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nanostructured thermosetting materials based on PS-b-PEO block copolymer-dispersed liquid crystal: Curing behavior and morphological variation
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Acta Materialia 56 (2008) 5112–5122 www.elsevier.com/locate/actamat
Thermoresponsive meso/nanostructured thermosetting materials based on PS-b-PEO block copolymer-dispersed liquid crystal: Curing behavior and morphological variation Agnieszka Tercjak *, Elena Serrano, In˜aki Garcia, In˜aki Mondragon * ‘‘Materials + Technologies” Group, Escuela Polite´cnica, Dpto. Ingenierı´a Quı´mica y M. Ambiente, Universidad Paı´s Vasco/Euskal Herriko Unibertsitatea, Pza. Europa 1, 20018 Donostia-San Sebastia´n, Spain Received 13 January 2008; received in revised form 19 June 2008; accepted 27 June 2008 Available online 26 July 2008
Abstract The main aim of this contribution was the generation of novel intelligent materials, in this case meso/nanostructured thermoresponsive thermosetting materials, based on an epoxy resin modified with two amphiphilic poly(styrene-b-ethylene oxide) block copolymers (PSEO) and a low molecular weight liquid crystal, 40 -(hexyl)-4-biphenyl-carbonitrile (HBC). A strong influence of the addition of a small amount of PSEO block copolymer as the third component in PSEO–HBC–(DGEBA/MXDA) cured blends on both curing behavior and morphology has been observed. The introduction of 5 and 10 wt.% PSEO in the thermosetting ternary blends results in, depending of both PS-block and HBC content, thermoresponsive meso/nanostructured thermosetting materials with high contrast ratios between transparent and opaque states. It is envisaged that these novel meso/nanostructured blends may lead to new materials due to both the ability of self-assembly of block copolymers and the fact that block copolymer-dispersed low molecular weight liquid crystals behave as thermoresponsive materials in a thermosetting matrix. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thermoresponsive materials; Block copolymers; Nanostructured thermoset; Self-assembly; Atomic force microscopy
1. Introduction Low molecular weight liquid crystals (LCs) can be thermally switched from opaque to transparent state due to the birefringence feature of LCs, when embedded in a polymer matrix [1–5]. For this reason, polymer-dispersed liquid crystals (PDLCs) have been extensively studied as promising new candidate materials for application in the field of thermo- and electrooptical devices, such as optical shutters, smart windows, optical sensors, memories and flexible display devices [2,5–11]. The ability to pattern LC nanodroplets at the nanoscale is not needed for conventional display applications. However, this property may find application in phase array optics, *
Corresponding authors. Tel.: +34 943017271; fax: +34 943017140. E-mail addresses: [email protected] (A. Tercjak), [email protected] (I. Mondragon).
e.g. as switchable birefringent phase shifters [12–14]. Generally, low molecular weight LCs have to show relatively high, and strongly temperature-dependent, solubility in polymer matrix. 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. As was first reported by Hoppe et al. [13–15], these drawbacks can be eliminated by addition of a small amount of a thermoplastic polymer to a solution of LC in thermoset precursor. In this strategy the thermoplastic polymer must have a refractive index matching that of the fully cured thermoset and should show a high compatibility with the LC and a low compatibility with the thermoset precursor. This would lead to phase separation of thermoplastic/LC solutions at low conversions in the polymerization reaction and would allow thermally reversible light scattering (TRLS) films to be obtained.
1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.06.024
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Subsequently, taking into account work done by Hoppe et al. [13–15] and our previous papers [16,17], we have studied thermoplastic/thermoset blends [18,19] modified with polystyrene (PS), poly(bisphenol A carbonate) or poly(methylphenylsiloxane) as thermoplastic, and with the low molecular weight nematic LCs 40 -(hexyloxy)-4biphenyl-carbonitrile (HOBC) or 40 -(hexyl)-4-biphenylcarbonitrile (HBC). The analysis of the morphologies generated in situ during switching from strong scatter light (OFF state), i.e. opaque state, to the transparent state (ON state) and studies of the effect of curing conditions on the reversibility of the thermoplastic/thermoset blends allowed us to conclude that the introduction of a third component yielded a narrow size distribution of nematic droplets and stabilized the system against the coalescence of LC droplets, resulting in the generation of materials suitable for electrooptical devices. On the other hand, block copolymers are widely used as templates for generating nanostructured epoxy or phenolic matrices with long-range order in both the uncured and cured states [20–40]. One feasible pathway for generating self-assembled thermosetting nanostructures is the use of amphiphilic block copolymers consisting of thermoset-miscible and thermoset-immiscible blocks. Several research groups, as well as our group, have successfully employed different amphiphilic block copolymers to generate stable nanostructures in epoxy matrices based on diglicidyl ether of bisphenol A (DGEBA) cured with different hardeners [20–40]. As is well known, nanostructured materials based on thermosetting matrices can find applications in many different fields of nanotechnology, such as nanostructured functional surfaces, nanolithography or building of nanostructured inorganic/organic materials. Whereas several studies have reported a new family of PDLCs [13–17], where LC is dispersed in a thermoplastic/ thermoset matrix, to the best of our knowledge, the possibility of using block copolymer-dispersed LCs as modifier for thermosetting systems and their ability to act as thermoresponsive nanostructured thermosetting materials has been recently reported by us for the first time [39]. Taking work done by Hoppe et al. [13–15] and our previous papers [16–19] into account, the main aim of our recently published paper [39] was to investigate the possibility of obtaining novel meso/nanostructured thermosetting materials, which can be thermoresponsive materials based on a thermosetting system modified with a block copolymer and a nematic low molecular weight LC. This work opened a new strategy for preparation of thermoresponsive nanostructured thermosetting materials. In this strategy, the amphiphilic block copolymers should contain one epoxy-miscible block and another immiscible block, serving as a self-assembly agent, and simultaneously the epoxy-immiscible block should act as a PDLC. Additionally, the epoxy-immiscible block must show higher miscibility with low molecular weight LC than with the epoxy matrix. Thermosetting materials, which follow these requirements, can find application in the field of thermo- and electrooptical devices.
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In the present study, bisphenol A-type epoxy resin, modified with two amphiphilic poly(styrene-b-ethylene oxide) block (PSEO) copolymers with different PEO-block contents and a low molecular weight nematic LC, HBC, has been cured with stoichiometric amount of an aromatic amine hardener, m-xylylenediamine (MXDA). The aim of this work is to systematically study the effect of the addition of PSEO block copolymers on the formation of meso/nanoordered thermosetting materials which maintain the ability to switch from opaque to transparent state when subject to external stimuli. In this context, the influence of the addition of small amount of PSEO block copolymer on the curing behavior of PSEO–HBC–(DGEBA/MXDA) has been studied by means of differential scanning calorimetry (DSC) and rheology. The morphological generated during network formation has been studied by atomic force microscopy (AFM) and related to the thermoresponsive behavior of the obtained cured blends. Additionally, the optical properties of the obtained ternary materials have been analyzed. 2. Materials and methods 2.1. Materials In this study, two amphiphilic PSEO diblock copolymers (Polymer Source Inc.) were used as self-assembling agents; these are hereafter denoted PSEO1 and PSEO2, respectively. Number-average molecular weights, Mn, for PS (PEO) block and the Mw/Mn of copolymer were 125,000 (16,100) g mol1 and 1.04, respectively, for PSEO1, and 58,600 (31,000) g mol1 and 1.03, respectively, for PSEO2. The low molecular weight nematic LC used in the present study was HBC, supplied by Sigma–Aldrich. As revealed by DSC and optical microscopy [18], this LC exhibits a nematic–isotropic (TN-TI) transition at about 34 °C and a crystal–nematic (TC-TN) transition at about 24 °C. 2.1.1. Thermoset precursors DGEBA (Dow DER 332, gifted by Dow Chemical) was used as reactive solvent. It has an epoxy equivalent of around 175 and an average number of hydroxyl groups per two epoxy groups n = 0.03. This epoxy resin was cured with a stoichiometric amount of an aromatic amine hardener, MXDA, supplied by Sigma–Aldrich. 2.2. Blending protocol Ternary block copolymer/LC/epoxy as well as block copolymer/epoxy and LC/epoxy cured blends were prepared in the following way: firstly, PSEO or/and HBC and DGEBA resin, both ternary and binary systems containing similar contents of PSEO and/or HBC, were dissolved in toluene. The resultant solution was heated at 80 °C in an oil bath until complete solvent removal was achieved. The curing agent MXDA was then added to the mixture and homogeneous ternary mixtures were
