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Relationships between the Morphology and Thermoresponsive Behavior in Micro/Nanostructured Thermosetting Matrixes Containing a 4′-(Hexyloxy)-4-biphenylcarbonitrile Liquid Crystal Agnieszka Tercjak* and Inãki Mondragon* Materials + Technologies Group, Escuela Politećnica, Departamento Ingenierıá Quı´mica y M. Ambiente, UniVersidad Paı´s Vasco/Euskal Herriko Unibertsitatea, Pza. Europa 1, 20018 Donostia-San Sebastiań, Spain ReceiVed May 18, 2008. ReVised Manuscript ReceiVed July 20, 2008 Meso/nanostructured thermoresponsive thermosetting materials based on an epoxy resin modified with two different molecular weight amphiphilic poly(styrene-block-ethylene oxide) block copolymers (PSEO) and a low molecular weight liquid crystal, 4′-(hexyloxy)-4-biphenylcarbonitrile (HOBC), were investigated. A strong influence of the addition of PSEO on the morphology generated in HOBC-(diglicydyl ether of bisphenol A epoxy resin/mxylylenediamine) was detected, especially in the case of the addition of PSEO block copolymers with a higher PEO-block content and a lower molecular weight. The morphologies generated in the ternary systems also influenced the thermoresponsive behavior of the HOBC separated phase provoked by applying an external field, such as a temperature gradient and an electrical field. Thermal analysis of the investigated materials allowed for a better understanding of the relationships between generated morphology/thermo-optical properties/PSEO:HOBC ratio, and HOBC content. Controlling the relationship between the morphology and thermoresponsive behavior in micro/ nanostructured thermosetting materials based on a 4′-(hexyloxy)-4-biphenylcarbonitrile liquid crystal allows the development of materials which can find application in thermo- and in some cases electroresponsive devices, with a high contrast ratio between transparent and opaque states.

Introduction The ability of block copolymers to self-assemble on the nanometric scale allows them to be widely used as templates for generating nanostructured epoxy or phenolic matrixes with longrange order in both the uncured and cured states.1-24 One practicable way for generating self-assembled thermosetting nanostructures is using amphiphilic block copolymers, which consist of thermoset-miscible and thermoset-immiscible blocks. Throughout the past decade many research groups,1-10,15-17,19-24 as well as our group,11-14,18 have effectively worked on the generation of nanostructured thermosetting systems by employing amphiphilic block copolymers mainly containing PEO-block copolymers,1-5,17-19 PMMA-block copolymers,6,7,20 or PCLblock copolymers.22-24 On the other hand, nanostructured thermosetting systems have also been generated by using the concept of chemical compatibilization,8-13 where block copolymers with reactive groups incorporated into one block can promote covalent or physical bonding with epoxy resin during network formation. The main advantage of nanostructured materials based on thermosetting matrixes is focused on their application in different fields of nanotechnology, such as nanostructured functional surfaces, nanolithography, or the building of nanostructured inorganic/organic materials. On the other hand, liquid crystals (LCs) are one of the most convenient materials to control light as they possess large optical and dielectric anisotropies, which are controllable by changes in the alignment of the molecules with external stimuli such as an electric field, thermal gradient, or light.25-29 Low molecular * To whom correspondence should be addressed. E-mail: inaki. [email protected] (I.M.); [email protected] (A.T.). Phone: +34943017271. Fax: +34-943017200. (1) Hillmyer, M. A.; Lipic, P. M.; Hadjuk, D. A.; Almdal, K.; Bates, F. S. J. Am. Chem. Soc. 1997, 119, 2749. (2) Lipic, P. M.; Bates, F. S.; Hillmyer, M. A. J. Am. Chem. Soc. 1998, 120, 8963.

