Letter pubs.acs.org/macroletters
Optical Autocatalysis Establishes Novel Spatial Dynamics in Phase Separation of Polymer Blends during Photocuring Saeid Biria,† Phillip P. A. Malley,‡ Tara F. Kahan,‡ and Ian D. Hosein*,† †
Department of Biomedical and Chemical Engineering and ‡Department of Chemistry, Syracuse University, Syracuse, New York 13244, United States S Supporting Information *
ABSTRACT: We report a fundamentally new nonlinear dynamic system that couples optical autocatalytic behavior to phase evolution in photoreactive binary polymer blends. Upon exposure to light, the blend undergoes spontaneous patterning into a dense arrangement of microscale polymer filaments. The filaments’ growth in turn induces local spinodal decomposition of the blend along their length, thereby regulating the spatially dynamics of phase separation. This leads to the spontaneous organization of a large-scale binary phase morphology dictated by the filament arrangement. This is a new mechanism for polymer blend organization, which couples nonlinear optical dynamics to chemical phase separation dynamics, and offers a new approach to light-directed patterning and organization of polymer and hybrid blends.
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spontaneously dividing into a congruent arrangement of light and polymer “filaments”, respectively, the latter of which constitute regions of high molecular weight and cross-linking.4 Herein, we present a fundamentally new nonlinear dynamic system by inducing optical autocatalysis in photoreactive polymer blends. The formation and growth of filaments in the blend now induces and directs spinodal decomposition, resulting in large-scale binary phase morphology whose structure mimics the spatial pattern of the polymer and light filaments. This coupling of nonlinear optical dynamics to the chemical dynamics of phase separation is a straightforward synthetic route to complex-structured binary and multiphase materials. We investigated blends of an acrylate monomer with different functionalities ( f = 1−3) and an epoxide-terminated polydimethylsiloxane (PDMS) oligomer in a 50/50 relative weight fraction. The acrylate monomers studied were methyl methacrylate (MMA), 1,6-hexanediol dimethacrylate (HDDMA), and trimethylolpropane triacrylate (TMPTA). A dual-component visible-light initiator system (λmax = 470 nm) was employed to enable concurrent free radical and cationic polymerization of the acrylate and epoxide functions,9,10 respectively (see Supporting Information for details). Blend volumes of 3 mm depth and ∼10 cm2 area were irradiated with an incandescent light source. A 1D periodic mask was employed to modulate the spatial intensity profile of transmitted light, in order to seed the optical autocatalytic process.
he discovery of new dynamic behavior in chemical systems is of fundamental importance for establishing novel synthetic routes to intelligent materials and functional structures.1 One approach that has proven fruitful is to couple nonlinear dynamics to polymer systems. Examples include dewetting of thin films, oscillatory gels from Belousov− Zhabotinsky reactions, frontal polymerization from traveling thermal fronts, and phase separation.1,2 A key characteristic responsible for polymer organization observed in these systems is positive feedback3 between the nonlinear dynamic process and a physical or chemical property of the polymer. This creates autocatalytic behavior that drives self-organization and patterning formation. Our4 and other studies5 recently demonstrated a new form of dynamic behavior that emerges through coupling the nonlinear dynamics of transmitted light6 to photo-cross-linking polymer media. At a suitable exposure intensity,4 positive feedback is established between light propagation and photopolymerization and results in amplification in polymer growth at spots that are strongly localized in space. This is seeded by weak spatial and temporal noise present in the transmitted light, which leads to spatially local, infinitesimally faster photopolymerization rates. Light consequently leaks into these regions due to the increase in refractive index associated with their higher molecular weight.7 The resultant higher light intensity increases the photopolymerization rate,8 thereby establishing the positive feedback loop (i.e., polymerization rate → molecular weight/ refractive index → intensity → polymerization rate). As a result, the photopolymer system exhibits optical autocatalytic behavior, accelerating polymer growth in randomly located high-intensity spots. This leads both the transmitted light and the photopolymer system to mutually undergo pattern formation, © XXXX American Chemical Society
Received: August 27, 2016 Accepted: October 24, 2016
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DOI: 10.1021/acsmacrolett.6b00659 ACS Macro Lett. 2016, 5, 1237−1241
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O to Si−CH3 peak intensities, which quantitatively identifies regions rich in either acrylate or PDMS:12 large ratio values indicate acrylate-rich regions. The Raman maps (Figure 2) show that the blends organized into a binary phase morphology, which consists of microscale, acrylate-rich filaments (i.e., red regions in maps shown in Figure 2) and PDMS-rich surroundings (dark regions in Figure 2). The acrylate-rich filaments align along the depth of the blend (zaxis) and correspond spatially to the optical and polymer filaments that induced their formation. The filaments are present, and extend over, the entire depth of the volume analyzed. We tracked the filament formation kinetics by measuring the average feature size in the optical patterns over time (Figure 3).
