Supercritical Fluid-Driven Polymer Phase Separation for Microlens

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Supercritical Fluid Driven Polymer Phase Separation for Microlens with Tunable Dimension and Curvature Youdi Yang, Xiaopeng Huang, Xinyue Zhang, Fuze Jiang, Xiaogang Zhang, and Yapei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01951 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 27, 2016

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ACS Applied Materials & Interfaces

Supercritical Fluid Driven Polymer Phase Separation for

Microlens

with

Tunable

Dimension

and

Curvature Youdi Yang, Xiaopeng Huang, Xinyue Zhang, Fuze Jiang, Xiaogang Zhang*, Yapei Wang* Department of Chemistry, Renmin University of China, Beijing, 100872, China KEYWORDS: microlens, polymer phase separation, supercritical carbon dioxide, variable focal length, microlens array

ABSTRACT: Microlenses are highly sought as reliable means for high-resolution optical imaging at low illumination intensities. Plano-convex configuration with tunable dimension and curvature is an essential feature in the microlens fabrication. In this study, we present a facile and green route for preparing well-defined microlenses based on polymer phase separation in the presence of supercritical carbon dioxide (scCO2). The behaviors of linear PMMA protruded from crosslinked silicone network in scCO2 environment are investigated from the perspectives of thermodynamics and kinetics. Microlenses with dimensions from 2 to 15 µm and contact angles from 55° to 112° are successfully obtained through the adjustment of the kinetic conditions and outgassing rate. With the tunable focal length, they exhibit intrinsic function of discerning submicroscale patterns which are unable to be observed directly under optical microscope. Moreover, size confinement on the substrate results in the generation of well-ordered microlens

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arrays, affording great promise for applications in bio-imaging, photolithography, light harvesting and optical nano-sensing.


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INTRODUCTION Lens-based optical microscopies are well acknowledged and extensively applied in a wide range of scientific fields like medicine, materials science and microelectronics. 1-3 Especially in life science, nearly 80% of the microscopic investigations are still dependent on the optical microscope due to the intrinsic advantages including cost-effectiveness, highly simple manipulation, real-time observation and low requirement for the sample preparation. 4 However, compared with modern electron microscope techniques, optical microscopies suffer from the limited resolution of around half of the light wavelength arising from the light diffraction, as first proposed by Ernst Abbe in 1873. 5 The fine sub-wavelength details in the objects are lost since the evanescent waves decay exponentially with the distance.

6-8

As an alternative improved

strategy, microlens has attracted massive attention in the past decades for the enhanced resolution, which could approach and even break through the Abbe diffraction limit. 9 Also, it can meet the demands of miniaturization and integration, playing a vital role in modern optics and microelectronics. Although a large number of methods have been well established for the fabrication of microlenses, flexible and facile strategies to tune the focal length of the plano-convex configuration

are

still

challengeable

and

urgently

wanted.

Photolithography

10-13

,

photodecomposition 14, microdroplet hydrolysis 15, and particle self-assembly 16-18, though widely used and highly reproductive, should rely on the prepared templates which already determine the dimension and curvature of the microlenses. Microlens based on stimuli-responsive microgel provides an interesting way to dynamically tune the focal length, yet the manipulation and application are confined to the aqueous environments. 19-21 Similar issue exists as well when using the pressure in the microfluidic chamber to adjust the focusing of the PDMS microlens. 22


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Dewetted droplets via direct transfer 9, inkjet printing 23, dip-pen nanolithography 24,25 as well as thermal annealing (also called reflow) 26-28 are reported for available tunable microlenses, while these adjustments should be accompanied with rigid variation in the substrate, roughness, curvature and constituent materials. A recent dewetting-based microlens can slowly increase contact angles of droplets leading to the control over the lens curvature, but a mixed solvent for both polymer annealing and interfacial regulation is necessary. 29 Herein, we disclose a simple and green protocol to prepare microlens with tunable focus length using environment-friendly supercritical carbon dioxide (scCO2). In our previous work, we presented the fabrication of hemispherical microcapsules via polymer perspiration in organic solvent atmosphere, offering a novel bottom-up pathway towards round surface microstructures. 30

