Optically Active Elastomers from Liquid Crystalline Comb Copolymers

Jul 27, 2017 - We report on the synthesis and properties of cholesteric liquid crystalline random terpolymers with comblike architecture as a modular ...
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Optically Active Elastomers from Liquid Crystalline Comb Copolymers with Dual Physical and Chemical Cross-Links Lalit H. Mahajan,‡ Dennis Ndaya,† Prashant Deshmukh,† Xiayu Peng,‡ Manesh Gopinadhan,§ Chinedum O. Osuji,§ and Rajeswari M. Kasi*,†,‡ †

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

ABSTRACT: We report on the synthesis and properties of cholesteric liquid crystalline random terpolymers with comblike architecture as a modular platform for preparation of stimuli-responsive photonic elastomers. Ring-opening metathesis of norbornene monomers bearing n-alkyloxy cholesteryl (Ch9), n-alkoxy cyanobiphenyl (CB6 or CB12), and poly(ethylene glycol) (PEG) side chains is efficient and quantitatively yields low polydispersity random terpolymers. This terpolymer scaffold selfassembles to form cholesteric mesophases (N*) in which microphase-segregated domains of PEG side chains are randomly embedded. The cholesteric mesophase provides a 1D photonic band gap structure at optical wavelengths, which is maintained during chemical cross-linking of the norbornene backbone to form elastomers. The presence of cyanobiphenyl mesogens leads to an increase in the helical pitch of the cholesteric mesophase, resulting in a red-shift of the reflectivity relative to the pure cholesteric mesophase. By contrast, the presence of PEG blue-shifts the reflectivity, such that the overall optical properties can be readily tailored by the composition of the terpolymer. Furthermore, the mechanical properties of the materials are enhanced by the presence of the microphase-separated PEG domains which act as physical cross-links and also provide plasticization of the system. The terpolymers described here provide a modular and versatile platform for the realization of photopatternable materials that exhibit shape memory and thermochromic properties.

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INTRODUCTION

Liquid crystalline block copolymers (LCBCPs), comprising a block of linear semicrystalline or amorphous units and another block of comblike side chain liquid crystalline moieties, form hierarchical structures due to two competing processes: (1) orientational ordering by LC mesogens and (2) microphase separation by the block copolymer superstructure.19−22 This supramolecular cooperative assembly results in a well-defined interface such that LC ordering is associated with 3−10 nm mesostructures and the blocks phase segregate into 10−100 nm microstructures. These LCBCPs present morphologies typical of traditional block architecture and respond reversibly to various combinations of external stimuli.23,19,24−26 However, morphologies containing helical mesophases with periodicities comparable to the wavelength of visible light, particularly

Polymer-based 1D photonic crystals are attractive due to their flexibility, processability, and tunable reflectivity over a broad wavelength range, including optical wavelengths.1 Compared to the small molecular analogues, polymer-based 1D photonic crystals show enhanced optical stability and the ability to selectively incorporate additives, other functionalities, or crosslinking units to enhance processing, elastomeric properties, and mechanical robustness.2−6 Polymer microstructures with periodic domain sizes comparable to wavelength of light can be obtained by (1) use of ultrahigh molecular weight linear block copolymers (BCPs),7 (2) use of small molecular additive to swell domain sizes of BCPs,8−12 (3) use of high molecular weight comb or brush random copolymers or BCPs,13−16 and (4) addition of metallic or semiconductor nanoparticles within brush or comb BCPs.17,18 Reflections appear as red, blue, green, or even near-IR depending on the interaction of periodic BCP microstructures with the electromagnetic spectrum. © XXXX American Chemical Society

Received: June 4, 2017 Revised: July 11, 2017

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Figure 1. Schematic of 1D photonic elastomer entrapping multilevel hierarchical structure by means of self-assembly and molecular cross-linking. (a) Liquid crystalline side-chain monomers (NBCh9 and NBCB12 or NBCB6) and semicrystalline PEG side chain containing monomer (NBPEG). (b) Random terpolymer architecture synthesized by ROMP using modified Grubbs second-generation catalyst at room temperature. (c, d) UV crosslinking at 74 °C to immobilize helical cholesteric mesophase (N*), PEG domains, and minor amounts of smectic polymorphic domains. (e) Hierarchical structure with the retention of photonic reflections along with improved mechanical features is made possible by of PEG physical junction points and chemical cross-linking to connect 1D photonic helical domains.

mesophase, shift to a larger pitch, and enable variation in optical band gap and color of reflected light, i.e., enable blue to red color shift.30−32 Furthermore, in addition to physical junctions from microphase-segregated PEG domains, olefinic units in the polynorbornene backbone can be chemically crosslinked resulting robust elastomers. Here, we report comb-type polynorbornene-based terpolymer single-component platform featuring tunable 1D photonic properties within robust, elastomeric networks due to presence of both physical junctions and chemical cross-links (Figure 1). This comblike random terpolymer platform comprises of two different LC side chains and PEG side chains. Mesophasic 1D photonic functionality is achieved by the virtue of cholesteric (N*) mesophase produced from one of the LC mesogens (cholesteryl bearing 9 methylene spacer, Ch9). Tunability of these photonic reflections is achieved by incorporation of a second LC mesogenic comonomer (cyanobiphenyl bearing 12 or 6 methylene spacers, CB12 or CB6, respectively), which cooperatively interacts with Ch9 mesogens. The PEG side chains function as an internal plasticizer and will help processing these materials.29,28 We systematically explore the effect of molecular engineering on the hierarchical structures in these polymers that results from the interplay of competing LC−LC interactions and the microphase segregation due to comblike PEG side chains and their thermal features. UV cross-linking polynorbornene backbone33 in situ or by quenching the samples from the cholesteric phase results in hierarchically structured elastomers with the retention of cholesteric mesophase and its 1D photonic features. These hierarchical elastomers present two

