Article pubs.acs.org/Macromolecules
Control of Morphology in Polymer Blends through Light SelfTrapping: An in Situ Study of Structure Evolution, Reaction Kinetics, and Phase Separation Saeid Biria and Ian D. Hosein* Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, United States S Supporting Information *
ABSTRACT: We report on how polymer morphology is controlled through the self-trapping of transmitted optical beams in photoreactive polymer blends. Self-trapped optical beams, characterized by divergence-free propagation, drives the growth of a congruent arrangement of polymer filaments in the blends. With suitable component weight fractions and exposure intensity, binary phase morphologies form in precisely the same pattern as the beams’ arrangement, thereby producing 2D structures in polymer blend volumes of large depth and area. Morphology evolution and the formation processes were observed by in situ microscopy. In situ confocal Raman measurements of polymer conversion and molecular weight increase along the filament regions reveal that polymerization undergoes autoacceleration, followed by the onset of mixing instability which leads to phase separation. These phenomena begin at the front end of the filament and propagate along its length over the depth of the blend. Control over morphology is discussed with respect to the competitive processes of phase separation and photo-cross-linking.
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congruent arrangement of polymer “filaments” of cylindrical geometry in the medium, and this process has been exploited to fabricate both 2D and 3D periodic structures.22,23,25−27 We recently showed that light can undergo self-trapping in a photoreactive polymer blend and that randomly arranged, filament-based binary phase morphologies can emerge over time.28 Given this potential for light self-trapping to dictate polymer structure, we performed in situ studies on morphology evolution, polymerization kinetics, mixing thermodynamics, and phase separation now in photoreactive polymer blends during irradiation with a periodic array of optical beams. We show coupling of the polymerization-induced self-trapping properties of light to polymerization-induced phase separation and elucidate the mechanisms of morphology evolution and the conditions under which polymer blend morphology correlates to the configuration of optical beams, that is, when morphology and light profile are spatially congruent with one another.
INTRODUCTION Polymer blends play critical roles as materials for ion1,2 and proton transport,3,4 thermoelectrics,5 solid-state lighting,6,7 and advanced composites.8,9 Controlling blend structure precisely and scalably remains a major challenge, yet is crucial for advancing their structure−property relationships. Photopolymerization-induced phase separation is one route to control morphology.10,11 Thus far, cocontinuous and “droplet” phase structures are produced with uniform irradiation, of either continuous12−15 or temporally modulated irradiation.16,17 However, the spatially random nature of phase separation, particularly with spinodal decomposition, renders achieving structures with specific arrangement quite difficult, unless directionality is specified to the phase separation dynamics. To this end, periodic morphologies are achieved using holographic polymerization,11,18,19 which directs diffusion of the components in or out of the bright regions. However, scalability is restricted by the limited volume over which a holographic field can be maintained (∼100 μm × 1 cm2).20 Hence, a new lightbased approach is needed that combines scalability, through use of wide-area sources, with a highly precise process, to produce large-scale, regular morphologies. It has been shown that transmitted optical beams undergo self-trapping in photopolymers, thereby propagating divergence-free, as a result of the dynamic balance between natural beam divergence and a self-focusing nonlinearity.21−23 The selffocusing effect emerges from the photopolymerization-induced changes in refractive index of the medium22 which creates a similar Kerr effect observed in nonlinear optics.24 In photopolymers, the self-trapped beams inscribe a permanent, © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Materials. Trimethylolpropane triacrylate (TMPTA) was purchased from Sigma-Aldrich, and epoxypropoxypropyl-terminated polydimethylsiloxane oligomer (PDMS, Mw = 363) was from Gelest. Note that while we refer to this oligomer by the acronym PDMS for simplicity, its polymerized form is not strictly polydimethylsiloxane, owing to the epoxypropoxypropyl ends groups. See Supporting Information for molecular structures. The visible-light photoinitiator system consisted of free-radical initiator camphorquinone (CQ) Received: March 6, 2017
