Domain Spacing and Composition Profile Behavior ... - ACS Publications

Sep 15, 2017 - Youngmi Seo,. ∥. Maxym Tansky,. § ... lithium salt to modulate effective segregation strength (χeffN). Based on X-ray reflectivity ...
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Domain Spacing and Composition Profile Behavior in Salt-Doped Cyclic vs Linear Block Polymer Thin Films: A Joint Experimental and Simulation Study Thomas E. Gartner, III,† Tomohiro Kubo,§ Youngmi Seo,∥ Maxym Tansky,§ Lisa M. Hall,*,∥ Brent S. Sumerlin,*,§ and Thomas H. Epps, III*,†,‡ †

Department of Chemical & Biomolecular Engineering and ‡Department of Materials Science & Engineering, University of Delaware, Newark, Delaware 19716, United States § George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States ∥ William G. Lowrie Department of Chemical & Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: Herein, we leveraged lithium salt doping of linear and cyclic block polymers (BPs) in thin film geometries to demonstrate how BP architecture influences self-assembled nanofeature sizes and interfacial widths in nanostructured thin films as a function of segregation strength. To mitigate potential issues with sample-tosample variability and time-consuming synthesis, we used a single linear BP specimen that was ring-closed to generate an analogous cyclic BP, and we mixed the cyclic and/or parent linear BP with a lithium salt to modulate effective segregation strength (χeffN). Based on X-ray reflectivity analyses, cyclic polystyrene-block-poly[oligo(ethylene glycol) methacrylate] BP thin film specimens had ∼20% smaller domain spacings than their linear counterparts at all χeffNs and lower absolute sensitivities to changes in χeff at identical molecular weights. We also report the first direct measurements of interfacial widths in cyclic BP assemblies, which quantitatively demonstrated that interfacial mixing in cyclic BPs was greater relative to linear BPs. Furthermore, the trends in domain characteristics with increasing salt concentration qualitatively agreed with results from molecular dynamics (MD) simulations with increasing χ, despite the fact that salt species were not explicitly included in the MD simulations. Our results underscore the utility of lithium salt doping to explore the BP phase behavior of synthetically challenging macromolecules and demonstrate key architecture/segregation strength relationships in cyclic BP thin films, which provides useful information to further evaluate cyclic BP suitability for nanoscale patterning and templating applications.

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INTRODUCTION

Flory−Huggins interaction parameter (χ)are not significantly affected by most processing methods.9−11 To satisfy the continual demand for smaller lithographic features, one possible approach to further decrease feature size is to alter the architecture of the BP, for example, by employing cyclic or star BPs.12 In particular, cyclic polymers are known to exhibit lower hydrodynamic volumes than their linear counterparts due to the lack of chain ends.13 This aspect permits cyclic BPs to be used to reduce feature size and maintain sufficient segregation strength (i.e., large enough N) to achieve microphase separation. Several groups have established that A−B cyclic BPs show a 5−10% reduction in domain spacing relative to analogous A−B−A triblock polymers and an ∼40% reduction relative to A−B diblocks.14−19 In addition to

The capacity of block polymers (BPs) to self-assemble into nanoscale periodic domains makes BPs attractive materials for a host of technological applications.1−4 For example, BPs in thin film geometries have been investigated widely as microelectronic templates due to their facile self-assembly into dot or line patterns, structures that are useful in the fabrication of memory devices and logic circuits.5−7 Key characteristics of an effective BP lithographic template include feature size, orientation, ordering, and defect density of the microphaseseparated BP domains.6 Significant progress toward controlling BP orientation, ordering, and defectivity has been attained with pre- and post-deposition techniques such as graphoepitaxy, chemical prepatterning, and solvent vapor annealing. 6,8 However, there are limited techniques available to tune the feature size in BPs, as the key determinants of the characteristic domain spacingthe degree of polymerization (N) and the © XXXX American Chemical Society