weight liquid crystals embedded in the polymer matrix (PDLC) can be optically switched from a highly light scattering state (off state) to a transparent state (on state) due to the mismatching of the refractive indices of the matrix and the oriented LC as a consequence of its birefringence features. The electro-optical properties of the PDLC films make them useful for applications (3) Mijovic, J.; Shen, M.; Sy, J. W.; Mondragon, I. Macromolecules 2000, 33, 5235. (4) Guo, Q.; Thomann, R.; Gronski, W.; Thurn-Albrecht, T. Macromolecules 2002, 35, 3133. (5) Guo, Q.; Thomann, R.; Gronski, W.; Staneva, R.; Ivanova, R.; Stu¨hn, B. Macromolecules 2003, 36, 3635. (6) Ritzenthaler, S.; Court, F.; David, L.; Girard-Reydet, E.; Leibler, L.; Pascault, J. P. Macromolecules 2002, 35, 6245. (7) Ritzenthaler, S.; Court, F.; David, L.; Girard-Reydet, E.; Leibler, L.; Pascault, J. P. Macromolecules 2003, 36, 118. (8) Grubbs, R. B.; Broz, M. E.; Dean, J. M.; Bates, F. S. Macromolecules 2000, 33, 2308. (9) Rebizant, V.; Abetz, V.; Tournilhac, F.; Court, F.; Leibler, L. Macromolecules 2003, 36, 9889. (10) Rebizant, V.; Venet, A. S.; Tournilhac, F.; Girard-Reydet, E.; Navarro, C.; Pascault, J. P.; Leibler, L. Macromolecules 2004, 37, 8017. (11) Serrano, E.; Martin, M. D.; Tercjak, A.; Pomposo, J. A.; Mecerreyes, D.; Mondragon, I. Macromol. Rapid Commun. 2005, 26, 982. (12) Serrano, E.; Tercjak, A.; Kortaberria, G.; Pomposo, J. A.; Mecerreyes, D.; Mondragon, I. Macromolecules 2006, 39, 2254. (13) Serrano, E.; Tercjak, A.; Ocando, C. J.; Larranãga, M.; Parellada, M. D.; Corona-Galvań, S.; Mecerreyes, D.; Zafeiropoulos, N. E.; Stamm, M.; Mondragon, I. Macromol. Chem. Phys. 2007, 208, 2281. (14) Ocando, C.; Serrano, E.; Tercjak, A.; Penã, C.; Kortaberria, G.; Calberg, C.; Grignard, B.; Jerome, R.; Carrasco, P. M.; Mecerreyes, D.; Mondragon, I. Macromolecules 2007, 40, 4068. (15) Meng, F.; Zheng, S.; Zhang, W.; Li, H.; Liang, Q. Macromolecules 2006, 39, 711. (16) Meng, F.; Zheng, S.; Liu, T. Polymer 2006, 47, 7590. (17) Meng, F.; Zheng, S.; Li, H.; Liang, Q.; Liu, T. Macromolecules 2006, 39, 5072. (18) Tercjak, A.; Larranãga, M.; Martin, M. D.; Mondragon, I. J. Therm. Anal. Calorim. 2006, 86, 663. (19) Guo, Q.; Chen, F.; Wang, K.; Chen, L. J. Polym. Sci., Part B: Polym. Phys. 2006, 46, 3042. (20) Maiez-Tribut, S.; Pascault, J. P.; Soule, E. R.; Barrajo, J.; Williams, R. J. J. Macromolecules 2007, 40, 1268. (21) Thio, Y.; Wu, J.; Bates, F. S Macromolecules 2006, 39, 7187.

10.1021/la8015244 CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

Micro/Nanostructured Thermosetting Matrixes

in the field of thermo- and electro-optical devices such as optic shutters, smart windows, optical sensors, memories, and flexible display devices.29-36 The main disadvantage of PDLC films is related to the fact that, in typical PDLC systems, low molecular weight liquid crystals show relatively high solubility in polymer matrixes, which strongly depends on the temperature. This leads to a low activation energy for the variation of optical transmission with temperature, which is one of the main drawbacks to their potential applications. This drawback can be successfully eliminated by the addition of a small amount of a thermoplastic polymer to a solution of a liquid crystal in a thermosetting precursor, as has been published by Hoppe et al.36-38 Following the work published by Hoppe et al.36-38 and our previous papers related to nanostructured thermosets,11-14,18 thermoplastic/ thermosetting,39,40 and thermoplastic/liquid crystal/thermosetting,41-43 materials we have recently employed, for the first time to the best of our knowledge, block copolymers44,45 instead of thermoplastic polymers in these ternary systems. In this case, the addition of the block copolymers to the liquid crystal/thermosetting systems opened up a new strategy for the preparation of thermoresponsive meso/nanostructured thermosetting materials. In this strategy the amphiphilic block copolymer, containing one epoxy-miscible block and one immiscible block, works as a selfassembly agent and simultaneously acts as a polymer for the dispersion of liquid crystals. The epoxy-immiscible block also must show a higher miscibility with a low molecular weight liquid crystal than with the epoxy matrix. In this research field, we have studied44,45 the influence of the curing condition on the morphology generated and the ability of the thermoresponsive behavior of the thermosetting materials based on 4′-hexyl-4biphenylcarbonitrile (HBC) as the liquid crystal. Taking the above into account and also the promising results published by us42 for 4′-(hexyloxy)-4-biphenylcarbonitrile/ poly(styrene-block-ethylene oxide) (HOBC/PSEO) binary systems, in the present investigation meso/nanostructured thermosetting systems modified with HOBC as the low molecular weight nematic liquid crystal have been studied. Atomic force microscopy (22) Xu, Z.; Zheng, S. Macromolecules 2007, 40, 2548. (23) Xu, Z.; Zheng, S. Polymer 2007, 48, 6134. (24) Meng, F.; Zhiguang, X.; Zheng, S. Macromolecules 2008, 41, 1411. (25) Craighead, H. G.; Chen, J.; Hackwood, S. Appl. Phys. Lett. 1982, 40, 22. (26) Nastał, E.; Zuran´ska, E.; Mucha, M. J. Appl. Polym. Sci. 1999, 71, 455. (27) de Gennes, P. H. Scaling Concepts in Polymer Science; Cornell University Press: Ithaca, NY, 1979; Chapter 5. (28) Coates, D. J. Mater. Chem. 1995, 5, 2063. (29) Sumana, G.; Raina, K. K. Curr. Appl. Phys. 2005, 5, 277. (30) Doane, J. W.; Vaz, N. A.; Wu, B. G.; Zumer, S. Appl. Phys. Lett. 1986, 48, 269. (31) Drzaic, P. S. Liq. Cryst. 1988, 3, 1543. (32) Herod, T. E.; Duran, R. S. Langmuir 1998, 14, 6956. (33) Zhou, J.; Petti, L.; Mormile, P.; Roviello, A. Opt. Commun. 2004, 231, 263. (34) Sumana, G.; Raina, K. K. J. Polym. Mater. 2002, 19, 281. (35) Karapinar, R.; O’Neill, M.; Hird, M. J. Phys. D: Appl. Phys. 2002, 35, 900. (36) Hoppe, C. E.; Galante, M. J.; Oyanguren, P. A.; Williams, R. J. J. Macromolecules 2002, 35, 6324. (37) Hoppe, C. E.; Galante, M. J.; Oyanguren, P. A.; Williams, R. J. J. Mater. Sci. Eng. 2004, 24, 591. (38) Hoppe, C. E.; Galante, M. J.; Oyanguren, P. A.; Williams, R. J. J. Macromolecules 2004, 37, 5352. (39) Tercjak, A.; Remiro, P. M.; Mondragon, I. Polym. Eng. Sci. 2005, 45, 303. (40) Tercjak, A.; Serrano, E.; Remiro, P. M.; Mondragon, I. J. Appl. Polym. Sci. 2006, 100, 2348. (41) Tercjak, A.; Serrano, E.; Mondragon, I. Polym. AdV. Technol. 2006, 17, 835. (42) Tercjak, A.; Serrano, E.; Garcia, I.; Ocando, C. J.; Mondragon, I. Acta Mater. 2007, 55, 6436. (43) Tercjak, A.; Serrano, E.; Larranãga, M.; Mondragon, I. J. Appl. Polym. Sci. 2008, 108, 1116. (44) Tercjak, A.; Serrano, E.; Mondragon, I. Macromol. Rapid Commun. 2007, 28, 937. (45) Tercjak, A., Serrano, E.; Garcia, I.; Mondragon, I. Acta Mater., in press.

