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Photothermal Nanoparticle Initiation Enables Radical Polymerization and Yields Unique, Uniform Microfibers with Broad Spectrum Light Rachel C. Steinhardt, Timothy Steeves, Brooke Marjorie Wallace, Brittany Moser, Dmitry A, Fishman, and Aaron P Esser-Kahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12230 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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ACS Applied Materials & Interfaces

Photothermal Nanoparticle Initiation Enables Radical Polymerization and Yields Unique, Uniform Microfibers with Broad Spectrum Light Rachel C. Steinhardt,†1 Timothy McCormick Steeves,†1Brooke Marjorie Wallace,2 Brittany Moser,1 Dmitry A. Fishman,2 Aaron P. Esser-Kahn*,1 1. University of Chicago, Chicago, IL 60637. 2. University of California, Irvine, Irvine, CA 92697. KEYWORDS: polymers, nanoparticles, photothermal heating, carbon black, radical polymerization

ABSTRACT

Photothermal processes are utilized across a variety of fields, from separations to medicine, and are an area of active research. Herein, the action of a solar simulator upon carbon black nanoparticles is shown to result in photothermally-initiated chain-growth polymerization of methyl acrylate, butyl acrylate, and methyl methacrylate initiated by benzoyl peroxide. Using methyl acrylate as the model system, products from this reaction are shown to be apparently indistinguishable on the molecular level, but result in unique microstructures relative to the 1 ACS Paragon Plus Environment


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thermal controls. The relative contribution of bands of the UV/visible spectrum to the polymerization initiation show that red/infrared wavelengths are most important for the initiation to occur. Kinetic analysis of the initiator homolysis indicate that the apparent reaction rate is accelerated in the photothermal condition. Introduction The photothermal effect of nanosurfaces is an area of interest for an increasing number of applications.1–4 In this process, the surface of a nanoparticle converts absorbed light energy to thermal energy. On the nanoscale, thermal diffusion from a superheated particle creates a nonequilibrium where the particle and its surroundings are heated well beyond the bulk temperature.5-6 The precise physical mechanisms resulting in this heating on the micro and bulk scales are an area of active investigation.6-8 The photothermal effect has found use in highly diverse fields, ranging from therapeutics and diagnostics to catalysis and separations.2,5,9-12 We were particularly excited by reports from the Scaiano, Linic, and Lear groups, among others, of the use of the photothermal effect to drive chemical reactions.13-14,18 Frequently in these works, plasmonic metal-associated nanoparticles (NP) generate “hot electrons” that directly participate in the reaction catalyzed by the NP.15 In contrast, we endeavored to use photothermally active particles to catalyze reactions purely through thermal energy. This nanoscale heating would allow us to understand parameters needed to power a macro-scale chemical reaction. Such processes might allow the nanoscale spatial and temporal control over heating, as well as nanoscale access to temperatures potentially far above those accessible in bulk reactors. Pioneering studies by the Lear group demonstrated that laserassisted photothermal processes could be employed to drive high energy-barrier chemical reactions, in particular the polymerization of urethanes and siloxanes.14,16 Inspired by this work, 2 ACS Paragon Plus Environment

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we endeavored to generate a system whereby broad-spectrum light–rather than a laser–could be the photon source for the photothermal initiation of thermally driven reactions. We accomplished this using full visible light spectrum-absorbing carbon black nanoparticles and a solar simulator to drive a radical chain-growth polymerization. We demonstrated the generality of this technique by observing a rate enhancement in the photothermally-initiated polymerizations of methyl acrylate, butyl acrylate, and methyl methacrylate. Using methyl acrylate as our model system, we found that the photothermal polymerization results in a polymeric material with highly aligned, microscale structures. Specifically, the system uses carbon black nanoparticles to initiate the homolysis of benzoyl peroxide, resulting in the chain-growth polymerization of methyl acrylate. The rate of this reaction is enhanced by the photothermal effect when carbon black NPs are included in the reaction mixture. Excitingly, the polymeric material that is generated by this process has highly distinct morphologies and altered glass transition temperatures as compared to thermally-synthesized polymers, while retaining similar molecular weights (Mw), and polydispersities (PDI).

