Dynamic Thiol–Michael Chemistry for Thermoresponsive Rehealable

Article pubs.acs.org/Macromolecules

Dynamic Thiol−Michael Chemistry for Thermoresponsive Rehealable and Malleable Networks Borui Zhang,† Zachary A. Digby,† Jacob A. Flum,† Progyateg Chakma,† Justin M. Saul,‡ Jessica L. Sparks,‡ and Dominik Konkolewicz*,† †

Department of Chemistry and Biochemistry and ‡Department of Chemical, Paper and Biomedical Engineering, Miami University, Oxford, Ohio 45056, United States S Supporting Information *

ABSTRACT: The thiol−Michael adduct is used as a thermoresponsive dynamic cross-linker in polymeric materials. Recently, the thiol−Michael reaction between thiols and conjugated alkenes has been used as a ligation reaction for polymer synthesis and functionalization. Here, the thiol− Michael reaction is demonstrated to be thermally responsive and dynamic. Small molecule model experiments demonstrate the potential for the thiol−Michael adducts to be used in dynamic covalent chemistry. Thiol−acrylate adducts are incorporated into a cross-linker to form a soft polymeric material. These thiol−Michael cross-linked materials display healing after being cut and malleability characteristics at 90 °C. Additionally, the data suggest that there is limited creep and stress relaxation at room temperature with complete recovery of creep once the strain is removed. These thiol−Michael cross-linked polymers show dynamic properties upon thermal stimulus, with long-term stability against mechanical deformation in the absence of this stimulus, opening the way for them to be used in various applications.

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INTRODUCTION Polymeric materials containing dynamic cross-links have received significant interest in the past decade due to their unique mechanical properties and potential applications.1 Unique features of these dynamically cross-linked materials include self-healing, or rehealability, malleability, enhanced fracture toughness, shape memory, and degradability, among others.1−4 Depending on the nature of the dynamic bond used, the exchange can be very rapid under ambient conditions, or it can be so slow that it requires external stimulus such as light, heat, or pH to activate the dynamic properties.2,5 Both rapidly and slowly exchanging dynamic bonds have their advantages.2 Highly dynamic bonds allow dynamic properties to be realized at all times, although this can also lead to stability and creep issues.2,5 Slowly exchanging and stimulus-responsive polymer networks have stability and resistance to creep due to their quasi-static character in the absence of stimulus, although they do not display any dynamic behavior until that stimulus is applied.2,6,7 This spectrum of reactivities implies that new dynamic bonds must be investigated and combined with existing bonds to create materials with properties targeted to a given application. In general, dynamically cross-linked materials fall into one of two classes: supramolecularly or dynamic-covalently crosslinked materials.2 Supramolecularly cross-linked materials utilize relatively rapidly exchanging noncovalent associations such as hydrogen bonding, host−guest interactions, cation− crown ether, and π-stacking interactions, among others, to create dynamically cross-linked materials.5,8−15 Polymers cross© XXXX American Chemical Society

linked with dynamic-covalent bonds utilize relatively exchangeable covalent bonds such as boronic esters, boronates, Diels− Alder adducts, anthracene dimers, disulfides, radical reshuffling reactions, and imines, among others.16−20 In many cases, dynamic-covalent interactions require external stimulus to activate the bond and make it dynamic, while supramolecular systems exchange under ambient conditions. High-yielding organic reactions have received significant interest in the field of polymer chemistry.21−23 Extension of this to dynamic covalent chemistry is a fascinating area where reactions are high yielding under one set of conditions and reversible under others.24 Already, Diels−Alder chemistry has been extensively explored in this context with various dynamically cross-linked materials based on exchange of dienes and dienophiles.5,25−30 The thiol-based additions, including the thiol−Michael,31−34 thiol−ene,35,36 and thiol−yne reactions,37−39 are utilized extensively in polymer chemistry due to their facile implementation and compatibility with functional groups commonly used in polymerization.40 The thiol−Michael reaction has received significant attention as a postpolymerization modification, ligation, and network synthesis tool.33 However, limited small molecule and polymer studies suggest that it has dynamic properties at elevated temperatures and pH values.31−33,41−44 The limited polymer studies have utilized the thiol−Michael adduct for degradation of hydrogel networks in Received: May 19, 2016 Revised: September 6, 2016