cholesteric or chiral smectic C (SmC*) mesophases, in conjunction with microphase-segregated domains are challenging to attain.24,22 Very high surface-to-volume ratio at the interface between the two blocks preferentially results in smectic ordering and inhibits helical 1D photonic mesophase formation in these LCBCPs. A major challenge to overcome is the development of a LC polymer platform that couples selfassembled tunable 1D photonic cholesteric mesophase properties and microphase-segregated BCPs features in addition to stimuli-responsive properties of liquid crystalline moieties. We previously reported the synthesis of polynorbornene random copolymer architecture containing comblike PEG side chains and cholesterol based LC side groups.27−29 In these random comb or brush polymers, side chains (LC or PEG) will phase segregate into separate PEG-containing microdomains and LC-containing mesodomains. Experimentally, we observe these random comb polymers self-assemble into hierarchical structures comprising PEG microphase-segregated domains (∼12−15 nm) with 1D photonic cholesteric mesophases (∼300−450 nm) above ∼80 °C.27 The cholesteric reflection bands originating from the Ch9 mesogen are limited to blue and green colors. PEG inclusions are exploited in two ways: (i) shift to a smaller helical pitch, and (ii) PEG physical junction domains serve as an internal plasticizer improving processability of these materials. However, films of these materials are still very brittle and unable to handle external stress or stimuli. We hypothesize that introduction of an achiral mesogen into the random copolymer will lead to LC−LC cooperative interactions between the achiral mesogen and cholesterol, which will in turn impact the helical twisting power of the cholesteric B

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Macromolecules Table 1. Polymer Composition and Molecular Weight weight percentageb entry

polymer

NBCh9

NBCB12

NBPEG

Mn,c kDa/mol; ĐM (PDI)

TP1_75-10-15 CP1_86-14-0 CP*_84-0-16

P(NBCh9-r-NBCB12-r-NBPEG) P(NBCh9-r-NBCB12) P(NBCh9-r-NBPEG)

74.58 85.59 84.47

9.94 14.41

15.48

72.32 (1.13) 66.52 (1.10) 85.8 (1.04)

a

15.53

a

Terpolymer TP1_75-10-15 has 75 wt % of NBCh9 (cholesteric) monomer, 10 wt % of NBCB12, and 15 wt % of NBPEG, and subscripts in Ch9/ CB12 represent number of methylene units. Copolymer CP1_86-14-0 has 86 wt % of NBCh9 and 14 wt % of NBCB12. CP*_84-0-16 has 84 wt % of NBCh9 and 16 wt % PEG. bWeight percentages of each monomer in random terpolymer and copolymer samples are determined by 1H NMR integrations of the peaks at 4.6, 3.36, and 6.96 ppm corresponding to NBCh9, NBMPEG, and NBCB12 monomers, respectively. cDetermined by GPC with ELSD detector, where THF was used as eluent and polystyrene standards are used to construct a conventional calibration. *Previously reported sample.27 optical properties. In method II, a mixture containing terpolymer, UV cross-linker (1.75 wt %), and UV-initiator (2 wt %) is maintained at various temperatures between 70 and 95 °C (sandwiched between the glass plate and silanized silica plate) for 15 min under UV irradiation. Several elastomers are prepared using these processing methods and varying concentrations of dithiol curing agents (1−5 wt %) (Table S3). The cross-linked samples are labeled as nXL-TP1_75-10-15 (temperature of cross-linking), where n denotes wt % of cross-linker used. Extent of cross-linking or gel fraction for each elastomer is determined from ratio of mass of elastomer after and before 24-h Soxhlet extraction using THF. Gel fraction values vary from 34 to 76% (Table S3). Samples cross-linked at temperatures between 70 and 95 °C containing same weight percentage of cross-linker (1.75 wt %) show higher gel fraction (76%) as compared to sample cross-linked at room temperature (41%). Higher extent of cross-linking here can be attributed to reduction in viscosity at higher cross-linking temperature, resulting in improved movement of polymeric chains than samples cross-linked at room temperature. We will mainly focus on the optimized cross-linked sample with higher gel fraction, 1.75XL_TP1_75-10-15 (74 °C). Composition, molecular characteristics, thermal, morphological, and mechanical properties of samples cross-linked at room temperature are discussed in the Supporting Information.

levels of cross-linking provided by (i) physical junctions from comblike PEG microphase-segregated domains and (ii) chemical networks provided by UV cross-linking (Figure 1). Finally, dual functional properties, i.e., shape memory effect, of patterned photonic films are presented. This combination of structural control by molecular level design of new polymer architecture and mechanical control by incorporating physical and chemical cross-linking is very versatile. This enables patterning of the terpolymer platform using PDMS templates, and cross-linking/processing in the cholesteric state can be reversibly deformed and reformed, resulting in thermal actuators with photonic features.34−36

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EXPERIMENTAL SECTION

Synthesis of Copolymers and Terpolymers. Monomers NBCh9,37 NBCB12,33 NBCB6,33 and NBPEG27 are synthesized as reported. Terpolymers and copolymers are synthesized by ringopening metathesis polymerization (ROMP) of these monomers using modified Grubbs generation II catalysts,38 and their compositions are presented in Table 1 and Table S1. We will focus on the synthesis of a terpolymer TP1_75-10-15 comprising ∼75 wt % of NBCh9, ∼10 wt % of NBCB12, and ∼15 wt % of NBPEG. Here subscripts 9 and 12 represent the number of methylene units between norbornene and the liquid crystalline molecule. Composition of terpolymers is determined by integrating the peaks in the 1H spectrum (Figure S1) at 4.6, 3.36, and 6.96 ppm corresponding to NBCh9, NBPEG, and NBCB12 monomers, respectively. In addition to this terpolymer, a copolymer CP1_86-14-0 is synthesized with ∼86 wt % of NBCh9 and ∼14 wt % of NBCB12 to elucidate the effect of PEG grafts in the terpolymer. Also, a previously reported copolymer CP*_84-0-1627 is synthesized to elucidate the molecular LC−LC interactions between Ch9 and CB12 side chain mesogens. Composition, molecular characteristics, and thermal, morphological, and mechanical properties of all other terpolymers and copolymers are elucidated in the Supporting Information. All polymers included in this study have statistically random architecture, which is established by studying the kinetics of individual monomers in ROMP (Figure S2).39,40,27 Mesogenic monomers NBCB6, NBCB12, and NBCh9 as well as NBPEG show comparable rates of ring-opening metathesis polymerization, which is responsible for the formation of statistically random copolymers and terpolymers. UV Cross-Linking. Random terpolymer TP1_75-10-15 is chosen as a representative sample to optimize UV cross-linking chemistry to obtain narrow photonic reflection band and produce robust elastomers. Two different cross-linking methods are employed. In method I, the reaction mixture containing terpolymer, UV cross-linker (1,10-decanedithiol, 1−5 wt %), and UV-initiator (2 wt %) is compression molded at 90 °C and quenched to room temperature (T < Tg), followed by UV exposure for 5 h at room temperature. The cross-linked samples are labeled as nXL-TP1_75-10-15 (rt), where n denotes wt % of cross-linker used and rt refers to room temperature cross-linking. This method is used to optimize the concentration of UV cross-linker required to attain elastomers with the retention of

