Conductive Properties of Switchable Photoluminescence

Nov 20, 2009 - -(hexyl)-4-biphenyl-carbonitrile. (HBC) liquid crystal and rutile TiO2 nanoparticles were successfully studied by means of tunneling at...
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Conductive Properties of Switchable Photoluminescence Thermosetting Systems Based on Liquid Crystals Agnieszka Tercjak,* Junkal Gutierrez, Connie Ocando, and I~naki Mondragon* Materials þ Technologies Group, Escuela Polit ecnica, Departamento Ingenierı´a Quı´mica y Medio Ambiente, Universidad Paı´s Vasco/Euskal Herriko Unibertsitatea, Plaza Europa 1, 20018 Donostia-San Sebasti an, Spain Received September 9, 2009. Revised Manuscript Received October 29, 2009 Conductive properties of different thermosetting materials modified with nematic 40 -(hexyl)-4-biphenyl-carbonitrile (HBC) liquid crystal and rutile TiO2 nanoparticles were successfully studied by means of tunneling atomic force miscroscopy (TUNA). Taking into account the liquid crystal state of the HBC at room temperature, depending on both the HBC content and the presence of TiO2 nanoparticles, designed materials showed different TUNA currents passed through the sample. The addition of TiO2 nanoparticles into the systems multiply the detected current if compared to the thermosetting systems without TiO2 nanoparticles and simultaneously stabilized the current passed through the sample, making the process reversible since the absolute current values were almost the same applying both negative and positive voltage. Moreover, thermosetting systems modified with liquid crystals with and without TiO2 nanoparticles are photoluminescence switchable materials as a function of temperature gradient during repeatable heating/cooling cycle. Conductive properties of switchable photoluminescence thermosetting systems based on liquid crystals can allow them to find potential application in the field of photoresponsive devices, with a high contrast ratio between transparent and opaque states.

Introduction Since the discovery of organic semiconductors, enormous efforts have been directed toward their application as active electronic devices in different electronic components such as light-emitting diodes,1,2 transistors,3 solar cells,4,5 capacitors,6,7 miniaturized circuitry,8 and others. Consequently, the production of smart materials that are able to respond to external stimuli such as temperature gradients, electrical fields, or lights is one of the most active fields in polymer science. In this field, special attention is paid to the development of hybrid inorganic/organic liquid crystal (LC) materials, since dispersion of different nanoparticles in low molecular weight LCs allows for the design of new liquid crystal devices (LCDs), given that semiconductor nanoparticles dispersed in the LC or polymer matrix can lead to large electrooptic9-14 or photorefractive effects. As is well-known, LCs are one of the most suitable materials to control light as they possess *Corresponding author. E-mail: [email protected] (I.M.); agnieszka. [email protected] (A.T.). Telephone: þ34-943017177. Fax: þ34-943017140.

(1) Lee, Ch. C.; Park, J. IEEE Photonics Technol. Lett. 2004, 16, 1706. (2) Zorn, M.; Bae, W. K.; Kwak, J.; Lee, H.; Lee, Ch.; Zentel, R.; Char, K. ACS Nano 2009, 3, 1063. (3) Hagfeldt, A.; Graetzel, M. Acc. Chem. Res. 2000, 33, 269. (4) Kumar, S. Curr. Sci. 2002, 82, 256. (5) Yamanaka, N.; Kawano, R.; Kubo, W.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. Chem. Commun. 2005, 740. (6) Yeh, J. A.; Chang, C. A.; Cheng, Ch.-Ch.; Huang, J.-Y.; Hsu, S. S. H. IEEE Electron Device Lett. 2005, 26, 451. (7) Holstein, P.; Bender, M.; Galvosas, P.; Geschke, D.; K€arger, J. J. Magn. Reson. 2000, 143, 427. (8) Rogers, J. A.; Bao, Z.; Meier, M.; Dodabalapur, A.; Schueller, O. J. A.; Whitesides, G. M. Synth. Met. 2000, 115, 5. (9) In, I.; Jun, Y.-W.; Kim, Y. J.; Kim, S. Y. Chemical Commun. 2005, 6, 800. (10) Jeng, S. Ch.; Kuo, Ch. W.; Wang, H. L.; Liao, Ch. Ch. Appl. Phys. Lett. 2007, 91, 061112. (11) Kaczmarek, M.; Buchnev, O.; Nandhakumar, I. Appl. Phys. Lett. 2008, 92, 103307. (12) Payne, J. C.; Thomas, E. L. Adv. Funct. Mater. 2007, 17, 2717. (13) da Cruz, C.; Sandre, O.; Cabuil, V. J. Phys. Chem. B 2005, 109, 14292. (14) Hegmann, T.; Qi, H.; Marx, V. M. J. Inorg. Organomet. Polym. Mater. 2007, 17, 483.

