Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based

Letter Cite This: ACS Macro Lett. 2018, 7, 1226−1231

pubs.acs.org/macroletters

Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based on Disulfide Exchange David J. Fortman,†,‡ Rachel L. Snyder,†,‡ Daylan T. Sheppard,† and William R. Dichtel*,† †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemistry and Chemical Biology, Cornell University, Baker Laboratory, Ithaca, New York 14853, United States

‡

ACS Macro Lett. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/20/18. For personal use only.

S Supporting Information *

ABSTRACT: Polymer networks that are cross-linked by dynamic covalent bonds often sacrifice the robust mechanical properties of traditional thermosets in exchange for rapid and efficient reprocessability. Polyurethanes are attractive materials for reprocessable cross-linked polymers because of their excellent mechanical properties, widespread use, and ease of synthesis, but their syntheses typically rely on harmful isocyanate precursors. Polyhydroxyurethanes (PHUs), derived from amines and cyclic carbonates, are promising alternatives to traditional polyurethanes. PHU networks are reprocessable via transcarbamoylation reactions even in the absence of external catalysts, but this process occurs over hours at temperatures above 150 °C. We have dramatically shortened the reprocessing times of PHU networks by incorporating dynamic disulfide bonds. Using cystamine as a comonomer gives materials with similar thermal stability and mechanical properties to other rigid cross-linked PHUs. Despite their excellent mechanical properties, these materials show rapid stress relaxation and have characteristic relaxation times as low as 30 s at 150 °C. This property enables reprocessing with quantitative recovery of cross-link density as measured by DMTA after only 30 min of elevated-temperature compression molding. Disulfide incorporation is a promising approach to obtain reprocessable, crosslinked PHU resins that are not derived from isocyanates.

T

reprocessing. Despite their widespread application, the synthesis of PUs from isocyanate-containing monomers is undesirable because they are hazardous.12 The development of isocyanate-free PUs would mitigate these drawbacks and is an active research target. Polyfunctional cyclic carbonates and amines polymerize into polyhydroxyurethanes (PHUs),13−15 which have been the basis of various isocyanate-free thermoplastics and thermosets,16,17 thermoplastic elastomers,18,19 and coatings.20 We and others have developed PHU polymer networks that can be reprocessed via dynamic exchange reactions between their urethane and hydroxyl groups. Crosslinked PHUs derived from six-membered cyclic carbonates were reprocessed by compression molding at elevated temperatures, albeit with long reprocessing times (4−8 h at 160 °C) and incomplete recovery of mechanical properties (typically ca. 75%).21,22 Torkelson and co-workers23 reported faster reprocessing and improved mechanical recovery of elastomeric PHU networks by embedding a transcarbamoylation catalyst, 4-dimethylaminopyridine, into the polymers. We hypothesized that PHU resins containing disulfide linkages would exhibit faster and more complete repair without

hermosets are cross-linked polymers that display excellent mechanical properties and chemical and thermal stability. Their covalent cross-links typically preclude repair or reprocessing; however, dynamic covalent bonds can impart malleability at elevated temperatures while maintaining stable mechanical properties under service conditions.1−9 Despite these developments, it remains challenging to achieve mechanical properties that are competitive with traditional thermosets alongside processability more typical of thermoplastics. Designing materials that balance robust mechanical properties and rapid relaxation times will enable new technologies and improve the sustainability of cross-linked polymers.10 Vitrimers are a class of dynamic covalent networks in which exchange reactions that occur within the polymer networks impart a gradual Arrhenius-type change in viscosity,2−5 resulting in materials that can be directly reprocessed without depolymerization to small molecules. Inspired by this approach, many dynamic covalent reactions1−4 have been employed in reconfigurable networks, but further development remains of high interest to enable simultaneous control of the mechanical properties, reprocessing rates, and thermochemical stability of these materials. Polyurethanes (PUs) are the sixth most abundant class of polymers11 and are among the most prevalent cross-linked polymers in use, making them important targets for © XXXX American Chemical Society

Received: August 31, 2018 Accepted: September 14, 2018

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DOI: 10.1021/acsmacrolett.8b00667 ACS Macro Lett. 2018, 7, 1226−1231

Letter

ACS Macro Letters

Figure 1. (A) Polyaddition of bis(cyclic carbonate) (bCC) with various ratios of cystamine and tris(2-aminoethylamine) (TREN) to yield crosslinked polymers 1−4. (B) Exchange of disulfides in the polymer backbone enables reprocessing.

