Thermally Functional Liquid Crystal Networks by Magnetic Field

Jul 27, 2016 - We carried out grazing incidence wide-angle X-ray scattering (GIWAXS) at the 12-ID-B beamline at the Advanced Photon Source using 14 ke...
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Thermally Functional Liquid Crystal Networks by Magnetic Field Driven Molecular Orientation Jungwoo Shin, Minjee Kang, Tsunghan Tsai, Cecilia Leal, Paul V. Braun, and David G. Cahill* †

Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: Aligned liquid crystal networks were synthesized by photopolymerization of liquid crystal monomers in the presence of magnetic fields. Grazing incident wide-angle X-ray scattering was used to characterize the degree of molecular alignment of mesogen chains and time-domain thermoreflectance was used to measure thermal conductivity. Liquid crystal networks with mesogenic units aligned perpendicular and parallel to the substrate exhibit thermal conductivity of 0.34 W m−1 K−1 and 0.22 W m−1 K−1, respectively. The thermal conductivity and orientational order of liquid crystal networks vary as a function of temperature. The thermal conductivity of liquid crystal networks can be manipulated by a magnetic field at above the glass transition temperature (65 °C) where the reduced viscosity enables molecular reorientation on the time scale of 10 min. hanges in the thermal conductivity Λ of a material with temperature are typically gradual; for example, in most cases, near room temperature, the strongest increase with temperature is the linear Λ(T) ∝ T temperature dependence of a metal alloy and the strongest decrease with temperature is the reciprocal Λ(T) ∝ 1/T temperature dependence of a dielectric crystal.1 Recently, materials that have more abrupt changes in Λ(T) have been studied for their potential as thermal regulators. For example, the thermal conductivity of VO2 increases abruptly by 50% at the metal-to-insulator transition, TMI ≈ 340 K.2 Materials that respond to a magnetic, electric, or elastic strain field with a change in thermal conductivity are also sought as thermal switches.3 The largest contrast that we are aware of for a solid-state thermal switch near room temperature, a factor of 2, is the giant magnetothermal resistance of Co/Cu multilayers.4 We refer to thermal regulators (materials with abrupt changes in conductivity with temperature) and thermal switches (materials with conductivity that responds to an external trigger) as “thermally functional” materials. Here, we describe thermally functional materials based on liquid crystal networks (LCNs). We report a ≈50% increase in thermal conductivity depending on the molecular orientation assisted by magnetic field. Although the thermal conductivity contrast of 1.5 is probably too small for practical applications as a thermal regulator or thermal switch, we believe our work provides an important step in the development of thermally functional soft materials. Liquid crystals (LC) have long been known to have an anisotropic thermal conductivity in the nematic phase which suggests the possibility of manipulating their thermal

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© XXXX American Chemical Society

conductivity through changes in nematic order.5 When polymerized, molecular ordering of mesogen units in LCNs can be manipulated by external stimuli such as thermal, electric, magnetic and optical fields.6 The LCN films we study are synthesized by photopolymerization of liquid crystal (LC) diacylate monomers (1,4-bis-[4-(3-acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene; RM257, Wilshire Technologies) with thiol-cross-linker (pentaerythritol tetrakis[3-mercaptopropionate]; PETMP, Sigma-Aldrich) in the presence of a magnetic field. The addition of PETMP and cross-linking by RM257 via a thiol−ene reaction increases the toughness of the material at T < Tg and creates an elastomer at T > Tg. Without cross-linking, RM257 is a brittle solid from room temperature to 250 °C. GIWAXS measurements of RM257 solidified in a magnetic field show similar molecular order as RM257 that is lightly cross-linked. RM257 is a nematic liquid crystal monomer in the temperature range of 67−130 °C and can be oriented by magnetic fields due to the anisotropic magnetic susceptibility of the mesogen units (Figure 1). A LC solution for photopolymerization was prepared by mixing RM257 with PETMP (5 mol %) and photoinitiator (2hydroxy-2-methylpropiophenone, 0.3 wt %, Sigma-Aldrich) at 80 °C. The solution was then cast onto a sapphire substrate that was previously prepared by spin coating with polyimide (PI) film followed by Al thin film deposition by magnetron Received: June 21, 2016 Accepted: July 25, 2016