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RESULTS Thermal Properties. Thermal transitions in terpolymer TP1_75-10-15 and corresponding elastomer 1.75XL_TP1_7510-15 (74 °C) with gel fraction of 76% are investigated using differential scanning calorimetry (DSC) (Figure 2). Both the terpolymer and the corresponding elastomers present depressed PEG crystallization transition temperature (Tc) around −40 °C, possibly due to confinement effects.27,41−43 In the terpolymer sample, two distinct LC phase transitions (TLC1 and TLC2) are observed during the cooling cycle in the temperature range 65−100 °C. The transition temperatures observed in TP1_75-10-15 are similar to that observed in CP*_84-0-1627 and LC transitions in CP1_86-14-0 (Figure 2). Elastomer 1.75XL_TP1_75-10-15 (74 °C) reveals two very broad LC transitions between 70 and 90 °C and LC clearing transition ∼125 °C. It is reasonable to assume that the clearing transition is shifted to a higher temperature due to cross-linking. Other terpolymers and copolymers also exhibit similar trend in LC transitions and PEG crystallization temperatures (Table S2 and Figure S3). In elastomers prepared by cross-linking at different temperatures, two very broad LC transitions between 70 and 90 °C and a third, probably, LC clearing transition around 125 °C are noted (Figure S5). Comparison of DSC profiles of 1.75XL-TP1_75-10-15 cross-linked at room temperature and at 74 °C shows that processing techniques and crosslinking temperature impacts extent of cross-linking and the C

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and both qLC1 and qLC2 reflections are present between 25 and 83 °C. Above this temperature, system transitions from the smectic to cholesteric mesophase, which then clears above 100 °C. It should be noted that cholesteric mesophase lacks characteristic X-ray reflections due to limited q-range accessible with lab scale SAXS as noted with CP*_84-0-16.27 However, existence of cholesteric mesophase domains is established by UV−vis reflectance and TEM which show well-known cholesteric signature for samples processed above 75 °C and quenched to room temperature as described in later sections.44,31,45,46 It should also be noted that samples with lower fractions of PEG, TP1_81-10-9 for example, containing ∼9−10 wt % of PEG, do not show microphase-segregated domains (Figure S6). Cross-linked terpolymer 1.75XL_TP1_75-10-15 (74 °C) with gel fraction of 76% is examined by temperature-controlled SAXS (Figure 3a). The SAXS profile shows q* reflection pertinent to PEG microphase-segregated domains and broad qLC2 reflection pertinent to SmA1 mesophase similar to TP1_7510-15 while qLC1 reflection is absent at room temperature. However, the qLC2 reflection is very broad and reduced in intensity compared to qLC2 reflection in TP1_75-10-15. The presence of low-intensity smectic reflections in the cross-linked materials maybe due to lower cross-linker concentration or lower cross-linking temperature that leads to incomplete locking of the helical structure locally at some portions of the film. DSC data along with T-SAXS study indicate that smectic to cholesteric mesophase transition occurs around 68 °C, while the LC clearing transition is observed close to the PEG disordering temperature ∼120 °C highlighting stabilization of the cholesteric phase upon cross-linking. Overall thermal characterizations using T-SAXS and DSC suggest that cross-linking/processing at higher temperature is more efficient compared to cross-linking at lower temperature (rt) to dominantly cross-link cholesteric helical domains over other LC domains (Figures S4, S5, and S11, Figure 3a). Thus, terpolymers cross-linked at 74−78 °C present a hierarchical structure with PEG microdomains serving as physical junctions and cholesteric helices predominantly enchained by chemical cross-links between helices. TEM Analyses. Samples for TEM are prepared by compression molding the materials at 85 °C and subsequently quenching the films with a jet of compressed air (T < Tg) to lock the hirarchichal structure. Compression-molded films of TP1_81-10-9 sample are cryo-microtomed at −80 °C and preferentially stained with ruthenium tetraoxide. TEM images show characteristic figerprint cholesteric structure47,46,48 with alternate bright and dark bands (Figure 4a), where the helical axis is perpendicular to the bright and dark bands. Each dark band in the TEM image indicates the layers in the cholesteric helix where mesogens are oriented parallel to the image plane, while the bright band represents the layers of the helix where mesogens are oriented perpendicular to the image plane. Halfpitch (1/2 P) of the cholesteric helix ∼170 nm is estimated by the average distance between the two adjescent dark or bright bands, suggesting the presence of light reflecting cholesteric domains. The estimated cholesteric pitch from TEM can be different from the corresponding visible reflection wavelength maxima obtained from the UV-reflectance measurements due to (a) broad reflectance wavelength bandwidth obtained from the reflectance measurements and (b) reflected wavelength being not only proportional to the domain size but also to the average refractive index of the medium. Terpolymers with

Figure 2. First cooling DSC thermogram of TP1_75-10-15, 1.75XL_TP1_75-10-15 (74 °C), and CP1_86-14-0 with a cooling rate of 10 °C/min exhibit four different transitions: (i) PEG crystallization transition (Tc), absent in CP1_86-14-0; (ii) glass transition temperature (Tg) of polynorbornene backbone and LC mesophase transition temperatures; (iii) TLC1 and (iv) TLC2. Tcl (clearing) transition ∼125 °C is seen for 1.75XL_TP1_75-10-15 (74 °C).

