Helical polymerisation using a polymer network derived from a blue phase as a template for chirality transfer

ABSTRACT We prepared a blue phase (BP) liquid crystal mixture consisting of a T-shaped nematic liquid crystal, a chiral dopant, a reacting monomer mixture and a photo-initiator. It exhibited a phase sequence of Iso-BPIII-cubic BP-N* on cooling. We produced the polymer stabilised cubic BP (PS-cubic BP) and BPIII (PS-BPIII). After washing low-molar-mass materials from each PS-BP, we obtained the corresponding polymer networks. Marked difference exhibited in a three-dimensional structure between the BPIII and cubic BP polymer networks. Although they were prepared by the photopolymerisation of achiral monomers, they showed optical activities. Furthermore, they proceed the helical polymerisation of achiral reactive monomers to produce a chiral polymer film. The optical handedness of the polymer film is corresponded to that of the original BP. We discuss how the chirality is transferred from each BP to the corresponding polymer film. Graphical Abstract


Introduction
Recently, some chiral polymers have been prepared from achiral components or racemates [1][2][3][4][5][6][7][8].How does the chirality appear in a higher ordered structure consisting of achiral components?These mechanisms are significant for producing chiral materials without a chiral component [9,10].The formation of chiral architectures from achiral small molecules has attracted a significant amount of attention [1,2,[11][12][13][14].When an achiral molecule has specific stereogenic centres, the self-assembled molecules can show one particular handedness.Chiral amplification by 'Sergeants and Soldiers effects' produces a single helical structure in the polymer from achiral monomers [4].Helix-sense-sensitive polymerisation proposed by Yashima et al. has been applied to the synthesis of chiral polymers composed of desired monomer units [5].Furthermore, chiral polymers have been obtained from achiral units under the influence of noncovalent interactions [1,6,8], and helical polyacetylene was synthesised in chiral nematic liquid crystals [15].It should be noted that distinct chiral sources, i.e. molecular chirality of a catalyst, polarised light and macroscopically helical ordering, are necessary in almost all cases.Recently, top-bottom chirality transfer has been recognised for its importance [16].Sevim et al. demonstrated the top-bottom chirality transfer from the chiral shape of a three-dimensional helical channel to the chirality of supramolecular aggregates of achiral molecules [17].Araoka and Choi et al. reported the transfer of chirality from inverse helical nanofilament networks to filling achiral nematic LCs [18].
We have designed the chirality transfer system using a blue phase (BP) as a three-dimensional template.BPs have a frustrated structure which is stabilised by the chirality-dependent defects [19,20].They are classified into three categories depending on the cylinder packing structure: blue phase I (BPI), blue phase II (BPII), and blue phase III (BPIII).BPI and BPII have a cubic structure, while BPIII has an amorphous one.Kikuchi et al. reported that specific polymer networks can stabilise the lattice defects of BPI [21].Castles et al. demonstrated the fabrication of self-assembled three-dimensional nanostructures by polymer templating blue phase I [22].Such polymer networks are thought to reflect defect structures in the original BP.We report here that the polymer network derived from a blue phase works a template for the helical polymerisation of achiral monomers to produce a chiral polymer film.We discuss how the chirality is transferred from each BP to the corresponding polymer film.The purpose of this paper is to present that the polymer network consisting of achiral molecules exhibits the super molecular chirality, which is transferred to the polymer film.

