Liquid Crystals

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Investigation on morphology of polymer structure in polymer-stabilized blue phase Hui-Yu Chen, Che-Kai Wu, Fang-Chi Chen & Chia-Sheng Chen To cite this article: Hui-Yu Chen, Che-Kai Wu, Fang-Chi Chen & Chia-Sheng Chen (2016): Investigation on morphology of polymer structure in polymer-stabilized blue phase, Liquid Crystals, DOI: 10.1080/02678292.2016.1173244 To link to this article: http://dx.doi.org/10.1080/02678292.2016.1173244

Published online: 24 Apr 2016.

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Date: 25 April 2016, At: 01:57

LIQUID CRYSTALS, 2016 http://dx.doi.org/10.1080/02678292.2016.1173244

Investigation on morphology of polymer structure in polymer-stabilized blue phase Hui-Yu Chena, Che-Kai Wub, Fang-Chi Chenb and Chia-Sheng Chenb

ABSTRACT

ARTICLE HISTORY

Polymer stabilization effect to increase the thermal stability of blue phases (BPs) for potential application in flat-panel display has been studied in detail though directly observing the morphology of the polymer network on the substrate formed under different experimental conditions during the polymerization process. The tiny and dense polymer network, in which the diameter of the polymer chain is not greater than 250 nm, is useful for stabilizing the BPI structure. Moreover, these experimental results can be explained by the free-energy density of BPI, where its temperature range depends inversely on the square of the polymer-chain diameter. In terms of the experimental results shown in this study, we can conclude an optimal photopolymerization process for a wide-temperature-range polymer-stabilized BP.

Received 16 February 2016 Accepted 29 March 2016

500 nm 500 nm

KEYWORDS

Blue phase; polymer stabilized; morphology; photopolymerization

500 nm

RM257 NOA65/EHA TMPTA/EHA

60 50 ΔTBP(°C)

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a Department of Physics, National Chung Hsing University, Taichung, Taiwan; bDepartment of Photonics, Feng Chia University, Taichung, Taiwan

40 30 20 10 0

200 400 600 Polymer-chaindiameter (nm)

1. Introduction Blue phases (BPs) have been recognized as a fast optical isotropic/anisotropic switching liquid crystal (LC) material in recent years, and they have become a possible candidate in fast-display (Ge et al. 2009; Chen & Wu 2013), three-dimensional laser (Cao et al. 2002) and photonics applications (Coles & Morris 2010; Li & CONTACT Hui-Yu Chen

[email protected]

© 2016 Informa UK Limited, trading as Taylor & Francis Group

800

Wu 2011) after improving their thermal stability. For most LC materials, BPs are usually stable within a very narrow temperature range between the isotropic liquid phase and the cholesteric phase with strong chirality. In the temperature range of a few degree Celsius, one may see three different structures of BPs stabilized by lattice defects (declination lines): amorphous BPIII, simple cubic BPII and body-centred cubic BPI. BPI and BPII

Department of Physics, National Chung Hsing University, Taichung 40227, Taiwan

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consist of double twist cylinders (DTCs), where the LC director rotates along two perpendicular directions. In terms of the defect theory, in BPI and BPII, DTCs have a lower elastic free energy than a uniaxially twisted director field in chiral nematic LC (Meiboom et al. 1981; Gleeson et al. 2015). Those DTCs self-assemble as three-dimensional cubic lattice structures, leading to the appearance of a disclination network. These disclination lines can be considered as cylinders filled with the isotropic liquid phase, and thus have a higher enthalpy and an additional interfacial energy, which causes the narrow temperature range of BPs (Nordendorf et al. 2014; Gleeson et al. 2015). In order to extend this temperature range, the most useful way is to introduce the polymer network into the BP declination lines to help achieve a temperature of more than 60 K, due to the pinning effect (Kikuchi et al. 2002; Ojima et al. 2009; Gleeson et al. 2015). Polymer-stabilized BP (PSBP) is a key for next-generation display and photonics application. Many papers (Haseba & Kikuchi 2007; Rao et al. 2011; Yan & Wu 2011a, 2011b) studied the effect of the material optimizations (Hsieh & Chen 2015), photoinitiator concentration, UV irradiation conditions, such as wavelength (Liu et al. 2014) and intensity of UV light, exposure time and curing temperature (Hirose & Yoshizawa 2015), and the stiffness of the polymer system (Zhu et al. 2014) on the electro-optical performance of the PS-BPLC composite. For an electrically addressable PSBP, the total monomer concentration is between 8% and 15% by weight (Fukuda 2010; Chen & Wu 2014; Nordendorf et al. 2014). To form a PSBP composite, the precursor, a mixture of nematic LC, chiral dopant and monomers, is cooled from the isotropic phase to the BP, and is then exposed to UV light. After UV irradiation at the BPs for 30 min to a few hours, one can get the PSBP sample. A previous study (Higashiguchi et al. 2008) showed that the thermal stability of BP enhancement is related to the polymer component accumulating in the disclination lines, reducing the enthalpy contribution from the isotropic liquid and the interfacial energy (Fukuda 2010). The sort and the concentration of reactive monomers are important for extending the BP temperature range (Kikuchi et al. 2002; Iwata et al. 2007; Fukuda 2010; Nordendorf et al. 2014). Another key point for achieving the stabilized effect is that the photopolymerization process should be completed during the BP phase. Usually, the thermal stability of BPI is easier to enhance as the UV explosion is done in BPI. By postulating that the polymer occupies the declination lines of radius r, and the dimension of the unit cell of BPI is l and is related to the radius of the polymer

