Segmented Thermoplastic Polymers Synthesized by ThiolâEne Click

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Segmented Thermoplastic Polymers Synthesized by Thiol−Ene Click Chemistry: Examples of Thiol−Norbornene and Thiol−Maleimide Click Reactions Kailong Jin,† Emily K. Leitsch,† Xi Chen,† William H. Heath,§ and John M. Torkelson*,†,‡ †

Department of Chemical and Biological Engineering and ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States § The Dow Chemical Company, Freeport, Texas 77541, United States S Supporting Information *

ABSTRACT: Thiol−ene click reactions are used to synthesize segmented thermoplastic materials for the first time via a soft segment + hard segment + chain extender approach that is commonly used to synthesize thermoplastic polyurethane elastomer (TPU). We employ a relatively long chain difunctional thiol (2500 g/mol) as soft segment, a small-molecule thiol as chain extender, and rigid cyclic-ene monomers, including norbornene (containing either urethane or urea linkages in the backbone) and maleimide, as hard segments to achieve thiol−norbornene and thiol−maleimide thermoplastics. The majority of the thiol−norbornene polymers synthesized with 45% or 55% urethane-based norbornene hard segments exhibit phase separation with broad interfaces as indicated by dynamic mechanical analysis (DMA) and hold promise as both thermoplastic elastomers competitive with TPUs and broad-temperature-range damping materials. Thiol−norbornene polymers synthesized with 50% urea-based norbornene hard segments are nanophase separated with sharp interfaces (as indicated by DMA and small-angle X-ray scattering) due to the stronger interurea hydrogen bonding as compared with interurethane interactions. The low strain at break (∼30%) and high Young’s modulus (200−300 MPa) suggest that the 50% hard segment forms the matrix in these polymers, disallowing elastomeric response. Segmented thiol−maleimide thermoplastics, synthesized without isocyanates at 45% and 50% hard-segment content, exhibit highly effective nanophase separation and properties indicating potential to be competitive with some thermoplastic non-isocyanate polyurethane (NIPU) elastomers and TPUs. (Tgs) at or below room temperature.2,5,23 To overcome these effects, previous studies have largely focused on synthesizing chemically cross-linked thiol−ene networks with elastomeric properties.3,5,29−33 Some examples include using photoinitiated thiol click reactions to synthesize single-phase two-component thiol−ene polymer networks for coatings, dental restoratives, and other applications.3,5,29−33,38,39 Such single-phase binary networks usually have limitations in physical or mechanical properties, particularly those related to toughness and hardness.38,39 A third component can be incorporated into these binary networks to improve their mechanical performance.40−53 For example, thiol−epoxy−acrylate hybrid networks13,53 with phase-separated morphologies (glassy epoxy minor phases) and enhanced thermal and mechanical properties have been synthesized. In addition to chemical cross-linking, the incorporation of physical cross-linking (induced by glassy epoxy minor phases) has resulted in enhanced material properties.13,53

1. INTRODUCTION Over the past decade, both academia and industry have shown great interest in thiol−ene click reactions (including, but not limited to, thiol−vinyl, thiol−acrylate, thiol−norbornene, and thiol−maleimide reactions) because of their simplicity, selectivity, efficiency, product yield, and rapid reactivity at ambient atmospheric conditions.1−5 Based on the initiating mechanism, thiol−ene click reactions can be categorized as radical-mediated thiol reactions (e.g., thiol−vinyl, thiol− norbornene, and some thiol−acrylate reactions6−12) and nucleophilic thiol reactions (e.g., base-catalyzed thiol−Michael addition reactions such as thiol−acrylate and thiol−maleimide reactions10,13−19). In the open research literature, thiol−ene click reactions have been employed for postpolymerization modification,4,20−23 substrate surface modification,4,23 microdevice fabrication and photolithography,4,23,24 production of monomers for polyurethane or polyhydroxyurethane synthesis,25−28 formation of polymer networks,3−5,23,29−35 hydrogel synthesis,4,23 and coatings.5,36,37 The main drawback of thiol−ene click chemistry is that the thioether linkage (CH2−S−CH2) formed tends to be flexible, leading to soft materials with glass transition temperatures © XXXX American Chemical Society

Received: March 16, 2018 Revised: April 25, 2018

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DOI: 10.1021/acs.macromol.8b00573 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 1. Reactions employed to synthesize urethane-based norbornene monomers (blue boxes, a−c), urea-based norbornene monomers (green boxes, d, e), and an allyl-functional monomer (red box, f). Reactions a, b, and c depict the syntheses of TDI-urethane-based norbornene-functional monomer (TDIN), MDI-urethane-based norbornene-functional monomer (MDIN), and PDI-urethane-based norbornene-functional monomer (PDIN), respectively. Reactions d and e depict the syntheses of TDI-urea- and MDI-urea-based norbornene-functional monomers (TDIUN and MDIUN), respectively. Reaction f depicts the synthesis of a vinyl-functional urethane-based monomer CYC-ene.

