Letter pubs.acs.org/JPCL
Formation of Hierarchically Structured Polymer Films via Multiple Phase Separation Mediated by Intermittent Irradiation Hideyuki Nakanishi,* Tomohisa Norisuye, and Qui Tran-Cong-Miyata Department of Macromolecular Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *
ABSTRACT: Hierarchically structured materials have drawn great attention to the integration of functionalities into a same material. Here, we demonstrate a method in which hierarchical structures of a polymer mixture can be formed and controlled via multiple phase separation mediated by successive UV irradiation through a temporal cessation. The UV irradiation causes the polymerization and the cross-linking of methyl methacrylate and polystyrene, inducing phase separation of the mixture. When the irradiation is ceased upon phase separation and is subsequently repeated after certain duration in the dark, the secondary phase separation is induced within the same mixture. This intermittent irradiation results in polymer films with hierarchical structures, whose feature is controlled by the UV light intensity and can be transformed into multimodal porous membranes and gels. SECTION: Glasses, Colloids, Polymers, and Soft Matter
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methods for designing hierarchical structures is needed but it is yet to be explored except for in a few examples.27,28 Here, we describe a method in which hierarchically structured polymer materials can be formed via multiple phase separation mediated by intermittent irradiation with UV light (Figure 1a). Our model system is based on polymer mixtures of photo-cross-linkable polystyrene (PS) and photopolymerizable methyl methacrylate (MMA). Under exposure to UV light, PS component is cross-linked and concurrently MMA component is polymerized, giving poly(methyl methacrylate) (PMMA) in the presence of photoinitiator. Starting from the homogeneous MMA/PS mixture, UV irradiation generates the second polymer, PMMA, which in turn destabilizes the miscible mixture, leading to phase separation with a two-phase structure.29 Upon phase separation, UV light is immediately turned off (the first cycle of irradiation), and the mixture is allowed to stay in the dark (cessation process). During this intermission, the emerging two-phase structure is further developed into two domains. Then, subsequent resumption of UV irradiation (the second cycle) restarts the photoreaction, and readily induces the secondary phase separation within each domain preformed in the cessation process. This intermittent irradiation results in polymer films with hierarchical structures. The remarkable feature of the light-induced hierarchical material is that the structural hierarchy can be engineered solely by varying the light intensity of UV light in the second
or several decades, morphology control has been of great interest in materials science since material properties can be controlled by their morphology.1 More recently, considerable efforts have been made to create hierarchical morphology with the expectation that elaborate performance of natural materials can be realized in synthetic ones, and in the general, efforts toward improvement and sophistication of material functionalities. For instance in porous materials, structures with multimodal pore size distribution can enhance mass transport owing to large pores and concurrently afford a large surface area thanks to the fine ones. Such hierarchical pore structures can find application in separation, and has been expected to show enhanced properties than that of single pore distribution.2 Approaches to fabricating polymer materials with hierarchical structure involve the use of molding,3,4 lithography,5,6 mechanical stimuli,7,8 colloidal assembly,9,10 and supramolecular chemistry.11,12 Alternatively, the use of reaction induced phase separation13−15 would be appealing to construct hierarchical structure since it is facile and versatile for many applications. Indeed, phase separation phenomena have manifested themselves in successfully designing the morphology with which materials exhibit superior performances for catalyst supports,16,17 antireflection coatings,18,19 photovoltaic cells/light-emitting diodes,20−22 tissue engineering scaffolds,23,24 and organic−inorganic hybrids.25,26 However, the majority of previous methods are limited to two-phase structures with a single characteristic length, and formation of hierarchical structure based on phase separation that often relies on chance. In this context, the development of rational © 2013 American Chemical Society