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Results and Discussion

Figure 1. Emission and absorption spectra and reaction schematic. (a) Solar emission spectrum (NIST). (b) Visible absorbance spectrum of a dilute (0.1 mg/mL) solution of carbon black. (c) Schematic of the setup used to create photothermally-initiated polymerizations Demonstration of photothermal effect using solar simulator The sun emits strongly across ultraviolet, visible, and infrared frequencies (Figure 1a). Thus a broad spectrum absorbance is a highly desirable property for a solar light-harvesting material.19 Carbon black NPs display such a broad spectrum absorbance making activation via a solar simulator ideal (Figure 1b).‡ Carbon black NPs are excellent light harvesters for our solarinitiated polymerization. Reinforcing this fact, we have previously shown carbon black NPs to be excellent converters of solar energy to photothermal processes.22–24 Additionally, carbon black NPs are cheap, commonly incorporated in materials, and have additive properties of strength enhancement, pigmentation, and electrical conductivity. In contrast to noble metal NPs, carbon black NPs are already inexpensively and regularly included in many composite materials. Therefore, our polymerization technique leverages a compound (carbon black NPs) often already included in polymeric materials for other reasons, as opposed to supplementing an expensive and/or additional catalyst into the polymer blend. 4 ACS Paragon Plus Environment

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For our model reaction system, we used the radical chain-growth polymerization of methyl acrylate, initiated by benzoyl peroxide. We selected, methyl acrylate (MA) and benzoyl peroxide (BPO) due to their widespread use in commercial applications. The resulting radical chaingrowth polymerization of poly(acrylate) represents both a polymerization methodology and polymer class used in current commercial applications.25 To demonstrate the rate-enhancement effect of photothermal heating of carbon black NPs on the MA-BPO polymerization process, we used an actinic solar simulator to irradiate a vial with MA, BPO, and commercially-available carbon black NPs (Cabot N115) (Figure 1c). The vials were equipped with a thermocouple, and the time to peak onset of thermal spike corresponding to the polymerization event was monitored. The vials were irradiated with an overall (400-1100 nm spectrum) intensity of 1700 W/m2, approximately equal to the irradiance of three suns. This intensity is easily achieved on larger scale using solar concentrating devices.17 Our results show a statistically significant rate enhancement of polymerization, reducing the onset time by approximately 15 minutes (Figure 2 a). To the best of our knowledge, this result represents the first instance of a solar-type photothermal initiation of a radical reaction, and the first instance of photothermal initiation of a chain-growth polymerization. Once we had honed our initial reaction parameters, we sought to determine the generalizabilty of the photothermal initiation process to other chain growth polymerizations. Employing butyl acrylate (BA) as the monomer, and using the exact same nanoparticle concentration and illumination parameters, we observed an analogous photothermal rate enhancement to that of the MA system (Fig. 2 a). Because of the steric and bulk physical property differences between the two monomers, this suggests that a broad variety of acrylates may be polymerized with our photothermal rate enhancement system.



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Encouraged by this data, we then tested our photothermal parameters with the extremely popular and industrially-relevant monomer, methyl methacrylate.25 Trials were conducted under an argon atmosphere, and used the same concentration of nanoparticles and illumination parameters as the acrylate monomers. The results indicated a dramatic rate enhancement when the nanoparticles were added to the illuminated system. This indicates the photothermal polymerization may be used with a methacrylate monomer as well. Overall, these studies indicate that CB nanoparticle-initiated photothermal rate enhancement has a broad scope in important materials syntheses.

