Alignment Control of Nematic Liquid Crystal using Gold Nanoparticles

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Applications of Polymer, Composite, and Coating Materials

Alignment Control of Nematic Liquid Crystal by Gold Nanoparticles Grafted Liquid Crystalline Polymer with Azobenzene Mesogens as Side Chain Ze-Yang Kuang, Yao-Jian Fan, Lei Tao, Ming-Li Li, Nie Zhao, Ping Wang, Er-Qiang Chen, Fan Fan, and He-Lou Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07483 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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ACS Applied Materials & Interfaces

Alignment Control of Nematic Liquid Crystal by Gold Nanoparticles Grafted Liquid Crystalline Polymer with Azobenzene Mesogens as Side Chain Ze-Yang Kuang a, Yao-Jian Fan a, Lei Tao a, Ming-Li Li a, Nie Zhao b*, Ping Wang a, Er-Qiang Chen c, Fan Fan d, He-Lou Xie a* a

Key Laboratory of Advanced Functional Polymer Materials of Colleges and

Universities of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China b

College of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, Hunan Province, China

c

Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China d

Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China E-mail:[email protected] (HLX) and [email protected]

Abstract: The gold nanoparticles highly grafted by liquid crystalline polymer (LCP) with azobenzene mesogens as side chain (denoted as Au@TE-PAzo NPs) are successfully designed and synthesized by the two-phase Brust-Schiffrin method. The chemical structures of the monomer and polymer ligands have been confirmed by NMR, and the molecular weight of the polymer is determined by gel permeation chromatography (GPC). The combined analysis of transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) shows the size of the nanoparticles is 2.5(±0.4) nm and the content of the gold in the Au@TE-PAzo NPs is ca.17.58%. The resultant Au@TE-PAzo NPs can well disperse in the nematic liquid crystal (NLC) of 1

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5CB. The well-dispersed mixture with appropriate doping concentrations can automatically form a perfect homeotropic alignment in LC cell. The homeotropic alignment is attributed to the brush formed by Au@TE-PAzo NPs on the substrate, wherein the Au@TE-PAzo NPs gradually diffuse onto the substrate from the mixture. On the contrary, the pure side chain LCPs cannot yield vertical alignment of 5CB, which indicates that the alignment of 5CB is ascribed to the synergistic interaction of the nanoparticles and the grafted LCPs. Moreover, Au@TE-PAzo NPs show excellent film-forming property on account of their periphery of high densely grafted LCPs, which can form uniform thin film by spin-coating. The resultant thin film also can prompt the automatical vertical alignment of the nematic 5CB. Further, upon alternative irradiation of UV and visible light the alignments of 5CB reversibly switch between vertical and random orientation due to the trans-cis photoisomerization of azobenzene group on the periphery of Au@TE-PAzo NPs. These experimental results suggest that this kind of nanoparticles can be potentially applied in constructing the remote-controllable optical devices. Keywords: gold nanoparticals, azobenzene mesogens, photoresponse, liquid crystal, alignment control

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Introduction Alignment of liquid crystal (LC) molecules along a preferred direction plays a crucial role in the fabrication of the high-performance devices, such as liquid crystal displays (LCDs), modulators, light shutters, and sensors.1-6 Thus, controlling LC alignment via tuning the surface environments becomes particularly important in both academic field and engineering application. The most conventional method is mechanical rubbing on the substrate coated with thin organic polymer film.7-10 Although the mechanical rubbing method is simple and robust, the obvious disadvantage is that mechanical frication can easily generate dust particles, mechanical damage, and electrostatic damage, which seriously limits the further application. In order to overcome these disadvantages, many novel functional materials and alternative free contact methods have been intensively developed. Photoinduced LC alignment is a typical contact-free method,11-18 which can be achieved

through

several

approaches,

such

as

photochemical

reversible

isomerization,19, 20 [2+2]-cycloaddition of dimerization reaction (or crosslinking),21, 22 and selective degradation of polymers by the irradiation of linearly polarized UV light.23,