particular, Walker et al. linked three macromonomers using thiol−norbornene chemistry and produced multiblock copolymers with apparent nanophase separation,65 and Li et al. employed thiol−ene chemistry to create phase-separated diblock copolymers of polystyrene and poly(ethylene glycol).66 Here, we designed segmented thiol−ene thermoplastic materials using an approach that is analogous to the synthesis of traditional thermoplastic polyurethane elastomers67−70 or TPUs and environmentally benign polyhydroxyurethane (isocyanate-free) thermoplastic elastomers.71−76 Thermoplastic polyurethane elastomers are commonly produced by capping a relatively long chain polyol soft segment with excess diisocyanate (resulting in urethane linkages which act as hard segments), followed by chain extension with a low molecular weight (MW) difunctional alcohol (often known as chain

As an alternative to chemical cross-linking, thermal and mechanical properties of thiol−ene-based materials can be enhanced by employing either stiff reactant backbones43,46,54−56 or rigid cyclic-ene monomers such as norbornene7,57−59 and maleimide.1,14,16,17,60 For example, the thiol−maleimide reaction1,14,16,17,34,60,61 has been used to synthesize relatively high-Tg (∼130 °C) thermosetting resins which are promising materials for load-bearing applications.61 Moreover, the inclusion of inorganic fillers36,62−64 such as silica particles and glass fibers has also been employed to increase the strength and Tg of thiol−ene-based materials. In contrast to the many studies focused on the synthesis of thiol−ene-based cross-linked polymer networks,3,5,29−35 few studies have synthesized segmented or phase-separated thermoplastic materials using thiol−ene linking reactions.65,66 In B


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Figure 2. (i) Example synthesis of a segmented thermoplastic thiol−norbornene polymer based on TDIN as the hard segment, PPGMP 2200 as the soft segment, and EDT as the chain extender. (ii) Example synthesis of a segmented thermoplastic thiol−maleimide polymer based on MI as the hard segment and the same soft segment and chain extender in (i). The polymeric structure depicted is only one possible structure obtained.

extender).67 The final material exhibits nanophase-separated morphology with hard segments dispersed in a rubbery softsegment matrix. The urethane linkages created during the isocyanate/alcohol reaction segregate from the soft segment due to polarity and inter-urethane hydrogen-bonding interactions; urethane linkages form the hard segments which act as physical cross-links promoting elastomeric-like behavior.77 Similar to traditional TPU synthesis, we employed a relatively long chain difunctional thiol (thiol-terminated polypropylene glycol; MW = 2500 g/mol) as soft segment and a small-molecule thiol (1,2-ethanedithiol) as chain extender. Both norbornene- and maleimide-functional monomers were employed as hard segments to achieve thermoplastic thiol−norbornene and thiol−maleimide materials, respectively. With regard to the thiol−norbornene materials, we designed norbornene-functional monomers containing urethane or urea linkages in their backbone which help to drive phase separation via hydrogen-bonding interactions. All components, including soft segment, hard segment, and chain extender, were reacted together in a single step via thiol−ene click reactions (radically initiated thiol−norbornene reaction7,57−59 and nucleophilic thiol−maleimide reaction1,14,16,17,60). The segmented thermoplastic thiol−ene-based materials synthesized exhibited phase separation and an elastomeric-like mechanical response with some degree of reversibility. Some of the synthesized polymers also showed potential utility as broad-temperature-range acoustic and vibration damping materials.

diisocyanate (TDI, Sigma-Aldrich, 95%), 4,4′-methylenebis(phenyl isocyanate) (MDI, Sigma-Aldrich, 98%), 1,4-phenylene diisocyanate (PDI, Sigma-Aldrich), and allyl isocyanate (AI, Sigma-Aldrich, 98%) were used as received to synthesize the norbornene- and vinylterminated hard segments. 5-Norbornene-2-methanol (Sigma-Aldrich, 98%) and 5-norbornene-2-methylamine (TCI America, 98%) were used as received to synthesize norbornene-terminated urethane- and urea-based monomers. 1,4-Cyclohexanedimethanol (CYC, SigmaAldrich, 99%) was used as received to synthesize the vinyl-terminated hard segment. 1,1′-(Methylenedi-4,1-phenylene)bismaleimide (MI, Sigma-Aldrich, 95%) was used as received and employed as the hard segment. Solvents, including anhydrous tetrahydrofuran (THF, SigmaAldrich, ≥99.9%), N,N-dimethylformamide (DMF, Sigma-Aldrich, 99.8%), hexane (Sigma-Aldrich, 95%), chloroform (Fisher Scientific, 99.8%), and m-cresol (Sigma-Aldrich, 99%), were used as received. Photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA, SigmaAldrich, 99%) was used as received. The catalysts dibutyltin dilaurate (DBTDL, Sigma-Aldrich, 95%) and triethylamine (TEA, SigmaAldrich, ≥99.5%) were used as received. 2.2. Synthesis. 2.2.1. Synthesis of Norbornene- and VinylTerminated Hard Segments. The norbornene-terminated monomers were produced using a 1:1 ratio of isocyanate to alcohol or amine groups. The vinyl-terminated monomer was synthesized with a 10 mol % excess of allyl isocyanate to account for volatility of the isocyanate and potential side reactions with water. The reaction steps and conditions to synthesize the monomers employed in this study are depicted in Figure 1, reactions a through f. In a typical reaction to produce a norbornene-terminated monomer, TDI (1.17 g, 6.7 mmol) was dissolved in 13.4 mL of anhydrous THF with magnetic stirring. Post dissolution, 5-norbornene-2-methanol (1.62 mL, 1.67 g, 13.4 mmol) and DBTDL (15 μL, ∼0.1 wt %) were added to the reaction mix. The reaction proceeded at room temperature for 3 h. The solution was heated to 70 °C to remove excess THF, and after removal of the majority of the solvent, the monomer was dried at 70 °C under vacuum for 12 h. Complete disappearance within error of isocyanate functionality was confirmed using a Bruker Tensor 37 MiD IR FTIR spectrophotometer equipped with an attenuated total reflectance (ATR) diamond/ZnSe attachment by monitoring the isocyanate

