Article pubs.acs.org/Macromolecules
Tricontinuous Morphology of Ternary Polymer Blends Driven by Photopolymerization: Reaction and Phase Separation Kinetics Toshiya Shukutani, Takahiro Myojo, 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: Phase separation of a ternary mixture composed of poly(ethyl acrylate) and poly(ethyl methacrylate) derivatives dissolved in methyl methacrylate monomer was induced and driven by photopolymerization. Upon varying the initial composition of the mixture and the molecular weight of the resulting poly(methyl methacrylate) via changing the light intensity, a wide variety of morphologies ranging from separated double droplets, Janus-like droplets, core−shell, salami, bicontinuous to tricontinuous structures emerged due to the competition between phase separation and polymerization. Among the emerging morphologies, the tricontinuous structures were significant, and the details of the formation process were particularly investigated by using laser-scanning confocal microscopy (LSCM). Time-resolved experiments reveal that these tricontinuous structures were generated via two steps of the consecutive phase separation process. The formation of these tricontinuous structures is discussed with respect to the polymerization kinetics, the resulting molecular weight, and the residual monomer in the polymerizing mixture.
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INTRODUCTION Multicomponent polymers are usually immiscible and consequently undergo phase separation upon mixing. The resulting morphology is mainly determined by three main factors: the interfacial tension between polymer pairs in the ternary mixture,1,2 the kinetic parameters such as viscosity ratio of the disperse morphology,3 the elasticity,4 and/or combination between the two (viscoelasticity)5 arising from the interconnectivity of chain molecules. The couplings among these factors often complicate the phase separation kinetics as well as the resulting morphology. Compared to binary mixtures, the phase separation kinetics of ternary polymer mixtures has not extensively investigated though there are a large number of studies on the equilibrium morphology of immiscible ternary polymer blends because of their practical importance.6 By numerical analysis of diffusion equation for the spinodal decomposition process, the kinetics and the morphology of nonreactive ternary polymer mixtures have been predicted and discussed.7,8 In order to control the morphology of ternary polymer mixtures, mechanical methods such as melt mixing have been used, and a number of research groups have interestingly demonstrated the variation of the morphology resulting from phase separation of ternary polymer blends.9,10 In practice, the melt mixing techniques utilized to prepare multicomponent polymer mixtures often require high temperatures and mechanical agitations. As a consequence, partial degradation of polymers could occasionally occur during the mixing process. From the viewpoint of kinetic studies, it would be very attractive if the phase separation kinetics of ternary polymer mixtures, particularly during the reaction process, © 2014 American Chemical Society
could be in situ monitored because most of the scanning (SEM) and/or transmission electron microscopes (TEM) cannot provide such the information. From the viewpoint of pattern formation in systems with competition between antagonistic interactions in polymer mixtures,11 we have examined the phase separation kinetics of binary polymer mixtures induced by photopolymerization. The roles of shrinkage12 and wetting associated with polymerization-induced phase separation were recently found and discussed in conjunction with the polymerization kinetics.13 Here, UV (ultraviolet) light was utilized as an efficient tool to start and terminate the polymerization independently from thermodynamic variables such as temperature and pressure. Furthermore, the light intensity can provide a fine control for the quench depth. The advantages of using light to drive these competition processes have been also extended to spatial14 and temporal modulations15 of the phase separation. In this study, we will show that by coupling photopolymerization to phase separation, a wide variety of morphologies can be produced and controlled by changing the light intensity and the initial composition of the mixture. Using laser-scanning confocal microscopy (LSCM), the formation kinetics of the tricontinuous morphology induced by photopolymerization was in situ monitored and compared to the polymerization kinetics obtained under the same conditions by Fourier-transform infrared spectroscopy (FT-IR). The Received: February 8, 2014 Revised: May 18, 2014 Published: June 25, 2014 4380
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illustrated in Figure 1a where the phase boundary is indicated by the dotted curve which distinguishes the immiscible (shaded area) from
relations between polymerization and phase separation are discussed. Finally, the role of residual monomer is discussed for the correlations between the resulting morphologies and the interfacial properties.
