Interplay between Liquid Crystalline Order and Microphase

Oct 14, 2013 - Interplay between Liquid Crystalline Order and Microphase Segregation on the Self-Assembly of Side-Chain Liquid Crystalline Brush Block...
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Interplay between Liquid Crystalline Order and Microphase Segregation on the Self-Assembly of Side-Chain Liquid Crystalline Brush Block Copolymers Prashant Deshmukh,† Suk-kyun Ahn,‡,∥ Ludovic Geelhand de Merxem,§ and Rajeswari M. Kasi*,†,∥ †

Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Department of Chemistry, University of Rouen, Rouen, Haute Normandie, France 76821 ∥ Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States ‡

S Supporting Information *

ABSTRACT: Herein we investigate the influence of competing self-organizing phenomena on the hierarchical self-assembly of liquid crystalline brush block copolymers (LCBBCs). A library of LCBBCs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising (1) cholesteryl mesogen with nine methylene spacer and (2) semicrystalline poly(ethylene glycol) (PEG). The self-assembly of LCBBCs with variations in LC block content (7−80 wt %) are investigated in their melt state. All LCBBCs show two distinct thermal transitions corresponding to PEG semicrystalline phase and LC mesophases. Interestingly, the LCBBCs display a multilevel hierarchical structure evidenced by the results from X-ray scattering and transmission electron microscopy (TEM): (1) smectic A (SmA) mesophases (d = 3−7 nm) by the assembly of cholesteryl side chains and (2) microphase segregation into lamellar or cylinder (d = 40−75 nm) resulting from the incompatibility between LC moieties and PEG side chain. Surprisingly, the presence of microphase-segregated domains in LCBBCs prevents the formation of cholesteric mesophase in sharp contrast to side-chain liquid crystalline homopolymer (SCLCP) bearing the same mesogen and the flexible spacer. This could be attributed to very high surface to volume ratio at intermaterial dividing surface (IMDS) in LCBBCs, by which only LC layers (i.e., SmA mesophase) are favored to form at the IMDS. On the fundamental side, these LCBBCs are an interesting scaffold to explore the impact of interactions between LC order and microphase segregation of side-chain polymeric brushes on the self-assembly of LCBBCs. Moreover, these new LCBBC scaffolds will serve as a tool box for rational design of hierarchically organized functional materials for stimuli responsive applications. microphase-segregated domains.16−18 Conversely, the microphase segregation in the LCBCPs also impacts the types of LC mesophases that may be produced.19,20 LCBCPs incorporating both LC moieties and semicrystalline polymers have received great attention due to the additional structure forming phenomena (crystallization) built within the system.1,21−26 In some of these systems, it has been found that LC order strongly obstructs the crystallization within the microphase-segregated domain, especially when the LCBCPs display spherical or cylindrical morphologies.21,27,28 In such systems, most research has been concentrated on the LCBCPs comprising linear semicrystalline polymers (e.g., poly(ethylene glycol), PEG) as the first block and LC units in the second block.21,29,30 However, the impact of semicrystalline PEG having nonlinear brush architectures on the self-assembly of LCBCPs has yet to be explored. These densely grafted brushtype polymers with unique untangled side chains are known to

1. INTRODUCTION Control of polymeric self-assembly by exploiting molecular and supramolecular interactions is a useful strategy to create functional materials with hierarchical order for various applications.1−4 For example, block copolymers (BCPs) with microphase-segregated structures on the length scale of 10−100 nm comprising spherical, cylindrical, gyroid, and lamellar morphologies are excellent candidates to create such selfassembled materials.5 The next generation of BCPs which contain moieties exhibiting self-organizing phenomenon such as hydrogen bonding,6−8 π−π interactions,9,10 and liquid crystalline (LC) order11,12 can result in complex hierarchical functional materials. BCPs containing LC segments (LCBCPs) exhibit a hierarchical order due to the microphase separation in the length scale of 10−100 nm and the LC ordering in the length scale of 3−10 nm. The interplay between microphase segregation and LC order has a strong influence on the resulting self-assembled structure.1,5,13−15 The presence of LC clearing transition can alter the interfacial curvature in the LCBCPs, resulting in an order−order transition within the © XXXX American Chemical Society