Received: October 13, 2013 Accepted: November 11, 2013 Published: November 11, 2013 3978
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separation and quantified by the decline in the integrated fluorescence intensity (see section 2 in the SI). As soon as phase separation starts, the UV light was turned off and the mixture was allowed to stay in the dark for 60 min. Subsequently, UV irradiation was resumed for 60 min with a desired light intensity. This procedure gave rise to multiple phase separation in which the secondary morphology is incorporated into large sea-island (matrix-droplet) structure, as demonstrated in the left portions of Figure 1b,c. In particular, structural features in the matrix strongly depended on the light intensity of the second cycle of irradiation. As illustrated in the right portions of Figure 1b,c, the irradiation with weak and intense UV light resulted in droplet and cocontinuous structures, respectively. These behaviors were in sharp contrast to the control experiment carried out with the same MMA/PS mixture continuously exposed to UV light. Upon continuous irradiation shown in Figure 2a, the mixture showed no noticeable hierarchical structures, but instead formed trivial sea-island morphologies with a unimodal size distribution of PMMA-rich droplets with the mean lateral diameter, DA ∼ 20 μm, as depicted in Figure 2f. In comparison, the intermittent irradiation with the same light intensity, I = 10 μW/cm2, displayed in Figure 2b gave a unique multimodal size distribution within the same material (DA ≲ 2 μm for PS-rich droplets, and DA ∼ 4 μm and DA ∼ 60 μm for PMMA-rich droplets as shown in Figure 2g). These experiments suggest that the formation of the multiple phase-separated structures can be attributed to the temporal cessation of the photoreaction (UV irradiation) during the course of phase separation. To understand the occurrence of the multiple phase separation, we first examine the interplay between the chemical reaction and the phase separation. When the reaction is continuously promoted by UV irradiation with I = 10 μW/cm2, the size of the resulting PMMA-rich droplet increases to some extent (Figure 2c, top), but immediately reaches a stationary state (DA ∼ 20 μm, within t − t0 ∼ 500 s; Figure 2h, inset). This circumstance arises because of the rapid increase in the viscosity caused by the polymerization and the cross-linking of MMA and PS. In addition to the suppression of the droplet growth, the chemical reaction gives rise to an important effect on phase separation. The bottom portion of Figure 2c shows the fluorescence intensity distributions (histograms) of the corresponding reacting mixture. Before phase separation (t − t0 < 0), the mixture exhibits a single fluorescence intensity distribution, which reflects the concentration fluctuation of the mixture in the miscible state. When, however, phase separation takes place (t − t0 > 0), the histogram splits into two peaks with lower (darker) and higher (brighter) intensities, which respectively represent the apparent concentration of the constituents (PMMA and PS) of the PMMA-rich droplet and the PS-rich matrix. Subsequently, these peaks oppositely shift to lower and higher intensities. This peak shift uncovers that the PMMA-rich droplet and PS-rich matrix are enriched respectively in PMMA and PS as phase separation proceeds with UV irradiation (chemical reaction). By contrast, according to the traditional nucleation-and-growth mechanism for nonreacting, conserved mixtures, the concentration of constituents (of droplet and matrix) does not effectively change during the course of phase separation.30 As a control experiment, while the UV light is turned off (Figure 2b), the horizontal shifts of the peaks in the histograms become unnoticeable and vertical shifts only occur (Figure 2(d),
Figure 1. (a) Schematic representation of the intermittent irradiation performed in this study. (b,c) (left) Typical LSCM images of the mixtures. The insets are the optical magnifications of the open squares marked with numbers. The scale bars are 200 μm, and 5 μm for the insets. (right) The 3D images corresponding to the area marked with the number 2. The images in (b) and (c) are for the mixtures exposed to the weak (I = 10 μW/cm2) and the intense (I = 750 μW/cm2) UV light in the second cycle of the irradiation, respectively. The whole intensity profiles for (b) and (c) are demonstrated in Figures 2b and 3a. The green and black (or transparent) regions respectively represent PS-rich and PMMA-rich domains.
