Log-Rolling Block Copolymer Cylinders · 2016. 11. 21. · Log-Rolling Block Copolymer Cylinders Ye Chan Kim,†,⊥ Dong Hyup Kim,†,⊥ Se Hun Joo,† Na Kyung Kwon,† Tae Joo

Log-Rolling Block Copolymer CylindersYe Chan Kim,†,⊥ Dong Hyup Kim,†,⊥ Se Hun Joo,† Na Kyung Kwon,† Tae Joo Shin,§

Richard A. Register,∥ Sang Kyu Kwak,† and So Youn Kim*,†

†School of Energy and Chemical Engineering and §UNIST Central Research Facilities & School of Natural Science, Ulsan NationalInstitute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 44919, Republic of Korea∥Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States

*S Supporting Information

ABSTRACT: Shear is an effective method to create long-rangeorder in micro- or nanostructured soft materials. When simpleshear flow is applied, particles or polymer microdomains tend toalign in the shear direction to minimize viscous dissipation; thus,transverse alignment (so-called log-rolling) is not typically favored.This is the first study to report the transverse alignment ofcylinder-forming coil−coil block copolymers. Poly(styrene-b-methyl methacrylate), PS−PMMA, where the PS blocks formthe matrix, can adopt a metastable PMMA hemicylindricalstructure when confined in a thin film, and this hemicylindricalstructure can orient either along the shear direction or transverseto the shear direction depending on the shearing temperature. Amonolayer of PS−PMMA forming full cylinders exhibits log-rolling alignment. This unusual log-rolling behavior is explained bythe low chain mobility of the cylinder-forming PMMA block at low temperatures, which is the critical quantity determining thedirection of shear alignment.

■ INTRODUCTIONSelf-assembly is of particular interest in soft matter physics, andthe nanoscale patterning of soft materials such as polymers orordered colloids has been widely studied and employed inmany applications.1,2 Among the various methods for creatinglong-range order in materials, shear is universal, effective, andreadily approachable. Liquid crystals,3−5 carbon nanotubes,6,7

and nanoparticles8−10 can all be aligned using shear.The alignment of block copolymers (BCPs) using flow fields

was first demonstrated by Keller et al.11 Early studiesinvestigated the structural changes in BCPs in bulk underunidirectional or oscillatory shear.12−14 More recently, shearalignment has been applied to BCPs in thin films, successfullycreating long-range order. In these experiments, shear wasapplied to BCP thin films via cross-linked poly(dimethyl-siloxane) (PDMS) pads mechanically,15−17 by thermalexpansion18,19 or solvent swelling20,21 of the overlying PDMSpad, or laser heating.1,19,22 Lamellae, cylinders, and spheres allhave been reported to align in shear.In principle, two macroscale alignments are possible for

cylinder-forming BCPs in thin films, both with the cylindersparallel to the substrate (Figure 1): parallel and transverse. Inlamella-forming BCPs, transverse alignment is rarely reportedsince the domain spacing is disturbed by shear.23 Transverselyaligned cylinders, which roll along the shearing direction (“log-rolling”, Figure 1b), are thermodynamically unfavorablecompared to those with parallel alignment due to the increased

viscous dissipation and chain mixing24 when shear stress isstrong enough to neglect other parameters such as normalstress. Molecular dynamics simulations conducted by Arya andPanagiotopoulos25 predicted log-rolling cylindrical micelleswhen the micelles couple strongly with the confining surfaces.The authors suggested that chain entanglements, present in thehigh-molecular-weight BCPs typically employed in experi-ments, would hinder log-rolling.25 Subsequently, Chremos etal. studied the shear alignment of cylinder-forming BCPs withcoarse-grained Langevin dynamics simulations; the authorsreported a transition from parallel to log-rolling alignment assegregation strength increased.26

