Letter Cite This: ACS Macro Lett. 2019, 8, 582−587
pubs.acs.org/macroletters
Tunable and Processable Shape-Memory Materials Based on Solvent-Free, Catalyst-Free Polycondensation between Formaldehyde and Diamine at Room Temperature Hengxin Lei,†,‡ Shengnan Wang,†,§ Der Jang Liaw,∥ Yilong Cheng,‡ Xutong Yang,⊥ Jidong Tan,‡ Xingxing Chen,‡ Junwei Gu,*,⊥ and Yanfeng Zhang*,‡
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‡
Department of Applied Chemistry, School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter (Xi’an Jiaotong University), Xi’an Key Laboratory of Sustainable Energy Materials Chemistry and State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China § Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan 621900, China ∥ Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ⊥ MOE Key Laboratory of Material Physics and Chemistry Under Extraordinary Conditions, Shaanxi Key Laboratory of Macromolecular Science and Technology, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China S Supporting Information *
ABSTRACT: Compared with traditional thermosets, malleable thermosets have more applications in aerospace, biotechnology, and construction. Here we report a one-step, solvent-free, catalyst-free polycondensation method between diamine and formaldehyde to prepare a series of malleable hemiaminal dynamic covalent networks (HDCNs). The materials have excellent malleability and reprocessability by hot pressing. The Young’s modulus and breaking strength of HDCNs obtained by the polycondensation of formaldehyde and 4,4-diaminodiphenylmethane (MDA) are as high as 1.6 GPa and 60 MPa, respectively, which can be facilely adjusted through the introduction of polyetheramine-400 (PEDA). Moreover, the HDCNs feature the shape memory ability with a recovery ratio above 93.5% and can be recycled by the addition of different monomers. This promising HDCN, prepared from economical raw materials, may have vast applications in industries.
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and environmentally friendly thermosets, such as carbon radical exchange,6−8 Diels−Alder reaction,9−11 disulfide bond formation,12−15 disulfide-thiol exchange,16,17 imine chemistry,18−21 amine chemistry,22,23 transcarbamoylation,24 triazolinedione chemistry,25−27 hindered urea bonds,28,29 transalkylation,30 thiol-aldehyde addition,31 transesterification,32 and boronic ester formation.33−38 The thermosets synthesized with the above-mentioned bonds can be processed repeatedly in molds at a specific temperature. Based on this concept, scientists have also tried to introduce shape memory ability into the malleable thermosets, which greatly expands the application range of dynamic networks. The programmed shapes of shape memory polymers (SMPs) can be fixed temporarily and recovered as required. Currently, a variety of SMPs based on dynamic bonds have been reported, which performed shape memory under the driving of heat,39−46 light,47−52 solvent,53 and change of pH.42 However, the above methods mostly required the addition of catalysts or harsh
hermoset materials have a wide range of applications in aerospace, construction, and daily life, mainly due to the excellent mechanical properties, solvent and wear resistance, and load carrying capacity of the covalently cross-linked structures.1 However, it is well-known that traditional thermosets are difficult to recycle and reprocess, since they cannot be remolded once cured.2 On the contrary, the thermoplastics could be converted into a flowing liquid under heating and be processed again, as well as good reprocessability and recyclability, but the mechanical properties are much poorer compared with thermosets. Hence, the development of materials featuring good mechanical strength, reprocessability, and recyclability is of importance for industrial applications and environmental protection. Researchers have been trying to impart plasticity to thermosets by introducing dynamic covalent bonds that promote stress relaxation and reversible depolymerization through the bond exchange.3−5 In 2011, Leibler and coworkers reported the reversible and malleable thermoset for the first time based on transesterification.5 Since then, various dynamic bonds have been introduced into cross-linked networks to develop malleable, self-healing, reprocessable, © XXXX American Chemical Society
Received: March 18, 2019 Accepted: April 29, 2019
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DOI: 10.1021/acsmacrolett.9b00199 ACS Macro Lett. 2019, 8, 582−587
Letter
ACS Macro Letters
probe the viability of hemiaminal (HA) formation further, we studied aniline with formaldehyde as a small molecule model system by NMR analysis in Figure S2. Compared with the formation of 1,3,5-hexahydro-1,3,5-triazine at 185 °C, the products formed at 25 °C showed −OH and −NH2 signals at δ 6.0 ppm and δ 4.6 ppm, respectively, which confirmed successful conditions for HA formation in the model system (Figure S2a). In addition, there was no signal observed corresponding to the carbon in imine, which features a signal around 150−170 ppm, as shown in Figure S2b. The Tg of MDA HDCN was 123 °C (Figure S3), and about 10 wt % of MDA HDCN were decomposed when the temperature got to 250 °C (Figure S4). In addition, it was found that the increasing of PEDA content in the HDCNs led to the decreasing of the Tg, which may enable the adjusting of the flexibility of HDCNs, as shown in Figure S5. In order to test mechanical properties, dumbbell-liked sheets were prepared by the hot pressing of the HDCNs powder. The tensile results in Figure S6 show that MDA HDCN exhibited a high Young’s modulus (1.6 GPa) and breaking strength (60.3 MPa). We also performed the SEM to observe the morphology of fracture surfaces; the image showed that the brittle fracture surface was homogeneous (Figure S7). We also tested the mechanical properties of HDCNs incorporating the flexible segments (PEDA). With the content of PEDA increasing from zero to 50%, the Young’s modulus was adjustable from 1.6 to 0.1 GPa with a 3 times increase of the strain, which proved that the flexibility of the materials was obviously improved (Figure S8). It was shown that when the molar ratio of PEDA was above 30%, the Young’s modulus of the materials was still higher than 0.8 GPa. Additionally, it was found that MDA HDCN exhibited good solvent resistance (Figure S9), and only swelling was observed in organic solvents instead of dissolving. MDA HDCN also exhibited a low swelling rate after immersing in water, and the mechanical properties were not significantly reduced, as shown in Figure S9b,c. First, we investigated the shape memory behavior of the HDCNs. In Figure 1a, the quantitative shape memory cycle for MDA HDCN was obtained by dynamic thermomechanical analysis (DMA). The sample was stretched at 140 °C with a pressure of 0.7 MPa, and the strain was fixed upon cooling to room temperature. The shape recovery rate (Rr) was calculated to be 93.5% at 140 °C under stress-free conditions, confirming a good shape-memory property. When the temperature was higher than the Tg of HDCN, the movement of the polymer was set free to allow the shape programming of the materials upon the stress. Subsequently, a temporary shape was obtained below the Tg due to the frozen molecular chain after removing the stress; once the material was heated above Tg, the locked molecular chain was activated again, and the shape returned back to its original state. A visualized version for the shape memory behavior is provided in Figure 1b. The square MDA HDCN sample was processed into a windmill shape at 140 °C, followed by shape fixing at room temperature. When heated to 140 °C again, the sample recovered to its original square shape within 30 min. Subsequently, it was reprogrammed (at 140 °C) to a more complex shape (windmill-shaped) and it could also recover to the original windmill shape after a temporary shape fixing. The permanent shape reconfiguration and temporary shape fixing were achieved in a combined shape deformation cycle. To investigate if the incorporation of flexible segments into HDCNs improved shape memory ability, we also tested the
conditions, and the commodity raw materials are rarely available. Recently, the Hedrick group reported a simple one-pot polycondensation at 50 °C that formed a kind of nitrogenbased thermoset with hemiaminal dynamic covalent networks, which could be digested at low pH (
