Letter Cite This: ACS Macro Lett. 2018, 7, 482−486
pubs.acs.org/macroletters
Vitrimeric Silicone Elastomers Enabled by Dynamic Meldrum’s AcidDerived Cross-Links Jacob S. A. Ishibashi and Julia A. Kalow* Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: Current vitrimer technology uses only a handful of distinct reactions for cross-linking. New dynamic reactions can diversify vitrimer functionality and properties. In this paper, reversible cross-links formed by conjugate addition−elimination of thiols with a Meldrum’s acid derivative enable compression− remolding of silicone elastomers. After 10 remolding cycles, there is no discernible deterioration of mechanical properties (Young’s modulus, Tg, rubbery plateau E’), nor is there a change in stress relaxation activation energy. This robust new cross-linker could be implemented in any number of systems that currently use permanent thiol−ene cross-linking, expanding the scope of recyclable materials.
V
Scheme 1. Reversible Addition of Alkyl Thiols Enables Vitrimer Synthesis
itrimers are an emerging class of materials that combine the mechanical advantages of cross-linked polymers with the recyclability of thermoplastics.1 Traditional elastomers and thermosets have permanent cross-links that prevent recycling or remolding,2 but vitrimers address this problem by introducing reversible, covalent cross-links. Under standard operating conditions, the cross-links are static, but exchange by an associative mechanism upon heating renders vitrimers recyclable. Since Leibler’s initial disclosure of vitrimeric materials,3 researchers have used a limited number of dynamic chemistries including transesterification and similar carbonylcentered additions,4 olefin metathesis,5 dioxaborolane exchange,6 and transalkylation7 or transalkoxylation8 of main group elements. However, many existing vitrimer exchanges rely upon catalysts to increase the rate of exchange, and these catalysts can leach or deactivate over multiple recycling cycles. Uncatalyzed reactions4a,6,7,8 avoid these issues and simplify network composition. We present here the application of an uncatalyzed reversible conjugate addition−elimination reaction to a Meldrum’s acid alkylidene, enabling dynamic covalent cross-linking in silicone elastomers. We were inspired by Anslyn and co-workers, who used a reversible “click” addition-elimination of thiols into a Meldrum’s acid alkylidene derivative for applications such as room-temperature peptide conjugation (Scheme 1).9 This conjugate addition of thiols, which does not require a catalyst, is mechanistically distinct from other reversible thiol−ene or thiol−Michael reactions traditionally used in polymer crosslinking.10 This cross-linker would have the added benefit of avoiding any strongly acidic or basic moieties, making it attractive for implementation in silicone elastomers. The presence of acids or bases can induce Si−O exchange11 and can actually effect stress relaxation and self-healing.12 In this sense, acid- or base-induced Si−O exchange of siloxane networks may contribute to the © XXXX American Chemical Society
disruption of polymer chains and may lead to eventual loss of network integrity.13 Silicone elastomers offer the possibility of low-temperature operation. Most vitrimeric elastomers are hydrocarbon based, with relatively high glass transition temperatures (Tg), and thus do not function well as elastomers at lower temperatures. Importantly, silicones are generally not recycled except to recover siloxane oligomers, producing downgraded material.14 There are two recent reports of silicone vitrimers:15 Leibler, Nicolaÿ, and co-workers demonstrated the efficacy of the vinylogous urethane exchange reaction in silicones,16 while Xu, Received: February 26, 2018 Accepted: March 27, 2018
482
DOI: 10.1021/acsmacrolett.8b00166 ACS Macro Lett. 2018, 7, 482−486
Letter
ACS Macro Letters Zhao, and co-workers used zinc(II)-catalyzed transesterification reactions to induce vitrimer-like behavior in silicones.17 One potential complication of our approach is the low thermal stability of Meldrum’s acid and many of its derivatives.18 Hawker and co-workers used Meldrum’s acid derivatives to generate reactive ketenes for cross-linking.19 If in situ formed ketenes result from the decomposition of our proposed cross-linker, the resulting networks would be permanently cross-linked. To test the thermal stability of 1, we heated it to 110 °C in C6D6 in a J-Young tube sealed under ambient conditions and observed little change by NMR integration after 16 h of heating (5% change, using 1,4cyclooctadiene as an internal standard). We observed no new products (see Supporting Information (SI)). Thermogravimetric analysis/mass spectrometry (TGA/MS) of 1 under a helium atmosphere shows that 5% mass loss occurs at 178 °C; this mass loss is accompanied by the evolution of volatile compounds with masses corresponding to carbon dioxide and acetone. TGA under an atmosphere of air gives similar