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Sep 23, 2016 - ABSTRACT: The synthesis of a novel high sulfur content material possessing improved thermomechanical properties is reported via the inv...
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High Refractive Index Copolymers with Improved Thermomechanical Properties via the Inverse Vulcanization of Sulfur and 1,3,5Triisopropenylbenzene Tristan S. Kleine,†,# Ngoc A. Nguyen,‡,# Laura E. Anderson,† Soha Namnabat,§ Edward A. LaVilla,§ Sasaan A. Showghi,§ Philip T. Dirlam,† Clay B. Arrington,† Michael S. Manchester,† Jim Schwiegerling,§ Richard S. Glass,† Kookheon Char,*,⊥ Robert A. Norwood,§ Michael E. Mackay,*,‡,∥ and Jeffrey Pyun*,†,⊥ †

Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States ‡ Department of Materials Science and Engineering, University of Delaware, 201 Dupont Hall, Newark, Delaware 19716, United States § College of Optical Sciences, University of Arizona, Tucson, Arizona 85721, United States ∥ Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, Delaware 19716, United States ⊥ School of Chemical and Biological Engineering, Program of Chemical Convergence of Energy and Environment, The National Creative Research Initiative Center for Intelligent Hybrids, Seoul National University, Seoul 151-744, Korea S Supporting Information *

ABSTRACT: The synthesis of a novel high sulfur content material possessing improved thermomechanical properties is reported via the inverse vulcanization of elemental sulfur (S8) and 1,3,5-triisopropenylbenzene (TIB). A key feature of this system was the ability to afford highly cross-linked, thermosetting materials, where the use of TIB as a comonomer enabled facile control of the network structure and dramatically improved the glass transition temperature (relative to our earlier sulfur copolymers) of poly(sulfurrandom-(1,3,5-triisopropenylbenzene)) (poly(S-r-TIB)) materials over a range from T = 68 to 130 °C. This approach allowed for the incorporation of a high content of sulfur− sulfur (S−S) units in the copolymer that enabled thermomechanical scission of these dynamic covalent bonds and thermal reprocessing of the material, which we confirmed via dynamic rheological characterization. Furthermore, the high sulfur content also imparted high refractive index (n > 1.75) and IR transparency to poly(S-r-TIB) copolymers, which offered a route to enhanced optical transmitting materials for IR thermal imaging applications with improved thermomechanical properties (n ∼ 1.5−1.6).1 While the preparation of high n-polymers or polymer composites has been conducted, these materials typically exhibit high n-values with concomitant optical absorption or exhibit high optical losses.2−4 Hence, there remains a strong need for new polymeric materials that exhibit very high refractive indices (n > 1.75), low optical losses and possess high IR transparency. We have previously developed a new class of ultrahigh refractive index, IR transmitting materials that possessed a high sulfur content (50−90 wt % sulfur) through a process termed inverse vulcanization of elemental sulfur (S8) with 1,3diisopropenylbenzene (DIB);5−7 this polymerization and

T

he development of wholly polymeric transmitting IR materials has historically been a tremendous technical challenge since the majority of polymers intrinsically possess low refractive indices and strongly absorb in the IR spectral window (due to the presence of carbon−hydrogen or heteroatom−hydrogen covalent bonds). While inorganic IR transmitting materials have been demonstrated to exhibit very high refractive indices (n = 2−5)1 and high transparency, these materials are relatively expensive and require high processing temperatures for device component fabrication. Hence, the development of polymeric IR transmitting materials is highly desirable from a melt or solution processing standpoint to enable the fabrication of complex or unique device architectures for IR and other photonic applications. However, it has been a longstanding challenge to increase the refractive index of organic polymeric materials, which typically exhibit low values © XXXX American Chemical Society

Received: August 4, 2016 Accepted: September 19, 2016

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DOI: 10.1021/acsmacrolett.6b00602 ACS Macro Lett. 2016, 5, 1152−1156