However, the use of organic solvent is an imperfection in consideration of environmental

protection and large-scale production. ScCO2 characterized by low critical temperature (31.2 °C) and accessible pressure (7.38 MPa), is a typical supercritical fluid which proves to be little harm to the environment and special interest for the industrial production. Its outstanding performance as a benign solvent for organic molecule has been demonstrated in the chemical separation, particles removal, synthetic reaction and material preparation. 31-33 In this contribution, scCO2 is used as the solvent of hydrophobic linear polymer, inducing polymer perspiration from crosslinked silicone. Focus-tunable microlenses with the dimension from 2 to 15 µm and contact angles from 55° to 112° were obtained and then transferred to discern the nano patterns which are inaccessible only with the employment of conventional optical microscope. EXPERIMENTAL SECTION Materials. All commercially available chemicals were used without further purification unless otherwise noted. The methyl methacrylate (MMA) was purified by the extraction of 5 wt. %


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NaOH aqueous solution for three times. The Sylgard 184 PDMS was purchased from Dow Corning Co.. The 1-Hydroxycyclohexyl Phenyl Ketone (HCPK) was provided from Aldrich. CO2 (>99.999% purity) was bought from Beijing Analytical Instrument Factory. All other reagents and solvents were purchased from Sinopharm Chemical Reagent Beijing Co.. Instruments. A JEOL 7401 scanning electron microscope was used to observe the morphology of microlenses. The samples needed to be sputtered with a thin gold film before observation. Magnified optical images of the microlenses were acquired from an optical microscope (Zeiss Axio Scope A1). AFM graphs and profiles were captured by Bruker Multimode 8 microscope. The annealing by scCO2 was performed in an 8 mL of high pressure reactor. An accessory TRC-1 temperature controller was used to control the temperature of the water bath with an accuracy of ± 0.1 °C. The temperature of the system was maintained at 35 °C. After the thermal equilibrium, the CO2 was supplied to the pressure cell at a given pressure with the assistant of ISCO Series D pump controller. Preparation of Microlenses. PDMS slices with thickness of 0.4 mm were first prepared as the matrix of the PMMA microlens. Typically, PDMS slices with different crosslinking degrees were formed by mixing the prepolymer and the crosslinker with the ratio of 5:1, 10:1, 20:1, being deflated in the vacuum and finally being cured at 75 ℃ for 2 h. Then this as-prepared crosslinked PDMS was cut into 0.5×0.5 cm2 slices which were further immersed in CH2Cl2 solution containing MMA monomer and photoinitiator, HCPK (0.3 wt. %), for about 15 min. These swollen PDMS slices were subjected to UV light (16.8 mW/cm2) for 7 min. After the photopolymerization, the slices were put in the reactor under various high pressures (9 MPa, 11 MPa and 13 MPa) at 35 °C for 2 h. The flow rate of outgassing process was controlled by adjusting the discharging time.


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Transfer of Microlenses. The microlenses were placed between the object and the objective lens of the microscope by compressing the annealed PDMS slice on the compact disk surface with a wet thin layer of PVA (0.5 wt. % aqueous) film for half an hour. After peeling off the slice, the microlenses were transferred to the compact disk (CD) surface successfully. Preparation of Microlens Arrays. PDMS slices with micropits were formed by mixing the prepolymer and the crosslinker with the ratio of 10:1, being deflated in the vacuum and finally being cured a templated silicon wafers at 75 ℃ for 2 h. The slides were peeled off from the silicon wafers and blended with PMMA as previously stated. The treated PDMS slices were put in the reactor under 11 MPa at 35 ℃ for 5 h. Finite Difference Time Domain (FDTD) Simulation. The FDTD simulations were performed using the FDTD Solutions. Planar light source was used, and the wavelength of the incident light ranges from 400 nm to 800 nm. Periodic boundary conditions were set for the x and y directions, and perfectly matched layer boundary conditions were selected for the z direction. The monitor was placed behind the microlenses. RESULTS AND DISCUSSIONS The Formation of PMMA Microlenses by Polymer Perspiration Using ScCO2 Annealing. Polydimethylsiloxane (PDMS) is used as the substrate material on account of its low surface energy and crosslinked network structure. Polymethylmethacrylate (PMMA) is carefully selected as the microlens material because of its excellent transmittance for the visible light (>92%). As shown in Figure 1a, the crosslinked PDMS slices were immersed into a dichloromethane (CH2Cl2) solution in which the monomer, methylmethacrylate (MMA), and the photoinitiator, 1hydroxycyclohexylphenyl ketone (HCPK) were both dissolved. The CH2Cl2 solution would swell the PDMS slices, which enabled the MMA monomers to homogeneously disperse inside


