Sub-10 nm Self-Assembly of Mesogen-Containing ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Sub-10 nm Self-Assembly of Mesogen-Containing Grafted Macromonomers and Their Bottlebrush Polymers Yekaterina Rokhlenko,† Ken Kawamoto,‡ Jeremiah A. Johnson,*,‡ and Chinedum O. Osuji*,† †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

‡

S Supporting Information *

ABSTRACT: We explore the morphology and phase behavior of branched diblock macromonomers and their polymers. A series of macromonomers was synthesized based on a disubstituted norbornene. The first branch consists of polydimethylsiloxane (PDMS) while the second branch is a quasi-mesogenic structure incorporating one or more cyanobiphenyl (CB) moieties. Bottlebrush polymers with varying degrees of polymerization were prepared by “graftthrough” ring-opening metathesis of the macromonomers. The molecules in the resulting library of macromonomers and bottlebrush polymers self-assemble to form classically observed microphase-separated structures, including spheres, hexagonally packed cylinders, bicontinuous gyroid, and lamellae. The systematic variation of molecular structure, molecular weight of each branch, and degree of polymerization of the polymers results in a diverse set of structures and properties. We report the observation of well-ordered lamellae and cylinders with dspacings as low as 6.1 and 8.0 nm, respectively. The system displays an asymmetric phase diagram, with large deviations from the canonical phase behavior of linear coil−coil diblocks. Hexagonally packed cylinders and lamellae are observed at remarkably small mass fractions of the mesogen-containing block of 0.07 and 0.21, respectively. The samples are highly birefringent, and polarized optical microscopy revealed the formation of well-developed textures in microphase-separated states formed by cooling samples through the order−disorder transition. The textures are reminiscent of the classic fan-like or focal-conic textures observed in small molecule liquid crystal mesophases, highlighting the formation of unusually large and well-ordered grains of the microphase-separated PDMS and CB microdomains. Apparent crystallization of the CB units in systems with two or three CB moieties per monomer results in distortion of the microphase-separated structure. The small d-spacings and large grain sizes observed here highlight the versatility and potential utility of this molecular architecture for designing and engineering new functional materials.

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Cheng et al.,7,8 in liquid crystal dendrons by Ungar and Percec et al.,9,10 and in lyotropic small molecule systems by Mahanthappa et al.11 The effects of molecular architecture on block copolymer (BCP) self-assembly are myriad, as documented in numerous experimental and theoretical studies.1,12 Branched polymers, including miktoarm star polymers,13−15 dendrimers,16 comblike grafted, and bottlebrush BCPs,17−19 have expanded the range of macromolecular architectures from which new materials can be conceived. In these systems, structural, mechanical, and rheological properties can be tuned by altering the branch type (graft or star) as well as the density, number, and length of the branches.20−23 Recent work by Johnson et al. demonstrated the synthesis and subsequent graft-through ringopening metathesis polymerization (ROMP) of norbornene macromonomers functionalized by two immiscible polymer branches.24 The resulting macromolecule is a Janus bottlebrush

INTRODUCTION Macromolecular self-assembly provides a versatile means of creating nanostructured materials with a diverse range of useful properties.1 The canonical case of high molecular weight (i.e., large degree of polymerization, N) near-monodisperse linear diblock copolymers has been studied extensively, and the associated phase behavior has been well characterized both theoretically and experimentally.2,3 Advances in polymer chemistry have enabled the synthesis of macromolecules with more complex architectures (e.g., stars, branched, gradient copolymers, etc.) and with tailored molecular weight dispersity.4 Architectural complexity and departures from the high-N near-monodisperse linear diblock motif complicate the task of predicting self-assembled structure. At the same time, such changes greatly enrich the landscape of structure and therefore function that can be realized in block copolymers. A rich bounty of unusual Frank−Kasper and quasi-crystal phases has been uncovered in small N macromomolecules, in particular in classic linear diblock copolymers of polyisoprene−polylactide by Lee and Bates et al.5,6 Such phases have also been observed in dendron-like “giant surfactants” by © XXXX American Chemical Society

Received: February 2, 2018 Revised: April 19, 2018

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DOI: 10.1021/acs.macromol.8b00261 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. (a) Synthetic scheme of CB BBCPs polymerized from macromonomer precursors using ROMP. (b1−b3) Cartoons of the macromonomers containing 1, 2, and 3 cyanobiphenyl mesogens, respectively. The green diamond represents the polymerizable norbornene, the red squiggle represents the PDMS chain, and the blue cylinder represents the rigid cyanobiphenyl mesogen. Assuming equal mass densities, ϕCB was calculated as massCB/masstotal, where massCB is the molecular weight of the CB-containing branch beginning with the carbon atom next to the triazole group closest to the branch point. The relevant portions are shaded light blue in (a) and in (b2).

mesogen-containing group and the other is a “soft” low molecular weight coil polymer. A library of norbornene-based macromonomers was synthesized consisting of a 1, 2.5, or 5 kDa (k) polydimethylsiloxane (PDMS) homopolymer branch and a branch containing 1, 2, or 3 cyanobiphenyl (CB)-based mesogens. The library was further expanded by polymerizing the macromonomers to varying degrees of polymerization (N = 10, 25, or 50). The value N refers to the targeted degree of polymerization; the experimentally estimated values are tabulated in Table S1 of the Supporting Information. Most of the macromonomers are polymerized with 90−95% conversion. The samples are referenced as (xPDMS-yCB)z, where x is the molecular weight of the PDMS chain, y is the number of CB units, and z is the degree of polymerization. Macromonomers are designated as z = 1 samples. All the samples self-assemble into classically observed structures, including spheres, hexagonally packed cylinders, gyroid, and lamellae. The apparent high segregation strength of the two branches leads to lamellae and cylinders with d-spacings as small as 6.1 and 8.0 nm, respectively. We observe cylinders at a CB mass fraction of 0.07 and lamellae at a mass fraction of 0.21, which reflects a significant skewing of the phase behavior relative to that observed for traditional linear diblocks as well as for liquid crystalline brush block copolymers.32 Macromonomers and CB BBCPs with two and three CB units are found to undergo crystallization induced distortion of the self-assembled microstucture.