24

Utilizing the cis-trans isomerization of molecules such as azobenzene

units25-28 to realize the LC orientation is the most commonly used method. The typical representative is the photoinduced isomerizations of azobenzene as “command surface” to trigger reversible orientations of LC molecules.29-31 For instance, Seki et al32 used Langmuir-Blodgett (LB) film fabricated by the polymers with azobenzene 3



mesogens as the side chain to control the LC alignment upon alternative irradiating UV and visible light. This method not only overcomes the previous disadvantages but also realizes more complex patterns. Compared with the photoinduced LC alignment method, the self-assembled monolayers (SAMs) as the reference surface do not require the special isomerization molecules or instrument. For example, the organic thiol molecules immobilized on gold surfaces can induce the nematic liquid crystal (NLC) alignment along a specific direction.33 It is noting that this method can be easily implemented in different surfaces and achieve many different alignment scenarios of NLC. Meantime, controlling the chemical structure, the molecular chain length and the deposited direction of the anchoring molecules can effectively adjust the LC directed orientation.34, 35 Different from previous command surfaces, the macroscopic LC orientation by nanoparticles only requires doping the nanoparticles in LC molecules without any surface treatment. The commonly used nanoparticles are oligomeric silsesquioxanes (POSS)36-38, metal nanoparticles39, fullerenes40, and carbon nanotubes41 and so on. The critical shortcoming for the undecorated nanoparticles is the poor solubility with LC molecules, which can lead to strongly diffuse scattering and even worse the deposition of nanoparticles from the mixture. Thus, to improve the solubility, the modification of nanaoparticles becomes a requisite approach. The most common method is using alkyl or mesogens to modify the nanoparticles. As the representative, gold nanoparticles modified by alkyl have been widely used to induce the alignment of the NLC via the doping method.42, 43 The LC alignment behavior is significantly 4


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dependent on the concentration of the suspended alkyl thiol-capped gold nanoparticles in the NLC host.44 It is worth mentioning that the gold nanoparticles grafted by photoresponsive azobenzene show the alignment behavior different from the ordinary nanoparticles. The initial LC molecules doped with the gold nanoparticles show the planar alignment, but UV irradiation results in vertically alignment.45 Other metal nanoparticles, such as CuS nanoparticles46, Nickel nanoparticles47-49 are also employed to direct the LC alignment. The semiconducting SiO2 nanoparticles with the adjustment of the refractive index of LC host can be exploited to develop hybrid LC nanocomposites with promoted photoelectric properties. Functionalized SiO2 nanoparticles doped with LC nanocomposites are able to dynamically self-assemble into a helical configuration and exhibit multi-stability, namely, homeotropic (transparent), focal conic (opaque), and planar states (partially transparent), hinging on the frequency applied at stabilized low-voltage50. However, for the general nanoparticles the poor solubility and film-forming property limit their further applications. In this work, we designed a kind of gold nanoparticles with good solubility and film-forming ability. This uniform-sized gold nanoparticles were grafted by liquid crystalline polymer (LCP) with azobenzene mesogens as side chain (denoted as Au@TE-PAzo NPs) (see Figure1). The chemical structures of the monomer and polymer ligands have been confirmed by NMR, the average core diameter of the Au@TE-PAzo NPs and the weight fraction of Au in Au@TE-PAzo NPs were determined by transmissionelectronmicroscope (TEM) and thermogravimetric 5



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analysis (TGA), respectively. Polarizing optical microscope (POM) experiment results revealed that the 5CB doping with Au@TE-PAzo NPs at the appropriate concentration range could form homeotropic alignment. Further, the LC cells with substrate coated by the Au@TE-PAzo NPs thin film by spin-coating also could form vertical alignment. Due to the trans-cis isomerization of azobenzene, both 5CB in mixtures and thin film coated cells could be reversibly switched between vertical and random alignment upon the alternative visible and UV irradiation.

Figure 1. Schematic diagram of Au@TE-PAzo NPs and the chemical structure of ligands (TE-PAzo).

Experimental Section Materials.