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the PDMS crosslinked network. Afterwards, a photopolymerization process was performed to rapidly cure MMA and avoid phase separation between the PDMS and PMMA. This behavior should be maximally inhibited during this step in order to obtain better dispersion of PMMA in PDMS for the following microlens preparation. Microlenses were formed by directing the macroscopic phase separation between PDMS and PMMA in the scCO2 environment. Typically, the as-prepared slices were placed in a high-pressure reactor, followed by the introduction of scCO2 at a given pressure and temperature. After a period time of annealing, the PMMA would migrate from the interior of PDMS to its surface as a result of phase separation between PMMA and PDMS. This led to the protrusion or “perspiration” of the PMMA droplets, which would be finally solidified and turned into plano-convex microlens structures after the deflation of carbon dioxide. Figure 1b-e summarize several scanning electron microscope (SEM) images of the as-prepared PMMA microlenses observed from different angles. As seen from the top view and side view, all the microlenses from the same PDMS present the similar configurations, which possess round smooth surface owing to the interfacial tension. The cross section gives a perfect profile of plano-convex microlens structure with solid and flawless interior structure, therefore, favorable for less scattering when light passes through the microlenses. In addition, the flat and glossy undersurfaces (Figure 1d), observed by transferring the microlenses onto a thin PVA film, are helpful to minimize the reflection leakage and improve the imaging quality. These demonstrations reveal the great potential of utilizing these protruded PMMA microstructures as excellent candidates for high-performance microlenses. We also demonstrates the perspiration method is also applicable for the growth of poly(tert-butylmethacrylate) (PtBMA) and polystyrene (PS) from PDMS network (Figure S1). In principle, once the polymer meets the


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following requirements, they are expected for the fabrication of microlens based on the perspiration pathway. Firstly, excellent transmittance and homogeneous refractive index for the visible light are required. Secondly, the monomer of the linear polymer should be compatible with another immobilized polymer network (e.g. PDMS in this work). Thirdly, the linear polymer is able to be rapidly solidified. Last, scCO2 is able to soften both linear polymer and crosslinked polymer, inducing the reorganization of the polymer chains based on polymer phase separation. It should be noted that this kind of polymer perspiration is adaptable for substrates with a diversity of geometries including flat, cambered and step surfaces (Figure S2), which are of great interests for the fabrication of 3-dimentional hierarchical structures such as bionic compound eye devices. 34


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Figure 1. (a) A schematic procedure of the preparation of microlens by polymer perspiration. (bd) The scanning electron microscope (SEM) images of the as-prepared microlens observed from the top (b), side (e), 45-degree tilted direction (c) as well as the backside after transferred by a thin PVA film (d). Thermodynamics of the Macroscopic Phase Separation between PMMA and PDMS. As mentioned above, macroscopic phase separation between linear PMMA and crosslinked PDMS


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plays the main role in the formation of PMMA microlenses. A proposed process of the phase separation by scCO2 annealing is depicted in Figure 2, including the softening of linear PMMA, the migration, the dewetting and the final solidification step. ScCO2 is a good solvent for the hydrophobic organic molecules and here, for linear PMMA, scCO2 would soften and mobilize the polymer chains. However, regarding to PDMS, scCO2 can swell but cannot mobilize its polymer chains freely due to the restriction of the 3D crosslinked network. As a consequence, the softened PMMA chains tend to flee from the PDMS network, thus significantly reducing the mixed free energy. With more and more protrusion, PMMA chains become aggregated and merge into PMMA droplets, dewetting on the substrate surface because of the low-surfaceenergy nature of PDMS. In the end, the deflation of the CO2 leads to the solidification of PMMA droplets, after which the final configuration of the PMMA microlenses is settled.