Figure 2. (A) FT-IR spectra of bCC and PHU 3 showing the conversion of cyclic carbonate (CO stretch at 1732 cm−1, black dashed line) to hydroxyurethane (CO stretch at 1688 cm−1, red dashed line). (B) Dynamic mechanical thermal analyses of polymers 1 (blue), 2 (green), 3 (red), and 4 (black), showing the storage modulus and tan δ as a function of temperature, are consistent with a cross-linked architecture, wherein decreasing TREN loadings correspond to decreasing storage moduli of the rubbery plateau.

Cystamine was selected as a disulfide-containing monomer for PHUs because it is a readily available diamine that can be derived from the amino acid cysteine.41 Cystamine is commonly used as a redox-responsive linker in applications such as controlled drug release42,43 but has not been used previously within reprocessable cross-linked polymers. We reacted cystamine with a bis(cyclic carbonate) monomer (bCC) and tris(2-aminoethyl)amine (TREN) as a crosslinking agent (Figure 1). The six-membered cyclic carbonate was chosen over an analogous five-membered cyclic carbonate because it reacts more rapidly44 and yields PHU networks with greater thermal stability.22 The monomers were dissolved in CH2Cl2 and allowed to react at room temperature, and then the resulting films were placed under vacuum at 80 °C for 48 h, followed by postcuring at 150 °C for 1 h to ensure complete cross-linking and solvent removal. We varied the ratio of cystamine:TREN while maintaining the same total concentration of primary amines to study the effects of cross-linking density and disulfide content on the properties of polymers 1− 4. The Fourier-transform IR (FT-IR) spectra of each polymer sample showed complete disappearance of the cyclic carbonate CO stretch at 1732 cm−1, along with the appearance of a hydrogen-bonded urethane CO stretch at 1688 cm−1, a

requiring an embedded catalyst, which might leach from the network or degrade under realistic service conditions. Disulfides undergo both radical-based24 and base-mediated25 exchange processes, whose rates are tunable by altering the disulfide substituents.26−29 Several strategies have emerged to control the dynamics of disulfide bonds in cross-linked elastomers,30−32 in part inspired by the presence of disulfide linkages in commercial tires. These strategies have been applied less frequently to rigid materials, although incorporation of aromatic disulfides into rigid epoxy resins was recently introduced to address these limitations. These materials exhibited rapid reprocessability at elevated temperatures,33,34 albeit with an increased cross-link density33 after reprocessing, which may suggest that side reactions occur under certain reprocessing conditions. Therefore, while the incorporation of disulfides into polyurethane elastomers is established for self-healing of materials with high chain mobility,35−40 here we evaluate the ability of aliphatic disulfides to enable the efficient bulk reprocessing of densely cross-linked PHUs and study their rheology at elevated temperatures. To enable these studies, we developed a new strategy that incorporates disulfide bonds into cross-linked PHUs (Figure 1) with control of their concentration within the network. 1227


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ACS Macro Letters Table 1. Properties of Cystamine-Containing Polyhydroxyurethanes polymer

gel %

Td (°C)a

Tg,DSC (°C)

Tg,DMTA (°C)

E′ (MPa)b

1 2 3 4

97 99 99 >99

273 278 279 274

49 51 56 59

53 57 63 66

1.24 1.59 2.14 2.84

Ea (kJ/mol)c 62 73 78 126

± ± ± ±

2 4 4 7

Tv (°C)d −18 9 24 75

5% mass loss as determined by TGA. bE′ at 120 °C as determined by DMTA. cArrhenius activation energy of stress relaxation. dTopology freezing temperature, determined by the literature method.45,46 a