Hydrogen

tetrachloroaurate(iii)

trihydrate,

tetra-n-octylammonium

bromide (TOAB), 4-cyano-4'-pentylbiphenyl (5CB), 4-butylaniline(97%), ethylene glycol,

2-bromo-2-methylpropionyl

bromide,

1,6-dibromo-hexane(98%)

were

obtained from Energy Chemical and directed used without any treatment. Methylbenzene (Aldrich, 99%) was purified according to the previous works.51 And other solutions and reagents (analytical pure) were directly used.

6


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Instruments and Measurements. Bruker ARX400 spectrometer were used to carried out 1H NMR experiments, where tetramethylsilane (TMS) and deuterated chloroform (CDCl3) were the internal standard and the solvent, respectively. The apparent number average molecular weight (Mn) was determined by GPC (Waters-GPC1515), where the eluent was THF. In order to calibrate the standard curve, the linear polystyrene as standard was used. TEM experiment was conducted using a JEM-2100F instrument operating with 100 kV accelerating voltage. Dilute solution of the Au@TE-PAzo NPs in chloroform was deposited in copper grid coated with carbon. Excess solvent was absorbed by a small piece of filter paper. The grid was then air-dried at room temperature. TA SDT Q600 instrument was used to carried out TGA under nitrogen atmosphere at a heating rate of 20 ºC/min. TA DSC Q10 calorimeter were used to perform differential scanning calorimetry (DSC) experiment on a with a programmed heating procedure in nitrogen. POM and conoscopic images were taken under a Leica DM-LM-P polarizing optical microscope with a Mettler FP82HT heating stage. Contact angle measurements were implemented with Powereach JC2000D1 dynamic contact angle measuring instrument. NanoScope III multimode SPM were used to perform atomic force microscrope (AFM) experiment, wherein tapping mode were adopted under ambient air using single-crystal silicon cantilevers. NanoScope software was used to evaluate data. Cary 100 was used to perform the UV-Vis absorption spectra with the Flashing xenon lamp as the light source. Grazing-incidence small-angle X-ray scattering (GISAXS) was 7



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performed at MAR165 at Shanghai Synchrotron Radiation Facility (SSRF). An incident X-ray beam of 10 keV (λ = 0.154 nm) were used under vacuum,. The GISAXS spectrum were the sum of 10 one-second exposures.

Synthesis.

The

phenoxy}hexyl

synthetic

route

methacrylate

of

monomer

(MAzo)

and

6-{4-[(4-butylphenyl)diazenyl]

the

initiator

2-hydroxyethyl

2-bromoisobutyrate (HEBI) is shown in Scheme S1, the relevant synthetic data are presented in Supporting Information, and the 1H NMR spectra of MAzo and HEBI are shown in Figure S1 and Figure S2, respectively. The synthetic route of the polymer ligands (TE-PAzo) and the gold nanoparticles (Au@TE-PAzo NPs) is shown in Scheme 1. The detailed synthesis information is described as follows.

Scheme 1. The synthetic route of the polymer ligands (TE-PAzo) and the gold nanoparticles (Au@TE-PAzo NPs).

Synthesis of ligands precursor (HO-PAzo). The ligands precursor of HO-PAzo was synthesized by atom transfer radical polymerization (ATRP). MAzo (5.0g, 8


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11.8mmol), PMDETA (0.24mmol), CuBr (0.24mmol), HEBI (0.24mmol) and chlorobenzene (7.5g) were added to a reaction tube. In order to eliminate the oxygen, the tube was sealed under vacuum after five-times freezing-thaw cycles. Then the reaction tube was placed in magnetic stirring apparatus with silicone oil at 80 oC for 8 hours. The polymerization was quenched with ice-water mixture, then added a large amount of THF solvent. The mixed solution was passed through a basic alumina column and precipitated into a large excess of methanol. Further the precipitation process was repeated several times, until the peak of the monomer was not observed by GPC. The resultant product was dried under vacuum at 35oC for 48 h. Yield: 4.15g, 83%. Synthesis of polymer ligands (TE-PAzo). 3.0 g (0.2mmol) of HO-PAzo polymer, 0.618 g (3mmol) of thioctic acid, 37 mg (0.3mmol) of DMAP (4-(dimethylamino) pyridine), and 70mg (0.6 mmol) of TEA (anhydrous triethylamine) were dissolved in 200 mL of anhydrous tetrahydrofuran. Then, the mixture was cooled in an ice bath, and 62 mg (0.3 mmol) of DCC (1,3-dicyclohexyl carbodiimide) was added. After stirring 3 h at 0 °C, the mixture was stired at room temperature for another 24 h. The precipitate was removed by filtration, and the filtrate was dried in vacuum. The crude product was redissolved in a small amount of THF and precipitated in methanol. The resultant product was dried under vacuum at 35oC for 48h.Yield: 2.41g, 80%. Synthesis of Au@TE-PAzo NPs. An aqueous solution of HAuCl4·3H2O (1.0mL, 30 mM) and the toluene solution of the tetra-n-octylammonium bromide 9