stabilization of various LC mesophases (Table S3, Figures S4 and S5). Temperature-Dependent Structural Evolution. The terpolymer TP1_75-10-15 at room temperature features PEG phase-separated domains coexisting with two distinct layered smectic mesophases induced by LC−LC molecular interactions between the two dissimilar mesogens, Ch9 and CB12. In the scattering profile of TP1_75-10-15 containing ∼15 wt % of PEG, a broad q* peak corresponding to correlations between PEG domains of ∼12 nm is observed but lacks higher order reflections due to poor long-range order (Figure S6). The morphology of PEG domains from scattering studies is inconclusive but may be better determined by TEM. PEG domains are amorphous in nature as evidenced by 2D WAXS measurements where sharp reflections indicative of crystalline PEG domains is absent (Figure S7). Apart from the primary scattering reflection q* due to the microphase-segregated PEG domains in TP1_75-10-15, two noncorrelated LC scattering peaks, at qLC1 reflection of lower intensity and at qLC2 of higher intensity, are noted. These two LC reflections indicate two distinct smectic phases with different extent of interdigitation, i.e., smectic interdigitated (SmAd, qLC1) and smectic monolayer (SmA1, qLC2) mesophases with corresponding domain layer spacings dLC1 = 6.44 nm and dLC2 = 3.66 nm. This assignment is based on the periodicities relative to the lengths of the mesogens where smectic monolayer and interdigitated domain spacings are mainly dependent on the length of the longer CB12 mesogen. During the heating cycle of TP1_75-10-15 (Figure S8), qLC1 reflection gains in intensity while q* and qLC2 reflections are also present. The q* peak still retains its intensity, while smectic to cholesteric phase (N*) transition occurs around 76 °C, as accounted by the disappearance of small-angle scattering LC peaks (Figure S8). Unlike TP1_75-10-15, T-SAXS measurements of CP*_84-0-16 27 material show that the q* disappearance is coincident with LC clearing temperature, D

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Figure 3. (a) Temperature-dependent 1D-SAXS of 1.75XL_TP1_75-10-15 processed at 74 °C. Between 25 and 45 °C, qLC2 peak is very low intensity and qLC1 is absent; i.e., minor amounts of smectic mesophases are present in the cross-linked polymer. This suggests that in the cross-linked polymer predominantly cholesteric mesophases are enchained with minorty smectic domains. Smectic−cholesteric transition peak appears at 68 °C during heat cycle, which is reversible. (b) First cooling DSC cycle with LC transitions and PEG crystallization transition (c) phase evolution of crosslinked sample during heat cycle. (d) Reflectance as a function of cross-linking temperature for the bulk photonic samples cross-linked with same cross-linker percentage (1.75XL_TP1_75-10-15) at selected temperatures ranging from 70 to 95 °C. For example, cross-linking at 74 °C temperature enables entrapping the characteristic cholesteric mesophase as a function of temperature.

phase.33 In the random terpolymer containing ∼75 wt % of Ch9, ∼15 wt % PEG and ∼10 wt % of CB12 or CB6 are present as minor components in the form of comonomers and appear to act simply as diluents that provide the ability to smoothly vary the properties of the Ch9 cholesteric mesophase (Table 1 and Table S1). The cholesteric reflection band originating from the Ch9 mesogen is tuned by the introduction of molecular interactions with the addition of dissimilar length LC mesogens (CB12 or CB6), as highlighted by the red-shift in the reflectance peak (Figure 5). LC copolymers CP1_86-14-0 and CP2_85-15-0 do not have PEG side chains and present reflection bands with λmax ∼ 570 nm (Figure 5). However, the inclusion of PEG side chains, which self-assemble into amorphous domains, leads to smaller helical pitch and blue-shift in PEG-containing terpolymers and copolymers. The blue-shift indicates that the presence of PEG

smaller amount of PEG (9−10 wt %, TP1_81-10-9) did not show microphase-segregated domains in TEM (Figure 4a) consistent with the SAXS data (Figure S6). To investigate the morphology of microphase-segregated PEG domains in TP1_75-10-15 microtomed samples are stained with ruthenium. Ruthenium also stains the double bonds in the polynorbornene backbone over LC mesogens and methylene spacers. Dark irregular spherical-like PEG domains with a size around ∼10−12 nm are observed along the edge direction with cholesteric helical domains (Figure 4b). These PEG domain lack long-range order, consistent with the SAXS data. Photonic Properties. Homopolymer PNBCh9 presents cholesteric mesophase with 1D photonic reflections that can be tuned with temperature49 while homopolymer PNBCB12 presents a smectic mesophase and PBNCB6 presents a nematic E

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Figure 4. Representative TEM image of TP1_81-10-9 sample (a) exhibiting fingerprint structure of cholesteric mesophase normal to helical axis. (b) Spherical-like PEG domains in TP1_75-10-15 when viewed along the helical axis as highlighted with red circles coexisting with cholesteric mesophase, suggesting hierarchical morphology of the system. Scale bar is 500 nm.

of the terpolymers probably due to it being a minority component. Reduction in PEG content in terpolymeric samples (TP1_81-10-9) induces small red-shift in the reflection wavelengths in comparison with samples having higher PEG content (Table S4). The modular processing and cross-linking protocols developed for the terpolymers enable the preparation of elastomers with customized 1D photonic properties. Elastomers with narrow 1D photonic reflection are prepared with 1.75 wt % of UV cross-linker at different temperatures (70−95 °C) (Figure 3d). As the temperature of cross-linking is decreased from 95 to 70 °C, the bandwidth of the reflection spectrum becomes narrower, suggesting better retention of cholesteric state. For example, when for terpolymers processed and cross-linked at 95 or 90 °C, the cholesteric domains lost their anisotropy and reflection bands became very broad. However, samples processed between 78 and 74 °C present blue-shift with narrower peak profiles which can be used to prepare optically active, mechanically robust, responsive elastomers. Terpolymers cross-linked at room temperature present low gel fraction and show red-shifted reflectance peaks compared to corresponding non-cross-linked sample (Figure S12, 1 and 1.75 wt % of UV cross-linker). With increase in extent of crosslinking for the room temperature cross-linked samples, elastomers exhibit a very broad reflection band, suggesting disruption and elongation of helical cholesteric domains (Figure S12, 2.5 and 5 wt % of UV cross-linker).51−53 Viscoelastic Properties. To understand the influence of backbone cross-linking on the linear viscoelastic properties, TP1_75-10-15 is compared with 1.75XL_TP1_75-10-15 (74 °C) (Figure 6). Broad Tg range obtained from tan δ curve in Figure 6 is in agreement with the broad Tg range obtained from DSC curves (Figure 2, 30−50 °C). At temperatures below Tg, all samples displayed typical storage modulus encountered for entangled glassy polymers (E′ ∼ 1 GPa), and no significant changes in loss modulus (E″) and tan δ are observed. As temperature approaches Tg, E′ decreases dramatically, and a characteristic peak associated with Tg of the polynorbornene backbone is observed from both E′ and tan δ plots. Above Tg, random terpolymer and random copolymer samples fail at ∼80 °C, suggesting that these samples are not mechanically robust.