urethane N−H deformation at 1530 cm−1, and a free hydroxyl O−H stretch at 3300 cm−1 (Figure 2A, Figure S1). These spectral features are all consistent with the formation of PHUs. High gel fractions (>0.97; Table 1) obtained after swelling the polymers in CH2Cl2 indicated that the curing reaction reached high conversion and that the presence of the disulfides did not interfere with the polymerization. Dynamic mechanical thermal analysis (DMTA) of 1−4 exhibited plateaus in the storage moduli above the glass-transition temperature (Tg), as expected for cross-linked polymers. The plateau modulus at 120 °C increased from 1.24 to 2.84 MPa (Figure 2B) as the TREN content of the networks increased from 20% to 66% of the total amine monomer feed. These values indicate that the materials are densely cross-linked and that our synthetic approach successfully provided polymers of varying cross-link density. Differential scanning calorimetry (Figure S2) and DMTA both showed that the Tg increased from 49 to 59 °C with increasing cross-link density, a trend that is typical for cross-linked polymer networks. The polymers all degraded above 270 °C (Figure S3), which is similar to the decomposition temperatures of PHUs that do not contain disulfides.21,22 Therefore, disulfides do not significantly affect the thermal stability of the resins. Polymers 3 and 4 had mechanical properties competitive with high performing rigid, reprocessable cross-linked polymers, as characterized by uniaxial tensile testing, with Young’s moduli reaching 2.0 GPa and tensile strengths surpassing 35 MPa. However, increasing the cystamine loading resulted in weaker, more brittle materials, as polymers 1 and 2 had tensile strengths of approximately 25 MPa, possibly because of the higher concentration of disulfide bonds in the networks (Figure S4, Table S1). Stress relaxation experiments, which indicate the rate of cross-link reorganization in dynamic covalent networks, were performed for each PHU at elevated temperature. The samples were equilibrated at the desired temperature, after which a 5% strain was applied rapidly, and the decay of the resulting stress in the material was monitored. This process was repeated at least three consecutive times for each sample to ensure reproducibility and that stress relaxation is not associated with decomposition. The FT-IR spectra of the polymers obtained after stress relaxation (Figure S1) were unchanged, which further confirmed that decomposition is not significant under these conditions. The polymers displayed rapid and reproducible stress relaxation at temperatures above 120 °C (Figure 3, Figures S5−7, Table S2), which demonstrated that the covalent bonds are dynamic under these conditions. Polymer 1 displayed a characteristic relaxation time (τ*, the time to relax to 1/e of the initial stress) of 29 ± 1 s at 150 °C, which is among the fastest reported for a rigid, densely cross-linked material.6,47−49 The relaxation time slows with increasing TREN loading (i.e., increased cross-link density and decreased disulfide concentration), but polymers 2 and 3 still exhibited

Figure 3. (A) Representative stress relaxation curves of polymer 3. (B) Arrhenius plot relating the characteristic relaxation time, τ*, to inverse temperature; the Arrhenius energy of activation of stress relaxation increases with increasing TREN concentration in the networks. (C) Arrhenius plot comparing the stress relaxation of polymer 3 to that of a control sample made with an identical 1:1 ratio of hexamethylenediamine:TREN.20 This comparison demonstrates the necessity of incorporating disulfides for rapid stress relaxation.

relatively fast relaxation (τ* < 80 s) at 150 °C. Polymer 4, which had the lowest disulfide concentration, relaxed stress more slowly than the other polymers (τ* = 445 ± 39 s at 150 1228


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ACS Macro Letters

Figure 4. (A) Ground polymer 3 and the reprocessed sample after compression molding for 30 min at 150 °C. This cycle was repeated multiple times on the same sample. (B) The results of dynamic mechanical thermal analysis of polymer 3 as synthesized (black) and after one (red), two (blue), and three (green) reprocessing cycles.

mold under 5−10 MPa of pressure at 150 °C for 30 min to yield a homogeneous, reprocessed material (Figure 4A). The FT-IR spectra of the polymer were indistinguishable before and after reprocessing (Figure S8), which demonstrated that the recycling process had no significant effect on the chemical composition of the network. DMTA was used to quantitatively evaluate the efficiency of this process because it quantifies the storage modulus in both the glassy and rubbery states, as well as the homogeneity of the network. The results showed similar glassy moduli and an identical rubbery storage modulus within error (Figure 4B, Figure S9), which demonstrated that the full cross-link density of these materials is recovered. Furthermore, no change was observed in tan δ as a function of temperature, which suggested that full homogenization of the networks occurred within this time frame. Given the short characteristic relaxation times and thermal stability, quantitative reprocessing can likely be achieved more quickly and/or at higher temperatures with further optimization of the reprocessing parameters. Reprocessing can be repeated at least three times without any detectable decrease in the storage modulus as measured by DMTA, which suggests that these materials could undergo a significant number of reprocessing cycles. Despite the quantitative recovery of the cross-link density, diminished tensile strength was observed during the first reprocessing cycle (∼65% recovery of tensile strength), followed by quantitative recovery within error upon further recycling (Table S3, Figure S10). This observation may be attributable to physical or environmental aging of the samples, as annealed samples display a similar drop in tensile strength, although a full investigation into the cause of this behavior is ongoing. Attempted reprocessing of control samples lacking disulfides under the same conditions yielded inhomogeneous materials with little evidence of polymer homogenization (Figure S12), which is consistent with the significantly longer relaxation times observed during their corresponding stress relaxation experiments. The results reported herein demonstrate the facile incorporation of disulfide-containing cystamine into PHU networks and show that these networks can be reprocessed with negligible changes in their chemical structure, good recovery of their cross-link density, and moderate recovery of their mechanical properties. Notably, these materials show very rapid relaxation, despite their high cross-link densities, and can be reprocessed with high efficiency under milder conditions