(TOAB, 4mL, 50 mM) were mixed. Stir the two-phase mixture vigorously until all the tetrachloroaurate was transferred into the organic layer. After removing the water layer, the polymer ligands (TE-PAzo, 225mg, half equiv to HAuCl4) were added to the organic phase, and then an aqueous solution of NaBH4 (10 equiv to HAuCl4) was slowly added with vigorous stirring. The mixture was allowed to stir for another 3h. The organic phase was separated, and precipitated in a large excess of ethanol. The mixture stood for a while. The final product was dried under vacuum at 35oC for 48h. Yield: 125 mg, 54%.

Result and Discussion Synthesis and Characterization of the Au@TE-PAzo NPs. The monomer used to prepare the polymer ligands was first synthesized according to Scheme S1 and its chemical structure was confirmed by the combined techniques as shown in Supporting Information. Then the resultant monomer was polymerized using HEBI as initiator by ATRP. GPC results revealed that the molecular weight (Mn) and polydispersity index (PDI) were 15000g/mol and 1.15, respectively. The resultant polymer was further reacted with thioctic acid. As shown in Figure S3, the peak at 3.11-3.17 ppm reveals the polymer ligands with thiol group are successfully synthesized. Finally, the polymer ligands were used to synthesize the gold nanoparticles. As shown in Figure 2a, the TEM image indicates that the homogeneous solutions without any aggregation and the average diameter of the Au@TE-PAzo NPs is approximately 2.5(±0.4) nm. The TGA result (Figure 2b) shows the weight loss 10


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ratio at 800°C is 82.42%, implying that the weight fraction of Au in Au@TE-PAzo NPs to be 17.58%. Based on the results of the average diameter and the weight fraction, we could estimate the average number of Au atoms per particle is 482, and the total number of polymer ligands per particle is 32 according to the previous calculation method.52 DSC and POM results reveal that TE-PAzo and Au@TE-PAzo show obvious LC behavior as shown in Figure S4 and Figure S5.

(a) (b) Figure 2. TEM image (a) and TGA curve (b) of Au@TE-PAzo NPs. inset: image of chloroform solution of Au@TE-PAzo NPs.

Alignment behavior of 5CB doping with the Au@TE-PAzo NPs. At the concentration range of 0.01 ~ 1 wt%, the Au@TE-PAzo NPs show good dissolubility in mixtures with 5CB, but further increasing the concentration of Au@TE-PAzo NPs (>1 wt%) leads to the uneven distribution as shown in Figure S6. The Au@TE-PAzo NPs/5CB mixtures could be filled into the hybrid cells by capillary force above the Tiso/N temperature (The hybrid cells were fabricated with quartz plate as top surface and silicon wafer as bottom surface. The cell gap was 2.5µm maintained by mylar film). After slowly cooling (~ 1 °C min–1) to the N-phase, the opitcal properties of the mixture of Au@TE-PAzo NPs and 5CB were examined. Figure 3 shows the images of the Au@TE-PAzoNPs/5CB mixtures under POM at 28 °C. The mixture with the 11



