Kinetically Controlled Photoinduced Phase Separation for Hybrid

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Kinetically Controlled Photoinduced Phase Separation for Hybrid Radical/Cationic Systems Erion Hasa,† Jon P. Scholte,† Julie L. P. Jessop,† Jeffrey W. Stansbury,‡ and C. Allan Guymon*,† †

Department of Chemical & Biochemical Engineering, University of Iowa, Iowa City 52242, United States Department of Chemical & Biological Engineering, University of Colorado Boulder, Boulder 80309, United States

‡

Macromolecules Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/03/19. For personal use only.

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ABSTRACT: Controlling phase separation in polymer systems has shown significant promise in combining properties of different components into an integrated polymer system. In this work, we investigate the effect of photoinduced phase separation on polymer morphology and properties in hybrid radical/cationic systems composed of butyl acrylate (BA) and difunctional oxetane (DOX). We show that the irradiation intensity has a significant effect on the formation of phase-separated domains. By increasing the irradiation intensity at fixed co-monomer composition, the morphology changes from one with a continuous soft BA domain to one with co-continuous BA (soft) and DOX (hard) domains. At higher irradiation intensity, the domain size of each phase is decreased because of fast photopolymerization, which significantly limits monomer/polymer diffusion. The smaller domain size enhances the flexibility and strength of the phase-separated polymers. On the other hand, the irradiation intensity has little to no effect on the polymer structure or properties for systems that do not phase-separate. Dynamic mechanical analysis demonstrates that phase separation associated with higher irradiation intensity during cure contributes to a 40-fold increase in toughness and up to fivefold higher elongation at break. This behavior is attributed to the formation of polymers with co-continuous hard/soft domains and decreased domain size. This study demonstrates that the morphology and properties of photoinduced phase-separated materials can be controlled by altering the initiation irradiation intensity for hybrid radical/cationic materials.

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INTRODUCTION Photopolymerization has been used extensively both academically and industrially for more than five decades. Because of the inherent spatial and temporal control offered via photopolymerization, this process has been widely used in applications including coatings, biomaterials, 3D printing, adhesives, dental fillings, contact lenses, and microelectronics.1−5 In most applications, either multifunctional radical or cationic monomers are chosen to obtain cross-linked polymer networks with high tensile modulus, stress at break, and hardness.6 Unfortunately, photopolymerization of monomers that generate highly cross-linked materials can also induce structural heterogeneities, resulting in very broad glasstransition temperatures and material brittleness. 7 One approach that has been suggested to overcome these drawbacks is to polymerize hybrid mixtures of free-radical and cationic monomers (orthogonal chemistries) that lead to interpenetrating polymer networks or controlled phase separation.8−10 For example, controlled phase separation has © XXXX American Chemical Society

demonstrated that specific polymer morphologies can enhance mechanical properties (e.g., toughness) because of better dissipation of crack propagating forces.11 Photopolymerization-induced phase separation (PhIPS) is one promising technique to generate polymer morphologies with multiple domains and controllable properties.12−16 PhIPS offers direct control over polymerization kinetics, which directly impacts polymer network formation and phase separation through irradiation conditions, photoinitiator type/concentration, and formulation. With this approach, phase separation is developed during the polymerization of an initially homogeneous hybrid radical/cationic formulation. As the polymers form during photopolymerization, phase separation will be induced based on thermodynamic incompatibilities in the two systems.10 The final polymer Received: January 23, 2019 Revised: March 15, 2019