Received: July 10, 2013 Revised: October 1, 2013

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Table 1. Molecular Characterization of LCBBCs and Homopolymers weight percentagec (wt %) b

entry

polymer

PNBCh9a LCBBC78 LCBBC55 LCBBC28 LCBBC16 LCBBC7 PNBMPEGa

P(NBCh9)125 PNBCh9135-b-PNBMPEG15 PNBCh980-b-PNBMPEG20 PNBCh965-b-PNBMPEG35 PNBCh935-b-PNBMPEG65 PNBCh925-b-PNBMPEG85 P(NBMPEG)50

NBCh9 100 78 55 28 16 7

Mn (kg/mol)

NBMPEG

theord

GPCe

PDIe

22 45 72 84 93 100

84 122 96 119 163 201 108

61 74 41 58 72 92 69

1.09 1.06 1.17 1.20 1.24 1.20 1.12

a

PNBCh9 and PNBMPEG represent homopolymer of NBCh9 and NBMPEG (macro)monomer, respectively, reported from previous publications.36,37 bSubscript represents the degree of polymerization calculated based on monomer to catalyst ratio. cDetermined by 1H NMR analyses. dTheoretical molecular weight calculated by Mn = {[MNBCh9]/[I] × molar mass of NBCh9 + [MNBMPEG]/[I] × molar mass of NBMPEG}, where [MNBCh9], [MNBMPEG], and [I] are moles of NBCh9, NBMPEG, and mG2nd catalyst, respectively. eDetermined by GPC with RI detector, where THF was used as eluent and polystyrene standards were used to construct a conventional calibration.

Figure 1. (a) On the left, synthesis of LCBBC by sequential ROMP using side-chain functionalized NBCh9 and NBMPEG monomer and (b) on the right, the illustration of LCBBC architecture.

exhibit unprecedented self-assembled structures to create nanoobjects, organic nanotubes, and photonic and nanoporous materials.31−33 In this paper, we investigate the impact of LCBCP architecture comprising molecular brush-type semicrystalline PEG units in one block and side-chain LC units in the other block on hierarchical assembly and phase behavior. To this end, we synthesize a series of well-defined poly(norbornene)-based side-chain liquid crystalline brush block copolymers (LCBBCs) bearing cholesteryl mesogens in the first block and semicrystalline PEG (Mn = 2000 g/mol) in the second block. Morphologies of LCBBCs investigated by scattering and microscopic techniques reveal various hierarchical structures as a result of the interplay between the microphase segregation in brushlike macromolecules and the LC order. Our current study demonstrates the role of microphase segregation from brushlike PEG domains on LC mesophase behavior. The synthesis of LCBBCs and their structure−property relationship

study can provide a useful template for the creation of stimuli responsive polymers.34,35

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. Two monomers αmethoxy-ω-norbornenyl-PEG (NBMPEG) 36 and 5-{9(cholesteryloxycarbonyl)nonyloxycarbonyl}bicyclo[2.2.1]hept2-ene (NBCh9)37 are synthesized according to previously reported procedures. A series of LCBBCs with various compositions of NBCh9 and NBMPEG are synthesized by sequential ring-opening metathesis polymerization (ROMP) using a modified Grubbs catalyst second generation (mG2nd) ((H2IMes)-(pyr)2(Cl)2RuCHPh),38 and their characterizations are summarized in Table 1: details of polymerization can be found in the Supporting Information. The compositions of LCBBCs are determined by integrating characteristic peaks of NBCh9 (a proton at 4.6 ppm) and NBMPEG (three protons at 3.36 ppm) in 1H NMR spectra and by comparing the ratio of their integration values (Figure S1). We will use the following B