block copolymer (BBCP) in which microphase separation of the immiscible A and B polymer branches on each monomer gives rise to a wide range of structures, including a gyroid phase which had not been observed previously in BBCPs. Notably, changes in the degree of polymerization N could be used to drive changes in bulk morphology and the order−disorder transition, but without significant changes in the d-spacing of the system. Such decoupling of the characteristic structural length scale from N is compelling, particularly in the context of BCP lithography. The incorporation of mesogenic species into BCPs has been pursued both as a means of modifying macromolecular architecture and as a path for imparting desirable mechanical, electronic, optical, or magnetic properties to the polymer. These properties are associated with the formation of liquid crystalline (LC) mesophases by the mesogens, and common embodiments of LC BCPs include rod−coil25−28 and sidechain29−32 systems. LC BCPs possess hierarchical order due to the dual self-assembly of the LC mesophase and its enveloping BCP superstructure. Antagonistic and synergistic effects can be observed depending on the symmetry of the preferred BCP microdomain structure, the LC anchoring condition at the microdomain interface, and the sequence of ordering.33 Here, we explore the self-assembly of a novel macromolecule that reflects several of the aforementioned characteristics. The system is a A-branch-B cyanobiphenyl bottlebrush block copolymer (CB BBCP) in which one branch is a “rigid” B


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Macromolecules Table 1. Summary of Molecular Compositions, Morphologies, and Domain Spacings for (xPDMS-yCB)z Library samplea

Mnb

Đb

TODTc (°C)

ϕCBd (wt %)

(1kPDMS-1CB)1 (1kPDMS-1CB)10 (1kPDMS-1CB)25 (1kPDMS-1CB)50 (1kPDMS-2CB)1 (1kPDMS-2CB)10 (1kPDMS-2CB)25 (1kPDMS-2CB)50 (1kPDMS-3CB)1 (1kPDMS-3CB)10 (1kPDMS-3CB)25 (1kPDMS-3CB)50 (2.5kPDMS-1CB)1 (2.5kPDMS-1CB)10 (2.5kPDMS-1CB)25 (2.5kPDMS-1CB)50 (2.5kPDMS-2CB)1 (2.5kPDMS-2CB)10 (2.5kPDMS-2CB)25 (2.5kPDMS-2CB)50 (2.5kPDMS-3CB)1 (2.5kPDMS-3CB)10 (2.5kPDMS-3CB)25 (2.5kPDMS-3CB)50 (5kPDMS-1CB)1 (5kPDMS-1CB)10 (5kPDMS-1CB)25 (5kPDMS-1CB)50 (5kPDMS-2CB)1 (5kPDMS-2CB)10 (5kPDMS-2CB)25 (5kPDMS-2CB)50 (5kPDMS-3CB)1 (5kPDMS-3CB)10 (5kPDMS-3CB)25 (5kPDMS-3CB)50

2.3 16.8 38.1 58.5

1.06 1.06 1.11 1.65

49 172 186 200

0.21 0.21 0.21 0.21

4.0 38.4

1.04 1.15

134 176

4.6 28.7 54.0 80.4 6.0 39.3 77.9

1.07 1.08 1.16 1.26 1.05 1.07 1.45

137 227 239 243

6.5 36.4 66.3 92.4 6.8 46.4 89.0 170.0 8.3 46.6 95.0 121.0 8.7 40.0 75.4 115.0

1.06 1.09 1.23 1.47 1.05 1.05 1.08 1.18 1.05 1.07 1.08 1.45 1.03 1.08 1.17 1.26

132 176 199 221

morphologye (TOOT °C)

d-spacing at 35 °Cf (nm)

d-spacing at 150 °C (nm)

LAM LAM LAM LAM

6.1 6.6 6.6 6.5

DIS 6.4 6.6 6.2

0.41 0.41

G→L (≈70) C→L (≈120)