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large optical and dielectric anisotropies, which are controllable by changes in the alignment of their molecules with external stimuli.15-19 Polymer dispersed liquid crystals (PDLCs) can be switched from a highly light scattering state (OFF-state) to a transparent state (ON-state) as a result of the mismatching of the refractive indices of the matrix and the oriented LC as a consequence of their birefringence features. As well known from the literature survey, the switching of PDLCs (based on nematic LCs) from OFF to ON states is carried out by external stimuli such as electric or magnetic field, temperature gradient, light, and others. For this reason, PDLCs have been extensively studied as promising candidates as new materials for application in the field of thermo- and electrooptical devices, such as optic shutters, smart windows, optical sensors, memories, and flexible display devices. However, as is well known, low molecular weight LCs show relatively high and strongly temperature-dependent solubility in polymer matrices. This leads to low activation energy for the variation of optical transmission with temperature, which is one of the main drawbacks to their potential application. One of the possible strategies to reduce this inconvenient behavior is, as recently published by Tercjak et al.,20-25 the use of block copolymers as polymer templates for the preparation of novel nanostructured (15) Craighead, H. G.; Chen, J.; Hackwood, S. Appl. Phys. Lett. 1982, 40, 22. _ (16) Nastaz, E.; Zura nska, E.; Mucha, M. J. Appl. Polym. Sci. 1999, 71, 455. (17) de Gennes, P. H. Scaling Concepts in Polymer Science; Cornell University Press: Ithaca, NY, 1979; Chapter 5. (18) Coates, D. J. Mater. Chem. 1995, 5, 2063. (19) Sumana, G.; Raina, K. K. Curr. Appl. Phys. 2005, 5, 277. (20) Tercjak, A.; Serrano, E.; Mondragon, I. Polym. Adv. Technol. 2006, 17, 835. (21) Tercjak, A.; Serrano, E.; Mondragon, I. Macromol. Rapid Commun. 2007, 28, 937. (22) Tercjak, A.; Serrano, E.; Garcia, I.; Ocando, C.; Mondragon, I. Acta Mater. 2007, 55, 6436. (23) Tercjak, A.; Garcia, I.; Mondragon, I. Nanotechnology 2008, 19, 275701. (24) Tercjak, A.; Serrano, E.; Garcia, I.; Mondragon, I. Acta Mater. 2008, 56, 5112. (25) Tercjak, A.; Mondragon, I. Langmuir 2008, 24, 11216.

