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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Reprocessable Acid-Degradable Polycarbonate Vitrimers Rachel L. Snyder,† David J. Fortman,†,‡ Guilhem X. De Hoe,§ Marc A. Hillmyer,§ and William R. Dichtel*,† †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States § Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡
S Supporting Information *
ABSTRACT: Vitrimers are cross-linked polymer networks containing linkages that undergo thermally activated, associative exchange reactions, such that the cross-link density and overall network connectivity are preserved. Polycarbonates are industrially relevant polymers that, to our knowledge, have not yet been explored as vitrimers. We developed hydroxylfunctionalized polycarbonate networks that undergo transcarbonation exchange reactions at elevated temperatures in the presence of catalytic Ti(IV) alkoxides. The rate of transcarbonation within the networks, estimated through stress relaxation experiments, was tuned by adjusting the catalyst loading or hydroxyl group concentration in the networks. The polymer networks exhibit recovery of their tensile strength and plateau storage modulus (71−133%) after reprocessing. In addition to being reprocessable, the networks were hydrolyzed and decarboxylated in aqueous acid to recover 80 wt % of the precursor to the bifunctional cyclic carbonate monomer. These observations demonstrate that PC vitrimers are a novel class of strong, repairable polymers with more facile end-of-life degradation compared to other vitrimers and conventional thermosets. These characteristics, along with the high likelihood of deriving their monomers from bio-based sources, make PC vitrimers outstanding candidates for sustainable manufacture and use.
1. INTRODUCTION Thermosets are cross-linked polymer networks with high mechanical strength, chemical resistivity, and thermal stability; however, conventional thermosets contain static covalent crosslinks that preclude the reprocessing of cured polymer networks. Synthesizing cross-linked networks that can be repaired, reshaped, and reprocessed after curing is a key challenge to enable common manufacturing techniques such as welding,1 injection/compression molding,2,3 and three-dimensional printing for thermoset-like materials. The availability of viable reprocessing techniques will also decrease the environmental impact of these traditionally nonrecyclable materials.4,5 One approach to overcome these limitations is to introduce dynamic cross-links into polymer networks.6−8 Reversible reactions, such as Diels−Alder cycloadditions,9−15 allow cross-linked networks to be depolymerized and re-formed in response to an external stimulus such as heat or light. However, dynamic networks based on dissociative reactions typically undergo depolymerization to monomers or oligomers; as a result, they dissolve when heated in solvents. In contrast, networks featuring dynamic reactions that occur through associative intermediates remain insoluble and exhibit gradual changes in viscosity as a function of temperature. In 2011, Leibler and co-workers16 demonstrated these concepts in polyester networks containing hydroxyl groups and catalytic Zn(OAc)2 and named them “vitrimers”. At service temper© XXXX American Chemical Society
atures, these materials behave as typical thermosets, but at elevated temperatures, transesterification reactions allow for stress relaxation through the topological reorganization of the cross-links. Different classes of vitrimers based on polyester or other linkages17−33 have since been synthesized,34−42 studied to gain mechanistic insights,43−50 and modified for specific applications such as shape-memory behavior.38,51−64 Although these materials reflect a broad range of common polymer compositions and dynamic covalent bonds, ongoing challenges for vitrimers include improving mechanical properties and identifying more efficient exchange reactions. The development of new vitrimer networks that combine robust exchange reactions with simple monomers and catalysts to produce strong materials is an ongoing challenge. Carbonates undergo transcarbonation exchange reactions with free hydroxyl groups, analogous to transesterification.65−69 We have studied and optimized transcarbonation to develop polycarbonate (PC) vitrimers (Figure 1). PCs have great potential for sustainability because carbonates can be synthesized from CO2,70 incorporate biorenewable polyols, and be degraded to nontoxic compounds.5 Additionally, Received: October 29, 2017 Revised: December 16, 2017
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DOI: 10.1021/acs.macromol.7b02299 Macromolecules XXXX, XXX, XXX−XXX
