Article pubs.acs.org/Macromolecules
Remoldable Thiol−Ene Vitrimers for Photopatterning and Nanoimprint Lithography Gayla Berg Lyon,† Lewis M. Cox,‡ J. Taylor Goodrich,† Austin D. Baranek,† Yifu Ding,‡ and Christopher N. Bowman*,† †
Department of Chemical and Biological Engineering, University of Colorado, 3415 Colorado Avenue, Boulder, Colorado 80303, United States ‡ Department of Mechanical Engineering, University of Colorado, 1111 Engineering Drive, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: Here, we introduce photocuring as a tool for the spatiotemporal control of vitrimer network synthesis via a photoinitiated thiol−ene polymerization. A difunctional norbornene monomer was synthesized containing ester linkages and pendant alcohol groups to participate in transesterification bond reshuffling reactions. The transesterification catalyst 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) was shown to be highly effective in promoting transesterification in these networks at high temperatures, without interfering with external spatiotemporal, photoinitiated control over the thiol−ene polymerization and associated network formation. A strong Arrhenius dependence of the stress relaxation time with inverse temperature was observed from 145 to 175 °C, which suggests a relaxation controlled by the transesterification reaction rate, similar to previously studied thermally cured vitrimers. These thiol−ene vitrimers are implemented in nanoimprint lithography (NIL) for creating surface features, where imprinting may be performed repeatedly on the same sample due to the reversible nature of the bond exchange reactions. Because the networks are photocurable, hierarchical structures were generated by photopatterning and developing a microscale pattern and then performing NIL on the surface of this pattern.
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INTRODUCTION Among the many dynamic covalent chemistries studied in the literature, the transesterification reaction has been demonstrated as a robust and convenient choice for thermally activated adaptable networks. Transesterification, which involves an exchange between an alcohol and an ester, has been widely applied by laboratory chemists and in industrial processes for many decades.1,2 In 2011 and 2012, Leibler et al. published several seminal papers describing epoxy acid stepgrowth networks, synthesized from inexpensive, off-the-shelf monomers.3,4 In these studies, the resulting networks behaved as typical cross-linked networks at low temperatures but became remoldable at temperatures above 150 °C. These “vitrimer” networks could be manipulated in ways that are normally exclusive to thermoplasticsincluding repeatable injection molding and welding at high temperaturesand subsequently reverted to thermoset-like properties when cooled to low temperatures. The viscosity of these polymers was © XXXX American Chemical Society
shown to have an Arrhenius dependence at high temperatures because in this case the reaction rate of the alcohol−ester exchange was rate-limiting.5 This behavior contrasts with the classic Williams−Landel−Ferry dependence, seen in most linear polymer melts, where the viscosity is dictated by the rate of chain diffusion.6 Others have developed alternative chemistries for vitrimer materials with similar thermal relaxation behavior, such as the vinylogous urethanes developed by Denissen et al.7 The overall temperature dependence of the behavior is reminiscent of vitreous silica, and thus, the term “vitrimer” was created to describe these thermally triggered dynamic networks.8 Further research has since been performed incorporating vitrimer behavior into different types of polymers, such as liquid crystal elastomers9 and compoReceived: June 24, 2016 Revised: November 9, 2016
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DOI: 10.1021/acs.macromol.6b01281 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 1. (A) Monomer, catalyst, and photoinitiator structures for compounds used to create thiol−ene vitrimer networks, including PETMP (1), BPAGDN (2), TBD (3), and DMPA (4). (B) Illustration of a portion of the network undergoing a bond exchange event with a tethered hydroxyl group within the network.
