Article pubs.acs.org/JPCB
Electroactive and Optically Adaptive Bionanocomposite for Reconfigurable Microlens Kishor Kumar Sadasivuni,† Deepalekshmi Ponnamma,† Hyun-U Ko,‡ Lindong Zhai,‡ Hyun-Chan Kim,‡ and Jaehwan Kim*,‡ †
Centre for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar Center for Nanocellulose Future Composites, Department of Mechanical Engineering, Inha University, 100 Inha-Ro, Nam-Ku, Incheon 22212, South Korea
‡
ABSTRACT: This paper introduces an electroactive bionanocomposite based on poly(diethylene glycol adipate) (PDEGA) and cellulose nanocrystals (CNCs). The bionanocomposites were made using CNCs extracted from cotton and by optimizing its concentration in terms of the optical transmittance and viscosity. The characteristic properties of the materials were analyzed using contact angle measurements and Fourier transformation infrared spectra. Using the PDEGA/CNC bionanocomposite at a very low concentration of CNCs, a configurable lens having a robust, selfcontained tunable optical structure was developed. The shape and curvature of the soft PDEGA/CNC device were controlled by applying voltage, and the focal length was measured. The simple structure, high optical transparency, biodegradability, thermal stability, high durability, and low power consumption make the new material particularly useful in fabricating a reconfigurable lens for future electronic and optical devices.
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their biodegradability and biocompatibility.20,21 CNCs can be isolated from various sources, for example, plants, algae, and micro-organisms, of which the one from cotton stands as the simplest and the easiest.22,23 Dufresne et al. is a pioneer in cellulose based bionanocomposite research, and their review explains different kinds of extraction processes for cellulose particles along with their characteristic properties and applications.24,25 A large number of studies explored the reinforcement of CNCs with a broad range of polymers and their wider applications in packaging films, microchips, actuators, sensors, field effect transistors, etc.26−28 In addition to the eco-friendliness, the widespread interest in polymer− CNC nanocomposites is because of the large availability, low cost, low density, and mechanical strength. We have investigated the significance of cellulose nanocomposites in the field of electronics in a series of contributions.29−35 The superior interactions between the functional groups on cellulose chains and the graphene oxide (GO) sheets were utilized to manufacture flexible sensors, actuators, and energy and memory storage devices.31−33 We have demonstrated that such electrically conducting materials are potential candidates for smart proximity sensors with faster response and higher sensitivity.35 However, depending on the number of sprayed layers, the sheet resistance and the optical transparency showed contradicting behavior; higher sheet resistance sacrificed the optical transparency.
INTRODUCTION Smart microlens systems have the unique ability to change their focal length when an external electric field is applied.1,2 Such electroactive microlens systems garnered significant consideration due to their wider applications in optics, electronics, and the biomedical field.3−6 A liquid crystal (LC) based microlens is used in optical interconnections, photonic devices, optical communication systems, high-density data storage, and energy-directing devices.7,8 The key feature exhibited by these microlens materials is their ability to change the focal length by varying either their refractive index or droplet curvature.9,10 Popular designs for these devices have included creating a lenslike distribution of the refractive index by a nonhomogeneous electric field, keeping the surface release microlens in LC, light induced reorientation, and photopolymerization.11,12 Various types of LC microlens systems claim many advantages such as non-necessity of mechanical actuators, small size in volume, and simplicity in structure over conventional solid lens modules.13 One recent approach designed a polymer based microlens which has the capability to precisely position in strategic locations and must make a revolution in the nanobiochip fabrication.14 As such, there is an increasing demand in developing unique microlens systems with superior performance.15,16 Polymer nanocomposites have demonstrated high transparency, excellent flexibility, enhanced mechanical strength, and stiffness due to the entanglement of high aspect ratio fillers with the long polymer chains through various interfacial interactions.17−19 Among many potential nanofillers, cellulose nanocrystals (CNCs) have been widely discussed because of © 2016 American Chemical Society
Received: February 9, 2016 Revised: May 9, 2016 Published: May 10, 2016 4699
DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
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The Journal of Physical Chemistry B
Figure 1. (a) TEM image (0.2 μm scale) and (b) AFM image of CNCs.