Figure 2. a) Time to peak onset of exotherm for photothermally- and thermally-initiated polymerizations. Reactions monitored by suspending a thermocouple probe in the reaction mixture. Vessels were illuminated using a full spectrum solar simulator with a luminous intensity of approximately three suns. MA thermal: methyl acrylate thermally-initiated (dark) polymerization, with CB nanoparticles; MA photo: methyl acrylate photothermally-initiated polymerization; BA thermal: butyl acrylate thermally-initiated (dark) polymerization, with CB nanoparticles; BA photo: butyl acrylate photothermally-initiated polymerization; MMA photo: 6 ACS Paragon Plus Environment

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methyl methacrylate photo-initiated polymerization; MMA CB photo: methy methacrylate photothermally-initiated polymerization. For methyl and butyl acrylate, solar simulator reactions containing initiator and monomer but lacking carbon black showed no reaction after 2 h. MA: methyl acrylate; BA: butyl acrylate; MMA: methyl methacrylate; CB: carbon black nanoparticles. b) thermal camera images of reaction vials illuminated under experimental conditions, showing change in thermal profile of reaction vial during exothermic polymerization.

Materials properties of photothermal polymers A full characterization was undertaken of the thermally- and photothermally-initiated polymers generated by the MA model system. Overall, these results showed that while the molecular composition of the polymer chains was identical between the two polymerization conditions used to generate the carbon black-containing polymers, the nano- and microscale compositions of the materials varied according to the initiation method used.



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a

b

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c

Mn: 216010 Da Mw: 264918 Da PDI: 1.23

104.5

105.0

272821.4688

105.5265312.0 106.0 4.0

Molecular Weight (Daltons)

d

3.5

3.0 2.5 2.0 1.5 268363.4

1.0

271418.8

ppm

e

f 272883.5625

Mn: 222000 Da Mw: 291677 Da PDI: 1.31

104.5

105.0

274469.2

Mass (m/z)

105.5

106.0 4.0

265957.0

Molecular Weight (Daltons)

3.5 3.0 2.5 2.0 268840.2

1.5

ppm

1.0 271723.4

274606.6

Mass (m/z)

Figure 3. GPC, NMR, and MALDI data for photothermally- and thermally-initiated polymerizations. (a) GPC trace of photothermally-initiated poly(methyl acrylate); (b) 1H NMR spectrum of photothermally-initiated poly(methyl acrylate); (c) MALDI spectrum of photothermally-initiated poly(methyl acrylate); (d) GPC trace of thermally-initiated poly(methyl acrylate); (e) 1H NMR spectrum of thermally-initiated poly(methyl acrylate); (f) MALDI spectrum of thermally-initiated poly(methyl acrylate). Molecular characterization of the polymers was accomplished using NMR, gel permeation chromatography (GPC), and MALDI-TOF. Solution 1H and 13C NMR spectra for polymers synthesized either through thermal or photothermal initiation were highly similar (Figure 3), suggesting the polymers synthesized have identical tacticities. GPC analysis of the polymers showed very similar results for the two polymers (Figure 3). Each polymerization resulted in a 8 ACS Paragon Plus Environment

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single dominant long chain polymer, and a small amount of short oligomers also observed. The predominant polymer of the photothermally-initiated polymerization had a number averaged molecular weight (Mn) of 216 kDa, a weight averaged molecular weight (Mw) of 265 kDa, and a PDI of 1.23. The predominant polymer of the thermally-initiated polymerization had an Mn of 222 kDa, a Mw of 292 kDa, and a PDI of 1.31 (Figure 3). These values are very close, and are consistent with a chain-growth mechanism for the polymerization. MALDI-TOF analysis showed an Mw of 272,821 for photothermally-initiated poly(methyl acrylate), and an Mw of 272,883 for thermally-initiated poly(methyl acrylate) (Figure 3).21 Taken together, these data suggest the molecular composition of the poly(methyl acrylate) generated by the two conditions is practically identical, and therefore that photothermal initiation does not measurably alter the molecular composition of the polymer synthesized by this methodology, as opposed to the traditional thermal method of initiation. In contrast, analysis of higher-order materials properties stemming from the two polymerization conditions were quite distinct. The glass transition temperatures (Tg) of the polymers was measured using differential scanning calorimetry. These data showed a Tg of 9.4 °C for thermally-initiated poly(methyl acrylate) with the addition of carbon black NP, and a Tg of 15.7 °C for photothermally-initiated poly(methyl acrylate). These results suggested that some differences in the microscale or nanoscale organization of the polymer might exist. Since carbon black is currently used as an additive to increase the stiffness, toughness, and conductivity of polymeric materials, the photothermal initiation may be of use in manufacturing carbon blackembedded materials because it suggests a way to lower carbon black loading to achieve the same advantageous material property.



