Introduction

Stimuli-responsive materials have been attracted substantial attention for a wide variety of potential applications such as sensors1, drug delivery systems2 and actuators3. Responsiveness to external stimuli, for example heat, light, electrical and magnetic field, vapour and pressure, involves changes of molecular conformation and packing structure of molecules that converts physical signals into optical, electrical, mechanical and thermal signals4.

One of the strategies to create stimuli-responsive materials is the incorporation of liquid crystal (LC) component in the molecular structure. LC is one of the self-organized soft materials originating from weak interactions of van der Waals force and the excluded volume effect5. LC materials spontaneously form organized structures and possess dynamic properties against external stimuli such as shear-stress, electrical and magnetic field. With these advantages, the materials incorporated with well-designed LC moiety into the molecular structure show mechanical-induced phase transition6, 7, and form spontaneous ordered structures with anisotropic functionalities8, 9 and photo- and magnetic field-assisted macroscopic orientations10, 11.

Molecular design of conjugated polymers (CPs) with high stimuli-responsiveness is challenging task. Because CPs possess strong π-π interaction between polymer chains in aggregation state, the film hardly shows responsiveness to external stimuli. To control conformation of the CP backbones, modifications of side chain have been carried out, which provide improvement of solubility, promotion for self-organization with higher order, and suppression of inter-chain interaction between the CP backbones12,13,14. However, it is still challenging to create CP films with high sensitivities to weak external stimuli, such as vapour and low pressure. CPs are attractive systems since they have π-electrons delocalized over the polymer backbone that are origins of electronic, optical and magnetic properties. In this context, “soft CPs” with high sensitivity to the above stimuli is greatly desired for future smart polymers. If such small stimuli are converted to the dynamic conformational change of the CPs, the signals are amplified, resulting in the drastic changes in colour, conductivity and electronic properties.

In this study, based on the recent advancement of supramolecular chemistry, we strategically designed block copolymer, poly(3-((3S)-3,7-dimethyl-octyl)-thiophene)-block-poly(4-octyl phenylisocyanide) (PTh- b -PPI), composed of a CP and a rod-type helical coiled polymer as shown in Fig. 1. Polythiophene is one of CPs and exhibits colour changes in visible range originating from the conformational changes in the polymer backbone between the planar state (purple) and the twisted state (yellow)15. The conformation of polythiophene that has chiral side chain has also been investigated, and it is considered to form helical packing of predominantly planar chains in the aggregation state driven by strong π-π interaction16. Introducing chiral side chain into polythiophene block also allows us to investigate the aggregation process in detail since we can track its process using circular dichroism (CD) spectroscopy. Therefore, we expect that the poly(3-((3S)-3,7-dimethyl-octyl)-thiophene (PTh) block can act as a colour-changing chiral chromophore in visible range by external stimuli. On the other hand, polyphenylisocyanide form rigid rod-like helical backbone. Polyphenylisocyanide with bulky side chains forms a rigid helical backbone not only in the solid state but also in solution. Because of its anisotropic rod-shaped structure, the polyphenylisocyanide segment can serve as a mesogen for liquid crystal superstructure in concentrated solution17. Recently, polyphenylisocyanide can also be used as an alignment medium to measure residual dipolar coupling in NMR studies18 since polyphenylisocyanide has magnetically anisotropic phenyl rings as side chains oriented in the peripheral position. Therefore, we envisioned that the poly(4-octyl phenylisocyanide) (PPI) unique structural motif has potential to be a novel responsive building block in block copolymers, which provides LC-like self-assembling and magnetic-field responsiveness.

Figure 1
figure 1

Molecular design. Synthetic route for PTh- b -PPI through Grignard metathesis (GRIM) reaction in one-pot. dppp = 1,3-Bis(diphenylphosphino)propane. THF = Tetrahydrofuran.

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Recently, Wu and Bielawski first reported the one-pot synthesis of polythiophene-block-polyphenylisocyanide, consisting of poly(3-hexylthiophene) and poly(decyl 4-phenylisocyanide)19. They have reported on a variety of morphologies of polythiophene-block-polyphenylisocyanide derivatives that form nanofibril, micelle and vesicle in a mixture of good and poor solvents and its pH-responsiveness20,21,22,23.

We coincidentally found potentially important phenomena of PTh- b -PPI film: vapour-induced liquid crystallinity and self-recovering mechanochromism. We found out PPI block performs crucial roles in the block copolymer, which are intrisically vapour-induced liquid crystallinity and self-reassembling property. PTh- b -PPI film possesses the softness and flexibility enough to show high sensitivity to solvent vapours and shear stresses.