Figure 2. A schematic of migration and dewetting of PMMA onto the surface of PDMS in scCO2 condition. In order to assess the solvent effect of CO2 on the blended phases, according to the classic Flory-Huggins theory, 35-37 it is necessary to determine Flory-Huggins interaction parameters (χ)


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between CO2 and the polymers.

38

The role of CO2 can be determined by the χ parameters

according to the Flory-Huggins expression in terms of the solvent activity in a given polymer, 39 ln = ln(1 − ) + 1 −

+

(1)

where A is the activity of CO2, r is a size parameter, and is the volume fraction of each polymer chain (poly represents PMMA or PDMS). In view of the negligible molecular weights of CO2 compared to that of polymers, we here assume the ratio of the size parameters ( ⁄ ) is equal to zero. Combining the Flory-Huggins theories with the regular solution model, the χ parameters can be calculated by the equation (2), χ = ! + " =

#($ %$ ) &'

+ "

(2)

where ! and " represent the interaction parameters of enthalpy and entropy, respectively. Generally, " is very small, and can be regarded as zero. The enthalpy is linked to the solubility parameters of two components, ( and ( . ( represents solubility parameters of PMMA (δPMMA = 19.0 MPa1/2, 25°C) or PDMS (δPDMS = 15.2 MPa1/2 , 25 °C). ( is solubility parameters of CO2, which can be figured out by the physical property of CO2 according to the following equation, ( = 8.04-. /⁄ (0 ⁄2.66)

(3)

where pc is the critical pressure of CO2 and ρr is relative density, namely the density ratio of CO2 in a given condition to that in the critical state. Therefore, ( is related to the density as the following equation, ( = 17.5080

(4)


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where 0 is the density of CO2. Thus through combing the above three equations, we can work out the solubility parameters of the polymers to carbon dioxide in a given supercritical state. For example, in a condition of 35 °C and 9 MPa, the 5667⁄ is 1.42, and 586"⁄ is 0.34. Treating the polymer blends with highly selective good solvents can induce phase segregation as a result of enthalpy change in the system. The phenomenon was described for spinodals and binodals according to the works of Patterson and Prausnitz.40, 41 However, when each of the binaries is miscible but the polymer-solvent interactions are asymmetric, the incompressible Flory model is used to explain the thermodynamic process in the polymer/polymer/solvent systems. The calculations indicate that, when polymer concentrations are high enough, the compatibility of these two polymers is governed by the interaction between the polymer segments (,:); whereas at low concentrations, the polymer-solvent interaction parameters (/, and /,: ) offer more contribution to deciding the compatibility in this system. In the latter case, if there exists a large distinction in the strength of the polymer-solvent interactions, phase separation can thermodynamically occur. Patterson referred to this phenomenon as the“|∆|” effect (|∆| = =/, − /,: =). 42, 43 In our system, ∆χ is 1.08 (35 °C, 9 MPa), which suggests phase separation would theoretically happen for a mixture of PMMA and PDMS when they are exposed to scCO2 atmosphere. Kinetic Analysis of the Polymer Perspiration. The perspiration process can be deemed as the diffusion of PMMA chains from the interior to the surface of PDMS slice. According to the Fick’s law, 44 the degree of diffusion, or the mean diffusion distance (x), is determined by the diffusion coefficient (D) and the diffusion time (t). Based on the Stokes-Einstein relation, 45 the diffusion coefficient (D) can be estimated as a function of temperature (T) and friction coefficient (>). The friction coefficient (>) in polymer motion is related to the crosslinking