2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene glycol-based dithiol, THIOCURE PPGMP 2200 with Meq = 1250 g/mol (Bruno Bock Thiochemicals), was used as received and employed as the soft segment. Low molecular weight dithiol, 1,2-ethanedithiol (EDT, TCI America, 99%), was used as received and employed as a chain extender. Tolylene 2,4C


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Macromolecules stretch at ∼2200 cm−1. All norbornene-terminated monomers were produced similarly, with some of the reactions occurring at a lower overall concentration in THF due to solubility issues. The vinylterminated monomer was produced similarly; 1,4-cyclohexanedimethanol (5 g, 34.7 mmol) was dissolved in 69.4 mL of anhydrous THF, and post dissolution, a 10 mol % excess of allyl isocyanate (6.34 g, 76.3 mmol) and DBTDL (∼0.1 wt %) were added to the reaction solution under magnetic stirring. The solution reacted at room temperature for 3 h. Complete disappearance within error of isocyanate functionality was confirmed by monitoring the isocyanate stretch at ∼2200 cm−1 by ATR-FTIR spectroscopy. The solution was heated to 70 °C to remove excess THF; after removal of the majority of the solvent, the monomer was dried at 70 °C under vacuum for 12 h. 2.2.2. Synthesis of Segmented Thermoplastic Thiol−Norbornene and Thiol−Vinyl Materials. In a typical one-pot synthesis of segmented thermoplastic thiol−norbornene materials (Figure 2, reaction i), TDIN (TDI-based urethane norbornene-functional hard segment, depicted in Figure 1, reaction a, 1.800 g, 4.261 mmol) was dissolved in 20 mL of DMF in a 100 mL sealed clear glass reaction flask. Equation 1 was used to calculate the thiol-functional chain extender and soft segment reactant amounts required to achieve a hard-segment content of 45%:71,72

hard segment wt % = 100% ×

materials with a hard-segment content of 55% were synthesized in a similar manner using the CYC-ene monomer (pictured in Figure 1, reaction f) as the hard segment. 2.2.3. Synthesis of Segmented Thermoplastic Thiol−Maleimide Materials. In a typical one-pot synthesis of segmented thermoplastic thiol−maleimide materials (Figure 2, reaction ii) with a hard-segment content of 50% (see eq 1), 0.940 g (9.997 mmol) of EDT, 4.300 g (12.000 mmol) of MI, and 5.000 g (1.999 mmol) of PPGMP 2200 were dissolved in certain amounts (∼20 mL) of m-cresol in a reaction vessel to adjust the overall concentration of the reaction mixture to 0.6 M. A stoichiometric balance between thiol groups and maleimide groups was maintained. The mixture was vigorously stirred for 10 min and exhibited a transparent amber color. 200 μL of TEA catalyst (0.7 wt %) was then added to the mixture dropwise. The mixture was maintained at room temperature with continuous stirring overnight. The viscosity of the mixture increased after reaction. The polymer was precipitated in a hexane solution (1 L) acidified with hydrochloric acid (2 mL). The precipitated polymer was redissolved in chloroform and precipitated again in hexane twice. The washed polymer was then vacuum-dried overnight at 100 °C. The as-dried material was pressed into 1 mm thick sheets at 140 °C at 5 psi for 5 min for sample testing. 2.3. Characterization. The resulting materials were characterized for molecular weight, thermal and mechanical properties, and phase separation. Apparent MWs of thiol−vinyl and thiol−norbornene materials were determined using gel permeation chromatography (GPC) at 30 °C using a high-pressure liquid chromatography pump (Waters 510) and a refractive index detector (Waters 2410); reported MWs are relative to polystyrene standards in THF. Apparent MWs of thiol−maleimide materials were not determined in this study. Thermal properties were determined by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The DSC characterization was done using a Mettler Toledo DSC 822e. After annealing the samples at 150 °C for 5 min, the materials were quenched to −80 °C, and heat flow as a function of temperature was collected on heating from −80 to 150 °C at a ramp rate of 10 °C/min. Glass transition temperature (Tg) values reported by DSC are the Tg at one-half ΔCp. The DMA measurements were done using a TA Instruments 2980 dynamic mechanical analyzer operating in straincontrolled mode (0.03% strain) at a frequency of 1 Hz. Temperaturedependent tensile storage modulus (E′), tensile loss modulus (E″), and damping ratio (tan δ = E″/E′) values were obtained during heating at 3 °C min−1 from −80 to 160 °C. The soft-segment Tg reported using this method is the peak in the loss modulus (E″). The hard-segment flow temperature (Tflow) was determined by the onset of inconsistent tan δ data (tan δ fluctuations ≥0.1) at high temperature. Tensile properties, including Young’s modulus, ultimate strength, and elongation at break, were determined via uniaxial tensile testing using a Sintech 20/g tensile tester equipped with a 100 N load cell and tensile testing grips. Dog-bone-shaped samples with a gauge length of 22 mm were cut using a Dewes-Gumbs die from hot-pressed sheets, and these samples were pulled under tension at 125 mm min−1. Seven samples were tested for each formulation; the reported errors for tensile properties are standard deviations. Phase separation in the materials was evaluated using small-angle Xray scattering (SAXS) employing as-synthesized materials and a Rigaku S-MAX 3000 SAXS system emitting X-ray with a wavelength of 0.154 nm (Cu Kα). The sample-to-detector distance was 1640 mm with silver behenate calibration. The 2D scattering patterns were azimuthally averaged to produce a 1-D plot of intensity vs scattering vector q, where q = 4π(sin θ)/λ, θ is 1/2 of the scattering angle, and λ is the X-ray wavelength.