Received: June 22, 2017 Revised: September 9, 2017

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PS−POEGMA with or without lithium salt. Coarse-grained MD simulations of symmetric linear and nonconcatenated cyclic block polymers, each with 100 beads per chain, also were performed. The domain characterization results from XRR were compared to MD results and show qualitative agreement. For example, trends in domain spacing and interfacial width as a function of increasing salt concentration, obtained via XRR analyses, match trends noted by increasing χ in MD simulations. This study leverages click macrocyclization and lithium salt doping to examine the thin film phase behavior of identical linear and cyclic BP specimens and reports the first direct measurements of cyclic BP interfacial width as a function of effective segregation strength. These strategies enable the exploration of cyclic BPs to achieve decreased characteristic feature size, opening new avenues to control the domain characteristics of nanostructured block polymer materials for advanced technological and templating applications.

decreases in domain spacing, these studies demonstrated increases in interfacial curvature,17 decreases in order−disorder transition temperature,18 and weaker scaling with molecular weight19 for cyclic BPs in comparison to linear BPs. These findings agreed qualitatively with theory20−22 and Monte Carlo simulations23−26 that suggested similar trends in domain spacing, molecular weight scaling, and critical segregation strength for phase separation in cyclic BPs. All of the above work examined cyclic BP phase behavior in the bulk, and corresponding thin film studies must be completed to advance cyclic BPs in nanotemplating applications. However, there are relatively few comparative reports on cyclic BP thin films, in part due to the synthetic complexity of cyclic BPs.13 Recently, Grayson and co-workers pioneered the use of click chemistry to link the ends of linear homopolymer and block copolymer precursors in a macrocyclization process.27,28 The speed and specificity of click chemistry permit the fabrication of cyclic BPs with controlled molecular weight and composition by tuning the molecular weight of the linear precursors. Furthermore, the relative ease of end-functionalization widens the list of available monomer types and possible polymerization processes.29 Hawker and co-workers used this click macrocyclization approach to synthesize cyclic PS-block-poly(ethylene oxide) [PS−PEO] BPs and presented the first report of cyclic BPs in the thin film geometry.12 They examined domain spacing through grazing-incidence small-angle X-ray scattering (GISAXS) on thin film samples of these polymers. Their thin film results agreed with prior reports in the bulk; cyclic PS− PEO had an ∼10% decrease in domain spacing relative to a PS−PEO−PS linear triblock and an ∼33% decrease relative to a linear PS−PEO diblock of the same molecular weight. Moreover, they successfully demonstrated the potential use of thin films composed of cyclic BPs as lithographic templates. However, all of the above experimental work was limited to a single or small set of molecular weights and relative volume fractions of BP components and primarily focused on the effects of the cyclic architecture on domain spacing. To our knowledge, the interfacial widths and domain composition profiles of cyclic BPs have been explored only via theoretical studies.20,22 Herein, we report the synthesis and characterization of cyclic and linear PS-block-poly[oligo(ethylene glycol) methacrylate] (PS−POEGMA) doped with lithium triflate (LiCF3SO3) salt along with coarse-grained molecular dynamics (MD) simulations of an analogous system. Lithium ions associate with the ethylene oxide groups in the POEGMA side chains, and this association increases the effective segregation strength (χeffN) of the block polymer, swells the POEGMA domain, and sharpens the interfacial profile.30−32 The ability to manipulate interactions through salt doping allows us to interrogate a range of χeffN and relative volume fractions of a BP by mixing in varying amounts of the lithium salt to a single synthetic sample, thus circumventing common issues related to sample-to-sample variability such as changes in the molecular weight or dispersity between batches. This aspect is particularly advantageous when studying synthetically challenging macromolecules, e.g., BPs with atypical architecture such as cyclic or star. Well-defined linear PS−POEGMA was synthesized via successive atom transfer radical polymerization (ATRP) reactions, and macrocyclization of the linear sample afforded the analogous cyclic PS−POEGMA in a high purity. BP domain spacings and interfacial widths were determined through X-ray reflectivity (XRR) analyses on thin films of linear and cyclic