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degree of PDMS (?@ ), the molecular weight of PMMA (Rs) and the interaction (∆δ) between the solvent and the polymers, that is A = 2BC (5) B = D E⁄> = DE⁄(6F?@ GH ∆() (6) where k is the Boltzmann constant. The impacts of those parameters are all investigated, as illustrated in Figure 2. The interaction between the polymer and the solvent (∆δ) is closely associated with the dissolving capacity of the scCO2, which is proportional to the density of the scCO2. As is well known, scCO2 is a green solvent whose density can be tuned by temperature and pressure.46, 47 The density of scCO2 is improved at higher pressure. Therefore, the PMMA chains were much easier to migrate onto the surface of PDMS, forming PMMA microlenses with large size (Figure 3a-c). On the contrary, the increase of molecular weight would result in the decrease of the diffusion coefficient, diminishing the dimension of the protruded microlenses (Figure 3d-f). It is not only because the elongation of PMMA chains restricts its movement, but also owing to the concomitant influence of reduced solubility in scCO2. Moreover, as shown in Figure 3g-i, the PDMS substrate with a greater crosslinking degree generates microlens with smaller size, which is a result of the slower migration of PMMA chains in a much more compact PDMS network. The influence of temperature is complex. When the temperature was raised, on one hand, the thermal motion of molecules became more violent, which would facilitate the migration. However, on the other hand, the dissolving capacity of CO2 would decrease as well, which in return discouraged the movement of the molecules. As the experiments show, no obvious changes were observed in this situation (Figure S3). It is also worth mentioning that the


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CO2 in supercritical state is very critical for the appropriate phase separation for microlenses, since liquid CO2 with higher solubility would bring uncontrolled phase-separation spots while gaseous CO2 failed to mobilize the polymer chains (Figure S4).

Figure 3. The SEM images of the PMMA microlenses in different conditions. (a-c) The operation pressure for the scCO2 annealing is 9 MPa (a), 11 MPa (b) and 13 MPa (c), respectively. The molecular weight of PMMA is kept as 2147 g/mol as well as the crosslinking ratio of PDMS is fixed as 20:1 (m:m). (d-f) The molecular weights of PMMA are 2147 (d), 3041 (e) and 3707 (f) g/mol, respectively. The annealing condition is kept under 11 MPa, 35 °C and the crosslinking ratio of PDMS is fixed as 20:1 (m:m). (g-i) The crosslinking ratios of PDMS between the prepolymer and the crosslinking agent are 5:1 (g), 10:1 (h), 20:1 (i) (m:m), respectively. The annealing condition is 11 MPa, 35 °C and the molecular weight of PMMA is 2147 g/mol.


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In addition to the diffusion coefficient, another vital factor influencing the process is the annealing time. As shown in Figure 4, the size of particles gradually became larger with the increase of annealing time. At the early stage of annealing, the microlenses on the substrate were small and the spaces between each other were large. With prolonged annealing time, the 8shaped microlenses were emerged, revealing that two adjacent droplets would fuse before solidification when the small PMMA droplets became larger (Figure S5). In this stage, two growth behaviors, protrusion of individual PMMA from the PDMS substrate as well as fusion of neighboring PMMA droplets on PDMS surface, were involved. The inserted graphs in Figure 4 suggest that the dimension distribution of the microlenses is concentrated in a small range. This provides an effective and efficient strategy to obtain well-defined plano-convex configuration with highly controllable dimension ranging from 2 to 15 micrometers, beneficial for the preparation of desirable microlenses.

Figure 4. (a-f) The SEM images of the PMMA microlenses with different growing times including 30 min, 1 h, 2 h, 5 h, 10 h and 15 h, respectively. Insert: The corresponding size analysis of the microstructures.


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The average size of microlenses is statistically analyzed with the annealing time in Figure 5a, presenting a best-fit curve between the experimental diameter (D) and annealing time. The dimension of the microlenses tends to be stable after a period of growth time. The growth rate is high at the initial stage, and then maintains at a lower level (Figure 5b). Considering the growth mass of the microlenses is directly proportional to the volume (R3), here, the time dependence of the cubical radius is further analyzed. As illustrated in Figure 5c and 5d, at the beginning stage, the process can be described by the nucleation and growth process.48 Compared with the traditional solvent used for phase separation (toluene, chloroform, etc.), the nuclei appear soon because of the rapid permeability of scCO2. The microlenses start to grow along with the nuclei. When the microlens is large enough to touch neighbor ones, they would merge into a larger one, which can be described as the collision and coalescence process.