Figure 5. Selective light reflection of non-cross-linked film samples prepared by compression molding at 90 °C. Reflections in the visible region are indicative of 1D photonic N* mesophase. Red-shift in reflection is observed due to molecular interactions introduced due to addition of mesogens (CB12 or CB6) along with cholesteric mesogens in random terpolymer architecture as compared to sample containing only cholesteric mesogen (CP*_84-0-16). Blue-shift is observed due to PEG domains in random terpolymers in comparison with copolymers without PEG (CP1_86-14-0 and CP2_85-15-0).

results in an increase in the twist angle between neighboring mesogens, consistent with a reduction in the helical pitch of the mesophase, i.e., presuming the packing distance of the mesogens remains the same. For example, CP*_84-0-16 with PEG content of ∼16 wt % shows peak maxima at ∼450 nm (Figure 5). However, the presence of CB12 or CB6 in the terpolymer TP1_75-10-15 leads to a red-shift. This suggests that the mesogen interactions lead to less strong twisting, resulting in a larger pitch and a red-shift of the reflectivity curve30,50 compared to CP*_84-0-16 (∼30−40 nm) (Figure 5). Therefore, addition of mesogens CB12 or CB6 alone in terpolymer causes lower twisting power in such a way that length of a pitch in TP1_75-10-15 is between the pitch lengths of CP*_84-0-16 and CP1_86-14-0. The length of spacer in the CB mesogen (CB12 vs CB6) does not impact reflection spectra F

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To understand the impact of randomly placed PEG domains formed in the terpolymer TP1_75-10-15, viscoelastic properties of cross-linked terpolymer samples containing microstructured PEG domains are compared with cross-linked copolymers CP1_86-14-0 and CP*_84-0-16 (Figure S14). These samples are cross-linked at room temperature and also have similar cross-linker content (5 wt %); therefore, similar gel fraction values are assumed. Elastomers without PEG microdomains have poor mechanical properties, relatively speaking, and fail slightly above Tg. Elastomer CP*_84-0-16 is mechanically robust until 100 °C while elastomer from the terpolymer is mechanically very robust until 150 °C. This trend in viscoelastic property suggests that the two levels of cross-linking in the terpolymer platform, PEG microdomains which serve as physical junctions and chemical backbone cross-linking, are both essential to improve its thermomechanical features compared to copolymers lacking PEG domains. Combined Shape Memory and Photonic Properties. Nanostructured thermally responsive photonic platform is developed from TP1_75-10-15 by exploiting its molecular characteristics in conjunction with cross-linking and processing techniques (Figure 7). In the first step, a nanostructured photonic material is fabricated by pressing TP1_75-10-15 against a hexagonally ordered 2 μm diameter PDMS pillar stamp at 100 °C and subsequently cooling to the cholesteric phase at 73 °C. The film is UV cross-linked at 73 °C to lock in the cholesteric state. This temperature is chosen for processing as cross-linked films at this temperature show a narrow green photonic band gap in films. The resultant film is released from the PDMS pillar stamp leaving hexagonally ordered 2 μm hole imprints on the film surface, as seen in Figure 7A. In the second step, uniaxial elastic deformation of the film at 110 °C results in distortion of the circular interfaces into elliptical shapes in the direction of the applied force, which then subsequently cooled to room temperature to entrap elliptical shapes (Figure 7B). In

Figure 6. Linear viscoelastic properties of cross-linked elastomer 1.75XL_TP1_75-10-15 (74 °C) in comparison with TP1_75-10-15 (un-cross-linked, blue solid line) and CP1_86-14-0 (control, green solid line) sample. Samples cross-linked at 74 °C (black solid line) show better elastomeric proprties exhibited by E′ plateuing stability up to higher temperatures (145 °C) due to higher extent of cross-linking.

UV cross-linking polymer backbones improve the elastomeric nature of the terpolymer samples, and plateauing of E′ and tan δ curve is observed above 80 °C. Cross-linked terpolymer 1.75XL_TP1_75-10-15 (74 °C) is stable until 150 °C (Figure 6) while elastomer 1.75XL_TP1_75-10-15 (rt) is stable until 110 °C (Figure S13). Similar plateauing of E′ and tan δ curve is observed in the samples cross-linked at room temperature with higher cross-linking extent (Figure S13). However, mechanical robustness is attained at the expense of disruption of cholesteric domains due to dense cross-linking networks as suggested by the broadening of reflectance peak bandwidth in UV reflectance data (Figure S12).

Figure 7. Illustration of shape memory effect in the photonic nanostructured elastomeric thin film (1.75XL_TP1_75-10-15 (73 °C)) material. The film was prepared by pressing the polymer−cross-linker mixture against a hexagonally ordered PDMS pillar stamp at 100 °C and subsequently cooling to 73 °C to achieve cholesteric phase with greenish reflection. Subsequently the film was UV-cross-linked to lock the cholesteric phase. Resultant film was released from the PDMS pillar stamp leaving hexagonally ordered 2 μm holes on the film surface as seen in (A). Then the film was uniaxially stretched at 110 °C (in the direction of white arrow) and subsequently quenched. The structures show deformation in the direction of the applied force (B). Upon reheating to 110 °C and the structures were relaxed back to the original permanent shape (C). G

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Macromolecules the final step, upon reheating the structure relaxes back to the original equilibrium shape (Figure 7C). Thus, the ability to prepare mechanically robust materials with built-in structure and functionality paves a new platform for optimized photonic and elastomeric features.

