1. Introduction

Chiral liquid-crystalline (LC) materials may exhibit a marvelous variety of liquid crystalline phases, including the chiral smectic C* phase, the cholesteric phase, and the blue phases (BPs, including BPI, BPII, and BPIII), etc. Blue phases are thermodynamically stabile mesophases, which appear in chiral systems between isotropic and liquid crystal phases, such as smectic BPs^{1, 2,} cholesteric BPs ^{3, 4}. Some lyotropic liquid crystals ⁵ and biomolecules ⁶, such as DNA, can also form BPs. Some of the chiral smectic C* phase, the cholesteric phase, and all of the BPs show brilliant colours in the visible light area and they are optically isotropic which mean no birefringence phenomena⁷. Recently, H. Kikuchi, et al ⁸ reported polymer-stabilized liquid crystal blue phases with a BP temperature range approximate 60 °C. M. N. Pivnenko et al ⁹ reported a kind of general family of small molecule liquid crystal which blue phases are stable over 40–50 °C temperature range. Many endeavors are focusing on increase blue phases including BPs temperature range ^8–11 in the visible light for requirement of the study and application. Recently, we reported a series of cyclosiloxane-based cholesteric liquid crystalline polymers (LCPs) synthesized from a cholesteric LC monomer and a nematic LC monomer, and some of the polymers exhibit cholesteric phases and blue phases with 20 °C temperature range¹¹. However, all of the blue phases appeared in high temperature, and the blue phase temperature range at which still could not satisfy for requirement of the study and application. Now we describe a series of novel liquid crystal polymers which use a cholesteric LC monomer and a nematic LC monomer with cyanophenyl, exhibiting PBPs in wide temperature range, some of the PBPs even demonstrate temperature range over 300 °C in cooling cycles. We propose that the unique optical property based on their structure and different twists formed and expect to open up new photonic application and enrich liquid-crystalline phase contents and theory, we should spend more stamina on the field of polymer blue phases (PBPs) as they are different comparing with small molecule blue phases in some phenomena and structures.

In the present study, a series of cyclosiloxane-based cholesteric LC polymers were graft polymerized by use of cholest-5-en-3-ol(3β)-4-(2-propenyloxy)benzoate and 4-cyano-phenyl (4-(2-Propenyloxy)benzoate, exhibiting wide temperature range PBPs. The generic structure of the polymers is shown in Scheme 1.

 

Scheme 1. General structure of the polymers.

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2. Experiment

Cyclo(methylhydrogeno)siloxane (CMHS) (Mn=200-300 g/mol) and the liquid crystalline monomer cholest-5-en-3-ol(3β)-4-(2-propenyloxy)benzoate (monomer M1) was prepared according to previously reported synthetic method ^{13, 14}. The monomers and polymers were studied by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), etc. X-ray measurements of the samples were performed using Cu Kα(λ=1.542 Å) radiation monochromatized with a Rigaku DMAX-3A X-ray diffractometer (Rigaku, Japan). Thermal transition properties were characterized by a NETZSCH Instruments DSC 204 (Netzsch, Wittelsbacherstr, Germany) at a heating rate of 10 °C min^-1 under nitrogen atmosphere. The thermal stability of the polymers under atmosphere was measured with a NETZSCH TGA 209C thermogravimetric analyzer. Visual observation of liquid crystalline transitions and optical textures under cross polarized light was made by a Leica DMRX (Leica, Wetzlar, Germany) polarizing optical microscope equipped with a Linkam THMSE-600 (Linkam, Surrey, England) hot stage. Ultraviolet-visible spectrophotometry was measured using PerkinElmer instruments Lambda 950 (PerkinElmer, Foster City, CA). FTIR imaging and optical rotation were acquired using a Spectrum spotlight FT-IR imaging system and a Model 341 Polarimeter (PerkinElmer).

The series novel cyclosiloxane polymers are designed and synthesized based on a cholesteric liquid crystal monomer cholest-5-en-3-ol (3β)-4-(2-propenyloxy) benzoate (M₁) and a monomer 4-cyano-Phenyl-[4-(2-Propenyloxy)]-Benzoate (M₂). The detailed polymerization experiments were summarized in Table 1. Most of the polymers display PBPs with wide stable temperature ranges in cooling cycles. Their reflectance colors, such as green and blue could be observed by naked eyes at temperature range.

