of high etch contrast since the siloxane-containing block has lower oxygen reactive ion etching rate than the hydrocarbon block, thus enabling pattern transfer. However, the siloxane-containing block has an inherently much lower surface energy, making it difficult to orient the microdomains normal to the surface without the use of chemical patterning of the substrate.27−29 Ross and co-workers21 developed highly ordered lamellar patterns using solvent-annealed poly(styreneb-dimethylsiloxane) (PS-b-PDMS) having a large χ. They achieved a 17 nm pitch using deep periodic trench patterns that can be transferred into a tungsten film to generate sub-10 nm nanowire arrays. Willson and co-workers27 reported poly(styrene-b-trimethylsilylstyrene-b-styrene) (PS-b-PTMSS-b-PS) and poly(trimethylsilylstyrene-b-lactide) (PTMSS-b-PLA) demonstrating lamellar morphologies oriented normal to the substrate using a top coat process, where the top coat balanced the interfacial energies. By use of this process, the well-formed perpendicular lamellar structures were obtained with L0 of 30 and 19 nm, corresponding to line widths of 15 and 9 nm, respectively. To replace the low surface energy siloxanecontaining block, Gopalan20 and Bates30 used poly(4-tertbutylstyrene-b-2-vinylpyridine) (P(tBuSt)-b-P2VP) and poly(4tert-butylstyrene-b-2-vinylpyridine) (P(tBuSt)-b-PMMA), respectively. The P(tBuSt) block is more hydrophobic than PS, which increases χ significantly. Hillmyer and co-workers31 investigated poly(cyclohexylethylene-b-methyl methacrylate) (PCHE-b-PMMA) having a large effective χ and produced a 9 nm lamellar domain pitch. Another approach to designing high χ−low N BCP has been the use of hydroxyl-functionalized blocks. Mahanthappa and co-workers32 explored the use of dimethylazlactone-containing polymers as a modular system for developing a series of highly immiscible BCPs, exhibiting an L0 as small as 7.6 nm. Kim and co-workers33 used poly(dihydroxystyrene-b-styrene) (PDHS-b-PS) copolymer to

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EXPERIMENTAL SECTION

Materials. PSM-b-PS BCPs were synthesized by a sequential reversible addition−fragmentation chain-transfer (RAFT) polymerization of SM and styrene. Detailed synthetic procedures for the monomers and polymers were reported in our previous work.34 1H NMR was utilized to determine the polymer molecular weight. The molecular characteristics and the sample codes of PSM-b-PS are summarized in Table 1. Trifluoroacetic acid (TFA) and 1,4-dioxane were purchased from Sigma-Aldrich Co. and used directly without purification. 37% hydrochloric acid (HCl) from Fisher Scientific was also used as received. Acid Hydrolysis of PSM-b-PS Copolymer. PSM-b-PS copolymers were dissolved into 1,4-dioxane (3 wt %) followed by slow addition of 1 N HCl solution to the stirred polymer solution using a syringe (10 vol % 1 N HCl). Acetone, generated as a byproduct of the B

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out from 4000 to 650 cm−1 with a universal ATR sampling accessory. Scheme 1 depicts the acid hydrolysis reaction of PSM-b-PS copolymer.

Scheme 1. Acid Hydrolysis Reaction To Transform PSM-bPS into PGM-b-PSa

a

Acetone, a byproduct of the hydrolysis, was purged by dry nitrogen gas during the reaction.