Results and Discussion

Synthesis and characterisation of PTh-b-PPI

PTh- b -PPI was synthesised according to the previously reported literature using the Grignard metathesis (GRIM) reaction19, 24, 25. First, we prepared Ni-terminated PTh macro-initiator from 2,5-dibromothiophene in a flask, then PPI monomer was added. When the polymerisation ceased, the resultant polymer was washed with a large volume of methanol and collected by filtration. Gel-permeation chromatography (GPC) showed a number average molecular weight M n = 16000 and a polydispersity PDI = 4.5. The PDI value was relatively high because the GRIM reaction usually proceeds in living fashion. The high value of PDI is probably caused by the broad polydispersity nature of Ni-teriminated PTh macroinitiator as reported in the literature26. PTh- b -PPI was thoroughly characterised by NMR, IR, UV, CD, and PL (Figures S1 and S2, Fig. 2a,b,c) and all the basic properties were well in accordance with those previously reported for polythiophene-block-polyphenylisocyanide derivatives19,20,21,22,23.

Figure 2
figure 2

Chiral aggregation process of PTh-b-PPI in solution driven by interaction of PTh blocks. (a) UV-vis absorption spectra, (b) Photoluminescence (PL) spectra (λex = 420 nm) and (c) Circular dichroism (CD) spectra of PTh-b-PPI in CHCl3/CH3OH solution (0.02 mg/ml) in various ratios. In high ratios of methanol, the spectra show film-like aggregation state. (d) Schematic illustration of PTh-b-PPI chiral aggregation in good/poor solvent mixture.

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We first investigated the aggregation formation of PTh- b -PPI in good/poor solvent-mixture. This characterisation gives us the insights of film formation process and the interactions of PTh- b -PPI. In UV-vis spectra (Fig. 2a), PTh- b -PPI in chloroform shows absorption maxima at 255 nm (mainly from the PPI block) and 441 nm (derived from twisted state of the PTh block). Addition of methanol to the polymer solution decreases the absorption peak at 411 nm, and new signals appear near 520, 570, and 620 nm. This large red-shift indicates conformational change of the PTh block into J-aggregation states with planar main-chain conformation. The spectra strongly support formation of PTh- b -PPI aggregation driven by π-π interaction between the PTh blocks26. PL spectra (Fig. 2b) also reveals that the chloroform solution state PTh- b -PPI shows luminescence at 572 nm with a shoulder peak at ~630 nm. The PL signal is derived from twisted PTh blocks. As methanol is increased in the solution the PL intensity decreases, indicating formation of aggregation of PTh blocks. Furthermore, the chiroptical properties of PTh- b -PPI were investigated by CD spectroscopy (Fig. 2c). The PTh- b -PPI in chloroform shows no CD signal, indicating random conformation of PTh- b -PPI. In the chloroform/methanol (40/60 v/v), the CD signals appeared at 605, 570 nm (positive) and 490 nm (negative). Additionally, the wavelength at the cross section from positive to negative (at 523 nm) corresponds to the maximum absorption wavelength of PTh- b -PPI (at ~520 nm), indicating right-handed helical aggregation of PTh chromophores27. In high ratios of methanol, the CD intensity decreases because of the precipitation. These optical measurements suggest that J-aggregation of PTh- b -PPI is driven by π-π interaction of PTh blocks.

Vapour-induced liquid crystallinity of PTh-b-PPI

We prepared PTh- b -PPI film by a drop-casting method from chloroform solution (2.0 mg/mL) onto a quartz substrate. Subsequently, PTh- b -PPI film was exposed to chloroform vapour annealing at room temperature. Very interestingly, the film colour changed from purple to transparent yellow in ~1 min in chroform vapour (Fig. 3a and Supplementary Video 1). More surprisingly, polarising optical microscopy (POM) observation evaluated the texture to be Schlieren structure of nematic liquid crystal phase (Fig. 3b,f). Other solvent vapours, such as tetrahydrofuran and dichloromethane, also induced the vapour-induced liquid crystal for the sample. After the removal of the vapour, the PTh- b -PPI film recovered their colour (yellow to purple) in ~1 sec. This reversible colour change is repeatable.

Figure 3
figure 3

Vapour-induced liquid crystallinity of PTh-b-PPI. (a) Schematic representation of PTh- b -PPI film preparation and vapour exposure. The film shows purple in solid state and yellow in liquid crystal (LC) state (under chloroform vapour). (b) Polarising optical microscopy (POM) image of PTh- b -PPI film in LC state (when exposed to chloroform vapour). (c) Absorption, (d) CD and (e) PL spectra (λex = 420 nm) of PTh- b -PPI film in solid state and LC state. (f) Schematic illustration of reversible phase-transition behaviour between solid state and nematic-like LC state of PTh- b -PPI. PPI blocks serve as mesogens and have orientation along the director.