A

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morphology is dictated by competition between phase separation and gelation during photopolymerization.17−19 The photopolymerization rate, and thus time to gelation, will play a deterministic role on the development of different phase-separated domains.20 After polymer gelation, the polymerization of the remaining reactive groups occurs locally and, based on restricted diffusion, the local composition will not be significantly altered through PhIPS.20 Many parameters that affect both photopolymerization kinetics and gel point including irradiation intensity, chemical structure of monomers, reaction temperature, ambient conditions (e.g., oxygen and moisture), and monomer composition can favor or suppress phase separation during photopolymerization.1,15,19−22 Previous work has utilized light-induced phase separation in mixtures consisting of dimethacrylate monomers and nonreactive linear prepolymers of different molecular weights.20 During photopolymerization, the nonreactive prepolymers facilitated the formation of phase-separated domains, which resulted in reduced volumetric shrinkage. Other studies have also shown that phase separation can be induced and controlled using reactive prepolymers as well.7,23,24 Recent work has shown that phase separation depends not only on the relative concentration of the different components but also on the reactive group placement.7 Phase separation was induced when the reactive groups were included only in the end segments of the oligomer chain, but not when they were randomly distributed. By inducing phase separation, both the elastic modulus and creep recovery were significantly enhanced. Other work has studied miscibility before and after polymerization to predict the final morphology in hybrid radical/cationic systems.15 By altering the monomer ratio, it was possible to move from a miscible IPN to a totally phaseseparated one due to changes in the formulation viscosity and the thermodynamic immiscibility of monomers prior to photopolymerization. Although prior studies have examined the use of photopolymerization to induce phase separation, little is known about the effect of photopolymerization kinetics on polymer morphology and properties.15,17,25,26 In this study, we investigate the effect of irradiation intensity and monomer composition on polymer structure and properties of reactive hybrid radical/cationic systems composed of butyl acrylate (BA) and difunctional oxetane (DOX). These monomers were selected because of their inherently different properties (e.g., glass-transition temperatures, moduli, chemical structure, and reactivity) as well as their orthogonal polymerization mechanisms that may facilitate phase separation during polymerization. Specifically, we alter significantly the irradiation intensity to control polymer morphology and thermomechanical properties. Monomer conversion was examined as a function of irradiation intensity, composition, and thermal postcure. The relationship between phase separation and gelation was monitored using a UV/vis spectrometer and photorheometer, respectively. Mechanical properties such as maximum stress, elongation at break, and overall toughness were characterized at all irradiation intensities using dynamic mechanical analysis. Atomic force microscopy was also used to characterize the surface morphology of polymer films formed at different irradiation intensities and monomer compositions. This work clearly demonstrates that PhIPS may be used to manipulate polymer morphology and thermomechanical properties.

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EXPERIMENTAL SECTION

Materials. Monomers used in this study were BA (Sigma-Aldrich) and 3-ethyl-3[[(3-ethyloxetane-3-yl)methoxy]methyl]oxetane (DOX, Synasia) (Figure 1). Photoinitiators 2,2-dimethoxy-1,2-diphenylethan-

Figure 1. Monomer chemical structures of BA (left) and DOX (right).

1-one (DMPA, Ciba Specialty Chemicals) and mixed triarylsulfonium hexafluoroantimonate salts (TSA, Sigma-Aldrich) were used to initiate the radical and cationic polymerizations, respectively. The latter initiator is diluted in propylene carbonate (50 wt %). All chemicals were used as received. Hybrid systems with ratios of 1:1, 7:3, and 9:1 w/w DOX/BA were examined. The concentration of the radical and cationic photoinitiators for each formulation was 0.25 and 0.75 wt %, respectively. Samples were photopolymerized at 10 and 100 mW cm−2 (320−390 nm) for 10 min using a high-pressure mercury arc lamp (Omnicure S1500 spot cure system) unless otherwise stated. For photopolymerizations at 1500 mW cm−2 (320−390 nm), samples were passed under a belt lamp (Fusion UV Systems, LC-6B) with a belt speed of 3 ft/min. After photocure, all samples were also thermally postcured at 90 °C for 4 h to ensure full cure. Polymer films, unless stated otherwise, were approximately 0.2 mm thick. Formulations with light-absorbing photoinitiators or monomers may develop light intensity gradients throughout the sample thickness during photopolymerization.27,28 To ensure that the absorbed irradiation is relatively uniform through a 0.2 mm thick sample, the Beer−Lambert Law was applied using inherent light absorption of the photoinitiator and emission from the high-pressure mercury arc lamp at 340 and 365 nm. From this information, at least 93% of light transmits through the sample at both wavelengths and each UVintensity tested. Other work has shown that much greater gradients are needed to change composition or polymer morphology. Thus, we can assume that the samples exhibit uniform morphology throughout the sample thickness. Methods. Fourier Transform Infrared Spectroscopy. The realtime conversion of oxetane and BA functional groups was examined using a Nexus 670 Fourier transform infrared spectroscopy (FTIR) adapted to enable UV-curing at room temperature. The formulations were deposited between two salt (NaCl) plates separated by 15 μm spacers. For photopolymerizations, samples were exposed to a highpressure mercury arc lamp (EXPO Acticure) at 10 mW cm−2 for 10 min at approximately 23 °C (room temperature). In addition, the conversion of these systems was measured during thermal postcure at 90 °C on the FTIR with a modified stage that allows control of temperature. Conversion was obtained by measuring the decrease of the acrylate CC stretching band at 1637 cm−1 and the oxetane C− O−C stretching band at 980 cm−1.29,30 Monomer conversion was calculated by eq 1.