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Then, the reaction mixture was poured into a circular aluminum weigh dish (2 in. diameter) on a hot plate at 105 °C to remove solvent. The reaction mixture was not disturbed to minimize solvent bubbling. Next, the clear, colorless reaction mixture was transferred to a convection oven at 105 °C for 18 h, during which time the reaction mixture passed the gel point. The temperature was then increased to 150 °C for 10 h to complete curing. While hot, the polymers were removed from the weigh pan. All samples were stored in a desiccator. Tensile bars and dog bones were cut from the sample using a cutting die or straight-edge blade on a hot plate at 100 °C. (See the Supporting Information for full characterization data.) Instrumentation. Infrared spectra were recorded on a Thermo Nicolet iS10 equipped with a ZnSe ATR attachment. Spectra were uncorrected. Gas chromatography/electron impact mass spectrometry was performed on an Agilent 6890N Network GC system equipped with a DB5 30 m column with a time-of-flight mass spectrometer in electron impact ionization mode. Tribromobenzene was used as an internal standard. All samples were diluted with anhydrous dichloromethane to a 1 mg/mL solution prior to analysis. Solution-phase NMR spectra were recorded on a Varian 400 MHz or an Agilent DD MR-400 400 MHz spectrometer using a standard 1 H/X Z-PFG probe at ambient temperature. Solid state NMR spectra were recorded on a Varian 400 MHz VNMRS system using an HXY 5 mm probe. Samples were spun at 5000 MHz, and spectra recorded at ambient temperature. Thermogravimetric analysis (TGA) was performed on a Mettler Toledo SDTA851 thermogravimetric analysis system using 10−15 mg of sample. Dynamic samples were heated under a nitrogen atmosphere at a rate of 5 °C/min from 25 to 500 °C. Isothermal samples were heated to designated temperature at 15 °C/min and held at that temperature for 6 h under N2 atmosphere. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q1000 differential scanning calorimeter or a Mettler Toledo DSC822 differential scanning calorimeter. Samples were heated at a rate of 10 °C/min to at least 90 °C to erase thermal history, cooled to −30 °C at 10 °C/min, and then heated to at least 110 °C. All data shown are taken from the second heating ramp. The glass transition temperature (Tg) was calculated from the maximum value of the derivative of heat flow with respect to temperature. Dynamic mechanical thermal analysis (DMTA) was performed on a TA Instruments RSA-G2 analyzer (New Castle, DE) using rectangular films (ca. 1.0 mm (T) × 3 mm (W) × 6 mm (L)). The transducer was set to spring mode. The axial force was adjusted to 20 g (sensitivity 1.0 g) before the test to ensure the sample was in tension and not buckling. The minimum axial force was set to 5 g, and a force tracking mode was set such that the axial force was twice the magnitude of the oscillation force during the test. A strain adjust of 30% was set with a minimum strain of 0.05%, a maximum strain of 10%, a minimum force of 1 g, and a maximum force of 20 g to prevent the sample from going out of the specified strain. A temperature ramp was then performed from −30 to 180 °C at a rate of 5 °C/min, with an oscillating strain of 0.05% and an angular frequency of 6.28 rad s−1 (1 Hz). The glass transition temperature (Tg) was calculated from the maximum value of the loss modulus (E″). The molar mass between cross-link junctions (Mx) was estimated according to the literature procedure using measured densities and the storage modulus (E′) at 100 °C;1 these values are summarized in Table S1. Uniaxial tensile testing was conducted using dog-bone-shaped tensile bars (ASTM D-1708 1.0 mm (T) × 5 mm (W) × 25 mm (L) and a gauge length of 16 mm). The samples were aged for at least 48 h at ambient temperatures in a desiccator prior to testing. Tensile measurements were performed on a Sintech 20G tensile tester with 250 g capacity load cell at ambient temperatures at a uniaxial extension rate of 5 mm/min. Young’s modulus (E) values were calculated using the TestWorks software by taking the slope of the stress−strain curve from 0 to 1 N of force applied. Reported values are the averages and standard deviations of at least five replicates. Stress relaxation analysis (SRA) was performed on a TA Instruments RSA-III analyzer (New Castle, DE) using rectangular
Figure 1. (A) Transcarbonation exchange reaction whereby a hydroxyl nucleophile reacts with a carbonate, forms an associative intermediate, and releases the exchanged carbonate and hydroxyl group. (B) Topology of a cross-linked polymer network containing carbonates and hydroxyls can be adjusted via transcarbonation exchange reactions.
