Phase Behavior of Polylactide-Based Liquid Crystalline Brushlike

Nov 16, 2015 - We explore the morphology and phase behavior of a recently introduced architecture of liquid crystalline brushlike block copolymer (LCB...
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Phase Behavior of Polylactide-Based Liquid Crystalline Brushlike Block Copolymers Youngwoo Choo,†,# Lalit H. Mahajan,§,# Manesh Gopinadhan,† Dennis Ndaya,‡ Prashant Deshmukh,‡ Rajeswari M. Kasi,‡,§ and Chinedum O. Osuji*,† †

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States Department of Chemistry and §Polymer Program, Institute of Material Science, University of Connecticut, Storrs, Connecticut 06269, United States

‡

S Supporting Information *

ABSTRACT: We explore the morphology and phase behavior of a recently introduced architecture of liquid crystalline brushlike block copolymer (LCBBC) as functions of composition and molecular weight. Lowpolydispersity materials were prepared by ring-opening metathesis polymerization of n-alkyloxycyanobiphenyl and poly(DL-lactide) (PLA) functionalized norbornene monomers. Well-ordered block copolymer mesophases were observed with transitions from spheres to hexagonally packed cylinders, lamellae, inverse cylinders, and inverse spheres on increasing the weight fraction of the liquid crystalline block, f LC, from 0.15 to 0.85. The microdomain spacing displays a power-law scaling with molecular weight with an exponent of 0.6, L0 ∼ MW0.6. The simple occurrence of microdomains with curved interfaces, spherical and cylindrical, and the sublinear scaling of microdomain spacing with molecular weight set this system clearly apart from bottlebrush block copolymers. We observe a peculiar morphology dependence of the liquid crystal anchoring condition with the cyanobiphenyl mesogens adopting planar anchoring at cylindrical microdomain interfaces while both homeotropic and planar anchoring were displayed at the block interface in lamellar systems.

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INTRODUCTION The implications of chain architecture on the morphology and phase behavior of block copolymers (BCPs) have been a topic of persistent concern in polymer science. Conformational asymmetry notwithstanding, linear diblocks represent the canonical case, and their phase behavior has been systematically explored and is to a large degree well understood.1,2 A substantial number of variations have been considered against the reference state represented by linear diblocks, including nonlinear BCPs such as comblike grafted BCPs,3 side-chain liquid crystalline BCPs (LC BCPs), 4−9 miktoarm star copolymers,10,11 and bottlebrush BCPs.12−14 The phase behavior of side-chain LC BCPs largely follows that of linear diblocks but moderated by the presence of the LC. The leading order effect of the LC is in defect energies associated with potential symmetry mismatches between the LC mesophase and the desired interfacial curvature dictated by the composition of the BCP, resulting for example in suppression of spherical microdomains and bicontinuous phases in many cases.15,16 A secondary effect arises due to commensurability issues regarding the dimensions of the mesogen and the microdomains in which they are located.17 Both of these considerations depend intimately on the anchoring condition of the mesogens at the block interface. Bottlebrush BCPs display much stronger departures from the canonical linear diblock behavior. Here, the severe backbone stretching due to steric crowding of side chains leads to a preponderance of lamellar morphologies by suppression of © XXXX American Chemical Society

microdomains with curved interfaces. Indeed, only a few cases of cylindrical microdomains have been reported.18−22 The current understanding suggests that microdomain curvature can be stabilized however in asymmetric brush block copolymers, i.e., in systems with dissimilar side-chain lengths.12,23,24 While bottlebrush BCPs lack morphological variety, they exhibit fast dynamics relative to linear chains of the same overall molecular weight (MW) due to the comparative absence of entanglements. Further, the extended nature of the backbone leads to linear scaling of microdomain spacing (d-spacing) with MW. Taken together, the fast dynamics and linear scaling of dspacing with MW of bottlebrush BCPs have led to exciting applications in photonics.25−27 Liquid crystalline brushlike block copolymers (LCBBCs) possess in effect a hybrid architecture, combining side-chain LC character with a brushtype block. One may therefore envision that such systems could display greater morphological variety than bottlebrush BCPs while perhaps retaining some of the beneficial character of bottlebrushes, including higher chain mobility and larger domain spacings relative to linear diblocks. Though intriguing, with the exception of a recent report on a semicrystalline cholesteryl-based system,28 the morphology and phase behavior of this class of materials have not been systematically described. Received: September 11, 2015 Revised: October 28, 2015