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concentration of Au@TE-PAzo NPs at 0.01wt% shows a bright field under orthoscopic POM (see Figure 3a), indicating the random orientation of LC domains. Other ones with higher concentration (at 0.05 wt%, 0.1 wt% and 1 wt%) present the dark field as shown in Figure3b-d. Meantime, the typical black crosses can be observed under the conoscopic POM (the insets in Figure3b-d), indicating that 5CB maintains vertical alignment. Furthermore, the higher concentration mixture with the uneven distribution shows bright field again (see Figure 3e), which is ascribed to the Au@TE-PAzo NPs aggregation. Obviously, the NLC sequentially experienced optical ON, optical OFF, and optical ON states with increasing the doping concentration, corresponding to the initially random alignment, homeotropic alignment and finally back to random alignment, respectively.

(a)

(b)

(c)

(d) (e) Figure 3. POM images of LC cells using different concentration of Au@TE-PAzo NPs doped in 5CB host at: (a) 0.01 wt%, (b) 0.05 wt%, (c) 0.1 wt%, (d) 1 wt%, (e) 5 wt%. insets: conoscopic images.

In

our

previous

works,

we found

that the

LCP

12


brush

fabricated

by

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hydroxyl-terminated

poly(6-(4-methoxy-azobenzene-4’-oxy)hexyl

methacrylate)

(PMMAZO) also could induce the homeotropic alignment of LC due to the anchoring effect.53 In this work, the polymer TE-PAzo with similar chemical structure as PMMAZO was used as the ligands of the nanoparticles. For comparison, the alignment behavior of TE-PAzo/5CB mixtures at different doping concentration was first investigated. As shown in Figure S7, TE-PAzo shows very good compatibility with 5CB. However, the typical LC birefringence phenomenon is observed at any doping concentration of TE-PAzo under POM (see Figure S8), meaning that the ligands of TE-PAzo without the gold nanoparticle core fail to induce the vertical alignment of 5CB. Clearly, the gold core in the Au@TE-PAzo NPs plays a crucial role in the LC vertical alignment. The interaction between the gold core and the hydrophilic substrate led the Au@TE-PAzo NPs to deposit at the substrate and then the nanoparticles further induced the LC molecules alignment. On the other hand, the textures of bright stripe domains surrounded by dark field were observed in NLC doped with the alkyl thiolate-capped gold nanoparticles.42 As shown in the POM images of Figure 3b-3d, the whole fields are dark and any birefringent stripe defects are not observed, which means that the polymer ligands acted as important constituent participate in the LC alignment. Evidently, this phenomenon was the outcome of polymer ligands and gold core, namely, the LC homeotropic alignment was induced by the synergistic effect of the gold core and the polymer ligands periphery. Meanwhile, note that the relatively low concentration Au@TE-PAzo NPs could not induce the LC vertical alignment, which might be attributed to that the low 13



concentration led to the lack of the Au@TE-PAzo NPs depositing on the substrate. The small amount of Au@TE-PAzo NPs on the substrate could not form uniform polymer brush, which further led to the random NLC arrangement.

Alignment behavior of the spin-coating Au@TE-PAzo films. It is very useful that the Au@TE-PAzo NPs could well dissolve in organic solvents such as chloroform, THF, chlorobenzene, and toluene. And the resultant solution possessed good film-forming property through spin-coating. To investigate the alignment behavior of the Au@TE-PAzo film, the thin films were prepared by spin-coating on the cleaned silicon wafers or quartz slides. The thickness of the film could also be controlled by varying the solution concentration at the same speed of rotation (3000 rpm). The thicknesses of thin film with the concentration of 0.025 wt %, 0.5 wt %, 1 wt %, 2 wt % and 5 wt % were about 1, 8, 17, 28 and 38 nm, respectively. After annealing the films at 80°C，the LC hybrid cells were prepared by silicon wafer and quartz slide coated with Au@TE-PAzo NPs thin film as substrates. Then, the pure 5CB was filled into the hybrid cells by capillary force in the isotropic liquid phase, and then the samples were slowly cooled (~ 1 °C min–1) to the N-phase. As shown in Figure 4a-4b, the filled LC hybrid cell with 1~ 8nm-thickness substrate shows the typical schlieren texture, indicating that the LC molecules are random arranged. However, with the increase of the thin film thickness, as shown in Figure 4c-4e the dark fields are observed, and the black crosses can be observed from the conoscopic experiment, indicating that 5CB maintains vertically alignment. Apparently, the LC 14


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alignment behavior was strongly dependent on the thickness of thin film on the substrate.