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data. The thickness of the PI layer does, however, play a dominant role in the steady-state temperature rise. We calculated that the fraction of steady-state temperature rise due to the PI layer is 84% by comparing ΔTSS with and without the PI layer. The Al layer thickness (∼50 nm) was measured by picosecond acoustics. (Supporting Information, Figure S3). The electric resistivity of Al film measured by four-point probes was converted to thermal conductivity using the Wiedemann− Franz law.11 The per-pulse heating and steady-state heating by accumulation of pulses are ΔTP = 3.6 K and ΔTSS = 8.4 K, respectively, by 5 mW pump and probe beams, which pass through the sapphire substrate. Examples of measured and fitted TDTR data are shown in Figure 2B. First, we measured the thermal conductivity of RM257 monomers in the nematic phase at 80 °C and found 0.14 W m−1K−1 when a magnetic field was applied parallel to the substrate (B∥) and 0.24 W m−1 K−1 when a magnetic field was applied perpendicular to the substrate (B⊥; Supporting Information, Figure S4). This degree of anisotropy, approximately a factor of 2, is similar to other nematic LCs.5 After polymerization, the thermal conductivity increases by 0.1 W m−1 K−1 in both directions with the formation of chemical bonds:12 the thermal conductivity of LCNs aligned perpendicular and parallel to the substrate are 0.34 ± 0.05 W m−1 K−1 and 0.22 ± 0.02 W m−1 K−1 for 10 samples, respectively. The standard deviation of multiple measurements is 10%. We carried out grazing incidence wide-angle X-ray scattering (GIWAXS) at the 12-ID-B beamline at the Advanced Photon Source using 14 keV photons to characterize the molecular orientation of our LCNs. The 2D scattering data were collected with a PerkinElmer XRD 1621 CN detector. We mounted the samples on a temperature-controlled sample stage with the incident angle of beam varying from 0.1° to 0.25°. The 1D scattering profiles were generated using Igor-pro based software NIKA.13 Figure 3 shows GIWAXS data and corresponding schematics of the mesogenic unit ordering. Note that chains connecting mesogens are omitted in the figure for clarity. We observe scattering peaks typical of magnetically aligned LCNs.14 Since three phenyl rings at the core of RM257 are rotated by 24° with respect to each other8a the primary GIWAXS peak at q ∼ 1.53 Å−1 corresponds to a plane spacing of 4.1 Å, 0.3 Å larger than typical for π−π stacking.15 With the magnetic field applied perpendicular to the substrate (B⊥) during polymerization, the π−π stacking peak appears along the direction parallel to the substrate, indicating the vertical alignment of mesogens in the LCN, see Figure 3A. Perpendicular and parallel GIWAXS measurements with respect to the magnetic field direction (B∥) were performed to prove in-plane anisotropy of mesogens in LCN films (Figure 3B,C). The different features of π−π stacking orientation distribution in Figure 3B,C reveal the nature of in-plane anisotropic alignment of mesogen units. The long optical axis of mesogens in each domain is aligned in the direction of the magnetic field, while π−π stacks have no preferential orientation around the optical axis, resulting in the isotropic ring pattern with an incident beam along with the aligned direction (Figure 3C). We checked that the molecular orientation of LCNs polymerized with a 4× higher cross-linker concentration (20 mol % PETMP) is reduced as expected.14,16 GIWAXS data confirm the loss of orientational ordering at the high crosslinking density (Supporting Information, Figure S5). The

Figure 1. Chemical structure of RM257.