chain, the volume fraction of the polymer chain in BPI is φ. Moreover, the interface energy σg after pffiffiffi introducing the polymer is described by σ g ¼ 4σð 3πÞ1=2 =lf0 , where σ is the interfacial energy without a guest component (~10–5 J/m2) and f0 is the free-energy density of BPI (Fukuda 2010). One realizes that, for a certain polymer and LC, the interface energy σg depends on the dimension of the unit cell of the BPI. It indicates that we should examine other polymerization conditions to control the morphology of the polymer network, and then discuss their influence on the thermal stability of BPI. However, the detailed investigation of the morphology of the polymer network constructed in different fabrication conditions, such as the photo-polymerization temperature (i.e. the phases of LC), the intensity of the UV irradiation and the photo-polymerization time, and discussion of the influence of the polymer-network structure on the thermal stability of the three BPs are still lacking. Although the scanning electron microscopy (SEM) pictures of the polymer network formed in BPI after washing out the LC materials by solvent were offered in the previous studies (Lu & Chien 2010; Hussain et al. 2011, Kwak et al. 2011; Yang & Yang 2011), those pictures are used only to show the formation of the polymer network inside the cell and there is no detailed discussion. Study on the morphology of the polymer network form in the LC is a fundamental and important work to develop and optimize the widetemperature-range PSBP materials for further application. In this study, we would like to investigate the influences of the LC phase, the intensity of the UV irradiation and the irradiation time during the polymerization process on the BP temperature ranges. The intensity of the UV irradiation affects the speed of the phase separation between the polymer and LC materials. The degree of polymerization of the monomer is decided by the intensity and time of the UV irradiation. From varying the irradiation time, one may know how long the polymerization takes. In order to explore the effects of the polymerization process, the morphology of the polymer network formed in BPLC is studied by SEM. We prepared PSBP samples, where the polymer network is constructed in cubic BPI, amorphous BPIII and the chiral nematic phase, with the intention of comparing the temperature range of the BPs in these samples. From these SEM photographs, the radius of the polymer chain becomes tiny when the photopolymerization temperature and the intensity of the UV irradiation are increased. These experimental results show that the different LC phases will affect the morphology of the polymer network. Referring to the free-energy density of BP (Meiboom et al. 1981),

LIQUID CRYSTALS

the temperature range of BP will depend inversely on the square of the disclination diameter, as well as the polymer-chain diameter. We can obtain experimental results which agree with this theoretical prediction. The study can offer suggestion to modify the PSBP fabrication and will be helpful for developing the PSBP device.

