The regulation of electro-optical properties and polymer morphology of polymer-dispersed liquid crystal films with silicon nanostructure

ABSTRACT In this work, the thiol modified silica nanostructures (SiO2-SH) were doped into polymer-dispersed liquid crystal (PDLC) films. The SEM was used to observe the morphology of the polymer microstructure. The electro-optical (E-O) property test was determined by a liquid crystal parameter tester. The SEM images suggested that the SiO2-SH nanoparticles can react with acrylate groups to form the polymer matrix. The saturation voltage decreased by half in the sample when the dosage of the SiO2-SH nanoparticles was 7.5 wt%, which may be due to the reduced anchoring strength of the polymer matrix incorporated with silicon nanoparticles resulting from the lower surface energy and the enhanced steric repulsions of LC droplets and polymer matrix. In addition, the preparation conditions like polymerisation temperatures and UV light intensity also effectively regulated the E-O properties and polymer microstructure of the PDLC film with silica nanostructure. Thus, the results showed that the E-O properties and the polymer morphology of PDLC films with silicon nanostructures can be effectively regulated by doping SiO2-SH nanoparticles and regulating the preparation conditions such as the polymerisation temperature and UV light intensity. This work can provide practical guidance for modulating the properties of PDLC films. GRAPHICAL ABSTRACT


Introduction
Polymer-dispersed liquid crystals (PDLCs) are those materials in which liquid crystal (LC) droplets of micro-or nano-sizes are embedded into a polymer matrix sandwiched between two transparent electrodes [1].Because of the anisotropic nature of LC, the PDLC films can exhibit two different optical states, i.e. transparent and opaque states, by the action of electric field [1][2][3][4].In general, the films scatter the light and are opaque due to the randomly dispersed LC droplets in the polymer and the mismatch refractive index between LC and polymer matrix.When under an enough electric field, the LC droplets align to the direction of the electric field and the refractive index are matched, thus the films switch to a transparent state and allow light to pass through [5,6].PDLC is usually fabricated by the phase separation method because of the simple operability and controllable morphology of polymer [7].Among the three main phase separation methods, i.e. thermalinduced phase separation (TIPS), solvent-induced phase separation (SIPS) and polymerisation-induced phase separation (PIPS), the PIPS method especially the ultraviolet (UV) polymerisation induced phase separation with prompt, solvent-free and eco-friendly properties is widely reported in the literatures [8][9][10].In recent decades, the electrically switchable characters allow the PDLC to be used in areas like displays [11], light shutter [12], diffuse film [8,13], solar cells [14,15], smart windows [16], and energy harvesting [17,18].
Although PDLC is commercially available, its electro-optical (E-O) performance, especially high driving voltage, is still a major factor limiting the extensive use of PDLC.Thus, many efforts have been devoted to optimising the E-O properties of PDLC films, such as doping nanomaterials [19][20][21][22][23][24][25][26][27][28], adjusting the morphology of polymer [11,[29][30][31] or regulating dielectric behaviour of LC [3,16,[32][33][34][35].Lu et al. [21] fabricated a PDLC film with low voltage and high contrast ratio by creating a controllable microsphere polymer morphology.The results suggested that the microstructure of the polymer could be controlled by the reaction time and free radical initiator.Nasir et al. [22] reported the influence of photo-initiator concentration on the properties of thiol-acrylate-based PDLC smart windows.They found that the photo-initiator content affected the phase separation process and thus determined the performance of PDLC films.Shi et al. [23] optimised the E-O properties of PDLC film by doping BaTiO 3 nanoparticle-laden nanofibers.The employment of nanomaterials could weaken the interaction between the polymer and LCs.SiO 2 is a type of nanomaterial that has attracted widespread attention because it is easy to prepare, has good light and thermal stability, and chemical inertness.Beroguiaa et al. [24] studied the E-O and thermophysical performances of SiO 2 nanoparticles-doped PDLC lenses and found that doping a small amount of the SiO 2 nanoparticle could achieve good E-O and thermophysical properties.Kim et al. [25] reported the enhancement of the E-O properties of PDLC film via doping SiO 2 nanoparticles.Thus, the employment of SiO 2 inorganic nanomaterials could help to optimise the E-O performance of PDLC film.
In this work, thiol modified silica nanoparticles (SiO 2 -SH) were doped into PDLC film.The effect of the dosage of SiO 2 -SH nanoparticles and the preparation conditions such as preparation temperature and UV light intensity on the electro-optical (E-O) properties and polymer morphology were also studied.The SEM results confirmed that the thiol groups allowed the SiO 2 -SH nanoparticles to act as the crosslinker to form the polymer matrix in the system.The E-O results suggested that doping of SiO 2 -SH nanoparticles can enhance the E-O properties of PDLC films.The E-O properties and morphology of the PDLC films silicon nanostructure can also be regulated by the preparation conditions, such as polymerisation temperatures and UV light intensity.This work can provide practice guidance for regulating the properties of PDLC films.