sputtering (Supporting Information, Figure S1). The PI film provides thermal insulation between the Al transducer and the sapphire substrate that increases the sensitivity of the measurements to the thermal conductivity of the LC layer.7 Photopolymerization was performed for 20 min using UV lamps (nominal wavelength 365 nm with a nominal power at the substrate of 3−20 mW cm−2), forming a randomly crosslinked liquid-crystal network (LCN) film. A magnetic field was applied parallel (B∥) or perpendicular (B⊥) to the Al/PI/ sapphire substrate using N52-grade NdFeB permanent magnets during photopolymerization (Supporting Information, Figure S2).8 The thermal conductivity of aligned LCNs was measured by time-domain thermoreflectance (TDTR).9 TDTR data were collected as in-phase voltage (Vin) and out-of-phase voltage (Vout) signals measured by a lock in amplifier with a modulation frequency of 9.1 MHz. The heat penetration depth in the LCN polymer layer, d = Λ1/2(πf C)−1/2 is approximately 60 nm; Λ is the thermal conductivity, f is the modulation frequency of the pump beam, and C is the heat capacity per unit volume. Due to the small thermal penetration depth compared to the laser spot size (10 μm), heat flow in the LCN is nearly one-dimensional in the direction normal to the film surface. Thus, the thermal conductivity we measure by TDTR is for the direction normal to the LCN/substrate interface. During TDTR studies, we manipulate molecular orientation using the magnetic field generated by SmCo permanent magnets, which can operate at elevated temperatures up to 320 °C (Figure 2A).

Figure 2. Thermal conductivity measurement of aligned LCNs by TDTR. (A) Schematic illustration of TDTR measurement for LCN/ Al/polyimide/sapphire samples under magnetic field B⊥ applied by a permanent magnet. (B) Measured and fitted TDTR data for LCNs polymerized under parallel (B∥) and perpendicular (B⊥) magnetic field.

We fit the data to the thermal model10 by adjusting the thermal conductivity of the LCN as the only free parameter. Thermal conductivities of sapphire (35 W m−1 K−1) and PI (0.2 W m−1 K−1) were obtained from literature and independent measurements, respectively. The thickness of the PI layer (400 ± 40 nm) was measured by ellipsometry. This value is greater than the heat penetration depth and can be considered as an infinitely thick insulating layer in our modeling of the TDTR 956

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Figure 3. Effect of magnetic field during polymerization on chain alignment. (A) GIWAXS data for LCN polymerized with a magnetic field perpendicular to the substrate (B⊥). (B, C) GIWAXS data for LCN polymerized with a magnetic field parallel to the substrate (B∥). X-ray incident direction is represented as red arrows. In the figure, scattering vector q(2πλ−1) ranged from −3 to 3 Å−1 for the xy plane and −0.5 to 4 Å−1 for the z plane.

thermal conductivity is 0.2 W m−1 K−1, independent of magnetic field orientation during polymerization as well. We also checked the effects of surface interactions on our experiments. Near a surface, mesogenic units can exhibit local orientation that deviates from the bulk. The thickness of the boundary layer can make up a significant portion of the thermal penetration depth of the TDTR measurement.17 We do not observe a significant difference in thermal conductivity for LCNs with or without surface functionalization of substrate by fluorosilane (1H,1H,2H,2H-perfluorodecyltriethoxysilane, Sigma-Aldrich) which greatly lowers surface anchoring energy. The lack of dependence on surface anchoring energy indicates that surface interactions are not driving the changes in thermal conductivity. We construct an effective medium model to provide insights on the dependence of thermal conductivity on molecular orientation. Our model describes the LCN material as comprised of partially ordered, cylindrical mesogens embedded in an isotropic amorphous polymer matrix. The fundamental components of the in-plane and out-of-plane thermal conductivity tensors of the mesogen are Λin‑plane and Λout-of-plane. The thermal conductivity of a partially aligned aggregate of mesogens is a function of the order parameter S:18

where the orientational distribution function of the mesogen molecules f(β) is obtained from the azimuthal intensity I(θ) of the primary scattering peak, as shown in Figure 2. In both cases, we calculate S = 0.6 as the order parameters for LCNs aligned parallel (B∥) and perpendicular (B⊥) to the substrate (Supporting Information, Figure S6). We model the flexible chain segments of the mesogens and cross-linkers as an isotropic polymer matrix. An effective medium model for cylindrical LC molecules embedded in an isotropic matrix20 predicts that the effective thermal conductivity of LCNs under magnetic field parallel (B∥) and perpendicular (B⊥) to the substrate is