Small-Angle X-ray Scattering (SAXS) Analysis. The morphologies of PSM-b-PS (partially to fully converted) samples were investigated by SAXS. The sample powder was compression molded into a small steel washer, 0.5 mm in thickness, at 150 °C for 10 min, and then Kapton films, 0.06 mm in thickness, were placed on both sides of the washer to seal in the sample. All samples were thermally annealed at 140 °C for 24 h under vacuum to equilibrate their morphologies. SAXS measurements were performed using a Ganesha SAXS-LAB with Cu Kα radiation (λ = 0.1542 nm) and an incident beam diameter of ∼0.3 mm under vacuum. SAXS patterns were recorded as a function of the scattering vector (q = (4π/λ) sin θ) from 0.05 to 3.0 nm−1 using a 2-dimensional detector (Pilatus 300K), where λ is the X-ray wavelength and 2θ is the scattering angle. With a heating stage (Linkam Scientific) connected to temperature controller, temperature-dependent SAXS measurements were performed at a constant heating rate of 1.0 °C/min from 120 to 270 °C under vacuum. Before the SAXS data were collected for 10 min at each temperature with 10 °C increase intervals, the sample was allowed to thermally equilibrate for 10 min. Absolute intensities were determined using a PIN diode and a Pilatus detector to measure the intensity of the incident beam and transmission for the sample and the scattered intensity.34,35

Figure 1. SAXS absolute intensity profiles for a series of PSM-b-PS copolymers as a function of the scattering vector (q) after thermal annealing at 140 °C for 24 h. All profiles were measured at room temperature for 10 min. The intensity profiles are vertically shifted for clarity, and the arrows indicate the position of the primary (q*) and higher order reflections.

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RESULTS AND DISCUSSION The SAXS profiles of a series of PSM-b-PS copolymers with total molecular weights ranging from 2200 g/mol (N = 16) to 43 000 g/mol (N = 316) were measured at room temperature after thermal annealing, as shown in Figure 1. P(SM21-S22), the highest molecular weight copolymer, shows an intense primary scattering peak at q* = 0.264 nm−1 along with higher order reflection (3q*) indicative of an ordered lamellar structure with L0 = 2π/q* of 23.8 nm. A decrease in the polymer molecular weight resulted in the disappearance of the higher order reflection for P(SM13-S14), showing only a q* = 0.379 nm−1 with an L0 of 16.6 nm. For the three lowest molecular weight copolymers, P(SM5-S5), P(SM2-S2), and P(SM1-S1), the absence of scattering peaks suggests a phasemixed (or disordered) morphology. Previously, we conducted the transformation of PSM segments into PGM segments in the solid state using TFA vapor.34 However, since the TFA vapor diffuses into the sample from the surfaces, PSM segments in the PSM-b-PS copolymer did not convert randomly to PGMthus making the morphology within the sample nonuniform, which gave rise to broadening or even splitting of the primary scattering peak. To provide a better understanding on the effect of PSM conversion with the block incompatibility, acid hydrolysis was performed in solutions of PSM-b-PS to ensure that the conversion of the PSM occurred randomly and was uniform for all copolymer molecules. The solution route was also

Figure 2. 1H NMR spectra in DMSO-d6 of P(SM21-S22) as a function of the degree of conversion calculated by the ratio of the peak areas between the methyl group of the backbone (b; 0.65−1.05 ppm) in SM segments and the hydroxy group (h, i; 4.55−4.96 ppm) in GM segments. The spectra are vertically shifted for clarity. hydrolysis, was removed by purging with dry nitrogen gas, since it can cause the copolymer to precipitate at high conversion. The hydrolysis of the copolymer was arrested at different time intervals by freezing the solution in liquid nitrogen. Subsequently, the polymer was dried for 24 h at room temperature under vacuum. 1H NMR spectra (Bruker 500 NMR spectrometer) were recorded in DMSO-d6 to quantify the degree of conversion of the copolymer. Fourier transform infrared spectra (FT-IR, PerkinElmer 2000) of the copolymer were also carried C