(a)

(b)

(c)

(d) (e) Figure 4. POM photographs of 5CB alignment in Au@TE-PAzo NPs brush coated hybrid cells at different spin-coating concentration, (a) 0.025 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 2 wt%, (e) 5 wt%. insets: conoscopic images.

The water contact angle (θw) can be used to evaluate the coverage rate of Au@TE-PAzo NPs on the surface. The value of θw for the bare Si substrate is 28.8° (see Figure S9), and for the thin films of Au@TE-PAzo NPs with 1 and 8 nm are 68° and 80°, but the others with achieving or more than 17 nm thin films almost exceed 90° (Table 1). The higher θw meant the sufficient coverage of nanoparticles. For the very thin film the Au@TE-PAzo NPs could not completely cover the whole substrate and thus the random alignment of 5CB was observed. Once the Au@TE-PAzo NPs on the substrate reached to a certain thickness, the dense film could cover the whole substrate, then the synergistic effect of the gold core and the polymer periphery induced the homeotropic alignment of NLC. This result well agreed with the previous 15



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speculation about the previous mechanism of the Au@TE-PAzo NPs/5CB mixtures. It was noteworthy that contact angles for water (θw) would be changed before and after annealling. After annealing, the θw values were obviously increased, which indicates the change of the orientation of mesogens on the surface. Table 1. The Water Contact Angle (θw) on Au@TE-PAzo NPs brush surface. Concentration(wt%) Contact angle θw(deg)

0.025

0. 5

1

2

5

68

80

98

95

93

We investigated the surface morphology of the thin films by AFM. Figure 5 shows the AFM image of the Au@TE-PAzo NPs thin film coated on Si substrate. The surface of bare glass substrate is fairly smooth (see Figure S9). However, as shown in Figure 5a, the film with 8nm thickness exhibits continuous protrusion layer in each domain, and the average height of protrusions is in the range of 3 nm, with the root mean-square (RMS) surface roughness about 0.71 nm (see Figure 5b). The AFM image of the film with 17 nm thickness shows similar results as shown in Figure S10. Note that increasing the thickness of the thin film hardly influenced the roughness of the Au@TE-PAzo NPs coated surface, the RMS values remained almost unchanged at about above-mentioned value range. Compared with the result reported by Kang,54 LC molecules were induced to introduce vertical orientation by the formation of small island with increasing the surface roughness. Their corresponding values of root mean square (RMS) roughness were 22.8 and 55.4nm, which were at least 20 times larger than our results. Evidently, there were no significant small islands on the Au@TE-PAzo NPs-coated surface at the concentration range of 0.5 to 5 wt%. So, it 16


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further indicated that the physical interaction, i.e., the topography effect of nanoparticles, was not the primary driving force for the homeotropic alignment in our system. Therefore, the increasing water contact angle and the uniform layer with excellent surface roughness indicated that the ligands grafted on gold nanoparticles participated in the vertical alignment of LC.

(a) (b) Figure 5. 3D topographic AFM image (a) and corresponding height profiles (b) of the 0.5 wt % of Au@TE-PAzo NPs film on the bare silicon substrate. inset: water contact angle image of the 0.5wt % of Au@TE-PAzo NPs film.

The photoalignment of Au@TE-PAzo NPs. The photochemical behavior of Au@TE-PAzo NPs film was investigated by UV-Vis spectroscopy. As shown in Figure 6a, the intensity of absorption peak at 350nm gradually decreases under the irradiating of 365nm UV light, and meanwhile the intensity for the peak at 450 nm slightly increases. Under the irradiation of visible light, the two absorption peaks present reversible variation. Similar behaviors of Au@TE-PAzo NPs in a chloroform solution are shown in Figure S11. The variation of the absorption peaks was typical tans-cis transformation of the azobenzen mesogens, which indicated that the gold core

17



did not affect tans-cis transformation of the side chain azobenzen mesogens of the ligands grafted on the gold core.