Conversion (%) =

Ao − A(t ) 100 Ao

(1)

where Ao is the initial height of each peak before photocuring and A(t) is the peak height at any time during photocuring.31 Atomic Force Microscopy. The surface morphology of polymers was obtained using an Asylum Research Molecular Force Probe 3D Classic atomic force microscopy (AFM) and analyzed with Igor software. Phase images were obtained in the tapping mode at a rate of 1 Hz. All systems were sandwiched between two glass slides separated by adhesive tape spacers to create a film thickness of approximately 0.2 mm for analysis. B

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Figure 2. Real-time conversion of acrylate and oxetane functional groups for (a) neat BA and DOX, and for systems with (b) 9:1, (c) 7:3, and (d) 1:1 DOX/BA photocured at 10 mW cm−2. Samples were exposed to UV radiation at 30 s. DMPA and TSA initiator concentrations were 0.25 and 0.75 wt %, respectively. Photorheometry. Samples were sandwiched between two 0.5 mm parallel quartz disc plates, attached to a rheometer (ARES, TA Instruments). To ensure that the experiments were conducted within the linear viscoelastic regime, the shear was set at a frequency of 1 Hz with 1% maximum strain. The crossover between G′ (storage modulus) and G″ (loss modulus) was used to determine the gel point time and conversion.32 Optical Density. To measure optical properties during polymerization, a UV/Vis portable spectrometer (Ocean Optics, USB2000) was used. A disc-shaped sample (thickness = 0.5 mm, diameter = 10 mm) was secured so that a UV curing light source could transmit through the material. The light source emits between 400 and 800 nm, a range at which the monomer and photoinitiator do not absorb. The intensity of 600 nm light transmitted was monitored in real time. Dynamic Mechanical Analysis. A dynamic mechanical analyzer (DMA; Q800 DMA TA Instruments) was used to characterize the ultimate thermomechanical properties of polymers. In order to create samples with dimensions of approximately 8 × 6 × 0.2 mm (length × width × thickness), small amounts of liquid mixture were photopolymerized between two glass plates with 0.2 mm thickness. A temperature range of −60−150 °C was applied to investigate glasstransition temperature behavior. The DMA tensile mode was used under constant strain at a frequency of 1 Hz and a heating rate of 3 °C/min. Stress and strain values were evaluated at room temperature in the tensile mode with a force rate of 0.5 N/min.

ultimate structure and properties, the real-time conversion of both oxetane and acrylate groups at different monomer ratios is shown in Figure 2 when photocured at 10 mW cm−2. When BA is polymerized independently, double bond conversion reaches 90% conversion in about 100 s of curing with final conversion reaching approximately 100%. In contrast, the neat DOX polymerization does not start immediately when exposed to UV irradiation. An induction time of over 2 min is observed because of the formation of relatively stable tertiary oxetanium ions during the initiation step of cationic ring-opening polymerization at room temperature.34 These tertiary ions require a large activation energy in order to react with oxetane monomers that lead to chain growth. Additionally, the conversion of oxetane groups from DOX is only about 30% after photocuring for 10 min. The lower oxetane conversion is likely due to the low UV absorbance of the cationic initiator (TSA) and inherently low propagation rate constant at this temperature based on the high stability of the intermediate tertiary oxetanium ions.19,29,34,35 Interestingly, the induction time of oxetane is eliminated with BA copolymerization (Figure 2b−d). This change may result from the BA exothermic reaction. The increased temperature from the BA copolymerization would result in increased and greater initial reaction rate. Additionally, the conversion of BA and DOX appears to be different after 1 min of UV curing. In all cases, BA reaches 75% conversion or greater, indicating that its reaction is not significantly affected by the presence of oxetane. On the other hand, DOX conversion is much lower, between 15 and 20%, for the same reaction time. Even though DOX polymerization does not exhibit an inhibition time and starts reaction immediately upon irradiation exposure in hybrid formulations, its final conversion remains below 25%. The lower oxetane conversion in these hybrid systems is associated with the decreased diffusion of oxetane active centers because of likely increased viscosity of the medium early in the reaction.