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2. Experimental method BPLC samples were prepared by mixing nematic LCs and chiral materials, and these BP mixtures’ phase sequences were determined by the reflection spectra and are summarized in Table 1. BPIII phase is confirmed by using crossed polarization microscope with a spectrometer, to record the reflecting spectrum of the sample. Compared to the reflection characteristic of the cubic BPs, BPIII reflects a very weak and broad region at short wavelengths. We record the intensities of the reflection peak from isotropic phase, BPIII, BPI to chiral nematic phase. A discontinuous change in the reflection intensity is observed (Iwata et al. 2007) when the LC mixture transits from isotropic phase to BPIII, and then we can find a rough temperature range of BPIII. Moreover, the temperature range of BPIII was double-checked by applying an external field on the sample. When the electric field is applied to BPIII cell, the reflected wavelength does not shift obviously, but the intensity of reflection increases or decreases in a few milliseconds (Chen et al. 2013). Based on these examinations, we can confirm and determine the temperature range of BPIII. In these BP mixtures, BPII was not observed in the cooling or heating process. Previous studies have shown that BPII only appears in a specific chiral concentration (Bowling et al. 1993; Chen et al. 2013). Usually, two types of monomers (monofunctional plus cross-linker monomers) are used in stabilizing BP (Yan & Wu 2011b). According to Kikuchi’s report, they compare the differential scanning calorimetry (DSC) thermal spectra of different monomer fractions between monofunctional monomer nC12A and cross-linker monomer RM257 (Iwata et al. 2007). When the concentration of RM257 increases, the baseline of the DSC thermal spectra shifts and the peak near 263 K due to the glass transition disappears. Table 1. Mixtures of the liquid-crystal blue phase and their temperature sequences. Nematica Chiral dopant Temperature sequence (ºC) S1 LCM05 (Δε = 4.8) R811 (30 wt%) Iso.-103.7-BPIII-103-BPI-97-N* S2* LCM10 (Δε = 2.7) S811 (25 wt%) Iso.-99.4-BPIII-98-BPI-91.8-N* Δε Denotes dielectric anisotropy of nematic mixtures and is measured at 25ºC with a 1-kHz electric field. *The sample was cured in the BPIII.

a

3

It means that the thermal molecular mobility of polymer main chain is depressed, and then small molecular mobility reduces the thermal fluctuation in the BP to enhance the pinning effect of the lattice structure. The polymer-stabilizing effect of the BP was enhanced by depressing the chain mobility of the polymer network formed in BP, thus the cross-linker monomer plays the important role in stabilizing BP. In this paper, to widen the temperature ranges of the BPs, only a 6-wt% crosslinker monomer diacrylate (RM257, Sigma-Aldrich China Inc., Shanghai, China) was added into these BP mixtures which then formed precursors. After mixing the BPLC and RM257 homogeneously, the precursors were heated to their isotropic state and filled into empty cells with a cell gap of 10 μm. In order to keep the samples on the specific LC phases (N*, BPI, BPIII or isotropic phase), they were placed on a temperaturecontrolled stage (HSC402, Instec Inc. Boulder, CO, USA) and then irradiated by UV light (~365 nm) for 10–180 min.

3. Results and discussion Previous studies have given an explanation of the mechanism of PSBP (Fukuda 2010). Because the stabilized effect is formed through a phase separation induced by the polymerization of the photoreactive monomer dispersed in the LC mixture, the morphology of the polymer network may be affected by the 3D cubic structure of the BPs. According to the molecular-aligned structures of the different BPs, we know that BPIII is amorphous and may have random DTC distribution (Chen et al. 2013), and BPI and BPII are body-centred cubic and simply cubic, respectively. One may conjecture that polymerizing the LC samples in the different BPs could extend their temperature ranges. However, no experimental study was provided to examine this idea. In this paper, we control the photo-polymerization temperatures corresponding to the temperatures when the samples were in the isotropic phase, BPIII, BPI and the chiral nematic phase, and study the effect of the LC phase during the polymerization process on the thermal stability of different BPs after polymerization. Because the temperature range of BPIII in S1 is very narrow (~0.6ºC), the BP mixture was replaced with S2 (in Table 1) to induce a wider temperature range for BPIII (~1.4ºC). This wider range prevented the samples from undergoing phase transition during the polymerization process. The irradiating intensity is kept at 100 μW/cm2 and the photo-polymerization time is 3 h. During the polymerization process, the phase of the sample was checked by the cross-polarization microscope to make sure that the polymerization was done

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Table 2. Temperature ranges of BPI and BPIII before and after polymerization when these samples were cured by different LC phases. ΔTBPIII (ºC)

ΔTBPI (ºC)

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Sample S1(N*) S1(BPI) S2(BPIII) S1(Iso)