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To examine the vapour-induced conformation change of PTh- b -PPI films, we performed optical measurements on the solid state (without vapour) and the LC state (with chloroform vapour). As shown in UV-vis spectra (Fig. 3c), PTh- b -PPI in solid state showed absorption peaks ~250 nm (π-π* transition of PPI block), 400 nm (n-π* transition of imine unit), and 527, 569, and 615 nm (PTh block). On the other hand, the LC-state PTh- b -PPI showed blue shift with the characteristic absorption peaks at ~250 nm (π-π* transition of PPI block) and ~427 nm (the sum of n-π* transition of imine unit and PTh block). The large blue shift was due to the reduced effective π-conjugated length, which is probably caused by permeation of the solvent vapour molecules intruded between main chains. In contrast, the absorption maximum of the PPI block at 250 nm did not change even exposed to the solvent vapour, implying that the PPI rigid helical coil was not subjected to intrusion by the vapour, and maintained a stable helical conformation. CD spectroscopy (Fig. 3d) shows PTh- b -PPI in solid state exhibited first-positive and second-negative Cotton effect, while the LC state of PTh- b -PPI showed no CD signals. The disappearance of CD spectra suggests the interruption of chiral interactions between PTh block by chloroform vapour. Additionally, PL spectroscopy (Fig. 3e) shows PTh- b -PPI in solid-state exhibited no signal in the visible region, while PTh- b -PPI in LC state showed photoluminescence at ~610 nm, which is ascribed to PTh block (excitation wavelength: 420 nm). The optical measurements suggest that the molecules in the vapour phase intrude between main chains of PTh- b -PPI, and depress the π-π interaction of PTh and expand the distance between the polymers, resulting in the blue-shift in UV-vis, the disappearance of the CD signal and the relaxation of the aggregation-induced quenching. It should be noted that PTh- b -PPI in LC state has blue-shifted absorption and red-shifted luminescence peaks compared to PTh- b -PPI in solution state (Fig. 2a,b). It implies that PTh chain in LC state is more twisted than in solution state, and the excited energy transfer between PTh chain may occur in the LC state

Magnetic orientation of PTh-b-PPI with exposed to chloroform vapour

To demonstrate the merit of vapour-induced LC properties of PTh- b -PPI film, magnetic orientation of the film exposed to chloroform vapour was carried out (Fig. 4a). As shown in Fig. 4b, an intense magnetic field of 12 Tesla for 4 hours during the vapour-annealing successfully formed unidirectional orientation of PTh- b -PPI films (Fig. 4b). To investigate the unidirectionally oriented polymer structure, we employed linear dichroism (LD) spectroscopy. From the LD spectroscopy, we can determine the orientation of the sample,

$${\rm{LD}}={{\rm{OD}}}_{//}-{{\rm{OD}}}_{\perp }={\mathrm{log}}_{10}({{\rm{I}}}_{\perp }/{{\rm{I}}}_{//})$$

where, OD is optical density, I is intensity of transmitted light in parallel (//) and perpendicular (⊥) direction relative to the magnetic field. Figure 4c shows the negative signal (550–750 nm) derived from the PTh block, and positive peak (300–500 nm) coming from the PPI block. Generally, PTh (conjugated polymer) possesses π-π* electron transition moment along its backbone. That is, the positive signal indicates the PTh main chain is oriented perpendicular to the magnetic field. On the other hand, because the phenyl rings of the PPI helical block extend at the peripheral position of the helix core, the transition moment is perpendicular to the helical cylinder28. Therefore the negative signal means that the phenyl group in the side chain aligns parallel, and the helical axis perpendicular relative to the magnetic field (Fig. 4d). This alignment was realized by vapour-induced LC of PTh- b -PPI and the magnetically anisotropic susceptibility of PPI block.

Figure 4
figure 4

Magnetic orientation of PTh-b-PPI film. (a) Schematic representation of magnetic orientation of PTh- b -PPI film with exposure to chloroform vapour. (b) POM image of magnetically oriented PTh- b -PPI film. (c) Linear dichroism (LD) spectrum of magnetically oriented PTh- b -PPI film. (d) Schematic illustration of magnetically oriented PTh- b -PPI. Here, the parallel direction is along the direction of the magnetic field.