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DOI: 10.1021/acs.macromol.7b00484 Macromolecules XXXX, XXX, XXX−XXX
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to excite a region of ∼40 μm diameter. Spectra collected over time consisted of line scans over the depth of the blend acquired along the central axis of a beam’s path length at a step interval of 200 μm, using the automated x, y, z translation stage of the Leica microscope (z direction only). See the Supporting Information for a schematic of the setup. A single scan was acquired in ∼30 s (collection time for each spectra was 1 s), and scans were collected over time at intervals of 5− 15 min depending on the total irradiation time. No beam damage was observed in samples over the duration of experiments. The two conditions for tracking polymer conversion of each blend component using Raman spectroscopy are that (1) each component has spectroscopically resolvable peaks that are characteristically different from one another, in order to properly associate each with their respective polymer, and (2) the peaks must be sufficiently separated in the spectra to be differentiated from one another. These two conditions are met by the Raman-active bands of the carbonyl bond31 (CO, 1730 cm−1) of TMPTA and epoxide32 (908 cm−1) of PDMS. The carbonyl peak was chosen due to its efficacy in tracking conversion of polyfunctional acrylates.33,34 The characteristic band at 908 cm−1 corresponds to the asymmetric ring deformation vibration of the epoxide function. Decreases over time in the integrated peak intensities of CO and epoxide were used to calculate the local conversions (p) (i.e., reaction yield) and degrees of polymerization (i.e., number-average molecular weight, N = Xn). The conversion was determined by
purchased from Sigma-Aldrich and cationic initiator (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate (OPPI) purchased from Hampford Research Inc. All chemicals were used as received. Preparation of Photopolymerizable Blends. We selected a two-component blend because it is the simplest mixture with which to study phase separation. TMPTA and PDMS are particularly selected as an appropriate polymer pair for this study because (1) they form miscible mixtures in their monomer−oligomer state, (2) their chemistries are different so as to discern spectroscopically their individual polymerization and spatial distribution in blends (vide inf ra), (3) their reactive epoxy and acrylate chemistries ensure no copolymerization, (4) their respective measured refractive indices enable the self-trapping and waveguiding of light upon phase separation (vide inf ra), and (5) they show relatively high transmittance at the excitation wavelength (470 nm) for polymerization (∼95%). Photocurable blends were prepared by mixing TMPTA and PDMS (both in liquid state at room temperature) of different relative weight fractions and dissolving in it CQ (2.5 wt % of total mixture) and OPPI (1.5 wt % of total mixture). Herein, the amount of PDMS and TMPTA in a blend is expressed as a relative weight percent ratio of PDMS to TMPTA (wt %/wt %). Mixtures were continuously stirred for 24 h, while protected from exposure to ambient light. To carry out an experiment, the mixture was injected into a homemade cell consisting of a circular Teflon frame sandwiched between two plastic coverslips. The thickness of the frame for all experiments was fixed to 3 mm, which sets the path length for transmitted light as well as the blend thickness. CQ sensitizes the photoreactive blend to blue light (λmax = ∼470 nm), initiates the free-radical polymerization of TMPTA, and facilitates free-radical decomposition of OPPI29 to initiate cationic polymerization of the epoxide functions on the PDMS oligomer. See Supporting Information for the absorbance