9.1 11.1

DIS 8.0

0.14 0.14 0.14 0.14 0.31 0.31 0.31

CYL CYL CYL CYL GYR LAM LAM

8.0 8.3 8.2 8.2 12.7 11.7 11.7

DIS 7.7 7.6 7.5 10.0 10.0 9.9

0.37 0.37 0.37 0.37 0.07 0.07 0.07 0.07 0.15 0.15 0.15 0.15 0.19 0.19 0.19 0.19

LAM LAM LAM LAM SPH S→C (≈120) CYL CYL CYL CYL CYL CYL CYL CYL CYL CYL

13.2 14.0 13.7 13.7 9.2 10.0 10.0 10.0 14.3 14.4 14.4 14.4 14.7 15.2 15.2 15.2

10.7 11.3 11.4 11.4 DIS 9.7 9.4 9.4 11.5 12.0 12.2 12.7 12.5 13.2 13.3 14.5

a Samples are identified in the form (xPDMS-yCB)z where x is the molecular weight of the PDMS homopolymer, y is the number of CB subunits, and z is the targeted degree of polymerization. bMn and Đ were determined with GPC using polystyrene standards. cTODT values were determined from X-ray scattering data as the temperatures at which the primary Bragg scattering peak loses 50% of its intensity transitioning from the disordered to ordered state on cooling. dϕCB was calculated as massCB/masstotal, where massCB is the molecular weight of the CB-containing branch beginning with the carbon atom next to the triazole group closest to the branch point. eThe first morphology listed is one initially observed by SAXS after cooling through TODT or that observed at 260 °C if the TODT is inaccessible. S: SPH; C: CYL; G: GYR; L: LAM. Order−order transitions are indicated by an arrow. The location of the OOT is indicated in parentheses. Distorted morphologies of crystallized 2 and 3 CB samples are not indicated. fSamples derived from 2 and 3 CB macromonomers show microstructure distortions due to crystallization. The d-spacing listed at 35 °C was determined from the first-order peak which was often broadened and/or shifted due to mesogen crystallization.

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mm (230−400 mesh), on a BiotageIsolera Prime flash purification system. Gel permeation chromatography (GPC) was performed with a concentration of 0.1−1.0 mg/mL on an Agilent 1260 Infinity system in THF, calibrated with linear polystyrene standards and equipped with a UV diode array detector and a differential refractive index (dRI) detector. The GPC was run at a flow rate of 1 mL/min at 35 °C, and three columns were assembled in series: Agilent Technolgies PLgel 5 μm 10E5A, 10E4A, and 10E3A, all of which are 300 × 7.5 mm in dimension. 1 H and 13C nuclear magnetic resonance (1H and 13C NMR) spectra were acquired on 500 MHz Varian INOVA or 600 MHz Bruker AVANCE spectrometers. Spectra were calibrated by the residual solvent signal in deuterated solvent (CDCl3), which was purchased from Cambridge Isotope Laboratories, Inc. NMR spectra were processed in MestReNova 10.0.2.

EXPERIMENTAL SECTION

Synthesis of Macromonomers and Graft-Through Polymers. The graft-through ROMP of macromonomers are depicted in Figure 1. Full details of the synthesis of macromonomers and polymers are provided in the Supporting Information. All anhydrous and HPLC grade solvents were purchased from Sigma-Aldrich or Alfa Aesar and used as supplied unless otherwise stated. Grubbs’ second-generation catalyst was received from Materia. PDMS with Mn = 1000 and 5000 were purchased from Gelest while Mn = 2500 material was purchased from ShinEtsu. Anhydrous, degassed dichloromethane (DCM) and tetrahydrofuran (THF) were used from a J.C. Meyer solvent purification system. HPLC grade DCM and THF were sparged vigorously with argon for at least 1 h before being connected to the solvent purification system. All reagents were purchased from SigmaAldrich or Alfa Aesar unless otherwise stated. All chromatography was performed on EMD Millipore silica gel 60, particle size 0.040−0.063 C


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Macromolecules Ring-opening metathesis polymerizations were conducted in a nitrogen-filled glovebox. A representative procedure is as follows: A macromonomer containing one to three mesogens and PDMS was dissolved in anhydrous THF to form a 500 mg/mL solution. The solution was dispensed into glass vials. Then, following the protocol described in Liu and Johnson et al., the requisite volume of a modified Grubbs’ third-generation catalyst in THF was dispensed.34 After stirring for 2 h, the polymerization was removed from the glovebox and quenched with three drops of ethyl vinyl ether. Sample Characterization. The domain spacing, order−disorder transition temperature (TODT), and phase behavior were characterized using small-angle X-ray scattering (SAXS) at one of two facilities. Bulk samples were sandwiched between Kapton sheets with a silicone rubber spacer to form disk-like specimens ∼2 mm thick and ∼3 mm in diameter. The in-house setup (Rigaku SMAX3000), with a collimated pinhole of Cu Kα radiation (λ = 1.542 Å) and accelerating voltage of 45 kV, was used for preliminary data collection. The accessible scattering vector, q, for this system is ≈0.015−0.2 Å−1. Higher resolution data were acquired with synchtron radiation at Brookhaven National Lab (NSLS-II, 11-BM CMS). A silver behenate standard was employed for the calibrations of all the resultant 2-D data. The 2-D scattering patterns were integrated into 1-D plots of scattering intensity I versus q, where q = 4π sin(θ)/λ and the scattering angle is 2θ. Thermal behavior was characterized by differential scanning calorimetry (DSC) using a TA Instruments Q200 instrument, with a 10 °C/min scanning rate under an N2 atmosphere. Samples underwent two heating and cooling cycles each, in the range of −40 to 300 °C. For transmission electron microscopy (TEM), samples were dissolved in chloroform at 1 wt % and drop-cast onto carbon supported copper grids (Ted Pella, 400 mesh/in.). The grids were annealed at either 100 or 150 °C for 30 min on a hot plate and left unstained. The specimens were imaged on a FEI Tecnai Osiris TEM with accelerating voltage of 200 kV. Polarized optical microscope (POM) studies were done on samples placed between two glass slides using a Zeiss Axio Observer microscope equipped with a Pike CCD camera. Temperature-resolved measurements were done at various scanning rates with a Linkam THMS600 hot stage.