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(AFM) has been used to study the morphological architecture generated in the epoxy blends. The influence of the final morphology on the thermoresponsive behavior of the systems modified with HOBC and two PSEO block copolymers has been studied to establish thermo-optical curves in the isotropization region by registration of the light transmission when an external gradient of temperature has been applied during the cooling/ heating cycle. Electrostatic force microscopy (EFM) has also been applied to verify whether microphase-separated domains of HOBC/PS responded to the voltage applied to the EFM tip. Thermal analysis of the investigated systems has been performed by means of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA).

Experimental Section Materials. A diglicydyl ether of bisphenol A epoxy resin (DGEBA) (Dow DER 332, gifted by Dow Chemical) was used as the reactive solvent. It has an epoxy equivalent of approximately 175 and an average number of hydroxyl groups per two epoxy groups (n) of 0.03. This epoxy resin was cured with a stoichiometric amount of an aminic hardener, m-xylylenediamine (MXDA), supplied by Sigma-Aldrich. Two amphiphilic poly(styrene-block-ethylene oxide) diblock copolymers, denoted as PSEO1 and PSEO2, Polymer Source Inc., were used to generate nanostructured epoxy systems. Number average molecular weights, Mn (Mw), for PS (PEO) blocks and Mw/Mn of block copolymers were 125 000 (16 100) g mol-1 and 1.04, respectively, for PSEO1, and 58 600 (31 000) g mol-1 and 1.03, respectively, for PSEO2. The low molecular weight nematic liquid crystal, 4′-(hexyloxy)-4-biphenyl-carbonitrile, used in this study was supplied by Sigma-Aldrich and used as received. As has been published by us,41 on the basis of differential scanning calorimetry and optical microscopy (OM) measurements, this liquid crystal exhibits a nematic-isotropic (TN-I) transition at about 70 °C and a crystal-nematic (TC-N) transition at about 49 °C. Blending Protocol. Ternary block copolymer/liquid crystal/epoxy as well as block copolymer/epoxy and liquid crystal/epoxy cured blends were prepared according to the procedure published by us elsewhere.44,45 First, adequate PSEO or/and HOBC and DGEBA resin contents, both ternary and binary systems containing similar contents of PSEO and/or HOBC, were dissolved in toluene. The resultant solution was heated to 80 °C in an oil bath until a complete solvent removal was achieved. The curing agent MXDA was then added to the mixture, and homogeneous ternary mixtures were obtained. After that, the mixtures were immediately degassed at 80 °C in a vacuum and cured at this temperature for 15 h. The precuring temperature has been chosen to perform curing above the nematic-isotropic transition of the HOBC. After curing (in air), the plaques were slowly cooled to room temperature, demolded, and postcured for 2 h at 160 °C under vacuum. The weight percentage of PSEO or HOBC in the ternary and binary mixtures was calculated with respect to the DGEBA/MXDA system. To avoid the possible sublimation process of the liquid crystal, the cured blends were prepared in a parallelepipedic mold of 1 ( 0.1 mm thickness. Morphological Analysis. The morphologies of both the PSEO1-HOBC-(DGEBA/MXDA) and PSEO2-HOBC-(DGEBA/ MXDA) systems as well as DGEBA/MXDA, neat and modified only with the corresponding PSEO or HOBC content, were investigated by atomic force microscopy. AFM images were obtained operating in tapping mode (TM-AFM) with a scanning probe microscope (Nanoscope IIIa, Multimode from Digital Instruments) equipped with an integrated silicon tip/cantilever having a resonance frequency of ∼300 kHz from the same manufacturer. To obtain repeatable results, different regions of the specimens were scanned. Similar images were obtained, thus demonstrating the reproducibility of the results. Representative pieces of each epoxy composite sheet were microtomed at room temperature using a Leica Ultracut R microtome equipped with a diamond knife.