properties of the photoinitiator system. Details on the mechanism of this dual photoinitiator system for facilitating concurrent free-radical and cationic polymerization can be found elsewhere.29,30 In Situ Microscopy of Photopolymerization. In situ observation was carried out with a Zeiss Axioscope equipped with an Axiocam 105 color camera operated by Zeiss imaging software. The photoreactive blends contained in the cells were placed onto a stage of the optical microscope and irradiated from below with collimated blue light from a light-emitting diode (LED) (λmax = 470 nm, Thorlabs Inc.) at an exposure intensity within 1−15 mW/cm2. The peak emission wavelength of the LED was selected to correspond to the peak absorbance of CQ in order to maximized photon absorbance. LED light was first passed through a photomask (Photosciences Inc.) consisting of a square array of circular apertures (40 μm diameter) spaced 200 μm apart, which generates the array of microscale beams that are subsequently transmitted through the blend. The microscope imaged the blend (transmission mode) using LED light as the imaging source. To measure filament growth during photopolymerization in the first 30 min. of irradiation, a set of samples were removed from the setup at different times, and the unreacted liquid blend was removed by washing with ethanol. To monitor and confirm light self-trapping, in separate experiments the spatial intensity profile of transmitted light was captured at the “exit plane” of the cell by passing light through imaging optics and then focusing onto a charge-coupled device (CCD) camera with pixel resolution of 3.2 × 3.2 μm (Dataray, WinCAMDXHR). Refractive Index Measurements. Refractive index changes for blends under irradiation over time were measured with an Abbe refractometer (Atago, NAR-1T SOLID). Approximately 0.1 mL of mixture was placed onto the analyzing prism surface (1 cm × 3 cm) of the refractometer and irradiated with LED light. Refractive indices were measured to an accuracy of ±0.001. In Situ Raman Spectroscopy. Raman spectra of the irradiated blends were acquired with a confocal Raman microscope (Renishaw, InVia) using a 785 nm continuous wave (CW) diode laser. The system combined the Raman spectrometer and a Leica DM2700P microscope. Similar to in situ microscopy experiments, the blends were irradiated with LED light from below, and the blend was probed with the confocal Raman laser from above. The laser power was adjusted so as
p=
2(I0 − I ) If
(1)
where I0 is the integrated peak area at t = 0 min, I is the integrated peak area any time thereafter, and f is the number of functions on the molecule (3 for TMPTA, 2 for PDMS). The degree of polymerization (N) was determined from p using Carothers’ equation.35 Raman volume images (i.e., 3D maps) of the final morphology were created by acquiring spectra at multiple positions in a sample (with 10 μm step size) using the automated translation stage of the microscope (x, y, and z directions). Owing to the long collection time to generate a single, high-resolution volume image (∼16 h), it was not possible to in situ map polymer morphology over the course of irradiation. Furthermore, to collect volume images within a reasonable time (10 cm 2 area (see Supporting Information). As there is no inherent scale limitation with this process, these morphologies may be created over areas as large as the light source.
Figure 2. Self-trapping of an array of optical beams in a photoreactive blend. (a) CCD image of 40 μm optical beams (at z = 0) and its strongly divergent profile after propagating z = 3 mm. (b) CCD image of self-trapped optical beams that form over the duration of exposure as compared to (a). (c) Growth of polymer filaments associated with the self-trapping of light. (d) Filament length over the duration of exposure until it reaches the other side of the blend (3 mm). Data shown correspond to an 80/20 blend at an exposure intensity of 15 mW/cm2. Scale bars = 200 μm. C
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Figure 3. Final morphologies observed by in situ study of blends irradiated with an array of self-trapped optical beams: (a−d) 90/10, (e−h) 80/20, and (i−l) 50/50. Exposure intensities are 1, 5, 10, and 15 mW/cm2 for columns left to right, respectively. Scale bars = 400 μm.