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Figure 4. SEM of photothermally- and thermally-initiated polymers. (a) photothermally-initiated MA polymer at 2499× magnification; (b) photothermally-initiated MA polymer at 5000× magnification; (c) photothermally-initiated MA polymer at 850× magnification; (d) thermallyinitiated polymerization at 2000× magnification; (e) lamellae of photothermally-initiated polymer with 50 mg/mL carbon black loading, 1500× magnification; (f) lamellae of photothermally-initiated polymer with 20 mg/mL carbon black loading, 650× magnification. Panels (a)–(c) show samples taken from three separate polymerization reactions. Intrigued by the difference in Tg that we observed, we analyzed the micro- and nanostructure of our polymers via scanning electron microscopy (SEM). To our surprise, SEM analysis of the thermally- and photothermally-initiated PMA revealed completely different microstructural morphologies between the two polymerization conditions (Figure 4). Photothermally-initiated PMA demonstrated a highly organized, fiber-aligned structure (Figure 4). This aligned morphology was not evident in the thermally-initiated polymerization which, in contrast, showed 10 ACS Paragon Plus Environment

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a highly reticulated surface (Figure 4). The fibers resulting from photothermal initiation, had an average width of 1.16 ± 0.04 µm, and an average length >100 µm, based on over 120 measurements taken over 3 samples. Similar microstructural features have been observed in block co-polymer and phase-separated systems, but not, to our knowledge, simply by changing the heat source of initiation.26,

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Additionally, samples with carbon black loadings above 5

mg/mL showed aligned fibers further arranged in a lamellar 3-dimensional format (Figure 4e–f). Each lamella of aligned fibers was 4.58 ± 1.30 µm. Intrigued by this phenomenon, we sought to determine the dependence of the fiber alignment on the carbon black photothermal initiation. We examined the relationship between carbon black concentration and fiber-aligned microstructure formation, finding that a critical concentration of 0.25 mg/mL of CB was needed to observe the fiber-aligned morphology (see Supplementary Information for details). Applying this method of polymerization initiation may offer a new methodology to create regular microstructures without the need for additional polymers or extra processing. Investigation into the precise mechanism of this microstructure formation is ongoing and will be reported in due time. It is possible that in addition to increasing the local concentration of active initiator, the nanoscale temperature differentials may have a direct relationship to polymer microdomain formation by altering polymer phase separation.

Fundamental investigation of photothermal effect I. Determination of wavelengths contributing to polymerization To investigate this process on more fundamental levels, we monitored the contribution of different bands of the UV/vis spectrum to the rate enhancement of reaction. Such data could provide insight into the mechanism by which the photothermal acceleration is occurring, and 11 ACS Paragon Plus Environment


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elucidate the sections of the solar spectrum most important for obtaining a photothermal effect with carbon black nanoparticles. Carbon black absorbs across the solar spectrum with high efficiency. However, evidence suggests that the infrared band is a particularly important contributor to the photothermal effect of carbon black nanoparticles.20 We assayed the contribution of the infrared band of the spectrum by placing an 850 nm shortpass filter in line with our solar simulator to filter out near-infrared and longer wavelengths of light in line with our solar simulator. Similarly, it is well known that both BPO homolysis and acrylate polymerization can be initiated with UV light. Thus, we investigated the contribution of both short- and longwave UV by placing a 400 nm longpass filter in line with our solar simulator. We performed these studies using the MA-BPO-CB system. Filtering infrared frequencies using a shortpass filter resulted in diminished heating of the reaction mixture. The bulk solution temperature held at 42.3 °C, and no polymerization was observed after 2 h. This data was consistent with previous observations made by our group, which suggest a modest threshold bulk solution temperature rise is needed to initiate a bulk photothermal process.20 However, when a 400 nm longpass filter was employed, removing UV light, photothermal polymerization occurred, with a slight delay–peak onset time occurred at 6.70 min ± 0.17. The delay in onset in time is likely an artefact–light transmission through the filter is reduced by 13% due to the absorptive qualities of the filter material. These results suggest that the IR region of the spectrum is a critical contributor to the photothermal effect observed, and that there is little to no contribution to polymerization initiation from UV region of the spectrum.