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that the peaks from the hydroxy group become more pronounced with increasing reaction time, while the peaks from the ketal group (f ,g; 1.18−1.43 ppm) decreased with conversion. The peaks at 3.65−4.30 ppm (c, d, and e) correspond to the five protons of the solketal moiety,34,38 which persist even after acid hydrolysis. These peaks gradually shift upfield to 3.33−4.07 ppm with the increasing degree of hydrolysis due to the carbon atom being replaced with a hydrogen atom.36 Five P(SM21-S22) samples, hydrolyzed to different extents (0%, 31%, 49%, 78%, and 100%), were successfully prepared for SAXS measurements. Characterization of the transformation of SM units to GM in P(SM21-S22) by FT-IR is presented in Figure 3. The most notable difference in the FT-IR spectra of the pristine copolymer sample with the hydrolyzed samples was the gradual increase in the intensity of a broad peak at 3100−3600 cm−1 arising from the −OH stretching vibration as GM content of poly(methacrylate) segment increases. Conversion of the ketal group to diol groups also resulted in a reduction in the peak intensity of the CH3 stretching and bending vibrations at 2987 and 1372 cm−1, while that of the CH2 stretching vibration at 2922 cm−1 from the backbone was maintained. After 100% conversion, the weak peaks from the CH3 vibrations emanating from the polymer backbone remained. The disappearance of the absorption peaks at 1218 and 1086 cm−1 from the ether linkages in the ketal group (C−O−C) is concomitant with the evolution of C−O stretching vibration at 1118 cm−1 characteristic of a secondary alcohol. Broadening and shifting of the C O absorption peak from 1730 to 1714 cm−1 was also observed after complete hydrolysis of SM to GM owing to the increase in the hydrogen-bonding interactions between the hydroxy and the carbonyl groups.36,39 Absolute intensity SAXS profiles for P(SM21-S22) with different degree of hydrolysis were measured at room temperature after thermal annealing to examine the changes in the self-assembly behavior (Figure 4a). A single microphase separated lamellar morphology was evident in all the SAXS profiles, indicating that conversion of SM to GM during the acid hydrolysis reaction occurred randomly along the PSM block. The pristine P(SM21-S22) initially only had two scattering peaks (q* and 3q*), as shown in Figure 1. However, after 31% conversion, a second order peak (2q*) was observed because the random conversion of SM to GM changed the width of the PSM-r-PGM microdomain breaking the symmetry. As the degree of conversion increased, multiple higher order reflections were detected, indicative of the strongly microphase separated morphology between the hydrophobic PS and the hydrophilic PGM blocks. Up to the eighth order (8q*) scattering signals were displayed after 100% conversion. For a quantitative analysis of the SAXS data, Figure 4b shows the inverse of a maximum intensity (1000/I(q*)), full width at halfmaximum (FWHM), and L0 at each conversion. The decrease in 1000/I(q*) with increasing degree of conversion indicates a stronger microphase separation of the copolymer. From the FWHM results of P(SM21-S22), it was evident that the randomly converted PSM segments caused a change in the symmetry of the lamellar structure. By 49% conversion, the FWHM increased from 0.028 to 0.031 nm−1 and then began to decrease, eventually returning to a sharp scattering peak with increasing conversion (0.027 nm−1 at 100% conversion). The noticeable shift in q* to a lower value during the conversion is due to the increased stretching of the two blocks at the interface as a consequence of the increase in nonfavorable

Figure 3. FT-IR spectra for P(SM21-S22) as a function of the degree of conversion. The spectra were measured with universal ATR sampling accessory. 32 scans, ranging from 4000 to 650 cm−1, were taken to collect the data with a resolution of 4 cm−1.

effective in controlling the degree of conversion by the reaction time.36,37 PSM-b-PS copolymers were dissolved in 1,4-dioxane and were reacted with 1 N HCl under dried nitrogen gas for different periods of time and then freeze-dried using liquid nitrogen. To quantify the transformation of SM into GM, 1H NMR spectroscopy was used. Figure 2 shows the 1H NMR spectra of P(SM21-S22) in DMSO-d6 as a function of the degree of hydrolysis with reaction time from 0 to 18 h. The degree of conversion (C) was determined using C = 3A(h, i)/ 2A(b) × 100, where A(h, i) and A(b) are the signal integral areas for the hydroxy protons in the GM units (4.55−4.96 ppm) and the methyl hydrogens of the backbone (0.65−1.05 ppm), respectively. From the 1H NMR spectra, it was evident

Figure 4. (a) SAXS absolute intensity profiles for P(SM21-S22) as a function of the degree of conversion after thermal annealing at 140 °C for 24 h. The intensity profiles are vertically shifted for clarity, and the arrows indicate the q* and higher order reflections of the lamellar morphology. (b) Inverse of a maximum intensity (1000/I(q*)), full width at half-maximum (FWHM), and domain spacing (L0 = 2π/q*) determined from the SAXS profiles are marked at each conversion. D