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Also, in sharp contrast, liquid crystalline comb- or brushlike block copolymers containing 75−85 wt % of Ch9 and 25−15 wt % of PEG presents a well-defined intermaterial dividing surface (IMDS) with microphase-segregated domains similar to traditional LCBCPs. Thus, statistically random architecture of copolymers, which lack a strong IMDS, is essential to attain hierarchical 1D cholesteric domains and disordered PEG domains. The preparation of statistical copolymer is feasible because Grubbs’ second-generation catalyst is capable of polymerizing monomers with dissimilar side chain lengths and different moieties at the similar polymerization rates into the random copolymer architectures.54−56 The comblike random terpolymer scaffold examined here comprises NBCh9, NBCB12/NBCB6, and NBPEG. Each monomer provides a specific function in the terpolymer: (1) NBCh9 and NBCB12 or NBCB6 provide cholesteric mesophase with built-in LC−LC interactions; (2) NBPEG acts as an internal plasticizer and may assemble into microdomains which function as physical junctions; (3) polynorbornene backbone which can be chemically cross-linked to produce elastomeric and shape memory properties. This scaffold enables optimal coupling of photonic and elastomeric properties resulting in unique optically active shape memory materials and actuators. All monomers used in this study (NBCh9, NBCB12, NBCB6, and NBPEG) exhibit similar rates of polymerization (Figure S2). Terpolymers and copolymers used in this study are nearly statistical random polymers,39 which prevents the formation of a strong interface or IMDS between incompatible moieties. Fast assembly kinetics of comblike LC side chains and PEG side chains enables formation of hierarchical structures, which can be immobilized by chemical cross-linking. Chemical connectivity is introduced between the segregated domains without disturbing the microphase-segregated morphology or cholesteric helix through optimum interchain covalent bonds. Olefinic units in polynorbornene backbone provide the crosslinking sites while the concentration of the multifunctional cross-linker decides the cross-linking density (number of connections between the polymeric chains). To preserve the delicate helical cholesteric mesophase, we use bifunctional 1,10decanedithiol, where 10 carbon chain between two thiols acts as a flexible spacer between two polymeric chains. Parameters including weight fraction of PEG in the terpolymer, concentration of UV cross-linker, and cross-linking temperatures are optimized to prepare elastomers with ideal levels of mechanical robustness as well as 1D photonic band gap. The gel fraction or gel content is a direct measure of extent of crosslinking and impact both mechanical and optical properties of these terpolymers. Evolution of Phase Behavior of Copolymers and Elastomers with Temperatures. The impact of interactions (LC−LC and microphase segregation of PEG) within the random terpolymer architecture is explored by using temperature-controlled SAXS in addition to DSC. Furthermore, the effects of processing and cross-linking temperatures in comparison with the un-cross-linked samples provide important information about the changes in mesophasic structures while cross-linking does not impact nanoconfinement of PEG (Figures 2 and 3). Addition of PEG downshifts the SmA-N* transition temperature in terpolymers (TP1_75-10-15, Figure S8) in comparison with copolymers (CP1_86-14-0, Figure S10), indicating the plasticizer-like role of PEG confined domains on the mesophasic structure. CB12 mesogen stabilizes in smectic mesophase, and thus inclusion of CB12 mesogen in

DISCUSSION

Synthesis of Comb-Shaped Polymers. In a previous study, using comb-shaped liquid crystalline homopolymers (PNBChn), in which cholesteryl mesogens are linked to a polynorbornene backbone by different lengths of methylene spacer (n = 9, 10), we investigated the impact of the molecular structure on the formation of cholesteric 1D photonic mesophase. We found that the extent of mesogen interdigitation and motional decoupling of the backbone from the side chain due to the comb shape are critical to the assembly of 1D photonic structures.49 Chemical cross-linking of these 1D photonic homopolymers results in materials that fail on stretching at temperatures lower than TLC1. In another study, comb-shaped homopolymers containing cyanobiphenyl mesogens with different spacer lengths, PNBCB12 and PNBCB6, are synthesized to examine the impact of mesogen spacer length.33 Both PNBCB12 and PNBCB6 form smectic or nematic meopshases, respectively, illustrating the dependence of LC assembly on the spacer length. We envisioned preparation of statistically random copolymers wherein norbornene containing achiral CB12 or CB6 mesogens are copolymerized along with cholesteric structure yielding norbornene-containing Ch9, which will enable LC−LC interactions between cyanobiphenyl moieties and cholesterol moieties. This LC−LC interaction in these copolymers, for example CP1_86-14-0, will directly impact the helical twisting power and the pitch of the helix can be systematically manipulated and tunable 1D photonic color reflections could be produced. Yet, again, chemical cross-linking of these random copolymers resulted in materials lacking elastomeric properties. To overcome these problems, an internal plasticizer comprising is needed. Better yet would be a covalently attached side chain plasticizer, including norbornene bearing side chain PEG, which can form selfassembled domains and will improve processability of these materials for cross-linking and formation of elastomers. We synthesized comb-shaped copolymers from ROMP of NBCh9 and NBPEG.24 Random comb-shape copolymers containing 75−85 wt % of Ch9 and 25−15 wt % of PEG presented 1D photonic features arising from Ch9 and some elastomeric properties from disordered PEG domains. In samples with higher PEG content (above 25 wt %) light reflection is not obtained even after annealing for longer time. Crystalline PEG lamellar domains are observed at room temperature for samples with higher PEG content which restricted the formation of cholesteric mesophase.24 In order to have optimal balance between optical and mechanical properties, PEG content in terpolymer is kept at around 15 wt %. Despite some progress in the molecular engineering of hierarchical blue light reflecting 1D photonic structures containing PEG microdomains, several major challenges remained to be addressed: (1) ability to tune the 1D photonic band gap and color of light reflected, (2) ability to produce mechanically robust films by immobilizing cholesteric structure, and (3) creation of single-component materials with built-in structure and elastomeric properties for applications in patternable responsive photonic materials. H