A

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Scheme 1. (a) Synthetic Scheme for Ring-Opening Metathesis Polymerization for the Poly(DL-lactide) (PLA)-Based Liquid Crystalline Brushlike Block Copolymers (LCBBC); (b) Schematic Illustration of Molecular Architecture of LCBBCs

Table 1. Molecular Composition and Thermal Properties of Liquid Crystalline Brush Block Copolymer (LCBBC) Library mol wt (kg/mol) sample

a

15LC-25 27LC-25 34LC-25 40LC-25 60LC-25 66LC-25 73LC-25 85LC-25 50LC-25 50LC-50 50LC-100 50LC-200 50LC-400

f LC (wt %) 15 27 34 40 60 66 73 85 50 50 50 50 50

GPC (PDI)b 29 30 29 30 31 32 29 34 32 53 94 190 487

(1.06) (1.07) (1.13) (1.13) (1.14) (1.14) (1.06) (1.15) (1.10) (1.09) (1.19) (1.19) (1.18)

theor

Tgc (°C)

26 25 25 28 27 25 26 25 26 50 101 202 400

45.0 45.0 44.8 44.4 42.7 42.6 45.0 42.0 47.3 45.2 46.4 46.5 42.4

TI (°C) (ΔH (J/g)) 70.0 71.7 74.8 75.8 74.3 82.2 76.6 77.7 76.8 85.9 88.4 83.5

(0.4) (0.8) (1.8) (1.6) (5.2) (7.0) (6.2) (4.1) (4.2) (5.2) (5.7) (4.9)

ΔHLC (J/g) 1.5 2.4 4.5 2.7 7.9 9.6 7.3 8.2 8.4 10.4 11.4 9.8

a

The library consist of two parts: (i) LC block copolymers having total molecular weight of 25 kg/mol with varying LC fraction (15LC-25, 27LC-25, 34LC-25, 40LC-25, 60LC-25, 66LC-25, 73LC-25, and 85LC-25) and (ii) symmetric block copolymers with f LC = 0.50 with increasing MWs (50LC25, 50LC-50, 50LC-100,50LC-200, and 50LC-400). bMW determined by GPC relative to PMMA standards using ELSD detector with THF as eluent. cGlass transition and LC clearing transition are determined from the second heating cycle of DSC measurements. The transitions were reversible upon successive heat−cool cycles.

the anchoring condition of the liquid crystal at the block interface exhibited striking variability. Planar anchoring was observed for cylindrical microdomains of both PLA and the LC block. Conversely, lamellar samples exhibited homeotropic anchoring but only for weight fraction asymmetric compositions, whereas symmetric systems displayed planar anchoring.

In this report, we investigate the effect of composition and molecular weight on the morphology and phase behavior of LCBBCs constituted by a norbornene backbone side functionalized in one block by cyanobiphenyl (CB) mesogens with a 12 methylene spacer and in the other by poly(DL-lactide) (PLA) side chains. Apart from the fundamental interest regarding the role of architecture on phase behavior, this system represents an attractive and modular platform for the development of functional materials, as recently described.29 The LC block provides an effective handle for magnetic field alignment of the microdomains while the brush block enables the formation of nanoporous domains through selective etch removal of the polylactide side chains.30 We observe a display of conventional morphologies involving assemblies of spherical, cylindrical, and lamellar microdomains of both PLA and LC blocks. Further,