stirring (1000 rpm) the mixture for 5 h. This one-step procedure of simple solution mixing ensured a good physical interaction between the PDEGA chains and CNCs on its surface. The final composite was dried in an oven at 80 °C for 12 h for getting a solvent free bionanocomposite. PDEGA/ CNC composites with higher concentrations (0.001, 0.005, and 0.01 wt %) of CNCs along with the neat PDEGA (for reference purposes ) were also prepared in the same way of solution mixing and at 80 °C drying. Characterization Methods. For each characterization method described below, three sets of measurements were done and an average was reported. The morphology of the CNCs was checked using a transmission electron microscope (JEOL, JEM 2100F) and an atomic force microscope (Veeco, Dimension 3100). For the TEM, very dilute suspensions of CNCs in IPA (0.01% w/v) were coated on the carbon grids. AFM was taken with spin coated CNCs on pyranha-treated silicon wafers. Contact angle measurements were done by online microscopy (GBX Digidrop intelligent version, France) using Windrop+2 software. The samples were spin-coated on clean glass plates and dried, and the contact angle measurements were carried out under water contact. The water drop volume was maintained as 6 μL in all cases. The Fourier transformation infrared (FTIR) spectra of the composite samples were obtained in the range 500−4000 cm−1 (Bruker Optics, Billerica, MA) by averaging 16 scans (resolution was 4 cm−1) at 1 min intervals to minimize the effects of dynamic scanning. Optical transmittance of the samples was studied using a UV−visible spectrophotometer. For this, the spectra of the films in the range of 200−800 nm wavelengths were recorded with a Hewlett-Packard (8452A) diode array. The Brookfield Viscometer (LV-II viscosity range) measured fluid viscosity at given shear rates. The focal length of the lens was measured by actuating the lens at a voltage from 50 V to 1 kV. For this, a collimated laser beam was shone through the lens onto a translucent screen behind which a CCD camera was placed. The focal length was determined by translating the tunable lens to minimize the size of the spot on the screen. Automation of the procedure was done both by moving the lens and repeating the process at different actuating voltages.
Investigation on CNC/polymer nanocomposites realized the significance of biodegradable and transparent materials in electronics, optics, and biomedical engineering.33 Our initial experiments on polydimethylsiloxane (PDMS) filled CNC composites suggest that a transparent reconfigurable microlens with adaptive focal length can be achieved by applying an electric field of 800 V under 5 mm electrode distance.36 However, this electric field, 160 V/cm, is very high as far as the practical application is concerned. Furthermore, the realization of a biodegradable lens in electronic devices will benefit significantly if optically transparent electroactive materials with variable focal length at lower voltage could be made available. Thus, we have explored the possibility of the biodegradable thermoplastic plasticizer poly(diethylene glycol adipate) (PDEGA) in fabricating the lens system. The design assumes that the nontoxic, thermally stable, biodegradable PDEGA macromolecule37,38 with low glass transition point promotes significant interaction with CNCs that would result in transparent bionanocomposites.39 In addition, the PDEGA has a similar structure of CNC and the developed electroactive “green” composite of PDEGA/CNC is supposed to regulate its curvature to form a tunable microlens. Moreover, this biopolymer needs low voltage for its activation, thus making the possibility of a lens operating at low voltages. While the previous works have reported a less biodegradable polymer nanocomposite lens, we herein investigate biodegradable and reconfigurable lens devices applicable in the optical systems for imaging, sensing, optical communication, lab-on-a-chip, parallel optical switches, and electronic devices.10
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EXPERIMENTAL SECTION Materials. Cellulose cotton pulp of 98% purity was obtained from Buckeye Technologies Inc. Other reagents like PDEGA, H2SO4, NaOH, isopropyl alcohol (IPA), and all minor chemicals were procured from Sigma-Aldrich and used without further purification. Extraction of Cellulose Nanocrystals from Cotton. Cotton pulp of 98% purity was bleached in NaOH in order to remove the noncellulosic components in it. CNCs were prepared by acid hydrolysis (175 mL of 30% (v/v) H2SO4) of such bleached material (20.0 g) by mechanically stirring the mixture (200 rpm, 6 h) at 60 °C. The suspension obtained after the hydrolysis left was successively centrifuged with distilled water to attain neutrality and then homogenized (10 min, 11000 rpm) and dialyzed overnight.35 Preparation of PDEGA/CNC Bionanocomposites. The homogenized CNCs were dispersed in IPA at a concentration of 50 mg/mL by sonicating for 1 h. This was added to the PDEGA at a concentration of 0.001 wt % by magnetically
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RESULTS AND DISCUSSION Morphology of Cellulose Nanocrystals. The CNCs used in this study were isolated from cotton pulp using the protocols described in the Experimental Section. Figure 1 shows the TEM and AFM images of the synthesized CNCs, both of which confirm rod-like morphology.35 The dimensions of the CNCs determined from the TEM micrographs were an average length 4700
DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
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The Journal of Physical Chemistry B and width of 200 ± 50 and 40 ± 5 nm, respectively. The average aspect ratio was calculated to be 5. The charge density created on the surface of CNCs during hydrolysis modulates the interactions between individual CNCs and results in good dispersion. The morphology of CNCs was further addressed using AFM images shown in Figure 1b. The uniform, needlelike structure of the CNCs with an individual whisker’s length and diameter of around 100−300 and 35−45 nm, respectively, is observed. Surface Wettability of the PDEGA/CNC Bionanocomposites. Pristine CNCs are hydrophilic, and they tend to absorb moisture because of the high density of the hydroxyl groups appearing on its surface. Contact angle measurement is usually utilized to evaluate the wettability of neat PDEGA and the nanocomposite with various CNC contents. As shown in Figure 2a, for the pristine PDEGA, the water droplet showed a
Figure 2. Contact angle measurement of a 6 μL water drop on (a) PDEGA, (b) PC001, (c) PC005, and (d) PC01.