Λ−B⊥1 =

1 −1 2 (Λ in‐plane + 2Λ−out1 ‐of‐plane) + S(Λ−in1‐plane − Λ−out1 ‐of‐plane) 3 3 (1)

Λ−B 1 =

1 1 −1 (Λ in‐plane + 2Λ−out1 ‐of‐plane) − S(Λ−in1‐plane − Λ−out1 ‐of‐plane) 3 3 (2)

where MLC, MRC, and MC are the molecular weights of the LC monomer, rigid core of mesogen, and cross-linker, respectively; ρLC, ρRC, and ρC are their mass densities; and C is the crosslinker concentration. The volume fraction taken by rigid cores is Φ = 0.6 for LCN with 5 mol % cross-linker. MLC, MRC, and MC we used are 588.6, 488.7, and 362 g mol−1; and ρLC, ρRC, and ρC are 1.22, 1.28, and 1.20 g cm−3, respectively. Here, we adjusted Λin‑plane and Λout‑of‑plane to match ΛLCN,B⊥ for different values of S and Φ. Assuming Λmedium = 0.2 W m−1 K−1, we estimate the fundamental in-plane thermal conductivity, Λin‑plane, and out-of-plane thermal conductivity, Λout‑of‑plane, of perfectly aligned rigid core of LCN of 1.5 W m−1 K−1 and 0.15 W m−1 K−1. Thus, we conclude that the contrast in the thermal conductivity change can be improved with a larger order parameter S and smaller isotropic component (1-Φ) but the upper limit on the contrast will be

1 (3⟨cos2 φ⟩ − 1) 2

S=

⟨cos φ⟩ =

∫0

π /2

π /2

f (β)sin β dβ

ΛLCN, B⊥ = (1 − Φ)Λ medium + ΦΛ B⊥

(6)

Φ=

(3)

f (β)cos 2 β sin β dβ

∫0

(5)

where Φ is the volume fraction of rigid core of mesogen and Λmedium is the thermal conductivity of the isotropic polymer matrix. We estimate Φ by

where φ is the angle between the long axis of the molecules and the magnetic field.19 2

ΛLCN, B = (1 − Φ)Λ medium + ΦΛ B

(4) 957

(1 − C)(MRC/ρRC ) (1 − C)(MLC/ρLC ) + C(MC/ρC )

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ACS Macro Letters a factor of 10. Additional details of the effective medium modeling of thermal conductivity are described in Supporting Information, Figure S7. Due to the thermotropic nature of LCNs, the molecular ordering and thermal conductivity of LCNs depend on temperature. Figure 4 shows two sets of TDTR measurement

Figure 5. Temperature effect on molecular ordering and magnetic field reorientation. (A) GIWAXS data for LCN aligned perpendicular to the substrate (B⊥) at 25 °C. (B) GIWAXS data for LCN aligned perpendicular to the substrate after heating up to 150 °C. Order− disorder transition was observed. (C) GIWAXS data for LCN after magnetic reorientation (B⊥ → B∥). LCN aligned perpendicular to the substrate (B⊥) was heating up to 150 °C then cooled down to the room temperature in the presence of magnetic field parallel to the substrate (B∥). (D) Azimuthal intensity of LCN aligned perpendicular to the substrate at (A), 25 °C, (B) 150 °C, and (C) 30 °C after reorientation. Φ represents azimuth angle from the substrate.

Figure 4. Temperature effect on thermal conductivity of LCNs. TDTR data for perpendicularly aligned LCN (B⊥) measured at fixed temperature from room temperature to 200 °C. Thermal conductivity at each temperature converted from Figure S7 (open squares) and continuous temperature measured by in situ continuous TDTR with a heating and cooling rate of 4 °C min−1 (dots). Continuous thermal conductivity is converted from −Vin/Vout at fixed time delay t = 80 ps.