Published on Web 11/20/2009

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thermosetting systems based on low molecular weight LCs. This new strategy was proposed taking into account that thermosetting systems can be nanostructured by the addition of block copolymers consisting of a thermoset-miscible and thermoset-immiscible blocks.21-25 Specifically, amphiphilic block copolymers were used as both a nanostructured agent for epoxy network and as a dispersing agent for low molecular LCs. Following this strategy, Tercjak et al.26,27 proposed a method for the preparation of hybrid inorganic/organic thermosetting materials based on LCs and titanium dioxide nanoparticles. The main aspects governing the electro-optical performance of these systems are the concentration, the morphology of the material, and the anchoring conditions of the LC phase at the polymer interface as well as the dielectric properties of the materials in alternating electric fields. Nevertheless, the mechanism of the orientation of the LC phase in the PDLCs systems seems to still be an open question, as it has still not been well understood due to technical difficulties in the analysis of the anchoring structure at the nanoscale level. From this point of view, the advance tunneling atomic force microscopy (TUNA) method can be an interesting approach in order to better understand the physical and electrical behavior of the LC molecules embedded in the polymeric matrices. This method allows for a better understanding of the current conduction mechanism through dielectric materials at the nanoscale when a DC voltage is applied between the conductive AFM probe and the back side of the sample.28-32 In our previous work,21,24,25 we reported the relationships between morphologies generated in thermosetting systems modified with a low molecular weight nematic LC, 40 -(hexyl)-4biphenyl-carbonitrile (HBC) or 40 -(hexyloxy)-4-biphenyl-carbonitrile (HOBC), and amphiphilic poly(styrene-b-ethylene oxide) block copolymers (PSEO) also analyzing their thermo-responsive behavior. Additionally, we also reported that those thermoresponsive nanostructured thermosetting systems show high contrast between charged LC/PS-block microphase-separated domains and uncharged epoxy-rich matrices.23,25 Moreover, we have recently published26 an effective method for the preparation of hybrid inorganic/organic thermosetting materials based on rutile TiO2 nanoparticles, materials that can show large electrooptic effects since TiO2 seems to be covered by LCs being well dispersed in LC/PS-block domains.27 Taking the above into account, in the present work, local electrical properties of those thermosetting systems have been successfully investigated by means of TUNA. Photoluminescence properties of the investigated materials confirmed their reversible photoluminescence switching ability while increasing/decreasing the temperature gradient.

Experimental Section Materials. A diglicydylether of bisphenol A epoxy resin (DGEBA) (Dow DER 332, gifted by Dow Chemical) was used as reactive solvent. It has an epoxy equivalent of approximately 175 and an average number of hydroxyl groups per two epoxy (26) Tercjak, A.; Gutierrez, J.; Peponi, L.; Rueda, L.; Mondragon, I. Macromolecules 2009, 42, 3386. (27) Tercjak, A.; Gutierrez, J.; Ocando, C.; Peponi, L.; Mondragon, I. Acta Mater. 2009, 57, 4624. (28) Tal, S.; Blumer-Ganon, B.; Kapon, M.; Eichen, Y. J. Am. Chem. Soc. 2005, 127, 9848. (29) Yanev, V.; Erlbacher, T.; Rommel, M.; Bauer, A. J.; Frey, L. Microelectron. Eng. 2009, 86, 1911. (30) O0 Neil, K. D.; Semenikhin, O. A. J. Phys. Chem. C 2007, 111, 14824. (31) De Wolf, P.; Brazel, E.; Erickson, A. Mater. Sci. Semicond. Process. 2001, 4, 71. (32) O0 Neil, K. D.; Heming, H.; Keech, P.; Shoesmith, D. W.; Semenikhin, O. A. Electrochem. Commun. 2008, 10, 1805.

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Figure 1. TUNA current map of DGEBA/MXDA system obtained by applying different voltages. Insets: top right: scale color bar corresponding to the positive voltage, bottom right: typical vertical cross-section profile. Scheme 1. Schematic of the Experimental Setup of the TUNA Measurements