°C). The slowing of relaxation rates with increasing cross-link density and decreasing disulfide concentration is consistent with previous studies of stress relaxation in vitrimer systems.50,51 The energy of activation of polymer stress relaxation was determined by applying an Arrhenius model to the temperature dependence of τ*. Ea also increased with increasing cross-link density, which is consistent with previous reports.50,51 This observation provides further evidence that the kinetics of the exchange reaction are not the only factor governing the relaxation rate of dynamic cross-linked materials and that the diffusion of the reactive groups, as governed by the polymer chain mobility, likely plays a role in the activation energy of stress relaxation in densely cross-linked systems. Changing the cystamine:TREN ratio from 1:1 to 1:2 (i.e., decreasing the disulfide concentration and increasing the crosslink density) resulted in a substantial increase in Ea from 78 to 126 kJ/mol and significant slowing of the relaxation rate. This outcome suggests the need for a percolated network of disulfide-containing cross-links to ensure rapid relaxation in these materials. In these polymers, the cross-links may reorganize through at least two pathways: disulfide exchange or transcarbamoylation of the urethane linkages. To assess the contributions of each of these processes, we compared the cystamine-containing polymers with identical networks in which the disulfide linkages were omitted by replacing cystamine with hexamethylenediamine. In polymers containing disulfides, stress relaxation occurred multiple orders-of-magnitude more rapidly at similar temperatures (Figure 3C) and with significantly lower Ea, which suggested that the disulfide exchange is dominant and the contribution of transcarbamoylation to stress relaxation is negligible. The relaxation rate of these materials is also faster than that of materials based on typical aliphatic-substituted disulfide bonds, possibly because the tertiary amines in the network catalyze exchange,25 though additional studies are required to fully understand the magnitude of this effect. Having established the rapid dynamic nature of these networks, we sought to demonstrate that these materials could be reprocessed at significantly faster rates than those of previous PHU systems. Polymer 3 was chosen for this demonstration because it showed the best balance of robust mechanical properties and rapid relaxation times. The polymer was ground into a fine powder and heated in a compression 1229


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ACS Macro Letters (150 °C, 30 min) than those required by other rigid PHU systems (160 °C, 4−8 h).21,22 This unique combination of mechanical robustness and rapid relaxation suggests that cystamine could be an attractive diamine feedstock for reprocessable cross-linked polymers. Additional efforts will aim to explore more industrially viable processing techniques, such as injection molding, that take advantage of the rapid and reproducible reprocessability of these materials and to incorporate cystamine into five-membered cyclic carbonatebased networks, which are generally considered more sustainable because of their facile synthesis from CO2.13