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RESULTS AND DISCUSSION Polymerization Kinetics. Photopolymerization kinetics significantly impact polymer structure formation. Both monomer composition and irradiation intensity directly affect reaction rate and final conversion which, in turn, influence polymer morphology and subsequently the thermomechanical properties of polymers.9,15,32 Additionally, the polymerization kinetics often give information that facilitates understanding of polymer structure evolution.33 In this study, we examine the copolymerization of a hybrid system with two orthogonally polymerizing monomers, radically polymerized BA and cationically polymerized DOX. To determine the effect of this copolymerization on photopolymerization kinetics and C

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Macromolecules Reactive Group Conversion. The reaction temperature significantly affects oxetane conversion. As described previously, the exothermic BA polymerization provides sufficient heat to eliminate the induction time, but the final DOX conversion remains low likely because the formed stable tertiary oxetanium ions remain less active at room temperature and do not lead to high propagation reaction rates.34,35 To reinitiate the propagation process of these stable intermediate species and thus increase DOX conversion, the samples must be thermally postcured at elevated temperatures.36 Higher temperatures should provide enough energy to overcome the activation energy barrier for propagation with the oxetanium intermediate species. Figure 3 shows the effect of thermal

and active centers, resulting in higher oxetane conversion. After thermally postcuring the samples, DOX final conversion reaches approximately 70% for all irradiation intensities. Although some small variations are observed when using different irradiation intensities and monomer compositions, these changes are not statistically significant. For these systems, the polymers are highly cross-linked, likely limiting final DOX conversion. Moreover, it is reasonable to believe that all three formulations will reach the gel point during the photopolymerization even with the low DOX conversion, possibly establishing any phase-separated morphology during photocure. Thus, additional reaction during thermal postcuring will likely have limited impact on ultimate polymer structure and properties. These small variations of UV-induced BA and DOX conversion may result in significant changes in the formation of polymer structure. Phase Separation. The development of different phaseseparated polymer structures should be highly dependent on photopolymerization kinetics. Higher irradiation intensities increase polymerization rate and UV-induced conversion, leading to reduced time to gelation and limited diffusion prior to gelation. To investigate how different photopolymerization conditions (e.g., irradiation intensity and monomer composition) affect phase separation, AFM was used to characterize polymer surface morphology and phase distribution. Typically, phase-separated polymers will exhibit distinct domains because of the different local stiffnesses of the polymeric phases that can be observed using AFM in the tapping mode.15 In AFM, the cantilever experiences a delay in the tapping oscillation when it touches the somewhat adhesive polymer surface. The greater the adhesion between the cantilever and the polymer, the greater the delay in oscillation. This delay is represented by a degree value in the AFM images with larger degree values corresponding to greater adhesion. In this study, different inherent properties of each polymer (e.g., Tg and chemical structure) will change the local stiffness or modulus of BA- and DOX-rich phases. For stiffer, less adhesive domains, the delay in the oscillation will be much less, leading to a lower degree of oscillation delay than that of softer more adhesive domains which will lead to a higher degree of oscillation delay. Thus, harder domains are represented with negative angle values and softer domains with positive angle values. Additionally, AFM investigation showed that both polymer surfaces (bottom and top) demonstrate similar morphologies for all hybrid systems and irradiation intensities. Taking into account the insignificant intensity gradient as described above, it is reasonable to believe that the polymer morphology observed at the surface is consistent throughout the sample thickness. Figure 4 shows the surface morphology and phase distribution for systems with 1:1 DOX/BA photocured at different irradiation intensities. Significantly different phaseseparated morphologies are observed by altering irradiation intensity. At lower irradiation intensity, continuous soft BA (white or blue) and dispersed hard DOX (red) domains appear to form. This behavior is likely due to slower polymerization, which allows enough time for high degrees of phase separation between the two polymer systems. With intermediate irradiation intensity, both soft BA and hard DOX domains appear more co-continuous. It is reasonable to believe that the co-continuous domains are formed because of faster polymerization and increased UV-induced DOX conversion, resulting in shorter time to gel point that will suppress

Figure 3. Final (a) BA and (b) DOX conversion after UV cure (striped) and thermal postcure (solid) with different DOX/BA ratios. The formulations were photopolymerized at approximately 10 (gray), 100 (red), and 1500 (blue) mW cm−2 and thermally postcured at 90 °C for 4 h. Experiments were conducted four times to determine reproducibility with the error bars representing the standard deviation. DMPA and TSA concentrations were 0.25 and 0.75 wt %, respectively.

postcure on conversion of DOX and BA when the systems were heated to 90 °C for 4 h after initial irradiation at different intensities. A temperature of 90 °C should also allow greater diffusion of reactive monomers and conversion because it is higher than the glass-transition temperature of both monomers.37,38 UV-induced BA conversion is up to 10% lower at higher DOX concentrations, which may be due to increased viscosity during polymerization with higher cross-link density. The application of thermal postcure facilitates the reaction of remaining carbon−carbon double bonds. Additionally, DOX conversion for all formulations is low when cured at low irradiation intensities. This conversion increases approximately by a factor of 2 with increasing irradiation intensity. At high irradiation intensities, the temperature during photopolymerization is increased through reaction and heating from the light source, overcoming the propagation activation energy barrier of oxetanium ions and facilitating higher oxetane conversion. In addition, the increased temperature enables greater diffusion of monomer D