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Macromolecules response to an added thiol at elevated pH44 or the thiol−ene reaction for the synthesis of the polymer backbone.16,45 Recent work has also shown that the oxygen based Michael adduct is reversible, albeit at very high temperatures.46 In contrast, the goal of this article is to develop the thermally responsive thiol− Michael (TM) adducts as a new dynamic polymer system. The thermally induced TM exchange reaction is shown in Scheme 1. This article will use TM adducts to create stimulus-responsive healable and malleable polymeric materials that are stable under ambient conditions even for 2 days under strain.

Typical Synthesis of a Thiol−Michael Adduct for Kinetic Studies. The synthesis of thiol−Michael adducts was adapted from the literature.48−50 Briefly, 4-hexen-3-one (0.655 g, 0.0078 mol) was added to a vial equipped with a magnetic stirrer bar. To this, 2mercaptoethanol (0.6094, 0.0078 mol) was added along with an equal amount of dimethylformamide (DMF) by total weight (1.3749 g). Lastly, 0.01 equiv of triethylamine (TEA) (0.007 255 g, 7.170 × 10−5 mol) was added, and the solution was capped and left to stir for 24 h to create the thiol−Michael adduct with 4-hexen-3-one (TM−VK). Thiol−Michael adducts using trans-2-hexen-1-al (TM−VA adduct) and 2-hydroxyethyl acrylate (TM−HEA adduct) with 2-mercaptoethanol were synthesized using a similar procedure. Efficient formation of the adduct was evidenced in each case by >90% decrease of the vinyl protons. Typical Kinetic of Thiol−Michael Adduct Exchange. To a vial of Thiol−Michael adduct and a stoichiometric amount of 2-hydroxyethyl acrylate (HEA), 4-hexen-3-one (VK) or 2-hexen-1-al (VA) was added with its equivalent weight of DMF. This solution was then stirred for 5 min to ensure a homogeneous solution, and then 1 mL of solution was diluted into a vial containing 9 mL of DMF and a stirrer bar. This solution was separated into sample vials, which were then capped and heated to 90 °C for 24 h with 0.1 mL samples taken at specified times. Kinetic studies were performed with the TM−HEA adduct with VK, TM−VK adduct with HEA, TM−VK adduct with VA, and TM−VA adduct with VK. The exchange between VK and VA adducts was monitored by the relative ratio of the H of the free vinyl aldehyde (VA) at 9.5 ppm and the corresponding thiol−Michael (TM−VA) adduct at 9.8 ppm. Similarly, for the exchange between HEA and VK, the reaction was monitored by following the relative ratios of the one vinyl proton of the HEA at 6.43 ppm relative to the one vinyl proton of the VK at 6.65 ppm. Typical Synthesis of Poly(HEA) Materials Cross-Linked by TMADA. Azobis(isobutyronitrile) (AIBN) (40 mg), HEA (4 g), and N,N-dimethylformamide (DMF) (8 mL) were added to a vial. Compound 2 (0.06, 0.16, or 0.32 g) was then added respectively to make 1.5%, 4.0%, or 8.0% materials. The solution was transferred to a Teflon mold to process polymerization. The polymerization proceeded at 65 °C for 30 min. The material was removed from the mold and allowed to dry for 2 days at room temperature and pressure followed by 16 h in a vacuum oven at 35 °C. Monomer conversion was determined by gravimetry and found to be greater than 90%. A similar process was applied for the synthesis of PHEA materials cross-linked by divinylbenzene (DVB) or poly(ethylene glycol) diacrylate (PEGDA, Mn = 250 for the cross-linker). Methods and Techniques. Analytical Methods. Infrared (IR) spectroscopy was performed on a PerkinElmer Spectrum 100 spectrometer. All nuclear magnetic resonance (NMR) was performed on a Bruker 300 or 500 MHz spectrometer. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q20 system. Direct injection ESI-MS spectra were collected on a Bruker EsquireLC mass spectrometer (Billerica, MA) operating in positive ion mode. Tensile Testing. Materials were subjected to tensile testing using an Instron 3344 apparatus equipped with a 100 N load cell at ambient temperature. The extension was increased at the rate of 1 mm/s. All samples were measured until the material broke. Young’s Modulus Calculation. The tensile response of the material was modeled using an incompressible neo-Hookean hyperelastic constitutive law (eq 1):