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(8) Mukherjee, S.; Cash, J. J.; Sumerlin, B. S. Responsive Dynamic Covalent Polymers. In Dynamic Covalent Chemistry: Principles, Reactions, and Applications; Wiley VCH: Weinheim, 2017; pp 331− 368. (9) Imato, K.; Otsuka, H. Self-Healing Polymers through Dynamic Covalent Chemistry. In Dynamic Covalent Chemistry: Principles, Reactions, and Applications; Wiley VCH: Weinheim, 2017; pp 369− 398. (10) Fortman, D. J.; Brutman, J. P.; De Hoe, G. X.; Snyder, R. L.; Dichtel, W. R.; Hillmyer, M. A. Approaches to Sustainable and Continually Recyclable Cross-linked Polymers. ACS Sustainable Chem. Eng. 2018, 6, 11145−11159. (11) Market Study, Polyurethanes (PUR) and Isocyanates (MDI & TDI); Cerasana, January 2016. (12) Methylene Diphenyl Diisocyanate (MDI) and Related Compounds Action Plan; United States Environmental Protection Agency, 2011. (13) Nohra, B.; Candy, L.; Blanco, J.-F.; Guerin, C.; Raoul, Y.; Mouloungui, Z. From Petrochemical Polyurethanes to Biobased Polyhydroxyurethanes. Macromolecules 2013, 46, 3771−3792. (14) Blattmann, H.; Fleischer, M.; Bähr, M.; Mü lhaupt, R. Isocyanate- and Phosgene-Free Routes to Polyfunctional Cyclic Carbonates and Green Polyurethanes by Fixation of Carbon Dioxide. Macromol. Rapid Commun. 2014, 35, 1238−1254. (15) Rokicki, G.; Parzuchowski, P. G.; Mazurek, M. Non-Isocyanate Polyurethanes: Synthesis, Properties, and Applications: Non-Isocyanate Polyurethanes: Synthesis, Properties, and Applications. Polym. Adv. Technol. 2015, 26, 707−761. (16) Schmidt, S.; Ritter, B. S.; Kratzert, D.; Bruchmann, B.; Mülhaupt, R. Isocyanate-Free Route to Poly(carbohydrate−urethane) Thermosets and 100% Bio-Based Coatings Derived from Glycerol Feedstock. Macromolecules 2016, 49, 7268−7276. (17) Schimpf, V.; Ritter, B. S.; Weis, P.; Parison, K.; Mülhaupt, R. High Purity Limonene Dicarbonate as Versatile Building Block for Sustainable Non-Isocyanate Polyhydroxyurethane Thermosets and Thermoplastics. Macromolecules 2017, 50, 944−955. (18) Leitsch, E. K.; Beniah, G.; Liu, K.; Lan, T.; Heath, W. H.; Scheidt, K. A.; Torkelson, J. M. Nonisocyanate Thermoplastic Polyhydroxyurethane Elastomers via Cyclic Carbonate Aminolysis: Critical Role of Hydroxyl Groups in Controlling Nanophase Separation. ACS Macro Lett. 2016, 5, 424−429. (19) Beniah, G.; Fortman, D. J.; Heath, W. H.; Dichtel, W. R.; Torkelson, J. M. Non-Isocyanate Polyurethane Thermoplastic Elastomer: Amide-Based Chain Extender Yields Enhanced Nanophase Separation and Properties in Polyhydroxyurethane. Macromolecules 2017, 50, 4425−4434. (20) Ma, Z.; Li, C.; Fan, H.; Wan, J.; Luo, Y.; Li, B.-G. Polyhydroxyurethanes (PHUs) Derived from Diphenolic Acid and Carbon Dioxide and Their Application in Solvent- and Water-Borne PHU Coatings. Ind. Eng. Chem. Res. 2017, 56, 14089−14100. (21) 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. (22) Fortman, D. J.; Brutman, J. P.; Hillmyer, M. A.; Dichtel, W. R. Structural Effects on the Reprocessability and Stress Relaxation of Crosslinked Polyhydroxyurethanes. J. Appl. Polym. Sci. 2017, 134, 44984. (23) Chen, X.; Li, L.; Jin, K.; Torkelson, J. M. Reprocessable Polyhydroxyurethane Networks Exhibiting Full Property Recovery and Concurrent Associative and Dissociative Dynamic Chemistry via Transcarbamoylation and Reversible Cyclic Carbonate Aminolysis. Polym. Chem. 2017, 8, 6349−6355. (24) Tobolsky, A. V.; MacKnight, W. J.; Takahashi, M. Relaxation of Disulfide and Tetrasulfide Polymers. J. Phys. Chem. 1964, 68, 787− 790. (25) Nevejans, S.; Ballard, N.; Miranda, J. I.; Reck, B.; Asua, J. M. The Underlying Mechanisms for Self-Healing of Poly(disulfide)s. Phys. Chem. Chem. Phys. 2016, 18, 27577−27583. (26) Rekondo, A.; Martin, R.; Ruiz de Luzuriaga, A.; Cabañero, G.; Grande, H. J.; Odriozola, I. Catalyst-Free Room-Temperature Self-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00667. Experimental procedures and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David J. Fortman: 0000-0002-0422-3733 Rachel L. Snyder: 0000-0002-0569-0704 William R. Dichtel: 0000-0002-3635-6119 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (NSF) through the Center for Sustainable Polymers (CHE-1413862). This research made use of the Materials Characterization and Imaging Facility, which receives support from the MRSEC Program (NSF DMR-1121262) of the Materials Research Center at Northwestern University and the Integrated Molecular Structure Education and Research Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental Resource (NSF NNCI-1542205), the State of Illinois, and the International Institute for Nanotechnology.

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Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based

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