from room temperature to 200 °C for LCNs aligned perpendicular to the substrate (B⊥). First, individual measurements were done at fixed temperatures (Supporting Information, Figure S8) and converted to thermal conductivity (open squares), see Figure 4. The time between each measurement is 10 min. We also measured the thermal conductivity using a constant heating and cooling rate of 4 °C min−1 (small circles). In this form of TDTR measurement, the time-delay between pump and probe is fixed at 80 ps and the ratio −Vin/Vout is recorded as a continuous function of temperature and converted to thermal conductivity. Both measurements show a decreasing thermal conductivity at elevated temperature. We attribute the decrease in thermal conductivity to the loss of molecular order at above glass transition temperature (Supporting Information, Figure S9).14 Figure 5A,B shows GIWAXS data for LCNs aligned perpendicular to the substrate. We observed an isotropic ring at 150 °C suggesting loss of orientational and positional order. Unlike sharp nematic-toisotropic transition of small molecule LCs, the order−disorder transition of LCNs is gradual with temperature.21 Although there might be a small amount of local, residual nematic ordering in thermally disordered LCNs, we do not see distinct peaks in the X-ray scattering data (Supporting Information, Figure S10). In contrast, LCNs maintain their thermal conductivity up to 200 °C when a magnetic field is applied in aligned directions during the measurements (Supporting Information, Figure S11). We examined chain reorientation kinetics of LCNs by switching the magnetic field direction. A LCN aligned perpendicular to the substrate (B⊥) was first heated to 200 °C and slowly cooled to room temperature in the presence of a magnetic field applied parallel to the substrate (B∥). The GIWAXS patterns (Figure 5C) and azimuthal intensity at q ∼ 1.53 Å−1 (Figure 5D) show that the scattering peak becomes

wider and the center is tilted by 32° with respect to the initial state (Figure 5A). Figure 6 shows thermal conductivity of LCN during magnetic reorientation. First, LCN polymerized under

Figure 6. Effect of magnetic field on thermal conductivity of aligned LCN. (A) Thermal conductivity of LCNs before and after magnetic reorientation. LCN polymerized under a parallel magnetic field (B∥) was heated up to 150 °C with B∥. After magnetic reorientation (B∥→ B⊥) it is slowly cooled down to room temperature. Heating and cooling rate is 4 °C min−1. (B) Thermal conductivity of a parallel aligned (B∥) LCN in response to an orthogonal magnetic field (B⊥) at 150 °C. The magnetic field was switched to the perpendicular direction (B⊥) at 150 °C at t = 0 s.

magnetic field parallel to the substrate (B∥) was heated to 150 °C (Figure 6A) followed by switching the orientation of the magnetic field (B⊥). The thermal conductivity of LCN increases gradually over a time span of 1500 s, see Figure 6B. The thermal conductivity shows pronounced hysteresis upon cooling. After cooling, the room temperature thermal conductivity is 0.30 W m−1 K−1, comparable to LCNs polymerized under magnetic field (B⊥, 0.34 W m−1 K−1). The GIWAXS pattern after reorientation shows vertical 958

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of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

molecular alignment as well (Supporting Information, Figure S12). The reorientation kinetics is controlled by the competition between magnetic torque, ΓM = −1/2 μ0−1ΔχB sin(2φ) and viscous drag, Γvis = −η1(dφ/dt), where μ0 is the permeability of vacuum, η1 is the rotational viscosity, Δχ is the anisotropy of the magnetic susceptibility of the mesogen, and B is the applied magnetic field.22 The relaxation time constant τ is23 ⎛ 2μ η ⎞ τ = ⎜ 0 2⎟ ⎝ ΔχB ⎠