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ACS Macro Letters other related processes have also been used by others to prepare a broad range of high sulfur content polymers.8−11 Poly(sulfur-random-(1,3-diisopropenyl-benzene)) (poly(S-rDIB)) materials exhibited one of the highest refractive indices reported for a polymeric material (n = 1.75 to 1.85 from 600 to 1500 nm) and was sufficiently IR transparent to enable use in IR thermal imaging.6 While the incorporation of a high content of S−S bonds was the key to imparting the desirable optical properties into poly(S-r-DIB) for IR applications, these polysulfide units also intrinsically lower the glass transition temperature of these copolymer materials (Tg of poly(S-r-DIB) ranges from T = 43.5 to 49.2 °C).5 Herein, we report on the synthesis of a novel IR optical polymer via the inverse vulcanization of sulfur with a trifunctional isopropenyl comonomer, based on 1,3,5-triisopropenylbenzene (TIB). This approach afforded a high sulfur content copolymer possessing high refractive index and high IR transparency with improved thermomechanical properties. Rheological characterization of poly(sulfur-random-(1,3,5-trisopropenylbenzene)) (poly(S-r-TIB)) confirmed the formation of highly cross-linked polymer networks, along with a dramatic increase of the copolymer glass transition temperature (Tg from 67.1 to 130 °C) over a broad window of TIB composition (30− 50 wt %). The properties of this new sulfur copolymer thermoset are a significant improvement over those from our earlier (poly(S-r-DIB)) copolymer which possessed relatively lower Tg’s (T = 43.5−49.2 °C). A useful feature of these sulfur cross-linked thermosets was the presence of S−S units in the copolymer which allowed for self-healing or thermal reprocessing of these materials, making them a unique addition to the expanding field of dynamic covalent polymers.12−17 Furthermore, the incorporation of a high content of S−S bonds in poly(S-r-TIB) enabled both the retention of high refractive indices (1.721 ≤ n ≤ 1.836 from 633 to 1554 nm) and mid-IR transparency required for thermal imaging. A key milestone in this research is the preparation of high refractive index sulfur copolymers possessing comparable thermomechanical properties to those of known optical polymers, such as poly(methyl methacrylate). The key step in the synthesis of the poly(S-r-TIB) copolymers was driven by the need to discover new comonomers for the inverse vulcanization of S8 that possessed a functionality of at least three vinylic groups (f ≥ 3) and was miscible with molten sulfur. Hence, a logical progression from our initial success with the inverse vulcanization of S8 was the use of TIB, which was designed to possess an additional vinylic group relative to DIB to afford more highly cross-linked materials while remaining miscible in liquid sulfur. Since TIB was not commercially available, the target comonomer was prepared in multigram quantities per batch via a Suzuki crosscoupling strategy using a 1,3,5-tribromobenzene core with isopropenyl boronic acid pinacaol ester (Scheme 1a) which proceeded with high regioselectivity and reasonable yields (80%). With TIB in hand, inverse vulcanizations at T = 180 °C in liquid sulfur were initially conducted in glass vials in a thermostated oil bath. TIB comonomer feed ratios of 30 wt % and 50 wt % were investigated, and copolymerizations with liquid sulfur were observed to rapidly vitrify within 10 min. Upon cooling to room temperature, red, transparent glassy materials were observed (Scheme 1b). We found that poly(S-rTIB) copolymers with both 30 wt % and 50 wt % organic comonomer (herein referred to as poly(S-r-TIB30) and poly(S-

Scheme 1. (a) Synthesis of TIB; (b) Inverse Vulcanization of DIB or TIB with S8 to Afford Poly(S-r-DIB) or Poly(S-rTIB) Copolymers; and (c) Melt Processing of Poly(S-r-TIB) Copolymer Resins with PDMS Replicas to Form Molded Poly(S-r-TIB)

r-TIB50)) were completely insoluble in organic solvents, such as tetrahydrofuran (THF) and toluene. This is in stark contrast to poly(S-r-DIB) copolymers that were partially soluble in these solvents (for the 30 wt % sulfur content copolymer) and found to be completely soluble at higher DIB compositions (50 wt % DIB) when prepared under similar conditions. Hence, from these vast solubility differences, it was inferred that the poly(Sr-TIB) copolymers were heavily cross-linked in comparison to their poly(S-r-DIB) analogues as a direct consequence of the higher functionality of the TIB comonomer. In order to quantitatively confirm this, dynamic rheological frequency sweeps (see Supporting Information Figure S7) were conducted on the poly(S-r-TIB) copolymers that prove they are in fact more heavily cross-linked than poly(S-r-DIB) copolymers. It was also observed that reaction mixtures from the inverse vulcanization of S8 and TIB could directly be used (as was the case of S8 and DIB) as prepolymer resins for melt processing into PDMS replicas to prepare molded discs and windows of poly(S-r-TIB) (Scheme 1c). The refractive indices of the resulting free-standing films of poly(S-r-TIB) were obtained by prism coupling optical measurement and were observed to retain the very high refractive index (n = 1.836 to 1.792 from 633 to 1554 nm) (see Supporting Information Figure S2) as observed for poly(S-r-DIB) (n = 1.856 to 1.810)6 at similar compositions. Furthermore, the poly(S-r-TIB) copolymers remained transmissive in the near-IR regime (see Supporting Information Figures S3 and S4) indicating that these materials are in fact a viable replacement for DIB-based sulfur copolymers as high refractive index, IR transparent materials. Differences in the thermomechanical properties of poly(S-rTIB) and poly(S-r-DIB) copolymers, as a result of the increased cross-link density, were interrogated by isochronal oscillatory shear measurements conducted at T = 30 to 180 °C (Figure 1a). We observed a progressive increase in the glass transition of poly(S-r-DIB30) and poly(S-r-DIB50) at T ≤ 30 and 55.4 °C, respectively, which were reasonably consistent with our earlier DMA analyses of poly(S-r-DIB) copolymers.5 However, for poly(S-r-TIB30 ) and poly(S-r-TIB50 ), we observed a considerably wider range of accessible Tg values 1153