Figure 3. Microstructure of PDEGA/CNC bionanocomposite.
small contact angle (θ ≈ 52.6°) and was absorbed by the polymer in less than 60 s. In the case of CNC composites, the contact angle again decreased due to the sufficiently large number of hydroxyl groups on the filler surface.40 This decrease was in accordance with the concentration variation from 0.001 to 0.01 wt % (from θ ≈ 43.6 to 32.5°). For the PDEGA/ CNC0.001 wt % (PC001) bionanocomposite shown in Figure 2b, the water droplet was absorbed in less than 30 s, and for the PDEGA/CNC0.01 wt % case (PC01), as shown in Figure 2d, it was readily absorbed in less than 10 s. This substantiates the enhanced hydrophilicity of PC01 due to the large number of accumulated hydroxyl groups.21 Note that high hydrophilicity is an important factor in improving its biocompatibility and biodegradability.38 PDEGA/CNC0.005% (PC005) is shown in Figure 2c. Nanocomposite Microstructure. The morphology of the CNCs suggests short rod-like structures of high hydrophilicity in which the incorporation of PDEGA chains caused network formation. The biopolymer PDEGA was semichemically immobilized on the surface of CNC through a high level of hydrogen bonding and van der Waals interactions.41 This was evidenced by the very good level of dispersion observed for the PDEGA/CNC. Figure 3 suggests a possible microstructure of the bionanocomposite. The chemical structure of PDEGA is also shown in the schematic diagram in which a large number of hydroxyl groups, as well as the lone pairs from double bonded oxygen, is seen. This forms the basis of interfacial interactions existing within the bionanocomposite.
The whole physical reaction happens in the presence of IPA medium (Experimental Section), and this again strengthens the physical interactions within PDEGA/CNC. The pendant hydroxyl groups act as chelating agents for the CNCs, and the final material PDEGA/CNC forms the basis for fabricating the lens material and was further characterized using FTIR. Figure 4a demonstrates the FTIR spectra of neat PDEGA and nanocomposites. The peaks for PDEGA at 1000, 1750, and 1170 cm−1, respectively, correspond to the C−H stretching, carboxylic (ester) strong stretching, and C−O (ester) stretching vibrations. This is attributed to the existence of ester groups in PDEGA. Apart from a few common peaks with PDEGA, CNCs show a broad band at 3411 cm−1 which is assigned to the O−H stretching (intramolecular hydrogen bonds) vibration. Additional peaks at 2900 and 1060 cm−1 are due to the C−H stretching and the C−O−C pyranose ring skeletal vibrations, respectively.35 For the PDEGA/CNC bionanocomposites, obtained peaks were at similar wavenumbers indicating the absence of any chemical interactions. Moreover, the broad band of CNC at 3411 cm−1 decreased much in intensity which is attributed to the presence of intermolecular hydrogen bonding between the PDEGA and CNCs. The intensity variation observed for almost all of the peaks suggests a similar kind of physical interactions, confirming the hydrogen bonding and London forces existing in the bionanocomposite. Optical Transmittance and UV−Visible Spectra of Nanocomposites. The neat PDEGA and all of the 4701
DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
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Figure 4. (a) FTIR of PDEGA/CNC bionanocomposite and (b) optical transmittance.