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of 1 °C/min from 120 to 270 °C. For P(SM21-S22), the intensity of the primary scattering peak barely changed up to 270 °C, even at 0% conversion (Figure S1). Only a second order scattering peak at 2q* emerged after increasing the temperature to 240 °C due to the change in the thickness of the lamellae by thermal expansion.40,41 Consequently, the molecular weight of the P(SM21-S22) was too high to obtain a phase-mixed state for measuring χ. Figure 5a shows SAXS absolute intensity profiles for P(SM13-S14) with different degrees of conversion at room temperature after thermal annealing. Each degree of conversion was calculated by 1H NMR analysis using the ratio of the peak areas for the hydroxy group and the methyl group similar to P(SM21-S22) (Figure S2). The pristine copolymer sample features an intense primary scattering peak with no higher order reflections (Figure 1); however, upon converting 27% of the SM units to GM, a lamellar morphology displaying reflections at 2q* and 3q* relative to the primary reflection was observed. With increasing acid hydrolysis, multiple higher order reflections were detected with the completely hydrolyzed sample having scattering peaks as high as 6q* suggestive of strong microphase separation consistent with the observation for P(SM21-S22). In Figure 5b, 1000/I(q*), FWHM, and L0 of P(SM13-S14) are shown as a function of the degree of conversion. 1000/I(q*) gradually decreased whereas the FWHM increased from 0.026 to 0.036 nm−1 as the conversion increased to 61%, and then it decreased to 0.025 nm−1 at 100% conversion. The primary scattering peak for P(SM13-S14) also shifted from q* = 0.379 nm−1 (L0 = 16.6 nm) to q* = 0.229 nm−1 (L0 = 27.4 nm) as the conversion increased due to an increased stretching of the copolymer chains at the interface. Previously, we described χ for PSM and PS by χ = 0.0196 + 4.69/T (∼0.035 at 25 °C) using temperature-dependent SAXS profiles of P(SM13-S14).34 However, the phase-mixed state was not accessible even for P(SM13-S14) with 27% of the SM hydrolyzed to GM. Furthermore, the intensity of the q* remained unchanged, indicating that χN > 10.5 even up to 270 °C due to the increase in χ (Figure S3). To obtain χ for the randomly converted (PSM-r-PGM)-b-PS, the lower molecular weight samples were evaluated using SAXS. In Figure 6a, the evolution of lamellar ordered structure from a disordered P(SM5-S5) after the hydrolysis reaction was monitored at room temperature. 1H NMR analysis was also used to determine the degree of conversion (Figure S4). At 18% conversion, a disordered state was observed with a broad, diffuse scattering profile. However, a sharp primary scattering peak was detected at q* = 0.601 nm−1 with an L0 of 10.5 nm after 40% conversion. The stronger microphase separated copolymer (over 40% conversion) showed multiple order reflections, and four sharp scattering peaks were observed upon complete hydrolysis of the copolymer. The values of 1000/ I(q*) and FWHM decreased gradually, and the q* value shifted to q* = 0.460 nm−1 with an L0 of 13.7 nm, as shown in Figure 6b. Using P(SM5-S5) with 40% conversion, the correlationhole scattering analysis at a phase-mixed symmetric BCP melts was performed to determine χ by the mean-field theory.42−45 In this analysis, a 118 Å3 reference volume was used to calibrate to a volume-based degree of polymerization (Nv)20,30,31,46 because of the differences in the densities among PSM (1.148 g/cm3), PGM (1.127 g/cm3), and PS (1.033 g/cm3), which were determined with a pycnometer. By fitting the incompressible scattering function, S(q) = (F(q) − 2χ)−1, to the scattering intensity profile, I(q) = kn × S(q), χ was determined, where

interactions. At 49% conversion, the primary scattering peak of P(SM21-S22) is located at q* = 0.244 nm−1 with an L0 of 25.8 nm, which increased slightly in comparison to the pristine sample (L0 = 23.8 nm). Subsequently, it changed significantly to q* = 0.163 nm−1 with an L0 of 38.5 nm at 100% conversion, indicating that χ between PGM and PS is much larger than that

Figure 5. (a) SAXS absolute intensity profiles for P(SM13-S14) as a function of the degree of conversion after thermal annealing at 140 °C for 24 h. The intensity profiles are vertically shifted for clarity, and the arrows indicate the q* and higher order reflections of the lamellar morphology. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at each conversion.