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obtained. After that, the mixtures were immediately degassed at 80 °C in vacuum, and cured at this temperature for 15 h. After curing (in air), the plaques were slowly cooled to room temperature, demolded and post-cured for 2 h at 160 °C under vacuum. The percentage weight of the PSEO or HBC in the ternary and binary mixtures was calculated with respect to DGEBA/MXDA system. In order to avoid possible sublimation of LC, the cured blends were prepared in a parallelepiped mold 11.2 mm thick. It should be noted that, even taking into account the partial miscibility between uncured epoxy resin and PSblock, after evaporation of toluene and at the cure temperature, PSEO block copolymers and LC were fully miscible with uncured epoxy resin, as revealed by rheological measurements (initially, while the curing sample is homogeneous liquid, G” was higher than G0 and the value of both of them were around 0, as described below). 2.3. Transparency test The transparency test was performed at room temperature and at 16 °C. Sixteen degree Celsius was chosen taking into account the DSC results of the PSEO–HBC– (DGEBA/MXDA) ternary and HBC–(DGEBA/MXDA) binary samples published in our previous paper [39], which confirms that at this temperature the HBC phase is in the nematic state. 2.4. Morphological analysis The morphological behavior of both PSEO1–HBC– (DGEBA/MXDA) and PSEO2–HBC–(DGEBA/MXDA) systems as well as DGEBA/MXDA neat and modified only with the corresponding PSEO or HBC content was investigated by AFM. AFM images were obtained operating in tapping mode (TM–AFM) with a scanning probe microscope (Nanoscope IIIa, MultimodeTM, Digital Instruments) equipped with an integrated silicon tip/cantilever having a resonance frequency of 300 kHz, from the same manufacturer. The height and phase images were obtained under ambient conditions with typical scan speeds of 0.8– 1.6 line s1, using a scan head with a maximum range of 16 16 lm. Height and phase images were recorded simultaneously during scanning. In order to obtain repeatable results of the blend morphology, different regions of the specimens were scanned. Similar images were obtained, thus demonstrating the reproducibility of the results. For the analysis of the observed surface structures, Nanoscope image processing software was used. Representative pieces of each epoxy composites were microtomed at room temperature using a Leica Ultracut R microtome equipped with a diamond knife. 2.5. Differential scanning calorimetry DSC measurements were carried out on a Mettler Toledo DSC 822 differential scanning calorimeter
equipped with a Sample Robot TSO 801 RO. Nitrogen was used as a purge gas (10 mL min1). Temperature and enthalpy were calibrated by using an indium standard. Curing was performed in sealed aluminum pans containing a sample weight of around 10 mg. The curing behavior of the different epoxy-based blends was analyzed by means of isothermal experiments performed at 80 °C for 140 min. All samples were tested immediately after preparation. 2.6. Rheological analysis Curing behavior of epoxy-based systems was also analyzed by using a TA Instruments Ares rheometer equipped with parallel plates of 50 and 25 mm diameter (upper and lower plate, respectively). Dynamic oscillatory shear measurements were performed at 80 °C as a function of curing time at a constant frequency of 1 Hz. Samples were prepared as described in blending protocol and MXDA was added just before loading the sample in the rheometer. The parallel plates were preheated at 80 °C before a zero gap was set. The upper plate was then raised, always ensuring that the normal force was close to zero, and the liquid uncured sample was put on the lower plate. The gap between the plates was about 1 mm and the applied strain was changed during the experiment to ensure linear viscoelastic response. The evolution of the storage (G0 ) and loss (G00 ) moduli was recorded as a function of reaction time. Consequently, the complex viscosity magnitude was derived from the complex modulus (G*) through the relation g* = G*/x, where x is the frequency. Taking into account the sensibility of the Ares rheometer, the error of the value of the storage and loss moduli for each data point was ±5%. The transducer operating range was set to 0.2–200 or 0.2–2000 g cm1 depending on the measured torque values. Data were collected and analyzed using Rhios Rheometrics software. 2.7. Optical properties In the experiment the 488.0 nm line from an Ar+ laser was used as a pump (or writing) beam, and the 632.8 nm line from a He–Ne laser was used as a probe (or reading) beam. To carry out the experiments, a film sample was placed between a pair of crossed polarizers. To achieve maximum signal, the polarization vector of the writing beam was set to 45° with respect to the polarization vector of the probe reading beam. The sample thickness was 1–1.2 mm. Samples were fixed in the vertical position by way of the beam lines which impinged perpendicularly on the cured epoxy plaque. Temperature was set at 27 °C (stability ±1 °C), exposure time was fixed at 120 s and light power was varied in the range 6–400 mW. No electric field was applied. The diameter of the laser beam on the sample was 1.5 mm.
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3. Results and discussion 3.1. Curing behavior of epoxy resin modified with PSEO block copolymer and HBC Evolved heat vs. curing time plots during isothermal tests at 80 °C for ternary PSEO–HBC–(DGEBA/MXDA) systems with different PSEO block copolymer and HBC contents as well as for DGEBA/MXDA neat and modified only with the corresponding PSEO or HBC contents are shown in Figs. 1 and 2 for PSEO1 and PSEO2 copolymer, respectively. As can be seen in both Figs. 1a and 2a, the addition of 5 wt.% of PSEO1 or PSEO2 block copolymer results in non-significant delay in curing reaction probably due to a small plasticization effect arising from both the partial miscibility of PEO-block with the epoxy system [25,26,41,42] and the low amount of block copolymer and consequently the low amount of PEO-block with respect to the overall composition. As expected, as PEO-
Fig. 2. (a) Isothermal DSC thermograms recorded at 80 °C for DGEBA/ MXDA system neat and modified with different PSEO2 or/and HBC contents. (b) Details of the curing behavior of the DGEBA/MXDA system modified with 50 wt.% HBC and 5 wt.% PSEO2–50 wt.% HBC.
Fig. 1. (a) Isothermal DSC thermograms recorded at 80 °C for DGEBA/ MXDA system neat and modified with different PSEO1 or/and HBC contents. (b) Details of the curing behavior of the DGEBA/MXDA system modified with 50 wt.% HBC and 5 wt.% PSEO1–50 wt.% HBC.
block content in block copolymer increases (PEO-block content higher in PSEO2 than in PSEO1, 11 and 34 wt.%, respectively), curing reactions are shifted to longer times. On the other hand, the addition of 30 or 50 wt.% HBC to DGEBA/MXDA system leads to significant delay in curing reactions. Introduction of 50 wt.% HBC into DGEBA/MXDA system results in longer curing reaction times if compared to the analogous system containing 30 wt.% of LC; the peaks of maximum exothermicity appeared at 28 and 43 min, for 30 and 50 wt.% HBC, respectively. Moreover, in Figs. 1b and 2b it can be clearly observed that for sample containing only 50 wt.% of HBC, in addition to the exothermic peak of epoxy polymerization, another exothermic peak appears (indicated by an arrow in (Figs. 1b and 2) that can be related to phase separation of HBC phase from the epoxy matrix. It can be noted that the appearance of this shoulder for other epoxy blends modified with thermoplastic [18,43,44] or block copolymers [41,42] was attributed to the phase separation process. Here, it should be pointed out that cured blend
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containing 50 wt.% HBC was opaque, instead of the curing blends containing 30 wt.% HBC, thus indicating the presence of macroscopic phase separation in this system. As described below, the macrophase separation in the case of 50 wt.% HBC–(DGEBA/MXDA) samples has been confirmed by the morphology generated in fully cured blends investigated by AFM. Regarding the PSEO–HBC–(DGEBA/MXDA) ternary blends, for 5 wt.% PSEO–30 wt.% HBC-modified systems, curing reactions are shifted to longer times than those corresponding to 5 wt.% PSEO–(DGEBA/MXDA) blends; these curing times are insignificantly shorter compared to that of the epoxy system containing only 30 wt.% HBC. Thus the polymerization rate of PSEO–HBC–(DGEBA/ MXDA) ternary blends decreases slightly with the addition of 5 wt.% PSEO. The slightly lower reaction times in the case of ternary system when compared to systems modified only with 30 wt.% HBC indicate that the initial homogeneous solutions follow the polymerization-induced phaseseparation process, leading to phase separation of HBC/ PS-block-rich phase in the HBC/DGEBA-rich phase. This phenomenon is related, on the one hand, with the plasticization and the dilution effect of HBC addition into the epoxy system, which results in a delayed curing time compared to the neat epoxy system. On the other hand, the separation of the HBC phase within the PS-block results in shorter curing reaction times when compared with the 30 wt.% HBC–(DGEBA/MXDA) system and longer times when compared with the 30 wt.% HBC–(DGEBA/MXDA) system. Although not shown here, at the polymerization temperature PSEO and HBC are soluble throughout the composition range and HBC is a better solvent for PSblock than the epoxy resin. Taking both facts into account, one can deduce that the lower curing time of 5 wt.% PSEO–30 wt.% HBC–(DGEBA/MXDA) is due to the HBC phase mainly separates within separate PS-block domains. However, taking into account that the initial amount of PS-block is low, a large fraction of HBC remains dissolved in the epoxy. Consequently, at full conversion two isotropic phases coexist: the first one is an epoxy resin swollen with HBC diluted by PEO-block and the second one is a HBC/PS-block solution. As pointed out also by Hoppe et al. [15] for the PS–EBBA– (DGEBA/BDMA) system, for PSEO–HBC–(DGEBA/ MXDA) at full epoxy conversion a neat HBC phase cannot exist in equilibrium due to the HBC phase is completely soluble in the PS-block at the curing temperature. Furthermore, the introduction of 5 wt.% PSEO1 or PSEO2 as the third component into the 50 wt.% HBC– (DGEBA/MXDA) blend does not change significantly the curing reaction time compared to that of the DGEBA/MXDA system modified only with 50 wt.% HBC. Nevertheless, for the 5 wt.% PSEO2–50 wt.% HBC–(DGEBA/MXDA) system, the additional exothermic peak is very difficult to distinguish if compared to the 50 wt.% HBC-based cured blend, which suggests a lack of macrophase separation in this ternary system. Nonethe-
less, this blend was opaque after curing and, as shown below, AFM images confirm the macrophase separation of PS-block/HBC phase from the epoxy matrix. Additionally, the insignificantly lower curing reaction times for this blend when compared with the 50 wt.% HBC–(DGEBA/ MXDA) system imply that, in this case, the HBC phase separates within the PS-block phase. The evolution of G0 and G00 moduli and complex viscosity profiles measured at 1 Hz during isothermal curing at 80 °C for the neat DGEBA/MXDA system and those modified with PSEO or/and HBC are shown in Fig. 3a and b, respectively. Taking into account that DGEBA/MXDA modified with PSEO1 and PSEO2 block copolymers show similar viscoelastic behavior, only viscoelastic behavior of DGEBA/MXDA modified with PSEO2 or/and HBC is
Fig. 3. Viscoelastic properties measured at 1 Hz during isothermal curing at 80 °C for neat DGEBA/MXDA system (-j-) and its blends containing different PSEO2 or/and HBC contents: (-H-) 5 wt.% PSEO2, (-d-) 5 wt.% PSEO2-30 wt.% HBC, (-4-) 30 wt.% HBC, (--), 5 wt.% PSEO2–50 wt.% HBC, (-N-) 50 wt.% HBC. (a) Storage shear modulus, G0 (filled symbols), and loss shear modulus, G0 0 (open symbols), vs. curing time, and (b) complex viscosity profile vs. curing time.