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(1) (a) Slack, G. A. Phys. Rev. 1962, 126 (2), 427−441. (b) Slack, G. A. Phys. Rev. 1961, 122 (5), 1451−1464. (c) Ho, C. Y.; Ackerman, M. W.; Wu, K. Y.; Oh, S. G.; Havill, T. N. J. Phys. Chem. Ref. Data 1978, 7 (3), 959−1177. (2) (a) Kizuka, H.; Yagi, T.; Jia, J.; Yamashita, Y.; Nakamura, S.; Taketoshi, N.; Shigesato, Y. JPN J. Appl. Phys. 2015, 54 (5), 053201. (b) Oh, D.-W.; Ko, C.; Ramanathan, S.; Cahill, D. G. Appl. Phys. Lett. 2010, 96 (15), 151906. (3) (a) Dhara, S.; Solanki, H. S.; R, A. P.; Singh, V.; Sengupta, S.; Chalke, B. A.; Dhar, A.; Gokhale, M.; Bhattacharya, A.; Deshmukh, M. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84 (12), 121307. (b) Kulkarni, A. J.; Zhou, M. Nanotechnology 2007, 18 (43), 435706. (c) Keyes, R. W.; Sladek, R. J. Phys. Rev. 1962, 125 (2), 478−483. (4) (a) Kimling, J.; Wilson, R. B.; Rott, K.; Kimling, J.; Reiss, G.; Cahill, D. G. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91 (14), 144405. (b) Kimling, J.; Nielsch, K.; Rott, K.; Reiss, G. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87 (13), 134406. (5) (a) Urbach, W.; Hervet, H.; Rondelez, F. Mol. Cryst. Liq. Cryst. 1978, 46 (3−4), 209−221. (b) Ahlers, G.; Cannell, D. S.; Berge, L. I.; Sakurai, S. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1994, 49 (1), 545−553. (6) White, T. J.; Broer, D. J. Nat. Mater. 2015, 14 (11), 1087−1098. (7) Ge, Z. B.; Cahill, D. G.; Braun, P. V. Phys. Rev. Lett. 2006, 96 (18), 186101. (8) (a) Broer, D. J.; Mol, G. N.; Challa, G. Makromol. Chem. 1991, 192 (1), 59−74. (b) Hikmet, R. A. M.; Lub, J.; Broer, D. J. Adv. Mater. 1991, 3 (7−8), 392−394. (c) Thiem, H.; Strohriegl, P.; Shkunov, M.; McCulloch, I. Macromol. Chem. Phys. 2005, 206 (21), 2153−2159. (9) (a) Cahill, D. G. Rev. Sci. Instrum. 2004, 75 (12), 5119−5122. (b) Kang, K.; Koh, Y. K.; Chiritescu, C.; Zheng, X.; Cahill, D. G. Rev. Sci. Instrum. 2008, 79 (11), 114901. (10) Cho, J.; Losego, M. D.; Zhang, H. G.; Kim, H.; Zuo, J. M.; Petrov, I.; Cahill, D. G.; Braun, P. V. Nat. Commun. 2014, 5, 4035. (11) Avery, A. D.; Mason, S. J.; Bassett, D.; Wesenberg, D.; Zink, B. L. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92 (21), 214410. (12) Yamamoto, O. Polym. J. 1971, 2 (4), 509−517. (13) Ilavsky, J. J. Appl. Crystallogr. 2012, 45, 324−328. (14) Barclay, G. G.; Mcnamee, S. G.; Ober, C. K.; Papathomas, K. I.; Wang, D. W. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (9), 1845− 1853. (15) (a) McGaughey, G. B.; Gagne, M.; Rappe, A. K. J. Biol. Chem. 1998, 273 (25), 15458−15463. (b) Janiak, C. J. Chem. Soc. Dalton 2000, 21, 3885−3896. (16) Dey, S.; Agra-Kooijman, D. M.; Ren, W. T.; McMullan, P. J.; Griffin, A. C.; Kumar, S. Crystals 2013, 3 (2), 363−390. (17) Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V. Handbook of Liquid Crystals Set; Wiley-VCH Verlag GmbH, 1998. (18) (a) Kurabayashi, K. Int. J. Thermophys. 2001, 22 (1), 277−288. (b) Hennig, J. J. Polym. Sci., Part C: Polym. Symp. 1967, 16, 2751− 2761. (c) Choy, C. L. Polymer 1977, 18 (10), 984−1004. (d) Mergenthaler, D. B.; Pietralla, M.; Roy, S.; Kilian, H. G. Macromolecules 1992, 25 (13), 3500−3502. (e) Choy, C. L.; Wong, Y. W.; Lau, K. W. E.; Yang, G. W.; Yee, A. F. J. Polym. Sci., Part B: Polym. Phys. 1995, 33 (14), 2055−2064. (19) Davidson, P.; Petermann, D.; Levelut, A. M. J. Phys. II 1995, 5 (1), 113−131. (20) (a) Cox, S. J.; Reshetnyak, V. Y.; Sluckin, T. J. J. Phys. D: Appl. Phys. 1998, 31 (14), 1611−1625. (b) Pietrak, K.; Wisniewski, T. S. J. Power. Technol. 2015, 95 (1), 14−24. (21) Thomas, D. C.; Cardarelli, M.; Sánchez-Ferrer, A.; Mbanga, B. L.; Atherton, T. J.; Cebe, P. Liq. Cryst. 2016, 43 (1), 112−123.