cycle of irradiation as depicted in Figure 1b,c. Moreover, one of the polymer domains (PMMA) forming hierarchical structures can be selectively washed off with common solvents to leave behind a multimodal pore structure of the other polymer (PS). Thanks to cross-linking of PS, the prepared multiporous structures appear in free-standing membranes in polar solvents and in free-floating gels in nonpolar solvents. All experiments were carried out at room temperature, 25 °C, and at a constant weight fraction MMA/PS (5/95). To this mixture, a small amount of photoinitiator (Lucirin TPO) and ethyleneglycol dimethacrylate were added respectively at 2 wt % and 4 wt % with respect to MMA to facilitate the polymerization of MMA. The prepared MMA/PS mixture was inserted into a homemade glass cell whose thickness was kept at ∼30 μm using a spacer. To observe the phase separation process, we employed an inverted laser scanning confocal microscope (LSCM) and in situ monitored the mixture. PS used in this study bears anthracene (photo-cross-linker) and a trace of fluorescein (fluorescent tag), which gives a contrast between PS-rich and PMMA-rich domains; PS and MMA (also, the resulting PMMA) appear in bright and dark under the microscope, respectively. For further experimental details, see section 1 in the Supporting Information (SI). As a typical procedure for irradiation, an MMA/PS (5/95) mixture was first exposed to 365 nm UV light with an intensity of 10 μW/cm2 as indicated in Figures 2b and 3a. The mixture underwent phase separation after an induction time, t0 (typically, t0 ≈ 850 s), which is the onset time of phase 3979
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Figure 2. Typical behaviors of phase separation. (left column: a,c,f) The MMA/PS mixture continuously exposed to UV light and (right column: b,d,e,g,h) the same MMA/PS mixture under intermittent UV irradiation. (a,b) Intensity-irradiation time diagrams. The mixtures undergoes phase separation at t = t0. (c,d,e) Time courses of the phase separation process and the corresponding fluorescence intensity distribution. The difference, t − t0, indicates the elapsed time of phase separation. (f,g) The stationary morphologies and the size distributions of the PMMA-rich droplet. (h) Time variation in the mean lateral diameter, DA, of the PMMA-rich droplet for the continuous (circles) and the intermittent (squares) irradiation. It is noting that the DA for the red squares in the second irradiation cycle corresponds to the time course of the PMMA-rich droplet preformed in the cessation process as represented in the left-top LSCM image of (e). The scale bars for all the LSCM images are 50 μm.
unreacted monomer across the interface of the phase-separated domains. Next, we consider the effect of the temporal cessation of the reaction on the multiple phase separation derived from the second cycle of irradiation. While the mixture is placed in the dark as shown in the top portion of Figure 2d, the nucleated PMMA-rich droplet dramatically coarsens for 60 min in comparison to the case of the continuous irradiation (DA ∼ 40 μm for the dark versus DA ∼ 20 μm for the continuous irradiation; Figure 2h, blue squares and red circles). This droplet coarsening arises to minimize the total interfacial area and the unfavorable droplet-matrix (PMMA-PS) interfacial energy. This process is realized by the low viscous environment produced by the temporal cessation of the reaction. The droplets eventually exceed sample thickness (∼30 μm) and deform into ellipsoids owing to the spatial confinement imposed by the glass cell. Under this particular condition, the diffusion of the constituents is not only limited to the lateral direction, but also slowed due to the increased interspacing distance of the droplets. When, therefore, the second cycle of irradiation is carried out, the diffusion can no longer catch up to the sudden change in the overall composition, leading to the secondary phase separation as seen in the top portion of Figure 2e. Additionally, a certain mass transfer across the dropletmatrix interface is involved upon the second cycle of irradiation. This situation is evidenced by the horizontal shifts in the bottom portion of Figure 2e and by the sudden increase in the size of the PMMA-rich droplet preformed in the dark as shown with the red squares in Figure 2h. One of the most remarkable features of our method is that the morphology of the secondary structure can be controlled solely by changing the light intensity in the second cycle of
Figure 3. (a) Profile for the irradiation with intense UV light (I = 750 μW/cm2) for the second cycle of irradiation. (b) The time course of phase separation during the second irradiation cycle. The scale bars are 25 μm. (c) Time dependence of the distribution of the circularly averaged 1D Fourier intensity, I(q) where q is wavenumber. Note that the Fourier transform is conducted only for the PS-rich domain in (b).