sites.9−11 An excellent review of recent work on various vitrimers has been recently published by Denissen et al.8 In this work, we seek to incorporate transesterification into photocurable polymer networks, which would greatly enhance the scope of this dynamic chemistry. Photocuring is an invaluable tool that is applied industrially for coatings, adhesives, stereotlithography, and many other uses. Introducing transesterification into a photocured resin would extend the possibility of reshaping, welding, and imprinting a covalently cross-linked network toward a variety of applications, utilizing the ubiquitous and easily incorporated functionalities of alcohols and esters. To this end, we chose to use a radicalmediated thiol−ene “click” reaction, which is known for its extremely rapid kinetics and ability to proceed rapidly under mild conditions.12−15 This reaction involves the addition of a thiyl radicaloften photogeneratedto an electron-rich or strained double bond, resulting in a carbon-centered radical. If the olefin does not readily homopropagate, as is the case with groups such as allyl ethers and norbornenes, a proton is abstracted from another thiol group to form a second thiyl radical, which can then continue the cycle with another double bond. When multifunctional thiol and olefin monomers are used, well-defined and homogeneous step-growth networks can be radically polymerized to quantitative conversion within seconds under low-intensity UV irradiation. The step-growth nature of these polymerizations is essential for achieving significant dynamic behavior, since esters and alcohols are readily incorporated along the backbone throughout the entire network, rather than being confined to side-chain cross-links as in a chain-growth polymer. The norbornene group in particular has been previously investigated for its particularly fast reaction kinetics in the thiol−ene reaction as well as the rigidity that it adds to the polymer backbone, which is useful for making glassy, high-Tg materials.16 In addition, we demonstrate that thiol−ene vitrimers have highly desirable characteristics and behavior for nanoimprint
lithography (NIL). Generating nanoscale patterns at reduced size scales is essential to the forward progress of many emerging applications; however, lithographic techniques to generate sub100 nm features (including deep-UV, E-beam, and others) come with many challenges and are often quite expensive.17−19 Originally developed in the 1990s by Chou and co-workers,20,21 NIL has emerged as a promising, low-cost alternative to highenergy lithographic techniques. In this method, a master negative mold, usually made of silicon or rigid metals, is applied to a polymeric resist material at a controlled temperature and pressure. As originally developed, thermoplastics such as PMMA were used as a resist material.20,21 An alternative method called “step-and-flash” lithography (SFIL), developed by Willson et al. in 1999,22 involves imprinting onto a liquid photoresist and photopolymerizing while the mold is in place. Cheng and Luo used SFIL in combination with a photomask to generate mixed-scale patterns, avoiding the issue of irregularities in flow that can occur when doing multiscale imprinting onto a thermoplastic.23 In this study, we show that thiol−ene vitrimers are also suitable for postpolymerization NIL with a number of interesting advantages. Because these polymers are photocurable, hierarchical patterns are generated by first creating a 2D microscale pattern by conventional photolithographic methods, followed by the application of NIL to generate nanoscale surface features. The adaptable nature of the network means that the pattern can be remolded as many times as desired after the initial imprint, allowing for the nanoscale surface topology to be altered multiple times on the same sample.
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EXPERIMENTAL SECTION
Commercially available reagents were used without further purification. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), 5norbornene-2-carboxylic acid (NCA; mixture of endo and exo), D.E.R. 332 (bisphenol A diglycidyl ether, BADGE), trimethylolpropane diallyl ether (TMPDAE), tetrapropylammonium bromide (TPAB), hydroquinone monomethyl ether (MEHQ), and 1,5,7-triazabicyclo[4.4.0]B
DOI: 10.1021/acs.macromol.6b01281 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Dynamic Mechanical Analysis. Dynamic mechanical analysis was performed on a Q800 DMA (TA Instruments), and the glass transition temperature (Tg) was defined as the peak of the tan delta curve. For glassy films containing BPAGDN, the storage modulus and tan delta were monitored from 20 to 150 °C, using a ramp rate of 3 °C/min, a frequency of 1 Hz, and a fixed oscillatory strain of 0.025%. To determine the ultimate Tg of the polymer films, multiple scans were performed until the tan delta peak was within 1 °C of the previous cycle. Stress Relaxation. Stress relaxation experiments were performed on a Q800 DMA. The sample was equilibrated at the selected temperature for 10 min, followed by an immediate application of strain and monitoring of stress decrease over approximately 30 min. For temperature step experiments, where multiple temperatures were tested, a small strain of 1% was used to avoid excessive sample deformation over multiple cycles. Nanoimprint Lithography and Photopatterning. Nanoimprint lithography (NIL) was performed on an Eitre 3 (Obducat, Lund, Sweden), using a silicon mold to stamp surface patterns onto samples. Prior to NIL, the silicon mold was cleaned using piranha solution and then coated with a low-surface-energy, self-assembled monolayer of CF 3 (CF 2 ) 5 (CH 2 ) 2 SiCl 3 (tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane, Sigma-Aldrich) by vapor deposition to facilitate mold release. The surface patterns imprinted using this mold are actually submicron in size; the term nanoimprint lithography, however, which