Figure 1. Filamentation of incandescent light in photoreactive blends of (a) MMA/PDMS, (b) HDDMA/PDMS, and (c) TMPTA/PDMS. Images captured after 60, 45, and 12 min of irradiation, respectively. (d) Typical intensity pattern at the beginning of exposure, before filamentation. Scale bars = 480 μm.
Filamentation in the blends is achieved at a suitable exposure intensity (12 mW/cm2) (Figure 1a−c). The presence of filaments is evidenced by the high intensity spots in the spatial profiles of transmitted light at the “exit” surface of the blend. They form out of the bright stripe patterns created by the mask (Figure 1d). These bright spots correspond to the transverse cross-section of the light filaments and indicate the positions of the polymer filaments to which light is confined, owing to their wave-guiding properties.11 The filaments are densely packed within each stripe region, although at random positions relative to filaments in adjacent stripes. The corresponding polymer filament structures in the blend are observed in microscopy images (see Supporting Information). The morphology induced by optical autocatalytic pattern formation is revealed by using confocal Raman spectroscopy over an analyzed volume (3D) to probe a characteristic Raman intensity peak for the acrylate (CO, 1720 cm−1) and PDMS (Si−CH3, 685 cm−1). We spatially mapped the ratio of the C
Figure 3. Kinetics of morphology formation as determined by the measured feature size for polymerizing (a) MMA/PDMS, (b) HDDMA/PDMS, and (c) TMPTA/PDMS blends. Asterisks indicate the onset of gelation, and range bars indicate the duration over which filamentation proceeds.
Figure 2. Raman 3D maps of the CO to Si−CH3 peak intensity ratio for (a−c) MMA/PDMS, (d−f) HDDMA/PDMS, and (g−i) TMPTA/ PDMS blends. Maps acquired after 100 min of irradiation. (b,e,h) Slices in the x−y plane at Z = 0. (c,f,i) Slices in the y−z plane, at x positions chosen to show the profile of selected filaments. Z-axis corresponds to the propagation direction of light. The Z = 0 plane corresponds to the blends’ exit surface. Arrows aid in identifying the position of select filaments. 1238
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lengths. To substantiate that filamentation causes the homogeneous blends to become unstable, to allow for demixing, we studied the mixing thermodynamics of the blends under irradiation. Assuming that the Flory−Huggins free energy of mixing14 and elastic free energy are additive, the total free energy per unit volume of the polymer blend is15
All blends show an induction period before filamentation, during which the measured feature size (∼120 μm) corresponds to the bright stripes shown in Figure 1d. For MMA/PDMS, filamentation occurs gradually, with a sharp spot pattern corresponding to filaments evident after 60 min and filament size decreasing thereafter. For HDDMA/PDMS and TMPTA/PDMS, the induction period is followed by gelation, visually indicated by gel fronts propagating across the sample. Gelling corresponds to a rise in the measured feature size. Filamentation occurs immediately following gelation, as visually indicated by the breakup of the stripe pattern into bright spots corresponding to the filaments. This is indicated in Figure 3b and c by a duration over which the measured feature size decreases. The onset time of filamentation decreased with increased acrylate functionality, i.e., approximately 60, 45, and 10 min for MMA/PDMS, HDDMA/PDMS, and TMPTA/ PDMS, respectively. After ∼100 min no further changes were observed in the blends. The MMA/PDMS blend retains the filament structure over the duration of irradiation. HDDMA/ PDMA and TMPTA/PDMS blends show a gradual filament size increase, and the sharp spotted pattern in the morphology begins to deteriorate (see Supporting Information). Hence, with polyfunctional acrylates the filament morphology is visually most clear at an intermediate time during photocuring, where upon further polymerization the morphology begins to change. The filament formation kinetics and Raman 3D maps reveal the competition between photopolymerization and phase separation in determining final morphology, which is wellknown in the photocuring of polymer blends.13 Namely, diffusion-based demixing drives the blend into two phases, while photopolymerization/photo-cross-linking tends to keep the blend components together, owing to increasing viscosity and elasticity. The lack of cross-linking with MMA affords the system