11218 Langmuir, Vol. 24, No. 19, 2008 Electrostatic force microscopy was used to study the positional and orientational order of low molecular weight liquid crystal microphase-separated domains in the block copolymer/thermosetting matrix. Measurements were performed using scanning probe microscopy (Nanoscope IVa, Dimension 3100 from Digital Instrument) in the lift mode (the lift height was 200 nm) in ambient conditions and equipped with an integrated Co/Cr-coated MESP tip having a resonance frequency of approximately 75 kHz. The secondary imaging mode derived from the tapping mode which measures the electric field gradient distribution above the sample surface was detected by applying a voltage to the cantilever tip. Locally charged LC phase domains on the surface of the samples were qualitatively mapped simultaneously with the height and phase AFM images. Differential Scanning Calorimetry. DSC measurements were carried out in a Mettler Toledo DSC 822 differential scanning calorimeter equipped with a TSO 801 RO sample robot. Nitrogen was used as the purge gas (10 mL/min). Measurements were performed in sealed aluminum pans containing approximately 10 mg of sample. The temperature and enthalpy were calibrated by using an indium standard. Measurements were performed in sealed aluminum pans containing approximately 10 mg of sample. To ensure comparable thermal histories, all samples were first heated to 150 °C, maintained at that temperature for 10 min, then cooled to -50 °C, and reheated to 150 °C. All the scans were performed at a constant rate of 5 °C min-1. DSC measurements allowed for determination of the thermal transitions. Dynamic Mechanical Analysis. DMA analysis was carried out on cured blends with a Perkin-Elmer DMA7 in the three-point bending mode to obtain dynamic mechanical spectra (storage modulus, E′, and loss factor, tan δ) between -120 and +180 °C. The scans were carried out at a frequency of 1 Hz and a heating rate of 5 °C min-1, using a span of 15 or 5 mm for low- and high-temperature ranges, respectively. The samples used were parallelepiped bars (24 × 3 × 1 mm3). During the scans the samples were subjected to a static force of 110 mN and a dynamic force of 100 mN. The glass transition temperature, Tg, was determined by means of the R relaxation maximum of the loss factor curve. Thermo-Optical Analysis. The thermo-optical behaviors of both the PSEO1-HOBC-(DGEBA/MXDA) and PSEO2-HOBC(DGEBA/MXDA) systems as well as DGEBA/MXDA cured blends modified only with the corresponding HOBC content were investigated by using a transmission optical microscope (TOM) (Nikon Eclipse E600) equipped with a hot stage (Mettler FP 82HT). To produce changes in the transmission light when the external gradient of temperature was applied, the thin film placed between two microscope slides was heated/cooled/heated from 30 to 90 °C at a rate of 1 °C min-1. The thickness of the samples was controlled by using a 1 mm aluminum spacer.

Results and Discussion Influence of the Morphology Generated in the Ternary Cured Blends on the Thermo-Optical Behavior. First, it has to be pointed out that the following results refer to the bulk behavior of these systems since the cured blends have been prepared in a parallelepipedic mold of 1 ( 0.1 mm thickness. The influence of the addition of a small amount of PSEO block copolymers on the morphology generated in HOBC-(DGEBA/ MXDA) cured systems has been studied by means of AFM. Though we have previously published some work concerning HBC,44,45 data related to HOBC low molecular weight liquid crystals are of great interest since in this case nematic/isotropic transition takes place at 70 °C. Consequently, we have been able to perform additional experiments to better understand the influence of both generated morphologies and thermal analysis on the thermo-optical response of these materials. Representative TM-AFM phase images of the epoxy system modified with 30 and 50 wt % HOBC are shown in Figure 1. Macrophase separation of the HOBC phase takes place for 30 and 50 wt %