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Figure 4. AFM phase images and distributions of polymer systems with 1:1 DOX/BA. Samples were photopolymerized at (a) 10, (b) 100, and (c) 1500 mW cm−2 and subsequently thermally postcured at 90 °C for 4 h. The dimension of each micrograph sample is 1 × 1 μm.

irradiation intensity, the soft BA domain appears continuous, whereas the hard DOX domains have a more droplet-like shape. This behavior may again be due to slow photopolymerization that allows enough time for creation of high degrees of phase separation. In addition, hard DOX domains appear larger than those of the system with 1:1 DOX/BA photocured at low irradiation intensity. In this case, the increased DOX concentration contributes to larger hard domains. By increasing the irradiation intensity to 100 mW cm−2, the morphology changes from one with a continuous softer BA domain to one with co-continuous BA (soft) and DOX (hard) domains. The formation of the co-continuous domains is associated with reduced gelation time because of faster UV-curing which prevents more complete phase separation between polymerized BA and DOX. At higher irradiation intensity, the domain size of each phase is decreased as a result of limited diffusion of monomers and polymerizing active centers. Additionally, photocuring at the highest irradiation intensity generates polymer structures with more mixed soft and hard domains, resulting in the appearance of multiple colors. Moreover, the phase distribution shows peaks for both negative and positive phase angles, indicating high degrees of

diffusion before the predominant morphology is established. For samples photopolymerized at the highest irradiation intensity, smaller domains are observed. The reduced domain size is not unexpected because of fast polymer gelation, which further reduces the time for diffusion and morphological development. Thus, irradiation intensity has a significant effect on phase-separated morphologies by forming co-continuous domains or smaller domain size. Additionally, phase distribution plots can be generated, which indicate the number of pixels for a specific phase lag degree. The peaks and broadness of distribution demonstrate the formation of different phases in the polymer network. At all irradiation intensities, phase distribution plots show two major peaks at negative and positive values, indicating high degrees of phase separation. By increasing the irradiation intensity, the positive degree peak from the soft phase is reduced and the overall distribution becomes broader consistent with the observations mentioned previously that increased irradiation intensity significantly alters the nano-/microdomains with more distributed and continuous soft and hard domains. Similar trends are observed in the morphologies and phase distributions for systems with 7:3 DOX/BA photocured at different irradiation intensities, as shown in Figure 5. At low E

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Figure 5. AFM phase images and distributions of polymer systems with 7:3 DOX/BA. Samples were photopolymerized at (a) 10, (b) 100, and (c) 1500 mW cm−2 and subsequently thermally postcured at 90 °C for 4 h. The dimension of each micrograph sample is 1 × 1 μm.

phase separation. For photopolymerizations at lower irradiation intensity, the phase distribution appears to be broader with a lower peak from the soft domain when compared to that of 1:1 DOX/BA. This behavior is likely due to higher DOX concentration, which contributes to faster gelation and larger hard DOX domains compared to that of 1:1 DOX/BA. Additionally, the overall distribution becomes broader with increasing irradiation intensity. Interestingly, the peak from the soft domain almost disappears when photocuring at the highest irradiation intensity. This behavior is associated with the higher DOX concentration and fast polymerization kinetics, which do not allow formation of large BA chain length, resulting to a more integrated polymer network than that of 1:1 DOX/BA. To better understand how phase separation is developed in polymers with likely lower gel point conversions/time, the effect of irradiation intensity on the morphology of hybrid polymer systems with higher oxetane concentration was also investigated. Figure 6 shows the surface morphology and phase distribution of hybrid polymer systems with 9:1 DOX/BA photopolymerized at different irradiation intensities. Interestingly, only a small degree of structural heterogeneity is

observed when photocuring at low irradiation intensity (10 mW cm−2). Although this heterogeneity may be due to phase separation, the oscillation degree range is dramatically lower than that for other systems, indicating that any phase separation is on vastly different scale. With increased irradiation intensity, the phase images show little variation, indicating single-domain structures. With greater photopolymerization rates at higher irradiation intensity, no observable phase separation occurs likely due to higher oxetane cross-linker concentration and reduced time to gelation. Increasing the irradiation intensity to 100 or 1500 mW cm−2, phase distributions appear very narrow and relatively independent of photocuring intensity. This behavior suggests that irradiation intensity has little to no effect on the structure of polymers with high cross-linking oxetane concentration, which significantly reduces gelation time and suppresses phase separation. Gelation and Phase Separation. The interplay between gelation and phase separation determines the basic polymer structure during photopolymerization. These parameters are significantly affected by irradiation intensity, monomer composition, and chemical structure of monomers. To better F

