Design of Phosphate-based Dynamic Covalent Network for Fused Deposition Modeling

Abstract

Additive manufacturing (AM) is considered
an environmentally friendly manufacturing process that builds solid 3D
structures layer-by-layer from a computer-aided design (CAD), resulting in reduced
waste as opposed to conventional subtractive manufacturing. However, a
significant challenge in AM is the extensive use of plastics, which lack a
standardized recycling process. The insufficient adhesion between printed
layers contributes to waste generated with AM. Therefore, it is important to
prioritize sustainability as a design parameter for future AM materials. Dynamic
Covalent Networks (DCNs) offer a potential solution by combining the desirable characteristics
of thermosets and the recyclability of thermoplastics. DCNs undergo bond rearrangement
reactions influenced by external stimuli like heat or light, leading to changes
in their topology. Bond exchange reactions can occur through two mechanisms:
dissociative, involving separate steps of bond breaking and forming, or
associative, involving simultaneous bond breaking and forming. Recently, a
phosphate triester based DCN was developed obtaining a neighboring 𝛽-hydroxyl group which could perform transesterification exchange
reactions within the network via the formation of a cyclic phosphate triester
intermediate in a dissociative manner. Based on this network rearrangement, we
synthesized a network by reacting phos- phoric acid with a diglycidyl ether to
perform ring-opening reactions which creates a pendent 𝛽-hydroxyl functional group assisted in neighboring group
transesterification. The reversibility of the bond exchange reaction within the
network was investigated with a heating and cooling cycle with Variable
temperature (VT) 31P solid-state NMR (SSNMR). Furthermore, the network was
reprocessable with compression molding at 130 ◦C. Fast re- laxation times of 5
seconds at 200 ◦C were observed. Additionally, frequency sweep and dynamic
mechanical temperature analysis (DMTA) experiments showed profiles which were expected
for a dissociative bond exchange mechanism. However, an increase of the storage
modulus at 150 ◦C was observed, indicating a curing process of the network.
Subsequently, similar experiments were performed on the cured network, in which
a reduction of the dynamic properties of the network was noted, with a stress
relaxation time of 114 seconds at 200 ◦C. DMTA and frequency sweep experiments
confirmed its increased storage modulus as well. Nevertheless, the uncured
network was reprocessable via extrusion at 200 ◦C, however, it required at
least 50 min for the network to obtain a viscous flow behavior for optimal
extrusion.