Scheme 1. Thiol−Michael Adduct Forming Reaction (Top) and Thermally Induced Dynamic Equilibrium (Bottom)

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EXPERIMENTAL SECTION

Materials. All materials were purchased from commercial suppliers unless otherwise specified. All reagents were used as received unless otherwise specified. Synthesis of 2-Hydroxyethyl 3-((2-Hydroxyethyl)thio)propanoate (Compound 1). This synthesis was adapted from a method in the literature.47 To a 500 mL round-bottom flask equipped with a magnetic stirrer bar, 2-mercaptoethanol (2 g, 0.0256 mol) and 2hydroxyethyl acrylate (3 g, 0.0258 mol) were added. To this mixture, 0.1% triethylamine was added by weight. This reaction mixture capped with a glass stopper was stirred for 1 h at room temperature to give product 1 (5.0158 g, 0.0258 mol, >99% yield by NMR). The compound was confirmed by 1H NMR. 1H NMR (300 MHz, CDCl3, δ ppm): 4.25 (m, 2H), 3.836 (m, 2H), 3.736 (m, 2H), 2.828 (m, 2H), 2.737 (m, 2H), 2.672 (m, 2H), 2.435 (s, 2H). Synthesis of 2-((3-(2-(Acryloyloxy)ethoxy)-3-oxopropyl)thio)ethyl Acrylate (TMADA, Compound 2). Compound 1 (5.0158 g, 0.0258 mol) and N,N-dimethylaminopyridiene (DMAP, 1.5882 g, 0.013 mol) were added in a round-bottom flask containing a magnetic stirrer bar, and to these solids 200 mL of DCM and acrylic acid (7.75 g, 0.0608 mol) were added. The reaction mixture was cooled to 0 °C, and EDC (14.6884 g, 0.0945 mol) was added. The solution was allowed to warm to room temperature and stirred for 24 h at room temperature. After the reaction, the DCM phase was washed with 0.2 M HCl (7 × 150 mL), followed by water (2 × 150 mL) and brine (1 × 150 mL, 1 wt %). The solvent was removed under reduced pressure to give the compound 2 (4.5923 g, 0.0152 mol, 59% yield) as a pale yellow viscous liquid. The compound was confirmed by 1H NMR. 1H NMR (500 MHz, CDCl3, δ ppm): 6.42 (dd, J1 = 17.5 Hz, J2 = 7.66 Hz, 2H), 6.13 (m, 2H), 5.85 (dd, J1 = 17.6 Hz, J2 = 7.1 Hz, 2H), 4.36 (m, 4H), 4.31 (t, J = 6.8 Hz, 2H), 2.85 (t, J = 7.3 Hz, 2H), 2.8 (t, J = 6.9 Hz, 2H), 2.66 (t, J = 7.3 Hz, 2H). 13C NMR (125 MHz, CDCl3, δ ppm): 171.5, 165.9, 165.8, 161.6, 161.3, 128.1, 127.9, 63.6, 62.4, 62.2, 34.7, 30.5, 27.2. ESI/MS = 325 (TMADA + Na+).

Table 1. Properties of Materials, Stress at Break, Strain at Break, Tg, Modulus, and Swelling Ratio in Water, Dichloromethane, and Hexane swelling ratio sample

peak stress (kPa)

strain at break (mm/mm)

Young’s modulus (kPa)

Tg (°C)

H2O

DCM

hexane

PHEA−1.5% TMADA PHEA−4% TMADA PHEA−8% TMADA

220 ± 40 340 ± 90 370 ± 40

4.2 ± 0.6 1.8 ± 0.2 1.1 ± 0.2

100 ± 20 360 ± 80 620 ± 60

9±1 13 ± 1 16 ± 1

7.8 ± 0.1 4.6 ± 0.1 2.57 ± 0.01

1.48 ± 0.01 1.47 ± 0.01 1.32 ± 0.03

1.0 1.0 1.0

B

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Macromolecules ⎛ 1⎞ σeng = G⎜λ − 2 ⎟ ⎝ λ ⎠