centered cubic spheres (SPH). Though most of the molecules exist in only one self-assembled microstructure, some undergo thermotropic order−order transitions (OOTs). The morphology column in Table 1 indicates whether an order−order transition was discernible by SAXS and the approximate temperature at which the transition occurred. The first morphology listed is the morphology initially observed after cooling through TODT or that observed at a fixed reference temperature of 260 °C for cases in which TODT was inaccessible. Role of Overall Backbone Length. The architecture of these molecules imparts several noteworthy properties. In contrast to linear BCPs, the microdomain period scales with the length of the side chains instead of the overall backbone length. Thus, polymerization of the macromonomers is not expected to significantly alter the d-spacing. Moreover, for given x and y, changes in the degree of polymerization, z, do not alter the composition of the system, as gauged by the CB weight fraction (ϕCB). For the 1CB samples, we observe a modest initial increase in d-spacing on changing the backbone length from z = 1 to z = 10. Thereafter, the d-spacing is essentially invariant with the degree of polymerization. The initial increase of dspacing relative to the macromonomer is likely due to the stretching of the PDMS and CB-based branches caused by increased steric confinement present in the z > 1 samples. Departures from this general trend were observed in some 2CB and 3CB samples, as highlighted for example by the larger dspacing change (2 nm) of the polymer relative to the macromonomer for (1kPDMS-3CB)10 vs (1kPDMS-3CB)1 and the 1 nm decrease in d-spacing for (2.5kPDMS-2CB)10 vs (2.5kPDMS-2CB)1. These effects appear to originate due to crystallization-induced microstructure changes, which we address in a later discussion. Appreciable effects of polymerization include substantial increases in TODT and, in some cases, differences in selfassembled morphologies. From z = 1 to z = 50 in the 1CB variant, TODT increases from 49 to 200 °C, 137 to 243 °C, and 132 to 221 °C, for the 1k, 2.5k, and 5k samples, respectively. Figure 2 shows the TODT of the CB BBCPs normalized to the TODT of the respective macromonomer samples. In all cases, we find that polymerization stabilizes the ordered phase, with the most dramatic TODT increase occurring between z = 1 and z = 10. We speculate that steric barriers to chain mixing may be

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RESULTS AND DISCUSSION Phase Behavior. Details of the molecular composition, selfassembled morphologies, and domain spacings of the molecules in the (xPDMS-yCB)z library are shown in Table 1. The general synthetic scheme of the CB BBCPs and chemical structures and cartoons of the macromonomers are shown in Figure 1. Each macromonomer is composed of two branches attached to a norbornene unit. The first branch includes a linker and a PDMS homopolymer with a molecular weight of 1k, 2.5k, or 5k. The second branch is a side chain containing either 1, 2, or 3 cyanobiphenyl (CB) moieties. Multiple CB units in one macromonomer serve to change the composition of the system, increasing the relative fraction of the CB block while keeping the molecular weight of the PDMS chain constant. Graft-through ROMP of the macromonomers, (xPDMS-yCB)1, produced (xPDMS-yCB)z CB BBCPs with x = 1k, 2.5k, or 5k, with y = 1, 2, or 3, and with z = 10, 25, or 50. Of the 36 molecules in this (xPDMS-yCB)z sample space, 29 were synthesized. Fourteen of the 29 were found to have accessible order−disorder transition temperatures (TODT), listed in Table 1. TODT values were determined from scattering data in the conventional way as the temperatures at which the primary Bragg scattering peak achieves 50% of its intensity rise during the transition from the disordered to ordered state on cooling. All the species self-assemble into classically observed morphologies, including lamellae (LAM), hexagonally packed cylinders (CYL), bicontinuous gyroid (GYR), and body-

Figure 2. TODT values of CB BBCPs normalized to the TODT of their respective macromonomer (in kelvin) as a function of targeted degree of polymerization. D


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Macromolecules

Figure 3. (a) Effect of increasing the degree of polymerization in (5kPDMS-1CB)z. The macromonomer (bottom trace) organizes into BCC while z = 10, 25, and 50 (second, third, and fourth trace, respectively, from bottom) adopt a HEX morphology. (b) Effect of increasing the number of CB side chains in (2.5kPDMS-yCB)1. Macromonomers with 1, 2, and 3 CB units form CYL, GYR, and LAM, respectively. The peak in the 2CB trace at 1.25 nm−1 (5.0 nm) can possibly be attributed to a smectic layering of CB molecules. (c) Effect of increasing the length of PDMS block in (xPDMS1CB)1. Macromonomers with a 1k, 2.5k, and 5k PDMS self-assemble into LAM, CYL, and BCC, respectively (bottom to top). Panels a and c show data collected at 30 °C, while panel b shows data collected at 100 °C.

Role of Composition. The sample space explored in this work spans three different PDMS molecular weights (1k, 2.5k, and 5k), three CB side chains (containing 1, 2, or 3 CB subunits), and 4 degrees of polymerization (1, 10, 25, or 50). Modifications in each of these categories induce changes in ϕ CB , T ODT, self-assembled morphology, and d-spacing. Manifestations of these systematic changes are highlighted in Figure 3. Figure 3a describes the effect of changing z in (5kPDMS-1CB)z. Hexagonally packed cylinders are observed for z = 10, 25, and 50, while z = 1 self-assembles into SPH at room temperature. An increase in d-spacing in going from z = 1 (bottom, black trace) to z = 10 (second from the bottom, red trace) is visible in the shift to lower q of the primary Bragg peak. Figure 3b shows the effect of increasing the number of CB subunits, y, in PDMS(2.5kPDMS-yCB)1. Data were collected at 100 °C rather than 30 °C to avoid structural distortion of the BCP structure due to CB crystallization (discussed later). The increase in ϕCB from 0.14 to 0.31 to 0.37 (bottom to top), is accompanied by a change in morphology from CYL to GYR to LAM with 1, 2, and 3 CB subunits, respectively. Figure 3c follows the effects of increasing the PDMS chain length, x, from 1k through 2.5k to 5k (bottom to top) in (xPDMS-1CB)1. As the PDMS chain length is increased, ϕCB decreases from 0.21 to 0.14 and then 0.07, resulting in the formation of LAM, CYL, and SPH phases, respectively. With the associated increase in molecular weight, the d-spacing concomitantly increases from 6.1 to 8.0 and 9.2 nm. 1D SAXS traces for all the 1k, 2.5k, and 5k PDMS CB BBCP samples at 150 °C are shown in the Supporting Information (Figures S1, S2, and S3, respectively). Trends observed on changing composition as well as direct observation by TEM (discussed later) confirm that the SPH and CYL systems have CB as the minority component. For the gyroid sample, (2.5kPDMS-2CB)1 (Figure 3b), the large intensity of the prominent peak at q = 1.25 nm−1 (dspacing of 5 nm) relative to the surrounding low intensity higher order gyroid reflections strongly suggests that the 5 nm