results, with 5% mass loss occurring at 172 °C (see SI). These results are consistent with the fact that Hawker and co-workers’ crosslinking requires temperatures over 180 °C to generate ketenes.19 Having established an upper temperature limit, we proceeded to synthesize silicone networks. We used approximately 2 mol % of 1 (relative to siloxane repeat units) to cross-link commercially available polydimethylsiloxane (PDMS) grafted with 13−17% propylthiol functionality.20 Heating the mixture with xylenes (for greater homogeneity) to 120 °C for 18 h produced siloxane elastomer E1 (Scheme 1). TGA of E1 shows that 5% weight loss occurs at 278 °C (see SI). Elastomer E1 swells in hot toluene (70 °C, 24 h), but it does not completely dissolve (Figure 1). Addition of a β-mercaptoethanol (2) to the swollen network causes complete dissolution by permanently disrupting network cross-links (Figure 2a).21 Taken together, these observations suggest that cross-linking is dynamic and that the mechanism of cross-link exchange is likely associative. If the cross-links are indeed dynamic at elevated temperature, we reasoned E1 may be reprocessable by compression remolding. We placed cut and torn pieces of the rubber in a hot press (150 °C, 9 tons force, 15 min) and obtained a monolithic mass. Infrared spectra of virgin and recycled material are nearly identical (see SI). We performed shear rheology (10% strain) on recycled E1 to quantify stress relaxation and investigate differences that might result from reprocessing.22 E1 undergoes stress relaxation to 1/ e (37%) of the initial modulus on the order of hundreds of seconds at elevated temperatures (120−150 °C) even after recycling 10 times (Figure 2). We observed no significant stress relaxation at 25 °C. A control siloxane network E223 featuring more permanent thiol−ene derived cross-links relaxes to about 80% of the initial modulus after 5000 s at 150 °C. The Arrhenius activation energy for stress relaxation changes little after remolding; after recycling once, the activation energy was 63 ± 6 kJ/mol, while after recycling 10 times, the activation energy was the same (64 ± 3 kJ/mol, Table 1). These values are similar to the activation energy we determined for a smallmolecule model (89 ± 5 kJ/mol) under pseudo-first-order conditions (30−60 °C, see SI).24 Additionally, networks synthesized from PDMS with lower thiol content exhibit longer relaxation times, showing that vitrimer relaxation kinetics depend not only on the identity of the cross-linker but also on the concentration of free thiol in
Figure 1. Qualitative characterization of a siloxane network crosslinked with 1 (a yellow solid). (a) Swelling in toluene vs dissolution with 1-mercapto-2-propanol 2. (b) Compression remolding is easily accomplished in a hot press. (c) Reprocessed samples are clear and do not distinguishably change in hue or transparency after multiple reprocessing cycles.
the network (see SI). This is consistent with observations by Dichtel and co-workers showing that the stoichiometry of reactive partners affects the rates of relaxation and further affirms that the mechanism of exchange is likely associative.4c Tensile testing of the recycled rubbers shows very little change in Young’s modulus after 10 reprocessing cycles (Figure 3a, Table 1). To quantify the low-temperature performance, we subjected E1 to dynamic mechanical thermal analysis (DMTA). The rubbery plateau has storage modulus E′ ∼ 380 kPa, and the glass transition temperature (Tg, peak of tan(δ)), occurs at −106 °C and is accompanied by a large increase in the storage modulus.25 These properties were largely unchanged through ten reprocessing cycles and show that E1 retains rubbery character to very low temperature. The retention of these properties suggests that the microscopic structure of the network remains unchanged. In support of this hypothesis, we performed gel permeation chromatography (GPC) experiments (see SI). If base-catalyzed Si−O bond cleavage were occurring, the molecular weight distribution should change significantly.26 Comparing the GPC traces of the commercial PDMS starting material and the material obtained when we dissolved thricerecycled E1 with 2, we did not observe a shift of the elution time corresponding to lower-molecular-weight polymer. This suggests that Si−O bond exchange is not the major pathway for reprocessing. Using the Arrhenius plots obtained from stress relaxation experiments and E′ of the rubbery plateau, we determined the topology freezing temperature Tv for E1 to be −6 °C.27 This presents an unusual situation where ambient conditions are above both Tg and Tv, suggesting samples may creep 483
DOI: 10.1021/acsmacrolett.8b00166 ACS Macro Lett. 2018, 7, 482−486
Letter
ACS Macro Letters
Figure 3. Representative plots and table of physical characterization for recycled silicone elastomer E1: (a) Stress−strain curves (draw rate: 2 mm/min). (b) DMTA (solid lines = E′, dotted lines = tan(δ); cooling at 3 °C/min).