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EXPERIMENTAL SECTION

Synthesis of Brushlike Block Copolymers. The synthetic route is outlined in Scheme 1. Two monomers norbornenyl end-functionalized poly(DL-lactide) (NBPLA3), where Mn of PLA chains = 3 kg/ mol, and cyanobiphenyl liquid crystalline units attached with 12 methylene spacer to norbornene (NBCB12) were synthesized using slight modifications of previously reported procedures.29 The library of block copolymers with varying block lengths and overall molecular B

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Macromolecules weights is synthesized by sequential ring-opening metathesis polymerization (ROMP) using a modified Grubbs catalyst second generation (mG2nd) ((H2IMes)-(pyr)2(Cl)2RuCHPh),38 and their molecular and thermal characterizations are summarized in Table 1. Details of polymerization can be found in the Supporting Information. 1H NMR spectra (Bruker DMX 400 MHz NMR spectrometer) are recorded in CDCl3, and the 7.24 ppm peak is used as internal standard. Gel permeation chromatography (GPC) is performed using Agilent 1260 Infinity coupled with a PL-ELS1000 evaporative light scattering (ELS) with tetrahydrofuran (THF) as the eluent. Molecular weights are determined by using conventional calibrations, which are constructed with poly(methyl methacrylate) (PMMA) standards for THF. The sample nomenclature used here indicates both the LC content and molecular weight (MW) of the block copolymer. Samples are designated “xLC-y”, where x is the wt % of LC monomer in the reaction feed and y is the overall MW in kg/mol. MW in this report refers to the number-average molar mass (Mn). Sample Preparation and Characterization. Bulk samples were prepared by dissolving the copolymers in tetrahydrofuran (THF) with concentration of ∼5 wt % followed by casting the solution at 60 °C. Casted samples were thermally annealed in vacuum at 120 °C for 12 h. Thermal behavior of the preannealed samples was characterized by differential scanning calorimetry, DSC (TA Instruments Q200), on using a scanning rate of 10 °C/min under a N2 atmosphere. The domain spacing and morphologies were characterized by smallangle X-ray scattering (SAXS, Rigaku SMAX3000) using a pinhole collimated of Cu Kα radiation (λ = 1.542 Å) with an accelerating voltage 45 kV. The accessible scattering vector (q) ranges from 0.015 to 0.21 Å−1. The preannealed samples were sandwiched between Kapton windows to form disk-shaped specimen ∼2 mm thick and ∼3 mm in diameter. Scattering was recorded on a 2-D electronic gas-filled wire detector and calibrated using a silver behenate standard (dspacing of 58.38 Å). For TEM, preannealed bulk samples were sectioned at room temperature on a Leica EM UC7 with diamond knife (Diatome Ultra 45°). The specimen with thickness ∼70 nm was floated on a water trough and subsequently transferred to lacey carbon supported copper grids (Ted Pella, 400 mesh/in.). The specimen on copper grid was stained with 0.5% RuO4 solution for 5 min to preferentially stain LC phase. The specimens were imaged on a FEI Tecnai Osiris TEM with accelerating voltage of 200 kV.

Figure 1. DSC traces of (a) heating and (b) cooling of LCBBCs with various weight fractions of norbornenyl LC side-chain block. Glass transition temperatures (Tg) of PLA block appear consistently at ∼45 °C for most of the BCPs while the LC clearing transitions present marginal difference according to the self-assembled morphologies.