groups of n = 0.03. This epoxy resin was cured with a stoichiometric amount of an aminic hardener, m-xylylenediamine (MXDA), supplied by Sigma-Aldrich. PSEO (Polymer Source, Inc.) was used to generate nanostructured epoxy systems. Number-average molecular weights for polystyrene (PS) and poly(ethylene oxide) (PEO) blocks were 58 600 and 31 000 g mol-1, respectively, with the polydispersity index for this copolymer being 1.03. The low molecular weight nematic LC, HBC, used in this study was supplied by SigmaAldrich. As published by us,24 on the basis of differential scanning calorimetry and optical microscopy (OM) measurements, this LC exhibits a nematic-isotropic (TN-TI) transition at about 34 °C and a crystal-nematic (TC-TN) transition at about 24 °C. Hydrophobic titanium dioxide nanoparticles (rutile with crystal size around 20 nm) supplied by Kemira Pigments Oy were used. Blending Protocol. Quaternary TiO2/block copolymer/LC/ epoxy, ternary block copolymer/LC/epoxy, as well as block copolymer/epoxy and LC/epoxy-cured blends were prepared according to the procedure published by us elsewhere.24,26 First, 1 wt % of TiO2 nanoparticles was sonicated in toluene at room temperature by using a microprocessor sonicator 750 W, Vibracell 75043 from Bioblock Scientific with an amplitude range between 20 and 25%. After 2 h, adequate PSEO block copolymer or/and HBC LC were added and sonicated for an additional 30 min. Then a DGEBA epoxy monomer was introduced, and the resultant mixture was heated at 80 °C in an oil bath until complete solvent removal was achieved. Once the solvent had been DOI: 10.1021/la9034003

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Article removed, the curing agent, MXDA, was added to the mixture, and homogeneous quaternary or ternary mixtures were obtained. After that, the mixtures were immediately degassed at 80 °C in vacuum, and cured at this temperature for 15 h. After curing, the plaques were slowly cooled to room temperature, demolded and postcured for 2 h at 160 °C under vacuum. In order to avoid the possible sublimation process of LC, the cured blends were prepared in a parallelepipedic mold of 1 ( 0.1 mm thickness. Though not shown, thermogravimetric analysis (TGA) was used to confirm the incorporation of 1 wt % of TiO2 nanoparticles into the inorganic/organic thermosetting systems. Tunneling Atomic Force Microscopy. TUNA analysis was performed under ambient conditions using a Veeco Dimension 3100 scanning probe microscope equipped with a Nanoscope IVa controller and a TUNA-extended extension module. The TUNA current-sensing range is sensitive to low currents (50 fA up to 100 pA), while the TUNA-extended module allows for the

Tercjak et al. measuring of higher currents (up to 1000 nA). All TUNA measurements were carried out operating in contact mode using a conductive Co/Cr-coated MESP tip having a resonance frequency of approximately 75 kHz and with a cantilever spring constant about 40 N/m. In order to obtain repeatable results, the local electrical properties of the investigated samples were measured in different regions of the specimens. Similar TUNA images were obtained, thus demonstrating the reproducibility of the results. All measurements were carried out in the LC state, since, though not shown, all modified thermosetting systems showed a nematic-isotropic (TN-TI) transition above room temperature. Additionally, here it should be pointed out that, since the response of the samples is proportional to the applied bias and inversely proportional to the film thickness, the same dimension of samples was used (10  3  1 mm3). Conductance Properties. A semiconductor characterization system (Keithley model 4200-SCS) was used to study the conductive properties of the materials. Two-point probe experiments were carried out applying current from -100 to 100 pA and from 100 to -100 pA to verify the response of the investigated systems. Photoluminescence. The photoluminescence properties of the investigated materials were determined using a Felix32 spectrophotometer of Photon Technology International (PTI) equipped with a temperature controller. Fluorescence emissions spectra for all investigated materials were recorded during the heating/ cooling cycle from -10 to 60 °C at a rate of 2 °C/min. The excitation wavelengths were 380 and 400 nm for the systems without and with TiO2 nanoparticles, respectively.

Results and Discussion Conductive Properties of the Thermosetting Based Systems. First, it has to be pointed out that the following results refer to the bulk behavior of these systems since the cured blends were prepared in a parallelepipedic mold of 1 ( 0.1 mm thickness. The sample dimensions (10  3  1 mm3) were always the same in order to avoid confusion in results interpretation since the response of the samples is proportional to the applied bias and

Figure 2. (a) TUNA current map of a 40 wt % HBC-(DGEBA/ MXDA) system obtained by applying different voltages. Inset: scale color bar corresponding to the positive voltage. (b) Profile corresponding to the black line on panel a. (c) Profile corresponding to the white line in panel a.