large number of spatial networks via hydrogen bonding between −CNC−CNC and PDEGA−CNC− were formed within the material so as to increase the resistance to flow. An actuator changes its position upon an external stimulus, and here, the change in position is defined in terms of the focal length of the microlens fabricated.31 In general, the viscosity of the material influences the actuation in the presence of an electric field. More clearly, viscous liquids are difficult to be actuated at lower voltages. This indicates the inappropriateness of the samples with high viscosity (high CNC concentration) in lens manufacture. Here the PC001 bionanocomposite was selected to fabricate the lens based on its viscosity and optical transmittance. Together with good crystallinity, the hydrophilicity of this particular bionanocomposite was also beneficial in acceleration of the polymer chain movement under low electric field.3 Figure 6 represents the experimental setup and the mechanism of the actuation performed in the lens system. The PC001 was placed on a PDMS mold as a support to the curved surface of the lens, as shown in Figure 6a. In brief, the device consists of an elastic PDMS support, a reservoir of the material, and two ITO glass plates on both sides through which the electric field was applied. The circular hole in the PDMS on which the lens stands was of 15 mm diameter and 2 mm in thickness. The area of the PDEGA/CNC was calculated as 177 mm2. The distance between the ITO electrodes was adjusted to be 20 mm. The aperture of the bionanocomposite droplet can be scaled up to several millimeters while maintaining a spherical-like shape. Also, if the droplet is larger in diameter, the apex distance increases and thus requires a thicker cell gap. This necessitates higher applied voltage, on the application of which reduces the gap and pushes the active material toward the center of the lens changing its curvature.9 The direction of electric field (red arrows in Figure 6c) and the mechanism of actuation that illustrates the change of shape of the highly transparent lens and the subsequent alteration of its focal length are given in Figure 6c. The applied electric field pulling the contact line couple with the shape modes of the active material surface causes distortions in the lens curvature at high actuation frequencies.42,43 In microscale, the shape of a liquid lens is determined by the liquid volume and the container design. The optical properties of the lens highly depend on the nature of the materials as well as the device shape. The conventional structure of the tunable microdoublet lens array given in Figure 6 also confirms the deformation of the liquid interface upon dielectric force. When voltage is applied, the shape
bionanocomposites (PC001, PC005, and PC01) synthesized were analyzed for optical transmittance. UV−visible radiation of 200−800 nm wavelength was passed through the samples, and the results obtained were plotted as transmittance versus wavelength plots (Figure 4b). As the concentration of CNCs changes from 0 to 0.01%, a decrease in transmittance from 89 to 62% was observed specifically at 540 nm wavelength. This is attributed to the aggregation of CNCs at higher concentrations which enhances the turbidity and thus decreases the optical transparency. The increase in density and thus decrease in transmittance was in accordance with the previously reported results.35 Since the maximum optical transparency was achieved for PC001, this sample was selected to be the best for fabricating the lens device. Lens with Tunable Focal Length. Since the fabricated PDEGA/CNC bionanocomposite was supposed to have applications in lens devices, its viscous nature was analyzed by viscosity measurements. Figure 5 demonstrates the viscosity
Figure 5. Viscosity of the PDEGA/CNC bionanocomposites with filler concentration.
plots of the samples as a variation of viscosity (cP) with the filler concentration. There exists a quasi-linear relationship between the concentration of CNCs and the viscosity of the PDEGA/CNC: for example, at lower CNC concentration (PC001), the viscosity is very low. This substantiates the high level CNC dispersion and good CNC−polymer physical interfacial interaction existing in this particular sample. At higher CNC concentrations, the viscosity was high due to the increased turbidity and agglomeration effects. At this state, a 4702
DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
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Figure 6. (a) Experimental setup, (b) front view of the lens and transparency, and (c) mechanism involved in the lens.
Figure 7. (a) Focal length change vs voltage of PDEGA and PDEGA/CNC composite. (b) Lens profile picture of PDEGA/CNC.