contributing factor in the stability of the LC mesophase, in addition to any size or confinement related effects. TI for the symmetric samples also showed an overall upward trend with increasing molecular weight, signaling that molecular weight effects on the clearing temperature were not completely saturated. The order-to-disorder temperature (TODT) was inaccessible for the materials investigated here. Even for the low molecular weight materials (25K) the material encountered degradation before crossing TODT. Data from SAXS measurements are provided in Figure 2. The profile of the lowest LC content sample, 15LC-25, shows only a primary scattering peak (q*). The absence of higher order reflections prevented unambiguous determination of the morphology by SAXS alone in this case. However, from TEM of 15LC-25 (Figure 3a) we conclude that the system forms poorly ordered spherical microdomains of CB12 embedded in a PLA matrix. For 27LC-25 and 34LC-25 the SAXS profiles show peaks at q*, √4q*, √7q* and q*, √3q*, √4q*, respectively. This suggests clearly that these samples form hexagonally packed cylinders, as corroborated by TEM images as shown in Figures 3b and 3c. Samples 40LC-25, 60LC-25, and 66LC-25, show peaks in their SAXS profiles at q*, 2q* or 3q*, indicative of lamellar structure formation. Lamellar morphologies observed in TEM images of 40LC-25, 60LC-25, and 66LC25 (Figure 3d−f) confirm the SAXS data. For LC majority sample 73LC-25, peaks appearing at q*, √3q*, and √4q* imply that the system forms hexagonally packed PLA cylinders in the LC matrix. This is confirmed by TEM which shows wellordered cylindrical morphology (Figure 3g). Finally, although the SAXS profile did not present clear higher order peaks,

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RESULTS Phase Behavior. Details regarding the molecular composition and thermal properties of the NBCB12-b-NBPLA3k are summarized in Table 1, and the architecture is schematically illustrated in Scheme 1. The weight fraction of the LC block was varied from 0.15 to 0.85 across nine samples at a total fixed molecular weight of ∼25 kg/mol or 25K. A series of additional polymers provided five samples with log-spaced molecular weight variation from 25K to 400K with symmetric composition, f LC = 0.50. The thermal properties of the polymers measured by DSC are shown in Figure 1. For most samples a glass transition temperature (Tg) was resolved around 45 °C, which is in good agreement with the Tg of PLA homopolymers. The clearing transition (TI) was not sufficiently strong to be resolved by DSC in the case of the smallest LC weight fraction sample, 15LC-25. For other samples, TI exhibited small but readily discernible upward shifts on increasing weight fraction of the LC block. The normalized transition enthalpies ΔH (normalized by f LC) likewise displayed an overall increase with f LC from 0.4 to 7 J/g, suggesting that the LC mesophase was stabilized as the size of the microdomain in which it resided increased. For the symmetric samples with varying molecular weights, however, the enthalpy was approximately constant, ΔH = 4.82 ± 0.68, which points to morphological changes with f LC as a likely C

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Figure 2. Small-angle X-ray scattering (SAXS) data of LCBBCs with a total MW of 25K at different LC fractions f LC. The samples were thermally annealed at 120 °C for 12 h, well above the glass transition temperature and LC transition temperature, to achieve the equilibrium morphologies. The black arrows indicate the reflection from block copolymer microphase separated structure while the green arrows represent smectic mesophase. The q* and higher order peaks resolve the bulk domain spacing and morphologies of the system, respectively, and they are summarized in Table 2. Data are vertically shifted for clarity.

85LC-25 was seen to form spherical microdomains as evident from TEM (Figure 3h). Scattering peaks associated with the formation of a smectic A mesophase were observed at a d-spacing of 5.4−5.5 nm, for all but the lowest LC content samples (Table 2). Given that the smectic layer spacing of the neat monolayer-type 12CB liquid crystal is ≈3.7 nm, the system is considered to form interdigitated bilayers. Subsequent SAXS and wide-angle Xray scattering (WAXS) experiments on shear aligned samples confirm that the mesophase is smectic A. While the DSC measurements for 15LC-25 were unable to resolve any clearing transition of the LC, these measurements were successfully executed for 27LC-25 and 34LC-25. We speculate that the absence of clear observable scattering which could reasonably be associated with the expected smectic ordering of these mesogens reflects the presence of a very poorly ordered LC mesophases. This is consistent with the low enthalpies recorded for the clearing transitions in these samples. Molecular Weight Dependence of Domain Spacing. The effect of molecular weight on d-spacing was investigated using a series of symmetric composition materials, f LC = 0.50, with molecular weights of 25K, 50K, 100K, 200K, and 400K. Symmetric compositions were selected to obtain lamellar morphologies and thereby simplify comparisons with bottlebrush block copolymers where the lamellar structure is prevalent. TEM was used to evaluate the lamellar periods for the high molecular weight samples for which the d-spacings were beyond the accessible q-range of the SAXS instrument. Figures 4a−d provide representative TEM micrographs of the 50LC-25, 50LC-50, 50LC-100, and 50LC-200 samples, respectively. From the 2-D Fourier transformed images, we examined the average d-spacing of the lamellae which were 44.1, 73.3, and 103.6 nm, respectively, for the three largest molecular weights. The d-spacings for 50LC-25 and 50LC-50