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Figure 3. TUNA current map and typical cross-section profile of the 1 wt % TiO2/40 wt % HBC-(DGEBA/MXDA) system obtained by applying different voltages. Insets: top right: scale color bar corresponding to the positive voltage; bottom right: typical vertical cross-section profile. Langmuir 2010, 26(6), 4296–4302

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Figure 4. (a) TUNA current map and typical cross-section profile of the 1 wt % TiO2/40 wt % HBC-(DGEBA/MXDA) system obtained by applying different voltages. Inset: top right: scale color bar corresponding to the positive voltage; bottom right: voltagecurrent curves by Keithley. (b) Typical vertical cross-section profile.

inversely proportional to the thickness. The conductive properties of the thermosetting based materials were measured using atomic force microscopy (AFM) in the TUNA mode, as shown in Scheme 1. Conductive silver paste was used for sticking the samples. As was expected, the neat epoxy system (DGEBA/MXDA) did not show any TUNA-currents passing throughout the sample when the different voltages were applied (10, 7, 0, -7, and -10 V). As shown in Figure 1, after applying either positive or negative voltages between the conductive AFM probe and the AFM chuck (see Scheme 1), no current was detected. The sample response was on the level of the noise of the TUNA (40 fA).32 Addition of 40 wt % HBC into the DGEBA/MXDA system led to detection of the TUNA-current on the level of ∼0.5 pA at a voltage of 10 V, as shown in Figure 2. However, a voltage of -10 V did not provoke any changes of TUNA current if compared with the neat DGEBA/MXDA epoxy system. Additionally, for a 40 wt % HBC-(DGEBA/MXDA) system, a local TUNA-current was detected. Typical profile obtained for the corresponding TUNAcurrent map indicated a local current on the level of 6/-6 pA. This Langmuir 2010, 26(6), 4296–4302

Figure 5. TUNA current map of (a) 10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA) and (b) 1 wt % TiO2/40 wt % HBC(DGEBA/MXDA) systems obtained by applying different voltages. Insets: corresponding cross-section profiles.

fact can be explained taking into account that, as was published elsewhere by us,23 40 wt % HBC-(DGEBA/MXDA) systems revealed the macro-phase separation of the HBC-rich domains (phase separated as a typical polygon/hexagon-shaped LC phase23,24), which explains the different current levels passing throughout the systems visible in the TUNA-current map as well (Figure 2). Thus, the whole surface of the 40 wt % HBC(DGEBA/MXDA) system was conductive on a very low level when the positive voltage of 10 V was applied due to partial miscibility between epoxy resin and the HBC LC phase.21-23 Moreover, the macro-phase-separated HBC-rich phase showed an additional higher local level of the current since, as it is wellknown, the HBC LC phase can respond to the electrical field provoked by applying voltage.15-19 The TUNA current map and the corresponding profile of the 1 wt % TiO2/40 wt % HBC(DGEBA/MXDA) system is shown in Figure 3. As clearly shown DOI: 10.1021/la9034003

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Figure 6. Fluorescence emission spectra of (a) 40 wt % HBC-(DGEBA/MXDA), (b) 1 wt % TiO2/40 wt % HBC-(DGEBA/MXDA), (c) 10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA), and (d) 1 wt % TiO2/10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA) systems taken at 10 and 45 °C. The excitation wavelengths were 380 and 400 nm, respectively, for the systems without and with TiO2 nanoparticles.