Sample preparation
We reported that a chiral mixture consisting of T-shaped nematic liquid crystal I (55 wt%) [23] and a chiral dopant S-811 or R-811 (45 wt%) exhibited BPIII and cubic BP with relatively wide temperature ranges [23,24].Their molecular structures are shown in Figure 1.Some T-shaped compounds have been reported to stabilise BPs [23,[25][26][27][28].The molecular biaxiality plays an important role in stabilising BPs [25,28].Interestingly, T-shaped LC I stabilises not only a BPIII but also cubic BP [23,24].In order to prepare a single mixture exhibiting both BPIII and cubic BP with relatively wide temperatures, we used T-shaped LC I as a host nematic material.We prepared a BP mixture consisting of T-shaped LC I (49.3 wt%), S-811 (40.7 wt%), liquid-crystalline bifunctional reactive monomer 1,4-bis[4-(3-acryloyloxypropoxy)benzoyloxy]-2-methylbenzene (RM257, 7.3 wt%), monofunctional reactive monomer 12-dodecylacrylate (C12A, 2.4 wt%) and 2,2-dimethoxy-2-phenylacetophenone (DMPAP, 0.3 wt%) as a photo-initiator.Figure 2 shows their molecular structures.The polymer stabilisation was performed in the cubic BP and BPIII according to our reported procedure [29].For example, we obtained a polymer network from the cubic BP in the following way.The mixture on a glass plate with a cover glass was irradiated with 365 nm UV light at a power density of 1.0 mW•cm −2 for 1 min at 56.0°C in the cubic BP to give the PS-cubic BP.Then, the sample was immersed in toluene to remove low-molar-mass materials, i.e. blue-phase liquid crystals and unreacted monomers, until those materials were not detected by TLC analysis of the washing solution.The results of the TLC analysis are shown in Figure S1.It should be noted that TLC analysis is not sensitive enough to proof that all small molecules are removed.The obtained polymer network was then dried overnight at room temperature on the same slide.We also obtained a polymer network derived from the BPIII.We name them as cubic BP and BPIII polymer networks, respectively.
We performed the photopolymerisation of achiral reactive monomers using the cubic BP or the BPIII polymer network as a template in a similar method reported by us [30].We used 4-[(6-acryloyloxy)hexyloxy]-4'-cyanobiphenyl (AHCB, 14.0 wt%), 1,1,1-tris-(acryloyloxymethyl)propane (TMPTA, 81.0 wt%) and a photo-initiator (5.0 wt%).Their molecular structures are shown in Figure 3.The cubic BP network on a glass plate was covered with a glass plate.The cell gap was maintained at 200 µm using spacers.The monomer mixture was put on the cubic BP network.The sample within a sandwich cell was exposed to 365 nm UV light at a power density of 10 mW cm −2 for 60 sec at room temperature.After the photopolymerisation, the sample cell was immersed in chloroform (5 mL) under ultrasonic irradiation at a frequency of 40 kHz for 5 min.The polymer network possessing phenyl benzoate units was solved in chloroform [30].On the other hand, the polymer film possessing TMPTA units was not solved.This process was repeated with fresh chloroform until unreacted monomers and the polymer network were not detected in the washing solution by TLC analysis.The results of the TLC analysis are shown in Figure S2.After removing the glass plates, we took out the polymer film.A quartz glass plate was used for CD measurements instead of a glass plate.

Measurements
Optical textures were observed using a polarising optical microscope (POM, BX-51; Olympus Optical Co. Ltd.) equipped with a temperature control unit (LK-600PM; Japan High Tech Co. Ltd.).Infrared spectra were obtained using an infrared spectroscopy FTS-30 (Bio-Rad Laboratories Inc.).Field-emission scanning electron micrographs were obtained using a field-emission scanning electron microscope (JSM-7000 FE-SEM; JEOL) with accelerating voltages of 10 keV.A sample was coated with platinum before being analysed.Circular dichroism (CD) and UV-vis spectra were obtained using a circular dichroism spectrometer (J-1100DS; JASCO).A sample in a sandwich cell composed of quartz plates was used for CD measurements.The sample thickness was not adjusted.