Figure 3. TEM snapshots of (a) 15LC-25, (b) 27LC-25, (c) 34LC-25, (d) 40LC-25, (e) 60LC-25, (f) 66LC-25, (g) 73LC-25, and (h) 85LC25 to confirm the morphologies of LCBBCs. The PLA microdomains appear brighter in the images. Scale bars are 100 nm.

were 19.5 and 31 nm as evaluated by SAXS. A complete list of d-spacings is provided in Table 2. Figure 4e shows a plot of the lamellar period L0 as a function of molecular weight. The data demonstrate a clear power law dependence, with exponent 0.6, L0 ∼ MW0.6. Morphology-Dependent Mesogen Anchoring. The anchoring condition of the mesogen at the block interface, the intermaterial dividing surface (IMDS), was explored as a function of morphology. Samples were shear aligned and then allowed to relax by thermal annealing. The resulting SAXS and WAXS data were examined to determine the orientation of mesogens at the IMDS based on the orientation of the LC mesophase with respect to the block copolymer superstructure. Samples were manually shear aligned between two Kapton sheets, at 140 °C, for a few seconds, and then allowed to relax D

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Macromolecules Table 2. Summary of the Self-Assembled Morphologies and the Domain Spacing of Each LCBBCsa d-spacing (nm) sample

f LC (wt %)

morphology

BCP

SmA

15LC-25 27LC-25 34LC-25 40LC-25 60LC-25 66LC-25 73LC-25 85LC-25 50LC-25 50LC-50 50LC-100 50LC-200 50LC-400

15 27 34 40 60 66 73 85 50 50 50 50 50

SPH CYL CYL LAM LAM LAM CYL SPH LAM LAM LAM LAM LAM

13.2a 17.3a 19.3a 19a 22.6a 21a 22.5a 19a 19.5a 31a 44.1b 73.3b 103.6b

5.5 5.4 5.4 5.5 5.5 5.4 5.4 5.5 5.4 5.5

a

SPH, CYL, and LAM denote spheres, hexagonally packed cylinders, and lamellae, respectively. Domain spacing was estimated by (a) SAXS and (b) TEM.

at 100 °C for 1−2 min before subsequent cooling to room temperature over a period of 1−2 min during which time the LC mesophase formed. This procedure was established based on prior practice16,31,32 to provide reliable data on mesogen anchoring, i.e., free of any artifacts that could potentially arise due to the imposition of shear, including possible antagonistic interactions of the LC compared to the block copolymer superstructure under flow.33,34 In some cases, fibers were also pulled from a high-temperature melt as a secondary means of generating aligned morphologies (Supporting Information). 2-D SAXS data for shear aligned samples are shown in Figures 5a and 5c for a PLA cylinder forming sample, 73LC-25, and a lamellar sample, 60LC-25, respectively. The concentration of intensity along the equatorial direction in Figure 5a for 73LC-25 indicates that the cylindrical microdomains aligned along the shear flow direction indicated by the black arrow. The concentration of intensity at higher q along the meridional line originates from the smectic A layers and indicates that the LC mesophase formed on cooling with planar anchoring of the mesogens at the IMDS as illustrated schematically in Figure 5b. The WAXS data, inset of Figure 5a, confirm that the system is smectic A as the scattering due to side−side correlations of mesogens within the smectic layers is perpendicular to the smectic layer periodicity. Similar results showing planar anchoring were obtained for a sample which formed LC cylinders, 34LC-25 (Supporting Information). Quite intriguingly, the lamellar system 60LC-25 showed different behavior. The lamellae aligned parallel to the shear flow as shown by the 2-D SAXS data (Figure 5c) where scattering from the block copolymer is concentrated along the meridional line. The presence of the smectic scattering along the same direction indicates that the smectic layer normal is parallel to the lamellar normal, i.e., that the anchoring condition is homeotropic. The 2-D WAXS data (Figure 5c inset) confirm that the system is smectic A and that the LC director is aligned along the lamellar normal. The structural arrangement is shown schematically in Figure 5d. Homeotropic anchoring was also observed for 40LC-25 and 66LC-25. Surprisingly, however, planar anchoring was observed for 50LC-25 and 50LC-50. Additional results and a table summarizing the data are available in the Supporting Information.