responsible for the observed increase. The barrier is due to the loss of backbone flexibility on increasing degree of polymerization, as a result of which chain mixing becomes sterically unfavorable, particularly for side chains with a common backbone. The reduction of backbone flexibility and the resulting difficulty associated with chain mixing can be viewed as stabilizing the microphase-separated state by enhancing local order. A similar trend was reported by Kawamoto and Johnson et al. for their A-branch-B BBCPs.24 This exemplifies that changing z can be used as a handle to modify the effective interaction energy of the two branches (manifested as TODT changes), without changing the d-spacing, as observed in linear diblocks. Changes in morphology as a function of z are highlighted in Table 1. One notable example is the presence of a stable gyroid phase in (2.5kPDMS-2CB)1, which is absent for z = 10, 25, and 50. In another instance, (5kPDMS-1CB)1 displays a SPH phase at all temperatures at which it is ordered, whereas (5kPDMS-1CB)10 displays the SPH phase at higher temperatures with an OOT to CYL on cooling, while (5kPDMS-1CB)25 and (5kPDMS-1CB)50 form stable CYL phases at all temperatures examined. The systems displayed structural changes consistent with a larger effective ϕCB on increasing the effective interaction strength, by either increasing the degree of polymerization or decreasing temperature. This is apparent for example in the SPH → CYL transition on cooling for (5kPDMS-1CB)10 and the formation of CYL versus SPH in going from the macromononer, z = 1, to z = 25, and z = 50. These structural changes as a function of interaction strength imply concavity in the boundary between SPH and CYL in a conventional χN vs ϕ phase diagram for the system. The inability to access the disordered state for many of the macromonomers and polymers, despite their modest molecular weights, is qualitatively indicative of a large thermodynamic driving force for microphase separation, i.e., a large effective interaction parameter, by comparison with conventional linear diblock systems such as poly(styrene-b-methyl methacrylate). E


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Macromolecules periodicity does not originate from the gyroid structure. We speculate that this periodicity is due to the presence of a liquid crystalline (LC) mesophase. Given the dimensions of the mesogens concerned, the mesophase may feature a layered structure (e.g., smectic A), but the absence of higher order reflections and other indicators from WAXS and DSC measurements did not permit confirmation of this hypothesis. Similar LC-attributed scattering was observed from the CYL phases formed by z = 1, 10, 25, and 50 samples of the (5kPDMS-3CB)z system (Figure S3). While the formation of LC mesophases (nematic and smectic) in or around cylindrical BCP microdomains is common,33 LC ordering concurrent with a small d-spacing gyroid BCP structure has not been observed to date. Further investigation of this system may shed light on a potentially unusual hierarchical structure, given the uniaxial symmetry of LC mesophases and the highly curved nature of gyroid interfaces. It is worthwhile to note that in the GYR (2.5kPDMS-2CB)1 sample and the 4 CYL (5kPDMS-3CB)z samples the ∼5 nm LC-attributed peak was present even at the highest temperatures examined (up to 260 °C). As we discuss later, this suggests that the putative LC ordering may be significantly stabilized physically by confinement within the BCP superstructure. As evidenced from Table 1 and some of the examples provided in Figure 3, the phase behavior of the CB BBCPs departs significantly from that of conventional linear diblocks. For example, (5kPDMS-1CB)25 and (5kPDMS-1CB)50 form hexagonally packed cylindrical microdomains at ϕCB ≈0.07, whereas in linear diblocks hexagonally packed cylinders are generally not observed for block minority volume fractions smaller than ≈0.15. LAM phases were formed by our CB BBCPs for ϕCB as low as 0.21, a composition that is on the low end of the expected CYL phase space for linear diblocks. For this comparison with linear diblocks, we assume that the deviation between mass and volume fraction is expected to be small. The departure from conventional phase behavior provides a useful benefit as the presence of cylindrical microdomains at the small mass fractions observed here enables the generation of smaller microdomains than would otherwise be possible using a linear diblock of the same molecular weight or d-spacing. The diameter of cylindrical microdomains a = d(8ϕ/π 3 )1/2 where ϕ is the cylinder volume fraction and d is the d-spacing of the hexagonal lattice (Figure S4). For (2.5kPDMS-1CB)1, with ϕCB ≈ 0.14 and dspacing of 8.0 nm at room temperature, the diameter of the cylindrical microdomains is ≈3.6 nm, which is appreciably smaller than typically encountered in linear BCPs (>6 nm). Transmission electron microscopy (TEM) was used to directly visualize the CB BBCP microstructure. Figure 4 shows representative images of several samples with 5k PDMS chains drop-cast from chloroform (1 wt %) onto Cu TEM grids and then annealed at 150 °C for 30 min, except for (5kPDMS1CB)1, a low TODT sample that was annealed instead at 100 °C. Figures 4a and Figure 4b show poorly ordered vertical cylinders for z = 1 and z = 50, respectively. Figure 4c shows a relatively thick island of (5kPDMS-2CB)1 surrounded by thinner material. What appear to be perpendicular cylinders are observed on the edges in the thinner portion of the film, while parallel cylinders are observed in the central, thicker regions. In Figure 4d, the cylinders are poorly ordered, displaying a mixed orientation relative to the substrate. Figures 4e and 4f show parallel cylinders for (5kPDMS-3CB)1 and