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Figure 6. AFM phase images and distributions of polymer systems with 9:1 DOX/BA. Samples were photopolymerized at (a) 10, (b) 100, and (c) 1500 mW cm−2 and subsequently thermally postcured at 90 °C for 4 h. The dimension of each micrograph sample is 1 × 1 μm.

understand the impact of this interplay in creating different polymer morphologies with different formulations and intensities, both the gel point and the onset of turbidity were monitored as a function of time using a photorheometer and UV/vis spectrometer, respectively. The photopolymerization conditions and sample geometry for both instruments were identical, enabling direct comparison. Phase separation has a considerable impact on the optical properties during polymerization, increasing visible light scatter.20 The increase in scatter and corresponding decrease in transmittance is due to refractive index mismatch between the nano-/microdomains formed on size scales of the wavelength of visible light. In addition, the gel point onset largely determines the ability of the reaction species to diffuse, with very low mobility, especially of growing polymer chains once gelation has been reached.20 Figure 7 shows both measurements of storage/loss modulus and transmittance for a system with 7:3 DOX/BA during photopolymerization at 10 mW cm−2. The gel point is reached at the time that storage modulus equals loss modulus.20 The crossover between the storage and loss modulus occurs at around 175 s, essentially fixing the polymer network, and consequently forming the basic polymer morphology. Domains

Figure 7. Real-time development of storage/loss modulus and transmittance for a 7:3 DOX/BA system at 10 mW cm−2 as measured by a photorheometer and UV/vis spectrometer.

are therefore developed via orthogonal chemistry from the two monomer systems during the first 175 s of UV-curing. In addition, the light transmission is 100% for the initial 90 s, implying that the phases are initially compatible. Thereafter, the transmission decreases as a result of phase separation. The G

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1:1 and 7:3 DOX/BA generate co-continuous domains at increased irradiation intensity because of likely reduced time differences between gelation and phase separation onset. This smaller time difference suppresses complete phase separation and inhibits the formation of larger BA chains. At even higher irradiation intensity, gelation quickly follows phase separation, which results in small domain size of each phase due to limited time for diffusion and morphological development. On the other hand, the gel point already precedes phase separation for 9:1 DOX/BA systems at 10 mW cm−2. Thus, increasing photocuring intensity could further reduce the time to reach gel point, preventing any significant degree of phase separation. These results suggest that polymer morphology can be controlled in systems with 1:1 and 7:3 DOX/BA because phase separation precedes gelation and the time difference between gelation and phase separation can be easily manipulated by altering irradiation intensity. Conversely, for the 9:1 DOX/BA system, any phase separation follows gelation and consequently, polymerization light intensity does little to change polymer morphology. Thermomechanical Analysis. Our results demonstrate that the phase-separated domain morphology can be controlled by altering the irradiation intensity and monomer composition. These changes in polymer morphology may have a significant impact on macroscopic properties of polymer films. To determine the influence of polymer structure on thermo-mechanical properties, the tan(δ) profiles of polymers were evaluated utilizing dynamic mechanical analysis (DMA) to determine glass-transition temperature (Tg) behavior. The effect of irradiation intensity on tan(δ) profiles of different monomer ratios is shown in Figure 9. The Tg’s of neat BA and DOX have been reported at approximately −50 °C and 51 °C, respectively.38−40 For hybrid systems with 1:1 DOX/BA, two tan(δ) peaks are observed, indicating phase separation. One peak is observed at around −30 °C, consistent with a BA-rich phase Tg.37 The peak height at this temperature decreases with exposure to higher irradiation intensities, suggesting changes in the polymer network with a less distinct soft and low Tg phase. Another tan(δ) peak, more prominent at higher irradiation intensities, is seen around 60 °C and is likely from a hard DOX-rich domain.38 It again appears that the higher intensities induce formation of co-continuous and more distributed domains with the increase in the tan(δ) peak at higher temperatures and decrease in the tan(δ) peak at lower temperatures. For polymer systems with 7:3 DOX/BA, one tan(δ) peak is observed at around −30 °C and the other one at around 60 °C associated with the Tg’s of BA- and DOX-rich phases, respectively. This distinct phase separation appears for all irradiation intensities. By increasing the irradiation intensity, the higher temperature tan(δ) peak becomes more distinct, whereas the lower temperature tan(δ) peak height is reduced and shifts to slightly lower temperatures. On the other hand, polymer films with 9:1 DOX/BA exhibit one very broad tan(δ) peak for each irradiation intensity. In this case, all of the tan(δ) profiles are almost identical, having a peak around 45 °C, a value consistent with a DOX/BA interpenetrating polymer network system. These results corroborate that this system does not phase separate at any irradiation intensity because gelation occurs faster than the onset of phase separation, preventing the formation of distinct polymer domains. With the large effect of irradiation intensity on tan(δ) profiles for different monomer compositions, it is reasonable to