Figure 2. Stress relaxation to G/G0 = 1/e (∼37%, dotted line). (a) Representative stress relaxation curves for E1 and E2. (b) Representative Arrhenius plot.
properties; in this case, the cross-links may be stabilized kinetically because the space between reactive partners remains far enough that a reaction will not occur under ambient operating conditions. In conclusion, we have shown that a Meldrum’s acid derivative enables dynamic bond exchange in silicone networks and results in vitrimers. These elastomers can be recycled up to 10 times without erosion of their mechanical properties. The robust exchange reaction between thiols and the Meldrum’s acid derivative points to the possibility of translation to nearly any system based on thiol−ene cross-link chemistry. We are currently exploring avenues to improve the mechanical properties of polymer networks using this type of cross-linker.
Table 1. Progressive Physical Characterization of Elastomer E1 with Number of Reprocessing Cyclesa times recycled 1 2 3 10
EAb (kJ/mol) 63 63 65 64
± ± ± ±
6 3 4 3
Young’s modulusc (kPa) 443 529 502 527
± ± ± ±
3 42 14 5
strain at breakc (%) 16 11 17 15
± ± ± ±
3 3 4 6
rubbery plateau E′d (kPa) 350 333 410 413
± ± ± ±
14 15 14 8
a
All data are the average of three runs, and the error is taken to be the standard deviation. bObtained from τ* = A + exp(EA/(RT)) where τ* is the time at which G/G0 = 1/e (∼0.37). cTensile tests were performed with specimens of approximate dimension 20 × 5 × 2 mm. d DMTA data were collected using specimens of approximate dimension 20 × 5 × 1 mm while cooling from −60 to −130 °C.
■
ASSOCIATED CONTENT
S Supporting Information *
significantly at room temperature. We have previously discussed that no significant shear stress relaxation occurs at 25 °C. We hung a 56 g weight (a constant 37 kPa stress) on a 600 × 15 × 1 mm rectangle of E1 for 1 h to evaluate creep in this extended period (see SI for photos.). The constant stress produced an immediate 4 mm extension (6% strain), which recovered in total after removing the weight. Qualitatively, the lack of creep suggests that a simple view of characteristic temperatures such as Tv and Tg may be insufficient to predict
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00166. Experimental details and additional data (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 484
DOI: 10.1021/acsmacrolett.8b00166 ACS Macro Lett. 2018, 7, 482−486
Letter
ACS Macro Letters ORCID
Recycling of Highly Cross-Linked Ion-Conducting Networks through Transalkylation Exchanges of C-N Bonds. J. Am. Chem. Soc. 2015, 137, 6078−6083. (b) Hendriks, B.; Waelkens, J.; Winne, J. M.; Du Prez, F. E. Poly(thioether) Vitrimers via Transalkylation of Trialkylsulfonium Salts. ACS Macro Lett. 2017, 6, 930−934. (8) Nishimura, Y.; Chung, J.; Muradyan, H.; Guan, Z. Silyl Ether as a Robust and Thermally Stable Dynamic Covalent Motif for Malleable Polymer Design. J. Am. Chem. Soc. 2017, 139, 14881−14884. (9) (a) Diehl, K. L.; Kolesnichenko, I. V.; Robotham, S. A.; Bachman, J. L.; Zhong, Y.; Brodbelt, J. S.; Anslyn, E. V. Click and Chemically Triggered Declick Reactions Through Reversible Amine and Thiol Coupling via a Conjugate Acceptor. Nat. Chem. 2016, 8, 968−973. (b) Johnson, A. M.; Anslyn, E. V. Reversible Macrocyclization of Peptides with a Conjugate Acceptor. Org. Lett. 2017, 19, 