Table 1. Polymerization some properties of the series of polymers

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3. Results and discussion

All samples were measured by means of liquid crystal cell containing polymers. Every sample was monitored by POM on heating until no LC texture could be observed followed by cooling from the isotropic phase to -180 °C at which the lowest temperature is limited for the instrument. The rate of heating and cooling is 0.1–5 °C/min. With most of the polymers, except P1, PBPs with wide stable temperature ranges, around 300 °C were observed in cooling cycles. Birefringence phenomenon was not found in any observation by POM when samples were rotated. Fig. 1(a), 1(b) and 1(c) are the photos of polymer P4 taken at 150 °C and -174.5 °C on the first cooling individually in November 2002. From the textures observed, it is more likely to be BP. Fig.1(b) appears frost fractures, but it shows almost as same texture as Fig. 1(a), indicating the texture are stable from 150 °C and -174.5 °C. In order to confirm the fractures in texture of Fig.1(b) are merely caused by the sample structure, ¼ λ polarized lens was used to cover on the sample 1b, and the corresponding photo is shown as Fig.1(c), in which the uniform texture with clear fractures could be seen, which is only PBPs textures. The fractures disappeared around -20 °C gradually but the texture remained. Comparing the two pictures of the Fig.1(a) and Fig.1(d) of polymer P4 (Fig. 1(d), taken in November 2005 at room temperature; Fig. 1(a), taken in November 2002 at heating to 150 °C, respectively), a change occurred during the 3-year period for these PBPs textures, suggesting that PBPs textures could be observed with different patterns. Fig.1(d) was distinguished from typical BPI textures. Furthermore, there were two kinds of texture patterns in Fig. 1(d). Therefore Fig.1(d) is BPII and BPIII probably. Experiments above indicate that the PBPs exist over wide temperature ranges, from -180 °C to clear point (T_c), and it means the PBPs could exist alone, that is, we called ‘independent polymer blue phases’ (IPBPs). The IPBPs existence could happen with three BPs, or two of them, or only one blue phase. It further proves that PBPs are the thermodynamically stable phases. From the DSC curves the T_g and T_c are observed, but no Tm is detected for all of the samples. With the increasing of M₂ component (mole percent, 0%–60%) in the polymer systems, the glass transition temperature changes from 24.6 °C to 4.8 °C, but the clear point temperature dropped around 65 °C (225.5 °C-159.4 °C). It is apparent that as the biggest and helix group, M₁ decreased, the T_g and T_c decreased.

 

Fig. 1. The probable blue phase textures of the polymer P4 (200×). Pictures (a), (b) and (c) were on the first cooling taken by LEICA DMLP Polarizing Optical Microscope in Nov. 1^st, 2002: (a), 150 °C; (b), -174.5 °C; (c), the sample of (b) covered with λ/4 plate; (d), the same sample of November 2002 after 2 times heating to more than 180 °C and cooling to -180 °C, then has been setting in room temperature more than 3 years taken by LEICA, DMRX Polarizing Optical Microscope, with heating platform-THMSE 600 in Nov. 25, 2005.

