Experiments were performed in order to develop photopolymerizable acrylate oligomers which can be used in digital light processing (DLP) 3D printers. The primary objective was to determine the influence of chain-length of the oligomers on the mechanical and swelling properties of 3D printed objects.
Both poly(ethylene glycol)-dimethacrylates (PEGDMs) (with average molecular weight of 750, 4k, 6k and 8k Da) as well as lithium phenyl(2,4,6 trimethylbenzoyl) phosphinate (LAP) photoinitiator are commercially available only at a high price, therefore they were synthesized according to respective publications. Synthesized materials were characterized by infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy along with mass spectrometry. The chemical structures of the synthesized compounds were confirmed, the reactions took place with decent conversion rate, remaining poly(ethylene glycol) (PEG) together with PEG monomethacrylate reaction by product were found besides the expected PEGDMs. 3D printing materials were prepared by mixing 50 m/m% PEGDM, 0,5 m/m% LAP and water. Viscosity measurements were performed in order to verify the printability; then light exposure time during printing was optimized. Standard (ISO527-3B) specimens were printed for tensile tests.
Thermal properties of crosslinked polymers were measured by thermogravimetry and dynamic mechanical thermoanalysis (DMTA). Results did not show significant molecular weight dependence regarding thermal decomposition or glass transition temperature. Mechanical properties of equilibrium-swollen and dried specimens were assessed. Mass of swollen polymer hydrogels were 2 6 times larger than the mass of dried polymers. Based on results, clear correlation between the chain length of polymers and the mechanical properties cannot be drawn. Ambiguous data could be explained with the identified contaminants or other effects causing the decrease of cross link density. Tensile strength ranged from 180 kPa to 15 MPa while compression modulus values were between 0.6 and 438 MPa. Higher values were measured on dried specimens in regarding both parameters. Swollen specimen made of the highest molecular weight showed the highest tensile strain (35%). Modulus and tensile strain data of swollen 3D printed specimens containing PEGDMs with 4k, 6k and 8k Da molecular weights were similar to human skin tissue respective data. However tensile strength of the same specimens was 1-2 orders of magnitude smaller than the same parameter of human skin. Although dried specimens showed higher fracture toughness, these specimens had low modulus and elongation at break compared to biological tissues. Mechanical properties of specimens with longer polymer chains were proven to be undoubtedly better than specimens made of commercially available DM750.
In conclusion, aqueous solutions of synthesized polymers were photocross linked with visible-light photoinitiator using DLP 3D printer. Improving the purity of the synthesized PEGDMs would make them suitable for examining the effects of polymer chain length on mechanical properties of 3D printed polymer hydrogels. As a result, a rapid and simple method for fabricating complex structures with tailorable mechanical properties would become available, which could have a great significance to researchers working on the field of tissue engineering.