Tercjak and Mondragon

HOBC-(DGEBA/MXDA) epoxy systems. The HOBC phase separates on typical polygon/hexagon-shaped domains of approximately 200-620 nm. Moreover, as expected, macrophaseseparated HOBC domains are more closely packed in the case of the 50 wt % HOBC-(DGEBA/MXDA) system. Thermooptical properties of these systems in the isotropization region have been plotted in Figure 1c. The thermo-optical curves indicate that both systems modified with 30 and 50 wt % HOBC-(DGEBA/ MXDA) show nematic-isotropic transition (TN-I) of the HOBC phase in the range of neat HOBC, approximately 70-75 °C, thus confirming their response to an externally applied temperature gradient. However, for both systems, the TN-I transition takes place in a broad range of temperature and the thermo-optical curves during the cooling process are shifted approximately 2 °C with respect to the heating process, which indicates a drawback in these materials since hysteresis has been detected. To minimize the drawback in the thermo-optical response of the HOBC-(DGEBA/MXDA) systems, two different molecular weight PSEO block copolymers were added as a third component. The morphological features generated for the 30 and 50 wt % HOBC-(DGEBA/MXDEA) systems by the addition of 5 and 10 wt % PSEO1 or PSEO2 are shown in Figures 2 and 3, respectively. Additionally, for ternary systems, the insets at the top left show thermo-optical curves taken during the heating/ cooling cycle in the isotropization region. Figure 2a shows microphase-separated PS-block micelles, typical for 5 wt % PSEO-(DGEBA/MXDA), with the average size not higher than 35 nm. In AFM phase images of the epoxy system modified with 5 wt % PSEO1 and 30 wt % HOBC (Figure 2b), one can easily distinguish two different phase-separated domains. The first one corresponds to the microphase-separated HOBC/PS-block domains, since the phase-separated domains with an average size of approximately 120-180 nm are 3-4 times higher than those for the microphase-separated PS block in the PSEO-(DGEBA/ MXDA) epoxy system. The second one is related to the HOBC phase as it has a typical polygon/hexagon-shaped LC phase.42,45 Thermo-optical curves for this system identify that 100% optical transparency is reached in the isotropization region and that switching from the opaque to transparent state (off/on state) takes place at the same temperature range almost without hysteresis. Thus, addition of PSEO1 to this system minimizes the drawback between heating/cooling curves and consequently leads to obtaining thermoresponsive materials with higher contrast between the off and on states if compared to the system modified only with 30 wt % HOBC. Addition of 5 wt % PSEO1 as a third component to 50 wt % HOBC-(DGEBA/MXDA) also leads to two different phaseseparated domains. However, microphase separation of the PS block or HOBC/PS block is significantly lower if compared to that for 5 wt % PSEO-(DGEBA/MXDA). The second phaseseparated domain, the polygon/hexagon-shaped HOBC phase, is very similar in size and distribution to the macrophase-separated LC phase in 50 wt % HOBC-(DGEBA/MXDA). Here, it is worth noting that, similarly to the 50 wt % HOBC-(DGEBA/MXDA) epoxy system, this material is not optically transparent in the isotropization region. Following the thermo-optical curves taken for this ternary thermosetting material, one can easily conclude that almost 10-15% of transparency is lost. Additionally, thermooptical curves during the heating process are shifted 2 °C toward lower temperatures during the cooling process, being similar in shape to the curves established for the 50 wt % HOBC-(DGEBA/ MXDA) system. On the contrary, the addition of 10 wt % PSEO1 to the 30 wt % HOBC-(DGEBA/MXDA) system leads to microphase-


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Figure 1. TM-AFM phase images for the DGEBA/MXDA system modified with (a) 30 wt % HOBC and (b) 50 wt % HOBC. (c) Thermo-optical curves in the isotropization region during the heating/cooling cycle for 30 wt % HOBC (O/b) and for 50 wt % HOBC (]/[).

separated domains. As shown in Figure 2e, only one regular, uniformly dispersed phase with an average size of approximately 70-170 nm, 2-5 times higher than those for this system without the LC (see Figure 2d), can be detected. This fact confirms that reaction-induced microphase-separated PS blocks contain LCs within, as has been discussed in our previous work.44,45 Therefore, the addition of 10 wt % PSEO1 hinders macrophase separation of HOBC, thus avoiding its crystallization, which is in good agreement with thermal analysis of these systems described below. The thermo-optical curves of this system reveal the ability to switch from the off to the on state almost without hysteresis with high contrast and in a narrower range of temperatures compared to that corresponding to the 5 wt % PSEO1-30 wt % HOBC-(DGEBA/MXDA) system. The addition of 10 wt % PSEO1 means a higher content of PEO block miscible with epoxy resin and a higher content of PS block partially miscible with the HOBC phase, compared with that of 5 wt % PSEO1. As a consequence a higher amount of the HOBC phase can be microphase-separated within the PS block, and consequently, no macrophase separation has been observed. Following this explanation, higher contrast and faster switching between the opaque and the transparent state have been detected.

The thermosetting system modified with 10 wt % PSEO1 and 50 wt % HOBC presents a morphology similar to that of the system modified with 50 wt % HOBC. As shown in Figure 2f, only macrophase-separated domains with an average size of approximately 1-2.2 µm are observed. In this case, contrary to the other investigated systems mentioned above, the PS block separates within the polygon/hexagon domains of the HOBC phase since microphase-separated domains are not detected by means of AFM measurements. Thermo-optical curves for this material do not present significant variations when compared to those of the 50 wt % HOBC-(DGEBA/MXDA) epoxy system. The system lost almost 18% of its transparency and is not optically transparent in the isotropization region. Taking into account that a higher content of PSEO1 added as the third component leads to meso/nanostructured thermoset systems with higher contrast and faster switching between the on and off states; PSEO2, with a higher content of the PEO block, has also been used. The morphologies generated in the DGEBA/MXDA systems modified with 30 and 50 wt % HOBC after addition of 5, 10, and 15 wt % PSEO2 are shown in Figure 3. As can be clearly seen, the addition of 5, 10, and 15 wt % PSEO2 leads to microphase separation of the PS block, with the

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Figure 2. TM-AFM phase images of DGEBA/MXDA systems modified with different PSEO1 or/and HOBC contents: (a) 5 wt % PSEO1, (b) 5 wt % PSEO1-30 wt % HOBC, (c) 5 wt % PSEO1-50 wt % HOBC, (d) 10 wt % PSEO1, (e) 10 wt % PSEO1-30 wt % HOBC, and (f) 10 wt % PSEO1-50 wt % HOBC. The insets show thermo-optical curves during the heating/cooling cycle.