aromatic polycarbonates are an industrially relevant class of tough, transparent polymers used in airplane windows and safety glasses.71 Approximately 3 million tons of PCs are produced annually; however, to our knowledge, PC vitrimers are unexplored, and the only report of a dynamic polycarbonate network uses dissociative retro-Diels−Alder reactions for crosslink exchange.71,72 We designed and prepared PC vitrimers capable of stress relaxation at elevated temperatures by incorporating Ti(Oi-Pr)4 during their synthesis. We determined the impacts of catalyst loading and the concentration of free hydroxyls in the network on the rate of stress relaxation. These materials were reprocessed with chemical integrity and recovery of mechanical properties. Finally, the vitrimers were degraded via aqueous acid-catalyzed hydrolysis and subsequent decarboxylation to isolate the pure monomer precursor, di(trimethylolpropane), in 80 wt % yield by a simple extraction, which is advantageous for practical chemical recycling of these networks. This study demonstrates that PC vitrimers are a potentially sustainable class of cross-linked polymers that can be reprocessed for extended functional lifetimes and degraded to recover useful feedstocks.
2. EXPERIMENTAL SECTION Materials. All reagents were purchased from Sigma-Aldrich or Fisher Scientific. 1,4-Butanediol and bis(6-membered cyclic carbonate) were dried under high vacuum for 24 h before use. Titanium(IV) isopropoxide was filtered through a 0.45 μm filter to remove TiO2 particles and stored under N2. All other reagents were used without further purification. Dichloromethane (DCM) and tetrahydrofuran (THF) were purchased from Fisher Scientific and purified using a custom-built alumina-column based solvent purification system. Other solvents were purchased from Fisher Scientific and used without further purification. Bis(6-membered cyclic carbonate) (bCC) was prepared according to the procedure previously reported in the literature.73 Synthesis of Polycarbonate Networks. To a flame-dried scintillation vial charged with N2 at 45 °C were added 1,4-butanediol and anhydrous CH2Cl2 (0.5 mL). Titanium(IV) isopropoxide was added via Hamilton syringe. In a separate flame-dried scintillation vial, bCC was dissolved in anhydrous CH2Cl2 (2 mL) at 45 °C to give a clear, colorless solution. The solution of bCC was added to the reaction vial via syringe and sonicated for 30 s to ensure homogeneity. B
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Figure 2. (A) Model transcarbonation exchange reaction with di(n-propyl) carbonate and 1-decanol in the presence of Ti(Oi-Pr)4 (2 mol % to carbonate). (B) Gas chromatograms show nearly complete consumption of di(n-propyl) carbonate after 1 h, and the presence of the mono- and disubstituted products. No side products were observed after 48 h at 150 °C. (C) Temperature-dependent kinetics provide an Arrhenius activation energy (Ea) of 74 ± 1 kJ/mol for small-molecule transcarbonation. films (ca. 1.0 mm (T) × 3 mm (W) × 20 mm (L) and a gauge length of 6 mm). The SRA experiments were performed with strain control at specified temperature (150−180 °C). The samples were allowed to equilibrate at this temperature for approximately 10 min, after which the axial force was then adjusted to 0 N with a sensitivity of ±0.002 N. Each sample was then subjected to an instantaneous 5% strain. The stress decay was monitored, while maintaining a constant strain (5%), until the stress relaxation modulus had relaxed to at least 37% (1/e) of its initial value. This was performed three consecutive times for each sample. The activation energy (Ea) and freezing transition temperature (Tv) were determined using the methodology in the literature.35,37 Degradation Studies. Elevated temperature swelling tests were conducted by submerging a 0.1 g piece of polymer in 10 mL of diphenyl ether or ethylene glycol and heating to 170 °C on a hot plate while stirring for 24 and 4 h, respectively. For degradation studies, 0.1 g samples of polymer were submerged in sealed vials with 10 mL of HCl (1 M), KOH (1 M), and H2O. The vials were heated to 90 °C on a hot plate for 36 h. The HCl solution was extracted with n-butanol (20 mL). The solvent was removed via rotary evaporation to yield the recovered di(trimethylolpropane). Annealing. Annealed samples were prepared by heating assynthesized tensile bars of the cross-linked polymers in an oven at 140 °C for 7 h. The resulting tensile bars were aged for at least 48 h in a desiccator and subjected to DMTA to compare their rubbery storage modulus to as-synthesized or reprocessed samples. Reprocessing. To reprocess the materials, the polymer was ground into small pieces using a Cuisinart Grind Central coffee grinder and placed between two aluminum plates with a 1.0 mm thick aluminum spacer. This assembly was placed in a preheated PHI 30 ton manual compression hot press with 5−10 MPa of pressure. The material was thermally equilibrated for 1 h and removed from the press to check for homogeneity of the film (∼2 min). The assembly was then placed in the hot press again, and 5−10 MPa of pressure was reapplied. After removing the homogeneous, rectangular piece from the hot press, tensile bars were punched from the material while still hot (to prevent brittle fracture) using an ASTM D-1708 tensile die and allowed to age for 48 h in a desiccator. Samples were subjected to uniaxial tensile testing and dynamic mechanical thermal analysis to determine their recovery in mechanical properties. Specimens for DMTA were cut using a straight-edge blade on a 100 °C hot plate. All samples were reprocessed for 7 h unless otherwise indicated.