bottom). This condition indicates the successive growth of the domains with an equilibrium concentration, suggesting that the behavior of the phase separation shifts a conventional conserved system in the absence of the chemical reaction. In theory, the equilibrium concentration of the constituents is dictated by the overall composition of the mixture.31 In our system, the composition ceaselessly changes by the ongoing chemical reaction under irradiation and so does the equilibrium concentration of the constituents. Thus, under exposure to UV, the constituents of the droplet and the matrix continuously vary their concentration to minimize the mixing energy of the polymer mixture. This variation would be mediated by the diffusion (mass transfer) of the polymer components and 3980
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irradiation. Figure 3 demonstrates a typical case for a mixture exposed to intense UV light (I = 750 μW/cm2) in the second irradiation process. This UV irradiation gives rise to the spinodal decomposition process of the matrix as seen in Figure 3b, and provides the cocontinuous structure shown in Figure 1c. To our best knowledge, this is the first example of the hierarchical structure in which the cocontinuous structure is incorporated into the matrix observed for the phase separation of binary polymer mixtures. Generally, spinodal decomposition is seen in conserved systems when the polymer mixture is deeply quenched into an unstable region by a temperature jump near critical composition.32 In our system, upon the second irradiation cycle, the matrix contains excess PS (less PMMA) and unreacted MMA, which can be instantaneously photopolymerized under exposure to an intense UV light with I = 750 μW/cm2 (see section 3 in the SI). Thus, the PS-rich matrix can be deeply quenched into the unstable region near the critical composition and undergoes spinodal decomposition. The peak of the 1D Fourier intensity distribution shown in Figure 3c indicates the periodicity of the cocontinuous structure, whereas the growth process of this secondary phase separation is revealed by the increase in the peak intensity with irradiation time. In contrast to the matrix, the droplet within the same mixture comprises excess PMMA (less PS) and unreacted MMA. The photopolymerization of the MMA therefore chemically quenches the PMMA-rich droplet through the off-critical composition, resulting in the PS-rich droplets as a minor component in the preformed droplet regardless of the light intensity in the second cycle of irradiation (demonstrated in the area marked with the number 1 in Figure 1b,c). Lastly, the prepared hierarchical morphology can be transformed into multimodal porous PS films by dissolving the PMMA-rich domains using organic solvent (e.g., toluene). The resulting PS films reversibly change into hard membranes in polar solvents (e.g., methanol) and gels in nonpolar solvents (e.g., toluene) (see section 4 in the SI). Upon swelling, the film shown in Figure 1a absorbs toluene by ∼100 times within 5 s (in terms of the weight proportion to the dried form). Among nonpolar solvents, these values are comparable with those developed for sorbents of organic solvents recently.33 In particular, the rapid uptake (5 s) of the solvent would be ascribed to the presence of the large pores in the film. With further optimization, the proposed method would be applicable to the fabrication of the polymer films capable of removing hazardous, volatile organic compounds (VOCs) with high speed. In summary, we described a class of light-induced, hierarchically structured materials controllable by intermittent UV irradiation. The significant feature of our method is that the chemical reaction with the ability to induce phase separation can be instantaneously initiated and ceased independently by turning on and off light at room temperature. Furthermore, the morphology with varying hierarchies can be controlled by changing the light intensity. We believe that the method proposed in this study appears generic to other photoreactive polymers such as poly(vinyl cinnamate) combined with miscible vinyl monomers. The resulting polymers after suitable treatment, such as extracting the noncross-link component, would be potentially utilized for fabrication of scaffolds for a variety of applications such as absorbents of organic solvents.
Letter
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, method for the determination of the onset time of phase separation, polymerization kinetics of MMA, and preparation procedure for the porous PS membrane and gel. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Numbers 23550241 and 24710128.
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REFERENCES
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