Experimentally, all reported BCPs with simple unstructured,random-coil blocks (“coil−coil” block copolymers) alignparallel to the shear direction. In bulk, there are twodistinguishable types of parallel alignment, which correspondto having either the (10) or (11) planes of the hexagonalmacrolattice lying in the plane of shear;24 in thin films,containing only one or a few layers of cylinders, only the (10)orientation is observed.27 The sole report of transverse cylinderalignment is in a BCP where the matrix block forms a smecticliquid crystal, and in which the mesogens exhibit thehomogeneous boundary condition at the cylinder−matrix

Received: November 21, 2016Revised: April 12, 2017Published: April 20, 2017

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interface. In these materials, transverse cylinder alignmentallows for a constant smectic layer spacing during shear.28 Tanget al.29,30 reported transverse alignment of lameallar domainsrelative to the flow direction during a zone-casting process,resulting from directional solvent evaporation and consequentmicrophase separation along the casting direction.29 Theyfurther reported that the alignment direction could be changedto perpendicular when one of the blocks crystallizes duringzone-casting. In these cases, the orientation is not governeddirectly by shear, but rather by solvent evaporation or blockcrystallization during the casting process.While the log-rolling orientation has not been obtained by

shearing coil−coil BCPs, log-rolling objects are often observedin other types of soft matter, such as liquid crystals,31 ellipticalparticles,32 strings of particles,33 and emulsions.34 In theseexamples, the particles act as rigid objects with high aspectratio. Although these are intriguing observations, the conditionsrequired to obtain the log-rolling orientation in coil−coil BCPsby shearingif possible at allremain a mystery.This study provides the first demonstration of log-rolling

(transversely aligned) coil−coil BCPs via melt shearing, andinvestigate the shear alignment mechanism. We show thatcylinder-forming polystyrene-b-poly(methyl methacrylate)(PS−PMMA) can be aligned parallel or transverse to theshearing direction depending on the shearing conditions. PS−PMMA is a readily synthesized and processable polymer; thus,it has been employed in pioneering studies on nanopattern-ing35−39 as well as in many applications,40−43 and many studieshave characterized its behavior. Intriguingly, cylinder-formingPS−PMMA can form hemicylinders42,44 parallel to thesubstrate due to the similarity between the surface energies ofPS and PMMA at certain temperatures.45−47 PS and PMMAare known to have similar glass transition temperatures(Tg)

48,49 and a relatively low polymer interaction parameter(χ), which is only weakly temperature dependent.50

■ METHODSSample Preparation. All polymers were purchased from Polymer

Source, Inc.: BCPs PS−PMMA 64K−35K, 24K−12K, and 26K−68K,with number-average molecular weights (Mn) of 99 000 g/mol(dispersity Đ = 1.09), 35 500 g/mol (Đ = 1.06), and 94 400 g/mol(Đ = 1.18), respectively, and hydroxy-terminated PS homopolymer(Mn = 9000 g/mol, Đ = 1.03). ⟨100⟩ Si wafers (purchased from SiliconQuest International or Waferbiz) were employed as substrates; thewafer surfaces were rinsed with toluene before spin-coating and had a2.2 nm thick native oxide layer. A PS-preferential substrate wasprepared by spin-coating hydroxy-terminated PS homopolymer at 25nm thickness and annealing for 24 h at 160 °C in a vacuum oven.Ungrafted polymer was removed by repeated rinsing with toluene,yielding a final grafted thickness of 5 nm. BCP films of uniform

thickness were deposited by spin-coating, with the thickness controlledthrough the spin speed and solution concentration, and measuredusing a spectroscopic ellipsometer (J.A. Woollam Co., M-2000V).Films with thickness gradients were prepared by flowcoating; a bead ofpolymer solution was placed between a blade and the substrate, andthe substrate was moved with a programmed accelerationprofile.27,51,52 The local thicknesses of the flow-coated samples weremeasured using small-spot ellipsometry at 632.8 nm (GaertnerScientific LS116S300).

Shear Alignment. Shear alignment using cross-linked PDMS padswas performed on a hot plate under an applied lateral force.15−17,27

PDMS sheets were prepared from Sylgard 184 (Dow Corning) at a10:1 ratio of base to curing agent and baked at 60 °C for 24 h. PDMSpads with dimensions of 1.2 × 1.2 cm2 were cut from these sheets andpressed against the supported BCP thin film for shear alignment. Shearstress was controlled by tuning the lateral force (Fl) and was calculatedbased on the area of the PDMS pad (S) as Fl/S. The shear was givenfor 30 min, and the PDMS pad moved more than 100 μm in 30 min.