HOBC liquid crystal within. The microphase separation of the HOBC phase within the PS block is confirmed by an increase of the size of the microphase-separated domains in the ternary systems when compared to the binary epoxy systems modified only with PSEO2.44,45 The average size of the microphaseseparated domains changes from 14-18 to 32-50 nm for the 5 wt % PSEO2-30 wt % HOBC-(DGEBA/MXDA) epoxy system and from 19-30 to 35-65 nm for the 10 wt % PSEO2-30 wt % HOBC-(DGEBA/MXDA) system, compared to the corresponding epoxy systems modified only with the same PSEO2 content. Furthermore, addition of 5 and 10 wt % PSEO2 leads to the typical micellar nanostructure, while addition of 15 wt % PSEO2 to 30 wt % HOBC-(DGEBA/MXDA) allows wormlike nanostructured epoxy systems to be obtained. On the other hand, neither for the systems containing 5 wt % PSEO2 nor for systems containing 15 wt % PSEO2 and 30 wt % HOBC have macrophase-separated domains of the LC phase been detected. Indeed, the higher content of the PEO block in the block copolymer (compare PSEO2 to PSEO1), which is partially miscible with epoxy resin,4,5,19,44 leads to more stable HOBC/PS-block microphase-separated thermoset systems. In the case of the addition of 5 wt % PSEO2 to 50 wt % HOBC-(DGEBA/MXDA), the final morphology generated in the ternary system changes from a polygon/hexagon-shaped domain characteristic for macrophase-separated LCs to an almost cocontinuous structure (Figure 3c), which shows thermooptical curves rather similar to the thermo-optical curves for the 50 wt % HOBC-(DGEBA/MXDA). These thermosetting systems are not optically transparent in the isotropic state; they lose almost 12% of their transparency, and the thermo-optical

curve during the cooling process is shifted 3-4 °C to the lowtemperature region with respect to the curve plotted for the heating process. The addition of 10 wt % PSEO2 to the 50 wt % HOBC-(DGEBA/MXDA) system is not enough to lead to microphase separation. The macrophase separation of HOBC includes PS blocks within since the average size of the macrophase-separated polygon/hexagon domains increases from 600 nm to 1.2 µm to 1.2-2 µm when compared to that of the epoxy DGEBA/MXDA system modified only with 50 wt % HOBC. This behavior is very similar to the behavior of the ternary system modified with PSEO1. Moreover, the thermo-optical curves indicate that this material is not optically transparent and loses almost 34% of its transparency in the isotropic state. In the case of 15 wt % PSEO2-30 wt % HOBC-(DGEBA/ MXDA), the electroresponse of the separated HOBC phase has been checked by using EFM. The results obtained are shown in Figure 3h. The electroresponse of the HOBC phase domains can be easily recognized since applying 0 V does not lead to the detection of any changes on the ultramicrotomed surface of the ternary nanostructured epoxy system. On the contrary, applying 12 V to the EFM tip allows for the detection of liquid crystal domains in the EFM phase image with high contrast between the uncharged and charged states and fast response (see Figure 3h). Here it should be pointed out that, as has been recently published by us,46 neither PSEO-(DGEBA/ MXDA) nor DGEBA/MXDA shows any electroresponse to the voltage applied to the EFM tip. (46) Tercjak, A.; Garcia, I.; Mondragon, I. Nanotechnology 2008, 19, 275701.


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Figure 3. TM-AFM phase images of DGEBA/MXDA systems modified with different PSEO2 or/and HOBC contents: (a) 5 wt % PSEO2, (b) 5 wt % PSEO2-30 wt % HOBC, (c) 5 wt % PSEO2-50 wt % HOBC, (d) 10 wt % PSEO2, (e) 10 wt % PSEO2-30 wt % HOBC, (f) 10 wt % PSEO2-50 wt % HOBC, and (g) 15 wt % PSEO2-30 wt % HOBC. (h) EFM phase images of 15 wt % PSEO2-30 wt % HOBC. The insets show thermo-optical curves during the heating/cooling cycle.

Following the morphological features of the ternary systems obtained by adding as the third component block copolymers such as PSEO1 or PSEO2 to 30 or 50 wt % HOBC-(DGEBA/ MXDA), one can conclude that, by controlling the content of each compound, modifier, and liquid crystal, nano/mesostructured materials with high contrast in switching from an opaque to a transparent state (off/on state) can be obtained. These materials possess interesting properties and could find application in the field of thermoresponsive devices. Thermal Analysis. Thermo-optical properties and morphologies generated in DGEBA/MXDA epoxy systems modified with 30 and 50 wt % HOBC and 5 and 10 wt %

PSEO1 are in good agreement with thermal analysis of these systems investigated by means of DSC and DMA. Corresponding DSC thermograms are shown in Figure 4a. For the 50 wt % HOBC-(DGEBA/MXDA) epoxy system and similar systems modified with 5 and 10 wt % PSEO1, two independent endotherms can be easily detected. The first one at approximately 40 °C corresponds to the crystal-nematic transition of the macrophase-separated HOBC, and the second one at approximately 70 °C is related to the nematic-isotropic transition of this phase in the epoxy systems. Here it should be pointed out that the enthalpy of the crystal-nematic transition of the HOBC phase in both HOBC-(DGEBA/MXDA) and PSEO-HOBC-