Michael reactions.47−50 The TM−VK was diluted to 5 wt % in N,N-dimethylformamide (DMF) and combined with 2hydroxyethyl acrylate (HEA) and heated at 90 °C for different lengths of time. Similarly, the TM−HEA adduct was diluted to 5% and combined with the VK and heated at 90 °C for different lengths of time. The stacked NMR data may be found in Figure S1 for the HEA/VK exchange study showing the clear exchange of HEA and VK vinyl protons. As shown in Figure 1B, there is a gradual shift of composition from the vinyl

(1)

where σeng is engineering stress, G is the shear modulus, and λ is the stretch ratio. Shear modulus was found for each sample by fitting eq 1 to the experimental mechanical test data, and then elastic modulus (E) was found from eq 2:

E = 2G(1 + υ)

(2)

where ν represents Poisson’s ratio (taken as 0.5 for an incompressible material). The mean Young’s modulus for each cross-linker concentration is given in Table 1. Stress Relaxation Test. An Instron 3344 apparatus equipped with a 100 N load cell was used to analyze stress relaxation at ambient temperature. For the PHEA−1.5% TMADA material and the PHEA− 4% TMADA material, the extension was increased at the rate of 1 mm/s until a strain of ε = 1 mm/mm was achieved. This strain of 100% was maintained while the stress was measured over time. For the PHEA−8% TMADA material, the extension was increased at the rate of 1 mm/s until a strain of ε = 0.2 mm/mm was achieved. This strain of 20% was maintained while the stress was measured over time. Creep Test. An Instron 3344 apparatus equipped with a 100 N load cell was used to analyze the extent of creep under load at ambient temperature. The PHEA−1.5% TMADA material and the PHEA−4% TMADA material were extended at the rate of 0.25 mm/s until a stress of 100 kPa was achieved. This stress of 100 kPa was maintained while the strain was measured over time. The PHEA−8% TMADA material was extended at the rate of 0.05 mm/s until a stress of 50 kPa was achieved. This stress of 50 kPa was maintained while the strain was measured over time. Rheology. A TA Instruments (New Castle, DE) Discovery HR-1 rheometer with a 20 mm cross-hatched plate geometry was used for rheological frequency sweeps (10−3−102 Hz) at 1% applied strain fitted, which was found to be in the linear region. The storage modulus and loss modulus is measured at 25 °C. Cutting and Healing Procedures. Materials were cut with a razor blade. Cut samples were completely cut through the thickness of the material. The two sections were placed in contact by gentle pressure from fingers for several seconds. Materials were healed at elevated temperatures by placing them in a preheated oven at 90 °C. Reshaping Materials. A series of PHEA−1.5% TMADA dogboneshaped material were twisted by 360° and fixed in this configuration. The materials were heated at 90 °C for different times ranging from 1 to 16 h, after which point the sample was released and allowed to relax. The angle between the two ends of the polymer was determined. Long-Term Stability and Creep Recovery. The PHEA−1.5% TMADA material was extended to a strain of ε = 1 mm/mm and fixed at this strain for 2 days. After that, the strain was released and creep recovery was determined by measuring the length of the material, and comparing to the length before strain, at specified time points. Swelling Ratio Determination. A small sample of dried TMADA cross-linked polymer was weighed placed in a glass vial. This sample was immersed with a large excess of solvent. At specified periods, the samples were taken out of solution, blotted to remove excess solvent, and weighed. This process was repeated every 24 h until the mass of the polymer + solvent increased by less than 5%.

