Abstract
Developing recyclable, reworkable, and intelligent thermosetting polymers, as a long-standing challenge, is highly desirable for modern manufacturing industries. Herein, we report a polyhexahydrotriazine thermoset (PHT) prepared by a one-pot polycondensation between 4-aminophenyl disulfide and paraformaldehyde. The PHT has a glass transition temperature of 135 °C and good solvent resistance. The incorporation of dual stimuli-responsive groups (disulfide bond and hexahydrotriazine ring) endows the PHT with re-processability, re-workability, and damage monitoring function. The PHT can be repeatedly reprocessed by hot pressing, and a near 100% recovery of flexural strength is achieved. The PHT can also degrade in inorganic acid or organic thiol solutions at room temperature. The thermally reworkable test demonstrates that, after heating the PHT at 200 °C for 1 h, the residuals can be easily wiped off. Finally, the PHT exhibits a reversible mechanochromic behavior when damaged.
Highlights
Thermosetting polymers have widespread industrial applications such as structural adhesives, protective coatings, electronic packaging materials, and polymer matrices for advanced composites thanks to their remarkable mechanical and electrical performances as well as excellent heat, creep, and chemical resistance
Since both the disulfide bond and hemiaminal group are dynamic covalent bonds [50,51], uniform coherent polyhexahydrotriazine thermoset (PHT) bars could be obtained by pressing hemiaminal dynamic covalent networks (HDCNs) powders at 150 ◦ C under 0.3 MPa for 1 h
If the mixture is directly cured in a mold, this would generate bubbles in the PHT specimens owing to evaporation of the residue NMP solvent
Summary
Thermosetting polymers have widespread industrial applications such as structural adhesives, protective coatings, electronic packaging materials, and polymer matrices for advanced composites thanks to their remarkable mechanical and electrical performances as well as excellent heat, creep, and chemical resistance. Conventional thermosetting polymers have permanent cross-linked structures consisting of irreversible covalent bonds, making them extremely difficult for recycling [1,2,3]. Technological innovations greatly shorten the iterative circles of consumer products, especially for personal electronic equipment in which thermosetting polymers are largely used. Along with stricter environmental regulations, the intractability of thermosetting polymers has raised increasing concerns about disposal and recycling of discarded electronics and composite products. Reworkable thermosetting polymers, which can be removed during the manufacturing process, are desirable [2,4]. The use of reworkable thermosetting polymers for electronic encapsulation will allow removal of an individual chip from the printed circuit board for replacement or Polymers 2020, 12, 2375; doi:10.3390/polym12102375 www.mdpi.com/journal/polymers
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