1654−1657. (c) Sun, X.; Anslyn, E. V. An Auto-Inductive Cascade for the Optical Sensing of Thiols in Aqueous Media: Application in the Detection of a VX Nerve Agent Mimic. Angew. Chem., Int. Ed. 2017, 56, 9522−9526. (10) Recent examples: (a) Chakma, P.; Rodrigues Possarle, L. H.; Digby, Z. A.; Zhang, B.; Sparks, J. L.; Konkolewicz, D. Dual Stimuli Responsive Self-Healing and Malleable Materials Based on Dynamic Thiol-Michael Chemistry. Polym. Chem. 2017, 8, 6534−6543. (b) Zhang, B.; Digby, Z. A.; Flum, J. A.; Chakma, P.; Saul, J. M.; Sparks, J. L.; Konkolewicz, D. Dynamic Thiol-Michael Chemistry for Thermoresponsive Rehealable and Malleable Networks. Macromolecules 2016, 49, 6871−6878. (c) Lee, T. Y.; Carioscia, J.; Smith, Z.; Bowman, C. N. Thiol-Allyl Ether-Methacrylate Ternary Systems. Evolution Mechanism of Polymerization-Induced Shrinkage Stress and Mechanical Properties. Macromolecules 2007, 40, 1473−1479. (11) Kantor, S. W.; Grubb, W. T.; Osthoff, R. C. The Mechanism of the Acid- and Base-Catalyzed Equilibration of Siloxanes. J. Am. Chem. Soc. 1954, 76, 5190−5197. (12) (a) Osthoff, R. C.; Bueche, A. M.; Grubb, W. T. Chemical Stress-Relaxation of Polydimethylsiloxane Elastomers. J. Am. Chem. Soc. 1954, 76, 4659−4663. (b) Zheng, P.; McCarthy, T. J. A surprise from 1954: Siloxane Equilibration is a Simple, Robust, and Obvious Polymer Self-Healing Mechanism. J. Am. Chem. Soc. 2012, 134, 2024− 2027. (13) Hsiao, Y.-C.; Hill, L. W.; Pappas, S. P. Reversible Amine Solubilization of Cured Siloxane Polymers. J. Appl. Polym. Sci. 1975, 19, 2817−2820. (14) Colas, A. Silicones: Preparation, Properties, and Performance; White Paper for Dow Corning, 2005. (15) There are a number of silicones with dissociative dynamic crosslinks: (a) Fawcett, A. S.; Hughes, T. C.; Zepeda-Velazquez, L.; Brook, M. A. Phototunable Cross-Linked Polysiloxanes. Macromolecules 2015, 48, 6499−6507. (b) Fawcett, A. S.; Brook, M. A. Thermoplastic Silicone Elastomers Through Self-Association of Pendant Coumarin Groups. Macromolecules 2014, 47, 1656−1663. (16) Stukenbroeker, T.; Wang, W.; Winne, J. M.; Du Prez, F. E.; Nicolaÿ, R.; Leibler, L. Polydimethylsiloxane Quenchable Vitrimers. Polym. Chem. 2017, 8, 6590−6593. (17) Zhang, H.; Cai, C.; Liu, W.; Li, D.; Zhang, J.; Zhao, N.; Xu, J. Recyclable Polydimethylsiloxane Network Crosslinked by Dynamic Transesterification Reaction. Sci. Rep. 2017, 7, 11833. (18) Heat can produce ketenes: (a) Dumas, A. M.; Fillion, E. Acc. Chem. Res. 2009, 43, 440−454. Flash-vacuum thermolysis gives some interesting products: (b) Ben Cheikh, A.; Chuche, J.; Manisse, N.; Pommelet, J. C.; Netsch, K.-P.; Lorencak, P.; Wentrup, C. Synthesis of α-Cyano Carbonyl Compounds by Flash Vacuum Thermolysis of (Alkylamino)methylene Derivatives of Meldrum’s Acid. Evidence for Facile 1,3-Shifts of Alkylamino and Alkylthio Groups in Imidoylketene Intermediates. J. Org. Chem. 1991, 56, 970−975. (19) (a) Leibfarth, F. A.; Kang, M.; Ham, M.; Kim, J.; Campos, L. M.; Gupta, N.; Moon, B.; Hawker, C. J. A Facile Route to KeteneFunctionalized Polymers for General Materials Applications. Nat. Chem. 2010, 2, 207−212. (b) Leibfarth, F. A.; Wolffs, M.; Campos, L. M.; Delany, K.; Treat, N.; Kade, M. J.; Moon, B.; Hawker, C. J. LowTemperature Ketene Formation in Materials Chemistry Through Molecular Engineering. Chem. Sci. 2012, 3, 766−771.