49

Because of the surface

tension effect, the 8-shaped droplet would like to change its shape into spherical cap, which leads to the size increase. In later development stage, the derivative is getting smaller and smaller because of the decrease of PMMA content in PDMS slice. All these data show that the dimension of the PMMA plano-convex structures can be finely tuned by the operation pressure, crosslinking degree, and molecular weight as well as annealing time, very desirable for the application as microlenses.


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Figure 5. The time dependences of the (a) diameter (D) and (c) cubical radius (R3) of the PMMA microlenses (the dots for the experiment data and the line for the data fitting). (b, d) The derivative curves corresponding to the fitting curves in (a) and (c), respectively. Adjustable Focal Length of the Microlenses. The macroscopic phase separation causes the migration and dewetting of the PMMA droplets onto the PDMS surface, but the final topography and configuration of the microlenses are further influenced by the solidification process. Figure 6 reveals the regulation of the contact angles by changing the outgassing rates of CO2. When the gas valve was turned on, the depressurized CO2 would escape from the reactor rapidly. Specifically, there were continuous CO2 flows fleeing away the PMMA droplets from different directions. The flow rate of the gas above the surface is larger than that below the surface, where the interlaced PDMS polymer chains with limited gas permeability hinder the outflow of carbon dioxide. Hence, according to the Bernoulli’s principle that an increase in the speed of the fluid occurs simultaneously with a decrease in pressure, the gas pressure below the PDMS surface is


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higher than that above the surface, providing an additional force to push the uncured PMMA droplet to dewet on the surface. At the meantime, as the reactor environment varies from scCO2 to air atmosphere, the PMMA droplets start to be solidified into microlenses and form the final configurations. Thus, after the deflating process, the contact angle of obtained PMMA microlens structure becomes higher, accompanied with the corresponding variation on the curvatures and observed dimension. This effect is much more prominent when the outgassing flow is faster. With a higher flow rate, the unsolidified PMMA droplets were pushed by a stronger force, leaving the microlenses with larger contact angles. The relationship between flow rate and contact angle is shown in Figure 6f. The contact angle of the microlenses could reach 110° in the rapid deflation of 1.6 mL/s. Nevertheless, when the outgassing rate was reduced to 2.2×10-4 mL/s, the contact angle would drop to 55°. It is noteworthy that the microlenses obtained at distinct outgassing speeds possess nearly perfect spherical geometry, as illustrated in the AFM profiles.

Figure 6. (a-e) The SEM images of the PMMA microlenses using different outgassing speeds. The outgassing speeds of scCO2 are 1.6 mL/s (a, Lens 1), 8.0×10-1 mL/s (b, Lens 2), 4.4×10-2


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mL/s (c, Lens 3), 5.9×10-3 mL/s (d, Lens 4) and 2.2×10-4 mL/s (e, Lens 5), respectively. Insert: AFM images of the cross sections of the corresponding microlenses (black dot) and the depicted perfect spherical profiles (red line). (f) The relationship between the contact angles of PMMA microlenses on the substrate and the outgassing flow rate. To further elucidate the tunable geometric structure, the parameters of microlenses are quantified. Given a spherical cap shape of the microlens, the contact angle (I) can be expressed as tan I =

L

&%M

=

ML

(7)

L %M

where R is the radius of curvature, L is the lateral radius and h is the height of the microlens. All the changes of the contact angles and the aspect ratios with five different outgassing speeds are shown in Table 1. Since the focal length of a microlens is relevant to its dimension and curvature, Table 1 also gives the corresponding variations of the focal length with the outgassing speed. We used finite difference time domain (FDTD) simulation to accurately and vividly describe the optical characteristics of these microlenses. The diffraction propagations of the wavefront along the optical axis are present in Figure 7. The microlenses exhibit curvilinear trajectories of light, which are different from geometrical optical lenses, thus shortening the focal length. The light intensity distributions, in the transverse plane and along the optical axis, clearly identify that the focal length can be well tuned.