Figure 4. Representative TEM images of samples drop-cast from chloroform onto Cu TEM grids and then annealed at 150 °C for 30 min (100 °C for (5kPDMS-1CB)1): (a) (5kPDMS-1CB)1, (b) (5kPDMS-1CB)50, (c) (5kPDMS-2CB)1, (d) (5kPDMS-2CB)50, (e) (5kPDMS-3CB)1, and (f) (5kPDMS-3CB)50. Scale bar = 100 nm.

(5kPDMS-3CB)50. The traditional smooth fingerprint pattern is observed for (5kPDMS-3CB)50 while for (5kPDMS-3CB)1 we observe more jagged and generally less persistent cylindrical microdomains. Because of the relatively rapid nature of the sample preparation, the observed morphologies may not correspond to the thermodynamic equilibrium thin film morphologies for these materials. Nevertheless, we note a qualitative correspondence with the bulk morphologies deduced from SAXS for these samples: the TEM images clearly reflect the formation of well delineated microphaseseparated structures with characteristic dimensions comparable to those observed by SAXS. Birefringent Textures of CB BBCPs Observed by POM. Samples were observed using temperature-resolved polarized optical microscopy (POM). The images in Figure 5a−e show several representative optical micrographs observed after cooling samples through TODT, while the images in Figures 5f,g show data for two samples without an accessible TODT, (2.5kPDMS-3CB)1 and (5kPDMS-3CB)1. In all cases, the samples display strong birefringence, qualitatively speaking. The overall transmitted light intensities were more akin to intensities observed in thermotropic small molecule and polymeric LCs, or lyotropic surfactant LCs, than the small transmitted intensities observed for conventional non-LC BCPs, at similar thicknesses. Moreover, for samples cooled from the disordered state, the micrographs show well formed F


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Macromolecules

Figure 5. (a) Cylinder-forming (2.5kPDMS-1CB)1 at 139 °C cooled 0.5 °C/min through TODT. (b) Cylinder-forming (2.5kPDMS-1CB)50 at 248 °C cooled 0.5 °C/min through TODT. (c) Cylinder-forming (5kPDMS-1CB)25 at 217 °C cooled 0.5 °C/min through TODT. All three cylinder forming samples present fan-like textures. (d) Lamellar-forming (1kPDMS-1CB)10 at 163 °C displaying elongated birefringent grains or bâtonnets. Further cooling results in a loss of the bâtonnets and production of a near-uniformly dark image with isolated focal conic domains as depicted in the inset (shown at 100 °C). (e) Lamellar-forming (1kPDMS-3CB)1 at 70 °C heated 10 °C/min from the crystallized state at 60 °C. The sample shows a complex texture that appears to be a dense collection of focal-conic domains. (f) Cylinder-forming (2.5kPDMS-3CB)1 at 150 °C after heating 10 °C/min from from RT. (g) Lamellar-forming (5kPDMS-3CB)1 at 150 °C after heating 10 °C/min from RT. TODT is inaccessible for the samples shown in (f) and (g), and their textures are poorly developed. All scale bars are 100 μm. Note: heat losses from the POM sample stage lead to a temperature offset between POM and SAXS experiments, with ODTs occurring at higher indicated temperatures in POM than the nominal temperatures deduced by SAXS.

in 2D SAXS micrographs for such samples (Figure S5); i.e., the azimuthal intensity dependence displays large random variations associated with scattering from individual grains which are not negligibly small relative to the beam. As established in prior studies,35−37 the development of large grains leads to an ability to control the alignment of the CB BBCP microstructure using magnetic fields by cooling samples slowly though the ODT (Figure S6). The POM images in Figures 5a and 5b are of CYL phases of a macromonomer (2.5kPDMS-1CB)1 and its z = 50 polymer, (2.5kPDMS-1CB)50, respectively. The samples were prepared by slow cooling (0.5 °C/min) through their respective ODTs to a temperature a few degrees (∼2−10 °C) below the ODT. Likewise, the image in Figure 5c is for another CYL phase, that of the polymer (5kPDMS-1CB)25. In all three cases we observe well-formed fan-like or broken-fan textures, similar to those seen in hexagonal lyotropic surfactant mesophases38 and thermotropic discotic systems.39 Fan-like textures may also result from focal-conic arrangement of layered systems such as smectic A mesophases, but the absence of the characteristic elliptical or hyperbolic lines of optical discontinuity in the cases above is consistent with a fan texture associated with hexagonal as opposed to lamellar mesophases.40 Figures 5d and 5e show micrographs from lamellar states of (1kPDMS-1CB)10 and (1kPDMS-3CB)1. In the first case, we observe the formation of bâtonnets, elongated bright islands, during the ordering transition from the surrounding dark isotropic fluid. However, instead of an impingement of such bâtonnets and the formation of a fan-like texture, the system presents a near-uniformly dark image at lower temperatures