RMHS + (R − 1)MCE RMHS + (R − 1)MCE + MSS (1)

where R is the molar ratio of hard segment (e.g., norborneneterminated monomer) to thiol-terminated soft segment, MHS is the molar mass of the hard segment (e.g., norbornene-terminated monomer), MCE is the molar mass of the chain extender, and MSS is the molar mass of the soft segment. In 20 mL of DMF, EDT (0.304 g, 3.233 mmol) was dissolved and then added to the 100 mL reaction flask. Additionally, in another 20 mL of DMF, the soft segment PPGMP 2200 (2.571 g, 1.028 mmol) was dissolved and added to the 100 mL reaction flask. The overall concentration of the reaction mixture was about 0.1 M. After all reactants were added, under magnetic stirring, 0.1 wt % DMPA (4.7 mg, 0.02 mmol) was added. The mixture was illuminated at room temperature with 365 nm UV light at 10 mW/cm2 for 30 min, followed by 254 nm UV light for 4 h to react any residual monomer. The polymer was precipitated in excess hexane, redissolved in THF, precipitated in excess hexane again, and dried at 70 °C under vacuum. The as-dried material was pressed into sheets at 120 °C at 5 psi for 5 min using either 0.5 or 1 mm spacers for sample testing. The as-dried materials were scanned using ATR-FTIR to examine the extent of urethane hydrogen bonding occurring in each system by observing shifts in the carbonyl absorbance at ∼1700 cm−1. Shifts to lower wavenumbers are an indication of strong hydrogen bonding.78 Other thermoplastic thiol−norbornene materials were synthesized in a similar manner (see Table 1 for detailed formulations of synthesized materials in this study). As a control, thiol−vinyl

Table 1. Formulations for Thermoplastic Thiol− Norbornene and Thiol−Maleimide Materials sample name

soft segment (SS)

TDIN-45% TDIN-55% MDIN-45% MDIN-55% PDIN-45% PDIN-55% TDIUN-50% MDIUN-50% MI-45% MI-50%

PPGMP PPGMP PPGMP PPGMP PPGMP PPGMP PPGMP PPGMP PPGMP PPGMP

2200 2200 2200 2200 2200 2200 2200 2200 2200 2200

chain extender

hard segment (HS)

HS (wt %)

EDT EDT EDT EDT EDT EDT EDT EDT EDT EDT

TDIN TDIN MDIN MDIN PDIN PDIN TDIUN MDIUN MI MI

45 55 45 55 45 55 50 50 45 50

3. RESULTS AND DISCUSSION 3.1. Segmented Thermoplastic Thiol−Vinyl Materials. The first synthesis attempt of a segmented thiol−ene material involved the reaction of a thiol-terminated soft segment (PPGMP 2200), the CYC-ene monomer (Figure 1, reaction f) as the hard segment, and EDT as the chain extender. A hardsegment content of 55% was chosen for this thermoplastic D


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Figure 3. Representative DMA curves, including storage modulus, loss modulus, and tan δ of urethane-based thiol−norbornene materials: (a) TDIN-45% and TDIN-55%; (b) MDIN-45% and MDIN-55%; (c) PDIN-45% and PDIN-55%. The table depicted in (d) shows temperature ranges and ΔT for each sample over which tan δ ≥ 0.3, associated with good vibration and acoustic damping properties.