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terminology to define the block copolymers: LCBBCx represents LC brush block copolymer, where x represents weight percentage (wt %) of NBCh9 in the copolymers. 2.2. Thermal Properties. Thermal properties of LCBBCs are investigated using differential scanning calorimetry (DSC) as shown in Figure 2 and Table S1. During cooling cycle,

Figure 2. DSC traces of LCBBCs during the first cooling cycle exhibit crystallization of PEG (Tc) and LC mesophase transitions (T1 and T2) shown in the inset: (a) LCBBC7, (b) LCBBC55, and (c) LCBBC78.

Figure 3. SAXS diffractograms of (a) LCBBC7, (b) LCBBC16, (c) LCBBC28, (d) LCBBC55, and (e) LCBBC78 recorded at room temperature. Primary scattering peaks (q*) and their higher order reflections corresponding to microphase segregation are indicated as black arrows. qPEG (blue arrows) indicates PEG lamellar. qLC1 and qLC2 (red arrows) correspond to LC ordering.

crystallization temperature (Tc) of PEG and two LC mesophase transitions (T1 and T2) is observed, and the intensity of these phase transitions is largely influenced by the amount of LC content in the LCBBCs. On one hand, higher LC content in the LCBBCs increases the enthalpy of LC mesophase transitions (Figure 2, inset): for example, the mesophase transitions are more pronounced in LCBBC78 as compared to LCBBC55 and LCBBC7. On the other hand, the higher LC content in the LCBBCs decreases the enthalpy of PEG crystallization. In block copolymers, microphase-segregated domains tend to create different populations of crystalline and amorphous regions, and as a result, the crystallization temperature varies between 10 and 30 °C. However, the different populations of crystalline and amorphous regions do not have a significant impact on the melting temperature as reported previously.27,39 The type of mesophase in LCBBCs will be resolved by X-ray scattering analyses, which will be discussed in the following sections. 2.3. Microstructural Analysis of LCBBCs. The unique LCBBC architecture incorporates microphase segregation, PEG crystallization, and LC order. Therefore, the interplay of these structural parameters can be manipulated by tuning compositions of two blocks and this will determine the self-assembly of LCBBCs at multiple length scales. Hierarchical order of microphase-segregated domain PEG crystalline lamellar and LC mesophase is examined by X-ray scattering and transmission electron microscopy (TEM) on melt processed film samples. 2.3.1. Microstructural Analysis at Room Temperature. Figure 3 represents small-angle X-ray scattering (SAXS) diffractograms of LCBBCs where three types of scattering reflections are noted, including (1) microphase segregation (q* and its higher order reflections), (2) PEG crystalline lamellar (qPEG), and (3) LC order (qLC1, qLC2). (1) Microphase segregation (d = 40−73 nm): All LCBBC display microphase segregation (q*) with higher order reflections indicated by black arrows in Figure 3. Generally, the composition of BCPs governs the type of morphology for