Plasma Treatment. To obtain clear SEM and AFM images of PS−PMMA 24K−12K, some of the samples were softly etched withoxygen plasma (Harrick Scientific PDC-32G-2 plasma cleaner). Thesamples were treated for 40−50 s at a high RF power level (18 W)with an oxygen flow rate of 14 sccm under 300 mTorr.

Atomic Force Microscopy. Atomic force microscopy (AFM;Veeco Dimension 3000 and 3100) was used in tapping mode withBruker tips (RTESP), which have a drive frequency of 300 kHz, aspring constant of 40 N/m, a cantilever length of 125 μm, and a tipradius of 8 nm.

SEM. Before imaging by SEM (Hitachi S-4800 field emissionscanning electron microscope, high vacuum, 10 keV), the BCP thinfilms were exposed to the saturated vapor from 0.5 wt % rutheniumtetroxide (RuO4) aqueous solution for 30 min to stain the PMMAdomains and thereby increase the interdomain contrast. Cross-sectional images were obtained by scoring the back side of the waferusing a diamond scribe, perpendicular to the direction of alignment,and fracturing the wafer by flexing.

ToF-SIMS. Depth profiles were obtained using ToF-SIMS (IONTOF, Germany), with a 25 keV Bi3

+ analysis beam and a 0.25 keV Cssputtering beam. The raster sizes for analysis and sputtering were 90and 300 μm, respectively. The target currents of the analysis andsputtering beams were 0.4 pA and 11.0 nA, respectively. The ToF-SIMS instrument was operated in the noninterlaced mode, andnegative secondary ions were detected to analyze the compositionalong the out-of-plane direction. All of the samples were cut to 1 × 1cm2.

GI-SAXS. GI-SAXS measurements were performed at the 6DUNIST-PAL beamline of the Pohang Accelerator Laboratory in Korea.The energy of the X-rays was 10.0 keV (wavelength, λ = 1.2398 Å), theincident angle was 0.14°, and the sample-to-detector distance was3527 mm. Scattering patterns were collected using a 2D CCD detector(MX225-HS, Rayonix L.L.C., USA). Samples were measured with theincident beam parallel to the cylinder alignment direction observed byAFM.

Figure 1. Schematic of the alignment modes of cylinder-forming BCPs. The scheme shows two possible alignments of cylinders in thin films: (a)parallel and (b) transverse (i.e., log-rolling). The shearing direction is indicated in each figure.

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Nonequilibrium Molecular Dynamics. Nonequilibrium molec-ular dynamics (NEMD) simulations were performed using theLangevin thermostat.53 Bead−spring models representing PS (16beads) and PMMA (8 beads) were constructed by applying WCA andWCA-MM potentials, respectively (see Supporting Information forNEMD). A total of 1179 BCP chains were packed in a box with athickness of 10 times the bead size (σ). The top and bottom walls weremoved in opposite directions at velocities in the range of 0.01−0.2 σ/τ,where τ is the unit of time. NEMD simulations in the canonicalensemble (i.e., constant temperature) were run for 2 × 107 steps with atime step of 0.005τ at a temperature of kBT = ε.

■ RESULTS AND DISCUSSIONPS−PMMA 64K−35K with PS and PMMA blocks havingnumber-average molecular weights of 64 and 35 kg/mol,respectively, was annealed for 23 h at 170 °C under vacuum tocharacterize the unsheared film structure. PS−PMMA 64K−35K exhibits asymmetric wetting behavior on an untreated Siwafer substrate (Figure S1); the equilibrium thicknesses atwhich uniform films did not terrace were approximately t = (n− 1/2)L0, where t is the film thickness, n is a positive integer,and L0 is the cylinder interlayer spacing.