in the profile (the bottom right inset), under the same preparation conditions, the addition of 1 wt % TiO2 nanoparticles into the HBC-(DGEBA/MXDA) system led to the detection of a higher and more stable TUNA current if compared to the system without nanoparticles. The TUNA current reached 1.5 pA at a voltage of 10 V, whereas under reverse bias, the current was on the level of 1 pA. This means that, in this system, the TUNA current passed throughout the sample applying both positive and negative voltage values. Obtained results proved that, under the same measurement and preparation conditions, the current when provoked by a positive voltage was 3 times higher, and the system was able to respond not only to positive but also to negative applied voltage if compared with that detected for the same system without TiO2 nanoparticles. Here, it should be pointed out that the introduction of conductive TiO2 nanoparticles26 led to systems that continuously responded to the increasing or decreasing voltage cycles (see profile in Figure 4). Additionally, in order to confirm the conductive properties of this system, conventional measurements were performed using a semiconductor characterization system. As shown in the bottom-right inset of Figure 4, investigated systems responded to the low current level when the same range of voltage from -10 to 10 V and from 10 to -10 V was applied. These results are in good agreement with TUNA-based results. The relationship between the TUNA-current level for 10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA) and the 1 wt % 4300 DOI: 10.1021/la9034003

TiO2/10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA) systems was very similar to those for the same systems without PSEO block copolymers, as shown in Figure 5a,b, respectively. The TUNA-current in the case of the epoxy resin modified with 10 wt % PSEO and 40 wt % HBC reached the level of 0.5 pA at a voltage of 10 V. On the contrary, negative voltage applied between the conductive AFM probe and the AFM chuck did not provoke any currents passing through the sample. The detected current was on the TUNA noise level (40 fA). The introduction of TiO2 nanoparticles into the 10 wt % PSEO/ 40 wt % HBC-(DGEBA/MXDA) led to a higher TUNA current. Thus, the HBC stabilized by the addition of TiO2 nanoparticles allowed for the obtaining of systems that showed quite similar conductivity at positive and negative biases. The current values at positive bias were almost 3 times higher if compared to passing through the thermosetting system without nanoparticles. Obtained results indicated that the addition of conductive TiO2 nanoparticles stabilized the conductive properties of the investigated thermosetting systems by the fact that the absolute values of the TUNA current taken at the same positive bias were insignificantly higher than the negative bias and systems that responded continuously to the voltage applied. This phenomenon can be related to the good dispersion of TiO2 nanoparticles in the ternary and quaternary systems since, as was published by us elsewhere,26,27 in these systems LCs acted not only as conductive materials but also as surfactant-covering TiO2 Langmuir 2010, 26(6), 4296–4302

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Figure 7. Fluorescence intensity of maximum emissions peak of (a) 40 wt % HBC-(DGEBA/MXDA), (b) 1 wt % TiO2/40 wt % HBC-(DGEBA/MXDA), (c) 10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA), and (d) 1 wt % TiO2/10 wt % PSEO/40 wt % HBC-(DGEBA/MXDA) systems as a function of temperature.

nanoparticles, thus forming a core-shell structure that was responsible for the confinement of the TiO2/HBC-rich phase in microphase-separated PS-block domains. Consequently, under the same preparation and experiment conditions, finely dispersed conductive nanoparticles allow for the increase of the current passing through the thermosetting systems modified with HBC (with or without PSEO) and for the stabilization of the voltage level. Taking into account our previous study,20-27 this fact can lead to design conductive nano/mesostructured thermosetting inorganic/organic materials, which are able to switch from opaque to transparent states using thermal gradients as external stimuli. Switchable Photoluminescence Properties of the Thermosetting-Based Systems. As it is well-known, both nematic HBC LCs and TiO2 nanoparticles show photoluminescence behavior. Taking this fact into account, the photoluminescence properties of the investigated systems were studied. The fluorescence emission spectra of the DGEBA/MXDA epoxy-based systems are shown in Figure 6. First, though not shown here, the DGEBA/ MXDA system did not show an emission peak. On the contrary, as shown in Figure 6a, addition of 40 wt % HBC led to a narrow emission peak at around 452 nm at 45 °C, which almost completely disappeared at 15 °C, in which the separated HBC phase was in crystalline state. Obtained results seem to be strongly related with the thermo-responsive21,24,25 ability of these systems, which are able to switch from opaque (OFF state) to transparent state (ON state) by applying a thermal gradient. An addition of Langmuir 2010, 26(6), 4296–4302