the resulting microphase-segregated domains.1,11,25,40 Table 2 summarizes the domain (d) spacing values (40−73 nm) and speculated morphology based on the correlation between q* and higher order reflections of the principal scattering vector. For high LC content polymer (i.e., LCBBC78), primary scattering and its higher order reflections are in the ratio of 1:√3:√7, which suggests PEG cylinder within LC matrix with d-spacing of 43.7 nm. Based on this result, PEG cylinder to cylinder spacing can be estimated as d0 = 50.5 nm [using d0 = (4/3)1/2d]. The lower LC content polymer (i.e., LCBBC55) exhibits primary scattering and its higher order reflections in the ratio of 1:2:3, which may be attributed to lamellar morphology with d-spacing value of 54.7 nm. Meanwhile, polymers with even lower LC content (i.e., LCBBC7, LCBBC16, and LCBBC28) show unresolved or absent higher order reflections, which suggests the absence of long-range order. Thus, the morphologies of these polymers are difficult to be delineated. The lack of higher order peaks suggest that breakout PEG spherulite crystals disrupt the microphasesegregated domains, leading to weak long-range order.21,41,42 (2) PEG crystalline domains (d = 12−13 nm): The semicrystalline PEG side chains in LCBBCs can crystallize into lamella, and its thickness depends on the molecular weight of PEG. The periodic structure resulting from the alternate crystalline lamella and amorphous domains of PEG is known to show a signature reflection in SAXS.43−45 Similarly, LCBBCs display qPEG scattering peak (qPEG = ∼0.5 nm−1, d = 12−13 nm) indicated by blue arrows in Figure 3, except for LCBBC78. The qPEG disappears after melting transition of PEG (Tm = ∼50 °C) as shown in the temperature-dependent SAXS analyses (Figure S3), which suggests these peaks do not originate from microphase segregation of BCPs. The uncorrelated multiple scattering reflection imply the presence of hierarchical morphologies (structure-within-structure) in LCBBCs. InterC

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Table 2. Domain (d) Spacing Values for LCBBCs and Homopolymers Determined by SAXS smectic layers

PEG lamellar

polymer

LC1 (nm)

LC2 (nm)

PNBCh9 LCBBC78 LCBBC55 LCBBC28 LCBBC16 LCBBC7 PNBMPEG

6.3 6.0 6.1 6.0 6.5 7.2

3.4 3.5 3.5

dPEG (nm)

microphase segregation d-spacing (nm)

morphologya

hierarchical structure

43.7 54.7 73.1 49.9 40.3

cylinder lamellar not determinedb not determinedb not determinedb

PEG cylinder within LC matrix lamellar within lamellarc

12.5 12.8 13.2 13.6 14.1

a

Entire scattering reflections are reported in Table S2. bMorphologies in these polymers are unable to determine due to weak higher order reflections. cIllustration of LCBBC55 and thin film TEM images are shown in Figure S2.

Figure 4. Microstructural analysis of LCBBC78 at room temperature wherein the X-ray beam was focused on the edge of the compression-molded film. (a) 1D-SAXS pattern exhibits microphase segregation peaks (q*, √3q*, and √7q*) in low q region, and smectic layer peaks (qLC1, qLC2) in high q region. (b) 2D X-ray scattering patterns: (top) 2D-SAXS pattern shows the orientation of microphase segregation (I) in equatorial direction and the orientation of smectic layers (II) in meridian direction. (bottom) 2D-WAXS pattern shows the orientation of PEG crystalline domains in meridian direction and broad hollow (III) in lateral direction is associated with the lateral distance of the cholesteryl mesogens. (c) Proposed hierarchical structure in LCBBC78: (top) microphase-segregated structure where PEG crystalline cylinders are embedded within LC matrix, and (bottom) LC layers are perpendicular to the microphase-segregated domain where mesogen arranges parallel to IMDS leading to homogeneously anchoring condition.

content (i.e., LCBBC7 and 16), loosely packed smectic layers are developed due to the dilution effect, where boarder LC scattering reflection indicates poorly defined layers.47 In addition to qLC1, the qLC2 peak, which does not correlate with the qLC1, is also detected in higher LC content polymer (i.e., LCBBC55 and 78) with d-spacing of 3.5 nm. This is attributed to smectic monolayers (SmA1), similar to the our previous observation for the PNBCh9.37,46 Smectic polymorphism or coexistence of more than one type of smectic layer in thermotropic LC polymers has been previously reported.48−50 Because of the more pronounced LC transition, we will focus on the LCBBC78 sample. Hierarchical arrangement within LCBBC78 is determined by 2D-SAXS and WAXS analyses on a film sample prepared by compression molding followed by thermal annealing, by which an anisotropic orientation in the self-assembled structure is produced. In the SAXS diffractogram shown in Figure 4a, the microphase segregation scattering reflections are in the ratio of 1:√3:√7, suggesting a cylindrical packing of PEG domains within the LC matrix. Hierarchical arrangement within LCBBC78 is determined by 2D-SAXS and WAXS analyses. Figure 4b shows scattering patterns of smalland wide-angle regions in the top and bottom, respectively. In these two scattering patterns, the smectic layers (II) and PEG