3. RESULTS AND DISCUSSION Model Reactions. Dynamic cross-linked polymers require robust exchange reactions that proceed without side reactions for extended time periods. Therefore, selection of an efficient transcarbonation catalyst is critical to the development of PC vitrimers. We conducted model reactions in which di(n-propyl) carbonate was stirred in neat 1-decanol (20 equiv) with catalyst (2 mol % to carbonate) at 150 °C for 48 h, and the reaction progress was analyzed using gas chromatography−mass spectrometry (Figure 2a). Both acidic and basic transcarbonation catalysts were screened (Table S9). Ti(Oi-Pr)4 was identified as an inexpensive, nontoxic catalyst that afforded rapid transcarbonation with no detectable side reactions. In the presence of 2 mol % Ti(Oi-Pr)4 and excess 1-decanol, di(npropyl) carbonate is converted to mono- and didecylsubstituted carbonates within 1 h, and no side products are observed after 48 h at 150 °C (Figure 2b). After 48 h, 1-octanol (10 equiv) was added to the reaction mixture, and all expected substitutions were observed (Figure S1). Therefore, the Ti(IV) catalyst remains active at high temperature and under air over extended times. Control experiments omitting Ti(Oi-Pr)4 showed minimal transcarbonation (Figure S2), which confirms that a catalyst is required for exchange even at these elevated temperatures. The rate of disappearance of di(n-propyl) carbonate was monitored as a function of temperature (Figure S5) and used to determine an Arrhenius activation energy (Ea) of 74 ± 1 kJ/mol for small-molecule transcarbonation (Figure 2c). Polymer Synthesis. After selecting a viable transcarbonation catalyst, we designed cross-linked polymer networks that incorporate both carbonates and primary hydroxyl groups after a single polymerization step using a bis(6-membered cyclic carbonate) (bCC) and 1,4-butanediol (BD) (Figure 3a). Sixmembered cyclic carbonates have high ring strain, which renders them susceptible to ring-opening polymerization at elevated temperatures in the presence of a hydroxyl initiator (e.g., BD).74−77 Once ring-opened, bCC yields a polyfunctional, hydroxyl-terminated oligomer that continues to react with additional bCC and undergo transcarbonation with other C
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Figure 3. (A) Reaction of a bis(6-membered cyclic carbonate) and 1,4-butanediol to form cross-linked polycarbonate networks with various ratios of CO to OH ([ROH] series) and catalyst loadings (Ti(IV) series). (B) L-2, H-2, M-2, M-1, and M-4 as synthesized. (C) Representative tensile tests of the as-synthesized polymer networks.