Synthesis of SiO 2 -SH nanoparticles
For a typical synthesis of SiO 2 -SH nanoparticles by the gel-sol method, 1.0 mL of (3-Mercaptopropyl) tri-methoxysilane (MPTS) was added to 50 mL deionised water and the mixture was then stirred at 500 rpm until a clear transparent syrup was formed.The reaction occurs when ammonia was added to the solution, keeping the pH at 11. Subsequently, the solution was mechanically stirred at room temperature for 12 h.Finally, the resulting white precipitates were harvested and washed three times with deionised water and three times with ethanol.Finally, white power was obtained after drying with a vacuum at 60°C for 24 h.

Preparation of PDLC films
Five samples with different content of SiO 2 -SH nanoparticles were prepared by PIPS method.First, LC cells were constructed from gluing two facing ITO coated glasses, and the 19.0 ± 1.0 μm cell thickness was controlled by PET spacer.Then, the precursors with corresponding components (listed in Table 1) were prepared and stirred to a homogeneous mixture, and the mixture was injected into LC cells by capillary action subsequently.Finally, the uncured samples were exposed to UV light (365 nm, 4 mW/cm 2 ) and the PDLC films were acquired by PIPS method at room temperature.

Characterisation and measurements
The electro-optical (E-O) properties test was determined by a liquid crystal parameter tester (LCT-5016C, Northern Liquid Crystal Engineering Research and Development Centre).The PDLC films were driven by a square-wave modulated electric field with a 100.0 Hz frequency.The detector was set at 30.0 cm away from the sample holder.The transmittance was normalised with air as 100%.The parameters obtained from E-O test include (1) the threshold voltage (V th ) and the saturation voltage (V sat ), which are denoted as the voltage to meet the requirement of 10.0% and 90.0% of the saturation transmittance of the film, respectively; (2) the turnon response time (t on ) and turn-off response tome (t off ), which are denoted as the time to vary between the 10.0% and 90.0% of the saturation level in the presence of an electric field and after removal of the voltage; (3) the on-state transmittance (T on ) and the off-state transmittance (T off ).In addition, the contrast ratio (CR) of T on /T off is also calculated.
The morphology and microstructure of the SiO 2 -SH nanoparticles and polymer matrix were observed under scanning electron microscopy (SEM, HITACHI S-4800).Before observing the polymer matrix, the PDLC films were sliced into small pieces and then immersed in n-hexane solvent for a fortnight to ensure complete extraction of the LC molecules.Then the resulting film was dried and sprayed with gold on its surface.The formulation of acrylate monomers is CHMA (22.5 wt%), HPMA (22.5 wt%), IBMA (45.0 wt%) and PEGDA 400 (10.0 wt%).
The transmission spectra were carried out on a Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer.The transmittance of blank cell was normalised to 100%.