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EXPERIMENTAL SECTION
Materials. Polymers used in this study are poly(ethyl acrylate) doubly labeled with anthracene and fluorescein (PEAAF, Mw = 83 000, Mw/Mn = 2.1) and poly(ethyl methacrylate) labeled with rhodamine B (PEMAR, Mw = 1.4 × 105, Mw/Mn = 1.6). Fluorescein and rhodamine B were used as fluorescent marker to image the two polymers PEAAF and PEMAR in the mixture by laser-scanning confocal microscopy. In particular, ethyl acrylate was polymerized in acetone under reflux condition (57 °C) in 30 min to prevent the formation of branching structure caused by radical transfer mechanism at high temperature.16 The detailed synthesis schemes of PEAAF and PEMAR are shown respectively in Schemes S1 and S2 of the Supporting Information. Here, the absorption and fluorescence spectra of these two polymers are also illustrated in Spectra S1 together with the corresponding optical filters used for wavelength selection. It should be noted that the three polymer components of the mixture used in this study poly(ethyl acrylate), poly(ethyl methacrylate), and poly(methyl methacrylate)were chosen as samples because the maximum difference in their refractive indices are less than 1.3%. This small difference helps minimizing the scattering of a laser beam from different domain interfaces of the resulting morphology during the scanning process of the laser confocal microscopy. As a consequence, all the phase-separated ternary PEAAF/PEMAR/PMMA mixtures used in this study are almost transparent under visible light. Samples with the dimension (15 mm × 15 mm × 25 μm) were sandwiched between two coverslips with an aluminum spacer used to adjust the sample thickness. Morphology Imaging by Laser-Scanning Confocal Microscope (LSCM). The resulting morphology was in situ monitored under irradiation with UV light (405 nm, 350 W Hg−Xe lamp) on a laser-scanning confocal microscope (LSM 5, Pascal, Zeiss Inc.) equipped with a He−Ne laser (1 mW, 543 nm) and an Ar+ ion laser (25 mW, 488 nm). The former was used to excite the rhodamine B moieties on the PEMAR component, whereas the latter was for the fluorescein moieties labeled on the PEAAF chains. The morphology emerging from the polymerization was observed with several objective lenses (×20, N.A. = 0.5; ×40, N.A. = 0.75; ×63, N.A. = 1.4 oil immersion) depending on the structure length scales. The optical micrographs were taken with the dimension (512 pixels × 512 pixels). The maximum resolution along the Z-direction is 0.7 μm, and the images were recorded at 0.3 μm interval. Data analysis was performed by 2D-fast Fourier transform (2D-FFT) to obtain the characteristic length scales of the morphology. The imaging procedure and the data analysis are described in detail elsewhere.13,17 The multitracking techniques18 were also utilized to separately monitor and image the two components PEAAF and PEMAR in the ternary mixture, thus enabling the independent imaging of these two components in the ternary mixtures during the phase separation. Determination of the Phase Diagram for the Ternary Mixture. Before photopolymerization, the coexistence curve of the ternary mixtures PEAAF/PEMAR/MMA was determined by directly observing the dependence of its miscibility on the composition under the LSCM. At first, PEAAF and PEMAR were dissolved in MMA monomer to form a homogeneous ternary mixture with a given initial composition. The solution was stirred and allowed to stand at 25 °C for an hour. Subsequently, the liquid mixture was poured dropwise onto a coverslip and was quickly sealed using the second coverslip prior to the observation under the LSCM. Upon varying the composition, the phase boundary at 25 °C was determined as soon as the phase-separated domains start appearing in the uniform solution. Fluorescence of the rhodamine B moieties labeled on the PEMAR was used for this observation because compared to fluorescein, its fading is almost negligible under our experimental conditions. The composition dependence of the phase boundary is
Figure 1. (a) Triangular phase diagram of the ternary mixture PEAAF/PEMAR/MMA measured at 25 °C by using the fluorescence intensity of PEMAR components under a laser confocal microscope. The open and filled circles indicate the composition of the miscible and immiscible mixtures, respectively. The asterisk represents the experimental composition before the photopolymerization of MMA. (b) Sketch of the tetrahedronal phase diagram for the PEAAF/ PEMAR/PMMA/MMA mixture resulting from polymerization of MMA in the ternary mixture shown in (a). the miscible regions. Since PEMAR and PEAAF are not soluble in MMA at high composition and particularly the mixture with high polymer concentrations results in highly viscous solutions, the composition PEAAF/PEMAR/MMA (6/6/88) was chosen for the experiments described in this paper. The localization of the mixture before the photopolymerization is indicated by an asterisk in Figure 1a on the triangular phase diagram. As the photopolymerization proceeds, PMMA was gradually generated, and the reacting system becomes a quaternary mixture with the phase diagram sketched in Figure 1b. The arrow in the figure expresses the evolution of the mixture as the photopolymerization of MMA monomer proceeds. Determination of the Polymerization Yield by FT-IR. The polymerization kinetics of MMA in the ternary mixture was followed in situ by using a Fourier-transform infrared spectrometer (FT-IR Model 8400 S, Shimadzu Inc.) with a resolution of 4 cm−1. The consumption of MMA monomer during the polymerization process was monitored under different irradiation-time intervals by following the change in the characteristic absorption of the CC double bond at 1640 cm−1. In order to compensate for the shrinkage associated with the polymerization, all the absorbances were normalized to the absorbance of the CO stretching vibration at 1720 cm−1. The polymerization yield Φ of PMMA under irradiation was calculated using the following equation:
⎡ ⎢ Φ = ⎢1 − ⎢⎣
⎤ ⎥ × 100 (A CC)t = 0 ⎥ ⎥ (A CO)t = 0 ⎦ (A CC)t (A CO)t
(1)
Here A is the absorbance of a particular vibrational mode of MMA. ACC and ACO indicate the characteristic absorbance of these chemical bonds in the monomer. Since the CO bond does not involve in the polymerization process, the reaction kinetics can be determined through the change in the absorbance of the CC stretching mode of the MMA monomer.
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RESULTS AND DISCUSSION Variety of Morphologies upon Photopolymerization of MMA Monomer. In general, polymer mixtures undergoing phase separation induced by polymerization exhibit the typical behavior of a system with competing interactions.19,20 For ternary mixtures, the polymerization competes with phase separation of the three polymer components. Depending on the 4381
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Figure 2. Morphology map with six different kinds of morphologies obtained by polymerization of MMA in the ternary mixture PEAAF/PEMAR/ MMA with partial symmetric compositions under various light intensities. The colorful box is used to classify these morphologies.
These experimental results clearly indicate that one can use light intensity as an only control parameter to generate and manipulate a wide variety of morphologies in ternary polymer mixtures. It should be noted that since the fluorescein marker is easily bleached compared to rhodamine B under irradiation with visible light, the contrast of the PEMAR-rich (red) phase appears stronger than the PEAAF-rich (green) phase in the morphology map illustrated in Figure 2. It is also worth noting that the so-called “salami” structure is formed by the coupling between the spinodal decomposition process and wetting phenomena.13 The details are provided in Figure S1 of the Supporting Information. Tricontinuous Morphology Generated by Photopolymerization. Among various structures illustrated in Figure 2, we particularly pay attention to the tricontinuous morphology because of its structural uniqueness and the potential applications to materials science such as controlling the transport properties.23−26 In this study, the emergence process of the tricontinuous morphology developed by the polymerization of MMA was in situ observed by laser-scanning confocal microscopy (LSCM) and was analyzed by comparison to the corresponding polymerization kinetics in situ monitored by FT-IR. To determine the irradiation time ti corresponding to the onset of phase separation, the deviation from the average concentration of polymers in the mixture was calculated from the fluorescence intensity of the PEMAR component in the images using the equation
resulting molecular weight, the yield of PMMA generated by photopolymerization and the interfacial properties of each polymer pair, the polymerizing mixture can produce a wide variety of morphologies upon traveling along the edge MMA → PMMA of the tetrahedronal phase diagram drawn in Figure 1b. Since mobility of the mixture will greatly decrease as the polymerization of MMA proceeds, the phase separation process would be arrested in its intermediate state due to the increase in the glass transition temperature (Tg) of the mixture, restoring the footprints of the transient phase diagram. The richness of morphology driven by photopolymerization in the ternary mixture can be observed in the “morphological map” depicted in Figure 2 through the variation of the initial composition and the irradiation intensity. The latter plays the role equivalent to the initiator concentration in thermally induced polymerization.21 From the diagram of stationary morphologies displayed in Figure 2, it was found that upon increasing the light intensity, i.e., increasing the free radical concentration of MMA in the mixture, similar morphology emerges and shifts diagonally upward on the morphology diagram as seen in Figure 2. A similar kind of morphological transition has been found many years ago in the processing of binary polymer mixtures upon changing the viscosity ratio of the two polymer components.22 With all the partially symmetric compositions used to construct the morphology map shown in Figure 2, for irradiation time tirr > 60 min which corresponds to a polymer yield higher than 80%, six kinds of stationary morphologies double-droplets, bicontinuous, salami-like, tricontinuous, core− shell, and partial wetting structuresemerge, depending upon the light intensity and the initial composition of the mixture. 4382
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N
σ=
∑i = 1 (xi − m)2 N
(2)
where σ is the standard deviation from the average concentration, N is the total number of pixels, xi is the fluorescence intensity of the ith pixel, and m is the average fluorescence intensity before irradiation. As indicated in Figure 3, the standard deviation σ obtained from eq 2 is almost
Figure 3. Determination of the cloud point for the ternary mixture PEAAF/PEMAR/MMA (6/6/88) undergoing photopolymerization at 25 °C by using the standard deviation of the fluorescence intensity from the PEMAR component. ti indicates the irradiation time required for the mixture to enter the two-phase region.