Polymer network derived from cubic BP or BPIII
We prepared a chiral mixture of T-shaped LC I (55 wt%) and S-or R-811 (45 wt%).It exhibited a phase sequence of Iso-BPIII-cubic BP-N*.The optical textures of the BPIII and cubic BP of the mixture containing S-811 under uncrossed polarisers are shown in Supporting Information (Figure S3).Positive CD responses were observed for the BPIII and cubic BP (see Figure S4).Then we added a mixture of bifunctional monomer RM257, monofunctional monomer C12A and photoinitiator DMPAP to the chiral mixture for obtaining the polymer-stabilised blue phases.Their component ratios were T-shaped LC I (49.3 wt%), S-811 (40.7 wt%), RM257 (7.3 wt%), C12A (2.4 wt%) and DMPAP (0.3 wt %).The phase transition temperatures of the BP mixture on cooling were Iso 65.7°C BPIII 60.7°C cubic BP 51.6°C N*.The polymer stabilisation was performed in the cubic BP according to our reported procedure [28].Thus, the obtained PS-cubic BP showed a cooling phase sequence of Iso 66.3°C BPIII 63.6 cubic BP.The cubic BP did not change at room temperature.After the polymerstabilisation in the cubic BP, the sample was heated to the isotropic liquid, then it was cooled.The Iso -BPIII phase transition was detected with the naked eye, i.e. the sample changed from colourless to slightly blue.The consisted molecules are thought to coexist with the cubic BP polymer network in high temperatures above the PS-cubic BP.Therefore, the Iso and BPIII can appear above the polymer-stabilised cubic BP.We also obtained the PS-BPIII in a similar method described above.The BPIII appeared at 66.2°C and it did not change at room temperature.Figure 4 shows the optical textures of the PScubic BP and PS-BPIII under uncrossed polarisers.With respect to the PS-cubic BP, after rotating one analyser by 10° from crossed polarisers in the clockwise direction and in the anticlockwise direction, the texture colour changed.With respect to the PS-BPIII, by uncrossing the polarisers in opposite directions by the same angle, the darker and brighter textures were exchanged.The texture brightness did not change by rotation of the sample between the polarisers.These results indicate that both the PS-cubic BP and PS-BPIII have optical activities.We also prepared the PS-cubic BP and PS-BPIII using the chiral mixture containing R-811 instead of S-811.The polymerstabilised BP materials were immersed in toluene to remove low-molar-mass materials to give their corresponding transparent and colourless polymer networks.
Figure 5 portrays the FE-SEM images of the cubic BP and BPIII networks.Marked difference can be seen in a 3D structure between them.The pore sizes, defined as diameters, are 170 ± 70 nm and 180 ± 70 nm in the cubic and BPIII networks, respectively.On the other hand, the polymer chain widths are about 80 nm and 130 ± 50 nm in the cubic and BPIII polymer networks, respectively.The BPIII polymer network was found to be thicker than the cubic BP one.BPI and BPII are highly ordered, and their disclination networks form a regular cubic  lattice.By contrast, amorphous BPIII has fluidity with a level of symmetry equal to that of an isotropic liquid.The defect structure of BPIII is thought to be disorder with fluidity.Therefore, many reactive monomers accumulate in defect regions of the BPIII.The polymerisation of the monomers produces the thick polymer chain, which can induce a strong twist in the BPIII polymer network.Figure 6 presents the CD and UV-vis spectra of the cubic BP network.The strong positive CD response appeared at around 305 nm.The polymer network consisting of achiral materials was found to exhibit an optical activity.The optical sense is the same as that of the corresponding cubic BP.Interestingly, some weak signals were detected in the wavelength below 300 nm.
They are thought to be originated from a phenyl benzoate fragment of RM257.However, the corresponding UV peaks could not be detected.It is due to poor sensitivity of the UV measurement system built in a circular dichroism spectrometer (J-1100DS).
Figure 7 shows the CD and UV-vis spectra of the BPIII polymer network derived from the BP mixture containing S-811 or R-811.Both networks exhibited a strong CD signal at about 305 nm and their optical senses were opposite.The CD magnitude is different between them due to their different sample thickness.The strong signal is thought to reflect the supermolecular chirality of the polymer network.Their senses are the same as those of the corresponding BPIII.Interestingly, several weak but clear signals can be seen in the wavelength region below 300 nm.They are mirror images.Those peaks are thought to be originated from a phenyl benzoate fragment of RM257. Figure 8 shows the CD and UV-vis spectra of the thin sample derived from the BPIII mixture containing R-811.No strong peak at 305 nm was detected, suggesting that the polymer film is not fully produced at this step.Two peaks, 290 nm with negative sense and 272 nm with positive sense, can be seen.They are characteristic bisignate signatures.Although the signal-to-noise ratio is weak, two peaks, about 287 nm and 220 nm, were slightly detected in the UV-vis spectrum (Figure 8(b)).Although we could not evaluate the sample thickness quantitatively, the intensity of the CD signal at 305 nm, which is thought to be attributed to the supermolecular chirality of the BPIII polymer network, depends on the sample thickness.The qualitative sample-thickness dependent CD spectra of the (+)-BPIII networks are shown in Figure S5.As the thickness increases, the signal at about 290 nm is overlapped with the strong signal at 305 nm.We surmise that the bisignate appearance of the CD signals results from the Cotton effect originated from excitonic interactions at a molecular level.Although the comparison of the experimental results and their simulations is not performed, the CD measurements suggest that the molecular twist of the mesogenic core of bifunctional monomer RM257 occurs.Such weak signals were also detected in the cubic BP network (Figure 6(a)).Interestingly, the CD peaks below 300 nm of the BPIII network are markedly stronger than those of cubic BP one.This suggests that the BPIII network has a stronger molecular twist structure than the cubic BP one, as discussed about the SEM images of both networks.