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While un-cross-linked polymers processed at 90 °C present 1D photonic features (Table S4 and Figure 5), their mechanical properties are compromised, and these polymers are very brittle. The presence of PEG domains along with chemical cross-linking is essential for elastomers from terpolymers to serve in stimuli-responsive applications. Because of hierarchical assembling and immobilization of N* mesophase, these elastomers present excellent tunable 1D photonic properties, mechanical stability, and stimuli-responsive properties. This is in sharp contrast to (i) block copolymer gels which present good mechanical stability and optical properties but lack covalent network structure and functionalities required for reversible shape memory, actuation, or stimuli-responsive materials platform and (ii) brush block copolymers and brush block copolymer nanocomposites which present good tunability of 1D photonic properties but are very brittle due to lack of entanglements and lack reversible stimuli-responsive properties. To evaluate the ability of this terpolymer platform to function as single optically active elastomers, patterning, crosslinking experiments, and proof-of-concept actuation experiments are performed. Patterned cross-linked media in cholesteric mesophase exhibiting shape tunability with temperature with the retention of photonic properties. This optically active elastomer platform can be used to produce shape memory devices, actuators, and mechanochromics. In the future, we envision thermal control of phototunability in optimally cross-linked elastomeric and mechanically robust sample in conjunction with mechanical tuning of photonic reflections by means of uniaxial force stretching of film to obtain de novo thermo-mechanochromic systems.

TP1_75-10-15 shows increase in width between TLC1 and TLC2 (cholesteric window) clearing transitions in comparison with CP*_84-0-16 sample (Table S2 and Figure S3). Also, CB6 mesogen, which stabilizes nematic phase, is shown to decrease the width of cholesteric transition in TP2_77-9-14 in comparison with CP*_84-0-16 (Table S2 and Figure S3). Thus, addition of smectic or nematic mesogens along with cholesteric structure forming mesogens impacts the width of cholesteric window and processing parameters. In cross-linked sample, 1.75XL-TP1_75-10-15 (74 °C), two small transitions are noted below 100 °C, which may be attributed to SmA-N* and N* clearing of a few lower cross-link density networks (lower gel content) while higher cross-linking density networks (higher gel content) may present increase in the N* clearing transition (∼125 °C) (Figure 3 and Table S3). The cross-linking/processing method determines the extent of cross-linking (or gel content) as well as favoring enchainment of N* mesophase over other mesophases. In general, higher temperature leads to better cross-linking as suggested with higher gel fraction values in comparison with room temperature cross-linking, which is reflected in T-SAXS characterization of 1.75XL_TP1_75-10-15 (74 °C) samples (Figure 3a), where remnant small reflections of smectic peaks (qLC2) from un-cross-linked regions are seen at room temperature while most of the qLC1 peaks are absent. In elastomers processed at higher temperatures, two dominant features, microstructure from PEG domains and mesostructure from cholesteric helix, are immobilized within a single network. Temperature-dependent SAXS studies of polyacrylates comprising side chain cholesterol mesogens with 10 methylene spacers show small-angle reflections corresponding to layered structures within cholesteric mesophases. This twisted grain boundary (TGB) phase is attributed to continuously twisted smectic layers, which eventually yield cholesteric helical mesophase.57−59 In comparison, cross-linked samples at 74 °C do not show layered structure within N* mesophase while the remnant smectic peaks are probably due to lower crosslinked regions. In fact, these materials present a clear transition from SmA to N* mesophase as shown in T-SAXS studies. The rotational motion of the polynorbornene backbone is restricted compared to polyacrylates, and this allows the mesogens to form N* which is of lower order compared to TGB. The temperature-dependent evolution SmA and N* within homopolymers, copolymers, and terpolymer is due to the multicomponent nature based on composition and molecular weight distribution.60,61 However, by manipulating interactions and processing/cross-linking conditions within terpolymers, it is possible to increase the cross-link density to a point where N* mesophase may be predominantly enchained. Multifunctional 1D Photonic Elastomers and Shape Memory Materials. Here, the molecular engineered combtype terpolymers optimally combine mechanical and elastomeric properties with 1D photonic properties, and a multifunctional responsive platform is developed. Photonic property tunability is obtained by varying terpolymer composition, where (i) LC−LC interactions between cyanobiphenyl mesophase and cholesteric mesophase resulting into increased N* helix pitch length (red-shift) and (ii) PEG side chains act as an internal plasticizer by forming domains within the cholesteric pitch by reducing the pitch length of cholesteric helix (blue-shift). Furthermore, the plasticization effect of PEG domains allows easy processing and cross-linking of the terpolymer to better attain anisotropy of N* domains.

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CONCLUSIONS We demonstrate a modular strategy to prepare novel class of mechanically robust, 1D photonic materials with useful shapememory effects and stimuli-responsive properties. The materials feature dual cross-linking units consisting of physical junction points provided by randomly ordered microphasesegregated PEG domains and covalent bonds provided by UV cross-linking without disrupting the self-assembled 1D photonic cholesteric structures. In developing these versatile materials, first, statistically random terpolymers are synthesized and self-assemble to facilitate the formation of helical cholesteric mesophase formed primarily by Ch9 mesogen that coexists with microphase-separated PEG domains. Second, the presence of a second cyanobiphenyl mesogenic species in the system leads to an increase in the helical periodicity due to the interplay of LC−LC interactions in the random terpolymeric architecture which twists the helical structures and alters the pitch length and color of light reflected. Third, the spacer length of cyanobiphenyl mesogens (CB6 vs CB12) is found to have negligible impact on the optical properties while the presence of PEG domains in the system leads to a significant blue-shift of the reflection band by ∼80−100 nm. Finally, these hierarchically self-assembled terpolymers are UV cross-linked to produce materials with optimal photonic properties along with improved mechanical and elastomeric characteristics. The presence of PEG domains substantially influences the fracture toughness, and at optimized cross-linking densities the resulting materials possess a storage modulus of ∼1 GPa at room temperature. Ongoing investigations include exploring thermal tuning of the optical band gap in the elastomers and the I