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The small molecular BPs are known having double twist structure. BPI and BPII have body and simple cubic lattice respectively, and BPIII is amorphous, which is called gray phase or fog phase ¹⁴. For BPs, the helical pitch directly relate to the wavelength of visible light and selective reflection of Bragg. As same as small molecule blue phases, PBPs exhibits excellent optical properties. Their reflectance colours, such as blue, could be observed by naked eyes at temperature range and the yellow colour observed on the back of the plane sample. These phenomena are called selecting reflection and selecting transmission of Bragg. The UV-Vis-NIR spectrophotometer and polarimeter were applied to measure the optical properties, giving more precise data. The powder sample was usually placed between two pieces of glass plate and was heated to soft, then the glasses were pressed until the film was to be an even plane, and the sample was examined after it cooled to ambient temperature. Fig. 2(a) shows the biggest selective reflective wavelength, λ_max, of polymer P4 at different temperature on first heating and cooling cycles with heating and cooling rate of 5 °C/min. On the heating from 25 °C to 96 °C, the λ_max increased from 686.0 nm to 742.9 nm; but along the temperature rising from 96 °C to 181 °C continually, the λ_max decreased to 487.9 nm, so red shift followed by blue shift was observed. The reflective color changed from orange to blue. These results indicate that the helical pitch of polymer was getting bigger from RT to 96 °C, then turned to be smaller from 96 °C to 181 °C. While cooling, the λ_max changed slightly, stabled around 500 nm as the temperature decreased from 181 °C down to the room temperature, the helical pitch changed a little that might imply a self-organized net was formed, revealing the nature of blue phases for the polymers. In the heating and cooling process the intensity of the peaks increased from 25 °C to 96 °C slowly, kept stably until 141 °C, then getting down rapidly and the peak disappeared in the 186 °C; but the value of the peaks on the cooling changed a little. Fig. 2(b) shows the biggest selective reflective wavelength (λ_max) of polymer P4 at different reflective angle at room temperature. It indicates that λ_max decreases with increase of reflective angle. Fig. 2(c) shows optical rotation (α) for polymer P4 at different temperatures on heating and cooling cycles with heating and cooling rate of 5 °C/min, using sodium light source (λ=589nm). On the heating cycle, the optical rotation (α) of the sample is negative below 80 °C, increasing with the increasing of temperature until 70.93 °C (-16.140), but it becomes positive from this temperature and reaches the maximum at 96.0 °C (16.728), and then the α decreases with the increasing of temperature. On the cooling cycle, the optical rotation changed from 0.276 (185.97 °C) to 5.001 (29.92 °C), suggesting stable optical activity of the PBPs.

 

Fig. 2. Optical properties of polymer P4. (a) On heating cycles, the λmax increased from 25 °C to 96 °C; but the λmax decreased when the temperature continually rised from 96 °C to 181 °C. On the cooling, the λmax changed slightly, stabled around 500 nm as the temperature decreased from 181 °C down to the room temperature. (b) The biggest selective reflective wavelength (λmax) of P4 at different reflective angle at room temperature. The data of a and b were got by PE, Lambda 950 UV/VIS spectrophotometer. (c) On the heating cycle, α is negative below 80 °C, decreasing with increase of temperature, but it gets a maximum at 98 °C. On the cooling cycle, α remained almost the same, suggesting stable optical activity of the PBPs. This test was finished using PE, Model 341 Polarimeter. (d) WAXD of polymer P4 cooled from 230 °C (15.16°) to 10 °C (16.48°) then heating from 10 °C to 210 °C (15.58°). The intensity and the location of peaks changed little. The data were measured with a Rigaku DMAX-3A X-ray diffractometer (Rigaku, Japan) using Cu Kα (λ=1.542 Å).

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The information about WAXD of PBPs has not been reported. The existence of blue phases of the polymers has also been confirmed by XRD system. XRD studies can provide more detailed information on the liquid crystalline structure and type. The small angle was not found, and Fig. 2(d) shows the WAXD diagrams of the polymer P4 at different temperature with heating and cooling rate of 5 °C/min. The intensity of the diffraction peak at 2θ≈16° changes little, a broad peak appears at 2θ from 15.16° (230 °C) up to 16.48° (10 °C) on the first cooling, then on the second heating, 2θ was down to 15.58° (210 °C). Combined with POM observation, the polymer P4 showed PBP textures when it was cooled from isotropic state through room temperature to -180 °C, the similar X-ray diffraction diagrams demonstrate the stable blue phase of the polymer systems (see Fig.2(d)). In Fig. 2, the results are different between the first heating and cooling cycles. On the first heating, because the polymer films were prepared via coating of the polymers solution using toluene as solvent, the samples were not orientated. When it was heated, the orientation happened gradually, leading to the great changes of the λ, α and the WAXD diagrams. But on the cooling cycles, the orientated PBPs were stable, showing little changes of the λ, α and the WAXD diagrams.