In Situ Measurement of Polymer Conversion over the Course of Irradiation. We performed in situ confocal Raman spectroscopy to probe the local conversion of the blend components along regions of light self-trapping. Note that we plot all data hereon against blend “depth”. However, because filaments grow along the depth axis of the blend, and we probe the blend in a filament region along its central axis, filament “length” and blend “depth” are equivalent. Figure 4 shows p and N mapped against irradiation time and depth for a 90/10 blend. The gel times observed with in situ microscopy correspond to the initial rise in TMPTA conversions to ∼20%, at which a gel−sol medium is expected to form that consists of low monomer fraction incorporated in the network39 and the remaining monomer, oligomer, and polymer chains partaking in subsequent growth. This conversion at which a gel−sol medium is attained is different from the common gel point value (0.66 for trifunctional TMPTA and 0.5 for bifunctional PDMS) at which a solid infinite network is attained. Thereafter, sharp increases in p and N is characteristic of the Trommsdorff−Norrish (TN) autoacceleration effect,40−42 induced by the increase in viscosity of the mixture and whose onset begins at the front end of the filament (i.e., where light enters the blend) and “propagates” along its length over time. This depth-dependent onset of the TN effect has not been previously observed and is most likely a result of the confinement and absorption of light along the filaments. PDMS also shows autoaccelerated growth because accelerated freeradical polymerization of TMPTA can increase the decomposition of the cationic initiator.29 Blends of 80/20 and 50/50 show similar gel−sol and TN effects (see Supporting Information), although low conversion at greater depths may also be attributed to distortion of morphology, which consequently causes filaments to misalign with the path analyzed by the Raman laser. No appreciable shrinkages in
samples were observed over the course of irradiation. The TN effect is not observed in the refractive index changes shown in Figure 1 because refractive index is more dependent on solidification, which is achieved predominately by the transition of the medium into a gel−sol. This is indicated by the correlation between the rise then plateau in refractive index with the times at which the gel−sol forms. Figure 4 shows how higher exposure intensities yield a shorter portion of a filament that finally becomes highly converted. This is owing to cure-depth limitations in crosslinked media.43 Namely, at high intensities, high cross-linking degrees trap monomer and photoinitiator, rendering filaments strongly absorbing and reducing the dosage of light at greater depths. For these reasons, the TN effect is not observed at greater depths with high exposure intensities. Furthermore, this reduced intensity can also explain the initially higher conversion at greater depths that occurs after gel−sol formation, as observed with 5−15 mW/cm2, because the consequent lowviscosity conditions enable sufficient monomer diffusion to sustain polymerizationmonomer diffusion to sustain polymerization.44 The subsequent drop in conversion, thereafter, at these greater depths just ahead of the TN effect can be attributed to inward diffusion of monomer. The TN effect begins earlier over the course of irradiation with increased intensity from 5 to 15 mW/cm2; however, irradiation at 1 mW/ cm2 is an exception to this trend. One explanation is that 5−15 mW/cm2 is a range in which the rate of termination increases with intensity,44 and the associated high cross-linking inhibits monomer diffusion. Whereas at 1 mW/cm2, termination may be lower and monomer diffusion is higher, allowing for the faster conversion along the filament; however, the back end is still inhibited from undergoing autoacceleration, which keeps this location at a low conversion. The final polymerization degrees along the filaments also correlate to their growth as D
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Figure 4. Filament polymerization kinetics. A 90/10 blend shown for exposure intensities of (a−d) 1 mW/cm2, (e−h) 5 mW/cm2, (i−l) 10 mW/ cm2, and (m−p) 15 mW/cm2. Columns from left to right show respectively TMPTA conversion, PDMS conversion, TMPTA polymerization degree, and PDMS polymerization degree. The entrance of the light into the blend is at depth = 0 μm. Labels “Gelling” and “TN” show exemplary regions at which each effect occurs, for convenience in identification at locations elsewhere.
individual filaments. Namely, values of N for 90/10 and 80/20 blends irradiated with 5 mW/cm2 or less were greater than with 10 and 15 mW/cm2, which also can be explained by lower intensities enabling higher conversion. The similar filament sizes among 90/10 irradiated with 5 mW/cm2 and 80/20 irradiated with either 1 or 5 mW/cm2 is corroborated by their similar polymerization degrees (Nmax = ∼12). Onset of Thermodynamic Mixing Instability. We assessed the thermodynamic mixing stability of the blend along the self-trapped beams over the duration of exposure. The total free energy of a binary blend may be expressed as45
spinodal decomposition occurs and is determined by when the second derivative of the free energy is equal to zero: χc =
(1 − φ) ln(1 − φ) + χφ(1 − φ) N2
(3)