were achieved (Mw = 15 000−60 000 g/mol; Table S1) that were considerably higher than those for the thiol−vinyl system (Mw = ∼ 6000 g/mol; Table S1) and comparable to those from typical urethane step-growth polymerizations over a range of hard-segment contents. This result is consistent with the notion that the inclusion of norbornene functionality increases the reactivity of the double bond and results in higher MW polymers with robust mechanical properties compared to common thiol−vinyl materials.82,83 For thiol−norbornene materials obtained from urethane-based norbornene monomers, the overall MW decreased only slightly with increasing hard-segment content (Table S1). Additionally, the MDINbased samples reached apparent average MWs ∼ 1.5 times those of TDIN- and PDIN-based samples, which is in agreement with monomer MW. In addition to MW, characterization focused on the effects of hard-segment structure (urethane- vs urea-based norbornene) and content on thermal and mechanical properties of thiol−norbornene materials. 3.2.1. Thermoplastic Thiol−Norbornene Materials from Urethane-Based Norbornene. Figure 3 depicts the DMA curves obtained for the thiol−-norbornene materials synthesized from the urethane-based norbornene-functional monomers listed in Table 1; corresponding detailed thermal property values obtained via DMA and DSC characterization are summarized in Table 2. As shown in Figure 3 and Table 2, increasing the hard-segment content increased both the softsegment Tg and Tflow value of the hard segment (determined by inconsistent tan δ data with fluctuations ≥0.1). The hardsegment Tg is not discernible by DSC, but the values of the soft-segment Tgs are in approximate agreement between the

thiol−vinyl material. The resulting polymer was soft and readily flowed at room temperature. The apparent weight-average molecular weight (Mw) was determined to be 6200 g/mol for the thiol−vinyl material. Because this polymer had a relatively low MW, it was inadequately phase separated at room temperature to exhibit a robust mechanical response sufficient for testing. In the recent research literature, it has been demonstrated that thiol−vinyl reactions are not efficient in conjugating block copolymers because of side reactions, diffusion limitations,79,80 and low degrees of homopolymerization;81 these aspects of the thiol−vinyl reaction might explain the low MW achieved. In addition, the flexibility of the thiol−vinyl linkage likely contributed to the poor mechanical performance. Inclusion of norbornene functionality has been shown to increase the reactivity of the double bond82,83 and elevate Tg,7 and such inclusion has been successfully used to conjugate multiblock copolymers.65 Consequently, the substitution of vinyl-terminated monomers with norbornene-terminated monomers should result in higher MW polymers with more robust mechanical properties. Here, norbornene-terminated monomers with urethane or urea linkages in their backbone were investigated. 3.2. Segmented Thermoplastic Thiol−Norbornene Materials. Table 1 lists the formulations of the synthesized segmented thiol−norbornene polymers with different hardsegment contents using urethane-based and urea-based norbornene monomers; an example synthesis and potential polymer structure are depicted in Figure 2, reaction i. Using thiol−norbornene UV-initiated polymerizations, apparent MWs E


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Macromolecules Table 2. Thermal Properties of Urethane- and Urea-Based Thiol−Norbornene Materials as Well as Thiol−Maleimide Materials Characterized Using DSC and DMA DSC characterization

Table 3. Tensile Properties of Urethane- and Urea-Based Thiol−Norbornene Materials as Well as Thiol−Maleimide Materials Determined Using Tensile Specimensa

DMA characterization

sample

SS Tg (° C)

HS Tg (° C)

SS Tg (° C)

HS Tflow (° C)

TDIN-45% TDIN-55% MDIN-45% MDIN-55% PDIN-45% PDIN-55% TDIUN-50% MDIUN-50% MI-45% MI-50%

−26

not obsd

−20

not obsd

−31

not obsd

−59 −60

88 92

−38 −26 −27 −21 −23 −16 −52 −52 −53 −52

35 63 44 69 59 69 143 152 121 150

sample TDIN-45% TDIN-55% MDIN-45% MDIN-55% PDIN-45% PDIN-55% TDIUN-50% MDIUN-50% MI-45% MI-50%

Young’s modulus (MPa) 57 ± 8 57 4 85 260 215 8 27

± ± ± ± ± ± ±

7 1 29 70 25 2 5

tensile strength (MPa) too soft to test 3.6 ± 0.2 too soft to test 4.4 ± 0.8 1.1 ± 0.3 8.6 ± 0.8 11.0 ± 0.3 10.3 ± 1.5 6.2 ± 0.5 10.6 ± 1.1

strain at break (%) 370 ± 60 240 790 300 26 30 205 175

± ± ± ± ± ± ±

70 80 40 25 20 15 40

a

The reported errors are standard deviations from seven measurements.