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shown. As can be seen, for the neat DGEBA/MXDA system the magnitude of both G0 and G00 moduli intersect at around 9 min; this time is in good agreement with the abrupt increase of complex viscosity, which clearly indicates that the system reaches the gelation point. Here it should be pointed out that even though various methods are available to determine the gelation point, the simplest one is based on an extrapolation of g* to infinity [45], and the point of intersection of the dynamic storage and dynamic loss moduli [46]; the latter is valid only for some stoichiometric balanced networks. The addition of PSEO block copolymers does not significantly hinder the gelation process of the DGEBA/MXDA epoxy system. As is well known, microphase separation of PS-block can affect the viscoelastic behavior of the PSEO–(DGEBA/MXDA) system during curing, resulting in an increase in the magnitudes of both G0 and G00 moduli and complex viscosity [18,19,33]. However, for the investigated system, polymerization-induced microphase separation does not have any influence on the viscoelastic behavior, probably due to both the low content of PSEO block copolymer with respect to the overall composition and the low molecular weight of the PS-block in the PSEO block copolymers. For the 30 wt.% HBC-modified system, the G00 curve crossed the G0 curve at a network formation time of around 31 min, almost 8 min later than the DGEBA/MXDA system modified with 5 wt.% PSEO2 and 30 wt.% HBC. Although not shown here, similar behavior has been observed when PSEO1 was added as a third component. The viscoelastic behavior of the 5 wt.% PSEO–30 wt.% HBC–(DGEBA/MXDA) system when compared with the 30 wt.% HBC–(DGEBA/MXDA) confirms that addition of PSEO block copolymers has a strong effect on both the viscoelastic behavior of the generated system and curing kinetics during network formation, thus suggesting that the microphase separation of the PS-block from the PSEO/ HBC/epoxy homogeneous solution leads to partial phase separation of HBC in PS-block domains. Analogously, the DGEBA/MXDA epoxy system modified with 5 wt.% PSEO and 50 wt.% HBC shows the same viscoelastic behavior as the system modified with 5 wt.% PSEO and 30 wt.% HBC, and is similar to the viscoelastic behavior of the basic HBC–(DGEBA/MXDA) binary system. The viscoelastic behaviors of investigated binary and ternary systems during network formation are in good agreement with curing behaviors of these systems studied by DSC. However, it should be taken into account that the appearance of the additional shoulder in DSC thermograms corresponding to 50 wt.% HBC–(DGEBA/MXDA) and 5 wt.% PSEO–50 wt.% HBC-(DGEBA/MXDA) hinders the gelation process during network formation. 3.2. Final morphology of the cured blends and their thermal reversibility It should first be noted that the following results refer to the bulk behavior of these systems since the cured blends
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have been prepared in a parallelepiped mold 11.2 mm thick. The effect of addition of PSEO block copolymer as a third component on the morphology generated in the 5 wt.% PSEO2–30 wt.% HBC–(DGEBA/MXDA) system has been reported in our recently published paper [39], showing that microphase separation of PS-block leads to separation of HBC within this block, thus yielding thermoresponsive nanostructured thermosetting systems. In the present study, the influence of PEO-block content in PSEO block copolymer on the final morphology of PSEO–HBC– (DGEBA/MXDA) cured blends containing 30 and 50 wt.% HBC has been investigated. Before analyzing this system, the morphologies generated in DGEBA/MXDA system modified with 30 and 50 wt.% HBC are discussed. As observed in Fig. 4a, the addition of 30 wt.% HBC does not produce any macrophase separation, the plaque being completely transparent at room temperature (Fig. 4a inset). On the contrary, as shown in Fig. 4b, for 50 wt.% HBC– (DGEBA/MXDA) system macrophase separation of HBC-rich phase from the HBC/epoxy takes place, which is confirmed by the opacity of the plaque at room temperature (Fig. 4b inset). Thus polygon/hexagon shaped crystalline domains of macrophase-separated HBC phase can easily be distinguished on the AFM phase image. These results are in good agreement with the curing behavior of these systems studied by DSC, where in the case of 50 wt.% HBC–(DGEBA/MXDA), the additional exothermic peak indicates macrophase separation of HBC phase. Final morphologies generated in HBC–(DGEBA/ MXDA) systems after the addition of PSEO1 and PSEO2 are presented in Figs. 5 and 6, respectively. As can be seen in Figs. 5 and 6a,d, microphase separation of PS-block takes place for cured blends containing both 5 and 10 wt.% PSEO1 and PSEO2, confirming that the transparency of these cured blends is related to the absence of macrophase separation, and not to the refractive index of PS, which is similar to that of the epoxy resin. The size distribution of PS-rich spherical domains strongly depends on the PSEO type and content. For the 5 and 10 wt.% PSEO1–(DGEBA/MXDA) systems (Fig. 5a and d, respectively) the size distribution of the PS-rich phase was between 31–64 nm and 48–82 nm in diameter, respectively. In the case of PSEO2-modified systems (Fig. 6a and d), a narrower size distribution of the PS-rich spherical particles can be distinguished: 16–21 nm and 18–26 nm in diameter for 5 and 10 wt.% PSEO2, respectively. Here it should be pointed out that the nanoscale spherical particles formed in the fully cured epoxy network can be attributed to the PS-rich phase since PEO-block is miscible with the epoxy network due both to the similarity of solubility parameters [40] and intermolecular hydrogen bonding interactions between the hydroxyl groups of amine-cured epoxy and the ether oxygen atoms of PEO; this has been demonstrated by DSC and published elsewhere [24,25,37,41,42]. Consequently, in the AFM phase image, the brighter continuous regions are ascribed to the epoxy cured resin, which were miscible with the PEO-block, whereas the dark
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Fig. 4. TM–AFM phase images for DGEBA/MXDA system modified with: (a) 30 wt.% HBC and (b) 50 wt.% HBC. The inset shows digital images of the transparency of the blends at room temperature.
areas correspond to PS-rich microdomains. Moreover, it is also noteworthy that the spherical PS-rich domains are smaller and narrower in size distribution in the case of PSEO2 since the PS-block of this block copolymer has a lower molecular weight and lower mass ratio compared with PSEO1. For 30 wt.% HBC–(DGEBA/MXDA) cured blends modified with 5 and 10 wt.% of PSEO1, one can easily distinguish the black spherical microdomains with a size distribution between 98–218 nm and 36–96 nm in diameter, respectively, dispersed in the continuous PEO-block/ HBC/epoxy matrix (Fig. 5b and e). The average size of the black spherical microdomains for thermosetting systems modified with 5 or 10 wt.% PSEO1 and 30 wt.% HBC is almost 2–3 times higher than that corresponding to the system without HBC under the same conditions, thus confirming that microphase separation of PS-block of block copolymers during network formation leads to phase separation of low molecular weight LC within. As has been recently published by us [39], in such systems amphiphilic block copolymer plays a double role: it works as a self-assembly agent yielding nanostructured materials, and also as a PDLC. Interestingly, the 5 wt.% PSEO1– 30 wt.% HBC–(DGEBA/MXDA) cured system is opaque at room temperature (upper image in the inset of Fig. 5b). However, just by touching with a finger (right part of medium image in the inset of Fig. 5b) human body temperature is enough for these samples to switch from opaque to transparent state (the bottom image in the inset of Fig. 5d). This fact confirms that this multifunctional mesostructured thermosetting material is thermally reversible by contact with the human body and consequently it could find potential application in the same fields in which nanostructured block copolymers materials and LC-based materials are used. The behavior of the 5 wt.% PSEO1–30 wt.% HBC–(DGEBA/MXDA) as a TRLS material is illustrated in Fig. 7 for a 1.2 mm cured sample. As can easily be seen,
the switching time from the ON to the OFF state was very short, less than 1 s. This confirms the high contrast ratio between opaque (OFF) and transparent (ON) states, and indicates the potential application of these materials, especially given that in this case switching can take place simply by the touch of a finger. In this case fast switching between opaque and transparent states was confirmed by measurement of the optical properties. In the case of the 10 wt.% PSEO1–30 wt.% HBC–(DGEBA/MXDA) cured blend, the plaque is completely transparent at room temperature, but becomes completely white (Fig. 5e inset) at around 16 °C, thus also indicating thermal reversibility. Furthermore, when 5 wt.% PSEO1 is added to 50 wt.% HBC–(DGEBA/MXDA), macrophase separation of the HBC/PS-block results in spherical domains with a wider size distribution of 78–650 nm. Thus macrophase separation of the HBC phase is confirmed by opacity of this plaque at room temperature. In this case, the sample loses its thermal reversibility since the final cured blend is opaque as a result of macroseparation of the HBC phase from the HBC/epoxy-rich phase. Consequently, the thermosetting materials obtained cannot be optically transparent above the nematic/isotropic transition due to the different refractive indices of the polygon/hexagon shaped crystalline domains of the macrophase-separated HBC phase and the HBC/epoxy-rich phase. This phenomenon will be discussed in a future paper describing the thermomechanical behavior of these systems and their influence on thermoresponse behavior. Simultaneously, the addition of 5 and 10 wt.% PSEO2 into the 30 wt.% HBC–(DGEBA/MXDA) system leads to microphase separation of the HBC/PS-block. The sizes of HBC/PS-block phase are 40–75 nm and 22–38 nm, respectively, for 5 and 10 wt.% PSEO2 in epoxy-cured blends. The average size of these spherical microdomains (Fig. 6b and e) is almost 2–3 times higher than that corresponding to the PSEO–(DGEBA/MXDA) system under
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Fig. 5. TM–AFM phase images of the DGEBA/MXDA systems modified with different PSEO1 or/and HBC contents: (a) 5 wt.% PSEO1, (b) 5 wt.% PSEO1–30 wt.% HBC, (c) 5 wt.% PSEO1–50 wt.% HBC, (d) 10 wt.% PSEO1 and (e) 10 wt.% PSEO1–30 wt.% HBC. The inset shows digital images of the transparency of the blends at room temperature and, in case of the thermoreversible blends, at room temperature and at 10 °C, top and bottom, respectively.