Figure 4. Time evolution of the phase separation process in a PEAAF/ PEMAR/MMA (6/6/88) ternary mixture monitored by using the multitracking techniques for the two components PEMAR (a) and PEAAF (b) where the secondary phase separation is clearly observed at the late stage of polymerization. The scale bars are 20 μm. (c) An example for a stationary tricontinuous morphology obtained with a PEAAF (green)/PEMAR (red)/PMMA (black) by irradiating a PEAAF/PEMAR/MMA (6/6/88) mixture with 405 nm over 60 min at 25 °C: a, PEMAR continuous phase; b, PEAAF continuous phase.
unchanged at the beginning of irradiation and suddenly increases with irradiation time around ti = 1396 s, suggesting that the mixture enters the two-phase region at this stage of irradiation. It should be noted that to minimize the scattering of laser light upon scanning the resulting morphology under the LSCM, the three polymer components PEAAF, PEMAR, and PMMA were chosen so that they have almost the same refractive index. Accordingly, the scattering intensity from these ternary mixtures is almost negligible, preventing precise detection of the onset of phase separation by scattering techniques as previously reported.27 The fluorescence data shown in Figure 3 are consistent with the morphology data observed under LSCM for the same mixture as described below. Shown in Figure 4 is the irradiation-time dependence of the morphology obtained for a ternary mixture PEAAF/PEMAR/ MMA (6/6/88) at 25 °C. The time evolution of the two components PEAAF and PEMAR in the ternary mixture was separately monitored by the multitracking techniques on the LSCM using the fluorescence from fluorescein and rhodamine B markers. It was found that after 1394 s of irradiation the mixture is still in the one-phase region and the phase separation was not yet observed under the LSCM. By using the multitracking mode, the time evolution of the PEMAR-rich (red) and the PEAAF-rich (green) phases was separately monitored and illustrated respectively in the upper (a) and lower (b) parts of Figure 4. As irradiation continues, phase separation proceeds, and the bicontinuous structures became clearly observable under LSCM as shown in Figure 4 for tirr = 1574 s. It was found that during the coarsening process the PMMA-rich component (the black domains) gradually segregates inside the PEMAR-rich (red) phase and tends to merge with the PMMA-rich matrix.
Subsequently, as indicated in Figure 4b, the PEAAF-rich (green) phase gradually separates from the PEMAR-rich (red) phase and eventually localized at the interfaces between the PEMAR-rich (red) and PMMA-rich (black) phases, resulting in a tricontinuous structure. It should be noted that after 1800 s of irradiation the tricontinuous morphology was frozen by the increase in viscosity associated with a yield of PMMA Φ > 80% An example for a three-dimensional stationary tricontinuous morphology is illustrated in Figure 4c for a PEAAF/PEMAR/ MMA (6/6/88) irradiated with the light intensity 0.1 mW/cm2 (405 nm) in 60 min. Here, the three phases PEAAF-rich, PEMAR-rich, and PMMA-rich are spatially continuous. In order to analyze the time evolution of the morphology, the 2D-fast Fourier transform (2D-FFT) of the morphology was performed and shown in Figure 5 for the PEAAF-rich and the PEMAR-rich phases. As illustrated in Figure 5a, the 1DFourier intensity of the PEMAR-rich phase reveals the irradiation time-dependence process of the continuous PEMAR-rich (red) phase in the ternary mixture. As observed from the time evolution of the 1D-FFT spectra, the Fourier peak first appears around 0.7 μm−1 and gradually grows while shifting to the small q range as irradiation time increases. Eventually, the peak stops moving and no longer changes for irradiation time larger than 1800 s. On the other hand, as shown in Figure 5b, the 1D-FFT power spectra obtained for the PEAAF-rich (green) phase also exhibit a broad peak which gradually moves toward the small q range similarly to the 4383
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Figure 5. Irradiation-time dependence of the 2D-Fourier intensity distribution obtained for the PEMAR-rich phase (a) and the PEAAFrich phase (b) obtained by the multitracking techniques.