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Relaxation times in prior work range from a few minutes for liquid crystal elastomers to a few tens of hours for block copolymers.23b,24,14,22,25 In our experiments, τ ∼ 700 s, see Figure 6B. The theoretical value17 of Δχ of RM257 is 1.1 × 10−3 cm3 mol−1. We measured the dynamic viscosity of the LCN by rotational oscillatory rheometry at 150 °C and found 4.5 × 103 Pa s. Equation 7 predicts τ ∼ 8 × 103 s (Supporting Information, Figure S13) an order of magnitude higher than what we observe in the thermal conductivity measurements. An order of magnitude discrepancy between measurements and the theory has been observed in other liquid crystal polymer systems.22b In summary, magnetically aligned LCNs were synthesized via polymerization of LCs in the presence of magnetic fields. Magnetically aligned LCNs show thermal conductivity of 0.34 W m−1 K−1 and 0.22 W m−1 K−1, respectively, for magnetic fields applied perpendicular and parallel to the substrate during polymerization. The molecular ordering and thermal conductivity of aligned LCNs are correlated and can be manipulated by temperature and magnetic field at elevated temperatures where the reduced viscosity enables chain reorientation on a time scale of minutes.

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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/acsmacrolett.6b00475. Sample preparation detail, TDTR sensitivity, thermal conductivity of RM257, additional GIWAXS data, thermal model, DSC and rheological characterization of LCN (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS TDTR measurement was carried out in the laser facility of the Frederick Seitz Materials Research Laboratory at the UIUC. Thermal conductivity measurements were supported by AFOSR MURI FA9550-12-1-0002 and the National Science Foundation Engineering Research Center for Power Optimization of Electro Thermal Systems (POETS) with Cooperative Agreement EEC-1449548. We thank Tae Ahn Kim for the rheology measurements and Dr. Byeongdu Lee for the assistance with GIWAXS measurements. This research used resources of the Advanced Photon Source, a U.S. Department 959

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ACS Macro Letters (22) (a) Dejeu, W. H. Mol. Cryst. Liq. Cryst. 1981, 63 (1−4), 83− 109. (b) Moore, J. S.; Stupp, S. I. Macromolecules 1987, 20 (2), 282− 293. (23) (a) Schmidt, K.; Schoberth, H. G.; Schubert, F.; Hansel, H.; Fischer, F.; Weiss, T. M.; Sevink, G. J. A.; Zvelindovsky, A. V.; Boker, A.; Krausch, G. Soft Matter 2007, 3 (4), 448−453. (b) Gopinadhan, M.; Majewski, P. W.; Choo, Y.; Osuji, C. O. Phys. Rev. Lett. 2013, 110 (7), 078301. (24) (a) Rokhlenko, Y.; Gopinadhan, M.; Osuji, C. O.; Zhang, K.; O’Hern, C. S.; Larson, S. R.; Gopalan, P.; Majewski, P. W.; Yager, K. G. Phys. Rev. Lett. 2015, 115 (25), 258302. (b) Ohm, C.; Brehmer, M.; Zentel, R. Adv. Mater. 2010, 22 (31), 3366−3387. (25) (a) Bras, W.; Emsley, J. W.; Levine, Y. K.; Luckhurst, G. R.; Seddon, J. M.; Timimi, B. A. J. Chem. Phys. 2004, 121 (9), 4397−4413. (b) McCulloch, B.; Portale, G.; Bras, W.; Pople, J. A.; Hexemer, A.; Segalman, R. A. Macromolecules 2013, 46 (11), 4462−4471.

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