textures; i.e., domains are large, and topological defects are clearly visible. The textures are reminiscent of the fan-like or focal-conic textures observed in small molecule systems. For the ODT-inaccessible samples, however, the textures were not well developed. For the ODT accessible samples, the materials lost birefringence upon heating above ODT and regained birefringence, presenting well-developed textures, on slow cooling into the ordered states. While all the polymers in this study contain mesogenic CB groups, it was not clear in all cases that they formed LC mesophases. Indeed, the clearest evidence of mesophase formation was in the aforementioned cases of the GYR (2.5kPDMS-2CB)1 sample and the 4 CYL (5kPDMS3CB)z samples which showed smectic-like scattering with ≈5 nm periodicity. These five ODT inaccessible samples showed POM features similar to those in Figure 5f,g, i.e., of ODT inaccessible (2.5kPDMS-3CB)1 and (5kPDMS-3CB)1. By contrast, we found no evidence by SAXS or DSC to suggest that either the 1CB macromonomers or any of its polymers formed LC mesophases. Taken together with the coincidence of the loss/return of birefringence with ODT, the data indicate that the well-resolved textures observed here are a product of the microdomain morphology and are not necessarily reliant on LC mesophase formation within the microdomains. The large transmitted light intensities across all samples suggest there is an appreciable difference in the index of refraction of the BCP microdomains. The optically well-resolved textures of systems with accessible ODTs reflect the existence of large micrometerscale grains in these samples. The formation of large grains is further confirmed by the “spotty” nature of the X-ray scattering G


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Macromolecules

Figure 6. Data presented for (2.5kPDMS-2CB)25. (a) Temperature-dependent POM images showing large decrease in birefringence at 70 °C upon heating and 48 °C upon cooling. (b) DSC of the same sample upon multiple heating/cooling cycles. (c) Temperature-dependent SAXS showing corresponding BCP structural changes at the temperature of the DSC transition and POM loss of birefringence. Above transition, the BCP adopts a lamellar morphology. Data are shown at equally spaced intervals (5 °C).

associated with BCP microphase separation is generally difficult to resolve, but prior work, particularly on low molecular weight systems, has successfully used calorimetry to detect the latent heat associated with the weakly first-order BCP ODT.43 Subsequent ordering within the microphase-separated state due to formation of LC or crystalline states by backbone substituents, or crystalline states by backbone chain folding, can also be resolved by calorimetry as well as by scattering experiments. Confinement of polymer crystallization within BCP microdomains, or lack thereof, has been well studied in semicrystalline diblock copolymers.44 Crystallization of an A block is generally confined if it occurs at a crystallization temperature (TAx ) below the glass transition temperature (TBg ) of a conjugate high modulus B block. In this case the morphology of the BCP is unperturbed by the crystallization event. By contrast, so-called breakout crystallization occurs if TAx > TBg . Breakout crystallization often results in distortion of the BCP morphology. In the present case, microphase separation notwithstanding, the amorphous nature of PDMS suggests that any clearly observed thermal transitions (i.e., transitions not correlated to the ODT) would originate from ordering of other subunits of the system, specifically from ordering of the CB moieties.

with a few small bright domains dispersed throughout (inset). The loss of transmitted light is due in this case to near-uniform parallel alignment of the lamellar structure with the confining glass surfaces, while the bright areas are circular focal conic domains with their 4-fold symmetric extinction brushes visible. These are typical features for layered LCs in sufficiently thin cells with planar anchoring conditions. For (1kPDMS-3CB)1, we observe an unusual texture that bears some superficial similarity to the threaded texture observed in thick samples of nematic LCs.41,42 Closer inspection, however, suggests that the features are due to a multitude of tightly packed small focal conic domains. The fact that the lamellar state in this sample is produced after melting a crystalline state and the proximity of the gyroid phase may contribute to the complexity of the defect texture. For (2.5kPDMS-3CB)1 and (5kPDMS-3CB)1 shown in Figure 5f,g, though the samples are birefringent, the inability to disorder the materials and subsequently cool them slowly through TODT to nucleate and grow large grains results in a very fine texture that is difficult to resolve optically. It is possible that extended thermal annealing may allow resolvable textures to develop in these samples, but we did not pursue this possibility. Mesogen Ordering and LC Stability. Molecular ordering transitions may give rise to sensible heat that can be detected in appropriately conducted calorimetry experiments. The enthalpy H


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Macromolecules

As mentioned earlier, several systems displayed SAXS data that were consistent with the presence of a smectic LC mesophase. Above the melting temperature attributed to the CB units, we observed a prominent peak, significantly higher in intensity than would be expected for higher order BCP peaks, in all four cylinder-forming (5kPDMS-3CB)z samples (z = 1, 10, 25, and 50) as well as in the gyroid-forming (2.5kPDMS2CB)1. This peak occurred at ≈5 nm, roughly the length of the mesogenic side chain. The smectic peak was absent in other 3CB samples, which we note were lamellae-forming. It is unclear, however, whether this observation reflects anything more than coincidence. In the samples where it was present, the smectic-like peak was present up 260 °C, the highest temperature tested. This suggests an elevated stability of the smectic mesophase, by comparison with smectic mesophases formed by other cyanobiphenyl-based LCs. We speculate that the high thermal stability of the mesophase may originate from its confiment within the BCP superstructure, i.e., within the 3CB cylindrical microdomains and the 2CB gyroid network.