Before photo-polymerization 6 6 6.2 6

After photo-polymerization 0 15 17.7 14.7

under the chosen LC phase. After 3-h polymerization, we had four different samples: S1(Iso.), S2(BPIII), S1 (BPI) and S1(N*), where the parentheses denote the LC phases during the polymerization. Before and after the polymerization, these samples were slowly cooled and the textures of the BPs were confirmed with a polarizing optical microscope in reflection mode. The temperature ranges of these PSBP samples before and after polymerization are compared in Table 2. When the polymerization LC phase was chosen in the isotropic phase, BPIII and BPI, the total temperature range of the BPs was twice as wide as that of the unpolymerized BP samples. However, the temperature range of the BPs was reduced when the polymerization was done in the chiral nematic phase. Comparing the temperature ranges for each BP in Table 2, one can see that it is of no use to widen the temperature range of BPIII under this experimental

Before photo-polymerization 0.7 0.7 1.4 0.7

After photo-polymerization 0.6 0.5 1.7 0.5

condition, as the polymer network can only stabilize the structure of the cubic BPI and then increase its thermal stability. In order to understand the relation between the polymer network and the thermal stability of the BPs, we used a more direct method to see the morphology of the polymer network in these samples. To observe the morphology of the polymer network formed in PSBP, these PSBP samples were decapped and washed gently with n-hexane to remove the nematic LCs and chiral dopants. In order to prevent damage to the polymer network (Kwak et al. 2011), the PSBP cell was washed with n-hexane for 40 s at room temperature. Only the polymer networks were left on the glass substrates after washing; they were then ready for analysing by SEM. The morphologies of the polymer networks on the substrate in these samples were examined by SEM after washing out the LC mixture as shown in Figure 1.

(a)

(b)

(c)

ξ

200 nm

(d)

Figure 1. (colour online) Morphologies of polymer networks were taken by SEM and were formed by polymerizing in (a) the isotropic phase, (b) BPIII, (c) BPI and (d) the chiral nematic phase.

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As the curing LC phase moves to isotropic, BPI or BPIII, it is not hard to find that the morphologies of the polymer networks in S1(Iso.), S2(BPIII) and S1(BPI) are very similar in shape and size (Figure 1 (a–c)). Their polymer chains are built by stacking small beads where the diameter of these bead-like chains is about 300 nm or less. The coherence length of the closed pores (ξ) between these polymer chains is measured to be 300–400 nm. According to the cubic structure of the BPI, the maximum radius of the disclination line was assumed to be 100 nm (Meiboom et al. 1981) and the radius of the DTC is less than 400 nm. The dimension of the coherence length is close to the maximum diameter of the DTC theoretically. It means that these pores were filled with DTC before washing out the BP. The production of a polymer chain thicker than the theoretical value of the disclination might be because of the slow polymerization rate, as well as weak irradiation intensity. When the polymerization was done in the chiral nematic phase (Figure 1(d)), the polymer chain looks like pine needles and the polymer network does not have closed pores. The S1 (N*) sample exhibits the phenomenon of the LC/polymer phase separation macroscopically. Now we have experimental evidence to show that the temperature range of the BPI can be extended when the polymerization process is done in the BPI. Next, we wanted to see the formation of the polymer network within the 10-, 30- and 180-min irradiation times by controlling the irradiating intensity (~100 μW/cm2). Figure 2 displays the morphologies of the polymer network with different irradiation times and the temperature ranges of the PSBPI. In the first 10 min, the monomer in the sample just starts to react and is

10 min

1µm

30 min

5

converse to the polymer, thus no obvious network structure can be seen. When the polymerization time is extended to 30 min, the polymer network forms in some areas. Until 180-min irradiation, in the majority of the samples, the polymer network was constructed as we see in Figure 1. We can say that under this irradiation intensity, the morphology of the polymer is decided in the first 30 min, because the monomers are already conversed to polymers with small molecular weight and then the polymer chain length is short. Continuously irradiating the sample, these polymers with short chain length connect with each other to increase the chain length and then they reduce the chain mobility. The depression in the chain mobility of the polymer network is useful for increasing the thermal stability of the BP (Iwata et al. 2007), as we observe in this study. The BPI temperature ranges are 15.8ºC, 12.0ºC and >60ºC when the irradiation times are 10, 30, 180 min, respectively. After confirming the effect of the polymerization LC phase and the irradiation time on the thermal stability of BP and the morphology of the polymer network, we move our attention to studying the influence of the irradiating intensity of the UV light on the diameter of the polymer chain and then on the thermal stability of BPs. The sample was S1 and the polymerization was done at the same temperature in BPI for 3 h and the irradiating intensities were 85 ± 15 μW/ cm2 and 5 ± 1 mW/cm2. We summarize the temperature ranges of BPI after polymerization and the morphology of the polymer network in Figure 3. In the weaker irradiating intensity (85 ± 15 μW/cm2), the BPI temperature range is only 15ºC. However, the temperature range can be over 60ºC, and it covers room

1µm

180 min

1µm

Figure 2. (colour online) Morphologies of the polymer network taken by SEM and the temperature range of PSBPI when the photopolymerization times are different.