two measurement methods. The slight difference between softsegment Tg values reported by DMA and DSC is due to the nature of each testing method and is commonly observed with traditional urethane systems.84−87 As depicted in Figure 3, the E′ and tan δ profiles of urethanebased thiol−norbornene materials show characteristics of nanophase-separated systems having broad interphases with a wide range of local composition;72,88,89 e.g., E′ decreases very gradually with increasing T over a wide temperature range. Such profiles are analogous to those obtained with appropriately designed gradient copolymer systems which exhibit nanophase separation with broad interphase regions and broad ranges of glass transition response.89−92 Additionally, these materials exhibited tan δ ≥ 0.30 over a wide temperature range, which indicates potential for application as broadtemperature-range vibration or acoustic damping materials.72−75,89,93,94 (A broad-temperature-range damping polymer exhibits tan δ ≥ 0.30 over a continuous temperature range ΔT ≥ 50 °C.89,93,94) Figure 3d summarizes the range of temperatures with tan δ ≥ 0.30 and the total ΔT. As depicted in Figure 3d, regardless of hard-segment content studied, the synthesized urethane-based thiol−norbornene polymers have the potential to function as effective damping materials over a wide temperature range, analogous to appropriately designed gradient copolymers89−92 and segmented polyhydroxyurethanes.72−75 Additionally, the hard-segment content can be varied to tune the temperature range with effective damping. Tensile properties, including Young’s modulus, tensile strength, and strain at break, of the urethane-based thiol− norbornene materials are summarized in Table 3; corresponding example tensile curves are shown in Figure S1. Both the TDIN- and MDIN-based polymers at a hard-segment content of 45% were soft at room temperature and flowed over time scales of several hours; these materials were insufficiently robust for their mechanical properties to be tested. As shown in Table 2, the Tflow values for these materials were only 10−20 °C above the tensile testing temperature (∼25 °C). The symmetry of the PDIN-based monomer most likely allowed for greater packing efficiency95 which manifested itself in an increased hard-segment Tflow and measurable mechanical properties in the sample with 45% hard segment. All tested urethane-based thiol−norbornene materials exhibited an elastomeric-like tensile mechanical response with modest tensile strengths. There is an apparently large degree of tunability in mechanical properties associated with hard-segment content; in the PDINbased elastomers, increasing the hard segment from 45% to

55% increased the Young’s modulus by a factor of ∼21 and the tensile strength by a factor of ∼8 while the materials retained a strain at break of 300%. (The significant increase in elastic modulus going from 45% to 55% hard segment may be attributed to a combination of the higher level of hard segment and a greater degree of hydrogen bonding in the 55% sample.) The mechanical property values are similar to those observed in traditional TPUs67,77,87 and similar to or better than those reported for a range of thermoplastic polyhydroxyurethane elastomers.71−76 The materials exhibited some level of reversible extension (Figure S2), but it was imperfect. Further testing is warranted to quantify the hysteresis associated with reversible extension in segmented thiol−norbornene polymers and to offer comparisons with the hysteresis exhibited by TPUs96 and thermoplastic polyhydroxyurethane elastomers.75 Among the urethane-based thiol−norbornene materials at 55% hard-segment content, the PDIN-based polymers exhibited the most robust elastomeric-like response; this result is most likely due to the symmetry of the hard segment employed. Although MDIN is a symmetric molecule, the lower tensile strength of polymers made with MDIN may be due to decreased hydrogen bonding in the hard segment as a result of employing a very short chain extender. (In traditional segmented polyurethane, use of 1,4-butanediol as a chain extender results in much more efficiently packed/phaseseparated hard segment as compared to use of 1,3-propanediol or ethylene glycol.97) Additionally, MDIN-based polymers (with a methylene spacer between the phenyl groups) are expected to exhibit greater flexibility than PDIN-based polymers (which have a hard segment that is a more rigid pphenylene derivative). Segregation of hard segments was examined via ATR-FTIR by monitoring the urethane carbonyl absorbance at ∼1700 cm−1 (data not shown). Shifts to lower wavenumbers are indicative of large degrees of hydrogen bonding between urethane linkages.78,98 The PDIN-based samples showed the largest degree of hydrogen bonding in the hard segments, followed by the MDIN-based samples, with TDIN-based samples exhibiting the largest amount of nonbonded carbonyl groups. The decreased hydrogen bonding in the TDIN-based elastomers is attributed to the disruption in symmetry as a result of the methyl group off the aromatic ring95 and the fact that TDI exists as a mixture of 2,4- and 2,6-isomers. F


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Macromolecules The 55% hard-segment TDIN- and MDIN-based thiol− norbornene polymers were also characterized via SAXS.99 Figure 4 indicates the absence of any distinct peak in the SAXS

Figure 5. Representative DMA curves of urea-based thiol−norbornene materials, including storage modulus, loss modulus, and tan δ: (a) TDIUN-50% and (b) MDIUN-50%. Figure 4. SAXS patterns for selected urethane-based thiol− norbornene materials (TDIN-55% and MDIN-55%), selected ureabased thiol−norbornene materials (TDIUN-50% and MDIUN-50%), and thiol−maleimide materials (MI-45% and MI-50%).