Figure 1. Dynamics of exchange in the TM reaction. (A) Schematic of exchange between 4-hexen-3-one (VK) and 2-hydroxyethyl acrylate (HEA) and their adducts with 2-mercaptoethanol (TM−VK and TM−HEA, respectively). (B) Kinetics of exchange as determined by measuring the relative ratios of VK and acrylic vinyl protons.

ketone-based adduct to the acrylate-based adduct. This clearly indicates that the thiol−Michael adduct can be made dynamic with thermal stimulus. Within 24 h there is 95% conversion from the ketone to the acrylate adducts, consistent with literature data suggesting that vinyl ketone-based TM adducts are less stable than acrylic ester-based TM adducts.31,32 When starting from the TM−HEA adduct, the same equilibrium is reached as indicated in Figure 1B, although the shift is much smaller due to the TM−HEA adduct being more stable. Similarly, Figure 2 shows the exchange of 2-hexen-1-al, a conjugated vinyl aldehyde (VA), and 4-hexen-3-one, a conjugated vinyl ketone (VK), and their TM adducts. Aldehydes are useful in this study since the aldehyde proton is shifted downfield to ∼9 ppm, away from the other overlapping peaks. This allows determination of the relative fraction of adduct and free aldehyde, as indicated in Figure S2. These data clearly show a common equilibrium of 70% free 2hexen-1-al (VA) and 30% aldehyde adduct (TM−VA), regardless of whether the system started with the TM−VA exchanging with free VK or the TM−VK adduct exchanging with the free VA. These data are consistent with the idea of a dynamic equilibrium between the thiol−Michael adducts which is needed for dynamic covalent chemistry and materials.

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RESULTS AND DISCUSSION Dynamic Exchange in the Model Thiol−Michael Reactions. To prove the inherent dynamics of the thiol− Michael adducts under thermal stimulus, a series of model dynamic exchange reactions were performed. Initially, an adduct between 4-hexen-3-one, (VK), a conjugated vinyl ketone, and 2-mercaptoethanol giving a thiol−Michael vinyl ketone adduct (TM−VK) as well as an adduct between 2mercaptoethanol and 2-hydroxyethyl acrylate (HEA) giving TM−HEA was formed, adapting literature syntheses of thiol− C

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Scheme 2. (A) Synthesis of Acrylate-Based Cross-Linker (Compounds 2, TMADA); (B) Synthesis of Polymer Networks Containing Dynamic Thiol−Michael Linkages

Figure 2. Dynamics of exchange in the TM reaction. (A) Schematic of exchange between 4-hexen-3-one (VK) and 2-hexen-1-al (VA) and their adducts with 2-mercaptoethanol (TM−VK and TM−VA). (B) Kinetics of exchange as determined by measuring the relative ratio of aldehyde proton of VA and TM−VA.

Although the aldehyde and ketone TM adducts and vinyl compounds are electronically similar, the fact that the TM−VK adduct is preferred at equilibrium over the TM−VA is possibly due to increased steric interactions around the TM−VA compound. Creation of a Thiol−Michael-Based Cross-Linker and Properties of Materials with TM-Based Linker. A thiol− acrylate (TMADA)-based thiol−Michael cross-linker was synthesized as a platform for dynamic networks as shown in Scheme 2A. This thiol−acrylate-based cross-linker is simple to prepare synthetically, and due to the relatively slow dynamics of the thiol−acrylate bond, it is likely to lead to dynamic upon stimulus yet mechanically stable materials. This cross-linker was used to make cross-linked polymers of 2-hydroxyethyl acrylate (HEA) with various compositions of TMADA as shown in Scheme 2B. All polymerization was performed at 65 °C. Tensile testing, swelling studies, differential scanning calorimetry, and rheology assessed the fundamental properties of the TMADAbased PHEA materials. These polymeric materials are elastic and not very sticky once dry, and for polymerization the conversion is typically in excess of 95%. Polymers of HEA were synthesized with 1.5, 4, and 8 wt % of the TMADA cross-linker and denoted PHEA− 1.5% TMADA, PHEA−4% TMADA, and PHEA−8% TMADA. The infrared (IR) spectrum of the PHEA−1.5% TMADA is given in Figure S3 and is consistent with other polymers of HEA.5 The IR spectra of PHEA−4% TMADA and PHEA−8% TMADA are similar to the IR spectrum of PHEA−1.5% TMADA. As shown in Figure 3 and Table 1, as the degree of cross-linker is increased, the Young’s modulus of the material increases. Representative stress (σ)−strain (ε) curves are given in Figures S4−S6. The glass transition temperatures of the materials were measured by differential scanning calorimetry (DSC). As the cross-linker content increases, the glass transition temperature (Tg) increases from approximately Tg

Figure 3. Typical stress−strain curves for PHEA−1.5, 4, and 8% TMADA materials.