Figure 4. Effect of molecular weights on the domain spacing of lamellar forming LCBBCs. TEM images demonstrate symmetric LCBBCs with molecular weights of (a) 25, (b) 50, (c) 100, and (d) 200 kg/mol. Scale bars are 100 nm. Microdomain spacing was characterized from Fourier transformed TEM images. (e) The domain spacing (L0) and the molecular weight (MW) have scaling relation with L0 ∼ MW0.6.

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DISCUSSION Overall, the phase behavior of the LCBBCs is quite analogous to that of a linear coil−coil diblock copolymer system as judged by the rough location of the boundaries between phases. There is a transition from LC spheres to LC cylinders at a composition f LC between 0.15 and 0.27, a transition from LC cylinders to lamellae between 0.34 and 0.40, a transition back to cylinders, PLA, between 0.66 and 0.73, and a final transition to PLA spheres between 0.73 and 0.85. The observation of spherical microdomains in LC block copolymers is unusual due to the putative large defect energies associated with packing a mesophase with uniaxial symmetry into or around a spherical object. From this perspective it is perhaps unsurprising that no clearing transition could be discerned for 15LC-25, suggesting that the mesophase does not form or is disordered to the point of being undetectable using conventional measurement techniques. On the other end of the phase diagram, the LC character of 85LC-25 may be supported the formation of a disordered layer in close proximity to the surface of the E

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range of those considered in studies of NBPS-NBLA bottlebrush copolymers where linear scaling of d-spacing on MW is observed.35,36 As described earlier, prior work on bottlebrush block copolymers has often resulted in the formation of lamellar morphologies, independent of block volume fraction. This propensity to form lamellae is ascribed to a strong preference for flat interfaces driven by the rigidity of the backbone due to stretching induced by steric crowding among the brush or side chains. A notable exception occurs for systems in which the side chain lengths are asymmetric between the blocks, as described by Rzayev et al.21 This result has been captured also in simulation23 while recent work in nanocomposite systems has uncovered an order−order transitions between lamellae and cylinders.24 We speculate that in the present LCBBC system that the strong asymmetry between the size of the LC side groups (∼0.4K) and the PLA side chains (3K) contributes strongly to the ability of these block copolymers to form microdomains with curved interfaces. That the transition among various morphologies proceeds as a function of block volume fraction rather than side chain asymmetry represents another departure from what is currently understood to govern the behavior of bottlebrush systems. The origin of the sublinear scaling of domain size with molecular weight is unclear given the expected stretching of the norbornene backbone by the PLA side chains, particularly in the high molecular weight samples, and the expectation, similar to the case of rod−coil systems,37 that the interfacial area constraints imposed by a semirigid block should dictate the morphology of the system. It is entirely conceivable that different behavior might pertain at larger side-chain lengths than the 3K explored here, i.e., that a 3K side chain is insufficient in the present case to propel the system toward true bottlebrush chain conformations. This is despite the fact that similar molecular weight side chains (2.4K) result in bottlebrush behavior in NBPS-NBLA.35,36 One anticipates that critical insight can be provided here by studies examining the phase behavior of asymmetric high-MW samples and of systems with longer PLA side chains. Finally, the analogy between the chain structure of the low degree of polymerization samples and AnB linear−star block copolymers is worth noting, despite clear differences in the observed phase behavior. Early work in this area by Gido et al. (polyisoprene− polystyrene) I2S38 and I5S BCPs recorded shifted phase boundaries and outright suppression of spherical microdomains, respectively, as well as a useful propensity toward long-range ordering when the linear block was confined to the minority phase of the system.39 A recent review is available.40 We turn attention now to the anchoring condition of the LC. The LC block is formed by cyanobiphenyl species with a 12carbon spacer. The spacer decouples the mesogen dynamics from that of the polymer and prior work has established the importance of this decoupling in enabling LC assembly of mesogens in side-chain LC polymers.41 In particular, the use of long spacers favors the assembly of smectic mesophases, as observed also in our prior work on this LCBBC where a 6carbon unit resulted only in the formation of nematic mesophases.29 Further, the orientational relationship between the LC mesophase and the block copolymer superstructure as dictated by the anchoring condition of the mesogen is critical to understanding the self-assembly of LC block copolymers as it provides a point of reference for evaluating the nonequilibrium states which may arise in the LC mesophase. For example, homeotropic anchoring inside of or surrounding, cylindrical