54 For PS−PMMA64K−35K, the wetting layer thickness was determined to be∼19 nm and L0 was ∼42 nm. A wetting layer naturally formswhen the major block (PS here) is not favored to be at thesubstrate; the minor block is preferentially attracted to thesubstrate, forming a layer with a thickness of ∼0.5L0 (Figure2e). For this reason, films with 0.5 < t/L0 < 1.5 generallyterraced into regions with a local thickness of either 0.5L0 or1.5L0, typically with a micrometer-scale lateral length. However,we observed that films of PS−PMMA 64K−35K at t = 38 nmdid not terrace after 23 h of annealing, but instead remaineduniform, while films at other thicknesses (t ≠ (n − 1/2)L0)formed islands, holes, or bicontinuous structures (Figure S1).Furthermore, there was an abrupt transition from holes at 36nm, to a uniform film at 38 nm, to islands at 40 nm (Figure S1).When the film with t = 38 nm was annealed for a longer period

(83 h) under the same conditions, the film eventually terracedand showed a bicontinuous structure (Figure 2c). The filmstructure was confirmed by time-of-flight secondary ion massspectrometry (ToF-SIMS). In the SIMS depth profiles shownin Figure 2d, the depth distributions of PS and PMMA arerepresented by C6H

− and CH3O− ions, respectively.55 The

SIMS results revealed the presence of PMMA at the polymer−vacuum interface (film surface) and a wetting layer at thepolymer−substrate interface (Figure S9). AFM images (Figure2b) indicated that the PMMA at the surface was present not asa uniform layer, but as stripes, characteristic of the hemi-cylindrical structure observed previously.42,44

We also confirmed that hemicylinders formed over a widerange of film thicknesses (25−40 nm) under mild annealingconditions (below 150 °C for a couple of hours; Figure S9),though the hemicylinder structure eventually transitioned to aterraced film comprising a wetting layer at ∼0.5L0 and a full-cylinder structure at ∼1.5L0 (Figure 2c). We further examinedthe thin-film structure of another diblock, PS−PMMA 24K−12K, which has PS and PMMA block number-averagemolecular weights of 24 and 11.5 kg/mol, respectively. ThisBCP has a PMMA fraction similar to that of PS−PMMA 64K−35K and, thus, also forms PMMA cylinders. From the annealingand ToF-SIMS experiments (Figures S2 and S9), we confirmedthat PS−PMMA 24K−12K also has a thickness (24 nm) atwhich it forms metastable hemicylinders.We aligned PS−PMMA 64K−35K at 34−38 nm film

thickness by applying different shear stresses over an area of1.2 × 1.2 cm2 with a cross-linked PDMS pad.15 The shearingtemperature was 140 °C (Figure 3a) or 150 °C (Figure S6),and shear was applied for 30 min. Surprisingly, as shown inFigure 3a, the hemicylinders aligned transverse to the sheardirection (indicated by the arrow in Figure 3). Similarly, thehemicylinders in PS−PMMA 24K−12K aligned transverse tothe shear direction at 130 °C (Figure 3b). The higher theapplied stress, the greater the extent of alignment. To quantify

Figure 2. Metastable film structure of PS−PMMA 64K−35K hemicylinders for a film thickness of 38 nm. (a) AFM height image after 23 h ofannealing at 170 °C. (b) Enlarged image of (a). (c) Bicontinuous terrace structure after 60 h of additional annealing; the height trace taken along thehorizontal black line is shown as an inset. (d) ToF-SIMS depth profile of the film annealed at 150 °C for 24 h. (e) Structure of the metastable film.