Tflow values, consistent with more effective phase separation and stronger hydrogen bonding. This phenomenon is common in traditional urea-based thermoplastic elastomers; urea-based hard segments undergo bidentate hydrogen bonding100,101 and tend to exhibit a greater degree of phase separation, higher hard-segment Tgs and increased elastomeric mechanical performance as compared to urethane-based analogues.77,97,102 Given the important role of hydrogen bonding in these systems, future work is warranted to study the effect of humidity on the properties and morphologies of these materials. Tensile properties, including Young’s modulus, tensile strength, and strain at break, of the urea-based thiol− norbornene materials are summarized in Table 3. At the 50% hard-segment content investigated, the urea-based materials were relatively brittle, with ∼30% strain at break and a high Young’s modulus (∼200−300 MPa), and did not behave elastomerically regardless of hard segment selection (TDIUN vs MDIUN). This behavior may result from the soft segments being dispersed in the hard-segment matrix. Future study is warranted to consider systems with hard-segment content compositions below 50%, where the soft segment would form the matrix, potentially resulting in elastomeric response. 3.3. Segmented Thermoplastic Thiol−Maleimide Materials. The previous section discussed segmented thiol− norbornene thermoplastics with relatively good thermal and mechanical properties that were synthesized from rigid norbornene monomers containing urethane or urea linkages. However, the synthesis of such norbornene-terminated monomers involves the use of isocyanate molecules, which can cause adverse health effects (e.g., isocyanates are irritating to the skin and eyes and are a respiratory hazard where repeated exposure can cause sensitization and occupational asthma103,104). Because of such adverse effects, there has been increased regulatory scrutiny concerning the safe use and transport of isocyanates.105−107 In this section, we discuss the use of another commercially available rigid ene, maleimide, for synthesizing segmented thermoplastic materials via nucleophilic thiol−maleimide click reactions1,14,16,17,60 (depicted in Figure 2, reaction ii). Because this synthesis does not involve the use of isocyanates, it may offer materials that are relatively more environmentally benign with improved sustainability from a human health perspective.108

intensity as a function of q for these samples. In combination with the DMA and tensile properties and FTIR characterization discussed above, the SAXS results are consistent with very broad interphases accompanying phase separation which make scattering peaks indiscernible. 3.2.2. Thermoplastic Thiol−Norbornene Materials from Urea-Based Norbornene. In addition to the urethane-based thermoplastic thiol−norbornene materials, other formulations (Table 1) were synthesized employing the urea-based norbornene hard segments (depicted in Figure 1, reactions d and e). The 50% hard-segment urea-based thiol−norbornene materials synthesized in this study underwent nanophase separation, as confirmed by the presence of a peak in the intensity vs q spacing from SAXS measurements in Figure 4. The interdomain spacing, d, was calculated using d = 2π/qmax99 (where qmax is the magnitude of the scattering vector at maximum intensity) and determined to be ∼12−13 nm in these urea-based thiol−norbornene materials. Future work is warranted to characterize the nanophase-separated morphology via atomic force microscopy. (We note that the MWs obtained by GPC for these polymers are decreased as compared to the urethane-based analogues. The apparent decrease may be an artifact of the GPC analysis method; the more polar urea-based segments may interact with the column, resulting in longer elution times and reduced apparent MW values. In the future, obtaining absolute MW data would be beneficial in order to make more exact comparisons.) Figure 5 depicts DMA profiles for urea-based thiol− norbornene materials, TDIUN-50% and MDIUN-50%; corresponding thermal property values obtained from DMA are summarized in Table 2. As shown in Figure 5, the urea-based thiol−norbornene materials do not possess properties desirable for vibrational and acoustic damping applications. In fact, the step change in E′ from glassy to rubbery response and sharp tan δ peaks are consequences of relatively effective nanophase separation. Because of the strong hydrogen bonding100,101 between hard-segment urea units, the nanophase separation produces sharp interphases between the hard and soft domains. When compared to the urethane-based elastomers, the ureabased elastomers exhibit lower soft-segment Tgs and higher G


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same hard-segment contents the thiol−maleimide thermoplastics synthesized in this study exhibited superior tensile properties relative to all three thiol−norbornene thermoplastics made with urethane-based norbornene (45% hard-segment content) and tensile strength equal to those of the two thiol− norbornene thermoplastics made with urea-based norbornene (50% hard-segment content). Thus, the thiol−maleimide reaction can be used to prepare effectively nanophase-separated polymers with thermal and mechanical properties indicating good potential for application as thermoplastic elastomers. Finally, because these segmented thiol−maleimide polymers were synthesized in the absence of isocyanate, they may be potential competition for segmented non-isocyanate polyurethane (NIPU) materials,71−76,109−111 e.g., polyhydroxyurethane, with applications targeting those of TPUs. Unlike reactions leading to the synthesis of polyhydroxyurethane, which are inherently slow at ambient conditions,109−112 especially in comparison with isocyanate-based reactions leading to traditional polyurethane, the thiol−maleimide click reaction is inherently rapid.113 Future studies are warranted to provide comparisons of the relative reactivity and tunability of the rate of thiol−maleimide reactions as well as to assess more fully the circumstances under which polymers synthesized via thiol−maleimide reactions may exhibit properties competitive with traditional isocyanate-based polyurethanes and/or NIPUs.

