Frontally polymerizable shape memory polymer for 3D printing of free-standing structures

Four-dimensional (4D) printing is used to describe three-dimensional (3D)-printed objects with properties that change over time. Shape memory polymers (SMPs) are representative materials for 4D printing technologies. The ability to print geometrically complex, free-standing forms with SMPs is crucial for successful 4D printing. In this study, an SMP capable of frontal polymerization featuring exothermic self-propagation was synthesized by adding cyclooctene to a poly(dicyclopentadiene) network, resulting in switching segments. The rheological properties of this SMP were controlled by adjusting incubation time. A nozzle system was designed such that the SMP could be printed with simultaneous polymerization to yield a free-standing structure. The printing speed was set to 3 cm/min according to the frontal polymerization speed. A free-standing, hexagonal spiral was successfully printed and printed spiral structure showed excellent shape memory performance with a fixity ratio of about 98% and a recovery ratio of 100%, thereby demonstrating the 3D printability and shape memory performance of frontally polymerizable SMPs.

3D PRINTING OF SHORT CARBON FIBER COMPOSITES VIA FRONTAL POLYMERIZATIONMORTEZA ZIAEE

Additive manufacturing (AM) of polymer composites is a growing field in academic and industrial research environment. Majority of research in this field is focused on thermoplastic-based composites, as manufacturing of thermoset composites requires long cure cycles that make the additive manufacturing process quite challenging. Even though thermoplastic composites are easier to print, the ultimate performance of composites is limited by low fiber volume fraction, relatively high porosity, and low mechanical performance of host polymers. Recently, a novel curing strategy based on frontal polymerization (FP) has been developed that enables 3D printing of high-quality thermoset polymers. In this approach, a monomer solution with a gel-like viscosity is in-situ cured following the extrusion from printing nozzle by a self-sustaining reaction front. In the present work, we use dicyclopentadiene as a thermoset resin that can be frontally polymerized to a high-performance solid polymer. We add short carbon fiber reinforcements (L ~74 μm) to resin to fabricate mechanically robust 3D composite structures. Our results show that incorporation of short fibers substantially improves the flexural strength and flexural modulus of 3D-printed composites by ~50 % and ~410 %, respectively, compared to traditionally molded neat samples. Optical microscopy from the crosssection of flexural samples reveals that no voids was formed within deposition lines.