As χc decreases with increases in N1 and N2 over time, the onset of mixing instability is indicated by when χc < χFH,46 where χFH is the Flory−Huggins interaction parameter of the blend determined by the Hildebrand solubility parameters (δ) for TMPTA and PDMS:46
2φ ⎛ φ ⎞ 3 ΔG (φs 2/3φ1/3 − φ) + ln⎜⎜ ⎟⎟ = 2N1 RT fN1 ⎝ φs ⎠ +
⎛ ⎞ φ 2/3 1⎜ 2 1 ⎟ + s 5/3 + ⎜ 2 ⎝ fN1φ N2(1 − ϕ) ⎟⎠ N1φ
χFH =
Vr [δ TMPTA − δ PDMS]2 RT
(4)
Values for χFH are 1.974, 1.926, and 1.782 for 90/10, 80/20, and 50/50 blends, respectively, based on calculation procedures described previously.28 The χFH values are different among blends owing to our consideration of a weight-fractiondependent reference volume, Vr. Figure 5 shows χc mapped against irradiation time and depth. The onset of instability is induced first at the front end of the filament, and subsequently, like the TN effect, the boundary between stable and unstable filament regions, i.e., an instability
(2)
where φ is the volume fraction of TMPTA, φs is the network volume fraction, N1 and N2 are the degrees of polymerization for TMPTA and PDMS, respectively, f is the functionality of TMPTA, and χ is the interaction parameter. The PDMS oligomer is treated as a single polymerizing unit in eq 2. The critical interaction parameter, χc, indicates the point at which E
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Figure 5. Mixing thermodynamics in a filament region over the course of irradiation. A 90/10 blend shown for exposure intensities of (a−d) 1 mW/ cm2, (e−h) 5 mW/cm2, (i−l) 10 mW/cm2, and (m−p) 15 mW/cm2. First column shows χc mapped against blend depth and irradiation time. The dashed line indicates the contour level χc = χFH, below which the blend is unstable. Columns 2 to 4 respectively show calculated spinodal curves with increased polymerization degree at the front (0 μm), middle (1500 μm), and end (3000 μm) of the filament. Spinodal curves expand and move upward over time with increased polymerization. Blue circles indicate the T−φ coordinate of the blend (T = 298 K).
“front”, also propagates along the filament length over time. Similar trends are observed for 80/20 and 50/50 blends (see Supporting Information). The calculated spinodal curves at the front, middle, and end locations of a filament in a 90/10 blend show that an exposure intensity of 5 mW/cm2 allows the entire filament to enter into the two-phase region under the spinodal line. This is also the case with 1 and 5 mW/cm2 for an 80/20 blend, whereas thermodynamic instability is induced over the entire depth of a 50/50 blend at all intensities. Note that the concurrent, accelerated growth of TMPTA and PDMS results in the spinodal curves remaining relatively symmetric over time. These results show that light intensity and weight fraction are critical in determining the depth over which mixing instability is influenced. As blends can spontaneously phase separate after the onset of mixing instability, the continued rise in polymerization degrees would in part be an artifact of diffusion of components out of the filament region, which will contribute to decreases in Raman signal. Consequently, other than showing the onset of mixing instability, the further decrease in χc itself is not sufficient to indicate phase separation. To this end, we also tracked a Si−CH3 peak (685 cm−1) of PDMS32 to reveal the change in composition over time. This peak is unaffected by polymerization and can only show significant variation in intensity as a result of changes in local concentration of PDMS. Figure 6 shows maps of the Si−CH3 peak intensity against
depth and irradiation time. The maps show that immediately after the onset of thermodynamic instability a sharp drop in PDMS concentration occurs beginning at the front end of the filament and propagating over its length. Significant changes in the intensity are found in the regions predicted to be thermodynamically unstable. This data provides evidence for phase separation, in which PDMS is the component that diffuses out of the filament region. By conversation of mass of these close systems, it is reasonable to conclude that the diffusion of PDMS out of the filament region is accompanied by diffusion of TMPTA into it. Using the CO peak to similarly show TMPTA diffusion is inclusive because the CO peak also decreases with increased conversion. Overall, the data indicate that the final process by which binary phase morphology is achieved is phase separation along and coaxial to the filament axis. In terms of phase separation ceasing, the associated viscosity increase induced by the TN effect acts as a stopper to subsequent dynamics, as also observed in the studies of Miyata and co-workers.40−42 This is indicated by the Si−CH3 peak remaining constant over time soon after its sudden drop. Similar results are obtained for blends of 80/20 and 50/50 (see Supporting Information), which also correspond to their respective thermodynamic data. 3D Mapping of Polymer Blend Morphology. We created Raman volume images of composition over the filament region at its back end (last 500 μm), which is closest F
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Figure 6. Evolution of PDMS concentration along a filament over the course of irradiation based on the Si−CH3 integrated peak (intensity in arbitrary units). A 90/10 blend is shown for exposure intensities of (a) 1, (b) 5, (c) 10, and (d) 15 mW/cm2. The dashed line shows the contour level χc = χFH, below which phase separation occurs.