Figure 5. 2D SAXS and inset WAXS data of the shear aligned (a) PLA cylinder forming 73LC-25 and (b) lamellae forming 60LC-25 systems, along with respective schematic representations of the morphologies (b, d). The samples were shear aligned at 140 °C in the zx and xy planes, along the z and x directions, respectively, as indicated by σz and σx, then briefly kept at 100 °C, and finally slowly cooled to allow the mesogens to form the smectic mesophase. The resulting SAXS data of 73LC-25 indicate that the long axes of the PLA cylinders are parallel to the SmA layer normal, indicating planar anchoring as depicted schematically. Lamellae forming 60LC-25 shows parallel alignment of the lamellar and SmA layer normal, indicating homeotropic boundary condition at the IMDS as depicted in (d).

spherical microdomains to relieve the packing constraints that would necessarily follow from imposition of a strict mesogen anchoring condition at the block interface. The substantial length of the spacer unit, 12 carbons, may play a role in relieving the constraint associated with mesogen anchoring. Further, the observation of spherical as well as cylindrical microdomains in a system with a brushlike architecture is somewhat unexpected. Despite the substantive length of the 3K PLA side chains used in the present study, it is apparent that backbone stretching due to side chain crowding does not present as severe a limitation in the self-assembly of the LCBBC system as it does in bottlebrush block copolymers. This is clear also from the sublinear scaling of lamellar period with molecular weight. The average degree of polymerization for the PLA block ranges from ∼1 to 7 over the range of samples used for the composition-dependent morphology studies, i.e., with total molecular weights of 25K. It may be attractive therefore to ascribe the formation of curved block interfaces here to limited chain stretching simply on account of the small number of side chains that are involved in the PLA blocks. The appeal of this line of reasoning, however, breaks down considering the well-resolved MW scaling behavior recovered for the d-spacing, covering samples with MW up to 400K, where the number of side chains in the non-LC block is substantially higher, ∼65. Importantly, the MW dependence of lamellar periodicity is unchanged in moving from low- to highMW samplesthere is no indication in our data of a change from sublinear to linear in the MW scaling, suggesting that the same physics pertains at low and high MWs. It is worth noting that the PLA side chain MW of 3K and the degree of polymerization for the highest MW samples are squarely in the F

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CONCLUSION We have systematically explored the phase behavior and morphology of a series of liquid crystalline brushlike block copolymers with various volume fractions and molecular weights. Ring-opening metathesis polymerization of n-alkyloxycyanobiphenyl and PLA functionalized norbornene monomers provided precisely controlled LCBBCs. DSC, SAXS, and TEM measurements indicate that the block copolymers display a phase behavior that is very much akin to that observed in linear coil−coil diblock copolymers. The domain spacing scales with molecular weight as L0 ∼ MW0.6, which represents a strong departure from the behavior of bottlebrush block copolymers where L0 ∼ MW1. The findings suggest that the self-assembly of the LCBBCs is more analogous to that of side-chain LC diblock copolymers or graft−coil block copolymers than to that of bottlebrush block copolymers. The observation of cylindrical and spherical microdomains further underscores this point. The system features morphology dependence in the mesogen anchoring condition at the IMDS with planar anchoring observed in cylinder forming systems, but both planar and homeotropic anchoring in lamellar systems. We speculate that the preferred anchoring condition is homeotropic but that this is suppressed in cylinder forming samples due to the associated defect energies and in some lamellar samples due to commensurability constraints. This observation has important consequences on the response of these materials during directed self-assembly under magnetic or other external fields and may open up new routes for controlling morphology in these systems.