Jacob S. A. Ishibashi: 0000-0003-2045-4387 Julia A. Kalow: 0000-0002-4449-9566 Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge the Center for Sustainable Polymers (NSF Project No. CHE-1413862) for funding. The authors additionally thank Mr. Guilhem De Hoe, Prof. Marc Hillmyer (University of Minnesota), Mr. David Fortman, and Profs. Will Dichtel and John Torkelson (Northwestern) for helpful discussions and access to instrumentation. This work made use of the Integrated Molecular Structure Education and Research Center at Northwestern, which has received support from the NIH (S10-OD021786-01); the NSF (NSF CHE9871268); Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); and the State of Illinois and International Institute for Nanotechnology. Rheological measurements were performed at the Materials Characterization and Imaging Facility which receives support from the MRSEC Program (NSF DMR-1720139) of the Materials Research Center at Northwestern University.
■
REFERENCES
(1) An excellent review: Denissen, W.; Winne, J. M.; Du Prez, F. E. Vitrimers: Permanent Organic Networks with Glass-Like Fluidity. Chem. Sci. 2015, 7, 30−38. (2) Hopewell, J.; Dvorak, R.; Kosior, E. Plastics Recycling: Challenges and Opportunities. Philos. Trans. R. Soc., B 2009, 364, 2115−2126. (3) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Silica-Like Malleable Materials from Permanent Organic Networks. Science 2011, 334, 965−968. (4) Recent examples include: (a) Fortman, D. J.; Brutman, J. P.; Cramer, C. J.; Hillmyer, M. A.; Dichtel, W. R. Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers. J. Am. Chem. Soc. 2015, 137, 14019−14022. (b) Fortman, D. J.; Brutman, J. P.; Hillmyer, M. A.; Dichtel, W. R. Structural Effects on the Reprocessability and Stress Relaxation of Cross-Linked Polyhydroxyurethanes. J. Appl. Polym. Sci. 2017, 134, 44984. (c) Snyder, R. L.; Fortman, D. J.; De Hoe, G. X.; Hillmyer, M. A.; Dichtel, W. R. Reprocessable Acid-Degradable Polycarbonate Vitrimers. Macromolecules 2018, 51, 389−397. (d) Denissen, W.; Rivero, G.; Nicolaÿ, R.; Leibler, L.; Winne, J. M.; Du Prez, F. E. Vinylogous Urethane Vitrimers. Adv. Funct. Mater. 2015, 25, 2451−2457. (e) Denissen, W.; Droesbeke, M.; Nicolaÿ, R.; Leibler, L.; Winne, J. M.; Du Prez, F. E. Chemical Control of the Viscoelastic Properties of Vinylogous Urethane Vitrimers. Nat. Commun. 2017, 8, 14857. (f) Chen, X.; Li, L.; Jin, K.; Torkelson, J. M. Reprocessable Polyhydroxyurethane Network Exhibiting Full Property Recovery and Concurrent Associative and Dissociative Dynamic Chemistry via Transcarbamoylation and Reversible Cyclic Carbonate Aminolysis. Polym. Chem. 2017, 8, 6349−6355. (5) Lu, Y.-X.; Guan, Z. Olefin Metathesis for Effective Polymer Healing via Dynamic Exchange of Strong Carbon-Carbon Double Bonds. J. Am. Chem. Soc. 2012, 134, 14226−14231. (6) Röttger, M.; Domenech, T.; van der Weegen, R.; Breuillac, A.; Nicolaÿ, R.; Leibler, L. High-Performance Vitrimers from Commodity Thermoplastics Through Dioxaborolane Metathesis. Science 2017, 356, 62−65. (7) Notable examples: (a) Obadia, M. M.; Mudraboyina, B. P.; Serghei, A.; Montarnal, D.; Drockenmuller, E. Reprocessing and 485