= 9 °C for the PHEA−1.5% TMADA to approximately Tg = 16 °C PHEA−8% TMADA material. Raw DSC traces are given in Figures S7−S9, and the Tg was determined from the inflection point of the DSC curves. These results indicate that the materials were all soft at room temperature. These values are larger than those of T g = −15 °C for the PHEA homopolymer,51 presumably due to increased restriction to rotation due to the cross-linking. As shown in Table 1, as the material cross-link density increased, the ability to swell decreased in various solvents. As expected, since PHEA is a hydrogel forming material, it had the highest swelling ratio in water, with slight swelling in the organic solvent dichloromethane (DCM) and negligible swelling in hexanes. In addition to the properties outlined in Table 1, rheological studies were performed on the PHEA−1.5% TMADA and PHEA−8% TMADA materials. As indicated in Figure 4, the PHEA−1.5% TMADA material had a similar value of the storage and loss moduli that continuously increased with frequency. This is consistent with materials that are close to their gel point and observed in other dynamic covalently crosslinked materials.52 In contrast, the PHEA−8% TMADA had an D

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Figure 4. Rheology of sample materials PHEA−1.5% TMADA (A) and PHEA−8% TMADA (B) performed at 25 °C.

extended rubbery plateau in the storage modulus at lower frequencies typical of elastic cross-linked materials. Both samples had a faster rate of increase of loss modulus compared to storage modulus. This has been reported in elastomers with relatively low densities of cross-linkers and has been attributed to chain mobility in frequency sweep experiments.3,53,54 It is also important to note that the PHEA−8% TMADA had a larger storage and loss modulus than the PHEA−1.5% TMADA. Dynamics in a Polymer Network. Since the previous work indicated that thiol−Michael materials are able to effectively exchange, the dynamic properties of the thiol− Michael adduct in a polymeric material were investigated. A consequence of dynamic cross-linking is the ability to repair damages or “reheal”, also known as “self-healing”. Figure 5 shows the stress−strain curves for PHEA−1.5% TMADA materials, for an uncut material. Figure 5 also show stress− strain data well as materials that were cut in half, and reattached by pushing the two halves together, followed by heating at 90 °C for different times.

Figure 5A shows that the material is able to recover a significant proportion of its original mechanical properties. These represent the best performing materials studied. In fact, the recovery in the peak stress was ∼90%, and strain at break was ∼85% of the original material properties when comparing the best performing uncut material to the best performing healed materials. The stress−strain curves of the samples healed at 90 °C for 4 and 16 h PHEA−1.5% TMADA samples in Figure 5A are consistent with the stress−strain curves of uncut materials shown in Figure S4. In fact, several of the samples, the cut and healed PHEA−1.5% TMADA, broke in a place completely separate from the original fracture site as shown in Figure 5B. Similarly, Figure S10 indicates that the PHEA−4% TMADA material showed similar, but less successful, recovery of mechanical properties after healing. An important consideration is whether the PHEA material is itself intrinsically able to reheal. As shown in Figure S11, a poly(HEA) with the same fraction of the static cross-linker divinylbenzene (PHEA−1.5% DVB) showed a very limited extent of healing ca. 25% of the original mechanical properties after 4 h of heating. Similar results were observed when a diacrylate cross-linker (PEGDA) was used as a nondynamic cross-linker. This is significantly less than the ca. 85% recovery of mechanical properties of the TMADA material indicating that the dynamic thiol−Michael adducts are responsible for the observed properties. The ca. 25% recovery of mechanical properties in PHEA−1.5% DVB is most likely due to associations between the hydrogen bond donors and acceptors in the PHEA material. It is important to note that the material’s healing properties appear to be inversely proportional to the content of the crosslinker. This could be due to two factors. One observation for all of these materials is that they are not particularly sticky to touch. However, the PHEA−1.5% TMADA and PHEA−4% TMADA materials had the two ends stick together effectively almost as soon as the two ends were placed in contact. In contrast, the PHEA−8% TMADA containing material did not stick together to any appreciable extent as well as this material being very brittle. These data suggest that for PHEA-based materials there is additional binding that assists with the TMAbased healing. This additional linking is most likely due to supramolecular interactions caused by hydrogen bonding between the different PHEA units. The data suggest that with the higher content of the TMADA linker (PHEA−8% TMADA), the ability of the material to participate in rehealing is diminished. This is most likely due to the fact that with the higher cross-linker content the polymer chains are less mobile and the H-bonding assistance to the rehealing is rendered less

