Additive Manufacturing of Polymer Networks Based on Dissociative and Associative Dynamic Covalent Bonds

Additive manufacturing (AM) techniques allow rapid prototyping of complex 3D shapes and custom-made products, in fact they attracted soon the interest of many researches and the range of materials that can be used is rapidly expanding. Curing and melting are the two main characteristics taken into account when deciding which techniques used, but recently polymers networks based on dynamic covalent bonds have been introduced into this world. Two main mechanism can be differentiated: associative and dissociative covalent chemistries. In the former, the crosslink density of the network stays constant with temperature, but the speed at which these dynamic covalent bonds exchange increases. In the latter, the crosslink density changes as a function of temperature. The main drawback for additive manufacturing of dissociative networks is the sharp and drastic change from a rubbery network to a low viscosity liquid at temperatures above their gel transition. In recent work, this limitation has been overcome by the addition of fillers that allowed the printing of overhangs and eventually the addition of other properties to the network. The fillers improved the printability of these materials, but at the same time they had a stiffening effect of the mechanical properties and obtaining a homogeneous dispersion is challenging.<br/>In this study, dissociative and associative dynamic bonds were combined in the same network, providing the dissociative network the thermal and mechanical support needed during the AM processing at high temperatures.<br/>Thermoplastics polymers such as PLA and ABS are usually printed taking advantage of the melting temperature or the devitrification of the polymer to have a high viscous flow in material extrusion AM. When using a polymer network based on dissociative thermoreversible Diels-Alder (DA) bonds, the gel transition temperature (<i>T</i>_gel) is used. When heated above T_gel, the dynamic equilibrium shifts towards the dissociation of the DA adducts to the extent that the polymer can flow through the nozzle as a viscous melt. Once in contact with the printing bed at a temperature below T_gel, the dynamic equilibrium shifts back to the formation of the adducts, reforming the polymer network structure. Differently from PLA or ABS, this is a reactive type of printing based on the reaction kinetics of the DA bonds, which are not fast enough to hold the shape of the object and cure before starting to flow on the print bed. Associative covalent networks present different types of exchange reactions. Recent studies showed that in presence of the transesterification reaction it proved possible to extrude at high temperatures and pressures, though with a rough surface finish. There is no net change in the crosslink density, but the exchange speed is fast enough for the material to flow and they also present thermal stability.<br/>When combining DA bonds and esters in the same network, it is possible to print objects with sharps edges and smooth finish using material extrusion AM. When heated at a temperature higher than the T_gel, the materials behave like a high viscous melt because the DA part of the network is completely dissociated and, once it reaches the bed, the network is hold together by the associative bonds present due to the vitrimers. Differential scanning calorimetry was used to verify the absence of degradation and side reactions when printing at temperatures much higher than the Tgel. Rheology measurements, such as small amplitude oscillatory shears, have been conducted on the materials and on commercial PLA used as a reference. The shear thinning behavior at high shears ensure extrudability and printability. Different percentages of the two dynamic bonds have been tested to evaluate the effect on the printability and to find the optimum composition. The healing property is also preserved after printing, the healing temperature is far from the printing temperature.