light transmission reaches a minimum value of 50%, which is followed by a period where it slightly increases. This slight increase is most likely associated with the changes in optical density as the slower polymerizing DOX continues to polymerize, decreasing the difference between the refractive indices of both polymer phases.9 Such large and positive time differences (i.e., phase separation onset precedes gelation) may allow each polymeric phase to coalesce to a greater degree, forming more distinct and larger domains before gelation. For relatively small-time differences, phase separation is arrested upon gelation, potentially forming more co-continuous and smaller polymer domains. When, however, the time difference is negative (i.e., gelation preceded phase separation onset), the basic polymer morphology is created before any degree of phase separation occurs. The effect of composition on the time at onset of phase separation and gelation is shown in Figure 8 as determined by

Figure 8. Gelation and phase separation onset of varying monomer compositions photopolymerized at 10 mW cm−2. Each error bar represents the standard deviation of three experiments.

UV/vis spectroscopy and photorheometry. The onset of phase separation precedes gelation by approximately 150 s for the system with 1:1 DOX/BA, suggesting that enough time is available for diffusion of the incompatible monomers to generate a continuous BA domain and distinct small DOX domains, as observed in Figure 4. Interestingly, the time difference between gelation and phase separation onset is reduced by increasing the DOX concentration. For a system with 7:3 DOX/BA, this time difference drops to around 85 s, resulting in larger distinct DOX domains and broader phase distribution (Figure 5). These differences in phase-separated domains between 1:1 and 7:3 DOX/BA at lower UV intensity are also likely driven by the higher oxetane concentration, which leads to larger hard DOX domains. Conversely, gelation occurs more quickly than the onset of phase separation for 9:1 DOX/BA, resulting in a single-domain polymer. The singledomain structure is consistent with the fast gelation that prevents dual phase morphology. Thus, only one domain with narrow phase distribution is detected by AFM, as shown in Figure 6. Although the onset of gelation and phase separation cannot be tested at higher irradiation intensities because of instrument limitations, it is reasonable to believe that the time difference between the gel point and phase separation onset will be reduced significantly by increasing the irradiation intensity because of faster reaction rates and times to reach the gel point. On the basis of AFM images, the polymer systems with H

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Figure 9. tan(δ) profiles from DMA as a function of temperature from systems with DOX/BA ratios of (a) 1:1, (b) 7:3, and (c) 9:1. Samples were photopolymerized at 10 (black), 100 (red), and 1500 (blue) mW cm−2 and thereafter thermally postcured at 90 °C for 4 h.

Figure 10. Stress−strain profiles in tensile mode at room temperature of polymer systems with DOX/BA ratios of (a) 1:1, (b) 7:3, and (c) 9:1. Samples were photopolymerized at 10 (black), 100 (red), and 1500 (blue) mW cm−2 and then thermally postcured at 90 °C for 4 h. I

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thereby resulting in polymers that are not very stiff and break relatively easily. By increasing the irradiation intensity to 100 mW cm−2, toughness is increased by an order of magnitude for the same systems because of co-continuous harder DOX and softer BA domains, which contribute to both higher modulus and flexibility, respectively. At the highest irradiation intensity, polymer toughness of systems with 1:1 and 7:3 DOX/BA is increased by approximately 40 times when compared to those polymerized at 10 mW cm−2. This large increase in toughness is associated with the reduction of the domain size. The smaller domains are well dispersed throughout the polymer matrix and more adaptable because of interfacial interactions between the soft/hard domains, absorbing more effectively the deformation energy. In addition, the smaller domain size can dissipate potential cracks in multiple directions, which dramatically alleviates concentrated stress levels.41 This behavior is in accordance with the results reported in prior studies investigating polymers with nanodomains.40,42−45 On the other hand, polymer toughness of systems with 9:1 DOX/BA is affected very little by altering the irradiation intensity. The standard deviation shows that these small differences are not statistically significant. These results are certainly reasonable given the minimal effect of irradiation intensity on the polymer structure (i.e., single-domain polymer). Furthermore, toughness does not significantly change when comparing systems that are only UV-cured to those both UV and thermally cured (Figure S2). Therefore, the mechanical behavior is largely predetermined by the initial UV cure and the structure generated during this stage. The additional thermal-curing performed at 90 °C contributes to higher conversion without changing the polymer structure and consequently the mechanical properties.