estingly, only LCBBCs having lower LC content allows the development of periodic structure from crystalline lamellar and amorphous domains in PEG. In contrast, LCBBC78 does not show this scattering reflection probably because of the presence of PEG cylinders within LC matrix which may inhibit the formation of periodic crystalline lamella within these cylinders. (3) LC order (d = 3−7 nm): LCBBC architecture also features side-chain cholesteryl mesogens, which forms smectic A (SmA) layers. In the higher q range, SmA layer reflections (qLC1) are observed with layer spacing between 3 and 7 nm (Figure 3). For example, in typical 2-D SAXS and WAXS of LCBBC78 (Figure 4b), the LC layer reflections (II) are stronger along the meridian direction while the lateral distance of cholesteryl mesogen scattering (III) is stronger along the equator direction. The observed orthogonal arrangement indicates typical SmA mesophase. The length of cholesteryl side chain with nine methylene spacer is calculated to be 3.34 nm, and the thickness of smectic bilayer (SmA2) is calculated to be 6.68 nm.46 Comparison between the calculated values and the experimentally observed d-spacings of SmA layer suggests that LC orderings in LCBBCs consist of SmA2 for lower LC content polymers or interdigitated smectic layers (SmAd) for higher LC content polymers. For the LCBBCs having lower LC D

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regions showing dark rodlike area probably due to the side view of cylinders (Figure 5b). Morphology consisting of the circular and rodlike domains of PEGs indicates the absence of longrange order in LCBBC78,21 which may be associated with sample preparation. Average cylinder diameter of ∼20 nm is estimated from TEM micrographs, but the absence of longrange order makes it difficult to compare the data obtained from TEM to that from SAXS analysis. To summarize, the unique LCBBC scaffold consisting of semicrystalline PEG side chain and cholesteryl mesogen selfassembles into multilevel hierarchical nanostructures. Assuming moderate segregation between the blocks, the composition of LCBBCs governs the morphology of the resultant microphasesegregated domains. Specifically, in higher PEG content copolymers (i.e., LCBBC7, LCBBC16, and LCBBC28), periodicity of PEG crystalline lamellar is observed within microphase-segregated domains. However, in higher LC content copolymer LCBBC78 (i.e., lower PEG content) PEG cylinders are embedded within the LC matrix. The interplay between different types of self-organization (i.e., microphase segregation, crystallization, and LC order) results in a hierarchically ordered system, which was supported by X-ray scattering and TEM analyses: microphase-segregated domains (40−70 nm), PEG crystalline regions (∼13 nm), and LC mesophase (3−7 nm). We also synthesized polymers containing even higher LC content (i.e., LCBBC85 and LCBBC90) to explore the composition effect of the copolymer on microphase segregation (see details in the Supporting Information, Tables S3 and S4). LCBBC85 tends to weakly microphase segregated (broad primary reflection and lack of higher order reflections), whereas LCBBC90 does not microphase segregate (Figure S7).

crystalline reflection are present in the meridian direction, whereas the microphase-segregated domains (I) are present along the equatorial direction. On the basis of these orientations, we speculate that the smectic layers are perpendicular to the microphase segregation and the LC mesogens are parallel to the intermaterial dividing surface (IMDS)homogeneously anchoring13,21as illustrated in Figure 4c. TEM of the ultrathin sections of LCBBC78 are performed to further investigate the morphology of microphase-segregated domains. The samples are prepared by sectioning with ultrathin cryo-microtome and stained with RuO4 vapors. In Figure 5a,

Figure 5. TEM micrographs of LCBBC78: (a) top view of cylindrical PEG domain appeared as dark circular spots and (b) side view of PEG cylinders appeared as rodlike domains. Additional images are shown in Figure S8.