DSC studies were used to investigate the presence of ordered states of the CB groups that could be inferred from thermal transitions observed on heating and cooling the samples. Calorimetry data are shown in Figures S7, S8, and S9 for samples with 1k, 2.5k, and 5k PDMS chains, respectively. No transitions were observed that could be correlated to the ODT for samples that had accessible ODTs. None of the 12 samples possessing only a single CB unit (i.e., y = 1 samples) showed any transitions in DSC. By contrast, samples with two and three CB units, especially on first heating, all displayed an endothermic peak in DSC. Temperature-resolved POM and SAXS data along with DSC are shown for a representative sample, (2.5kPDMS-2CB)25, in Figure 6. POM images show a pronounced decrease of birefringence around 60−65 °C on heating, followed by a reemergence around 70−75 °C. The loss of birefringence is pronounced but not total, and it appears to coincide roughly with the temperature at which there is a transition observed upon first heating in DSC (Figure 6b). A transient loss of birefringence was observed on cooling, between about 50 and 45 °C. Interestingly, the cooling curve (first cool) and the subsequent heating and cooling cycles (second heat; second cool) do not show any thermal events. The endothermic transition on heating reappeared, however, when the sample was examined after storage at room temperature for 2 months. This time-delayed reappearance of what is likely a melting transition, and the absence of a clear exothermic counterpart on immediate cooling, strongly suggest that the sample undergoes a slow and perhaps frustrated crystallization event. Temperature-resolved SAXS data are shown in Figure 6c. At low temperatures below ∼100 °C, the CB BBCP is poorly ordered, as evident by the very broad peak centered at q* = 0.55 nm−1. As the sample is heated, this primary peak moves to higher q and sharpens considerably while developing a secondorder reflection at 2q*, suggesting the existence of a layered structure, i.e., lamellar microdomains. The inverse occurs on cooling. Temperature-resolved WAXS was conducted on this sample to observe changes on smaller length scales (Figure S10). The system displays subtle evidence of relatively small but sharper peaks lying atop broad amorphous humps between 1.2 and 1.8 Å−1 (3.7−5.2 Å), and a loss of these peaks with heating above ≈70 °C. These features are more clearly visible in the WAXS data for the macromonomer, (2.5kPDMS-2CB)1 (Figure S11). Overall, the POM, DSC, and SAXS data indicate that the microphase-separated BCP morphology is disrupted by molecular ordering of the CB units. This is not unexpected given the soft nature of the low-Tg PDMS microdomains. Although the ordering is slow, it is significant enough to distort the BCP microstructure as shown by the SAXS data in which there is a broadening of the primary Bragg reflection and loss of the second-order peak on cooling. Similar effects were observed in the macromonomer of this sample (Figure S12) as well as in the other 2CB and 3CB samples. It is apparent that the presence of multiple CB units results in the formation of crystalline states and that the crystallization disrupts the BCP morphology. The slow crystallization kinetics may result from geometric frustration due to steric effects associated with the tight clustering of the rigid biphenyl units evident from the structures shown in Figure 1. We speculate that the transient loss of birefringence is associated with mesogen crystallization and its subsequent effects on the BCP morphology. The data make clear that the decoupling of these events is subject to kinetic considerations.

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CONCLUSION

In this work we have synthesized novel, self-assembling macromomers containing one “soft” coiled PDMS branch and a branch with one, two, or three “rigid” CBs. Ring-opening metathesis polymerization of each macromonomer was performed to generate samples with degrees of polymerization of 10, 25, and 50. The resulting library of polymers was studied to elucidate the effects of PDMS branch length, CB number, ϕCB, and degree of polymerization on the structure and properties of the materials. Higher TODT values are attainable, without a corresponding increase in d-spacing, by increasing the degree of polymerization. The ability to modulate thermophysical properties (TODT, viscosity, etc.) independently of dspacing is compelling and potentially relevant for BCP lithography where a certain thermophysical property profile must be maintained, despite the small feature sizes required by the system’s self-assembly. Strong segregation between the PDMS and CB branches resulted in lamellae and cylinders with d-spacings as low as 6.1 and 8.0 nm, respectively. Though cylinders, lamallae, gyroid, and spheres were all observed, their occurrence was generally at compositions that represented a significant departure from the composition-dependent phase behavior of conventional linear coil−coil BCPs, with cylinders and lamellae observed at CB mass fractions of 0.07 and 0.21, respectively. ODT-accessible samples formed large grains with sizes on the order of 10 μm, resulting in readily observable defect textures by POM and highlighting the fast ordering kinetics of the system. The formation of large grains presents compelling opportunities for magnetic field control of these microstructures. Slow crystallization and morphology disruption by such crystallization were observed in 2CB and 3CB samples, while several samples with CBs confined in a minority cylinder or gyroid phase showed evidence of a highly thermally stable smectic LC structure. While the currently studied materials did not display the hallmark characteristic of LC BCPs (e.g., hierarchical ordering, optical activity), we anticipate that the molecular motif of grafted LC (mesogen-containing) and non-LC branches will provide new avenues for designing hierarchically ordered functional materials. I


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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.8b00261. DSC, additional SAXS and WAXS data, NMR spectra, and other characterization data for monomers and polymers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.A.J.). *E-mail: [email protected] (C.O.O.). ORCID

Jeremiah A. Johnson: 0000-0001-9157-6491 Chinedum O. Osuji: 0000-0003-0261-3065 Author Contributions

Y.R. and K.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF (DMR-1410568). Y.R. acknowledges fellowship support from the National Physical Science Consortium (NPSC). Facilities use was supported by YINQE and NSF (DMR-1119826). The authors thank Y. Choo, X. Feng, M. Gopinadhan, and M. Zhong for helpful discussions. This research used the CMS beamline (11-BM) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract DE-SC0012704. The authors thanks Drs. M. Fukuto and R. Li of CMS for their assistance with X-ray scattering studies.

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