believe that these polymers will also exhibit significantly different mechanical properties. To probe any such changes, stress was examined as a function of strain at a constant force ramp of 0.5 N/min for DOX/BA compositions polymerized at different UV curing intensities as shown in Figure 10. Interestingly, the stress−strain curves of the phase-separated systems (1:1 and 7:3 DOX/BA) are much more dependent on the irradiation intensity than those of single-domain materials (9:1 DOX/BA). At low irradiation intensity, these materials exhibit very low elongation at break because of continuous soft BA domain. The elongation at break of polymers with 1:1 DOX/BA increases up to 2.5 times by increasing the irradiation intensity, whereas Young’s modulus increases over three and sixfold when the irradiation intensity is increased to 100 and 1500 mW cm−2, respectively. For systems with 7:3 DOX/BA, the elongation at break increases even further with an increase of almost five times at the highest irradiation intensity. Additionally, Young’s modulus increases but to a lesser degree with a 1.5-fold increase at 1500 mW cm−2. The enhanced Young’s modulus can be related to the more prominent and continuous hard DOX domain as the irradiation intensity increases, contributing to increased local modulus. The increased elongation at break could be due to the wider phase distribution and co-continuous hard and soft domains combining the stiffness associated with the more heavily cross-linked domains with the flexibility imparted by the soft domain much like behavior in an IPN.10 In contrast, Figure 10c shows relatively small changes in the stress−strain profiles with increasing irradiation intensity for systems with 9:1 DOX/BA. Where phase separation does not occur appreciably, the irradiation intensity has a minimal effect on the morphology. Therefore, only relatively small changes in Young’s modulus and elongation at break are observed. Because of the significant increase in both Young’s modulus and elongation at break, these materials should absorb more energy before breaking. As a measure of this energy, polymer toughness was determined by calculating the area under the stress−strain curves. Figure 11 shows this calculated toughness as a function of monomer ratio and irradiation intensity. The systems with 1:1 and 7:3 DOX/BA have relatively low toughness when photocured at 10 mW cm−2 because of the formation of continuous soft and prominent BA domains with low Tg, whereas the harder DOX domains are more discrete,

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CONCLUSIONS This research highlights that irradiation intensity can be used to control phase separation and polymer properties in photocurable systems. Hybrid systems consisting of a low Tg monoacrylate (BA) and higher Tg dioxetane (DOX) are photocured at different irradiation intensities to examine the effect of cure speed on phase separation and polymer morphology. For polymer systems with 9:1 DOX/BA, the morphology is relatively uniform and does not change significantly with increased irradiation intensity. Additionally, tan(δ) profiles show a very broad Tg at all curing intensities, indicating that single-domain polymers are formed. Conversely, for systems with 1:1 and 7:3 DOX/BA, not only are two distinct domains and Tg’s observed but also these domains change significantly with increasing irradiation intensity. When the onset of phase separation occurs well before gelation, thereby allowing enough time for monomer/polymer diffusion before gelation, phase separation with continuous soft BA domains is observed. As this time difference decreases either with increased irradiation intensity or cross-linking density, smaller and more co-continuous hard/soft domains are formed. For these phase-separated systems, increasing irradiation intensity results in up to a fivefold increase in elongation at break with enhanced Young’s modulus. Consequently, polymer toughness is increased up to 40 times with increasing irradiation intensity based on the reduction of co-continuous hard/soft domain size. This investigation demonstrates that irradiation intensity and monomer composition can manipulate the relationship between gel point and phase separation in hybrid systems,

Figure 11. Toughness of monomer systems photopolymerized at 10 (black triangles), 100 (red squares), and 1500 (blue circles) mW cm−2, and then thermally postcured at 90 °C for 4 h. Each experiment was conducted four times with error bars representing standard deviation. J

DOI: 10.1021/acs.macromol.9b00177 Macromolecules XXXX, XXX, XXX−XXX

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resulting in controlled polymer morphologies and thermomechanical properties.

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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.9b00177. Polymer stress−strain and toughness behavior of systems UV-cured without postcure compared with those UV-cured followed by thermal cure (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 319-335-5015. Fax: 319-335-1415. ORCID

Erion Hasa: 0000-0002-3983-0138 C. Allan Guymon: 0000-0002-3351-9621 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of this research from Industry/University Cooperative Research Center (IUCRC) on Fundamentals and Applications of Photopolymerizations and the National Science Foundation (CBET-1438486).

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REFERENCES

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