Results and discussion
The SiO 2 -SH nanoparticles were used as the dopant in the PDLC system.Figure 2(a,b) displayed the morphology of the nanoparticles.The SEM images suggested that the SiO 2 -SH nanoparticles were spherical in shape with a diameter of approximately 1.5 micrometres.From the TEM images in Figure S1, the dimension of the KH570-POSS nanostructure was about 230 nanometres and the nanocomposite dispersed well.Previous studies have reported that the silicon nanoparticles tend to aggregate [37], but the SiO 2 -SH nanoparticles modified by the thiol groups used in this study were well dispersed with no significant aggregation.Furthermore, the existence of thiol groups allowed the SiO 2 -SH nanoparticles to act as the crosslinker to form the polymer matrix in the system.After polymerisation, the morphology of the silicon nanoparticles changed dramatically with bumps appearing on the surface as shown in Figure 2(c) due to the cross-linking of thiol groups.The disappearance of the vibrational bands at 2571 cm −1 (v (−SH) ) and 1636 cm −1 (v (−C=C) ) in the Fourier transform infrared (FT-IR) spectra of the mixture after curing indicated the reaction between the thiol groups in the SiO 2 -SH nanoparticles and the acrylate groups in the system.
The electro-optical (E-O) properties of PDLC samples A-E with different dosages of SiO 2 -SH nanoparticles were measured and displayed in Figure 3.In Figure 3(a), the transmittance of all the samples rose to a saturation level as the applied voltage increased due to the vertical alignment of LC in the presence of an electric field.However, the samples B-E with SiO 2 -SH nanoparticles showed more steeper shapes in comparison to sample A without SiO 2 -SH nanoparticles, which demonstrated that the LC in these samples was much easier to drive by electric field.It can be seen in Figure 3(b) that the contrast ratio (CR) was improved after doping SiO 2 -SH nanoparticles to the system caused by the reduced T off value.Although the samples A-E possessed the same high value of the on-state transmittance (T on ), the introduction of SiO 2 -SH nanoparticles could reduce the off-state transmittance (T off ) to less than 0.5%, which could lead to an increase in CR to above 200.Moreover, the T off of sample E doped with too high-content (15 wt%) SiO 2 -SH nanoparticles increased again, while the CR declined accordingly.The driving voltage is also of great importance for assessing the E-O characteristics of PDLC films [6].In Figure 3(c), the driving voltage including the threshold voltage (V th ) and saturation voltage (V sat ) showed a declining trend as the dosage of SiO 2 -SH nanoparticles increased.In particular, the V sat of samples D and E were below 40 V, less than half that of the sample A (88 V) without the addition of SiO 2 -SH nanoparticles.The value of response time also varied, as shown in Figure 3(d).The turn-on response time (t on ) showed an increasing trend and the turn-off response time (t off ) showed the opposite downward trend.
To better understand the variability of E-O properties, the microstructure of the polymer matrix in samples A-E was observed under SEM, because the morphology of the polymer has an important influence on E-O properties, as has been confirmed in many reports [9].The polymer morphology in the five samples is compared in Figure 4.The porous morphology observed under SEM was the result of phase separation between the LC droplets and the polymer.The sample A with no SiO 2 -SH nanoparticles presented a dense microstructure, causing a high driving voltage.When doping with SiO 2 -SH nanoparticles, the size of the void became enlarged and got larger as the amount of doping increased.The driving voltage decreased correspondingly with the larger void size from sample A to E, consistent with the results in Figure 2(c).Furthermore, from the microstructure of samples B-E, the large-sized SiO 2 -SH nanoparticles can be clearly seen in the SEM images, which demonstrated that the introduction of the SiO 2 -SH nanoparticles not only affected the crosslinking density of the polymer network but also implanted the silicon nanoparticle into the polymer.Therefore, the optimisation of the E-O properties of the PDLC films doped with SiO 2 -SH nanoparticles may be due to the reduced anchoring strength of the polymer matrix and the enhanced steric repulsions of LC droplets and polymer matrix [20,21].The above experimental results indicated that doping SiO 2 -SH nanoparticles into PDLC film was helpful to optimise their E-O properties and the sample D with the best E-O performance would be used for the subsequent studies in this work.The preparation conditions such as polymerisation temperature and UV intensity are also important parameters to affect the E-O performance of PDLC films [38], especially in this system where SiO 2 -SH nanoparticles are involved in the formation of the polymer matrix.Therefore, to better regulate the E-O properties of the PDLC sample containing SiO 2 -SH nanoparticles, a series of sample D doped with 7.5 wt % SiO 2 -SH nanoparticles, named samples D1-D5, were prepared at different polymerisation temperatures (30-50°C), and the test results of their E-O properties are shown in Figure 5.With the increase in temperature, the samples show a smaller slope of the E-O curves, as can be found in Figure 5(a).The T off first decreased and then increased as the temperature increased and the CR showed an opposite trend in the samples in Figure 5(b).Moreover, Figure 5(c) shows that the driving voltage of both V th and V sat increased from samples D1 to D5.In addition, the gradually increased t on and decreased t off in Figure 5(d) indicate that the anchoring forces produced by the polymer matrix increased as the polymerisation temperatures increased.
The surface morphology of the polymer matrix in five samples prepared with various polymerisation temperatures are shown in Figure 6.From the images, the SiO 2 -SH nanoparticles can be observed, suggesting the successful implantation of the nanoparticles into the polymer matrix.The results indicated that the polymer network in the sample gradually became denser and the void in the polymer that was the location of the LC droplets became smaller with the increase in preparation temperatures.It is worth noting that the separation rate between the polymer and LC molecules has a great influence on the LC droplets dimension [39].In this work, when the sample was prepared at a higher temperature, the photo-induced polymerisation between reactive monomers consisting of acrylate groups and thiol groups would rapidly react to form the polymer matrix, and the polymerisation rate was high.Thus, there was not enough time for the LC droplets to grow up to a large size.As a result, smaller polymer meshes, where the LC droplets were located, were observed on the surface of the polymer matrix when the sample was prepared at a higher temperature in Figure 6.Besides, the smaller the LC droplets, the stronger the anchoring effect of polymer matrix on LC droplets.Therefore, the LC droplets dispersed in a dense polymer matrix need more energy to overcome the anchoring force from the polymer matrix to rearrange, resulting in a higher driving voltage.The above experimental results indicate that the polymerisation temperature could affect the E-O properties and microstructure of PDLCs.In this work, the sample D2 prepared at 35°C showed lower driving voltage and higher CR and hence would be used for further studies.state was enhanced when prepared at higher UV light intensities.In addition, the driving voltage plot displayed an increasing trend as the UV intensity increased (Figure 7(c)).In particular, the sample D10 cannot be completely driven before the applied voltage reached nearly 100 V.Moreover, the t on increased and t off decreased gradually from samples D6 to D10, suggesting that the anchoring force of the polymer matrix was larger when the sample is prepared under high UV intensity (Figure 7(d)).
The microstructures of the polymer matrix in samples D6-D10 containing 7.5 wt% SiO 2 -SH nanoparticles prepared with various UV light intensities at (35°C) are displayed in Figure 8.The SiO 2 -SH nanoparticles can be clearly observed in the images.The results suggest that the polymer meshes became smaller if the sample was prepared under UV light with a higher intensity.The UV light intensity will also affect the separation process between the polymer and the LC droplets and thus affect the size of LC droplets [40].If the sample was cured under higher UV light intensity, the polymerisation rate in the system was fast and the polymer matrix was formed rapidly, which led to a smaller dimension of LC droplets because of the insufficient time.While the polymer with smaller pores has a stronger anchoring effect on LC droplets [35].From Figure 8, the size of polymer pores decreased as the UV light intensity increased.Thus, the anchoring forces from the polymer in the sample D10 under the highest UV light intensity were greatest, resulting in the maximum value of driving voltage.To sum up, the E-O properties and the polymer morphology of PDLC films with silicon nanostructure can be effectively regulated by the dosage of SiO 2 -SH nanoparticles and the preparation conditions such as polymerisation temperature and UV light intensity.
The UV/Vis/NIR spectrophotometer was used to evaluate the switchable transmittance performance of sample D7 containing 7.5 wt% SiO 2 -SH nanoparticles prepared under UV light intensities of 3 mW/cm 2 at 35°C.The visible transmission spectra in Figure 9(a) show that the device shows low transmittance (<5%) at 0 V and high transmittance (>80%) at 40 V, indicating an electric field-controlled transmittance switching capability.To visually show the variation in transmittance, Figure 9 also displays the photographs of samples prepared on glass and flexible substrates.In the OFF state, i.e. at 0 V, the sample was opaque and the picture behind it was blocked.While in the ON state, i.e. at 40 V, the sample was transparent and the picture behind it was clearly visible.Moreover, the flexible film maintained its optical and E-O performance despite bending.