microdomains results in a frustrated system due to the creation of a line of +1 defects located along the center line of the cylindrical microdomains. Likewise, homeotropic anchoring may have attendant commensurability issues in confined systems if the domain size is not in close proximity to an integer multiple of the preferred layer spacing. The anchoring condition can have profound impacts on the morphology of LC block copolymers. For example, shear alignment in a cylinder forming side chain LC BCP system was found to lead to transverse orientation of the cylinders and perpendicular orientation of the smectic layers. This morphology represented a compromise in a system in which the cylinders and smectic layers would both align parallel to the shear direction if allowed to do so independently. The intermediate state of transverse cylinders and perpendicular smectic layers resulted expressly from the preservation of the preferred planar anchoring condition in that system.31 Despite the importance of the anchoring condition in dictating morphology and phase behavior, a clear understanding of the role of spacer length on anchoring has not emerged. It seems reasonable to expect that longer spacers enable the system to adopt its preferred anchoring condition, whatever that maybe while short spacers have a propensity to yield planar anchoring in end-attached mesogens in side-chain LCs.16 Given the above considerations, the change in anchoring observed here is striking. The authors are familiar with only one other related instance in which anchoring changed as a function of stoichiometry and morphology in a side-chain LC BCP in which mesogens were hydrogen bonded to the polymer backbone.42 It is possible if not likely that the transition for the LCBBCs is driven by the unfavorable elastic distortion due to +1 defect formation that would be associated with homeotropic anchoring at the cylinder IMDS. That said, homeotropic anchoring in the lamellar system has an attendant commensurability constraint due to packing of the smectic layers along the lamellar domain spacing direction. We speculate that the thermodynamically preferred anchoring condition in the system is homeotropic but that such anchoring is suppressed in the case of cylindrical microdomains due to the attendant energetic penalty of the elastic distortions that would be generated by homeotropic anchoring at a curved interface. We surmise that homeotropic anchoring in the lamellar samples occurs only for those samples in which the mesophase can achieve commensurability within the microdomains. The fact that only the asymmetric lamellar systems displayed homeotropic anchoring may just be a product of coincidence. It is unfortunately not possible to say more regarding the dominant effects here as the relevant length scale which determines commensurability cannot be accurately specified and as the degree of interdigitation can change from sample to sample this is reflected in the variation of the smectic layer spacing from sample to sample in Table 2. Further, we observed some small variations of the d-spacing of sheared versus nonsheared samples (Supporting Information). The difference in anchoring condition, however, has important implications for the alignment of these materials under magnetic and other fields. In the case of magnetic fields, the coincidence of planar anchoring of mesogens with positive magnetic anisotropy in the case of the cylinders, and homeotropic anchoring for lamellae has the attendant benefit of yielding nondegenerate arrangements on field application.43

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02009. Figures S1−S4 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.O.O.). Author Contributions #

Y.C. and L.H.M. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF under CMMI-1246804. The authors thank Mike Degen (Rigaku Inc.) for technical support. C.O. acknowledges additional financial support from NSF (DMR-1410568). R.K. acknowledges additional financial support from NSF (DMR 0748398; DMR-1507045). Facilities use was supported by YINQE and NSF MRSEC DMR1119826. The authors thank personnel at the GPC Facility in PSE at UMass-Amherst for assistance with GPC analysis.

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DOI: 10.1021/acs.macromol.5b02009 Macromolecules XXXX, XXX, XXX−XXX