Figure 5. (A) Rehealing properties PHEA−1.5% TMADA after different times heated at 90 °C. (B) Photograph of a PHEA−1.5% TMADA indicating site of original cut and break point during mechanical testing after 4 h of healing at 90 °C. E

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Macromolecules effective. We anticipate that only a small fraction of the crosslinks will be dissociated under the conditions used in this study since the material retains its shape through the healing cycle. The small fraction of dissociated cross-links makes diffusion of reactive groups a determining factor in the rehealing. The greater mobility of chains and facility in entanglements offered by the lower cross-link density may assist the recovery of mechanical properties by entanglement and may facilitate healing by increased mobility. Malleability. An alternative measure of dynamics within the networks is the materials ability to be remolded into new shapes. This property is not present in traditional thermoset materials, although the introduction of dynamic bonds introduces malleability since the exchanging bonds will relieve stresses and allow a distorted shape to be the new permanent shape. To test this ability, the 1.5% TMDA material was twisted 360° and left in the oven at 90 °C for different times (1, 4, 9, and 16 h). The material was allowed to fully relax, and as shown in Figure 6A, the material became increasingly twisted and

Figure 7. (A) Creep and stress relaxation of the PHEA−1.5% TMADA materials. In the creep experiment a 100 kPa stress was applied, while in the stress-relaxation experiment the material was strained to a value of ε = 1. (B) Creep recovery as a function of time for the PHEA−1.5% TMADA material after being released from 100% strain for 2 days. Both studies were performed at ambient temperature.

experiment in Figure 7A shows that in the same time frame of 3 h the creep experiment settled to a constant strain of ε = 1.5 mm/mm under the 100 kPa tensile stress. This clearly shows that the PHEA−1.5% TMADA material displays a finite stress relaxation and creep, consistent with the idea of dynamic covalent bonding that requires external activation such as thermal stimulus. Similar results are seen in Figure S12 for the PHEA−4% TMADA material, although PHEA−4% TMADA material sample required just 2 h to reach equilibrium strain of ε = 0.5 mm/mm in the creep experiment and equilibrium stress of 0.8 of the peak stress (σpeak) in the same time frame of 2 h in the stress relaxation experiment. Finally, Figure S13 shows the stress relaxation and creep data for the PHEA−8% TMADA material under a strain of 0.2 for the stress-relaxation experiment and a stress of 50 kPa for the creep experiment, The PHEA−8% TMADA sample required less than 1 h to reach mechanical equilibrium of a strain of 0.25 in the creep experiment and 0.9 of the peak stress (σpeak) in the stress relaxation experiment. Since the PHEA−1.5% TMADA materials displayed a greater tendency to participate in creep and stress relaxation, these samples were subjected to long-term stability studies. The PHEA−1.5% TMADA material was extended to a strain of ε = 1 mm/mm and fixed in place for 2 days. After this time the strain was released the material was allowed to recover. As shown in Figure 7B, after releasing from the strain the material quickly recovered its original shape, returning to a strain of ε = 0.1 mm/mm within 1 h and within 16 h returning to its original size within experimental error. This suggests that the creep or stress relaxation observed in Figure 7A is not permanent. Likely contributions to the observed creep and stress relaxation are polymer chain rearrangement and rearrangements of hydrogen