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EXPERIMENTAL SECTION

Synthesis of Linear PS−POEGMA. A detailed report of the synthesis procedure for the linear PS−POEGMA is provided in Supporting Information section 1. Macrocyclization of Linear PS−POEGMA. (i) A solution of linear PS−POEGMA (240 mg, 7.50 × 10−3 mmol) and ascorbic acid (192 mg, 1.09 mmol) in N,N-dimethylformamide (DMF) [120 mL] was deoxygenated by purging with nitrogen for 30 min and taken up in a syringe. (ii) A solution of N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) [0.23 mL, 1.1 mmol] in DMF (250 mL) was deoxygenated by purging with nitrogen for 30 min and frozen in a liquid nitrogen bath. Cu(I)Br (196 mg, 1.37 mmol) and ascorbic acid (240 mg, 1.36 mmol) were added onto the frozen solution ii, and the flask was deoxygenated by purging with nitrogen for 30 min and warmed to room temperature. Solution i was added to a vigorously stirred solution ii via a syringe pump at the rate of 2 mL/ h. The resulting solution was diluted with saturated aqueous NH4Cl (300 mL) and extracted with dichloromethane (DCM) [200 mL × 3]. The combined organic layers were washed with saturated aqueous NH4Cl (200 mL × 2), saturated aqueous NaHCO3 (200 mL × 2), and brine/H2O (1/1, 200 mL × 3), dried (Na2SO4), filtered, and concentrated. The resulting materials were dissolved in DCM, precipitated into cold isopropanol, filtered, and dried to yield cyclic PS−POEGMA as an off-white solid (Mn = 32 000 g/mol, Mw/Mn = 1.10). Polymer Characterization. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on an Inova 500 spectrometer. Gelpermeation chromatography (GPC) was performed using N,Ndimethylacetamide with 50 mM LiCl as the mobile phase at 50 °C and a flow rate of 1.0 mL/min (Agilent isocratic pump, degasser, and autosampler; columns: PLgel 5 μm guard + two ViscoGel I-series G3078 mixed bed columns, molecular weight range 0−20 × 103 and 0−100 × 104 g/mol). Detection consisted of a Wyatt Optilab T-rEX refractive index detector operating at 658 nm and a Wyatt miniDAWN Treos light scattering detector operating at 659 nm. Absolute molecular weights and molecular weight distributions were calculated using the Wyatt ASTRA software. Intrinsic viscosity measurements were performed using GPC with tetrahydrofuran (THF) as the mobile phase at 35 °C at a flow rate of 1.0 mL/min (Viscotek GPCmax pump, degasser, and autosampler; columns: three PLgel 5 μm MIXED-D mixed bed columns, molecular weight range 200−400 000 g/mol). Detection consisted of a Wyatt Optilab rEX refractive index detector operating at 658 nm, a Wyatt miniDAWN Treos light scattering detector operating at 656 nm, and a Wyatt ViscoStar-II viscometer. Absolute molecular weights and molecular weight distributions were calculated using the Wyatt ASTRA software. Apparent molecular weights were calculated using nine PS standards from 327 300 to 1370 g/mol. Materials Purification and Storage. Following synthesis, the linear and cyclic copolymers were precipitated twice in petroleum B

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Figure 1. Synthesis and characterization data of the linear and cyclic BPs. (a) Copper-catalyzed azide−alkyne cycloaddition of the linear PS− POEGMA afforded the cyclic counterparts. (b) Gel permeation chromatography traces of the linear and cyclic BPs. Linear PS−POEGMA data are plotted with a black solid line, and cyclic PS−POEGMA data are plotted with a red solid line. Dotted lines denote deconvolution of overlapping peaks. ether to remove remaining impurities, and impurity removal was monitored by 1H NMR spectroscopy (Bruker, AVX400). Because of the hygroscopic nature of the PS−POEGMA polymer and the lithium triflate salt, all materials were dried under dynamic vacuum (at 85 °C) for at least 24 h prior to transfer to an argon glovebox for storage and sample preparation. Thin Film Preparation. The polymer and lithium triflate salt were dissolved in a mixture of 85 wt % THF and 15 wt % methanol (final solution ∼3.5 wt % polymer) and stirred overnight prior to deposition on silicon wafers via flow coating.33 The THF/methanol mixture was found to improve PS−POEGMA dissolution and polymer−salt mixing relative to a pure THF solution and also resulted in higher quality thin films. Silicon wafer substrates (Wafer World Inc.) were rinsed with toluene and cleaned in an ultraviolet-ozone oven (model 342, Jelight Co., Inc.) prior to film casting. Gradient thickness films (25.4 mm wide, 70 mm long, and 80−130 nm thick) were used; polymer film thicknesses were measured with a Filmetrics F20-UV interferometer operated in reflectance mode. After casting, films were held under dynamic vacuum overnight at room temperature prior to annealing under vacuum for 6 h at 135 °C. Sections of the films of thickness commensurate to the domain spacing (either 2.5, 3.5, 4.5, or 5.5 lamellar periods depending on the particular film) were used for subsequent XRR analyses. Thin Film Characterization. A Rigaku Ultima IV X-ray diffractometer was operated at room temperature to obtain XRR profiles; the measured profiles were fit using Rigaku Globalfit software. AFM images were captured on a Veeco Dimension 3100 operating in tapping mode with silicon probes (Tap150G, BudgetSensors), and a typical set point ratio was 0.75. Molecular Dynamics Simulations. Coarse-Grained Polymer Model. Linear or nonconcatenated cyclic polymers were simulated using an attractive Kremer−Grest model.34 Polymers had 100 beads per chain with two equally sized blocks, i.e., 50 A monomer segments and 50 B monomer segments. The following finitely extensible nonlinear elastic (FENE) potential describes interactions between bonded beads:

⎛ r2 ⎞ ⎟ UFENE(r ) = − 0.5kR 0 2 ln⎜1 − R 02 ⎠ ⎝

was 1.0ϵ between like beads and between bonded beads regardless of the type. Thus, the characteristic length, mass, and energy units of the simulation are σ, m, and ϵ, and the characteristic time unit is τ = σ(m/ ϵ)1/2. The salt species were not explicitly represented in our model; rather, the microphase segregation strength was controlled by adjusting ϵAB for nonbonded A−B interactions to mimic changes in salt concentration. ϵAB can be related to the Flory−Huggins interaction parameter (χ) using a first-order approximation χ ≈ 14.07(1 − ϵAB/ϵ) obtained from35

χ = (ϵAB−ϵ) +

∫0

2.5σ

⎡⎛ σ ⎞12 ⎛ σ ⎞6 ⎛ σ ⎞12 ⎟ ⎢⎜ ⎟ − ⎜ ⎟ − ⎜ ⎝r⎠ ⎝ 2.5σ ⎠ ⎢⎣⎝ r ⎠

⎛ σ ⎞6 ⎤ 2 ⎜ ⎟ ⎥g (r )4πr dr ⎝ 2.5σ ⎠ ⎥⎦

(3)

ULJ

( )

in which g(r) = exp − k T with ϵij = ϵ within the g(r) term (i.e., one B

fluid approximation), kB is the Boltzmann constant, and ρ is 0.88σ−3, which was the bead number density of homopolymers in our simulations. MD Initialization and Simulation Methods. All systems were initialized in a lamellar structure. For linear block polymers, four lamellae with 120 polymers per layer were obtained with initial interfacial coverage density 0.1σ−2 and total density 0.88σ−3, following the same procedure (growing random walks from the interfacial beads) as in previous work.36 If the initial configuration for cyclic polymers were generated analogously using constrained random walks (also being constrained to form a ring in some way), interlocking rings may be formed. To avoid this issue, each cyclic polymer was initialized with a rectangular shape in the x−z plane (the aspect ratio of the rectangle was 7:6 with the longer side in x) with one A−B interfacial joint starting from a randomly chosen point on the interfacial x−y surface. The initial box sizes for the cyclics were determined by the initial coverage density 0.1σ−2 and the length in the z direction needed to keep the rectangular initial configurations from overlapping, leading to a total initial density 0.22σ−3. For both linear and cyclic polymers, the above initialization methods were used for ϵAB ≤ 0.9 (or χ ≥ 1.41). Above this value of ϵAB, the last configuration from a simulation at a lower ϵAB was fed as an initial configuration for the next higher ϵAB system stepwise to carefully equilibrate the systems that were near the order-to-disorder transition point. Specifically, ϵAB was increased in increments of 0.025 or 0.01, and the maximum value of ϵAB used was 0.98 for linear polymers and 0.96 for cyclic polymers. At the highest ϵAB values, the lamellar structure became disordered. Bead overlaps that occurred in the initial state were removed using a brief simulation with a soft potential for all nonbonded interactions, A[1 + cos(πr/rc)]; r < rc, in which rc = 21/6σ. During this soft push-off step, A was increased linearly from 0 to 250 for 57.5τ. Then, the soft potential was replaced with the LJ potential, and both equilibration and main simulation were performed in an isothermal−isobaric (NPT) ensemble. A Nosé−Hoover barostat (thermostat) was used to maintain the pressure 0ϵσ−3 (temperature 1ϵ/kB) with a damping