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Figure 4. (a) DSC thermograms. (b, c) Dynamic mechanical spectra (storage modulus (full symbols) and tan δ (open sysmbols)) in the (b) high-temperature and (c) low-temperature ranges obtained at 1 Hz for the DGEBA/MXDA epoxy systems (9) modified with different PSEO1 or/and HOBC contents: (f) 5 wt % PSEO1, (b) 30 wt % HOBC, (2) 5 wt % PSEO1-30 wt % HOBC, (1) 10 wt % PSEO1-30 wt % HOBC, ([) 50 wt % HOBC, (right-pointing triangle) 5 wt % PSEO1-50 wt % HOBC, and (left-pointing triangle) 10 wt % PSEO1-50 wt % HOBC. The inset of (a) shows DSC thermograms for neat HOBC and 50 wt % HOBC-(DGEBA/MXDA).

(DGEBA/MXDA) is around 4 times lower than the enthalpy of the neat HOBC (inset of Figure 4a). The enthalpy of the second endothermic peak has not been higher than 4 J/g. Furthermore, the melting points and the nematic-isotropic transition temperatures of the LC phase in the LC-PSEO-(DGEBA/MXDA) epoxy blends are shifted to lower temperature compared with


those of the neat HOBC owing to partial miscibility both with the PS block42 and with the epoxy resin.37,38 Under the same preparation conditions, 30 wt % HOBC-(DGEBA/MXDEA) epoxy systems without and with 5 and 10 wt % PSEO1 only show one transition related to the nematic-isotropic process. In this case the melting transition of HOBC is almost undetectable (see Figure 4a). Taking into account both a narrower temperature range for the nematic-isotropic transition for these systems compared with the 30 wt % HOBC-(DGEBA/MXDEA) system and the bigger size of the microphase-separated domains compared with the microphase-separated domains in the PSEO1-(DGEBA/MXDA) systems, one can conclude that addition of PSEO1 to 30 wt % HOBC-(DGEBA/MXDA) leads to the separation of the HOBC phase within PS-block domains, being the reason to achieve thermoresponsive systems almost without hysteresis. Parts b and c of Figure 4 show dynamical mechanical spectra (temperature dependence of the loss factor, tan δ, and storage modulus, E′) for PSEO1 and/or HOBC-(DGEBA/MXDA) epoxy blends in both the high- and low-temperature ranges, respectively. The results obtained are in good agreement with the results reported above. All modified systems show only one glass transition temperature, Tg, of the epoxy/HOBC- or/and PSEO1modified matrix in the high-temperature range. For the 30 wt % HOBC-(DGEBA/MXDA) epoxy systems, the Tg of the epoxyrich phase is shifted to the lower temperature from approximately 120 °C for the neat DGEBA/MXDA system to 66 °C. This fact indicates the partial miscibility between the epoxy resin and HOBC and the consequent plasticization effect of the LC phase on the epoxy matrix. For the 50 wt % HOBC-(DGEBA/MXDA) system, the Tg is shifted to higher temperature compared to that for the system modified only with the 30 wt % HOBC. This behavior is related to the fact that, simultaneously with macrophase separation of the HOBC phase, less HOBC molecules are dissolved in the epoxy matrix. Furthermore, addition of 5 or 10 wt % PSEO1 shifts the Tg of the modified epoxy matrix from 120 to 112 and 102 °C, respectively. This shift is an effect of the partial miscibility of the PEO block with the epoxy matrix and the plasticization effect.4,5,19,44 Additionally, as expected, taking the final morphology generated in these systems into account, the Tg of the epoxy matrix in the ternary systems is higher and lower compared with the adequate Tg of the epoxy matrix in the 30 and 50 wt % HOBC-(DGEBA/MXDA) systems, respectively. This once again proves that the microphase separation of the PS block induces macro/microphase separation of the HOBC phase within the PS block. On the contrary, the Tg of the epoxy matrix in 50 wt % HOBC-(DGEBA/MXDA) systems modified with 5 and 10 wt % PSEO decreases with addition of PSEO1. This fact agrees with the DSC results since the enthalpy of the melting process of the HOBC phase decreases with addition of 5 and 10 wt % PSEO1. Thus, less amount of the HOBC phase is macrophaseseparated from the epoxy system during network formation because the PS block separated within (almost undetectable microseparation of the PS block/HOBC phase in the case of 5 and 10 wt % PSEO1-50 wt % HOBC-(DGEBA/MXDA) systems by means of AFM), and consequently, the plasticization effect of both the PEO block and nonseparated HOBC phase leads to a lower Tg of the epoxy matrix. In all the investigated epoxy systems modified with different amounts of PSEO1 and/or HOBC, the storage modulus drops 2 orders of magnitude after overcoming the glass transition and shows a broad plateau up to the end of the measurements. This rubberlike behavior confirms the maintenance of elastic properties,