The FT-IR Imaging (3D surface projection provides) information about the chemical structure and morphology of the polymers, in this work, we apply the FT-IR Imaging system technique to a spatial analysis of the morphology in the blue phase states of the P4. The samples were monitored using the Spectrum Spotlight FT-IR Imaging System from temperature of isotropic-anisotropic transition to room temperature on the cooling rate 1 °C/min. The C=O stretching in the polymer showed IR characteristic peaks around 1710 cm^-1, as shown in Fig. 3(a). The C=O stretching modes (corresponding hatchings in Fig 3(a)) were selected to show FT-IR imagings, which display distribution of the C=O groups in the polymer systems, as shown in Fig. 3(b). The planar axis represents the selected region of the polymer films, and the ordinate express the intensity of IR peaks. The colors from red to blue represent the intensity of IR absorption varied from strong to weak. The morphologies of 170 °C, 110 °C and 30 °C showed similar pattern on cooling cycle in Fig. 3(b), especially the morphologies of 110 °C and 30 °C. It showed the intensity of the C=O was almost the same in Fig. 3, indicating stable morphology of PBPs in cooling cycles. It confirms the results obtained with above measurements including the λ, α and the WAXD diagrams results.

We assume that the spatial arrangement of the cholesteric monomer, M1, with high chirality and a bulky group together with M2 containing polarized group, cyanophenyl, offers possibility for twists among segments and molecules of polymers. The van de Waale force, the interaction between the polarized units of the molecules, electric cloud of aromatic ring etc, all contribute to the formation of the three dimensional arrangement of the blue phases under broad temperature ranges. Meanwhile, the flexible -Si-O-main chain moves easily and this affects the side chain or the whole molecules, leading to the occurrence of different twines. First, The topological twists of polymer molecules stabilize the helical periodic defect structure of polymers and the helical pitch. It is also one of the main reasons why the lattice parameters of polymers did not change much during the cooling from T_c to lower temperature. Comparing with topological twists, the physical condensed twists occur normally with smaller three dimension scale. The polymer twists in the system could be accumulated and formed by partially parallel segments of several units of two or three of different molecules chains close to each other, The density of physical twines is more than topological, because the distance of two points is only several ten monomer units, and the interactive physical twines depend on the structure of polymer mainly. The force of condensed twist is small and easy to loose, only influent the physical properties under T_g, but the topological twines influent relatively big scale movement of molecule, the whole macromolecular twist gradually loosen approaching to T_c. As the consequence of all factors, the PBPs are stabilized and the independent existence again proves PBPs are the thermodynamically stable status. However, the accumulation on the study of PBPs is such less that the understanding of PBPs is still superficial and many phenomena could not yet be explained. Probably, it is why the beautiful blue phases are so attractive.

 

Fig. 3. Infrared images and spectra of polymer P4. (a) Spectra of P4 at at different temperature on the first cooling rate 1 °C/min. The C=O stretching absorbance intensity of sample changed slightly on first cooling from isotropic state to RT and all the morphologies showed similar shape on cooling cycle. The intensities of peaks in the 170 °C, 110 °C and 30 °C were same, meanwhile the locations of the peaks were 1712.5 cm^-1, 1711.9 cm^-1 and 1710.9 cm-1 corresponding the above temperatures. (b) Infrared images (3D Surface Projection) of P4 at different temperature on the first cooling rate 1 °C/min. From the comparison of spectra at different temperature, the differences in absorbance intensities in the C=O stretching modes were used as probes for FT-IR imaging contrasts. This experiment was carried out using PE, Spectrum spotlight 300 FT-IR imaging system

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4. Conclusion

The optical properties of polymer liquid crystal cell exhibiting PBPs have been determined using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray measurements, ultraviolet-visible spectrophotometry, FTIR imaging and optical rotation technique. PBPs are thermodynamically stabile mesophases, which appear in chiral systems between isotropic and liquid crystal phases. A series of cyclosiloxane-based cholesteric LC polymers were synthesized from a cholesteric LC monomer and a nematic LC monomer, some of the polymers exhibit wide blue phases. The unique property based on their structure and different twists formed and expect to open up new photonic application and enrich polymer blue phase contents and theory.