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where Tg∞ is the glass transition temperature for an infinite molecular weight polymer; ρx is the degree of cross-linking; and Ki is a constant specific to a given system (we will use KDIB for the DIB system and KTIB for the TIB). First, it is clear the TIB system promoted a larger Tg at a given concentration indicative of more cross-links. Second, we calculated Ki for these two systems (KDIB = 1.3 and KTIB = 3.2) using the DSC data, and it is clearly seen that the TIB system was a much more effective cross-linker. The fact that the values of Tg did not agree between the two measurement techniques was merely reflective of the different temperature ramp rates and was of little concern since we found KTIB = 3.1, derived from the rheological data (Figure 1b), was in good agreement with the DSC value. The temperature ramps could not be operated below room temperature, limiting its effectiveness for the 30 wt % DIB system. Nevertheless, the remarkable difference in the Tg of the poly(S-r-TIB) due to the higher cross-linking density of this material afforded, for the first time, a high sulfur content, high refractive index material (with n > 1.7) with a Tg > 100 °C. To confirm the presence of dynamic S−S units in poly(S-rTIB) copolymers (see Supporting Information Figure S9a) in situ rheological monitoring of shear-induced S−S bond scission at varying strain rates was conducted above Tg (see Supporting Information for comparative rheology of poly(S-r-TIB) vs poly(S-r-DIB), Figures S8, S9). Mechanical tensile testing of poly(S-r-DIB) and poly(S-r-TIB) copolymers was also conducted but was found to afford largely similar mechanical strength and toughness (see Supporting Information, Figure S1, Table S5). To further demonstrate the benefits of poly(S-r-TIB) copolymers for optical applications, flat panel discs were fabricated via melt processing and subjected to irradiation with a laser to access the dimensional stability of these materials under high optical powers. A comparative analysis of poly(S-rDIB30) and poly(S-r-TIB50) was conducted by direct laser diode irradiation at 405 nm (a wavelength of high absorption in both poly(S-r-TIB) (see Supporting Information Figure S3) and poly(S-r-DIB)6). Flat panel discs of both sulfur copolymers (25 mm × 1 mm samples of poly(S-r-DIB) and poly(S-r-TIB)) were fabricated and subjected to irradiation with a focused laser beam at 405 nm (on each a ∼100 μm diameter spot was exposed to 35 mJ, Figure 2c). After irradiation for five seconds, significant laser-induced heating and damage were observed using dark-field optical microscopy of the poly(S-r-DIB) sample, which was attributed to the lower Tg of the material (Figure 2d,e). Conversely, under identical conditions, the poly(S-r-TIB) disc remained undamaged due to the enhanced thermomechanical stability of the material (Figure 2f,g). The 405 nm wavelength was chosen as it ensured complete absorption of the laser beam by the 1 mm samples, subjecting them to the largest possible lased-induced heating. Finally, to confirm that poly(S-r-TIB) copolymers were sufficiently transparent for IR applications, mid-IR imaging experiments of a USAF target (Figure 2h) were conducted through poly(S-r-TIB30) and poly(S-r-TIB50) flat panel discs (1 mm thick, 25 mm diameter). It is important to note that the USAF target was transparent in the 3−4 μm regime which was suitable for imaging but was absorbing at longer wavelengths due to its glass substrate; the sulfur copolymers have even higher transmission in the 4−5 μm regime. In these experiments, a lens mount bearing the sulfur copolymer panel was placed in front of the IR camera. Control imaging of the USAF target without a sulfur copolymer panel through the lens