Figure 6. (a) SAXS absolute intensity profiles for P(SM5-S5) as a function of the degree of conversion after thermal annealing at 140 °C for 24 h. The intensity profiles are vertically shifted for clarity, and the arrows indicate the q* and higher order reflections of the lamellar morphology. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at each conversion.

between PSM and PS. To evaluate χ in the phase-mixed state, SAXS was performed at different temperatures at a heating rate E

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Figure 8. (a) SAXS absolute intensity profiles for P(SM2-S2) as a function of the degree of conversion after thermal annealing at 140 °C for 24 h. The intensity profiles are vertically shifted for clarity, and the arrows indicate the q* and higher order reflections of the lamellar morphology. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at 75% and 100% conversion.

be extracted and then fit to a linear relation with the inverse temperature as χ = α + β/T. Figure 7a shows SAXS intensity profiles for P(SM5-S5) of 40% conversion at various temperatures from 140 to 240 °C. At temperatures lower than ∼190 °C, a sharp primary scattering peak at q* = 0.601 nm−1 was observed. However, after heating to 200 °C, a diffuse scattering profile was observed, indicating a transition from the ordered to disordered state with temperature. Figure 7b illustrates the discontinuous changes of 1000/I(q*) and FWHM as a function of the inverse temperature (1/K) derived from the SAXS profiles for P(SM5-S5) at 40% conversion; the abrupt increase in FWHM indicates the ODT temperature (TODT) to be around 200−210 °C. When the temperature was greater than TODT, the 1000/I(q*) and FWHM increased continuously, and the L0 decreased gradually because χ between two blocks is decreased. In Figure 7c, the χ values for 40% converted PSM and PS as a function of the inverse temperature were fit to χ = 0.0511 + 13.33/T, normalized to Nv = 131 for P(SM5-S5) at 40% conversion. At 25 °C, ∼0.096 was obtained from this equation, which is 2.7 times larger than that for PSM and PS. To characterize χ at higher conversions, P(SM5-S5) with 58% conversion was also measured as a function of temperature from 160 to 270 °C. However, the change in the intensity at q* was not significant, and the second order peak at 2q* was maintained up to 270 °C due to the increase in χ (Figure S5). For the disordered P(SM2-S2), a diffuse scattering profile was observed even at 45% conversion, as shown in Figure 8a (1H NMR analysis in Figure S6). After 75% conversion, a primary reflection at q* = 0.882 nm−1 with an L0 of 7.1 nm was observed without higher order reflections. For 100% conversion, the lamellar ordered morphology was well-developed with a weak higher order peak at 2q*, and L0 was increased slightly to 7.7 nm (q* = 0.817 nm−1). On the other hand, both 1000/I(q*) and FWHM decreased with increasing degree of hydrolysis, indicating the formation of a sharper primary scattering peak (Figure 8b). To obtain χ for the copolymers with over 40% conversion, the SAXS measurement for P(SM2S2) of 75% conversion at various temperatures and its intensity profiles from 140 to 220 °C are shown in Figure 9a. The intensity of the primary peak gradually decreased with increasing temperature and completely disappeared upon reaching the ODT at 190−200 °C. A significant increase in 1000/I(q*) and FWHM of the 75% converted P(SM2-S2) was

Figure 7. (a) SAXS absolute intensity profiles for P(SM5-S5) of 40% conversion at various temperatures with a heating rate of 1.0 °C/min. The temperature-dependent profiles are vertically shifted in the range of 140−240 °C. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at each temperature. (c) At 40% conversion, the segmental interaction parameter (χ) between two blocks is plotted against the inverse temperature using the incompressible scattering function. χ is fitted to a linear relation as χ = 0.0511 + 13.33/T.