(a) (b) Figure 6. UV/Vis absorption spectrum of Au@TE-PAzo NPs film: (a) after irradiation with 365 nm UV light and (b) after irradiation with 470 nm Vis light.

The trans–cis configuration of Au@TE-PAzo NPs provided an opportunity to adjust LC molecular alignment through light irradiation. Under UV(365 nm) and visible (470 nm) light irradiation, POM images of the Au@TE-PAzo NPs/5CB mixtures with the concentration from 0.01 to 5wt% are shown in Figure 7. For the LC cell filled 0.01 wt% Au@TE-PAzo NPs/5CB mixtures, the birefringence always sustains under visible and UV irradiation as shown in Figure 7a and Figure 7f, suggesting that in this case Au@TE-PAzo NPs are unable to adjust the LC alignment under different light irradiation. For LC cells filled with Au@TE-PAzo NPs/5CB mixtures with the concentration of Au@TE-PAzo NPs at the range between 0.05 and 1 wt%, the dark and bright area form upon visible and UV irradiation alternatively (see Figure 7b-7d and Figure 7g-7i), indicating that the alignment of 5CB switches 18


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from homeotropic to random orientation. For LC cell filled with 5 wt% Au@TE-PAzo NPs, the field always remains bright no matter upon visible or UV irradiation as shown in Figure 7e and Figure 7j, suggesting that there is not LC alignment transition for the aggregation of Au@TE-PAzo NPs. Clearly, the concentration of Au@TE-PAzo NPs in mixtures was an important factor for the switch of NLC from homeotropic state to random director distribution under irradiating with visible and UV light. Because the transitions under light irradiation came from the trans–cis configuration of the side chain azobenzen mesogens, this result further demonstrated that the polymer ligands participated in the LC alignment. However, the pure ligands doping 5CB (TE-PAzo/5CB mixtures) do not show any photoresponse behavior (Figure S8). This combined results further demonstrated that the synergistic effect of the gold core and the polymer periphery induced the LC alignment rather than any individual component of the polymer ligands or gold core.

(a)

(f)

(b)

(g)

19



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(c)

(h)

(d)

(i)

(j) (e) Figure 7. POM images of the LC test cells with different weight ratios of Au@TE-PAzo NPs /5CBmixtures: (a)(f) 0.01 wt%, (b)(g) 0.05 wt%, (c)(h) 0.1 wt%, (d)(i) 1 wt%, (e)(j) 5 wt%. (a)-(e): upon 470 nm Vis irradiation; (f)-(j): upon 365 nm UV irradiation. insets: conoscopic images. Similarly, the photoresponsive LC alignment behavior on the thin films with different thickness fabricated by Au@TE-PAzo NPs was investigated. As shown in Figure 8, the filled LC hybrid cell with 1 nm substrate always shows the typical schlieren texture under UV and visible light(see Figure 8a and Figure 8f), which implies that the cell cannot adjust the LC alignment for too low coverage rate of nanoparticles in this case. For thin film with 8 nm the thickness, the schlieren texture of 5CB switches to dark under the UV irradiation as shown in Figure 8b and 8g, which implies the alignment of 5CB switches from random to homeotropic orientation. This result was similar to the experimental reported by Li et al.40 We 20


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speculated the transformation was the same mechanism as the previous reported work. However, while the thickness of the film was achieving or more than 17nm, the dark and bright field also alternately form upon visible and UV irradiation(see Figure 8d, 8i, 8e and 8j), suggesting the alignment of 5CB switches from homeotropic to random orientation. Evidently, only the relative thick film could enable the transformation between the homeoptropic and random alignment upon different light irradiation. Because the thick films with sufficient nanoparticles resulted in that their peripheral LCP cover the whole substrate. And the reversible trans–cis configuration transformation led to the change of the space filled 5CB upon visible and UV irradiation, which further yielded photoinduced reversible alignment switch of 5CB.