the same conditions, and narrower and smaller if compared to similar blends containing PSEO1. At first, the morphology generated in systems with 5 and 10 wt.% PSEO2 as the third component confirms that microphase separation of
the PS-block during network formation leads to phase separation of low molecular weight LC within, yielding multifunctional nanostructured thermosetting systems. Second, AFM-based results suggest that the PEO-block is partially
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Fig. 6. TM–AFM phase images of the DGEBA/MXDA systems modified with different PSEO1 or/and HBC contents: (a) 5 wt.% PSEO2, (b) 5 wt.% PSEO2–30 wt.% HBC, (c) 5 wt.% PSEO2–50 wt.% HBC, (d) 10 wt.% PSEO2, (e) 10 wt.% PSEO2–30 wt.% HBC and (f) 10 wt.% PSEO2–50 wt.% HBC. The inset shows digital images of the transparency of the blends at room temperature and, in case of the thermoreversible blends, at room temperature and at 10 °C, top and bottom, respectively.
miscible with epoxy matrix [24,25,37,41,42] and with HBC [39] (taking into account the solubility parameters of these components published by us elsewhere [39,40], it should be pointed out that HBC low molecular weight LC is partially
miscible with both PS- and PEO-blocks of PSEO; however, it shows higher miscibility with PS-block), which has a strong influence on microphase separation of HBC/PSblock in PEO-block/epoxy matrix. Addition of 10 wt.%
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has been reported for thermosetting systems modified with low molecular weight nematic LCs. It can be noted that this blend is translucent at room temperature, and can switch from translucent to opaque (Fig. 6f inset), thus confirming the possibility of obtaining a thermoresponsive thermosetting system. 4. Conclusions
Fig. 7. Optical response of the 5 wt.% PSEO1–30 wt.% HBC–(DGEBA/ MXDA) cured sample vs. time.
PSEO as a third component halves the domain size and results in a narrower distribution of HBC/PS-block phase domain. Nevertheless, it should be remembered that the amount of PS-block which can microphase separate from the HBC phase is lower in the case of PSEO2 compared to the amount of PS-block in PSEO1 copolymer. A very interesting result is that DGEBA/MXDA systems modified with 30 wt.% HBC and 5 and 10 wt.% PSEO2 are fully transparent at room temperature and switch from transparent to opaque state by decreasing the temperature to around 16 °C. In this case, although not shown here in order to avoid repeating data, the addition of PSEO2 as a third component enables not only the preparation of nanostructured thermosetting materials but also materials that can be switched from opaque to transparent state by application of external stimuli, e.g. thermal gradient or electrical field, similar to that shown in Fig. 7 for the analogous PSEO1-based systems. Therefore, these materials offer a new strategy for the generation of novel thermoresponsive nanostructured thermosetting materials. Finally, as shown in Fig. 6c, the addition of 5 wt.% PSEO2 into the 50 wt.% HBC–(DGEBA/MXDA) system leads to macrophase separation of polygon/hexagonshaped crystalline domains of PS-block/HBC with a size distribution in the range of 300–850 nm. Thus macrophase separation of HBC phase is confirmed by the opacity of this plaque at room temperature (Fig. 6c inset). Consequently, similar to what was observed for the ternary blend containing 5 wt.% PSEO1, this system does not possess thermal reversibility since the final cured blend is opaque and, accordingly, it cannot be optically transparent above the nematic/isotropic transition due to different refractive indices of the polygon/hexagon-shaped crystalline domains of macrophase-separated HBC phase and the HBC/epoxyrich phase. When the amount of PSEO2 in ternary blends is increased up to 10 wt.% (Fig. 6f), a typical Schlieren texture characteristic of PDLC can be observed. However, to our knowledge, this is the first time that this morphology
The addition of a small amount of PSEO block copolymers as the third component in epoxy-type thermosetting blends modified with nematic LCs leads to a new family of thermoresponsive thermosetting nanostructured materials. This fact has been demonstrated by using HBC as nematic LC and two different PSEOs with different PEOblock contents. In the obtained thermoresponsive meso/ nanostructured thermosetting materials, PSEO block copolymer is used as both a self-assembly agent and as a PDLC. The curing behavior of the systems studied by DSC and rheology, as well as the morphology generated after network formation, have shown that the introduction of an amphiphilic block copolymer as the third component yields a narrow, nanometer-scale distribution of the PSEO/ HBC phase and stabilizes the system against macrophase separation of the HBC fraction. For adequate PSEO and HBC content, microphase separation of the PS-block from the epoxy matrix leads to internal microseparation of the HBC phase, which is responsible for switching from the opaque (OFF) to the transparent (ON) state. Consequently, nanostructured thermoresponsive thermosetting materials can be obtained with potential applications in the same fields in which nanostructured block copolymerand LC-based materials are used. Thus this study opens a new strategy for preparation of novel meso/nanostructured termosetting materials, which are able to switch from the opaque to the transparent state with high contrast. Acknowledgments Financial support from Basque Country Governments in the frame of Grupos Consolidados (IT-365-07) and SAIOTEK (S-PE07UN39) projects is gratefully acknowledged. References [1] Craighead HG, Chen J, Hackwood S. Appl Phys Lett 1982;40:22. _ ´ ska E, Mucha MJ. Appl Polym Sci 1999;71:455. [2] Nastał E, Zuran [3] de Gennes PH. Scaling Concepts in Polymer Science. Ithaca, NY: Cornell University Press; 1979 [chapter 5, p. 131]. [4] Coates DJ. Mater Chem 1995;5:2063. [5] Sumana G, Raina KK. Curr Appl Phys 2005;5:277. [6] Doane JW, Vaz NA, Wu BG, Zumer S. Appl Phys Lett 1986;48:269. [7] Smith JW, Vaz NA. Liq Crystallogr 1988;3:543. [8] Drzaic PS. Liq Cryst 1988;3:1543. [9] Herod TE, Duran RS. Langmuir 1998;14:6956. [10] Chin WK, Hsin LP, Lu HL, Shau MD. J Polym Sci Part B Polym Phys 2000;38:2033. [11] Zhou J, Petti L, Mormile P, Roviello A. Opt Commun 2004;231:263.
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[12] Wowk B. In: Crandall BC, editor. Nanotechnology. Cambridge, MA: MIT Press; 1996. [13] Hoppe CE, Galante MJ, Oyanguren PA, Williams RJJ. Macromolecules 2002;35:6324. [14] Hoppe CE, Galante MJ, Oyanguren PA, Williams RJJ. Mater Sci Eng 2004;24:591. [15] Hoppe CE, Galante MJ, Oyanguren PA, Williams RJJ. Macromolecules 2004;37:5352. [16] Tercjak A, Remiro PM, Mondragon I. Polym Eng Sci 2005;45:303. [17] Tercjak A, Serrano E, Remiro PM, Mondragon I. J Appl Polym Sci 2006;100:2348. [18] Tercjak A, Serrano E, Mondragon I. Polym Adv Technol 2006;17:835. [19] Tercjak A, Serrano E, Larran˜aga M, Mondragon I. J Appl Polym Sci 2008;108:1116. [20] Grubbs RB, Dean JM, Bates FS. Macromolecules 2001;34:8593. [21] Hillmyer MA, Lipic PM, Hadjuk DA, Almdal K, Bates FSJ. Am Chem Soc 1997;119:2749. [22] Lipic PM, Bates FS, Hillmyer MA. J Am Chem Soc 1998;120:8963. [23] Mijovic J, Shen M, Sy JW, Mondragon I. Macromolecules 2000;33:5235. [24] Guo Q, Thomann R, Gronski W, Thurn-Albrecht T. Macromolecules 2002;35:3133. [25] Guo Q, Thomann R, Gronski W, Staneva R, Ivanova R, Stu¨hn B. Macromolecules 2003;36:3635. [26] Ritzenthaler S, Court F, David L, Girard-Reydet E, Leibler L, Pascault JP. Macromolecules 2002;35:6245. [27] Ritzenthaler S, Court F, David L, Girard-Reydet E, Leibler L, Pascault JP. Macromolecules 2003;36:118. [28] Dean JM, Lipic PM, Grubbs RB, Cook RF, Bates FS. J Polym Sci Part B: Polym Phys 2001;39:2996. [29] Rebizant V, Abetz V, Tournilhac F, Court F, Leibler L. Macromolecules 2003;36:9889.