Conclusions
In conclusion, the PDLC films with silicon nanostructures were fabricated, and the effect of the dosage of SiO 2 -SH nanoparticles and the preparation conditions such as preparation temperatures and UV light intensity on the electro-optical (E-O) properties and polymer morphology were also studied.The existence of thiol groups allowed the SiO 2 -SH nanoparticles to act as the crosslinker to react with the acrylate groups to form the polymer matrix in this system.The results indicate the enhancement of SiO 2 -SH nanoparticles on the PDLC properties.The preparation conditions can also affect the E-O properties and polymer morphology of PDLC films.In addition, the CR first increased and then decreased, and the polymer network in the sample gradually became denser and the void became smaller with the increase in polymerisation temperatures.While the CR increased and the driving voltage decreased as the UV intensity increased.In particular, the saturation voltage of the sample was reduced by more than half from the initial value by doping 7.5 wt% SiO 2 -SH nanoparticles and regulating preparation conditions.This work provides practical guidance for regulating the properties of PFLC films.

Figure 1 .
Figure 1.(Colour online) Chemical structures of the raw materials used in the experiment.

Figure 2 .
Figure 2. SEM images of SiO 2 -SH nanoparticles at (a) Low and (b) High magnification.(c) SEM image of SiO 2 -SH nanoparticles after polymerisation.

Figure 4 .
Figure 4. SEM images of the polymer matrix of samples (a-e).

Figure 3 .
Figure 3. (Colour online) The E-O properties of PDLC samples containing different dosage of SiO 2 -SH nanoparticles.(a) The transmittance curves as a function of applied voltage; (b) The contrast ratio (CR) and off-state transmittance (T off ); (c) The threshold voltage (V th ) and saturation voltage (V sat ); and (d) The turn-on response time (t on ) and turn-off response time (t off ).

Table 1 .
Composition of the samples.