1 wt % TiO2 nanoparticles into a 40 wt % HBC-(DGEBA/ MXDA) system provoked a shift of the emission peak to higher wavelength (around 488 nm at 45 °C). This significant shift and the peak broadening seem to be related to the influence of the photoluminescent TiO2 nanoparticles on the fluorescence emission spectrum of the hybrid inorganic/organic thermosetting materials. The effect of the addition of TiO2 nanoparticles was especially visible at 15 °C. At this temperature, the emission intensity was significantly lower. However, two interrelated peaks were clearly distinguished (around 491 nm and 524 nm). One possible explanation for this behavior can be that the systems contained two different TiO2 nanoparticle types, with strong and low interaction with HBC phase, mainly taking into account both the fact that, under the same conditions, practically no emission was detected for 40 wt % HBC-(DGEBA/MXDA) and that the emission peak of TiO2 nanoparticles was 550 nm. As shown in Figure 6c, the 10 wt % PSEO/40 wt % HBC(DGEBA/MXDA) system, similar to the same system without PSEO, presented a narrow emission peak at around 456 nm at 45 °C. This emission spectrum showed an intensity 4 times lower at 15 °C, once more indicating the ability for photoluminescence switching of this system as a function of temperature. The introduction of 1 wt % TiO2 nanoparticles to the system modified with PSEO and HBC shifted the emission spectra at 45 °C to higher wavelength, from 456 nm for the system without nanoparticles to 488 nm (Figure 6d). This shift can be related to interactions between TiO2 nanoparticles and HBC LCs since, as DOI: 10.1021/la9034003

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published by us,26,27 in this system, the HBC-rich phase can also act as surfactant for the inorganic nanoparticles and can surround the nanoparticle-formed core-shell structure, leading to good dispersion and location of the TiO2/HBC-rich phase in microphase-separated PS-block domains. Simultaneously, at 15 °C, the fluorescence intensity of the emission spectra became much lower than at 45 °C. Again, as in the case of the systems without PSEO block copolymer, two interconnected peaks were distinguished at around 490 and 522 nm. In order to verify the photoluminescence switching of investigated materials, as shown in Figure 7, the maximum of fluorescence intensity of the emission peak was plotted as a function of temperature in the range from -10 to 60 °C. As can be clearly observed, all investigated systems showed a well-pronounced increase/decrease during the heating/cooling cycle, thus confirming that these materials are photoluminescence active and that the photoluminescence switching of those systems is a reversible process almost without hysteresis. Indeed, the switching took place almost at the same temperature during both heating and cooling process. Through not shown here, this process was more than 10 times repeatable. Here, it should be pointed out that the photoluminescence switching of investigated materials is strongly related to the nematic-isotropic transition (TN-TI) of HBC phase in investigated epoxy resin systems. These conductive nanostructured materials, which simultaneously can be photoluminescence switched by applying a temperature gradient, can lead to the design of new photoresponsive LCDs.

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Conclusions TUNA was successfully applied in order to control a local current passing through thermosetting systems modified with nematic LC and conductive TiO2 nanoparticles. The HBC showed conductive properties since the measurement was performed in the LC state of the HBC phase in investigated thermosetting systems. Obtained results proved that both thermosetting systems modified with HBC (without and with PSEO) and corresponding systems with TiO2 nanoparticles showed a current passing through the sample under voltage applied by means of an AFM conductive probe. Moreover, the addition of TiO2 nanoparticles tripled the level of conductivity of the designed materials and stabilized the current passing through those systems. The photoluminescence properties of these materials indicated a reversible switching process during repeatable heating/ cooling cycles. Obtained results confirm that modification of thermosetting systems with both TiO2 nanoparticles and LCs allows one to maintain conductive properties of these components in designed materials, which are simultaneously photoluminescence switchable by applying a thermal gradient. Acknowledgment. Financial support from Basque Country Governments in the frame of ETORTEK inanoGUNE (IE08225) and Grupos Consolidados (IT-365-07) is gratefully acknowledged. The authors also wish to thank the Ministry of Education and Innovation for Projects MAT-2006-06331 and MAT-200912832.

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