Polymer films prepared from the photopolymerisation of achiral monomers using the BP polymer network as a template
We performed the photopolymerisation of achiral reactive monomers with the cubic BP polymer network as a template.We used a mixture of 4-[(6-acryloyloxy)hexyloxy]-4'-cyanobiphenyl (AHCB, 14 wt%), 1,1,1-tris-(acryloyloxymethyl)propane (TMPTA, 81 wt%), and a photo-initiator (5.0 wt%).The cyanobiphenyl unit of AHCB is expected to induce a CD signal.The sample cell was exposed to UV irradiation at room temperature.After the photopolymerisation, the cell was immersed in chloroform under ultrasonic irradiation.A transparent and slightly light brown film was obtained as shown in Figure 9.Although we tried to remove the polymer network from the film completely, we could not exclude the possibility that a piece of the network materials was still attached to the film.The polymer film exhibited a sharp peak at 2225 cm −1 in the IR spectrum, which is assigned to the CN stretching mode (Figure S6).The film was confirmed to have a CN unit.We also prepared the   polymer film from AHCB, TMPTA and photo-initiator without using a polymer network as a reference.The film also showed a light brown (Figure S7).Therefore, the light brown colour of the film with the BP polymer network is attributed to the photopolymerisation of the materials.It does not result from the polymer network.Although we cannot identify the origin of the colour, we can say that it is not due to the helicity.
Figure 10 presents the CD and UV-vis spectra of the film prepared by using the (+)-or (-)-cubic BP network.We name a cubic BP polymer network with a positive sense and that with a negative sense as (+)-cubic and (-)-cubic networks, respectively.The CD spectra reveal that the films prepared from achiral molecules have an optical activity and the sense is the same as that of the corresponding network.Such as weak signals below 300 nm observed for their networks were not detected.The CD activities are not originated from linear dichroism (LD).The LD spectrum of the film with the (-)-cubic BP network is shown in Figure S8.For comparison, the polymerisation of the monomer mixture (95 wt%) and R-811 (5 wt%) without using the network was carried out.The obtained film did not show any CD activity at all (Figure S9).The chiral compound itself was found not to induce the helicity in the film.
We have investigated the microstructures of the polymer film using a FE-SEM.Figure 11(a) shows the FE-SEM images of the microstructure at the air/polymer film Figure 10.(Colour online) CD and UV-vis spectra of the films prepared using the cubic BP networks with an opposite handedness.Blue shows the film prepared using the (+)-cubic BP network and red shows that prepared using the (-)-cubic BP one.
interface prepared using the (+)-cubic BP network.A porous structure can be seen inside of the film.Figure 11(b) shows the FE-SEM image of the crosssection area.It seems to be a granular-like structure.The microporous structure is thought to be originated from the cubic BP network structure.
We performed the photopolymerisation of the achiral reactive monomers with the BPIII network as a template.Figure 12 shows the CD spectra of the (-)-BPIII network and the obtained polymer film.The polymer film exhibited a negative CD signal with strong intensity at around 350 nm.If the biphenyl unit of AHCB formed a twisted conformation, weak bisignate CD signals are expected to be seen.However, it did not show such bisignate CD signals as observed for the network.We observed the microstructures of the polymer film by FE-SEM measurements.The results are shown in Figure 13.The interface structure is relatively smooth (Figure 13 (a)).Crater-like structures can be seen for the cross-section area (Figure 13(b)).On the other hand, we did not detect such a microporous structure observed for the film with the (-)-cubic BP network.