(1)

which effectively avoids chain crossing or bond breaking events with a spring constant of k = 30ϵ/σ2 and maximum length of R0 = 1.5σ.34 All pairwise interactions are of Lennard-Jones (LJ) form: 12 ⎡ σ 12 ⎛ σij ⎞6 ⎤ ⎛ ij ⎞ ⎛ σij ⎞6 ⎛ σij ⎞ ULJ(r ) = 4ϵij⎢⎜ ⎟ − ⎜ ⎟ − ⎜ ⎟ + ⎜ ⎟ ⎥, ⎝r ⎠ ⎢⎣⎝ r ⎠ ⎝ rc ⎠ ⎝ rc ⎠ ⎥⎦

4ρ kBT

r ≤ rc

(2) in which the cutoff distance rc = 2.5σ for nonbonded beads and 2 σ for bonded beads, and ULJ(r) = 0 for r > rc. ϵij and σij are the interaction strength and the characteristic length scale of the interaction between i and j beads, respectively. Both A and B segment types are of size 1.0σ and mass 1.0m, and the interaction strength ϵij 1/6

C

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Figure 2. (a, b) XRR profiles (black) and fits (red) for thin films of (a) linear PS−POEGMA and (b) cyclic PS−POEGMA; see also Table S2. Curves are shifted vertically (arbitrary units) for clarity. For panels a and b, the salt concentration increases from the bottom profile (neat [no salt] for the linear specimens, 48:1 for the cyclic specimens) to the top profile (6:1 in both cases). (c, d) Lamellar domain spacing (L0) (c) as a function of lithium triflate salt concentration (Csalt) from XRR on thin film samples and (d) as a function of χ from MD simulations. Linear PS−POEGMA data are plotted with closed diamonds, and cyclic PS−POEGMA data are plotted with open circles. Error bars in panel c are calculated via the standard deviation of the layer thicknesses obtained for the PS−POEGMA lamellae in the films. parameter of 10τ (1τ). The barostat used coupled x and y dimensions (the box kept a square cross section), but the z dimension was allowed to fluctuate independently; this strategy allows the system to find the proper lamellar spacing (L0) and volume fraction of interface (f int). The total simulation time was 230000τ with time step (Δt) of 0.0115τ. L0 started fluctuating around a constant value within ∼23000τ for all considered systems. The open-source MD package LAMMPS was used to implement all simulations.37 The spacing and density profiles were obtained by averaging over the last 500 configurations each 115τ apart.

possible uncertainty associated with this approach, we conducted further analyses to provide additional confirmation of cyclization extent. Examination of the ratio of apparent peak molecular weights (MWp) yielded MWp,cyclic/MWp,linear = 0.77, which is close to the expected ratio (Supporting Information Table S1).38 Additionally, analysis of the ratio of intrinsic viscosities at the peak maximum ([η]p) resulted in [η]p,cyclic/ [η]p,linear = 0.67, which further supports the formation of a more compact cyclic structure.39 These results demonstrated the successful synthesis of BPs with targeted molecular weight (Mn = 32 000 g/mol) and narrow dispersity (Mw/Mn = 1.05 and 1.10 for the linear and cyclic BPs, respectively). Thin films of linear and cyclic PS−POEGMA doped with varying amounts of lithium triflate salt were cast on silicon substrates by flow coating.33 The salt concentrations used were [EO]:[Li] = 48:1, 24:1, 12:1, and 6:1, for which [EO]:[Li] is the ratio of the molar concentration of oxygen in the ethylene oxide side chain of the polymer to the molar concentration of lithium ions. XRR was employed to characterize the domain spacing and interfacial width of the thin film samples. Fits of the XRR profiles to multilayer models, plotted in Figure 2a,b, indicated that the majority of the films formed parallel lamellae; the neat (no salt) cyclic film appeared disordered, and its XRR profile could not be fit with a multilayer model. The detailed layer thickness, density, and roughness information for all model fits is located in Table S2. AFM also was conducted on the films (shown in Figure S2) and supported the morphological assignments, as imaging the cyclic neat film did not show an identifiable ordered structure, whereas imaging all other films produced featureless micrographs indicative of parallel lamellae. The lamellar films displayed asymmetric wetting as reported previously;40 a half-layer of POEGMA wet the silicon substrate due to preferential interactions between the oxygens in the POEGMA side chains and the hydroxyl-