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the alignment quality, Hermans orientation parameter, S, was

calculated for both films where S is defined as = θ⟨ ⟩ −S 3 cos 12

2

.56

Here θ is the azimuthal angle obtained from the FFT images inF i g u r e 3 , a n d ⟨ c o s 2 θ ⟩ i s c a l c u l a t e d a s

θ⟨ ⟩ = θ θ θθ θ

∑

∑= °

°

= °°cos

I

I2 ( ) sin cos

( ) sinI

I

090 2

090 . When S has a value of 1, −0.5,

or 0, the cylinders are aligned perfectly parallel, perfectlyperpendicular, or randomly to the shearing direction,respectively. These sheared films retained their hemicylindricalstructure, as confirmed by ToF-SIMS depth profiles and cross-sectional SEM images (Figure 4a).Interestingly, when the shearing temperature was increased

to 180 °C, the cylinders aligned parallel to the shear direction,

as indicated by the grazing-incidence small-angle X-rayscattering (GI-SAXS) pattern (Figure 4b). PS−PMMA 24K−12K showed the same trend: the transverse alignment of thehemicylinders gave way to parallel alignment with increasingtemperature. However, the transition temperature for PS−PMMA 24K−12K was 150 °C, lower than that for PS−PMMA64K−35K (see Figure S3). The alignment quality of parallelalignment (S = 0.91) is better than that of transverse alignment(S = −0.40).Because the hemicylinder structure is only metastable, one

might ask whether the transverse alignment is transient, withthe alignment eventually shifting to be along the sheardirection. To show that parallel alignment is not favoredunder these conditions, after shearing at 140 °C to obtain

Figure 3. AFM height images of (a) PS−PMMA 64K−35K with a thickness of 34 nm sheared at 140 °C and (b) PS−PMMA 24K−12K with athickness of 23 nm sheared at 130 °C. Arrows indicate the direction of shear; fast Fourier transforms (FFTs) are given in the upper right cornerswith calculated orientation parameters (S). The AFM images are (a) 2 × 2 μm2 and (b) 1 × 1 μm2. Scale bars are 200 nm.

Figure 4. Temperature-dependent shear alignment of PS−PMMA 64K−35K hemicylinders (thickness = 38 nm) at (a) 140 °C and (b) 180 °C. TheToF-SIMS profile, GI-SAXS 2D pattern, AFM height image with FFT (and orientation parameter), and SEM image are given in each row from leftto right. A cross-sectional SEM image is provided in the upper right corner showing the PMMA hemicylinders (an enlarged image is available inFigure S12). Arrows indicate the direction of shear. Unlabeled scale bars are 500 nm. The beam was aligned parallel to the alignment direction in GI-SAXS measurements. The GI-SAXS 1D line profiles are given in Figure S10c.

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transversely aligned hemicylinders, a second shear was appliedorthogonally at 140 °C. If parallel alignment were favored atsteady state, no reorientation should occur during the secondshear; however, the hemicylinders rearranged to be transverselyaligned with the second shear direction, indicating thattransverse alignment is energetically favored under theseconditions (Figure S4).Experiments were also performed with a PS-preferential

substrate, achieved by grafting 5 nm of hydroxy-terminated PShomopolymer (Mn = 9000 g/mol) to the Si wafer substrateprior to BCP spin-coating. PS−PMMA 64K−35K, 26 nm thick(∼0.5L0), forms hemicylinders on this PS-grafted substrate andshows the same temperature-dependent shear alignment asdoes a 38 nm thick film on an ungrafted (PMMA-preferential)

wafer (Figure S5). Thus, the observed transverse alignment isnot a substrate-specific phenomenon.For PS−PMMA 64K−35K, the bulk Tg values of the PS and

PMMA domains reported by the manufacturer are 110 and 124°C, respectively. The PMMA blocks, which form the cylindersin PS−PMMA 64K−35K, have a high segmental frictionfactor57 and therefore a low chain mobility at 140 °C, even inbulk. In thin films, Tg can be modulated by polymer−surfaceand polymer−substrate interactions, with the Tg of PS lower inthin films than in the bulk,49,58 while the Tg in PMMA thinfilms can either increase or decrease due to substrateinteractions.48,49,58−63 Therefore, we expect that this differencein segmental mobility between PS and PMMA will bepreserved, or even enhanced, in thin films. We note that thelower-molecular-weight PS−PMMA 24K−12K shows trans-

Figure 5. Stepwise annealing results for PS−PMMA 64K−35K hemicylinders (thickness = 35 nm). (a) GI-SAXS 2D images taken every 10 °C from140 to 190 °C (enlarged 2D data are given in Figure S10a). (b) AFM height images, 2 × 2 μm2, of samples annealed for 30 min at 140 °C (left) and180 °C (right). Scale bars are 500 nm. (c) GI-SAXS 1D profiles extracted from (a). (d) Intercylinder spacings obtained from GI-SAXS 1D profiles.