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  1 Rmax F ¼ aΔTBP πR2 þ 2ρπR  πK þ πK ln 4 R

(1)

where a may be estimated from the latent heat of the isotropic – N transition, R is the radius of the core of disclination, ρ is the surface tension and can usually be neglected, Rmax is a cut-off radius of the core of disclination and K is (K11+K33)/2. The BP will be stable with respect to the cholesteric when F < 0. A minimum of F can occur for

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Figure 3. (colour online) Morphologies of the polymer network taken by SEM and the temperature range of PSBPI when the S1 sample was irradiated by (a) 85 ± 15 μW/cm2 or (b) 5 ± 1 mW/cm2.

temperature when the irradiating intensity is stronger (5 ± 1 mW/cm2). Figure 3 exhibits that the diameter of the polymer chain and the coherence length of the pores become smaller in the sample irradiated by the stronger UV intensity (5 ± 1 mW/cm2), and the morphology of the polymer network is like sponge, which is quite different from the morphologies formed in the sample cured by a weak irradiating intensity. The diameter of the polymer chain in Figure 3 is tinier, less than 50 nm, and the coherence length of the pores is not uniform and is in a range between 200 and 100 nm. The irradiating intensity may affect two polymerization factors: the heating effect on the cell and the polymerization rate of the LC material in the polymer. The heating effect is excluded by detecting the cell temperature through an individual thermocouple, where the cell temperature does not change during UV irradiation. In the stronger irradiating intensity experiment, the polymerization reaction is obvious in a short time, which increases the speed of the phase separation between the LC material and the polymer. Most of the monomer molecules can be polymerized inside the disclination lines. It causes a tinier polymer chain, and the pores of the polymer network are dense. These results exhibit that the reaction rate and conversion of the polymerization process are important for producing a useful polymer network for the intended stabilized effect. The free-energy density of the BP is contributed by the excess free energy of the disclination core (the first term), the surface energy at the interface between the core and cholesteric (the second term) and the DTC and elastic energies (the last two terms) in Equation (1) (Meiboom et al. 1981).

K (2) 8aR2 In Equation (2), K and a are the material parameters dominated by the LC material. When the LC material is chosen, Equation (2) shows that the temperature range of BP inversely depends on the square of the disclination diameter. Because the polymer chains grow along the disclination to stabilize the BP structure, R can also represent the polymer-chain diameter in PSBP. By gathering the polymer-chain diameters from the SEM pictures, we can demonstrate the relation in Equation (2), as shown in Figure 4, in which we examine three monomer mixtures, RM257, NOA65/ EHA and TMPTA/EHA, to form polymer networks in the S1 sample. Figure 4 displays that the tiny polymer chain is good for extending the temperature range of BP. As the polymer-chain diameter is wider than 250 nm, the stabilized effect is not obvious. Moreover, the BP disappears if the diameter is more than 1000 nm. Now, we can understand that the diameter of the polymer chain is a key point for stabilizing the BP. ΔTBP ¼

4. Conclusion We demonstrate the effect of the polymer-chain diameter on the thermal stability of BPI by observing the morphology of the polymer network formed under different experimental conditions during the polymerization process, including the LC phase, the intensity of the UV irradiation and the photo-polymerization time. When the photo-polymerization temperature and the intensity of the UV irradiation are increased, the diameter of the polymer chain becomes tiny, and then the polymer network can stabilize the BP structure effectively. These experimental results can be explained by the free-energy density of BP, where the temperature range of BP will depend inversely on the square of the polymer-chain diameter.

LIQUID CRYSTALS

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500 nm

7

500 nm

Figure 4. (colour online) Dependence of the polymer-chain diameter and the temperature range of BP.

Acknowledgements The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this research under grant nos. MOST 101-2112-M-005006-MY3 and MOST 104-2112-M-005-004.

Disclosure statement No potential conflict of interest was reported by the authors.

Funding The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this research [grant nos. MOST 101-2112-M-005-006MY3 and MOST 104-2112-M-005-004].

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