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RESULTS AND DISCUSSION Sequential ATRP reactions were performed using trimethylsilyl-protected alkyne-functional ATRP initiator to yield PS− POEGMA with controlled molecular weight (Supporting Information Scheme S1). On the basis of 1H NMR spectroscopy analysis, the weight fraction of PS block was 0.63 (Supporting Information Figure S1). Successive chain-end functionalization with sodium azide and tetrabutylammonium fluoride afforded the telechelic linear BPs. Macrocyclization via copper-catalyzed azide−alkyne cycloaddition enabled the preparation of the cyclic BPs (Figure 1a). High dilution of the reactive BPs was achieved by slow addition of reactive BPs into a solution of the copper catalyst to favor intramolecular cyclization over intermolecular oligomerization. GPC analysis showed a shift to a longer retention time for the cyclic sample, indicating a reduction in the hydrodynamic volume upon cyclization. A small higher molecular weight shoulder was noted after cyclization (Figure 1b), which could be attributed to a cyclic or linear dimer impurity formed during the macrocyclization process. Peak fitting and deconvolution of the GPC trace indicated the purity of the cyclized product was ∼95 wt %, assuming complete conversion of the linear diblock precursor to form either cyclized diblock polymers or dimer impurities. We believe our assumption is reasonable; however, due to any D

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Macromolecules terminated silicon substrate, and a half-layer of PS formed at the free surface with alternating lamellae of PS and POEGMA in between. The domain spacings (L0)s of the PS−POEGMA specimens obtained from the fits of the XRR profiles and through MD simulations are shown in Figures 2c and 2d, respectively. The domain spacing of the cyclic PS−POEGMA was 18%−21% lower than the linear PS−POEGMA domain spacing for all salt concentrations, as measured via XRR. This difference is slightly lower than the results obtained by Hawker and co-workers,12 potentially due to the lower molecular weight used in their study (as shorter chains have limited ability to stretch) or the effects of architectural differences between PEO and POEGMA monomers, as the side chains on the POEGMA monomers may prevent the cyclic PS−POEGMA used in our work from collapsing to the same degree as the cyclic PS−PEO. The L0 for both the linear and cyclic PS−POEGMA BPs increased as salt was added due to swelling of the POEGMA domain; however, this increase in L0 was minimal above salt concentrations of Csalt = 0.1 for the linear polymers and Csalt = 0.05 for the cyclic polymers. This behavior indicated that the topological constraints introduced by the cyclic architecture limit the ability of cyclic BPs to swell with changes in salt content. In other words, cyclic BPs are less sensitive than linear polymers to changes in χeffN. MD simulations agreed qualitatively with the XRR results, as the domain spacing increased as χ increased for both polymers; the cyclic polymers exhibited 37.1%−38.5% lower domain spacing and less sensitivity to χ than the linear polymers. The high-resolution information in domain density profiles obtained from XRR also enables the determination of interfacial widths between the PS and POEGMA domains for both the linear and cyclic BPs. The interfacial width (tint) was calculated as tint = (2π)1/2δ,41,42 in which δ is the mean of the roughness parameter obtained from the Globalfit XRR software for each layer in the film. Because the domain spacing (L0) was different for each sample in our study (adding salt swells the POEGMA domain), we compared relative interfacial widths by calculating the volume fraction of the interface, f int. This parameter can be interpreted as the fraction of each lamellar period that exists within the interfacial region between PS and POEGMA domains: fint =

2t int 2(2π )1/2 δ = L0 L0

Figure 3. Volume fraction of interface ( f int) (a) as a function of lithium triflate salt concentration (Csalt) from XRR on thin film samples and (b) as a function of χ from MD simulations. Linear PS− POEGMA data are plotted with closed diamonds, and cyclic PS− POEGMA data are plotted with open circles. Error bars in (a) are propagated uncertainties in the f int calculation based on the standard deviation of the layer thickness and roughness parameters obtained for the PS−POEGMA lamellae in the films.