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verse alignment only at temperatures lower than where PS−PMMA 64K−35K shows transverse alignment (Figure 3).Indeed, the bulk Tg values obtained from the manufacturer forPS−PMMA 24K−12K are 99 °C for the PS domains and 107°C for the PMMA domains, which are 11 and 17 °C lower thanthe values for PS−PMMA 64K−35K, respectively. Thecombination of lower Tg and shorter block length thus allowsPS−PMMA 24K−12K to transition to parallel alignment at alower temperature than PS−PMMA 64K−35K.We performed stepwise annealing experiments with PS−

PMMA 64K−35K at the hemicylinder thickness, wheretemperature was increased from 140 to 190 °C and GI-SAXSmeasurements were conducted every 10 °C. Each targetedtemperature was reached in 1 min and held for 30 min beforeeach measurement. Figure 5c shows the GI-SAXS 1D profilesextracted from the GI-SAXS 2D images in Figure 5a. Astemperature increased, the domain spacing calculated from thepeak position also increased. As mentioned above, the 64K−35K film formed metastable hemicylinders at 170 °C butterraced rapidly at 190 °C; thus, the reduced peak heights attemperatures above 160 °C were attributed to the macroscopicterracing of the films. We also found similar results for PS−PMMA 24K−12K (see Figure S11). Domain spacing (d)commonly decreases with increasing temperature, as usually theFlory−Huggins parameter χ is inversely related to temperature(T). For example, Hashimoto et al.64 reported that the lamellardomain spacing in neat PS-b-polyisoprene scales as d ∼ (1/T)1/3. For PS−PMMA, the change of d with T is much weaker,

due to the much weaker dependence50 of χ on T; nonetheless,d still decreases slightly as T is increased.65

The results of Figure 5 clearly show that the microdomainstructure in these films is not at equilibrium, particularly attemperatures below 170 °Canother reflection of low chainmobility. AFM also confirms the apparent increase of d with T:films annealed at 180 °C (vs 140 °C) for 30 mina somewhatdifferent thermal history than that employed for GI-SAXSshowed an increase of 9 nm in d. When shear was applied at180 °C (parallel alignment), the d-spacing was not significantlydifferent from that of the annealed but unsheared film.However, the d-spacing after shearing at 140 °C (transversealignment) was 13 nm larger than that for the unshearedsample at 140 °C, indicating that the process of shearingimparts sufficient mobility to the chains for the domain spacingto change. However, the low mobility of the PMMA blocksadversely affects the alignment quality in specimens exhibitingtransverse alignment, which is generally poorer than inspecimens exhibiting parallel alignment (compare Figures 4aand 4b). To further probe the influence of PMMA blockmobility, the temperature dependence of shear alignment wasalso examined in a third BCP, PS−PMMA 26K−68K, in whichPMMA is the major block (Figure S8). In this case, noalignment was observed after shearing at 150 °C, due to the lowmobility of the PMMA matrix; at 190 °C, where both the PSand PMMA blocks have sufficient mobility, the cylindersaligned in the shear direction, with the high degree oforientation typically observed for parallel alignment.

Figure 6. Temperature- and shear stress-dependent alignment in PS−PMMA 24K−12K full cylinders at 35 nm thickness. A shear stress of 30 kPawas applied at (a) 120, (b) 130, (c) 140, (d) 150, (e) 160, and (f) 170 °C for 90 min. In (d), the shear stress was varied from 10 to 30 kPa asindicated. The shear direction (white arrow) was vertical in all cases. 2D FFTs are shown as insets with calculated orientation parameters; all SEMimage scales are 1 × 1 μm2, and the scale bar indicates 200 nm.