PEMAR component. Using the Bragg formula for diffraction from periodic structures, ξ = (2π/qmax), where qmax is the wavenumber corresponding to the maximum Fourier intensity and ξ is the period of the structure, it was found that the timeevolution process of the PEMAR-rich phase can be approximately expressed by the phase separation kinetics under the influence of hydrodynamic interactions proposed by Siggia, ξ ∼ t.28 The kinetic data are shown in Figure 6 in the
Figure 7. Coalescence processes of the PMMA-rich phase in the PEAAF/PEMAR/MMA (6/6/88): (a) nonrestricted case; (b) restricted case. The scale bars are 10 μm.
separation, the so-called viscoelastic phase separation.5 This “necking” phenomenon of morphology, which was observed at tirr = 1766 s for the PEMAR-rich (red) domains as shown in Figure 7a, was originated from the coarsening process of PMMA-rich domains (black) formed at tirr = 1634 s as indicated by white dotted circles. After successive coarsening steps, this PMMA-rich domain approaches the boundary between PEMAR-rich and PMMA-rich phases at tirr = 1704 s and eventually merges into the PMMA-rich matrix phase at tirr =1766 s, resulting in the tricontinuous PEAAF/PEMAR/ PMMA morphology. However, this is not always the only coarsening process observed for the phase separation of this ternary mixture. There also exist some regions in the same mixture where the coalescence between the PMMA-rich phases and the PMMA matrix was forbidden as revealed in Figure 7b. Here upon coarsening, the PMMA-rich (black) domains were formed inside the PEMAR-rich domains by a secondary phase separation. However, these black domains were not able to coalesce with the outside PMMA-rich phase upon coarsening. As a consequence, these PMMA-rich black droplets were arrested at the boundary between PEMAR-rich and PMMAmatrix phases as shown in Figure 7b for tirr =1766 s. A clear example for such the arrested phase separation is illustrated under higher magnification in Figure 8 where the PMMA-rich spherical (black) domains were arrested inside the PEAAF-rich (green) domains and were eventually located in the PEAAFrich (green) phase at the boundary between the PMMA (black) matrix and the PEMAR-rich (red) phases. From the kinetic viewpoint, the coalescence ability of the PMMA-rich domains would depend on their resulting molecular weight because one of the factors controlling the segregation is diffusion. To understand this phenomenon, we examined the polymerization kinetics and the molecular weight of the resulting PMMA in the PEAAF/PEMAR/MMA (6/6/88) ternary mixture under the same irradiation conditions. The molecular
Figure 6. Time evolution of the characteristic length scales of PEMAR-rich phase in a PEAAF/PEMAR/MMA (6/6/88) ternary mixture under irradiation with I = 0.1 mW/cm2.
double logarithmic scales. Careful examination of the 1D-FFT data obtained for the PEAAF-rich phase reveals an interesting time-evolution process of the phase separation. Namely, under continuous irradiation, PMMA was generated in the mixture, leading to the secondary phase separation of the PMMA-rich (black) phase inside the PEMAR-rich (red) domains as indicated by the arrows in Figure 5b. These are the black domains marked by the white dotted circles inside the continuous PEMAR-rich (red) phase of Figure 7. There are two distinct cases for this secondary phase separation. In one case, these PMMA-rich phases moves to the boundary between PEMAR (red) and PMMA-rich (black) domains and gradually coalesce with the PMMA-rich matrix outside these PEMARrich phases, leaving behind the “skewed-arc”-like domains in the morphology of Figure 7a observed at tirr = 1766 s. Taking into account that PEAAF has the lowest glass transition temperature (Tg = −22 °C) among the three polymers, this peculiar stationary morphology probably arises from the mismatch in elasticity and mobility of the three polymers upon phase 4384
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Figure 10. Correlation between the polymerization kinetics (diamond) and the resulting molecular weight (triangle) of PMMA in a PEAAF/PEMAR/MMA (6/6/88) mixture irradiated with 405 nm UV light. The molecular weight (solid curve) was obtained from the peak of GPC chromatograms. ti and tf are respectively the time for the onset and the termination of the phase separation. The dotted curve represents the irradiation-time derivative of the reaction yield, expressing the average polymerization rate.