with reduced viscosity and increased polymer flexibility to enable phase separation to proceed relatively uninhibited. It is for this reason that MMA/PDMS shows a multitude of relatively shorter filaments (Figure 2a−c and, comparatively, Figure 2b, e, and h), which is most likely due to longer filaments splitting apart during phase separation. For HDDMA/PDMS, continuous filaments over the entire analyzed depth (∼1 mm) are present (Figure 2d and f), and split filaments are also present but are longer than in MMA/ PDMS. Longer filaments may be due to HDDMA providing the necessary cross-linking to resist splitting apart, yet still allowing for phase separation. However, the presence of short filaments along a common line in the z-direction (Figure 2f) indicates that cross-linking of HDDMA may also inhibit a growing filament phase from continuously extending over the blend. This latter effect appears stronger in TMPTA/PDMS, as most filaments are not clearly distinguished. Consequently, the increase in filament size over time in HDDMA/PDMS and TMPTA/PDMS, as shown in Figure 3b and c, is most likely due to acrylate cross-linking that inhibits demixing and distorts the filament phases upon continued polymerization. Thus, a balance between polymerization and phase separation is necessary to retain the filament morphology established by irradiation. To this end, the functionality of the acrylate monomer is critical. The observed binary phase morphology indicates not only that the autocatalytic process facilitates structural patterning through filamentation but also the associated molecular weight increase in the filaments induces phase separation along their
2φ ⎛ φ ⎞ 3 ΔG (φs2/3φ1/3 − φ) + ln⎜⎜ ⎟⎟ = RT 2N1 fN1 ⎝ φs ⎠ +
(1 − φ)ln(1 − φ) + χφ(1 − φ) N2
(1)
where φ is the volume fraction of acrylate; φs is the network volume fraction (taken as the initial volume fraction of acrylate); N1 and N2 are the degrees of polymerization for acrylate and PDMS, respectively; f is the functionality of the acrylate; and χ is the interaction parameter. Equation 1 accounts for both polymerization as well as associated changes in elasticity and shrinkage of the curing blend. Shrinkage during irradiation and the associated change in the volume fractions of the blend components were determined in separate experiments (see Supporting Information). N1 and N2 over the course of irradiation were determined from the extent of polymerization (p) measured with FTIR spectroscopy (see Supporting Information). The critical interaction parameter, χc, indicates the point at which spinodal decomposition occurs and is determined by the point at which the second derivative of the Gibbs free energy is equal to zero15 χc =
⎛ ⎞ φs2/3 1⎜ 2 1 ⎟ + + 2 ⎜⎝ fN1φ N2(1 − φ) ⎟⎠ N1φ5/3
(2)
The value of χ c decreases over the duration of the photopolymerization due to the increase in molecular weight of both acrylate and PDMS polymers. The onset of phase separation is indicated by when χc < χFH,14 where χFH is the Flory−Huggins interaction parameter of the blend determined from the Hildebrand solubility parameters (δ) for acrylate and PDMS.16,17 We calculated χFH to be 2.216, 1.981, and 1.782 for 50/50 weight fraction blends of MMA/PDMS, HDDMA/ PDMS, and TMPTA/PDMS, respectively (see Supporting Information). Figure 4(a,c,e) shows that irradiating all blends eventually yields the condition χc < χFH; therefore, photopolymerization associated with optical autocatalysis observed herein indeed results in spinodal decomposition of the blends. Phase separation is further confirmed by the upward shift and outward expansion of the calculated spinodal curve in the binary phase diagram, such that the temperature−composition coordinate (298 K, 0.5), assuming room temperature, enters the two-phase coexistence region (Figure 4b, d, and f). Our proposed mechanism for optical autocatalytic blend organization is as follows: After an induction period, during which only polymerization occurs, filamentation of light and the blend morphology begins. The growing filaments initially consist of both photopolymerizing acrylate and PDMS. As the molecular weight increase of the acrylate monomer is faster than the PDMS oligomer, owing to the faster free-radical polymerization, the rise in the refractive index of the acrylate is consequently faster, and thus growth is dominated by the acrylate. Sustained molecular weight increases in both acrylate and PDMS, owing to continued photoinitiation by transmitted light within the filaments, drive PDMS out of the filament 1239