LCBBC78 displays dark circular domains representing the top views of PEG cylinders since PEGs are expected to be preferentially stained by RuO4. In addition, there are also

Figure 6. Microstructural phase evolution of LCBBC78 using temperature-dependent SAXS analysis. (a) Temperature-dependent SAXS plot, where q*, qLC1, and qLC2 represent microphase segregation, SmAd, and SmA1, respectively. (b) Mesophase transitions (T1 and T2) observed in the first heating cycle of DSC. (c) Illustration of mesophase evolution observed in LCBBC78. (i) At room temperature: crystalline PEG cylinders embedded in the matrix of smectic layers, (ii) Tm < T < T1: crystalline PEG melts and smectic layers are retained (a picture is not shown), (iii) T1 < T < T2: ordering of smectic layers decreases and mesogens are homogeneously anchored at the IMDS of amorphous PEG cylinders, and (iv) T > T2: LC clearing transition occurs at T2 while the microphase-segregated domains are still retained. E

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2.3.2. Mesophase Evolution of LCBBC78 Investigated by Temperature-Dependent SAXS. To examine the combinational impact of microphase segregation, PEG crystallization, and LC order on the final morphology of LCBBC78, temperature-dependent SAXS is further performed, and the results are shown in Figure 6a. As previously discussed in thermal analysis (Figure 2), LCBBC78 displays two LC transitions (T1 and T2), which will be elucidated in the next paragraph. SAXS analysis at room temperature for LCBBC78 film sample shows three reflections (q*, √3q*, √7q*) due to microphase-segregated PEG cylinders in the lower q range and two smectic layer reflections (qLC1 and qLC2) in the higher q range. As sample is heated above Tm (50.5 °C), PEG crystallite melts within microphase-segregated domains creating amorphous PEG cylinders. Upon heating the LCBBC78 above T1 (92 °C), the intensities of both qLC1 and qLC2 gradually decrease, and only qLC1 remains between T1 and T2. On the basis of this result, we speculate that smectic to smectic transition occurs at T1, wherein the order of smectic layer is largely reduced. When the polymer is further heated above T2, all smectic layers completely disappear, implying that T2 is LC clearing temperature. It should be noted that the microphase segregation peaks are still preserved even after heating the polymer above T2. It has been suggested that the microphase segregation in chiral LCBCPs unwinds the pitch of helical mesophase (e.g., cholesteric and smectic C*) due to the presence of preferred anchoring condition at the IMDS of microphase-segregated domains.11,51 The 2D SAXS data (Figure S4) for LCBBC78 collected at 100 °C show “weak” orthogonal arrangement of microphase segregation peak (q*) and LC order (qLC1), implying homogeneous anchoring of cholesteryl mesogen at IMDS, and these smectic layers are perpendicular to the microphase-segregated domains. The primary microphase-segregated reflection (q*) is preserved up to 140 °C, while the d-spacing associated with microphase segregation gradually decreases from 54.2 nm (room temperature) to 49.5 nm (140 °C) as shown in Figure S5. The order−disorder transition temperature of BCP microphase segregation is not accessible within experimental temperature range, which may be due to the high molecular weight of LCBBCs and the presence of anisotropic LC mesogen.1,11 All LC transitions are reversible in the cooling cycle, where the reappearance of LC scattering peaks (qLC1 and qLC2) below T2 and the increase of intensity below T1 are clearly observed (Figure S6). Thus, two different LC transitions (T1 and T2) observed in thermal analysis originate from smectic-to-smectic mesophase transition (for T1) and LC clearing transition (for T2), respectively. 2.4. Influence of LC Polymer Architecture on Mesophase Behavior. In the present study, we synthesized new LCBBCs comprising semicrystalline PEG side chain and cholesteryl mesogen where the interplay between microphase segregation of BCPs, crystallization of PEG, and LC ordering play important roles on the final self-assembly as illustrated in Figure 7. The illustration compares the mesophase behavior between LCBBC78 and SCLCP, both of which incorporate cholesteryl mesogen attached to backbone with nine methylene spacer. Interestingly, both SCLCP and LCBBC78 exhibit two distinct LC transitions. In our previous study, LC transitions (T1 and T2) detected in the SCLCP originated from smectic to cholesteric transition and LC clearing transition, respectively.46