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Figure 7. (a-e) The light intensity distributions based on FDTD simulation for Lens 1, Lens 2, Lens 3, Lens 4 and Lens 5, respectively. Table 1. The characteristics of microlenses upon adjusting focal length. Lens 1

Lens 2

Lens 3

Lens 4

Lens 5

Flow Rate [mL/s]

1.6

8.0×10-1

4.4×10-2

5.9×10-3

2.2×10-4

Aspect Ratio

0.737

0.626

0.344

0.306

0.262

Contact Angle [°]

112

103

69.0

62.9

55.3

Focal Length [µm]

6.17

7.34

8.97

9.90

14.4

Enhanced Resolution by the Polymer Microlenses. In order to evaluate the magnification property of the microlenses, optical characterization was performed. Microlenses were transferred onto the top of the object with nanoscale patterns and then observed under an optical microscope (Figure 8a). Due to the weak adhesion between PMMA and PDMS, the microlenses could be easily transferred with the help of a wet and thin PVA film (0.5 wt. % in water). Figure 8b exhibits the large-area transfer of the microlenses onto a compact disk (CD), which was engraved with 780 nm wide parallel stripes containing dots and dashes. The dots on the CD surface have a minimum size of 380 nm, obviously observed under the SEM but unable to be seen under the optical microscope (Figure 8b). With the assistance of the microlens, the stripe


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was amplified obviously, as shown in Figure 8c-g. The five microlenses with different focal lengths in Table 1 were placed on the CD surface upside down, enabling the incident light to penetrate vertically through the lenses. The object was located within the focus, so the amplified images could be virtually observed. The distance between the centers of adjacent stripes observed directly by optical microscope is denoted as d, while the increased line spacing observed through the microlens is labeled as d’. As shown in Table 1, the focal length of Lens 1 is shortest; it is supposed to have the largest magnification. However, because of very large contact angle, Lens 1 is quite similar to a sphere, which is detrimental to the resolution on account of spherical aberration. As summarized in Table 2, all the five microlenses present a better resolution than observing directly under the microscope. Lens with appropriate focal length and curvature shows the best magnification factor as high as 130%, clearly differentiating the dots and dashes along the stripes. Furthermore, the facile transfer process also allows for the observation of biological sample (Figure S6).


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Figure 8. (a) A schematic of using the micolenses for microimaging. (b) The optical micrograph of micolenses on compact disk (CD) with aligned stripes. Insert: the SEM image of the CD. Scale bar: 2 µm. (c-g) Microimaging of the CD under the optical microscope with the help of microlenses, including Lens 1, Lens 2, Lens 3, Lens 4 and Lens 5, respectively. Table 2. The magnification based on the considerable increase of the line spacing (d) behind the lens (d’). Lens 1 Lens 2 Lens 3 Lens 4 Lens 5 d [µm]

1.57

1.50

1.52

1.53

1.53

d’ [µm]

1.84

1.95

1.81

1.77

1.75

magnification

117%

130%

119%

116%

114%


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The Preparation of the Microlens Arrays. A large-area imaging normally requires the use of microlens arrays, in which all the uniform microlenses are aligned into a regular pattern. Apart from controlling the configuration of individual microlens, we here demonstrate a formation of microlens array in scCO2 by using patterned PDMS slices. As shown in Figure 9, PDMS slices consisting of micropit arrays with the size and spacing ranging from 50 µm to 7 µm were used as the substrate for polymer perspiration. When the size and spacing of micropits are both 50 µm, the polymer microlenses are mainly engendered on the top surface. Inside the micropits, the protruded polymer microlenses were inclined to coalesce along the edges and walls instead of forming individual microlenses. As we discussed above, PMMA droplets would fuse with the adjacent ones after a period of annealing time, and the abrupt edges provided more possibilities for the fusion of the PMMA droplets. Most importantly, the droplets formed inside the micropits were difficult to migrate to the top surface. Consequently, as the micropits and the spacing distance went down, microlenses on the top surface would gradually reduce while PMMA droplets inside the micropits grew bigger. This trend was clearly observed when the sizes of micropits and spacing were both reduced to 7 µm. Interestingly, nearly uniform microlens arrays were obtained when the feature sizes of micropits and spacing match those of PMMA microlenses (around 8 µm as mentioned in Figure 6f), as a result of the edge effect or confining effect of the pre-patterns (Figure 9b-g). 50 Thanks to the high light transmission of PMMA, this ordered microlens array exhibited pronounced superiority on light harvesting and imaging performance. When the microlens array was illuminated by a beam of light passing through a 0.5 cm wide mask, an array of virtual images of line was clearly captured. The monodisperse and homogenous projected images manifest the rather uniform focal length and fine imaging property of the microlens array (Figure 9i).


