Slowing crystallization to enhance interlayer strength of 3D printed poly (ether ether ketone) parts by molecular design

The major challenge restricting the application of 3D printed PEEK parts is the weak interlayer strength. Herein, the fluorene-based poly (ether ether ketone)s (FD-PEEK) were designed and synthesized to solve the problem. A remarkable improvement in the interlayer strength of materials was achieved. By introducing 10 mol% fluorene groups into the polymer chain (10 %-FD-PEEK), the interlayer strength of 10 %-FD-PEEK was improved to 41 MPa, which is close to that of the commercial PEEK resin for 3D printing (Victrex, AM 200). The introduction of 15 mol% fluorene groups (15 %-FD-PEEK) enabled the interlayer strength and strain at break of 400 % and 500 % higher than that of PEEK, reaching 67 MPa and 11.23 %, respectively. The improvement of the interlayer strength was attributed to the slower crystallizing rate of FD-PEEK, which was verified by the isothermal and non-isothermal crystallization kinetics. Meanwhile, the chain rigidity caused by fluorene groups and the larger spherulite generated by the slower crystallization allowed FD-PEEK to reach a similar tensile strength to PEEK in the X-axis and Y-axis directions. Achieved by slowing crystallization and introducing rigid structure into the molecular chain, the results significantly promote the engineering application of 3D printing parts and provide new insights into the performance enhancement of semi-crystalline additive manufacturing materials.