We investigated the dependence of the dispersion of retardation on the UV-polymerization temperature and the molecular orientation in a self-organized smectic host-guest reactive mesogen (RM) compound. The positive dispersion of retardation was converted to the negative dispersion of retardation with decreasing the UV-polymerization temperature. From the Fourier-transform infrared (FT-IR) dichroism measurement, it was found that more fractions of the guest molecules were aligned parallel to the smectic layer plane with decreasing the UV-polymerization temperature. The guest molecules located in the inter-layer space absorb a longer wavelength of UV light compared to the host and induce the negative dispersion of retardation.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The negative dispersion (ND) retarder whose retardation is increasing with a longer wavelength of light λ has been actively studied in these days [1–17]. The in-plane retardation Rin(λ) is defined as Rin(λ)≡Δnd, where Δn≡nx-ny is birefringence of the substance, nx and ny are the refractive indices along the x-and y-axis, respectively, and d is the thickness of the retarder. The ND retarder can make the phase retardation Г(λ)≡2πΔnd/λ to be independent of λ. Consequently, the optical properties such as transmittance and reflectance can be independent of λ. In particular, the ND retarder is useful for the display compensation film such as the quarter-wave plate in the quasi-circular polarizer for the organic light emitting diode (OLED) displays [9, 13–17].
There have been various approaches for the fabrication of the ND retarder [1–17]. In terms of the film structure, the previous approaches can be classified into a single-layer type [2–9] and a multi-layers type [10–17]. Although the fabrication process of the single-layer ND retarder is more simple than the multi-layers type, the optical properties such as the achromaticity and the viewing angle dependence of the latter is superior than the former. With this reason, most of the present commercialized ND retarders are multi-layers type. However, the demand for the single-layer ND retarder has been contiuously increased for the application of the flexible displays. For the flexible displays, the thickness of the retarder should be thin for the reliability during bending and rolling. In terms of materials, the ND retarders can be classified to streched-copolymers [2–4] and the reactive mesogen (RM) type [5–9]. Because the birefringence of the RM material is greater than the stretched-copolymers, the RM-based ND retarder can be fabricated with a thinner thickness and is useful for the flexible displays application.
Our group has reported a single-layer-type ND retarder using self-organization of host-guest smectic RM molecules [7–9]. We have synthesized new kinds of RM molecules and obtained the ND of Rin(λ) using the host-guest mixtures. In this paper, we investigated the dependence of the Rin(λ) dispersion on the UV-polymerization tempeature and the molecular orientation. We found that the ND of Rin(λ) was more promoted with decreasing the UV-polymerization temperature. To understand the physical reason, we measured the Fourier-Transform Infrared (FT-IR) dichroism of the constituent molecules and investigated the physical correlation between the Rin(λ) dispersion and the molecular orientation.
2. Experimental procedure
Figure 1 shows the chemical structure of the host smectic RM HCM026 and the guest N2 molecules. The HCM026 (HCCH), N2 and a photonitiator Irgacure651 (Ciba Chem) were in a choloroform solvent and stirred at for 1 h. The weight fraction of each solid content to the total weight including the solvent was 3.1, 2.1, and 0.05 wt%, respectively. For the alignment of the RM molecules, a polyimide PIA-X189-KU1 (JNC) was coated onto a glass substrate and baked at 230 °C for 1 h. The substrate was then unidirectionally rubbed with a cotton cloth. The RM mixture was then coated on the polyimide-coated substrate using a spin coater. The solvent was evaporated at 80 °C for 3 min. The phase transition temperature of the RM mixture after evaporation of the solvent was Cry (83 °C) SmA (115 °C) N (133 °C) Iso in the cooling cycle.The RM-coated sample was then heated up to isotropic phase with a rate of 30 °C/min and then cooled with a rate of 10 °C/min. The fast heating rate was programmed to avoid a pre-polymerization by heat. We varied the UV-polymerization temperature as 90, 100, 110, 120, 125, and 130 °C. The sample was stayed at UV-polymerization temperature for 3 min and then UV light was exposed with nitrogen gas purged. The UV light with an intensity of 40 mW/cm2 was exposed to the sample for 3 min. The temperature of the sample was maintained using a heating stage (Linkam) with a variance less than 0.1 °C.
