Article pubs.acs.org/Macromolecules
Malleable, Mechanically Strong, and Adaptive Elastomers Enabled by Interfacial Exchangeable Bonds Zhenghai Tang,† Yingjun Liu,† Baochun Guo,*,† and Liqun Zhang*,‡ †
Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, P. R. China State Key Laboratory of Organic/Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China
‡
S Supporting Information *
ABSTRACT: Reinforcement, recycling, and functional applications are three important issues in elastomer science and engineering. It is of great importance, but rarely achievable, to integrate these properties into elastomers. Herein, we report a simple way to prepare covalently cross-linked yet recyclable, robust, and macroscopically responsive elastomer vitrimers by engineering exchangeable bonds into rubber−carbon nanodot (CD) interphase using CD as high-functionality cross-linker. The cross-linked rubbers can rearrange the network topology through transesterification reactions in the interphase, conferring the materials the ability to be recycled, reshaped, and welded. The relatively short chains bridging adjacent CD are highly stretched and preferentially rupture to dissipate energy under external force, resulting in remarkable improvements on the mechanical properties. Moreover, the malleable and welding properties allow the samples to access reconfigurable/multiple shape memory effects.
1. INTRODUCTION Covalently cross-linked thermosets possess outstanding mechanical properties and chemical resistance. However, reshaping and recycling thermosets are inherently difficult because they are insoluble and infusible once complete curing, in contrast to thermoplastics that can be repetitively processed and recycled. To combine the advantages of thermosets with plasticity, intense effort has been devoted to incorporating elegant dynamic covalent bonds into cross-linked polymer networks.1,2 These networks can rearrange themselves in a reversible manner under external stimuli, imparting the materials with adaptability, malleability, and self-healing.2−4 Typically, previous studies have largely focused on reversible chemical cross-links based on Diels−Alder chemistry. Unfortunately, the retro-Diels−Alder reaction results in a temporary decrease in network connectivity and gel-to-sol transition upon heating, which is undesirable for the majority of structural applications.5,6 Vitrimers, recently pioneered by Leibler and co-workers, are a novel class of covalent networks that are able to alter the topology through thermally triggered exchange reactions.7 Because of the associative nature of exchange reactions, the numbers of chemical bonds and cross-links remain constant during network rearrangements, and vitrimers are insoluble irrespective of temperature. In addition, the viscosity of vitrimers is essentially controlled by exchange reaction rate, which leads to Arrhenius-like gradual viscosity variations like those of vitreous silica, rather than the abrupt viscosity change near the glass transition of thermoplastics.8,9 The fascinating properties of vitrimers were initially demonstrated in an © XXXX American Chemical Society
epoxy−acid system through a catalytic transesterification reaction.7 Currently, many other chemical transformations such as olefin metathesis,10 transalkylation,11 disulfide exchange,12 imine exchange,13−15 transamination,16,17 boronic ester exchange,18−20 and transcarbamoylation21 have been explored to improve and expand both the performance and sustainability of vitrimer materials. Since their discovery, vitrimers hold great promising applications in liquid-crystalline elastomer actuators,22 nanoimprint lithography,23 shape memory polymers,24 recyclable 3D printing materials,25 and selfhealing materials.20,26,27 To date, various nanofillers such as silica,28 nanotubes,27 and nanofibers29,30 have been incorporated into vitrimers to improve their mechanical properties and/or impart functional properties. For examples, Ji et al. demonstrated that epoxy vitrimers with carbon nanotubes (CNTs) could be welded by light because CNTs can absorb light and transform the energy to heat to activate the transesterfication reaction.27 Legrand et al. prepared silicareinforced epoxy vitrimer nanocomposites and studied the effect of silica surface chemistry on the relaxation process. When epoxy group-functionalized silica were covalently linked to the network through exchangeable β-hydroxy ester bonds, the exchangeable bonds in the interphase underwent transesterification reactions and speeded up the relaxation of composites compared to nonfunctionalized silica. In their work, the vitrimer network are covalently cross-linked with fatty Received: June 14, 2017 Revised: September 18, 2017
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DOI: 10.1021/acs.macromol.7b01261 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
