Photoinitiator Free Resins Composed of Plant-Derived Monomers for the Optical µ-3D Printing of Thermosets

Abstract

In this study, acrylated epoxidized soybean oil (AESO) and mixtures of AESO and vanillin dimethacrylate (VDM) or vanillin diacrylate (VDA) were investigated as photosensitive resins for optical 3D printing without any photoinitiator and solvent. The study of photocross-linking kinetics by real-time photorheometry revealed the higher rate of photocross-linking of pure AESO than that of AESO with VDM or VDA. Through the higher yield of the insoluble fraction, better thermal and mechanical properties were obtained for the pure AESO polymer. Here, for the first time, we validate that pure AESO and mixtures of AESO and VDM can be used for 3D microstructuring by employing direct laser writing lithography technique. The smallest achieved spatial features are 1 µm with a throughput in 6900 voxels per second is obtained. The plant-derived resins were laser polymerized using ultrashort pulses by multiphoton absorption and avalanche induced cross-linking without the usage of any photoinitiator. This advances the light-based additive manufacturing towards the 3D processing of pure cross-linkable renewable materials.

Keywords: acrylated epoxidized soybean oil; direct laser writing; multi-photon processing; nanolithography; optical 3D printing; photocross-linking; two-photon polymerization (2PP); vanillin diacrylate; vanillin dimethacrylate.

Scheme 1

The chemical structure of acrylated epoxidized soybean oil (AESO), vanillin dimethacrylate (VDM) and vanillin diacrylate (VDA).

Figure 1

The explanatory scheme of a sample preparation and Direct Laser Writing (DLW) 3D lithography: (a) Kapton tape attached to the glass slide and working as an intermediate, resin’s droplet drop cast on such a substrate; (b) another glass slide used as a cover to squeeze the droplet and spread it uniformly through the substrate; (c) laser beam focused through the cover glass into the resin and initiating the 3D confined polymerization reaction.

Figure 2

The dependencies of storage modulus G’, loss modulus G”, loss factor tanδ and complex viscosity η* of AESO on irradiation time.

Figure 3

The irradiation time dependencies of the storage modulus G’ of AESO and the resin series AESO/VDM.

Figure 4

The irradiation time dependencies of the storage modulus G’ of the resin series AESO/VDA.

Figure 5

The FT-IR spectra of AESO, VDM, homopolymer pAESO and copolymer series pAESO/VDM.

Figure 6

The thermogravimetric curves of the polymers pAESO and the polymer series pAESO/VDM.

Figure 7

The SEM images of RB and 3D microporous woodpile structures: (a) a side view of RB at the angle of 45 degrees and 1800 magnification. The applied power P to produce bridges was 0.6 mW (2 TW/cm²), scan velocity v varied from 0.1 mm/s to 0.5 mm/s every 0.1 mm/s; (b) a top view of the other RB at 4000 magnification. P = 0.6 mW (2 TW/cm²), v = 2–6 mm/s every 1 mm/s; (c) 75 × 75 µm² woodpile structures with a 30 µm period, v = 5 mm/s, the scale at the top of the image demonstrates the applied I; (d) a 1065 × 1065 µm² woodpile with a 75 µm period, v = 5 mm/s, P = 0.4 mW (1.3 TW/cm²). (a–d) The structures were fabricated out of the resin AESO/VDM1; (e) 75 × 75 µm² woodpile structures with a 30 µm period, v = 5 mm/s. Scale on the left of image shows the distance d_xy between neighboring scans; (f) a 1095 × 1095 µm² woodpile with 120 µm period, v = 5 mm/s, P = 0.6 mW (2 TW/cm²). (e,f) structures were fabricated out of AESO.