By thermogravimetric analysis, the networks first lose mass around 260 °C (5% mass loss), which is well above the temperature required for transcarbonation (Figures S23 and S24). The glass-transition temperatures (Tg) of these materials were determined by differential scanning calorimetry to be generally near room temperature and decreased as either catalyst loading or [ROH] was increased. As more catalyst was incorporated, the Tg decreased from 34 °C to 25 and 19 °C for M-1, M-2, and M-4, respectively. This phenomenon may result from decreased hydrogen bonding or plasticization as more Ti(IV) is incorporated. Similarly, as more BD was incorporated, the Tg decreased from 35 °C to 25 and 15 °C for L-2, M-2, and H-2, respectively (Table S1, Figures S13−S18). In these cases, we attribute the decreasing Tg predominantly to decreasing cross-linking density as estimated through dynamic mechanical thermal analysis (DMTA) measurements. DMTA analysis of the [ROH] series showed that increasing the feed ratio of BD significantly increases the molar mass between cross-links (Mx) as calculated from the rubbery storage modulus at 100 °C (Table S1).37 This result was expected, as the linear BD linker acts as a chain extender to provide a less densely cross-linked network. The tensile properties of these materials vary with the nearroom temperature Tg values, as all tensile tests were conducted at ambient temperature. Networks in the glassy regime at room temperature (i.e., L-2) exhibit ultimate tensile strengths of 36 ± 2 MPa, which surpasses many previously reported vitrimers and is a relatively high value for an all-aliphatic polycarbonate network. Networks with low Tg values are rubbery at room temperature (i.e., H-2) with a lower ultimate tensile strength of 7 ± 1 MPa and higher elongation at break of 101 ± 20%. The other materials, with Tg values close to room temperature (i.e., M-1, M-2, and M-4), have intermediate properties and exhibit tensile strength values between 8 and 16 MPa after undergoing plastic-like deformation. Network Dynamics. Dynamic reactions in vitrimers are typically evaluated through elevated temperature stress relaxation analysis, in which a set strain is applied to a tensile specimen and the resultant stress dissipates over time (Figure 4a). The characteristic relaxation time (τ*) is defined as the
oligomers, eventually generating a cross-linked PC network. Crucially, the ratio of carbonate to hydroxyl groups in the polymer networks is identical to that of the monomers because each ring-opening or transcarbonation reaction consumes one hydroxyl group and liberates another. As such, the concentration of hydroxyls in the network ([ROH]) is controlled by modifying the feed ratio of bCC to BD. We anticipated that higher [ROH] would yield a faster rate of exchange. We developed a series of polymers ([ROH] series) with hydroxyl concentrations of 3.75 (low; L), 5.26 (medium; M), and 6.57 mol/L (high, H), each containing 2 mol % Ti(Oi-Pr)4 with respect to bCC (polymers L-2, M-2, and H-2, respectively). These networks were synthesized by adjusting the stoichiometric feed ratio (Figure 3a), which resulted in a 40% increase in [ROH] from L-2 to M-2 and a 31% increase in [ROH] from M-2 to H-2. Because the decrease in carbonate concentration with increasing BD loading is relatively small (6− 7%), we attributed any observed difference in stress relaxation kinetics to the increasing [ROH] (see below). A second series of networks (Ti(IV) series) were synthesized with 1, 2, and 4 mol % catalyst loading with respect to bCC at a constant “medium” [ROH] loading (M-1, M-2, and M-4, respectively) to study the impact of catalyst loading. To synthesize the PC networks, we homogenized bCC, BD, and Ti(Oi-Pr)4 in anhydrous CH2Cl2 and cast the mixture into a mold. The mixture was then heated to 105 °C to evaporate the CH2Cl2 and yield a homogeneous reaction mixture. The mixtures were cured at 150 °C for 8 h to give clear, colorless networks (Figure 3b). Polymer Characterization. Fourier transform infrared (FT-IR) spectra of the PC networks showed that the bCC carbonyl stretch shifted from 1739 cm−1 (cyclic carbonate) to 1741 cm−1 (linear carbonate), and O−H stretches were present at 3500 cm−1 for all polymers (Figure S6). 13C crosspolarization/magic-angle-spinning solid-state NMR spectroscopy showed the presence of carbonates (155 ppm) in the networks without other obvious functional groups (Figure S11). Gel fractions of 90−94% were measured by swelling the networks in CH2Cl2 to remove unincorporated monomers and oligomers as well as residual catalyst (Table S1). D
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Figure 4. (A) Schematic of a stress relaxation experiment in which a 5% strain is applied to a tensile specimen and the resultant stress is measured as a function of time. (B, C) Representative stress relaxation experiments for the Ti(IV) series (B) and [ROH] series (C) at 170 °C. The dotted line represents E/E0 = 1/e, which defines the characteristic relaxation time (τ*). As catalyst loading increases, τ* decreases, indicating a faster rate of exchange. τ* also decreases as the hydroxyl concentration increases. (C) Arrhenius activation energy (Ea) of stress relaxation for the Ti(IV) series (D) and [ROH] series (E). The insets show the correlation between Ea and molecular weight between cross-links (Mx).
