Fig 1 Synthetic route to BPU.
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Covalent crosslinking points within thermosets generally result in excellent mechanical properties and solvent resistance yet lead to limited degradability and recyclability. Those thermosets become degradable or recyclable if crosslinking points are cleavable or reversible. Following this principle, we report a kind of polyurea-urethane thermoset with borate ester as its crosslinking point to enable a controllable de-crosslinking in response to acetate acid. Such a thermoset presents remarkable mechanical properties as well as outstanding solvent resistance capability, due to the high crosslinking density and intermolecular hydrogen bonding. Furthermore, the de-crosslinked product can be reporcessed to generate a brand new thermoplastic material.
Recyclable thermosets;
Borate ester;
Polyurea-urethane;
Stimuli-cleavage
Owing to the excellent mechanical properties, solvent resistance, and chemical stability, thermosets have been widely applied in coatings, composites, electronic packaging, and so on.[
Amongst all types of promising stimuli-cleavable linkages, borate ester unit possesses unique tripodal molecular structure that can endow the polymer networks with considerable crosslinking density and controllable degradability. Yet the extreme water sensitivity inevitably causes the instability of borate-ester-based thermosets in humid environments.[
Herein, we leveraged the activity of borate ester to design a novel recyclable thermoset which serves as an eco-friendly polymer to cope with the problems of polymer waste. Using polyurea-urethane as the main chain, this thermoset possessed outstanding mechanical strength and solvent resistance capability. Based on the stimuli-cleavable feature of boronic ester linkages, the thermoset could be specifically de-crosslinked under acetic acid environment, and dissociation products could be further reprocessed to form a linear polyurea-urethane copolymer.
The well-designed one-step synthesis of borate-crosslinked polyurea-urethane (BPU) is depicted in
Fig 1 Synthetic route to BPU.
Poly(ethylene glycol) (PEG, Mw=400 g/mol), hexamethylene diisocyanate (HDI), acetic acid, dioxane, and dibutyltin dilaurate (DBDTL) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Boric acid was supplied by Beijing Chemical Reagent Company. All reagents were used without further purification.
The borate crosslinked polyurea-urethane was synthesized via one-pot reaction among PEG-400 (Mw=400 g/mol, 4.0 g, 10 mmol), HDI (1.060 mL, 13.75 mmol), and H3BO3 (155.0 mg, 2.5 mmol) under the catalysis by DBDTL (40 μL, 0.5%). PEG-400 and H3BO3 were dissolved in 4.0 mL of dioxane. Then HDI and DBDTL were successively mixed with the above solution and the reaction was carried out on a glass plate at 60 °C for 6 h. The cured film was then transferred to a vacuum oven at 80 °C overnight to evaporate the remaining solvent and cooled to room temperature to obtain the BPU film. Finally, the BPU film (named as PEG-4-400) was processed into predesigned shapes (the 4 referred to the molar ratio of PEG-400:H3BO3=4:1 and the 400 referred to the molecular weight of PEG). Other BPU films with different ratios of raw materials were prepared in the same way.
The solid-state 13C-NMR and 11B-NMR spectra of BPU were recorded on Bruker Avance 600 MHz NMR spectrometer. The reflection Fourier infrared spectroscopy (FTIR) spectra were measured on infrared spectrometer (Tensor 27, Bruker). Thermogravimetric analysis (TGA) was performed on a TA INSTRUMENTS Q50. Sample was typically loaded on a platinum pan and the temperature was ramped to 600 °C at a rate of 10 °C/min under nitrogen atmosphere. Differential scanning calorimetry (DSC) was performed on a DSC8000 (PerkinElmer) apparatus. Two heating cycles between −50 and 200 °C were collected at a heating rate of 10 °C/min. The tensile test of the material was carried out by Instron tensile tester, and the tensile rate was 10 cm/min at room temperature. The Loss factor and storage modulus were collected by a dynamic mechanical analysis (DMA) machine (Q800). Storage modulus and loss factor were specified via a temperature scan method with a temperature ramping rate of 5 °C/min at fixed frequency (1 Hz) and strain (0.05%). According to the DMA results, the crosslinking density of BPU-6-400, BPU-4-400, BPU-2-400 was calculated to be 566.4, 424.0, 59.1 mol/m3, respectively (detailed calculation method see the electronic supplementary information, ESI).