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A PS−PMMA 64K−35K thickness-gradient film, withthickness ranging from 20 to 60 nm, was prepared via flowcoating, and shear was applied at 150 °C for 30 minperpendicular to the thickness gradient. Figure S6 presentspostshear AFM images at various thicknesses; transverselyaligned BCPs were observed for film thicknesses ranging from25 to 40 nm, where metastable hemicylinders are formed,whereas no alignment was observed at other thicknesses. In allof the transversely aligned films, PMMA hemicylinders werepresent at the polymer/PDMS interface, as confirmed by ToF-SIMS (Figure 4a and Figures S9b,c,g).To achieve transverse alignment in films containing a layer of

full PMMA cylinders (vs hemicylinders), higher shear stresseswere required. Figure 6 shows SEM images of a 35 nm film ofPS−PMMA 24K−12K, containing a layer of full PMMAcylinders, after 90 min of shearing at various temperatures andstresses. While 10 kPa of shear stress is insufficient to producediscernible alignment, 30 kPa yields transverse alignment,observed over the whole range of shearing temperaturesemployed (120−170 °C). A transition to parallel alignment isexpected to occur at sufficiently high temperatures (evidently>170 °C), while this transition occurred at 150 °C in the samepolymer at the hemicylinder film thickness of 23 nm (FigureS3). For the higher-molecular-weight PS−PMMA 64K−35K,the alignment quality was poorer, but partial transversealignment was observed at 170 °C and 30 kPa (Figure S7e).The transition appeared to be abrupt; no intermediatealignment (e.g., absence of preferred orientation) was foundat temperatures between those yielding transverse and parallelalignments. We also summarize the shear alignment directionsobtained from all experiments in Figure 7.

As mentioned in the Introduction, block copolymer domainstend to orient along the shear direction to minimize viscousdissipation and chain mixing between dissimilar blocks.66

However, in the present case, we believe that the observedlog-rolling or transverse alignment is actually favored attemperatures modestly above the Tg of the cylinder-formingPMMA block. In a recent simulation study using Langevindynamics, Chremos and Panagiotopoulos26 predicted a

transition from log-rolling to parallel alignment as segregationstrength is reduced; however, for PS−PMMA, χ depends onlyweakly on temperature,50 so changes in segregation strengthcannot be the source of the temperature-dependent alignmenttransition observed here. Nonetheless, since simulations haveobserved both parallel and transverse alignment, we employedLangevin dynamics (see NEMD in Methods) to simulate theconfined BCPs under shear flow and to observe the behavior ofthe chains in the two alignment modes. Details of the NEMDsimulation method are given in the Supporting Information.Figure 8a (see also Supporting Information Movie 1) shows

parallel alignment with respect to the shear direction at Φ = 1.0.

Note that Φ represents the segregation strength, which iscorrelated with the depth of the potential well. When the twowalls slide in opposite directions, the polymer chains in theupper half of each cylinder and the polymer chains in the lowerhalf of the cylinder experience the same force but in oppositedirections. Figures 8c and 8d show the velocity profiles asfunctions of segregation strength for parallel and log-rollingalignments, respectively. The dilemma for cylinders orientedalong the shear direction is that they experience a significantvelocity difference between their upper and lower halves.Therefore, there is continuous interchain mixing inside the

Figure 7. Summary of shear alignment transitions of PS−PMMA thinfilms with temperature. Red circles, blue triangles, and green squaresrepresent transverse (log-rolling), parallel alignment, and no align-ment, respectively. Solid shapes represent confirmed structures,whereas transparent shapes represent expected structures underconditions not explored experimentally.

Figure 8. NEMD simulation snapshots of the (a) parallel and (b)transverse alignments. The black arrow represents the shear direction,vw is the velocity of the wall, and gray and yellow beads represent PS(S) and PMMA (M) segments, respectively. The polymer model isshown at the top of the figure. Average velocity profiles of the yellow(M) beads across the thin films (thickness of 10σ and vw of 0.05σ/τ)for (c) parallel and (d) perpendicular alignment. Note that ε is anenergy parameter that sets the scale of the short-ranged repulsion.