DOI: 10.1021/acsmacrolett.8b00166 ACS Macro Lett. 2018, 7, 482−486
Letter
ACS Macro Letters (20) Using 5 mol % of 1 results in a brittle network at room temperature. (21) Ansyln demonstrates the following reaction in ref 9a, using it to deconjugate peptides:
(22) Virgin rubbers were not investigated because of the difficulty in controlling thickness. (23) Preparation adapted from (a) Campos, L. M.; Killops, K. L.; Sakai, R.; Paulusse, J. M. J.; Damiron, D.; Drockenmuller, E.; Messmore, B. W.; Hawker, C. J. Development of Thermal and Photochemical Strategies for Thiol-Ene Click Polymer Functionalization. Macromolecules 2008, 41, 7063−7070. (b) Campos, L. M.; Truong, T. T.; Shim, D. E.; Dimitriou, M. D.; Shir, D.; Meinel, I.; Gerbec, J. A.; Hahn, H. T.; Rogers, J. A.; Hawker, C. J. Applications of Photocurable PMMS Thiol-Ene Stamps in Soft Lithography. Chem. Mater. 2009, 21, 5319−5326. (24) Arrhenius EA may be significantly lower than those derived from small-molecule models. See Dichtel and co-workers, ref 4a. (25) The low moduli and strain at break may be due to our use of low molecular weight silicones (3−4 kDa). The molecular weight of entanglement for PDMS ranges from 21 to 33 kDa. See: Kuo, A. C. M. Poly(dimethylsiloxane). In Polymer Data Handbook; Mark, J. E., Ed.; Oxford University Press: Oxford, 1999; p 421. Silicone networks synthesized with 2.5 and 3 mol % cross-linker 1 show progressively increased storage modulus E′, a typical characteristic of elastomers (see SI). High-performance silicone elastomers typically require fillers: Paul, D. R.; Mark, J. E. Fillers for Polysiloxane (“Silicone”) Elastomers. Prog. Polym. Sci. 2010, 35, 893−901. (26) Siloxane depolymerization and redistribution can proceed through random back-biting or interchain mechanisms. An overview of silicone equilibration: (a) Brook, M. A. Silicon in Organic, Organometallic, and Polymer Chemistry; John Wiley & Sons: New York, 2000; Chapter 9.2. Base-catalyzed degradation of silicone rubbers produces uneven roughness, implying oligomers cleaved from the surface are not of uniform size. (b) Brook, M. A.; Liu, L.; Zhao, S.; Mammo, Z. N. Etching of Silicone Elastomers: Controlled Manipulation of Surface Roughness. ACS Symp. Ser. 2010, 1051, 147−155. Oligomerization of Dx siloxanes (the microscopic reverse of silicone depolymerization) produces a distribution of larger oligomers and polymers (c) Chojnowski, J.; Kaźmierski, K.; Rubinsztajn, S.; Stańczyk, W. Transformation of Oligodimethylsiloxanols in the Presence of a Strong Base. Reactivity Enhancement of the Siloxane Bond by the Adjacent Hydroxyl Group. Makromol. Chem. 1986, 187, 2039−2052 See also Kantor et al. Ref 11.. (27) Brutman, J. P.; Delgado, P. A.; Hillmyer, M. A. Sustainable Thermoplastic Elastomers from Terpene-Derived Monomers. ACS Macro Lett. 2014, 3, 607−610.
486
DOI: 10.1021/acsmacrolett.8b00166 ACS Macro Lett. 2018, 7, 482−486