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now facilitates phase evolution to create binary phase structures commensurate to the filament pattern. This morphology, and how they form, is unlike that observed in previous photopolymerization-induced phase separations. While some thermal autocatalysis may aid phase evolution, heat evolution can neither induce nor direct the organization of microscale filaments, due, in part, to the isotropic nature of heat propagation in polymerizing systems. Therefore, the phase morphologies achieved are due purely to optical autocatalysis and represent a new nonlinear chemical dynamic phenomenon. In summary, we have demonstrated that optical autocatalysis in a photoreactive blend directs phase evolution toward a binary phase structure consisting of one phase in the form of filaments and the other polymer in the surroundings. Invoking this process in other polymer blends is straightforward because the refractive index increases of most photoinitiated polymers allow for optical autocatalysis. The generalizability of phase separation enables use of this process in a range of polymer and hybrid blends. This opens opportunities for studying novel light-directed materials organization and synthetic routes to complex, functional architectures in polymer and hybrid materials. The patterning of binary polymer phases can lead to, as examples, the synthesis of complex structures in filled and open cellular materials, advanced blend composites, as well as novel light-responsive and programmable soft materials.
Figure 4. Mixing stability of polymerizing (a,b) MMA/PDMS, (c,d) HDDMA/PDMS, and (e,f) TMPTA/PDMS blends. (a, c, e) show changes in χc (plotted against N of acrylate) with increased degree of polymerization of both polymers. Blue lines indicate the boundary set by χFH, below which blends become unstable. (b,d,f) are phase diagrams showing evolution of the spinodal curve. Blue circles indicate the initial T−φ coordinate and red circles the final T−φ coordinate after shrinkage.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00659. (1) Experimental details. (2) Interaction parameter calculations. (3) Thermodynamic calculations. (4) Microscopy images. (5) In situ microscopy data. (5) Shrinkage data. (6) FTIR peak, polymer conversion, degree of polymerization, and χc data over irradiation time (PDF)
region. As the light intensity is highest at the filament centers, this leads to the fastest molecular weight increases, and thus instability and phase separation is initiated first at the center. The instability region then expands radially outward from the filament central axis, resulting in coaxial directionality of the dynamics of PDMS diffusion out of and acrylate monomer into the filament regions. This mechanism explains how the filament pattern directs the large-scale organization of phase morphology and describes a new approach for controlling the phase evolution dynamics. The ability for the polymer filaments to extend over millimeters in a blend volume enables them to induce and regulate phase separation over significant depths; the ability to generate filamentation over wide areas enables the creation of, and control over, large-scale polymer blend morphologies. Light-driven phase separation in polymer blends using uniform UV illumination has been studied.18−21 These systems demonstrate thermal autocatalytic behavior due to the heat produced from the photopolymerization. However, the high reactivity and high absorbance associated with UV curing of polymers results in limited feedback from the blend so as to modulate the spatial distribution of transmitted light. Optical autocatalysis occurs with visible light (lower absorbance) at low polymerization rates (i.e., low exposure intensities) that allow the polymer blend to evolve the spatial refractive index differences (via molecular weight differences) that modulate light propagation. Blend morphologies have also been achieved using holographic fields.22,23 However, no feedback is present, as the holographic field remains static. Hence, the uniqueness of the nonlinear dynamic system herein is feedback between the photopolymerizing blend and transmitted light, which results in a mutual, dynamic interaction that drives pattern formation and
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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