Figure 8. Stationary morphology obtained for a PEAAF/PEMAR/ MMA (6/6/88) mixture irradiated with 405 nm light with PMMA-rich domains engulfed in the PEAAF-rich phase in the secondary phase separation process. Irradiation time is 60 min, and the light intensity is I = 0.1 mW/cm2.
broad peak as seen in Figure 9. Eventually, the molecular weight of this new component tends to approach a constant value. Therefore, PMMA with bimodal molecular weight exits in the polymerizing mixture at the stationary stage of the polymerization. From the time evolution of the phase separation shown in Figures 5 and 6, we concluded that the PMMA-rich component with high molecular weights resulting from the Norrish−Trommsdorff effect plays a role of a stopper for the phase separation induced by polymerization. Correlations between the Interfacial Properties and the Resulting Morphology in the Polymerizing Mixtures. Since the morphology resulting from mixtures of immiscible polymers is often predictable from their interfacial tension, we have attempted to use theories based on equilibrium surface tension29−31 such as Harkins equation and the Neumann triangle to explain the resulting morphology observed in this work. However, the calculation does not agree with the experimental results obtained by photopolymerization. The main reason for this inconsistency would be originated from the spatial distribution of the residual (nonreacting) MMA monomers in PEAAF- and PEMAR-rich phases as evidenced by the kinetics data of Figure 10 and the inhomogeneity shown in Figures 7 and 8. In order to elucidate the mechanism for the formation of various morphologies resulting from polymerization-induced phase separation, the distribution of the (transient) unreacted monomers inside the phase separated mixtures needs to be elucidated. In the context of generation of tricontinuous morphology, it should be noted that many years ago the idea of using a triblock copolymer ABC to emulsify blends of A and C for obtaining tricontinuous phases ABC was suggested.32 However, this is the case for the nonreacting and conserved systems, which is different from the reaction-diffusion coupling to phase separation described in this study. In the latter case, the mixtures are reactive and nonconserved due to the change in volume associated with polymerization.12 Further experiments using polymers with different Tg are in progress to clarify the roles of the glass transition and the elasticity of the polymerizing component in the resulting morphology for understanding the roles of interfacial phenomena in polymerization-induced phase separation.
weight of PMMA obtained after a given irradiation time was measured by using GPC with monodisperse polystyrene used as standard reference. As shown in Figures 9 and 10, before the
Figure 9. Variation of the chromatogram obtained by GPC for the polymerization of MMA in a PEAAF/PEMAR/MMA (6/6/88) mixture irradiated by 405 nm light with the intensity I = 0.1 mW/ cm2. THF was used as a diluent. The numbers in the figure indicate irradiation time.
rate of polymerization reaches its maximum at tirr =1500 s due to the Norrish−Trommsdorff effect,13 it was found that the primary molecular weight of the resulting PMMA in the ternary mixture is ca. 25 000. This molecular weight is almost unchanged during this period of irradiation as revealed by the broad peak on the left-hand side of the GPC chromatogram shown in Figure 9. It is worth noting that the broad peak appearing in the right-hand side of the chromatogram before irradiation (tirr = 0 s) comes from the two polymers, PEAAF and PEMAR, which were initially dissolved in MMA monomer. However, for irradiation time beyond the autoacceleration step around tirr = 1500 s, a new PMMA component with molecular weight of 1 order of magnitude higher (M ∼ 2 × 105) was generated and developed under irradiation as revealed by the 4385