glacial acetic acid were obtained from Tianjin Fuyu Fine Chemical Co. Ltd. All the chemicals are used as received. Preparation of CD. CD was synthesized using a simple bottom-up approach.42 Briefly, 1 mL of glacial acetic acid containing 80 μL of water was quickly added to 2.5 g of P2O5 without stirring. The mixture was dramatically foamed by the spontaneous heat. After cooling to room temperature, the obtained product was purified via dialysis over deionized water for 48 h with a dialysis bag (retained molecular weight 1000 Da). Finally, the CD aqueous solution was dried to obtain CD powder. The concentration of carboxyl groups on CD was titrated as 5.29 mmol/g. TEM image showed that CD had an average size of about 5 nm (Figure S1). The UV spectrum of CD exhibited characteristic absorption peaks at 250 nm and a shoulder at 304 nm, which were due to π−π* transition of aromatic CC and n−π* transition of CO, respectively (Figure S2).43 Preparation of Cured ENR Samples. Desired amounts of CD, Zn(Ac)2, and DMI were successively compounded with ENR on a two-roll mill for about 15 min. After mixing, the compounds were subjected to compression at 180 °C for the optimum curing time determined by a vulcameter. The stoichiometry of the oxirane and carboxyl groups was controlled to be 50, 40, 35, 30, and 25. The quantity of Zn(Ac)2 and DMI were 10 and 25 mol % relative to the carboxyl group, respectively. DMI acts as accelerator to promote the cross-linking kinetics,39,40 and Zn(Ac)2 can promote both esterification of epoxy/acids and subsequent transesterification between hydroxyl and ester groups.9,44 The formulations are listed in Table S1. In the context, sample code of CE-x refers to CD cross-linked ENR with oxirane group:carboxyl group of x. For comparison, the small molecule sebacic acid cross-linked ENR sample (SE, oxirane group:carboxyl group=35) was prepared according to the aforementioned protocols. Characterizations. Transmission electron microscopy (TEM) was performed on Tecnai G2 F30 S-Twin electron microscope. The sample was prepared by dropping CD aqueous solution onto a copper grid coated with carbon film. The concentration of carboxyl groups on CD was determined by the potentiometric titration method using a MetrOhm 888 Titrando autotitrator. The UV−vis absorption spectrum was recorded using a Scinco S3100 UV−vis spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Kratos Axis Ultra DLD with Al Kα radiation (1486.6 eV). Fourier transform infrared spectroscopy (FTIR) was collected on a Bruker Vertex 70 FTIR spectrometer equipped with a heating cell. Thermal gravimetric analysis (TGA) was performed on a TA Q50 thermogravimetric analyzer under a nitrogen atmosphere. Curing characteristics were determined at 180 °C using a U-CAN UR-2030 vulcameter. Tensile tests were performed on dogbone-shaped samples (ca. 60 mm × 4 mm × 1 mm and a gauge length of 25 mm) using a U-CAN UT-2060 instrument at room temperature. For uniaxial tensile tests, the extension rate was 500 mm/min, and Young’s modulus was calculated by taking the slope of the stress−strain curve from 0.5 to 3% strain. Six specimens were measured for each sample, and the average value and standard deviation were calculated. For cyclic tensile tests, both loading and unloading were conducted with an extension rate of 100 mm/min. The sample was first stretched to a strain of 100% and then underwent unloading. The sample was allowed to relax at room temperature for 10 min prior to the subsequent loading process. After three cycles, the sample was subsequently subjected to another three loading−unloading cycles with a strain of 300%. Equilibrium swelling experiments were conducted by immersing vulcanizations in toluene at room temperature for 3 days. The swelling ratio is defined as (m1 − m2)/m2, and sol fraction is determined as (m0 − m2)/m0, where m0 is the weight of the sample before swelling, m1 and m2 are the mass of the swollen and unswollen sample, respectively. The cross-linking density was calculated based on the classical Flory− Rehner equation.45 Details can be found in the Supporting Information. Three specimens were measured for each sample. The tests including stress relaxation experiments, creep experiments, cyclic strain recovery tests, dynamic mechanical analysis (DMA), and dilatometry experiments were performed on rectangular specimen (10
acids, and the mole ratio of epoxy groups in bisphenol A diglycidyl ether and carboxyl groups in fatty acids is 1.28 As a class of thermosetting polymers, cross-linked rubbers with high elasticity are strategically important materials due to their irreplaceable applications in tires, seals, and shock absorbers. Reinforcement, recycling, and functional applications are three important issues in elastomer science and engineering. First, the reinforcement of rubbers is essential as almost all neat rubbers suffer from poor mechanical properties. Many methodologies such as the addition of nanofillers31 and introduction of energy-dissipating mechanisms based on slidering structure32 and dual-cross-linking concept33 have been developed to improve the mechanical performance of rubbers. Second, the irreversible chemical cross-links make the recycling of rubber products practical impossibility. About 30 million tons of rubber products are scrapped each year, and the bulk of which is burnt for energy recovery and dumped as landfill, causing seriously environmental pollution and resource waste.34 Lastly, expanding the application galleries of rubbers for advanced functional applications, together with the benefit of high elasticity, has drawn growing attention.35−37 Epoxidized natural rubber (ENR) is obtained by epoxidation of