[30] Serrano E, Martin MD, Tercjak A, Pomposo JA, Mecerreyes D, Mondragon I. Macromol Rapid Commun 2005;26:982. [31] Serrano E, Tercjak A, Pomposo JA, Mecerreyes D, Mondragon I. Macromolecules 2006;39:2254. [32] Ocando C, Serrano E, Tercjak A, Pen˜a C, Kortaberria G, Calberg C, Grignard B, Jerome R, Carrasco PM, Mecerreyes D, Mondragon I. Macromolecules 2007;40:4068. [33] Serrano E, Tercjak A, Ocando C, Larran˜aga M, Parellada MD, Corona-Galva´n S, Mecerreyes D, Zafeiropoulos NE, Stamm M, Mondragon I. Macromol Chem Phys 2007;208:2281. [34] Meng F, Zheng S, Zhang W, Li H, Liang Q. Macromolecules 2006;39:711. [35] Meng F, Zheng S, Liu T. Polymer 2006;47:7590. [36] Meng F, Zheng S, Li H, Liang Q, Liu T. Macromolecules 2006;39:5072. [37] Tercjak A, Larran˜aga M, Martin MD, Mondragon I. J Therm Anal Cal 2006;86:663. [38] Maiez-Tribut S, Pascault JP, Soule ER, Borrajo J, Williams RJJ. Macromolecules 2007;40:1268. [39] Tercjak A, Serrano E, Mondragon I. Macromol Rapid Commun 2007;28:937. [40] Tercjak A, Serrano E, Garcia I, Ocando C, Mondragon I. Acta Mater 2007;55:6436. [41] Larran˜aga M, Martin MD, Gabilondo N, Kortaberria G, Corcuera MA, Riccardi CC, Mondragon I. Polym Int 2004;53:1495. [42] Larran˜aga M, Martin MD, Gabilondo N, Kortaberria G, Eceiza A, Riccardi CC, Mondragon I. Colloid Polym Sci 2006;284:1403. [43] Martinez I, Martin MD, Eceiza A, Oyanguren P, Mondragon I. Polymer 2000;41:1027. [44] Remiro PM, Riccardi CC, Corcuera MA, Mondragon I. J Appl Polym Sci 1999;74:772. [45] Macosko CW. Rheology: Principles, Measurements and Applications. New York: VCH; 1994.
Acta Materialia 56 (2008) 5112–5122 www.elsevier.com/locate/actamat
Thermoresponsive meso/nanostructured thermosetting materials based on PS-b-PEO block copolymer-dispersed liquid crystal: Curing behavior and morphological variation Agnieszka Tercjak *, Elena Serrano, In˜aki Garcia, In˜aki Mondragon * ‘‘Materials + Technologies” Group, Escuela Polite´cnica, Dpto. Ingenierı´a Quı´mica y M. Ambiente, Universidad Paı´s Vasco/Euskal Herriko Unibertsitatea, Pza. Europa 1, 20018 Donostia-San Sebastia´n, Spain Received 13 January 2008; received in revised form 19 June 2008; accepted 27 June 2008 Available online 26 July 2008
Abstract The main aim of this contribution was the generation of novel intelligent materials, in this case meso/nanostructured thermoresponsive thermosetting materials, based on an epoxy resin modified with two amphiphilic poly(styrene-b-ethylene oxide) block copolymers (PSEO) and a low molecular weight liquid crystal, 40 -(hexyl)-4-biphenyl-carbonitrile (HBC). A strong influence of the addition of a small amount of PSEO block copolymer as the third component in PSEO–HBC–(DGEBA/MXDA) cured blends on both curing behavior and morphology has been observed. The introduction of 5 and 10 wt.% PSEO in the thermosetting ternary blends results in, depending of both PS-block and HBC content, thermoresponsive meso/nanostructured thermosetting materials with high contrast ratios between transparent and opaque states. It is envisaged that these novel meso/nanostructured blends may lead to new materials due to both the ability of self-assembly of block copolymers and the fact that block copolymer-dispersed low molecular weight liquid crystals behave as thermoresponsive materials in a thermosetting matrix. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Thermoresponsive materials; Block copolymers; Nanostructured thermoset; Self-assembly; Atomic force microscopy
1. Introduction Low molecular weight liquid crystals (LCs) can be thermally switched from opaque to transparent state due to the birefringence feature of LCs, when embedded in a polymer matrix [1–5]. For this reason, polymer-dispersed liquid crystals (PDLCs) have been extensively studied as promising new candidate materials for application in the field of thermo- and electrooptical devices, such as optical shutters, smart windows, optical sensors, memories and flexible display devices [2,5–11]. The ability to pattern LC nanodroplets at the nanoscale is not needed for conventional display applications. However, this property may find application in phase array optics, *
Corresponding authors. Tel.: +34 943017271; fax: +34 943017140. E-mail addresses: [email protected] (A. Tercjak), [email protected] (I. Mondragon).
e.g. as switchable birefringent phase shifters [12–14]. Generally, low molecular weight LCs have to show relatively high, and strongly temperature-dependent, solubility in polymer matrix. 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. As was first reported by Hoppe et al. [13–15], these drawbacks can be eliminated by addition of a small amount of a thermoplastic polymer to a solution of LC in thermoset precursor. In this strategy the thermoplastic polymer must have a refractive index matching that of the fully cured thermoset and should show a high compatibility with the LC and a low compatibility with the thermoset precursor. This would lead to phase separation of thermoplastic/LC solutions at low conversions in the polymerization reaction and would allow thermally reversible light scattering (TRLS) films to be obtained.
1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.06.024
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Subsequently, taking into account work done by Hoppe et al. [13–15] and our previous papers [16,17], we have studied thermoplastic/thermoset blends [18,19] modified with polystyrene (PS), poly(bisphenol A carbonate) or poly(methylphenylsiloxane) as thermoplastic, and with the low molecular weight nematic LCs 40 -(hexyloxy)-4biphenyl-carbonitrile (HOBC) or 40 -(hexyl)-4-biphenylcarbonitrile (HBC). The analysis of the morphologies generated in situ during switching from strong scatter light (OFF state), i.e. opaque state, to the transparent state (ON state) and studies of the effect of curing conditions on the reversibility of the thermoplastic/thermoset blends allowed us to conclude that the introduction of a third component yielded a narrow size distribution of nematic droplets and stabilized the system against the coalescence of LC droplets, resulting in the generation of materials suitable for electrooptical devices. On the other hand, block copolymers are widely used as templates for generating nanostructured epoxy or phenolic matrices with long-range order in both the uncured and cured states [20–40]. One feasible pathway for generating self-assembled thermosetting nanostructures is the use of amphiphilic block copolymers consisting of thermoset-miscible and thermoset-immiscible blocks. Several research groups, as well as our group, have successfully employed different amphiphilic block copolymers to generate stable nanostructures in epoxy matrices based on diglicidyl ether of bisphenol A (DGEBA) cured with different hardeners [20–40]. As is well known, nanostructured materials based on thermosetting matrices can find applications in many different fields of nanotechnology, such as nanostructured functional surfaces, nanolithography or building of nanostructured inorganic/organic materials. Whereas several studies have reported a new family of PDLCs [13–17], where LC is dispersed in a thermoplastic/ thermoset matrix, to the best of our knowledge, the possibility of using block copolymer-dispersed LCs as modifier for thermosetting systems and their ability to act as thermoresponsive nanostructured thermosetting materials has been recently reported by us for the first time [39]. Taking work done by Hoppe et al. [13–15] and our previous papers [16–19] into account, the main aim of our recently published paper [39] was to investigate the possibility of obtaining novel meso/nanostructured thermosetting materials, which can be thermoresponsive materials based on a thermosetting system modified with a block copolymer and a nematic low molecular weight LC. This work opened a new strategy for preparation of thermoresponsive nanostructured thermosetting materials. In this strategy, the amphiphilic block copolymers should contain one epoxy-miscible block and another immiscible block, serving as a self-assembly agent, and simultaneously the epoxy-immiscible block should act as a PDLC. Additionally, the epoxy-immiscible block must show higher miscibility with low molecular weight LC than with the epoxy matrix. Thermosetting materials, which follow these requirements, can find application in the field of thermo- and electrooptical devices.