Solid samples of BPU-400 were placed in 20 mL vials followed by the addition of 10 mL of acetic acid. After 24 h, all the solid samples of BPU-400 were dissolved in the acetic acid and 10 mL of water was added to precipitate the dissociation products from the solution. The precipitated solid material was filtered and washed with H2O to remove acetic acid, and then dried in a vacuum oven for 24 h. The obtained colourless polyurea-urethane (PUU) was hot-pressed into a thin film. For the cyclic reprocessing test, the PUU film was chopped into small pieces and repossessed into another film for 3 times.
As shown in
Fig 1 (a) Fabrication of BPU film via blade-coating method; (b) 11B-NMR and (c) 13C-NMR spectra of BPU samples; (d) FTIR spectra of BPU samples with different ratios of PEG-400: H3BO3, where BPU-4-400 refers to the molar ratio of PEG-400 (400 g/mol):H3BO3=4:1.
Typically, the mechanical properties of crosslinked polymer could be manipulated by varying the crosslinking degree and intermolecular interaction such as hydrogen bond (
Fig 2 (a) Possible intermolecular hydrogen bonding in BPU; (b) Strain-stress curves of BPU-2-400, BPU-4-400 and BPU-6-400; (c) Young’s modulus and toughness of BPU-2-400, BPU-4-400 and BPU-6-400; (d) The comparison of mechanical properties between BPU and other reported recyclable cross-linked thermosets; (e) The storage modulus and loss modulus of BPU-2-400, BPU-4-400 and BPU-6-400 in temperature range from −50 °C to 150 °C. Black, red and blue lines refer to PEG-6-400, PEG-4-400 and PEG-2-400, respectively.
Classical cross-linked polymers are swellable because of the penetration of a solvent into the polymer network. The permanent covalent crosslinkers maintain the topology of polymer network and prevent the BPU from dissolving, which is essential for the material to work in complex conditions. To evaluate the chemical stability of BPU, the swelling tests by recording the mass swelling ratio of BPU in organic solvents and PBS were carried out. According to the swelling kinetics in
Fig 3 (a) Swelling tests of BPU in various solvents; (b) Swelling tests of BPU in PBS buffer with varied pH values from 1.00 to 9.18.
Based on the stimuli-cleavage behaviour of borate easter linkage, the covalent network can be de-crosslinked into linear polyurea-urethane in acidic condition (
Fig 4 (a) Dissociation process of BPU in acetic acid; (b) Optical images of BPU being dissociated and dissolved in acetate acid; (c) NMR spectra of the dissociated linear product; (d) FTIR spectra of BPU (black line) and the dissociated linear product (red line).
The dissociation product of linear PUU could be reprocessed by hot-pressing to achieve recycling products. To evaluate the recovered product as a thermoplastic material, the mechanical properties of PUU recycling products were tested, including storage modulus, loss modulus and stress-strain curves. The transition from crosslinked network to linear polymer provides the recycling product with thermoplastic capability. As shown in
Fig 5 (a) Temperature scanning spectra of the recycling products characterized by DMA; (b) Strain-stress curves of the recycling products; (c) Optical images of the recycling products.
In conclusion, a kind of recyclable polyurea-urethane thermosets was synthesized and well characterized. Benefiting from the considerable crosslinking density and intermolecular hydrogen bonding, such thermosets presented excellent mechanical properties as well as outstanding solvent resistance capability. Based on the stimuli-cleavage property of borate ester unit, the polyurea-urethane polymer network could be specifically dissociated in acetate acid to generate a linear polymer product. The de-crosslinking product could be further fabricated through hot-press process to achieve recycling thermoplastic products. It is envisioned that this particular class of recyclable polyurea-urethane thermosets with de-crosslinking capability may be readily extended to wide range of applications in optoelectronics, reinforcement, and encapsulation.
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