natural rubber and retains most of its properties. Because of the introduction of oxirane groups, ENR acquires improvements on oil resistance, gas barrier property, and wet grip. Recently, Norvez et al. reported that ENR could be cross-linked by dicarboxylic acids reacting with the oxirane groups, producing β-hydroxy esters along the chain.38 The crosslinking kinetics was greatly accelerated by adding 25 mol % (relative to carboxyl groups) or higher 1,2-dimethylimidazole (DMI) as an accelerator. The dicarboxylic acid-cross-linked ENR exhibited improvements on the thermal stability and dynamic performance, which could compete with conventional vulcanization processes using sulfur or peroxide formulations.39,40 More recently, Norvez et al. incorporated a transesterification catalyst (zinc acetate, Zn(Ac)2) into dodecanedioic acid-cross-linked ENR; the resulting networks could relax stress at high temperatures and exhibited vitrimerlike properties.41 In this work, we report a simple method to prepare covalently cross-linked yet recyclable, mechanically strong, and macroscopically responsive elastomer vitrimers by incorporating carboxyl group-functionalized carbon nanodot (CD) into ENR to engineer exchangeable bonds into the ENR−CD interphase. CD with numerous carboxyls on the surface are utilized as cross-linker to covalently cross-link ENR through exchangeable β-hydroxyl ester linkages. The prepared CD-cross-linked ENR samples (CE) can alter network topology and relax stress via transesterification reactions in the ENR−CD interphase at elevated temperatures, endowing them with the ability to be recycled, reshaped, and welded. Meanwhile, the relatively short rubber chains between adjacent CD preferentially rupture, i.e., function as sacrificial bonds under external load, to dissipate energy and reinforce ENR. Moreover, we demonstrate that CE can be used as reconfigurable/multiple shape memory materials.
2. EXPERIMENTAL SECTION Materials. ENR with an epoxidization degree of 50% was kindly provided by the Agricultural Products Processing Research Institute, Chinese Academy of Tropical Agricultural Science, China. Sebacic acid (99%), 1,2-dimethylimidazole (DMI, 99%), and zinc acetate (Zn(Ac)2, 98%) were purchased from Sigma-Aldrich. Phosphoric anhydride and B
DOI: 10.1021/acs.macromol.7b01261 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. (a) FTIR spectra of CD, uncured CE-35, and cured CE-35. (b) Evolutions of carboxyl groups (1820−1560 cm−1) and epoxy (960−760 cm−1) regions of FTIR spectra during the curing of CE-35 at 180 °C. mm × 4 mm × 0.5 mm) by using a TA-Q800 DMA apparatus. For stress relaxation experiments, the sample was initially preloaded by 1 × 10−3 N force to maintain straightness. After reaching the testing temperature, the sample was equilibrated for 15 min. The specimen was then stretched a constant strain of 5%, and the deformation was maintained throughout the test; the stress decay was monitored over time. For creep experiments, a nominal stress of 0.1 MPa was applied on the sample after a 10 min temperature equilibration. For cyclic strain recovery tests, the stress applied on the sample was alternated between 0.1 MPa for 60 min and 0 MPa for 10 min in each cycle. For DMA investigations, a tensile mode was adopted with a dynamic strain of 0.5%. The sample was scanned from −60 to 200 °C, and the frequency and heating rate were 5 Hz and 3 °C/min, respectively. For dilatometry experiments, the sample length was measured when it was heated from −50 to 250 °C at 3 °C/min, and a weak stress of 10 kPa was applied to avoid buckling throughout the experiment. Quantitative shape memory measurements were performed using a TA DMA Q800. After equilibrium at 70 °C, the rectangular specimen was stretched to a set force of 0.5 at 0.2 N/min. The sample was then cooled at a rate of 5 °C/min to −10 °C before the load was removed to fix the temporary shape. After that, the force exerted on the samples was unloaded at 0.2 N/min until it reached the preload force of 10−3 N. The permanent shape was recovered by heating at 5 °C/min to 70 °C and holding isothermal for 5 min. The permanent shape was reprogrammed by stretching the samples with a constant force of 0.2 N for 45 min at 200 °C followed by a recover time of 10 min. To run a second shape memory cycle, a new shape memory cycle was executed using the parameters described above.
groups during curing are monitored using FTIR at 180 °C. As the curing proceeds, the absorption for oxirane rings at 877 cm−1 gradually decreases while that for ester groups at 1724 cm−1 conversely increases. The traces showed that the reaction was complete within 20 min (Figure 1b). The evolutions of the FTIR absorptions of oxirane and ester groups exhibit a similar change trend in SE by heating at 180 °C (Figure S3). The cross-linking kinetics of CE-35 and SE are determined by monitoring the torque at 180 °C in a rheometer (Figure S4). A sharp increase in the torque indicates the beginning of the cross-linking reaction. Then a torque plateau is reached within 30 min, showing that the cross-linking reaction is complete. Equilibrium swelling tests show that CE and SE samples are insoluble in toluene, manifesting their covalently cross-linked molecular architecture. The low sol fraction (