describes the physical technique implemented to prepare the pattern is used here to be consistent with literature referencing the sample preparation technique at similar length scales. Imprinting was performed on fully annealed and dried samples at 60 bar pressure at a temperature of 175 °C for 10 min. Because the polymers in this study are glassy at ambient temperature, the films were briefly heated to 100 °C for 5 min after imprinting. This step was taken to eliminate any shape memory effects associated with limited mobility in the glassy state, so that when atomic force microscopy (AFM) was performed, the surface topography would be a direct result only of the network rearrangement associated with the transesterification. Atomic force microscopy was performed on a Dimension 3100 AFM (Veeco Instruments, Plainview, NY). Photopatterning was performed using a Model 500 mask aligner (Optical Associates Inc., San Jose, CA). For photopatterning formulations, 0.01 mol equiv per hydroxyl of the inhibitor MEHQ was included to enhance XY patterning resolution. The resin was cast on a glass slide functionalized with thiol groups to ensure good adhesion with the substrate. The collimated UV light was measured at about 10 mW/cm2, and patterning was performed through a Mylar photomask (Fineline Imaging, Colorado Springs, CO) for 12 s. Development of the pattern was performed using ethyl acetate to remove excess unreacted resin, followed by a drying and annealing of the film in a 100 °C vacuum oven for 2 days. Nanoimprint lithography was subsequently performed on the patterned film, as described above.
dec-5-ene (TBD) were purchased from Sigma-Aldrich. TBD was stored under inert atmosphere to avoid CO2 complexation. Thiocure GDMP (glycol di(3-mercaptopropionate)) was donated by Bruno Bock. Irgacure 651 (2,2-dimethoxy-1,2-diphenylethanone, DMPA) was donated by Ciba Specialty Chemicals (now BASF). Monomer Synthesis. The synthesis of bisphenol A glycerolate di(norbornenyl ester) (BPAGDN, Figure 1) was adapted from work done by Podgórski24 and is outlined in Figure S1. BADGE (1 equiv), NCA (2.1 equiv), and TPAB (0.1 wt %) were combined neat, heated to 100 °C while stirring, and allowed to react overnight. While cooling to ambient temperature, ethyl acetate was added to dissolve the mixture, which was then washed with 1.0 M HCl, water, and brine. The dissolved product was dried over sodium sulfate, followed by removal of solvent in vacuo to yield an amorphous semisolid, which readily flows upon heating. The product was confirmed by 1H NMR and mass spectroscopy. NMR peaks for protons within the norbornyl ester structure, both endo and exo forms, were assigned based on the analysis of endo and exo NCA by Happer et al.25 A detailed overview of the NMR spectrum for BPAGDN can be found in Figure S2 of the Supporting Information. Resin Preparation and FTIR Monitoring of Polymerization. Thiol−ene networks were formulated using a 1:1 thiol:ene stoichiometry based on functional groups. Monomer structures are shown in Figure 1A. Because of the extremely high viscosity of the dinorbornene monomer, excess ethyl acetate was used to facilitate mixing of BPAGDN, PETMP, transesterification catalyst (TBD), and photoinitiator (DMPA). After all components were dissolved, the ethyl acetate solvent was removed gradually in vacuo while periodically monitoring the mass of the solution. Solvent removal was stopped once the mixture contained 25 wt % ethyl acetate. It was found that the BPAGDN-containing resin was significantly easier to work with under this condition, due to its reduced viscosity, as compared with the neat resin. Additional resin compositions were formulated that included 15 and 25 wt % substitution of the BPAGDN monomer with trimethylolpropane diallyl ether (TMPDAE), adjusting the PETMP content to maintain a stoichiometric ratio of thiol and ene groups. Ethyl acetate was also used to facilitate mixing of the TMPDAEcontaining formulations; however, these formulations remained tractable after the solvent was completely removed. For collecting kinetic data on the photopolymerization, mid-FTIR experiments were conducted where a drop of resin was placed between NaCl salt plates to make a thin film and then set up in the IR beam path on a horizontal stage. The sample was irradiated with 365 nm filtered light at an intensity of 3 mW/cm2, using a mercury arc lamp (Acticure 4000, EXFO, Quebec City, Canada). To determine the conversion of thiol and ene functional groups, the peak areas for representative thiol (2569 cm−1, S−H stretch), norbornene (714 cm−1, =C−H bending vibration), and double bond (3137 cm−1, =C−H stretch) peaks were monitored. Thiol−Ene Thin Film Formation. Thiol−ene resins containing 25 wt % ethyl acetate were prepared as described above. Polymer films were cast between glass slides treated with Rain-X (ITW Global Brands, Houston, TX), using 250 μm plastic spacers (Small Parts Inc., Logansport, IN) to control film thickness. During the curing step, films were irradiated with 365 nm light at ∼10 mW/cm2 intensity for 5 min per side. Once cured, the samples were dried for 24 h in a vacuum oven at 100 °C. Solvent removal was confirmed by comparing the film weight before and after drying, as well as a DMA scan (as described below) on bulk films to confirm via tan delta curve analysis that the sample was homogeneous and had a peak within 5 °C of the ultimate Tg. Thermogravimetric Analysis. Thermogravimetric analysis was performed using a Pyris 1 TGA (PerkinElmer, Waltham, MA). From a film that had been fully annealed and dried, a small sample (2−5 mg) was obtained and loaded into the sample chamber. The sample mass was monitored as it was first held at 150 °C for 60 min, followed by a ramp at 10 °C/min from 150 to 850 °C. A second experiment was performed on a separate sample, which was heated to 190 °C for 2 h and 200 °C for an additional 2 h while monitoring the sample mass.