(4)

To compute an analogous quantity in the MD simulations, a local volume fraction profile (ϕA) was calculated from number density profiles (ρA) such that ϕA(z) =

ρA (z) ρA (z) + ρB (z)

for all considered χ, and the difference in f int between linear and cyclic BP analogues decreased as χ increased. By visual inspection of the MD snapshots (Figure 4), we found that the cyclic BPs had relatively long-ranged waviness of the interface which contributed to their increased interfacial width. Furthermore, we performed additional simulations to investigate the effect of a linker located at one contact point between the two types of monomer segments to gauge the effect of the linker chemistry on the domain characteristics (L0 and f int). In these simulations, a neutral bead of type C was inserted between the A and B blocks on the cyclic chains; the presence of this additional bead at the A−B interface had a minimal effect on the trends in L0 and f int reported herein, as discussed in detail in Supporting Information section 3. The composition of the films as a function of position across the PS−POEGMA lamellae were obtained from the XRR fits;

, and the f int

was obtained by fitting the averaged ϕA(z) of half a lamellar layer to α + β tanh(−4(z/L0 − z0)/f int), in which α, β, and z0 were adjustable parameters, and the factor of 4 was used to define the interfacial region approximately at around 10% to 90% of ϕA(z). The volume fraction of the interface from XRR is plotted in Figure 3a and from MD simulation in Figure 3b. As determined by XRR, f int decreased as salt concentration increased until plateauing at f int = ∼0.4. Notably, the cyclic polymers had slightly higher f int than the linear polymers below Csalt = 0.05, but as salt concentration increased, f int became approximately equal for the linear and cyclic polymers. The MD results also show that the cyclic BPs had wider interfaces than linear BPs E

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Figure 4. Snapshots from MD simulations of the equilibrated systems of linear and cyclic block polymers with typical chain conformations for χ = (a) 0.70, (b) 1.41, (c) 2.81, and (d) 4.92. Each individual chain is smoothed by averaging over 21 snapshots that were 115τ apart.

Figure 5. Domain composition profiles (volume fraction POEGMA or A, ϕPOEGMA or ϕA) as a function of location (z) across an average lamellar period for (a, c) linear and (b, d) cyclic PS−POEGMA obtained from (a, b) XRR and (c, d) MD simulations. The center of the POEGMA or A domain is at z = 0, with each curve extending to z = ±L0/2. The color of the profiles corresponds to either salt-doping level or χ: neat or χ = 0.42 (black); [EO]:[Li] = 48:1 or χ = 0.70 (blue); 24:1 or χ = 1.06 (green); 12:1 or χ = 1.41 (red); 6:1 (goldenrod). The profile for the neat (or χ = 0.42) cyclic film is not included in (b, d) because this film appeared disordered. Note the various χ values are chosen to show a range of composition profile behavior, not to quantitatively match χeffN of the salt-doped systems.

of the polymers used in this study was relatively low, the neat films exhibited mixing between the PS and POEGMA. As the salt content (and thus χeffN) increased, the POEGMA domains swelled and the interfaces sharpened as indicated by the larger

these composition profiles (volume fraction POEGMA, ϕPOEGMA, averaged over several lamellar periods) are plotted in Figure 5a,b. Composition profiles centered on the PS domain are plotted in Figure S3. Because the molecular weight F

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BP phase diagram efficiently with a single synthetic sample, and XRR provides high-resolution and nondestructive information about domain composition and interfacial properties in BP films of different architectures. This salt-doping strategy is particularly useful to explore the phase behavior of exotic chain architectures that may require complex and time-consuming synthesis and mitigates issues with sample-to-sample variability. Furthermore, these results underscore the potential of cyclic BPs to decrease feature size in the next generation of lithographic templates.