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Figure 9. (a) A schematic of fabrication of PMMA microlens arrays using patterned PDMS slices. (b-g) The micrographs of the polymer perspiration on PDMS micropit array with the size of 50, 40, 30, 20, 10 and 7 µm, respectively. (h) The SEM image of the microlens array prepared using PDMS micropit array with the size of 7 µm. (i) Uniform projected images obtained from the microlens array. CONCLUSION In summary, we have demonstrated the fabrication of the well-defined microlenses through directing the polymer perspiration in an environment-friendly supercritical CO2 environment. ScCO2 annealing was proved to be a practicable and effective approach for the protrusion of PMMA microlenses from the crosslinked PDMS network. The fine control over dimension and configuration by regulating kinetic parameters and outgassing speed allows the adjustment of focal length of the microlens without changing substrate, making up for the deficiency of


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conventional approaches. Its remarkable optical magnifying performance was evidenced by telling apart the sub-microscale patterns which failed to be distinguished directly by optical microscope. An amplification factor of the focus-optimized microlens can reach as high as 130%. Moreover, well-ordered microlens arrays are successfully constructed through confining the polymer perspiration in a patterned PDMS substrate, affording great promise for applications in bio-imaging, photolithography, light harvesting and optical nano-sensing. To the best of our knowledge, this is the first example of fabricating microlenses based on polymer phase separation driven by supercritical fluid, much simpler and greener than traditional approaches. It is envisioned that the powerful means to engineer tunable microlenses in this work will open up an avenue towards green and sustainable manufacture of advanced micro devices as well as other exquisite microstructures. ASSOCIATED CONTENT Supporting Information. SEM graphs of the microlenses produced using poly(tertbutylmethacrylate) (PtBMA) and polystyrene (PS), under gaseous and liquid state, grown on substrate with different geometries, fabricated at different annealing temperatures, 8-shape transitional state of the microlenses and optical micrograph of a stomata guard cell observed through the PMMA microlenses. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]； [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (51373197, 21422407, 20876169). REFERENCES (1) Wang, L.; Li. F.; Liu, H.; Jiang, W.; Niu, D.; Li, R.; Yin, L.; Shi, Y.; Chen, B. GrapheneBased Bioinspired Compound Eyes for Programmable Focusing and Remote Actuation. ACS Appl. Mater. Interfaces 2015, 7, 21416−21422. (2) Lee, J. Y.; Hong, B. H.; Kim, W. Y.; Min, S. K.; Kim, Y.; Jouravlev, M. V.; Bose, R.; Kim, K. S.; Hwang, I.; Kaufman, L. J.; Wong, C. W.; Kim, P.; Kim, K. S. Near-field Focusing and Magnification Through Self-assembled Nanoscale Spherical Lenses. Nature 2009, 460, 498-501. (3) Wang, Z.; Guo, W.; Li, L.; Luk'yanchuk, B.; Khan, A.; Liu, Z.; Chen, Z.; Hong, M. Optical Virtual Imaging at 50 nm Lateral Resolution with a White-light Nanoscope. Nat. Commun. 2011, 2, 218. (4) Hell, S. W. Far-field Optical Nanoscopy. Science 2007, 316, 1153-1158. (5) Abbe, E. Beiträgezur Theorie des Mikroskops und der Mikroskopischen Wahrnehmung. Mikroskop. Anat. 1873, 9, 413-418. (6) Zhang, X.; Liu, Z. Superlenses to Overcome the Diffraction Limit. Nat. Mater. 2008, 7, 435441.


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Supercritical Fluid-Driven Polymer Phase Separation for Microlens

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