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cylinders when they align along the shear direction. In Movie 1,a few PS chains in the upper (lower) part of the cylinders arecolored in black (pink); the chains with different colors movein opposite directions, and chains occasionally change color asthey move across the centerplane, indicating that the upper andlower parts of the PMMA cylinders mix.On the other hand, at an increased strength of interchain

attraction (i.e., Φ = 1.6) in the cylinders, log-rolling alignmentprevails, as shown in Figure 8b (see Movie 1). The velocityprofiles in Figures 8c,d show that the velocity gradient isnegligible across the cylinders when Φ > 1.6. These resultsclearly demonstrate that a strong attraction between thecylinder-forming blocks restrains their movement duringshear; this lower mobility then favors the log-rollingorientation, as observed experimentally at temperaturesmodestly above the PMMA block Tg. Thus, the principaleffect of increased Φ in the simulations is not its influence onthe interblock segregation strength, but its reduction in thecylinder block mobility. In both orientations, significant slip wasobserved at the walls due to the repulsive nature of the wall−polymer interactions. Simulation results at other segregationstrengths and wall velocities are given in Figure S13.

■ CONCLUSIONSIn this study, we reported temperature-dependent shearalignment of cylinder-forming coil−coil BCPs, including theobservation of transverse, log-rolling alignment. Under shear,the free energy is minimized when the cylinder axes alignparallel to the shear direction, but this orientation requiressufficient mobility of the cylinder-forming blocks. If themobility is insufficient, log-rolling alignment is favored instead.Notably, this unusual log-rolling alignment was observed forthe BCP chemistry (PS−PMMA) which is the most widelyused in directed self-assembly for nanofabrication. This impliesthat understanding the fundamental physics of these copoly-mers (e.g., mobility of the constituent blocks) can be critical indetermining their quality of alignment,67 which has greatimplications for nanopatterning.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.6b02516.

Film structure of PS−PMMA 64K−35K at variousthicknesses; film structure of PS−PMMA 24K−12K atvarious thicknesses; alignment transition of PS−PMMA24K−12K hemicylinders with temperature; alignmenttransition of PS−PMMA 64K−35K hemicylinders with asecond shear; transverse alignment of PS−PMMA 64K−35K on a PS-preferential substrate; shear alignment ofPS−PMMA 64K−35K at various thicknessesm; shearalignment of PS−PMMA 64K−35K full cylinders at 57nm; PS−PMMA 26K−68K cylinders at 36 nm; shearedat different temperatures; ToF-SIMS depth profiling; GI-SAXS results for PS−PMMA 64K−35K; GI-SAXS resultsfor PS−PMMA 24K−12K; cross-sectional SEM image ofPS−PMMA 64K−35K hemicylinders; results of non-equilibrium molecular dynamics simulations at variousconditions; nonequilibrium molecular dynamics(NEMD) simulation details (PDF)

Movie 1: simulation for parallel and transverse alignment(AVI)

■ AUTHOR INFORMATIONCorresponding Author*E-mail [email protected]; Ph +82 52 217 2558(S.Y.K.).ORCIDSe Hun Joo: 0000-0003-4507-150XRichard A. Register: 0000-0002-5223-4306Sang Kyu Kwak: 0000-0002-0332-1534So Youn Kim: 0000-0003-0066-8839Author Contributions⊥Y.C.K. and D.H.K. contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (NRF-2014R1A1A2056774) and Ministry of Science, ICT and FuturePlanning (NRF-2016M3A7B4905624). GI-SAXS experimentsat PLS-II 6D beamline of the Pohang Accelerator Laboratorywere supported in part by UCRF, MSIP and POSTECH.R.A.R. acknowledges financial support from the NationalScience Foundation (MRSEC Program) through the PrincetonCenter for Complex Materials (DMR-1420541). S.K.K.acknowledges financial support from KISTI (C17006) andcomputational support from UNIST-HPC and KISTI (KSC-2016-C2-0003). We gratefully acknowledge Dr. AlexandrosChremos for helpful discussions.

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