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In the present study, bisphenol A-type epoxy resin, modified with two amphiphilic poly(styrene-b-ethylene oxide) block (PSEO) copolymers with different PEO-block contents and a low molecular weight nematic LC, HBC, has been cured with stoichiometric amount of an aromatic amine hardener, m-xylylenediamine (MXDA). The aim of this work is to systematically study the effect of the addition of PSEO block copolymers on the formation of meso/nanoordered thermosetting materials which maintain the ability to switch from opaque to transparent state when subject to external stimuli. In this context, the influence of the addition of small amount of PSEO block copolymer on the curing behavior of PSEO–HBC–(DGEBA/MXDA) has been studied by means of differential scanning calorimetry (DSC) and rheology. The morphological generated during network formation has been studied by atomic force microscopy (AFM) and related to the thermoresponsive behavior of the obtained cured blends. Additionally, the optical properties of the obtained ternary materials have been analyzed. 2. Materials and methods 2.1. Materials In this study, two amphiphilic PSEO diblock copolymers (Polymer Source Inc.) were used as self-assembling agents; these are hereafter denoted PSEO1 and PSEO2, respectively. Number-average molecular weights, Mn, for PS (PEO) block and the Mw/Mn of copolymer were 125,000 (16,100) g mol1 and 1.04, respectively, for PSEO1, and 58,600 (31,000) g mol1 and 1.03, respectively, for PSEO2. The low molecular weight nematic LC used in the present study was HBC, supplied by Sigma–Aldrich. As revealed by DSC and optical microscopy [18], this LC exhibits a nematic–isotropic (TN-TI) transition at about 34 °C and a crystal–nematic (TC-TN) transition at about 24 °C. 2.1.1. Thermoset precursors DGEBA (Dow DER 332, gifted by Dow Chemical) was used as reactive solvent. It has an epoxy equivalent of around 175 and an average number of hydroxyl groups per two epoxy groups n = 0.03. This epoxy resin was cured with a stoichiometric amount of an aromatic amine hardener, MXDA, supplied by Sigma–Aldrich. 2.2. Blending protocol Ternary block copolymer/LC/epoxy as well as block copolymer/epoxy and LC/epoxy cured blends were prepared in the following way: firstly, PSEO or/and HBC and DGEBA resin, both ternary and binary systems containing similar contents of PSEO and/or HBC, were dissolved in toluene. The resultant solution was heated at 80 °C in an oil bath until complete solvent removal was achieved. The curing agent MXDA was then added to the mixture and homogeneous ternary mixtures were
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obtained. After that, the mixtures were immediately degassed at 80 °C in vacuum, and cured at this temperature for 15 h. After curing (in air), the plaques were slowly cooled to room temperature, demolded and post-cured for 2 h at 160 °C under vacuum. The percentage weight of the PSEO or HBC in the ternary and binary mixtures was calculated with respect to DGEBA/MXDA system. In order to avoid possible sublimation of LC, the cured blends were prepared in a parallelepiped mold 11.2 mm thick. It should be noted that, even taking into account the partial miscibility between uncured epoxy resin and PSblock, after evaporation of toluene and at the cure temperature, PSEO block copolymers and LC were fully miscible with uncured epoxy resin, as revealed by rheological measurements (initially, while the curing sample is homogeneous liquid, G” was higher than G0 and the value of both of them were around 0, as described below). 2.3. Transparency test The transparency test was performed at room temperature and at 16 °C. Sixteen degree Celsius was chosen taking into account the DSC results of the PSEO–HBC– (DGEBA/MXDA) ternary and HBC–(DGEBA/MXDA) binary samples published in our previous paper [39], which confirms that at this temperature the HBC phase is in the nematic state. 2.4. Morphological analysis The morphological behavior of both PSEO1–HBC– (DGEBA/MXDA) and PSEO2–HBC–(DGEBA/MXDA) systems as well as DGEBA/MXDA neat and modified only with the corresponding PSEO or HBC content was investigated by AFM. AFM images were obtained operating in tapping mode (TM–AFM) with a scanning probe microscope (Nanoscope IIIa, MultimodeTM, Digital Instruments) equipped with an integrated silicon tip/cantilever having a resonance frequency of 300 kHz, from the same manufacturer. The height and phase images were obtained under ambient conditions with typical scan speeds of 0.8– 1.6 line s1, using a scan head with a maximum range of 16 16 lm. Height and phase images were recorded simultaneously during scanning. In order to obtain repeatable results of the blend morphology, different regions of the specimens were scanned. Similar images were obtained, thus demonstrating the reproducibility of the results. For the analysis of the observed surface structures, Nanoscope image processing software was used. Representative pieces of each epoxy composites were microtomed at room temperature using a Leica Ultracut R microtome equipped with a diamond knife. 2.5. Differential scanning calorimetry DSC measurements were carried out on a Mettler Toledo DSC 822 differential scanning calorimeter
equipped with a Sample Robot TSO 801 RO. Nitrogen was used as a purge gas (10 mL min1). Temperature and enthalpy were calibrated by using an indium standard. Curing was performed in sealed aluminum pans containing a sample weight of around 10 mg. The curing behavior of the different epoxy-based blends was analyzed by means of isothermal experiments performed at 80 °C for 140 min. All samples were tested immediately after preparation. 2.6. Rheological analysis Curing behavior of epoxy-based systems was also analyzed by using a TA Instruments Ares rheometer equipped with parallel plates of 50 and 25 mm diameter (upper and lower plate, respectively). Dynamic oscillatory shear measurements were performed at 80 °C as a function of curing time at a constant frequency of 1 Hz. Samples were prepared as described in blending protocol and MXDA was added just before loading the sample in the rheometer. The parallel plates were preheated at 80 °C before a zero gap was set. The upper plate was then raised, always ensuring that the normal force was close to zero, and the liquid uncured sample was put on the lower plate. The gap between the plates was about 1 mm and the applied strain was changed during the experiment to ensure linear viscoelastic response. The evolution of the storage (G0 ) and loss (G00 ) moduli was recorded as a function of reaction time. Consequently, the complex viscosity magnitude was derived from the complex modulus (G*) through the relation g* = G*/x, where x is the frequency. Taking into account the sensibility of the Ares rheometer, the error of the value of the storage and loss moduli for each data point was ±5%. The transducer operating range was set to 0.2–200 or 0.2–2000 g cm1 depending on the measured torque values. Data were collected and analyzed using Rhios Rheometrics software. 2.7. Optical properties In the experiment the 488.0 nm line from an Ar+ laser was used as a pump (or writing) beam, and the 632.8 nm line from a He–Ne laser was used as a probe (or reading) beam. To carry out the experiments, a film sample was placed between a pair of crossed polarizers. To achieve maximum signal, the polarization vector of the writing beam was set to 45° with respect to the polarization vector of the probe reading beam. The sample thickness was 1–1.2 mm. Samples were fixed in the vertical position by way of the beam lines which impinged perpendicularly on the cured epoxy plaque. Temperature was set at 27 °C (stability ±1 °C), exposure time was fixed at 120 s and light power was varied in the range 6–400 mW. No electric field was applied. The diameter of the laser beam on the sample was 1.5 mm.
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3. Results and discussion 3.1. Curing behavior of epoxy resin modified with PSEO block copolymer and HBC Evolved heat vs. curing time plots during isothermal tests at 80 °C for ternary PSEO–HBC–(DGEBA/MXDA) systems with different PSEO block copolymer and HBC contents as well as for DGEBA/MXDA neat and modified only with the corresponding PSEO or HBC contents are shown in Figs. 1 and 2 for PSEO1 and PSEO2 copolymer, respectively. As can be seen in both Figs. 1a and 2a, the addition of 5 wt.% of PSEO1 or PSEO2 block copolymer results in non-significant delay in curing reaction probably due to a small plasticization effect arising from both the partial miscibility of PEO-block with the epoxy system [25,26,41,42] and the low amount of block copolymer and consequently the low amount of PEO-block with respect to the overall composition. As expected, as PEO-
Fig. 2. (a) Isothermal DSC thermograms recorded at 80 °C for DGEBA/ MXDA system neat and modified with different PSEO2 or/and HBC contents. (b) Details of the curing behavior of the DGEBA/MXDA system modified with 50 wt.% HBC and 5 wt.% PSEO2–50 wt.% HBC.
Fig. 1. (a) Isothermal DSC thermograms recorded at 80 °C for DGEBA/ MXDA system neat and modified with different PSEO1 or/and HBC contents. (b) Details of the curing behavior of the DGEBA/MXDA system modified with 50 wt.% HBC and 5 wt.% PSEO1–50 wt.% HBC.