F(q) indicates the interference function composed of the Debye scattering function for individual blocks containing the radius of gyration of a Gaussian chain (Rg) and q, and kn is the contrast factor. By nonlinear regression analysis, the χ value can F

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Figure 10. (a) SAXS absolute intensity profiles for P(SM1-S1) as a function of the degree of conversion after thermal annealing at 140 °C for 24 h. The intensity profiles are vertically shifted for clarity, and the arrows indicate the q* and higher order reflections of the lamellar morphology. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at only 100% conversion.

larger than that for the pristine copolymer sample. For P(SM2S2) having 100% conversion, the intensity at q* remained constant up to 270 °C, with the second order reflection at 2q* vanishing at 250 °C, indicating that the copolymer was still the microphase-separated, i.e., χNv > 10.5 (Figure S7). To evaluate the χ value between PGM and PS (1H NMR analysis in Figure S8), SAXS analysis on the polymer sample with the lowest molecular weight, P(SM1-S1), was conducted, as shown in Figure 10. Owing to its lower N, a disordered state was observed with a diffuse scattering profile even at 79% conversion. However, a well-ordered lamellar structure was detected upon complete hydrolysis of the PSM block featuring a strong primary scattering peak at q* = 1.164 nm−1 with an L0 of 5.4 nm, the smallest full pitch in this study, and a second order reflection at 2q*. This result arises from the massive increase in χ between PGM and PS blocks. Figure 11a shows the SAXS intensity profiles for P(SM1-S1) of 100% conversion as a function of temperature from 120 to 200 °C with ODT being evident from 150 to 160 °C, as evidenced by the discontinuous change of 1000/I(q*) and the FWHM (Figure 11b) and the disappearance of the primary reflection. With the reference volume of 118 Å3, χ for PGM-b-PS is plotted as a function of the inverse temperature and fit to χ = 0.3144 + 36.91/T, which was normalized to Nv = 26 for P(SM1-S1) of 100% conversion (Figure 11c). From this equation, the χ value for PGM and PS was calculated to be ∼0.438 at 25 °C, which is 13 times greater than that for PSM and PS. This value of χ is much greater than those reported for PS-b-PDMS (∼0.14 at 25 °C), P(tBuSt)-b-P2VP (∼0.18 at 25 °C), PCHE-b-PMMA (∼0.32 at 25 °C), and PTMSS-b-PLA (∼0.21 at 25 °C, recalculated) at a 118 Å3 reference volume.16,30,31 In Figure 12, the χ values of PSM-b-PS at 25 °C as a function of the degree of conversion are summarized. By transforming PSM into PGM, the χ increased exponentially from 0.035 to 0.438 as a function of the degree of hydrolysis. This enormous increase in χ resulted in highly ordered lamellar microdomains, as evidenced by the multiple higher order reflections, even for the smallest molecular weight sample. These χ values have an entropic (α) and enthalpic (β/T) contributions, which translates into interaction energies of the components at the interface and a stretching energy of BCP chains away from the interface, respectively.47 The microphase separation is governed by the competition between these two parameters. For the

Figure 9. (a) SAXS absolute intensity profiles for P(SM2-S2) of 75% conversion at various temperatures with a heating rate of 1.0 °C/min. The temperature-dependent profiles are vertically shifted in the range of 140−220 °C. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at each temperature. (c) At 75% conversion, the χ value between two blocks is plotted against the inverse temperature using the incompressible scattering function. χ is fitted to a linear relation as χ = 0.1587 + 22.51/T.

also observed over the same temperature range, and L0 decreased slightly above 200 °C (Figure 9b). At T > TODT, using the correlation-hole scattering analysis with Nv = 50 for P(SM2-S2) having 75% conversion, the χ values as a function of the inverse temperature were fit to χ = 0.1587 + 22.51/T, ∼0.234 at 25 °C, as shown in Figure 9c. This χ is 6.7 times G