Table 1. Comparison of the PC001 Microlens with the Reported Lenses type of lens
applied voltage
lens performance
dielectric liquid lens44 variable-focus liquid lens45 dielectric liquid lens46 liquid microlens47 liquid lenses utilizing electrowetting42 liquid lens48 liquid crystal lens49 PEDGA/CNC lens (present work)
0−45 Vrms 40 V 0−120 Vrms 0−200 V −1.7 V 150 V 75 V 50 V/cm
tunable focal length focus on objects away from 2.5 cm up to infinity tunable focal length at ∼ −8.28 ⩽ f ⩽ ∼ −4.4 mm focal length varied from 34 to 12 mm able to focus the hexadecanethiol (HDT) drop decrease in focal length to 20% of its initial value (3.7 mm) actuation and focal length variation actuation and focal length variation
polymer networks on CNCs. In order to clearly understand the focal length variation with applied voltage, as the voltage changes from the higher side to the lower side, the change in focal length moves from 0.35 to 0.25 mm for the neat polymer and from 0.3 to 0.25 mm for the composite. This behavior at higher voltages is attributed to the severe deformation of the surface profile of the lens, as shown in Figure 7b, and the possibility of the droplet to touch the opposite substrate surface.9,34 The durability of the PC001 lens was checked by testing over 1000 cycles, and no apparent change in the lens performance was observed. The lens reversibility was also perfect, as the curve obtained when decreasing the voltage exactly superimposed on the increasing one. It is also observed that the response time during focal length change varies with the applied voltage, as the contracting speed of the droplet depends on the dielectric force and its relaxing speed on the viscosity of the liquid and the related interfacial tensions. The dielectric force is expressed in terms of the permittivity of free space (ε0),
changes by the generated electrostatic force and this causes variation in focal length. From the movement of the PC001 bionanocomposite, the focal length and the curvature of the lens were measured by the computer and the data was analyzed from the focal length against voltage plots. From Figure 7a, the focal length variation (Δf) of the lens with applied voltage is very clear as per eq 1 Δf = f2 − f1
(1)
where f 2 and f1 are the final (after applying an electric field) and initial focal length of the sample. Theoretically, Δf is proportional to the square of the voltage. Initially, at V = 0, the focal length of the material was 33 mm. By slowly increasing the applied voltage, the focal length started to vary, but the initial point at which variation in focal length starts is different for the PC001 lens and the PDEGA. This threshold electric field was about 15 V/cm for the neat PDEGA and 2.5 V/cm for the PC001 lens. The relatively fast response time (frequency dependent) for the PDEGA/CNC lens compared to PDEGA is due to the anchoring effect of the 4703
DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
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The Journal of Physical Chemistry B liquid-1 (ε1), and liquid-2 (ε2) and the electrical field applied on the curved droplet surface (E), as represented in eq 2. ε F ⃗ = 0 (ε1 − ε2)Δ(E ·E) (2) 2
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF-2013M3C1A3059586, NRF2015R1A3A2066301).
It is clear from the equation that the difference between the permittivity values of the two liquids and the electric field gradient determine the dielectric force. Because of the complicated liquid−liquid and liquid−substrate interfacial interactions, theoretical studies of response time were not done here. Regarding the electrical power consumption, the present PDEGA/CNC exhibited 49 μW/cm when the electric field was 50 V/cm, which is much lower than the previous PDMS/CNC composite case, 210 mW/cm.36 In order to establish the applicability of the developed PC001 actuator in a lens, its performance has been compared with the reported works, as shown in Table 1. It should be noted that none of the reported works are about biodegradable material. On the basis of all of these observations, it is established that the fabricated PC001 bionanocomposite is capable as an actuator at lower electric field. The main important points toward this newly developed material are its significant transparency, low voltage required, and low viscosity (easily movable under an electric field). With tunable focal length, this microlens generates sharp images at different distances. The portable and compatible design, reasonable response time, low power consumption, and negligible heat dissipation make the lens applicable in many areas including lab-on-chip devices which require optical characterization. The liquid lens surface was very smooth and lens structure was insensitive to shocks and vibrations, since the surface profile of the liquid lens was determined by the surface tension. Thus, it appears that the cost-effective PDEGA/CNC bionanocomposite is suitable for both individual microlens, microlens array with reasonable resolution.
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CONCLUSIONS In summary, a new type of tunable focal lens based on a deformable PDEGA/CNC bionanocomposite actuated by an electric field was reported. Analysis of various composites by changing the CNC concentration resulted in an optimized PDEGA/CNC composite with 93% optical transmittance, moderate hydrophilicity, and viscosity at 0.001 wt % of the filler. This lens was capable of adjusting its focal length in a few seconds even at a lower applied electric field of 50 V. The high thermal stability of PDEGA makes this composite thermally strong enough to withstand temperatures up to 200 °C. The biodegradable and biocompatible reconfigurable microlens also showed fully reversible and highly reproducible behavior which could open new opportunities in the field of smart optics. The fully enclosed structure and electric field of operation allow the PDEGA/CNC lens to be tuned over a wide range of focal lengths. The microlens can also be readily integrated into arrays that may find useful applications in sensing, medical diagnostics, and lab-on-a-chip technologies. Ongoing work in PDEGA/CNC composites addresses the influence of temperature on the focal length change and expects to derive the potential benefits of this material in practical applications.
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REFERENCES
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[email protected]. 4704
DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
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DOI: 10.1021/acs.jpcb.6b01370 J. Phys. Chem. B 2016, 120, 4699−4705