block content in block copolymer increases (PEO-block content higher in PSEO2 than in PSEO1, 11 and 34 wt.%, respectively), curing reactions are shifted to longer times. On the other hand, the addition of 30 or 50 wt.% HBC to DGEBA/MXDA system leads to significant delay in curing reactions. Introduction of 50 wt.% HBC into DGEBA/MXDA system results in longer curing reaction times if compared to the analogous system containing 30 wt.% of LC; the peaks of maximum exothermicity appeared at 28 and 43 min, for 30 and 50 wt.% HBC, respectively. Moreover, in Figs. 1b and 2b it can be clearly observed that for sample containing only 50 wt.% of HBC, in addition to the exothermic peak of epoxy polymerization, another exothermic peak appears (indicated by an arrow in (Figs. 1b and 2) that can be related to phase separation of HBC phase from the epoxy matrix. It can be noted that the appearance of this shoulder for other epoxy blends modified with thermoplastic [18,43,44] or block copolymers [41,42] was attributed to the phase separation process. Here, it should be pointed out that cured blend
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containing 50 wt.% HBC was opaque, instead of the curing blends containing 30 wt.% HBC, thus indicating the presence of macroscopic phase separation in this system. As described below, the macrophase separation in the case of 50 wt.% HBC–(DGEBA/MXDA) samples has been confirmed by the morphology generated in fully cured blends investigated by AFM. Regarding the PSEO–HBC–(DGEBA/MXDA) ternary blends, for 5 wt.% PSEO–30 wt.% HBC-modified systems, curing reactions are shifted to longer times than those corresponding to 5 wt.% PSEO–(DGEBA/MXDA) blends; these curing times are insignificantly shorter compared to that of the epoxy system containing only 30 wt.% HBC. Thus the polymerization rate of PSEO–HBC–(DGEBA/ MXDA) ternary blends decreases slightly with the addition of 5 wt.% PSEO. The slightly lower reaction times in the case of ternary system when compared to systems modified only with 30 wt.% HBC indicate that the initial homogeneous solutions follow the polymerization-induced phaseseparation process, leading to phase separation of HBC/ PS-block-rich phase in the HBC/DGEBA-rich phase. This phenomenon is related, on the one hand, with the plasticization and the dilution effect of HBC addition into the epoxy system, which results in a delayed curing time compared to the neat epoxy system. On the other hand, the separation of the HBC phase within the PS-block results in shorter curing reaction times when compared with the 30 wt.% HBC–(DGEBA/MXDA) system and longer times when compared with the 30 wt.% HBC–(DGEBA/MXDA) system. Although not shown here, at the polymerization temperature PSEO and HBC are soluble throughout the composition range and HBC is a better solvent for PSblock than the epoxy resin. Taking both facts into account, one can deduce that the lower curing time of 5 wt.% PSEO–30 wt.% HBC–(DGEBA/MXDA) is due to the HBC phase mainly separates within separate PS-block domains. However, taking into account that the initial amount of PS-block is low, a large fraction of HBC remains dissolved in the epoxy. Consequently, at full conversion two isotropic phases coexist: the first one is an epoxy resin swollen with HBC diluted by PEO-block and the second one is a HBC/PS-block solution. As pointed out also by Hoppe et al. [15] for the PS–EBBA– (DGEBA/BDMA) system, for PSEO–HBC–(DGEBA/ MXDA) at full epoxy conversion a neat HBC phase cannot exist in equilibrium due to the HBC phase is completely soluble in the PS-block at the curing temperature. Furthermore, the introduction of 5 wt.% PSEO1 or PSEO2 as the third component into the 50 wt.% HBC– (DGEBA/MXDA) blend does not change significantly the curing reaction time compared to that of the DGEBA/MXDA system modified only with 50 wt.% HBC. Nevertheless, for the 5 wt.% PSEO2–50 wt.% HBC–(DGEBA/MXDA) system, the additional exothermic peak is very difficult to distinguish if compared to the 50 wt.% HBC-based cured blend, which suggests a lack of macrophase separation in this ternary system. Nonethe-
less, this blend was opaque after curing and, as shown below, AFM images confirm the macrophase separation of PS-block/HBC phase from the epoxy matrix. Additionally, the insignificantly lower curing reaction times for this blend when compared with the 50 wt.% HBC–(DGEBA/ MXDA) system imply that, in this case, the HBC phase separates within the PS-block phase. The evolution of G0 and G00 moduli and complex viscosity profiles measured at 1 Hz during isothermal curing at 80 °C for the neat DGEBA/MXDA system and those modified with PSEO or/and HBC are shown in Fig. 3a and b, respectively. Taking into account that DGEBA/MXDA modified with PSEO1 and PSEO2 block copolymers show similar viscoelastic behavior, only viscoelastic behavior of DGEBA/MXDA modified with PSEO2 or/and HBC is
Fig. 3. Viscoelastic properties measured at 1 Hz during isothermal curing at 80 °C for neat DGEBA/MXDA system (-j-) and its blends containing different PSEO2 or/and HBC contents: (-H-) 5 wt.% PSEO2, (-d-) 5 wt.% PSEO2-30 wt.% HBC, (-4-) 30 wt.% HBC, (--), 5 wt.% PSEO2–50 wt.% HBC, (-N-) 50 wt.% HBC. (a) Storage shear modulus, G0 (filled symbols), and loss shear modulus, G0 0 (open symbols), vs. curing time, and (b) complex viscosity profile vs. curing time.
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shown. As can be seen, for the neat DGEBA/MXDA system the magnitude of both G0 and G00 moduli intersect at around 9 min; this time is in good agreement with the abrupt increase of complex viscosity, which clearly indicates that the system reaches the gelation point. Here it should be pointed out that even though various methods are available to determine the gelation point, the simplest one is based on an extrapolation of g* to infinity [45], and the point of intersection of the dynamic storage and dynamic loss moduli [46]; the latter is valid only for some stoichiometric balanced networks. The addition of PSEO block copolymers does not significantly hinder the gelation process of the DGEBA/MXDA epoxy system. As is well known, microphase separation of PS-block can affect the viscoelastic behavior of the PSEO–(DGEBA/MXDA) system during curing, resulting in an increase in the magnitudes of both G0 and G00 moduli and complex viscosity [18,19,33]. However, for the investigated system, polymerization-induced microphase separation does not have any influence on the viscoelastic behavior, probably due to both the low content of PSEO block copolymer with respect to the overall composition and the low molecular weight of the PS-block in the PSEO block copolymers. For the 30 wt.% HBC-modified system, the G00 curve crossed the G0 curve at a network formation time of around 31 min, almost 8 min later than the DGEBA/MXDA system modified with 5 wt.% PSEO2 and 30 wt.% HBC. Although not shown here, similar behavior has been observed when PSEO1 was added as a third component. The viscoelastic behavior of the 5 wt.% PSEO–30 wt.% HBC–(DGEBA/MXDA) system when compared with the 30 wt.% HBC–(DGEBA/MXDA) confirms that addition of PSEO block copolymers has a strong effect on both the viscoelastic behavior of the generated system and curing kinetics during network formation, thus suggesting that the microphase separation of the PS-block from the PSEO/ HBC/epoxy homogeneous solution leads to partial phase separation of HBC in PS-block domains. Analogously, the DGEBA/MXDA epoxy system modified with 5 wt.% PSEO and 50 wt.% HBC shows the same viscoelastic behavior as the system modified with 5 wt.% PSEO and 30 wt.% HBC, and is similar to the viscoelastic behavior of the basic HBC–(DGEBA/MXDA) binary system. The viscoelastic behaviors of investigated binary and ternary systems during network formation are in good agreement with curing behaviors of these systems studied by DSC. However, it should be taken into account that the appearance of the additional shoulder in DSC thermograms corresponding to 50 wt.% HBC–(DGEBA/MXDA) and 5 wt.% PSEO–50 wt.% HBC-(DGEBA/MXDA) hinders the gelation process during network formation. 3.2. Final morphology of the cured blends and their thermal reversibility It should first be noted that the following results refer to the bulk behavior of these systems since the cured blends
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have been prepared in a parallelepiped mold 11.2 mm thick. The effect of addition of PSEO block copolymer as a third component on the morphology generated in the 5 wt.% PSEO2–30 wt.% HBC–(DGEBA/MXDA) system has been reported in our recently published paper [39], showing that microphase separation of PS-block leads to separation of HBC within this block, thus yielding thermoresponsive nanostructured thermosetting systems. In the present study, the influence of PEO-block content in PSEO block copolymer on the final morphology of PSEO–HBC– (DGEBA/MXDA) cured blends containing 30 and 50 wt.% HBC has been investigated. Before analyzing this system, the morphologies generated in DGEBA/MXDA system modified with 30 and 50 wt.% HBC are discussed. As observed in Fig. 4a, the addition of 30 wt.% HBC does not produce any macrophase separation, the plaque being completely transparent at room temperature (Fig. 4a inset). On the contrary, as shown in Fig. 4b, for 50 wt.% HBC– (DGEBA/MXDA) system macrophase separation of HBC-rich phase from the HBC/epoxy takes place, which is confirmed by the opacity of the plaque at room temperature (Fig. 4b inset). Thus polygon/hexagon shaped crystalline domains of macrophase-separated HBC phase can easily be distinguished on the AFM phase image. These results are in good agreement with the curing behavior of these systems studied by DSC, where in the case of 50 wt.% HBC–(DGEBA/MXDA), the additional exothermic peak indicates macrophase separation of HBC phase. Final morphologies generated in HBC–(DGEBA/ MXDA) systems after the addition of PSEO1 and PSEO2 are presented in Figs. 5 and 6, respectively. As can be seen in Figs. 5 and 6a,d, microphase separation of PS-block takes place for cured blends containing both 5 and 10 wt.% PSEO1 and PSEO2, confirming that the transparency of these cured blends is related to the absence of macrophase separation, and not to the refractive index of PS, which is similar to that of the epoxy resin. The size distribution of PS-rich spherical domains strongly depends on the PSEO type and content. For the 5 and 10 wt.% PSEO1–(DGEBA/MXDA) systems (Fig. 5a and d, respectively) the size distribution of the PS-rich phase was between 31–64 nm and 48–82 nm in diameter, respectively. In the case of PSEO2-modified systems (Fig. 6a and d), a narrower size distribution of the PS-rich spherical particles can be distinguished: 16–21 nm and 18–26 nm in diameter for 5 and 10 wt.% PSEO2, respectively. Here it should be pointed out that the nanoscale spherical particles formed in the fully cured epoxy network can be attributed to the PS-rich phase since PEO-block is miscible with the epoxy network due both to the similarity of solubility parameters [40] and intermolecular hydrogen bonding interactions between the hydroxyl groups of amine-cured epoxy and the ether oxygen atoms of PEO; this has been demonstrated by DSC and published elsewhere [24,25,37,41,42]. Consequently, in the AFM phase image, the brighter continuous regions are ascribed to the epoxy cured resin, which were miscible with the PEO-block, whereas the dark
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Fig. 4. TM–AFM phase images for DGEBA/MXDA system modified with: (a) 30 wt.% HBC and (b) 50 wt.% HBC. The inset shows digital images of the transparency of the blends at room temperature.