which identify the investigated systems as thermosetting materials, the epoxy-rich phase being the matrix. In the low-temperature range, a strong influence of the addition of the HOBC content on the β relaxation of the epoxy-rich phase, associated with the sum of localized motions of glycidyl ether groups in the epoxy network,47,48 has been detected. It is related to the fact that R relaxation of the HOBC, under the same measurement conditions, takes place at approximately -50 °C. Consequently, macrophase separation of the HOBC phase leads to the appearance of a better pronounced tan δ at low temperature (see Figure 4c). Moreover, it is worth noting that in this case the β relaxation of the epoxy-rich phase is almost undetectable, which confirms the partial miscibility of the epoxy systems with the HOBC phase. The results corresponding to thermal analysis of the PSEO2-HOBC-(DGEBA/MXDA), PSEO2-(DGEBA/MXDA), and HOBC-(DGEBA/MXDA) systems obtained by DSC and DMA are shown in parts a and b, respectively, of Figure 5. DSC thermograms of the ternary systems modified with PSEO2 confirm a lack of macrophase separation of the HOBC except for 10 wt % PSEO2-50 wt % HOBC-(DGEBA/MXDA), where a melting endotherm corresponding to the macrophase-separated HOBC has been detected, being similar to that of the macrophaseseparated HOBC phase in the 50 wt % HOBC-(DGEBA/MXDA) system. This is in good agreement with AFM phase images for these systems. Moreover, here it should be pointed out that the addition of PSEO2, with a high PEO-block content and the lower molecular weight of each block, leads to a higher partial miscibility among the PEO block, epoxy resin, PS block, and low molecular weight liquid crystal. Consequently, meso/nanostructured systems with thermoresponsive behavior can be obtained with the microseparated HOBC phase in a broader range of composition compared with that of PSEO1. This fact indicates that the microphase-separated PS block with a lower molecular weight than in the case of PSEO1 makes sure that a higher content of the HOBC phase is separated within, as a consequence of the higher partial miscibility with each other. Simultaneously, the addition of PSEO2 hinders the crystallization process of the HOBC chains since in ternary epoxy blends, except for 10 wt % PSEO2-50 wt % HOBC-(DGEBA/MXDA), neither melting endotherms nor crystallization exotherms have been detected. The temperature dependency of tan δ and E′ for the DGEBA/ MXDA epoxy systems modified with 5 or 10 wt % PSEO2 and/or 30 or 50 wt % HOBC in the high-temperature range is shown in Figure 5b. The tendency of the Tg of the epoxy-rich matrix in modified systems is similar to that for the DGEBA/ MXDA epoxy system modified with HOBC and PSEO1. Nevertheless, the maximum of tan δ of the ternary systems modified with 5 and 10 wt % PSEO2 and 30 wt % HOBC is shifted 8 and 6 °C, respectively, to lower temperature. This can be related to the stronger plasticization effect of the PEO block in ternary systems containing PSEO2 compared to similar systems with PSEO1. Here it should be pointed out that, though not shown in the case of the ternary systems obtained using PSEO2 as a third component, the β relaxation of the epoxy-rich phase has no significant changes, as macrophase separation of the HOBC chains is only detected in the 10 wt % PSEO2-50 wt % HOBC-(DGEBA/ MXDA) system. (47) Federolf, H. A.; Eyerer, P.; Moginger, P.; Mebus, C.; Jin, R.; Scheer, W. J. Polym. Sci., Part B: Polym. Phys. 1999, 19, 243. (48) Gerard, J. F. Polym. Eng. Sci. 1998, 28, 568.

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Figure 5. (a) DSC thermograms. (b) Dynamic mechanical spectra (storage modulus (full symbols) and tan δ (open sysmbols)) in the hightemperature range obtained at 1 Hz for DGEBA/MXDA epoxy systems (9) modified with different PSEO2 or/and HOBC contents: (f) 5 wt % PSEO2, (b) 30 wt % HOBC, (2) 5 wt % PSEO2-30 wt % HOBC, (1) 10 wt % PSEO2-30 wt % HOBC, ([) 50 wt % HOBC, (rightpointing triangle) 5 wt % PSEO2-50 wt % HOBC, and (left-pointing triangle) 10 wt % PSEO2-50 wt % HOBC.

Conclusions The thermoresponsive behavior of the epoxy systems modified with a low molecular weight liquid crystal, HOBC, has been investigated. Two different molecular weight poly(styrene-blockethylene oxide) (PSEO) block copolymers have been used as a third component to generate meso/nanostructured thermosetting systems and to control the macro/microphase separation of the HOBC phase in the ternary epoxy systems. Since two PSEO block copolymers used in this work have different PEO-block contents and molecular weights, addition of them to the HOBC-(DGEBA/MXDA) epoxy systems allows for the design of various micro/nanostructured morphologies and provides correlation among the morphology, thermoresponsive behavior, and thermal properties of these materials. Control of the morphology created by the addition of a small amount of PSEO block copolymers used as both a self-assembling agent for the epoxy matrix and a dispersing/miscibility agent for the liquid crystal allows thermosetting materials with both well-defined

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morphology and the ability of the LC to respond to external fields such as a thermal gradient or an electrical field applied to the EFM tip to be obtained. Some of the generated materials are well-defined meso/nanostructured systems in which the microphase separation of the PS block provokes separation of the HOBC phase and responds to the thermal gradient switching from the off state to the on state almost without hysteresis. This fact allows thermosetting materials with interesting properties,


which can find potential application in the field of thermo-optical devices, to be created. Acknowledgment. Financial support from the Basque Country Governments in the frame of SAIOTEK (Grant S-PE07UN39), ETORTEK nanoGUNE, and Grupos Consolidados (Grant IT365-07) is gratefully acknowledged. LA8015244

Relationships between the Morphology and Thermoresponsive

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