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(11) Tran-Cong, Q.; Harada, A. Phys. Rev. Lett. 1996, 76, 1162− 1165. (12) Tran-Cong-Miyata, Q.; Kinohira, T.; Van-Pham, D.-T.; Hirose, A.; Norisuye, T.; Nakanishi, H. Curr. Opin. Solid State Mater. Sci. 2011, 15, 254−261. (13) Kimura, N.; Kawazoe, K.; Nakanishi, H.; Norisuye, T.; TranCong-Miyata, Q. Soft Matter 2013, 9, 8428−8437. (14) Ishino, S.; Nakanishi, H.; Norisuye, T.; Awatsuji, Y.; Tran-CongMiyata, Q. Macromol. Rapid Commun. 2006, 27, 758−762. (15) Tran-Cong-Miyata, Q.; Nishigami, S.; Ito, T.; Komatsu, S.; Norisuye, T. Nat. Mater. 2004, 3, 448−451. (16) For details, see: the Special Issue on Acrylate Free Radical Polymerization: From Mechanism to Polymer Design. Macromol. Rapid Commun. 2009, 30 (No. 23). (17) Nakanishi, H.; Satoh, M.; Norisuye, T.; Tran-Cong-Miyata, Q. Macromolecules 2004, 37, 8495−8498. (18) Dickinson, M. E.; Bearman, G.; Tille, S.; Lansford, R.; Fraser, S. E. Bioimaging 2001, 31, 1272−1278. (19) Harada, A.; Tran-Cong, Q. Macromolecules 1997, 30, 1643− 1650. (20) Vedmedenko, E. Y. Competing Interactions and Pattern Formation in Nanoworld; Wiley-VCH: Weinheim, 2007. (21) Roffey, C. Photogeneration of Reactive Species for UV Curing; John Wiley: New York, 1997; Chapter 3, pp 127−186. (22) Renfree, R. W.; Nosker, T. J.; Morrow, D. R.; Van Ness, K. E.; Suttner, L. W. SPE ANTEC 1992; pp 2396−2400. (23) Fujimoto, T.; Ohkoshi, K.; Miyaki, Y.; Nagasawa, M. Science 1984, 224, 74−76. (24) Kurian, P.; Kennedy, J. P. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1209−1217. (25) Han, Y.; Zhang, D.; Chung, L. L.; Sun, J.; Zhao, L.; Zou, X.; Ying, J. Y. Nat. Chem. 2009, 1, 123−127. (26) Rhodes, C. P.; Long, J. W.; Pittigrew, K. A.; Stroud, R. M.; Rolison, D. R. Nanoscale 2011, 3, 1731−1740. (27) Nakanishi, H.; Satoh, M.; Norisuye, T.; Tran-Cong-Miyata, Q. Macromolecules 2006, 39, 9456−9466. (28) Siggia, E. D. Phys. Rev. A 1979, 20, 595−605. (29) Wu, S. Polymer Interface and Adhesion; Marcel Dekker Inc.: New York, 1982; Chapter 3, pp 67−132. (30) For Neuman triangle, see: Berg, J. C. An Introduction to Interfaces and Colloids; World Scientific: Singapore, 2010; Chapter 2, pp 23− 103. See also ref 29 , Chapter 1 , p 14. (31) Style, R. W.; Dufresne, E. R. Soft Matter 2012, 8, 7177−7184. (32) Fredrickson, G. H.; Bates, F. S. Eur. Phys. J. B 1998, 1, 71−76.
CONCLUSION We have shown that photopolymerization can be used as a useful tool to drive the phase separation of ternary polymer mixtures. From the static viewpoint, a wide variety of stationary morphologies which would be useful to practical applications can be generated and controlled by taking advantages of the competition between polymerization and phase separation. Depending upon the irradiation intensity and the initial composition, the resulting morphology can vary from multidroplet structures, salami, bicontinuous to tricontinuous structures. By laser scanning confocal microscopy, the phase separation kinetics was in situ monitored during the photopolymerization. It was found that the phase separation kinetics data reflect the effects of hydrodynamics on the domains growth as previously studied by Siggia. The phase separation took place in the vicinity of the Trommsdorff−Norrish effect which serves as a controller of the phase separation. Experiments on generation and control of morphologies in ternary polymer blends other than the tricontinuous structures are currently in progress. Practical applications of these structures to transport process, templating microporous materials, and so on are also under way and will be reported later.
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ASSOCIATED CONTENT
S Supporting Information *
Additional detail of the polymer synthesis, characterization of these samples, the salami structure, and the tricontinuous morphologies. 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] (Q.T.-C.-M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Ministry of Education, Japan (MONKASHO) through Grant-in-Aid Nos. 20350107 and 23550241 for Scientific Research Types B and C is gratefully acknowledged. We also greatly acknowledge our former graduate students Kazuhiko Nakayama (currently with Sekisui Chemical Co. Inc.), Yuta Takeda (currently with Duponts, Japan) and Masataka Fukuoka (currently with Asahi Kasei Co.) for their active participation at the beginning of the project.
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REFERENCES
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dx.doi.org/10.1021/ma500302k | Macromolecules 2014, 47, 4380−4386