areas correspond to PS-rich microdomains. Moreover, it is also noteworthy that the spherical PS-rich domains are smaller and narrower in size distribution in the case of PSEO2 since the PS-block of this block copolymer has a lower molecular weight and lower mass ratio compared with PSEO1. For 30 wt.% HBC–(DGEBA/MXDA) cured blends modified with 5 and 10 wt.% of PSEO1, one can easily distinguish the black spherical microdomains with a size distribution between 98–218 nm and 36–96 nm in diameter, respectively, dispersed in the continuous PEO-block/ HBC/epoxy matrix (Fig. 5b and e). The average size of the black spherical microdomains for thermosetting systems modified with 5 or 10 wt.% PSEO1 and 30 wt.% HBC is almost 2–3 times higher than that corresponding to the system without HBC under the same conditions, thus confirming that microphase separation of PS-block of block copolymers during network formation leads to phase separation of low molecular weight LC within. As has been recently published by us [39], in such systems amphiphilic block copolymer plays a double role: it works as a self-assembly agent yielding nanostructured materials, and also as a PDLC. Interestingly, the 5 wt.% PSEO1– 30 wt.% HBC–(DGEBA/MXDA) cured system is opaque at room temperature (upper image in the inset of Fig. 5b). However, just by touching with a finger (right part of medium image in the inset of Fig. 5b) human body temperature is enough for these samples to switch from opaque to transparent state (the bottom image in the inset of Fig. 5d). This fact confirms that this multifunctional mesostructured thermosetting material is thermally reversible by contact with the human body and consequently it could find potential application in the same fields in which nanostructured block copolymers materials and LC-based materials are used. The behavior of the 5 wt.% PSEO1–30 wt.% HBC–(DGEBA/MXDA) as a TRLS material is illustrated in Fig. 7 for a 1.2 mm cured sample. As can easily be seen,
the switching time from the ON to the OFF state was very short, less than 1 s. This confirms the high contrast ratio between opaque (OFF) and transparent (ON) states, and indicates the potential application of these materials, especially given that in this case switching can take place simply by the touch of a finger. In this case fast switching between opaque and transparent states was confirmed by measurement of the optical properties. In the case of the 10 wt.% PSEO1–30 wt.% HBC–(DGEBA/MXDA) cured blend, the plaque is completely transparent at room temperature, but becomes completely white (Fig. 5e inset) at around 16 °C, thus also indicating thermal reversibility. Furthermore, when 5 wt.% PSEO1 is added to 50 wt.% HBC–(DGEBA/MXDA), macrophase separation of the HBC/PS-block results in spherical domains with a wider size distribution of 78–650 nm. Thus macrophase separation of the HBC phase is confirmed by opacity of this plaque at room temperature. In this case, the sample loses its thermal reversibility since the final cured blend is opaque as a result of macroseparation of the HBC phase from the HBC/epoxy-rich phase. Consequently, the thermosetting materials obtained cannot be optically transparent above the nematic/isotropic transition due to the different refractive indices of the polygon/hexagon shaped crystalline domains of the macrophase-separated HBC phase and the HBC/epoxy-rich phase. This phenomenon will be discussed in a future paper describing the thermomechanical behavior of these systems and their influence on thermoresponse behavior. Simultaneously, the addition of 5 and 10 wt.% PSEO2 into the 30 wt.% HBC–(DGEBA/MXDA) system leads to microphase separation of the HBC/PS-block. The sizes of HBC/PS-block phase are 40–75 nm and 22–38 nm, respectively, for 5 and 10 wt.% PSEO2 in epoxy-cured blends. The average size of these spherical microdomains (Fig. 6b and e) is almost 2–3 times higher than that corresponding to the PSEO–(DGEBA/MXDA) system under
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Fig. 5. TM–AFM phase images of the DGEBA/MXDA systems modified with different PSEO1 or/and HBC contents: (a) 5 wt.% PSEO1, (b) 5 wt.% PSEO1–30 wt.% HBC, (c) 5 wt.% PSEO1–50 wt.% HBC, (d) 10 wt.% PSEO1 and (e) 10 wt.% PSEO1–30 wt.% HBC. The inset shows digital images of the transparency of the blends at room temperature and, in case of the thermoreversible blends, at room temperature and at 10 °C, top and bottom, respectively.
the same conditions, and narrower and smaller if compared to similar blends containing PSEO1. At first, the morphology generated in systems with 5 and 10 wt.% PSEO2 as the third component confirms that microphase separation of
the PS-block during network formation leads to phase separation of low molecular weight LC within, yielding multifunctional nanostructured thermosetting systems. Second, AFM-based results suggest that the PEO-block is partially
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Fig. 6. TM–AFM phase images of the DGEBA/MXDA systems modified with different PSEO1 or/and HBC contents: (a) 5 wt.% PSEO2, (b) 5 wt.% PSEO2–30 wt.% HBC, (c) 5 wt.% PSEO2–50 wt.% HBC, (d) 10 wt.% PSEO2, (e) 10 wt.% PSEO2–30 wt.% HBC and (f) 10 wt.% PSEO2–50 wt.% HBC. The inset shows digital images of the transparency of the blends at room temperature and, in case of the thermoreversible blends, at room temperature and at 10 °C, top and bottom, respectively.
miscible with epoxy matrix [24,25,37,41,42] and with HBC [39] (taking into account the solubility parameters of these components published by us elsewhere [39,40], it should be pointed out that HBC low molecular weight LC is partially
miscible with both PS- and PEO-blocks of PSEO; however, it shows higher miscibility with PS-block), which has a strong influence on microphase separation of HBC/PSblock in PEO-block/epoxy matrix. Addition of 10 wt.%
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has been reported for thermosetting systems modified with low molecular weight nematic LCs. It can be noted that this blend is translucent at room temperature, and can switch from translucent to opaque (Fig. 6f inset), thus confirming the possibility of obtaining a thermoresponsive thermosetting system. 4. Conclusions
Fig. 7. Optical response of the 5 wt.% PSEO1–30 wt.% HBC–(DGEBA/ MXDA) cured sample vs. time.
PSEO as a third component halves the domain size and results in a narrower distribution of HBC/PS-block phase domain. Nevertheless, it should be remembered that the amount of PS-block which can microphase separate from the HBC phase is lower in the case of PSEO2 compared to the amount of PS-block in PSEO1 copolymer. A very interesting result is that DGEBA/MXDA systems modified with 30 wt.% HBC and 5 and 10 wt.% PSEO2 are fully transparent at room temperature and switch from transparent to opaque state by decreasing the temperature to around 16 °C. In this case, although not shown here in order to avoid repeating data, the addition of PSEO2 as a third component enables not only the preparation of nanostructured thermosetting materials but also materials that can be switched from opaque to transparent state by application of external stimuli, e.g. thermal gradient or electrical field, similar to that shown in Fig. 7 for the analogous PSEO1-based systems. Therefore, these materials offer a new strategy for the generation of novel thermoresponsive nanostructured thermosetting materials. Finally, as shown in Fig. 6c, the addition of 5 wt.% PSEO2 into the 50 wt.% HBC–(DGEBA/MXDA) system leads to macrophase separation of polygon/hexagonshaped crystalline domains of PS-block/HBC with a size distribution in the range of 300–850 nm. Thus macrophase separation of HBC phase is confirmed by the opacity of this plaque at room temperature (Fig. 6c inset). Consequently, similar to what was observed for the ternary blend containing 5 wt.% PSEO1, this system does not possess thermal reversibility since the final cured blend is opaque and, accordingly, it cannot be optically transparent above the nematic/isotropic transition due to different refractive indices of the polygon/hexagon-shaped crystalline domains of macrophase-separated HBC phase and the HBC/epoxyrich phase. When the amount of PSEO2 in ternary blends is increased up to 10 wt.% (Fig. 6f), a typical Schlieren texture characteristic of PDLC can be observed. However, to our knowledge, this is the first time that this morphology
The addition of a small amount of PSEO block copolymers as the third component in epoxy-type thermosetting blends modified with nematic LCs leads to a new family of thermoresponsive thermosetting nanostructured materials. This fact has been demonstrated by using HBC as nematic LC and two different PSEOs with different PEOblock contents. In the obtained thermoresponsive meso/ nanostructured thermosetting materials, PSEO block copolymer is used as both a self-assembly agent and as a PDLC. The curing behavior of the systems studied by DSC and rheology, as well as the morphology generated after network formation, have shown that the introduction of an amphiphilic block copolymer as the third component yields a narrow, nanometer-scale distribution of the PSEO/ HBC phase and stabilizes the system against macrophase separation of the HBC fraction. For adequate PSEO and HBC content, microphase separation of the PS-block from the epoxy matrix leads to internal microseparation of the HBC phase, which is responsible for switching from the opaque (OFF) to the transparent (ON) state. Consequently, nanostructured thermoresponsive thermosetting materials can be obtained with potential applications in the same fields in which nanostructured block copolymerand LC-based materials are used. Thus this study opens a new strategy for preparation of novel meso/nanostructured termosetting materials, which are able to switch from the opaque to the transparent state with high contrast. Acknowledgments Financial support from Basque Country Governments in the frame of Grupos Consolidados (IT-365-07) and SAIOTEK (S-PE07UN39) projects is gratefully acknowledged. References [1] Craighead HG, Chen J, Hackwood S. Appl Phys Lett 1982;40:22. _ ´ ska E, Mucha MJ. Appl Polym Sci 1999;71:455. [2] Nastał E, Zuran [3] de Gennes PH. Scaling Concepts in Polymer Science. Ithaca, NY: Cornell University Press; 1979 [chapter 5, p. 131]. [4] Coates DJ. Mater Chem 1995;5:2063. [5] Sumana G, Raina KK. Curr Appl Phys 2005;5:277. [6] Doane JW, Vaz NA, Wu BG, Zumer S. Appl Phys Lett 1986;48:269. [7] Smith JW, Vaz NA. Liq Crystallogr 1988;3:543. [8] Drzaic PS. Liq Cryst 1988;3:1543. [9] Herod TE, Duran RS. Langmuir 1998;14:6956. [10] Chin WK, Hsin LP, Lu HL, Shau MD. J Polym Sci Part B Polym Phys 2000;38:2033. [11] Zhou J, Petti L, Mormile P, Roviello A. Opt Commun 2004;231:263.
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