between the Arrhenius Ea of small molecule exchange and vitrimer stress relaxation.16,33,78,79 In contrast, we observed that the measured Ea varied significantly across polymer networks (Figure 4d,e). Notably, the Ea of stress relaxation in the polymer networks is higher than that measured for smallmolecule transcarbonation (74 ± 1 kJ/mol) in all networks, which suggests that the origin of the increased Ea is associated with the diffusion-limiting topology of the cross-linked networks. As previously discussed, the most prominent change in the networks is Mx (measured by DMTA). In the [ROH] series, Mx decreased from H-2 to M-2 to L-2, as less BD was incorporated and the networks became more densely crosslinked. As Mx decreased, the Ea of stress relaxation increased (Figure 4e, inset). In agreement with observations of epoxy vitrimers made by Qi and co-workers,43 we hypothesize that this trend is related to relative chain mobility and thus the mobility of the reactive groups within the dense networks. At low cross-linking densities, the Ea is closer to that measured for model compounds. The diffusion of the reactive groups is more restricted in more densely cross-linked networks, which increases Ea. While no obvious trend in Ea was observed upon changing catalyst concentration (Figure 4d, inset), we hypothesize that the observed differences in activation energy are related to a variety of factors including differing crosslinking densities of the samples, the impact of alkoxide or proton concentration on the polarity of the networks, and potential changes in active form or oligomerization of the titanium catalyst at higher concentrations.80−82
time required for the material to reach 1/e (37%) of the initial stress and is interpreted as a measure of the rate of exchange (e.g., transcarbonation) under the testing conditions. Stress relaxation experiments were performed between 150 and 180 °C with an applied strain of 5%. Multiple relaxation experiments were performed on the same sample, and nearly identical relaxation rates were repeatedly observed (Figure S38), which suggested the absence of side reactions on the experimental time scale. As expected, values of τ* at 170 °C decreased in the Ti(IV) series (i.e., faster relaxation time) from approximately 1000 to 500 to 200 s in samples with 1, 2, and 4 mol % catalyst loading, respectively (Figure 4b). As the catalyst loading doubled, τ* approximately halved, suggesting that the rate of transcarbonation has a first-order dependence on [Ti(IV)]. In the [ROH] series, τ* at 170 °C decreased from approximately 1200 to 500 to 200 s as the hydroxyl concentration increased (see Figure 4b). However, differences in Ea (see below), slight changes in the concentration of carbonate, and the marked difference in cross-linking density make the rate order in [ROH] unclear. Notably, the observed rates of stress relaxation in both series are comparable to or faster than those of most previously reported vitrimers based on transesterification and transcarbamoylation reactions at similar temperatures and catalyst loadings.16,21 As the temperature increased from 150 to 180 °C, τ* consistently decreased, as expected (Figures S33−S37). The relationship between ln(τ*) and 1/T was fit to an Arrhenius model to determine an Ea for stress relaxation (Figure 4d,e). Several previous reports described quantitative agreement E
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Macromolecules Reprocessing. The reprocessing of PC vitrimers requires that the materials be exposed to elevated temperatures for extended periods. Annealing experiments confirmed the stability of the networks at high temperatures. Annealing M-2 at 170 °C for 6 and 24 h caused no change in its 13C solid-state NMR spectrum (Figure S12). Likewise, annealing all networks for 7 h at 140 °C resulted in no discernible change in the FT-IR spectra (Figure S7), indicating that chemical changes are minimal in these networks after thermal annealing. After annealing for 7 days at 170 °C, FT-IR analysis revealed a weak stretch at 1700 cm−1, which might arise from the partial oxidation of the hydroxyls to aldehydes that occurs after heating for extended times (Figure S8). This process occurs much more slowly than the reprocessing times used in this study. Each sample showed