Similar to the urea-based thiol−norbornene materials, segmented thiol−maleimide materials underwent nanophase separation as confirmed by the existence of a peak in the intensity vs q spacing from SAXS measurements depicted in Figure 4. The interdomain spacings were 11.5 and 12.5 nm in MI-45% and MI-50%, consistent with the interdomain spacing increasing with increasing hard-segment content. This result is in accord with previous observations regarding the correlation between interdomain spacing and hard-segment content in polyhydroxyurethane.72 Future work is warranted to characterize the nanophase-separated morphology via atomic force microscopy. Figure 6 depicts DMA profiles for the thiol−maleimide thermoplastic materials, MI-45% and MI-50%; corresponding

4. CONCLUSIONS Segmented thermoplastic thiol−ene materials were synthesized via a soft segment + hard segment + chain extender approach that is analogous to the synthesis of traditional TPUs. A relatively long chain, difunctional thiol was used as the soft segment and a small-molecule thiol as the chain extender. Rigid cyclic-ene monomers, including norbornene and maleimide, were employed as the hard segments to achieve thermoplastic thiol−norbornene and thiol−maleimide materials, respectively. With regard to thiol−norbornene materials, norbornenefunctional monomers were designed containing urethane or urea linkages in their backbone which aided in driving nanophase separation via hydrogen-bonding interactions. The majority of the segmented thiol−norbornene materials synthesized with 45% and 55% urethane-based norbornene hard segments exhibited nanophase separation with broad interfaces and potential for application as thermoplastic elastomers and broad-temperature-range acoustic and vibration damping materials. Because of stronger inter-urea hydrogen bonding relative to inter-urethane hydrogen bonding, the thiol−norbornene materials synthesized with 50% urea-based norbornene hard segments exhibited highly effective nanophase separation but relatively high Young’s modulus and low strain at break, likely associated with the hard segment forming the matrix. The segmented thiol−maleimide thermoplastic materials synthesized with 45% and 50% hard-segment content exhibited very good nanophase separation with narrow interfaces and thermal and mechanical properties associated with thermoplastic elastomers. Because thiol−maleimide polymers were synthesized without isocyanates via click reactions that are inherently relatively rapid in comparison with reactions used in synthesizing polyhydroxyurethanes, polymers synthesized by thiol−maleimide reactions may have an advantage over some NIPU materials as potential nonisocyanate-based substitutes for traditional isocyanate-based polyurethanes. Overall, we have demonstrated for the first time that thiol−ene click reactions can be employed to synthesize

Figure 6. Representative DMA curves, including storage modulus, loss modulus, and tan δ of thiol−maleimide materials: MI-45% and MI50%.

thermal property values obtained from both DMA and DSC are summarized in Table 2. DSC analysis indicated that two Tgs were present in the segmented thiol−maleimide thermoplastic materials, consistent with the presence of nanophase separation. The hard maleimide phase Tg was ∼90 °C for these nanophaseseparated thiol−maleimide thermoplastics, much higher than Tg values for common thiol−ene materials.2,5,23 Both the hardsegment Tg from DSC and Tflow value from DMA increased with increasing hard-segment content; e.g., Tflow increased from 121 to 150 °C when the hard-segment content increased from 45% to 50%. Compared to the urea-based materials with the same hard-segment contents, the segmented thiol−maleimide thermoplastic (MI-50%) exhibited similar soft-segment Tg and Tflow temperatures, indicative of equally effective nanophase separation in these two systems. Consistent with the presence of highly effective nanophase separation, the DMA profiles in Figure 6 indicated that segmented thiol−maleimide thermoplastics do not possess the properties needed for broadtemperature-range vibrational and acoustic damping applications. Tensile properties, including Young’s modulus, tensile strength, and strain at break, of the segmented thiol−maleimide materials are summarized in Table 3. These thiol−maleimide thermoplastics exhibited relatively good elastomeric character; e.g., a strain at break of ∼175% and a tensile strength of ∼11 MPa were achieved with the MI-50% sample. A comparison between MI-45% and MI-50% samples indicates that the tensile properties of thiol−maleimide polymers can be easily tuned via hard segment content. Also noteworthy is the fact that at the H


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segmented thermoplastic polymers with properties appropriate for applications ranging from broad-temperature range damping materials to thermoplastic elastomers.

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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.8b00573. Representative tensile curves; reversibility of extension of thiol−norbornene materials; detailed molecular weight values obtained by GPC analysis; domain spacing values determined from SAXS results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.M.T.). ORCID

Kailong Jin: 0000-0001-5428-3227 John M. Torkelson: 0000-0002-4875-4827 Author Contributions

K.J. and E.K.L. are co-first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the University Partnership Initiative between Northwestern University and The Dow Chemical Company and by discretionary funds from a Walter P. Murphy Professorship (J.M.T.). This work made use of central facilities supported by the MRSEC program of the National Science Foundation (DMR-1121262 and DMR1720139) at the Northwestern University Materials Research Science and Engineering Center as well as facilities supported by Northwestern University at the Integrated Molecular Structure Education and Research Center. We also gratefully acknowledge support in the form of a SMART Fellowship (E.K.L), a Terminal Year Fellowship (K.J.), and an ISEN Fellowship (X.C.).

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REFERENCES

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