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Discussion on vapour-induced liquid crystallinity of PTh-b-PPI

To investigate the origin of vapour-induced liquid crystallinity of PTh- b -PPI, homopolymers PTh and PPI were synthesised by the GRIM reaction and Ni-catalysed reaction, respectively. We prepared drop-cast film of PTh and PPI from the chloroform solution. Magnetic orientation of the homopolymers PTh and PPI under chloroform vapour was also carried out under chloroform vapour and the orientation was confirmed by the LD spectroscopy. Drop-cast PTh film showed no liquid crystal-like structure (Figure S4) and PTh chains could not be oriented by the magnetic field under chloroform vapour (Figures S5 and S6). This fact implies that PTh has no vapour-induced liquid crystallinity under chloroform vapour and thus does not have large domain size and/or moderate viscosity enough for susceptibility in alignment by magnetic field. On the other hand, drop-cast PPI film shows fan-shaped structure after chloroform vapour-annealing, indicating smectic-like super structure (Figure S4). Furthermore, PPI chains are macroscopically oriented by the magnetic field (Figures S5 and S6). This fact demonstrates PPI itself possesses intrinsically vapour-induced liquid crystallinity. Therefore, vapour-induced liquid crystallinity of PTh- b -PPI film is derived from intrinsically vapour-induced liquid crystallinity of PPI. We assume that the PTh- b -PPI film is very sensitive and it is swelled by the solvent vapour, resulting in low viscosity to form liquid crystal order. That is, the PTh- b -PPI film can form lyotropic LC state with slight amount of vapour molecules. Since lyotropic mesophase are seldom seen in CPs29,30,31, it is new strategy to introduce PPI block into conjugated backbone for realization of CPs with vapour-induced lyotropic LC.

Self-recover mechanochromism of PTh-b-PPI

Additionally, we performed a grinding test for the PTh- b -PPI solid film (Fig. 5a). Interestingly, we found that the film gradually turned to vermillion from purple during mechanical shearing. After the removal of the force, the colour recovered to purple under ambient conditions without any treatment in ~30 sec (Fig. 5b and Supplementary Video 2). In addition, the shear-stressed film showed anisotropic absorption between the parallel and perpendicular shear directions, which was confirmed by visible inspection through polarisers. As shown in the LD spectroscopy (Fig. 5c), the broad positive peak at 450–800 nm is ascribed to the PTh block. Thus the LD spectra indicates parallel orientation of the PTh chains to the shear direction. This implies that the rod shaped PTh block also can function as mesogen in shear-induced orientation. This shear-induced anisotropic orientation could be repeated, indicating a capability for overwriting/rewriting. This self-recovery mechanochromism is a unique phenomenon because many mechanochromism materials require aging treatment such as annealing, fuming and recrystallisation to regain original colour32. The colour change during the shear process probably comes from conformation and aggregation change of the PTh block. Although the detailed mechanism of the phenomenon is unclear, we assume that friction heat of shear stress causes conformation change of PTh block (Figure S7). After removal of the shear stress, the liquid crystalline PPI moiety probably facilitates the rearrangement to the original packing structure.

Figure 5
figure 5

 Self-recover mechanochromism of PTh-b-PPI. (a) Schematic representation of mechanical treatment of PTh- b -PPI films. (b) Photographs of PTh- b -PPI before and after mechanical stress. (c) LD spectrum of mechanically oriented PTh-b-PPI film and schematic illustration of mechanically oriented PTh- b -PPI. Here, the parallel direction is along the grinding direction.

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Conclusions

We demonstrated vapour-induced liquid crystallinity and self-recover mechanochromisms of PTh- b -PPI. We found out new promising properties of rod-type helical coiled polymer PPI block: intrinsically vapour-induced liquid crystallinity and self-reassembling property. Introduction of PPI block offers moderate viscosity enough for PTh- b -PPI film to possess high sensitiveness to solvent vapour and self-recovery property against shear stress at ambient condition. In addition, PPI block in PTh- b -PPI produces the anisotropic magnetic susceptibility for magnetic alignment. As far as we know, PTh- b -PPI is the first material that exhibits vapour-induced liquid crystallinity. PPI helical structural motif has been employed mainly as chiral catalyst33, chiral separation34, chiral recognition35 because PPI can form one-handed helical conformation. Our study will pioneer new functionalities of rod-shaped helical polymers. Furthermore, magnetic orientation for vapour-exposed films provides a simple and convenient method to examine the films, offering a new orientation approach for block copolymers that possess anisotropic magnetic susceptibility units. We believe this block copolymer can be applied in phase-transition-type vapour sensors, pressure sensors, and as a memory medium for shear directions.