The Rin(λ) was measured using the experimental setup in Fig. 2. The probe beam consecutively passed through the polarizer, the sample, and the polarimeter (Thorlab) [Fig. 2(a)]. The transmission axis of the polarizer was at 45° from the x-axis and the slow axis of the sample was parallel to the x-axis. Thus, the polarization state after passing through the polarizer is located at the S2 axis on the Poincaré sphere [Fig. 2(b)]. After passing through the sample, the polarization state is located on the meridian of the S2-S3 plane. By measuring the Stokes parameter of the output beam using the polarimeter, the Г(λ) and Rin(λ) of the sample can be measured. The Г(λ) is dependent on the sign of S3 and is given by ,Eq. (1-2) can be obtained from S1,
For the Fourier-transform infrared (FT-IR) dichroism measurement, CaF2 window was used as the substrate and the other fabrication conditions were same as described above. The absorption intensity of the RM mixtures were measured as a function of the rotation angle of the IR polarizer. For better understaning the correlation between the Rin(λ) dispersion and the molecules orientation, we defined the FT-IR dichorism as the ratio of the absorption along the layer-parallel direction A∥divided by the absorption to the layer-normal direction A⊥.
3. Results and discussion
Figure 3(a) shows Rin(λ) of the samples which were polymerized at 90, 100, 110, 120, 125, and 130 °C, respectively. The samples polymerized at the temperature over 120 °C showed a positive dispersion (PD) of Rin(λ), i.e., Rin(λ) was decreased with longer λ. On the other hand, the other samples polymerized below 110 °C showed ND of Rin(λ). In addition, the magnitude of Rin(λ) was suddenly decreased when the UV-polymerization temperature was below 110 °C. Figure 3(b) shows the normalized Rin(λ) with Rin(550 nm). A certain conversion of the sign of dRin(λ)/dλ was observed when the UV-polymerization temperature was changed from 120 to 110 °C.
As an aside, we need to mention that the magnitude of Rin(λ) of the sample showing ND property was smaller than the one in our previous paper . This is due to the reduced thickness of the film. In order to investigate the exact physical relation between the dispersion of Rin(λ) and the molecular orientation, we reduced the RM thickness, resulting in better alignment of the molecules.
To understand the physics underlying the conversion of the Rin(λ) dispersion and the reduction of the Rin(λ) magnitude, we measured the FT-IR dichroism of the samples which were polymerized at various temperatures. Figure 4(a) shows the FT-IR absorption spectrum of the pure host HCM026 and the pure guest N2 vs. wavenumber. It is seen that the pure N2 have non-overlapping absorption peak at the wavenumber of 1209 cm−1. Thus, the orientation of the guest N2 could be separately analyzed. We measured the FT-IR absorption spectrum of the UV-polymerized samples with the IR polarizer inserted [Fig. 4(b)]. The IR polarizer was rotated with a step of 10° and the absorption intensity was measured at each orientation of the polarizer. As shown in Fig. 4(b), the absorption intensity becomes the maximum when the IR polarization direction is parallel to the chemical bonding direction. As described in the experimental section, we defined the FT-IR dichorism as the ratio of the absorption along the layer-parallel direction A∥ divided by the absorption along the layer-nomral direction A⊥ for better understanding the underlying physics.
Figure 5(a) shows dRin(λ)/dλ vs. the UV-polymerization temperature. The dRin(λ)/dλ data was obtained from the results in Fig. 3(a). The positive sign of dRin(λ)/dλ means the ND of Rin(λ), while the negative sign of dRin(λ)/dλ indicates the PD of Rin(λ). It is clearly observed that the dispersion of Rin(λ) was converted from ND to PD with increasing the UV-polymerization temperature. The dRin(λ)/dλ value was nearly constant when the UV-polymerization temperature was less than 110 °C, but rapidly decreased as the UV-exposure temperature was further increased. As described in the experimental section, the SmA-N phase transition temperature of the RM mixture was 115 °C. Thus, the rapid decrease of dRin(λ)/dλ may be related to the different orientation of the constituent molecules in each phase.