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Self-Assembly into Large Domain Nanostructures. Macromolecules 2016, 49 (10), 3733−3738. (8) Noro, A.; Tomita, Y.; Shinohara, Y.; Sageshima, Y.; Walish, J. J.; Matsushita, Y.; Thomas, E. L. Photonic Block Copolymer Films Swollen with an Ionic Liquid. Macromolecules 2014, 47 (12), 4103− 4109. (9) Chan, E. P.; Walish, J. J.; Urbas, A. M.; Thomas, E. L. Mechanochromic Photonic Gels. Adv. Mater. 2013, 25 (29), 3934− 3947. (10) Chan, E. P.; Walish, J. J.; Thomas, E. L.; Stafford, C. M. Block Copolymer Photonic Gel for Mechanochromic Sensing. Adv. Mater. 2011, 23 (40), 4702−4706. (11) Kang, Y.; Walish, J. J.; Gorishnyy, T.; Thomas, E. L. Broadwavelength-range chemically tunable block-copolymer photonic gels. Nat. Mater. 2007, 6 (12), 957−960. (12) Noro, A.; Tomita, Y.; Matsushita, Y.; Thomas, E. L. EnthalpyDriven Swelling of Photonic Block Polymer Films. Macromolecules 2016, 49 (23), 8971−8979. (13) Sveinbjornsson, B. R.; Weitekamp, R. A.; Miyake, G. M.; Xia, Y.; Atwater, H. A.; Grubbs, R. H. Rapid self-assembly of brush block copolymers to photonic crystals. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (36), 14332−14336. (14) Rzayev, J. Synthesis of Polystyrene−Polylactide Bottlebrush Block Copolymers and Their Melt Self-Assembly into Large Domain Nanostructures. Macromolecules 2009, 42 (6), 2135−2141. (15) Miyake, G. M.; Weitekamp, R. A.; Piunova, V. A.; Grubbs, R. H. Synthesis of Isocyanate-Based Brush Block Copolymers and Their Rapid Self-Assembly to Infrared-Reflecting Photonic Crystals. J. Am. Chem. Soc. 2012, 134 (34), 14249−14254. (16) Piunova, V. A.; Miyake, G. M.; Daeffler, C. S.; Weitekamp, R. A.; Grubbs, R. H. Highly Ordered Dielectric Mirrors via the Self-Assembly of Dendronized Block Copolymers. J. Am. Chem. Soc. 2013, 135 (41), 15609−15616. (17) Song, D.-P.; Li, C.; Colella, N. S.; Xie, W.; Li, S.; Lu, X.; Gido, S.; Lee, J.-H.; Watkins, J. J. Large-Volume Self-Organization of Polymer/Nanoparticle Hybrids with Millimeter-Scale Grain Sizes Using Brush Block Copolymers. J. Am. Chem. Soc. 2015, 137 (39), 12510−12513. (18) Song, D.-P.; Li, C.; Colella, N. S.; Lu, X.; Lee, J.-H.; Watkins, J. J. Thermally Tunable Metallodielectric Photonic Crystals from the Self-Assembly of Brush Block Copolymers and Gold Nanoparticles. Adv. Opt. Mater. 2015, 3 (9), 1169−1175. (19) Yu, H.; Kobayashi, T.; Yang, H. Liquid-Crystalline Ordering Helps Block Copolymer Self-Assembly. Adv. Mater. 2011, 23 (29), 3337−3344. (20) Walther, M.; Finkelmann, H. Structure formation of liquid crystalline block copolymers. Prog. Polym. Sci. 1996, 21 (5), 951−979. (21) Anthamatten, M.; Hammond, P. T. A SAXS Study of Microstructure Ordering Transitions in Liquid Crystalline SideChain Diblock Copolymers. Macromolecules 1999, 32 (24), 8066− 8076. (22) Hamley, I. W.; Castelletto, V.; Parras, P.; Lu, Z. B.; Imrie, C. T.; Itoh, T. Ordering on multiple lengthscales in a series of side group liquid crystal block copolymers containing a cholesteryl-based mesogen. Soft Matter 2005, 1 (5), 355−363. (23) Komiyama, H.; Sakai, R.; Hadano, S.; Asaoka, S.; Kamata, K.; Iyoda, T.; Komura, M.; Yamada, T.; Yoshida, H. Enormously Wide Range Cylinder Phase of Liquid Crystalline PEO-b-PMA(Az) Block Copolymer. Macromolecules 2014, 47 (5), 1777−1782. (24) Deshmukh, P.; Ahn, S. K.; De Merxem, L. G.; Kasi, R. M. Interplay between Liquid Crystalline Order and Microphase Segregation on the Self-Assembly of Side-Chain Liquid Crystalline Brush Block Copolymers. Macromolecules 2013, 46 (20), 8245−8252. (25) Choo, Y.; Mahajan, L. H.; Gopinadhan, M.; Ndaya, D.; Deshmukh, P.; Kasi, R. M.; Osuji, C. O. Phase Behavior of PolylactideBased Liquid Crystalline Brushlike Block Copolymers. Macromolecules 2015, 48 (22), 8315−8322. (26) Mao, G.; Wang, J.; Clingman, S. R.; Ober, C. K.; Chen, J. T.; Thomas, E. L. Molecular Design, Synthesis, and Characterization of

potential utility of the system to be used as thermomechanochromic materials.

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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.7b01157. Detailed synthetic protocols of random terpolymer and copolymer synthesis; reperesentative 1H NMR spectrum of TP1_75-10-15; kinetics of polymerization for individual monomer; molecular characterization of random polymers and corresponding cross-linked samples (including gel fraction values); microstructural, mechanical, and optical (UV reflectance) properties of random terpolymers and samples cross-linked at room temperature (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Chinedum O. Osuji: 0000-0003-0261-3065 Rajeswari M. Kasi: 0000-0003-3872-1463 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF under DMR-1507045 and under CMMI-1246804. The authors thank Mike Degen (Rigaku Inc.) and AMI Inc. for technical support. C.O. acknowledges additional financial support from NSF (DMR0847534; DMR-1119826) and from 3M Nontenured Faculty Award. Facilities use was supported by YINQE and NSF MRSEC DMR-1119826. The central instrumentation faciltiies at the Insitute of Materals Science and Chemistry Department at UConn are acknowledged. The authors thank Prof. Steven Suib and his research group for help with UV−vis measurements.

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K

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