slopes of the interfacial composition profile as a function of salt doping level. A comparison of the purity (maximum value of the composition profile) of the PS and POEGMA domains between the linear and cyclic architectures is plotted in Figure S4. At low salt concentrations, the domains in the cyclic specimens were less pure than their linear counterparts; however, this difference decreased as the salt concentration increased, matching the volume of interface trends shown in Figure 3a. For comparison, composition profiles from MD simulations are shown in Figure 5c,d as a function of χ. We note that the profiles for the A-rich and B-rich domains were identical in the simulations because the model polymers were symmetric. At the lowest considered χ (χ = 0.42), linear BPs maintained a lamellar structure, whereas cyclic BPs became disordered despite being initialized in a lamellar structure. At χ values which resulted in microphase separation in both the linear and cyclic systems, linear BPs clearly showed sharper interfaces and flattened profiles relative to cyclic BPs. This result agrees well with the trends shown regarding interfacial width in Figure 3b. Taken together, the XRR and MD results indicate that the behavior induced experimentally through the introduction of lithium salts (increased domain spacing and reduced interfacial width) can be mimicked in MD simulations without the explicit inclusion of salt species simply by tuning the segregation strength. This aspect demonstrates key evidence that salt doping can be used to explore the phase behavior of BPs of both linear and cyclic architectures without having to synthesize a library of polymers with systematic variations in segregation strength (or block chemistry). Thus, salt doping may represent a significant time savings when studying synthetically challenging macromolecules, and it permits valuable comparisons between different architectures and segregation strengths with decreased sample variability issues such as changes in dispersity and molecular weight or the introduction of impurities. Furthermore, analyzing salt-doped BP materials in the thin film geometry allows us to leverage reflectivity techniques to probe the interfacial properties of these systems, capitalizing on this facile platform to expand upon our knowledge of the thin film phase behavior of BPs with alternative architectures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01338. Polymer synthesis and purification procedure, apparent molecular weights and intrinsic viscosity values at peak maximum, detailed XRR layer data, AFM images of select films, domain composition profiles centered on PS domain, domain purity as a function of salt content, and additional MD simulation results for cyclic chains with a linker bead (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (L.M.H.). *E-mail [email protected]fl.edu (B.S.S.). *E-mail [email protected] (T.H.E.). ORCID

Thomas E. Gartner III: 0000-0003-0815-1930 Lisa M. Hall: 0000-0002-3430-3494 Brent S. Sumerlin: 0000-0001-5749-5444 Thomas H. Epps III: 0000-0002-2513-0966 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.H.E. and T.E.G. thank the National Science Foundation (NSF) DMR-1610134 for financial support for thin film preparation and nanostructure characterization. T.H.E. also thanks the Thomas & Kipp Gutshall Professorship for financial support. Materials synthesis and characterization efforts were supported by an NSF DMR-1606410 award to B.S.S. L.M.H. and Y.S. thank NSF DMR-1454343 for financial support and the Ohio Supercomputer Center for computing resources. We also acknowledge the W.M. Keck Electron Microscopy Facility and the NMR Core Facility at the University of Delaware for use of the AFM and NMR instruments, respectively, which are partially supported by the Delaware COBRE program with grant NIGMS (5 P30 GM110758-02) from the National Institutes of Health. We thank the University of Delaware Advanced Materials Characterization Lab for the use of the XRR instrument.

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CONCLUSION In summary, we explored changes in domain spacing and interfacial width of lamellar-forming linear and cyclic BPs in thin film geometries upon doping with lithium salts. Efficient cyclization of the telechelic linear BP via click chemistry was exploited to synthesize the cyclic BP, and the success of the macrocyclization was corroborated by GPC and viscometry analyses. Cyclic PS−POEGMA thin films showed an ∼20% decrease in L0 relative to linear PS−POEGMA thin films across the range of compositions studied and demonstrated reduced swelling with increasing χeffN. Additionally, we report the first experimental analysis of the interfacial widths of cyclic BPs as a function of χeffN. Cyclic BP films had slightly wider interfaces with more interfacial mixing than their linear counterparts, but this difference was mitigated at high salt concentrations (or high χeffN). These trends obtained from XRR agreed qualitatively with MD simulations; the effects induced experimentally by increasing salt content were replicated in simulations without the explicit inclusion of salt species by directly increasing the χ between polymer blocks. Hence, this work demonstrates that salt doping can be used to explore the

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DOI: 10.1021/acs.macromol.7b01338 Macromolecules XXXX, XXX, XXX−XXX