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II. Kinetics of BPO homolysis

Equation 1. Decomposition of benzoyl peroxide.

The initial process resulting in the formation of the polymer is the photothermal heatingassisted homolysis of the radical initiator, BPO (Eq. 1). We sought to understand if acceleration of the BPO homolysis might explain the different onset times. We investigated the kinetics of the rate enhancement of BPO homolysis by photothermal heating from the carbon black NPs (Figure 5). Ethyl acetate was used as a solvent owing to its structural similarity MA–mimicking the conditions of the MA polymerization. Two conditions were compared: 1) photo-thermal, wherein BPO was subjected to the same solar simulator with the same intensity full-spectrum light as for the polymerization trials; and 2) thermal, wherein BPO was heated to the same plateau temperature as the photothermal reaction. Our results showed apparent first order kinetics with respect to BPO in both conditions, (Figure 5). These values provide direct evidence for the fact that the rate acceleration of MA polymerization observed is due to the photothermal acceleration of a chemical reaction–the homolysis of BPO.



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Figure 5. Rate study of BPO decomposition in the presence of CB. (a) Trend lines for rate trials of photothermally- vs. thermally-initiated BPO decomposition of BPO in the presence of CB. (b) Observed rate constants for the apparent first-order rate of decomposition of BPO in photohermally- and thermally-initiated conditions.

Conclusions In conclusion, we demonstrated that photothermal heating initiates a radical polymerization yielding unique microstructures. To the best of our knowledge, this is the first reported instance of a photothermally-accelerated polymerization using incoherent full spectrum light. Polymeric materials synthesized using this methodology resulted in altered microstructural morphology and Tg, while retaining similar molecular weights, Mn, and polydispersity as compared with thermally-initiated reactions. These data indicate that interesting microstructures may be created using photothermal initiation while leaving the molecular structure and distribution of the underlying polymers unchanged. Analysis of the effect of incident light upon the carbon black NPs revealed that the absorbance of visible and infrared frequencies results in the photothermal response. These data suggest that full spectrum, incoherent light may be used to drive a 14 ACS Paragon Plus Environment

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photothermal polymerization process, and further, that such a photothermal process results in unique polymeric materials with ordered microstructures. Such materials may be of particular use to applications employing regular microstructures, and applications requiring self-organized materials. Such a polymerization may also be employed as a turn-on probe for evidence of light exposure, or as facile way to quickly repair coatings on surfaces exposed to sunlight. The kinetics of the reaction indicated that the photothermal initiation accelerated the homolysis of the initiator BPO, likely via a nanoscale heating phenomenon. Overall, this work presents both fundamental information and novel results of a nanoparticle-generated photothermal effect. These data may inspire further use of photothermal phenomena to initiate chemical reactions, allowing the nanoscale control of heating, novel microstructure synthesis, and energy efficiency. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supplementary Information.doc (file type, i.e., PDF) AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. †Denotes equal contribution.



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Notes ‡ In

fact, carbon black is one of the blackest substances known.

ACKNOWLEDGMENT The authors wish to acknowledge Hemakesh Mohapatra for critical reading of this manuscript. We are supported by AFOSR 445910. Aaron P. Esser-Kahn thanks the Pew Scholars program and the Cottrell scholars program for generous support. Brittany A. Moser thanks NSF-GRFP DGE-1321846. This work was funded, in part, by the Alfred P. Sloan foundation. ABBREVIATIONS BPO, benzoyl peroxide; CB, carbon black; GPC, gel permeation chromatography; MA, methyl acrylate; MALDI-TOF, matrix-assisted laser desorption/ionization time of flight mass spectroscopy; Mn, number average molecular weight; Mw, weight average molecular weight; NMR, nuclear magnetic resonance; NP, nanoparticle; PMA, (poly)methyl acrylate; PDI, polydispersity; UV, ultraviolet; vis, visible REFERENCES (1)

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