Figure 5(b) is the dichroic ratio A∥/A⊥of the guest N2 at the wavenumber of 1209 cm−1. The absorption at 1209 cm−1 is from the C-N bond located in the core of N2 [Fig. 1(b)]. Although there was a deviation at 90 °C, the dichroic ratio was decreased with increasing the UV-polymerization temperature. Because we defined the dichroic ratio as the absorption along the layer-parallel direction divided by the absorption along the layer-normal direction, the smaller dichroic ratio means that more fraction of the N2 molecules were aligned to the layer-normal direction, vice versa. With increasing the temperature, the out-of-layer fluctuation of the host RM molecules along the layer normal direction becomes strong [17–21]. Consequently, the guest RM molecules are hard to retain the layer-parallel orientation due to the thermal fluctuation of the host molecules. Thus, the guest molecules change their orientation from layer-parallel to layer-normal direction with increasing the UV-polymerization temperature.
Figure 5(c) shows the dRin(λ)/dλ vs. the dichroic ratio of N2 using the results in Fig. 5(a)-5(b). It was observed that the dRin(λ)/dλ value was increased with a greater dichroic ratio of N2. This means that the ND of Rin(λ) is promoted with more fraction of N2 oriented to the layer-parallel direction. In addition, a sudden jump of dRin(λ)/dλ is also seen in Fig. 5(c).
As described in our previous papers [7, 9], the ND of Rin(λ) is shown when ny is more rapidly decreased than nx in the visible wavelength range [Fig. 6]. From the dispersion relation, the dn/dλ is dependent on the absorption wavelength of light. The refractive index is rapidly decreased in the visible wavelength range provided that the absorption wavelength of the material is close to the visible wavelength region. Figure 6(a) is the UV-Vis absorption spectrum of the host HCM026 and the guest N2 materials. Both of the host and the guest have common absorption peak at 270 nm due to the π-conjugated structure. Meanwhile, the guest N2 has additional absorption at λ = 370 nm due to the imine group. Consequently, the refactive index of N2 can be more rapidly decreased in the visible wavelength region compared to the host HCM026. Given the orientation of the guest molecules parallel to the layer plane in Fig. 6(b), ny is rapidly decreasing due to the absorption of a longer wavelength of UV light, while nx is smoothly decreased due to the absorption of a shorter wavelength of UV light. On the other hand, the guest molecules oriented to the layer-normal direction in Fig. 6(c), nx is rapidly decreased compared to ny, resulting in PD of Rin(λ).
We also need to mention that the sudden decrease of the magnitude of Rin(λ) in Fig. 3(a) can be interpreted with the model in Fig. 6. Because the core of the guest molecules were aligned orthogonal to the smectic host in Fig. 6(b), the net birefringence should be smaller than the case where both of the host and the guest molecules were aligned to the same direction in Fig. 6(c).
One can have a question about the depolarization effect of the samples which were polymerized at different temperatures. To answer this question, we measured the degree of polarization (DOP) of the samples which were polymerized at various temperatures. The DOP is defined as (s12 + s22 + s32)1/2/s0, where s1, s2, s3, and s0 are non-normalized Stokes parameters. The DOP of the samples polymerized at 90, 100, 110, 120, and 130 °C was 0.89, 0.91, 0.91, 0.92, and 0.91, respectively. Thus, the DOP of the samples showing the ND of Rin(λ) was nearly similar to the ones showing the the PD of Rin(λ). This means the HCM026-N2 mixture still keeps homogeneous orientation when it shows the ND property. Meanwhile, when the thickness of the RM layer was increased showing the quarter wave retardation, the DOP value was around 0.86. Thus, the DOP value needs to be improved by optimizing the alignment conditions and this remained as our future work.
We think that the results in this paper could elucidate the physical mechanism involved in the appearance of the ND of Rin(λ). The experimental results also indiate that the fabrication conditions such as the UV curing temperature should be carefully controlled to obtain the ND of Rin(λ). In addition, there can be other materials with different chemical structures showing better ND property. For instance, discotic guest molecules which can be well segregated to the inter-layer space with least disturbing of the host orientation can be developed based on the mechanism verified in this paper.
We investigated the dependence of the dispersion of Rin(λ) on the UV-polymerization temperature and the molecular orientation in the smectic host-guest RM compound. It was found that the guest molecules preferred their orientation parallel to the smectic layer plane with decreasing the UV-polymerization temperature. The guest molecules absorbing longer wavelength of UV light oriented to the layer-parallel direction can induce ND of Rin(λ).
Ministry of Trade, Industry, and Economy (MOTIE) and Korea Display Research Consortium (10051334); National Research Foundation (2016R1A2B4010361); Research Base Construction Fund of Chonbuk National University (2018).
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