Figure 7. Impact of LC polymer architecture on the mesophase behavior observed in side-chain liquid crystalline polymer (SCLCP) of PNBCh937,46 and LCBBC78.

The presence of cholesteric phase was further supported by observation of its characteristic light reflection property which resulted from the formation of helical structure. In the present study, however, observed LC transitions (T1 and T2) in the LCBBC78 are associated with smectic to smectic transition and LC clearing transition, respectively. We hypothesize that in the case of LCBBC78 the presence of microphase-segregated domains and very high surface to volume ratio at IMDS induce cholesteryl mesogens to preferentially anchor in homogeneous manner (i.e., parallel to the IMDS). This preferred anchoring condition of cholesteryl mesogens leads to the development of smectic layers leading to T1 transition. Overall, due to the anchoring of smectic layers, the formation of cholesteric mesophase is prohibited in LCBBCs, indicating that microphase segregation significantly impacts on the type of mesophase in copolymers as shown in Figure 7. To summarize, LCBBC copolymer architecture consisting of incompatible brushlike PEG and cholesteryl mesogen engenders a hierarchical structure. The presence of cylindrical PEG microphase-segregated domain remarkably increased the thermal stability of the LC order. Hence, in the case of LCBBC78, the smectic layers are preserved after T1 mesophase transition temperature. The systematic study of LCBBCs including synthesis characterization and thorough examination on their hierarchical morphologies in nanoscale will allow for the creation of new functional materials.

3. CONCLUSION We report the synthesis and the characterization of well-defined LCBBCs comprising semicrystalline PEG side chains and cholesteryl mesogen. In this LCBBC scaffold, the interplay F

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between PEG crystallization, LC ordering, and microphase segregation governs the overall self-assembly and produces hierarchically ordered nanostructures at multiple length scales. On the basis of temperature-dependent SAXS studies of LCBBC78, we speculate that the microphase-segregated domains dictate the anchoring of the LC mesogen at the IMDS, leading to the preferential development of smectic layers over chiral mesophases. Our synthetic method is a rational tool to incorporate different functionalities within the BCP architecture and thereby a methodology to control the phase behavior and properties of functional materials. The synthesis and morphological evaluation focusing on the interplay of selforganization phenomena on the final hierarchical self-assembly will serve as a tool to design more interesting and promising BCP functional materials,52−54 sensors, actuators,55,56 and stimuli responsive smart materials.57,58

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ASSOCIATED CONTENT

S Supporting Information *

Experimental/characterization procedures, additional figures/ tables including 1H NMR spectra of polymer, thermal analysis table, SAXS diffractograms, illustration of polymer selfassembly. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.M.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided University of Connecticut Research Foundation Faculty Grant, NSF CAREER Award to R.M.K. (DMR-0748398). Central instrumentation facilities in the Institute of Materials Science, Chemistry Department, and the TEM facility at Physiology and Neurobiology (PNB) are acknowledged. We also thank Stephen Daniels for sectioning samples and performing TEM experiments. The authors are very thankful to the NSF-MRSEC X-ray Scattering Laboratory at University of Massachusetts Amherst for using their RigakuMolmet SAXS equipment and Dr. Dhanasekaran Thirunavukkarasu for helpful assistance with SAXS data collection. We also thank Dr. Manesh Gopinadhan and Prof. Chinedum O. Osuji (Yale University) for helpful discussions.

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