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PMMA.30,44 With progressive transformation from PSM to PGM, the entropic contribution increased exponentially from 0.0196 to 0.3144 due to the formation of the hydrogen bonding, which generates stronger interactions (Table 2). Furthermore, the conversion of the hydrophobic PSM to the hydrophilic PGM blocks minimizes the domain interactions with the PS blocks at the interface, which increases the enthalpic contribution. The enthalpic coefficient (β) shows a huge increase from 4.69 to 36.91 upon hydrolysis. From these results, it is evident that PSM/PGM-b-PS system with controllable and large χ values has a high potential to be used for advanced nanoelectronics as a template with sub-3 nm feature sizes.

Figure 12. χ values of the PSM-b-PS copolymer as a function of the degree of conversion.

Table 2. Entropic and Enthalpic Contribution in χ = α + β/T for PSM-b-PS Depending on the Degree of Conversion

a

conversion (%)

entropic contribution (α)

enthalpic contribution (β/T)

χ at 25 °C

0a 40 75 100

0.0196 0.0511 0.1587 0.3144

4.69/T 13.33/T 22.51/T 36.91/T

0.035 0.096 0.234 0.438

Calculated in our previous work.34

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CONCLUSION We have demonstrated the self-assembly of BCPs using lamellar-forming PSM-b-PS copolymers synthesized by RAFT polymerization. Through ketal hydrolysis of the PSM blocks to the PGM blocks, 5.4 nm full pitch lamellar microdomains were achieved. The acid hydrolysis reaction was accomplished in solution to randomly convert ketal moieties in the PSM block to diol groups, which allowed us to control the degree of conversion with reaction time. SAXS analysis showed that disordered or weakly ordered PSM-b-PS was transformed into well-ordered PGM-b-PS displaying multiple higher order reflections due to a significant increase in χ. Using the correlation-hole scattering arising from a phase-mixed state in the BCP melts, χ for PSM and PS was found to be χ = 0.0196 + 4.69/T with a 118 Å3 reference volume, ∼0.035 at 25 °C, where T is the absolute temperature. Progressive conversion of SM units to GM resulted to an exponential increase in the χ value to ∼0.438 at 25 °C (χ = 0.3144 + 36.91/T), which is 13 times

Figure 11. (a) SAXS absolute intensity profiles for P(SM1-S1) of 100% conversion (P(GM1-S1)) at various temperatures with a heating rate of 1.0 °C/min. The temperature-dependent profiles are vertically shifted in the range of 120−200 °C. (b) The 1000/I(q*), FWHM, and L0 determined from the SAXS profiles are marked at each temperature. (c) At 100% conversion, the χ value between two blocks (PGM and PS) is plotted against the inverse temperature using the incompressible scattering function. χ is fitted to a linear relation as χ = 0.3144 + 36.91/T.

copolymers investigated here, the entropic contribution to χ is larger than the enthalpic contribution, as indicated by the weak dependence on temperature. The positive entropic contribution for PSM-b-PS arises from weak noncovalent interactions between the ester and the aromatic moieties similar to PS-bH

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Macromolecules greater than the χ value for the pristine copolymer, enabling the access to lamellar structure with 5.4 nm domain sizes from the smallest molar mass sample (Nv = 26). These results show that χ for PGM-b-PS is larger than those reported for most of the copolymers and, consequently, the sub-3 nm feature size achieved from a pure diblock copolymer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02221. Temperature-dependent SAXS intensity profiles, 1H NMR, TGA curves, and thermal expansion coefficients of homopolymers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.R.) E-mail jrzayev@buffalo.edu, Tel (716) 645-4314, Fax (716) 645-6963. *(T.P.R.) E-mail [email protected], Tel (413) 5771516, Fax (413) 577-1510. ORCID

Jose Kenneth D. Mapas: 0000-0002-9867-9917 Javid Rzayev: 0000-0002-9280-1811 Thomas P. Russell: 0000-0001-6384-5826 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research under Contract 16RT1602. J. Choi acknowledges Samsung Scholarship from the Samsung Foundation for financial support.

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