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RESULTS AND DISCUSSION Design and Synthesis of BPAGDN for Thiol−Ene Networks. The diene monomer BPAGDN was designed to form a highly glassy thiol−ene polymer that can undergo bond rearrangement via transesterification at high temperatures. Norbornene groups, noted for their extremely rapid kinetics in thiol−ene reactions, have previously been studied to create high-Tg thiol−ene networks.16 The synthetic procedure for BPAGDN, shown in Figure 1, is a one-step process that does not require column separations and gives an overall product yield of 77%. The epoxy acid reaction results in a monomer containing a 1:1 ratio of alcohol and ester groups, which are required for transesterification. In these thiol−ene networks, the tetrafunctional thiol monomer PETMP also serves as a source of ester groups for the transesterification reactions. Firdaus et al. have suggested that these mercaptopropionatebased esters could have increased reactivity for transC
DOI: 10.1021/acs.macromol.6b01281 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. Relevant sections of FTIR spectra of a PETMP−BPAGDN resin, both before (solid black line) and after (dashed red line) 10 min irradiation at a 365 nm wavelength and 3 mW/cm2 intensity. The resin contained 5 mol % TBD per OH group and 5 mol % DMPA per thiol group. Both thiol (2569 cm−1) and ene (714 and 3137 cm−1) groups were completely consumed during the polymerization. As a reference, the peak representing an aryl C−H stretch within the bisphenol A moiety (830 cm−1) and hydroxyl (broad peak between 3200 and 3600 cm−1) did not change significantly.
shelf test formulation containing 1 equiv of PETMP, 1 equiv of GDMP, and 3 equiv of TMPDAE. Varying amounts of DMPA (0 or 0.02 equiv per OH group), TBD (0, 0.01, or 0.05 equiv per OH group), and radical inhibitor MEHQ (0, 0.005, or 0.01 equiv per OH group) were included, and the stability of the mixtures was observed over time. All conditions that did not include TBD formed a gel when left in the dark overnight, while TBD appeared to stabilize the resin for up to 1 week after resin mixing. Despite its stabilizing effect, TBD-containing formulations that included DMPA readily polymerized upon UV light exposure. A more in-depth FTIR study of the test formulation (Figure S3) revealed that TBD appears to behave as a retarder, reducing the initial rate of the thiol−ene polymerization. Nonetheless, under the conditions tested, it did not prevent the reaction from eventually reaching quantitative conversion with sufficient exposure time. The primary formulation used in this work contained BPAGDN, PETMP, 0.05 mol equiv of TBD per OH, and 0.05 mol equiv of DMPA per thiol, using a stoichiometric ratio of thiol to ene groups. For the polymerization, 25 wt % ethyl acetate was included to improve the ease of resin processing and to prevent vitrification from occurring during ambient temperature photocuring. The polymerization was monitored using FTIR spectroscopy, and the reaction was confirmed to reach full conversion of both thiol and ene groups after 10 min irradiation (Figure 2). TGA experiments (Figure S5), which involved a temperature hold at 190 °C for 2 h and 200 °C for an additional 2 h, revealed a mass loss of