(a)

(f)

(b)

(g)

(c)

(h)

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(i)

(d)

(j) (e) Figure 8. POM images of 5CB alignment in Au@TE-PAzo NPs brush coated hybrid cells at spin-coating concentration: (a)(f) 0.025 wt%, (b)(g) 0.5 wt%, (c)(h) 1 wt%, (d)(i) 2 wt%, (e)(j) 5 wt%. (a)-(e) upon 470 nm Vis irradiation; (f)-(j) upon 365 nm UV irradiation. insets: conoscopic images. The above-mentioned results indicated that the LC homeotropic alignment was ascribed to the synergistic effect of the gold core and the polymer periphery rather than any individual component of the polymer ligands and gold core. As well known, both the gold core and the treated substrate were hydrophilic, which led to a strong interaction between the substrate and the gold core. Thus, the whole nanoparticles residing on the substrate led to the formation of the analogous polymer brush by the peripheral polymer ligands, wherein azobenzene moieties are homoetropically aligned as show in Figure S12. The schematic illustration is shown in Figure 9. The empty zones between the adjacent azobenzene groups on side chain offered the enough empty spaces for NLC. While LC molecules crawled into the space, the limited area only permitted LC molecules stand up, which led to the first homeotropic alignment. Further upon UV irradiation, the azobenzene mesogens transfered from trans isomer to cis isomer, which yielded smaller space between the side groups. Thus, the NLC 22


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molecules were forced to escape from the space between the side groups and adopt a random alignment. Due to the reversible trans-cis photoisomerization, the alignment process was reversible if the visible light was irradiated again. Although the ligands played a significant role in the homeotropic alignment, the pure ligands could not induce the homeotropic behavior. Different from general LC alignment controlled by nanoparticles, the NLC alignment adjusted by Au@TE-PAzo NPs was strongly dependent on the synergistic effect of the ligands of the nanoparticle and nanoparticle core. This adjustable nematic LC alignment was somewhat similar to LC behavior on the polymer LC brushes, but using LC brush generally required fabricating especial functional group on the brush through complicated chemical reaction and treating processes. Apparently, for Au@TE-PAzo NPs these processes became simple and easy accessibility, which helped to expand their application.

(a)

(b)

5CB cis-Azo trans-Azo gold core Figure 9. Schematic illustrations for the possible photoresponsive mechanism of Au@TE-PAzo NPs.(a) upon Vis irradiation; (b) upon UV irradiation.

Conclusion We have successfully fabricated a kind of Au@TE-PAzo NPs highly grafted LCP with azobenzene mesogens as the side chain by the two-phase Brust-Schiffrin method. 23



The resultant Au@TE-PAzo NPs could well dissolve in 5CB. This kind of mixture could automatically form a perfect homeotropic alignment. Meantime, the nanoparticles showed good solubility in common solvent and excellent film-forming property. The resultant spin-coating film also could make 5CB automatically achieve vertical alignment. Due to the trans-cis photoisomerization of azobenzene group on the nanoparticle periphery, Au@TE-PAzoNPs could make 5CB reversibly switch between vertical and random alignment upon Vis and UV irradiation. On the contrary, only LCPs could not produce vertical alignment of 5CB, indicating that the alignment of 5CB should be ascribed to the synergistic interaction of nanoparticles and the grafted LCPs. These experimental results suggested that this LCP nanoparticles could be potentially applied in constructing the remote-controllable optical devices for flexible LC displays.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21674088, 21374092 and 51503174), the Nature Science Foundation of Hunan Province(2016JJ2127) and Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization, and Hunan graduate scientific research innovation project (CX2017B299).

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For Table of Contents use only

Alignment Control of Nematic Liquid Crystal by Gold Nanoparticles Grafted Liquid Crystalline Polymer with Azobenzene Mesogens as Side Chain Ze-Yang Kuang, Yao-Jian Fan, Lei Tao, Ming-Li Li, Nie Zhao, Ping Wang, Er-Qiang Chen, Fan Fan, He-Lou Xie

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Alignment Control of Nematic Liquid Crystal using Gold Nanoparticles

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