over the entire length of a filament. As such, while Figure 7a,g,j shows discontinuous filament morphologies at the back end of the filament, the corresponding in situ Raman data indicate that continuous phases are attained at other locations, such as the center and front end. Our in situ microscopy, in situ Raman, and volume images data all corroborate with one another in confirming the specific component weight fractions and exposure intensities that lead to spatially correlated binary phase morphologies. Mechanisms Involved in Attaining Spatially Correlated Morphologies. Morphologies spatially correlated to the self-trapped light profile are achieved herein by a balance of two antagonist processes in photopolymerization-induced phase separation;10 namely, the system’s drive to phase separate is countered by the inhibitory effect of photo-cross-linking. At low intensities, phase separation can occur relatively uninhibited because photo-cross-linking is insufficient to stop filaments from growing beyond the regions dictated by the optical beams. Whereas, at high exposure intensities, high cross-linking inhibits phase separation, such that filaments (or portions of filaments) remain in a mixed state with discontinuous and distorted geometries. Lower volume fractions of TMPTA (φ) result in a greater initial χc; thus, higher polymerization degrees are required to reach thermodynamic instability. The consequent higher levels of viscosity and elasticity when the system finally becomes unstable will counter the drive to phase separate, which can either be deleterious if no phase separation occurs, or beneficial if it helps retain the filament’s cylindrical geometry. Smaller φ values also eliminate the tendency for neighboring filaments to fuse due to resistance by this cross-linked network and less available monomer. Based on the conditions herein, lower weight fractions result in correlated morphologies and individual filament phases, so long as the exposure intensity both allows for phase separation but also controls phases from over growing. Individual filament phases can be achieved with higher weight fractions; however, either the strong phase separation at low intensities or strong cross-linking at high intensities distorts their arrangement to some degree. Even
to the entrance of the Raman laser beam into the blend. Phase separation is evidenced by mapping the ratio of the CO peak to the Si−CH3 peak to identify TMPTA-rich regions. Figure 7 indicates the ability to attain continuous, acrylate-rich filaments, with PDMS-rich surroundings. For example, in a 90/10 blend continuous filament phases are achieved using 5 mW/cm2 (Figure 7d−f). All other exposure intensities for 90/10 yield discontinuous filament morphologies. Images of 80/20 blends show continuous filament phases for 1 and 5 mW/cm2 and discontinuous ones for 10 and 15 mW/cm2 (see Supporting Information). Note the continuous filament structures can still have waveguide properties, owing to the expected higher refractive index of the TMPTA-rich region compared to PDMS (e.g., refractive index measurements of cured TMPTA and PDMS were 1.480 and 1.446, respectively). This explains why in situ microscopy showed that light remains confined to the filaments over the duration in which phase separation occurs. The morphologies revealed in Raman volume images are corroborated by their corresponding in situ microscopy and thermodynamic data (specifically between depths of 2500− 3000 μm). Namely, continuous filament phases occur under conditions for which mixing instability and phase separation are indicated. Attaining 3D maps of 50/50 blends proved difficult, owing to the strong disorder in morphology. Overall, the maps show the effect of light intensity and weight fraction on influencing morphology, and that suitable conditions can result in binary phase morphologies that spatially correlate to the pattern of self-trapped beams over the depth of the blend. While the volume images reveal morphology for only the last 500 μm of the filament, it nevertheless confirms a correlation, observed in both 90/10 and 80/20 blends over all exposure intensities between resultant morphology (i.e., continuous or discontinuous filament phase) and mixing stability (i.e., phase separation or lack of it), which is further corroborated by changes in PDMS concentration. That is, a location along a filament that undergoes mixing instability and phase separation corresponds to a region showing a continuous TMPTA-rich phase. We can reasonably infer that this correlation holds true G
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Figure 7. Raman mapping of the final morphology for a 90/10 blend. The maps show the composition in the last 500 μm of the blend (i.e., from 2500 to 3000 μm): (a−c), 1, (d−f), 5, (g−i), 10, and (j−l) 15 mW/cm2. (a, d, g, j) show full volume maps. (b, e, h, k) show an xy-slice at the end of the filament (z = 3000 μm). (c, f, i, l) show a yz-slice through the filament central axis.