How does the chirality transfer occur?
Let us discuss why BP networks exhibit optical activities.The CD measurements of the BPIII network suggest that the mesogenic core of achiral monomer RM257 occurs to twist, as discussed in 3.1.We can say that the chiral field originated from the twisting power in the blue phases induces axial chirality of the mesogenic core of RM257.This phenomenon can be classified as the top-bottom chirality transfer [16,17].As the photopolymerisation progreses, the induced chirality propagates to form a homochiral polymer network.It produces the supermolecular chirality in those BP networks.The schematic image of the formation of the BPIII network is in Figure 14.We surmise that the formation of the cubic BP network has a similar process as that of the BPIII one.
We discuss how the chirality is transferred from a BP network to a polymer film.The polymer films with cubic BP or BPIII network showed a CD signal with a strong intensity at around 350-360 nm.The optical sense of each film was the same as that of the corresponding network.However, no other CD signal reflected to molecular twist was detected.Therefore, the polymer films exhibit only a macroscopic chirality.According to the SEM images of both networks (Figure 5), they have a porous structure and their pore sizes are about 100-200 nm.It seems to be possible that the reactive monomers gather in the pores, and they are photopolymerisation along the pores.We surmise that the supermolecular chirality of the BP network is transferred to the corresponding polymer film during the photopolymerisation without inducing a microscopic molecular twist.

Conclusion
We prepared the polymer network derived from the cubic BP or the BPIII of the chiral mixture containing T-shaped LCI and a chiral dopant.Both polymer networks, which were prepared from achiral monomers, have nano porous structures exhibiting optical activities.Their optical senses are the same as those of the corresponding BPs.The CD measurements of those BP networks give a possible explanation for the chirality transfer as follows.We used the blue phases containing a chiral compound S-811 or R-811 as the chiral sources.The chiral field in the blue phases induces the twist conformation of the mesogenic unit of bifunctional achiral monomer RM257.As the photopolymerisation progreses, thus induced chirality propagates to form the homochiral polymer network.Then, we prepared the polymer films by the photopolymerisation of achiral monomers using the BP polymer networks as a template.The obtained polymer films exhibited optical activities.The optical  sense was the same as the corresponding network.We demonstrate that the polymer network obtained from achiral monomers exhibits the supermolecular chirality, which is transferred to the polymer film.

Figure 1 .
Figure 1.Molecular structures and their phase transition temperatures of T-shaped LC I and S-811 [23].

Figure 4 .Figure 5 .
Figure 4. (Colour online) (a) Optical textures of the PS-cubic BP and (b) those of the PS-BPIII.under uncrossed polarizers at room temperature.Those PS-BP phases were obtained from the blue phase mixture containing S-811.

Figure 6 .
Figure 6.(Colour online) (a) CD and (b) UV-vis spectra of the polymer network derived from the PS-cubic BP containing S-811.An expansion of the CD spectra below 300 nm is inserted in (a).

Figure 7 .
Figure 7. (Colour online) CD and UV-vis spectra of the BPIII polymer network derived from the BPIII mixture containing S-811 (blue) or R-811 (red).

Figure 9 .
Figure 9. (Colour online) Photograph of the polymer film in a sandwich cell.

Figure 11 .
Figure 11.(Colour online) (a) FE-SEM image of the microstructure at the air/polymer film interface and its expansions.(b) That of the cross-section area near the interface.The polymer film was prepared using the (+)-cubic BP network.

Figure 12 .
Figure 12. (Colour online) CD spectrum of the (-)-BPIII network (red) and that of the obtained polymer film (green).

Figure 13 .
Figure 13.(a) FE-SEM image of the microstructure ar the air/polymer interface and (b) that of the microstructure of the cross-section area near the interface.The polymer film was prepared using the (-)-BPIII network.

Figure 14 .
Figure 14.Schematic image of the formation of the BPIII network.