Figure 6. (A) Visual depiction of malleability properties of PHEA− 1.5% TMADA. Samples were heated in a 360° twisted configuration at 90 °C for different times. (B) Measured angle between the top and bottom of the material displayed as a function of the heating time.

closer to the 360° target conformation with longer times. As shown in Figure 6B, the angle between the two flat ends of the dogbone-shaped material increases gradually over time, eventually reaching approximately 75% of the targeted 360°. This is broadly consistent with the recovery in stress−strain data, with differences due to the distinct nature of the experiments. Stability. An important factor to balance against the dynamics of the material is its stability. In fact, the tendency of dynamically cross-linked materials to creep under load is a significant limitation toward high stress applications. Therefore, the long-term stability of these materials was tested under stress and strain. Figure 7A shows the stress relaxation at 100% strain as well as creep under a stress of 100 kPa for the PHEA−1.5% TMADA material. As seen in Figure 7A, the stress relaxation reaches an asymptote of approximately 75% of the initial stress. This long-term stress value under a tensile strain of ε = 1 mm/ mm was reached after approximately 3 h. Similarly, the creep F

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bonds between units of polymerized HEA. However, the TMADA-based cross-links are essentially static due to the very slow exchange and room temperature. However, as seen in Figure 6, these bonds are dynamic and lead to rearrangement upon heating. This is similar to other dynamic-covalent-based materials which also have a balance of limited creep and stress relaxation, although the essentially static dynamic covalent bonds at room temperature lead to negligible permanent deformation over time.5 Proposed Mechanism of Dynamic Exchange in Polymeric Materials. The results above indicate that the PHEA materials cross-linked with the thiol−acrylate linker display dynamic properties such as heal ability and malleability. The proposed mechanism for this dynamic exchange is that there is first a thermally promoted retro-Michael reaction, creating a small amount of free thiol and acrylate in the polymer followed by rapid reassociation to achieve dynamic exchange. This is indicated in Scheme 3.

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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.6b01061. Additional figures of NMR data, tensile testing, differential scanning calorimetry, creep and stress relaxation data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Teresa Ramelot and Dr. Sameer AlAbdul Wahid for experimental assistance. This work was supported by Start-Up and CFR funding from Miami University.

Scheme 3. Proposed Mechanism of Dynamic Exchange of Thiol−Michael Adducts under Thermal Stimulus

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REFERENCES

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This mechanism of thermal dissociation of the thiol−Michael adduct is consistent with literature data, which suggests that thiol−Michael adducts can dissociate at elevated temperatures.32 This mechanism is also consistent with the dynamic exchange in the first section, since the TM adducts (TM−HEA, TM−VK, and TM−VA) could dissociate under thermal stimulus to give the thiol and the conjugated alkene.32 The formed thiol can subsequently react with conjugated double bonds giving rise to dynamic exchange. This implies that in the polymeric material a small fraction of the TM adducts will undergo retro-Michael addition under thermal stimulus, followed by dynamic exchange to giving the healable and malleable properties at elevated temperature. The creep and stress relaxation data suggest that at lower temperatures negligible dynamic exchange occurs due to full creep recovery even after 2 days at a strain of 1 mm/mm.

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CONCLUSIONS The thiol−Michael adduct has been shown to be dynamic at elevated temperatures of 90 °C. Small molecule and polymer network studies have demonstrated the dynamic nature of the thiol−Michael adduct. Polymers of 2-hydroxyethyl acrylate (HEA) were cross-linked with a thiol−acrylate-based crosslinker (TMADA), and these materials displayed the dynamic properties such as rehealing and malleability. The TMADAbased material’s ability to heal after being cut was found to be 3−4 times more efficient than control experiments using divinylbenzene as the cross-linker. Additionally, the materials were determined to be stable under stress and strain under ambient conditions, suggesting that the thiol−Michael linkage is thermally activated. This work paves the way for the easily synthesized thiol−Michael adducts to be used as a thermally activated dynamic linker in various applications such as coatings, sealants, and high performance elastomers, especially those where stability against creep is needed at room temperature. G

DOI: 10.1021/acs.macromol.6b01061 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01061 Macromolecules XXXX, XXX, XXX−XXX