Figure 1. (a) Plot of storage modulus (G′) as a function of temperature performed with a 3 °C min −1 ramp rate and oscillation frequency of 6.28 rad/s for copolymers poly(S-r-DIB) and poly(S-rTIB) of varying compositions. (b) Plot of tan δ as a function of temperature for the same copolymers, obtained under the same conditions as the storage modulus plots.

ranging from T = 67.1 to 130 °C over the same copolymer compositions, which is markedly apparent in tan δ peaks of these measurements (Figure 1b). DSC measurements (Table 1) were similarly conducted and corroborated the dynamic rheological (temperature ramp) measurements. Fox and Loshaek18 demonstrated for an infinite molecular weight polymer (eq 1) Tg = Tg ∞ + K iρx

(1)

Table 1. Glass Transition Temperatures Determined by Differential Scanning Calorimetry (Tg(DSC)) and Dynamic Rheological Characterization (Oscillatory Shear) (Tg(OS)) as a Function of Comonomer Concentration comonomer content

Tg (DSC) (°C)

Tg (OS) (°C)

DIB 30 wt % DIB 50 wt % TIB 30 wt % TIB 50 wt %

9.9 36.5 51.2 116

53 68 130 1154

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Figure 2. Dark-field optical microscopy images of pristine (a) poly(S-r-DIB30) and (b) poly(S-r-TIB50) 1 mm flat panel discs; (c) a 405 nm laser diode was set up to irradiate both sulfur copolymers with an optical output power of 7 mW for 5 s; (d) digital image of poly(S-r-DIB) after irradiation; (e) dark-field optical microscopy image of poly(S-r-DIB30) panel after irradiation (35 mJ) revealing the damaged region (white circle) commensurate with spot size of laser; (f) dark-field microscopy image and digital image (g) of poly(S-r-TIB50) panel showing no damage after irradiation; (h) digital image of USAF target; images captured with a mid-IR camera, operating at 3−5 μm, of a USAF target, transparent at 3−4 μm: (i) IR thermal image of USAF target imaged through a lens mount without sulfur copolymer panel; (j) IR thermal image through the poly(S-rTIB30) panel (1 mm thick, 25 mm wide), and (k) IR thermal image through the poly(S-r-TIB50) panel (1 mm thick, 25 mm wide).

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mount (as seen as the yellow circular feature) was performed as a reference (Figure 2i). For both poly(S-r-TIB30) (Figure 2j) and poly(S-r-TIB50) (Figure 2k) panels, high-quality imaging was achieved, which confirmed that these materials retained the desirable IR optical properties required for imaging while possessing improved thermomechanical stability. In conclusion, we have demonstrated the preparation of high refractive index sulfur copolymers via the inverse vulcanization of sulfur with a new comonomer, 1,3,5-triisopropenylbenzene (TIB), to afford highly cross-linked polymer networks. The thermal properties of these new poly(S-r-TIB) copolymers were dramatically improved and were tuned to exhibit Tg’s above 100 °C, which is now comparable to those of widely used optical polymers, such as poly(methyl methacrylate) (PMMA). Furthermore, we demonstrate the ability to use these sulfur copolymers as novel transmitting materials for IR thermal imaging.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions #

Both authors contributed equally to this manuscript.

Notes

The authors declare the following competing financial interest(s): JP declares an actual or potential financial conflict of interest and is co-founder/equity holder in Innovative Energetics, a licensee of University of Arizona (UA) intellectual property. This relationship has been disclosed to the UA Institutional Review Committee and is managed by a Financial Conflict of Interest Management Plan.

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ACKNOWLEDGMENTS We acknowledge the NSF (DMR-1607971), Kuraray, for support of this work. KC acknowledges the support from NRF for the National Creative Research Initiative Center for Intelligent Hybrids (2010-0018290). N.A.N. acknowledges the NIST award 70NANB10H256 through the Center for Neutron Science at the University of Delaware. University of Delaware’s funding provided through the Department of Materials Science and Engineering. S. Namnabat acknowledges

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00602. Experimental details for the preparation and characterization of sulfur copolymers (PDF) 1155

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ACS Macro Letters the support of AFOSR Phase I SBIR contract FA9550-15-C0046.

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DOI: 10.1021/acsmacrolett.6b00602 ACS Macro Lett. 2016, 5, 1152−1156