mm thickness to demonstrate the approach; yet, light selftrapping can persist over centimeter length scales, and hence it may be possible to control blends of significantly greater thicknesses. It is quite possible that at greater depths intensity gradients may emerge over the path length of light, such that the filament geometry may have a depth dependence. Herein, our aim was to describe the general mechanisms of morphology formation in terms of weight fraction and exposure intensity; dependencies on beam size, spacing, and depth limits are the subject of future study. In situ study of the propagated growth mechanisms along the filaments through side-view imaging is also planned.
higher weight fractions exacerbate either phase separation or crosslinking, consequently leading to random morphology. Our results indicate that morphology evolution occurs through sequential processes that propagate over the blend depth: (1) a liquid-to-solid transition (filament growth), (2) autoacceleration (increase molecular weight), (3) mixing instability and phase separation, and (4) ceasing of phase separation. The occurrence of these phenomena along multiple, parallel, linear pathways is a new mechanism by which the binary phase morphology may be precisely patterned over significant depths and over a wide area. The ability for light to self-trap over a range of intensities enables this approach to vary the underlying photopolymerization reaction, whereby these phenomena can be controlled to attain spatially correlated morphologies. We also expect morphology to depend on the beam size and spacing. Smaller interbeam spacing may cause the reacting regions of neighboring filaments to overlap, which may induce significant cross-linking to inhibit phase separation, or on the other hand favor fusion of the filament phases. In terms of filament size, theoretical predictions place a limit on the smallest self-trapped beam to a little under 1 μm.22 However, the low overall exposure associated with such small beams may require significantly higher light intensities to control morphology. In terms of depth, we selected volumes with 3
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CONCLUSION In summary, we have shown a new approach to control morphology in binary polymer mixtures by exploiting the selftrapping properties of light. We demonstrated that photoreactive blends can provide the rise in refractive index to induce light self-trapping. The divergence-free, linearly propagating nature of an array of self-trapped optical beams drives the growth of filaments in the blend along each beam’s path length. Through transmission optical microscopy, the evolution of morphology was in situ monitored to reveal the influences of exposure intensity and component weight fractions, whereby structures can form that spatially correlate to the self-trapped H
DOI: 10.1021/acs.macromol.7b00484 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
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light pattern. Using confocal Raman spectroscopy, the selftrapped beams are shown to driven conversion along the filaments, consequently leading to the onset of the Trommsdorff−Norrish effect, followed closely in time by thermodynamic mixing instability and phase separation. Confocal Raman mapping of composition shows that binary phase morphologies can be achieved that also spatially correlate to the self-trapped light pattern. Hence, self-trapped beams can specify the locations and directionality of the polymerization kinetics and phase separation, thereby providing regularity in the spatial organization, to induce precise morphologies with consistent structure over an entire volume. Spatially correlated structures are achieved owing to a balance in the underlying competition between phase separation and photo-cross-linking. Experiments on synthesis and control of morphologies with different length scales, depths, and symmetries as well as for other polymer compositions are currently in progress. Practical uses of these materials in applications that benefit from their 2D highly directional morphologies will be reported in the future.
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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/acs.macromol.7b00484. Figures S1−S17 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(I.D.H.) E-mail:
[email protected]. ORCID
Ian D. Hosein: 0000-0003-0317-2644 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the College of Engineering and Computer Science at Syracuse University. REFERENCES
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DOI: 10.1021/acs.macromol.7b00484 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.7b00484 Macromolecules XXXX, XXX, XXX−XXX
