2.2    Sample Preparation and Material Characterization
To characterize the properties of the two-stage resins, tensile, dynamic mechanical analysis (DMA), Fourier transform infrared spectroscopy (FTIR), and rheological tests were performed. Dogbone-style samples for tensile and (DMA) characterization experiments were cast in a silicone/polydimethylsiloxane (PDMS) mold. Samples with varying resin compositions (P, PE30, PE50, PE70, E) were subjected to a 60 second UV curing (intensity = 66 mW/cm2) followed by a 72 hour room temperature cure. Additionally, PE70 tensile samples were subjected to only UV curing for 1, 15, 30, and 60 seconds. FTIR and rheological experiments were conducted using only PE70 resin.
To characterize mechanical properties of the two-stage resin at various resin composition ratios (P, PE30, PE50, PE70, E), we performed tensile tests (n = 3) using a universal mechanical testing machine (Criterion, MTS, Eden Prairie, MN, USA) to acquire Young’s Modulus (E) as well as yield and ultimate engineering stresses and strains. Furthermore, tensile tests of PE70 resin samples subjected to 66 mW/cm2 UV irradiation for 1, 15, 30, and 60 seconds were performed. Additionally, we carried out DMA tests using a DMA tester (Q800, TA Instruments, New Castle, DE, USA) to obtain the glass transition temperature (T g), storage (E’) and loss (E”) moduli, and tanδ of the two-stage resin. Rheological measurements of PE70 two-stage resin were obtained at room temperature using an ARES G2 rotating rheometer (TA Instruments). Viscosity (η), shear storage (G’) and loss moduli (G”) were obtained over a range of shear rates (0.1 – 200 1/s). Finally, two sets of FTIR experiments were preformed to determine the DoC for both photopolymerization and room-temperature epoxy cure reactions in PE70 resin. For photopolymerization, FTIR scans were taken on resin samples subjected to 66 mW/cm2 UV irradiation for (0, 1, 3, 5, 7, 15, 30, and 60 seconds). Likewise – for the room-temperature epoxy cure – FTIR scans were performed on fully photopolymerized PE70 samples 0, 12, 24, 36, 48, and 72 hours after creation.
To understand the bonding strength of printed structures, we printed 5 x 10 x 20 mm pillars onto acrylic, wood, glass, aluminum, and concrete substrates. Substrates with pillars directly after printing (0 hour) as well as after 72 hours were constrained to a plate affixed to a universal mechanical testing machine (Criterion, MTS). Next, the pillars were constrained in a tensile test grip and a load applied until the pillar delaminated from the substrate. The critical stress measured was determined to be the adhesion strength.
2.3    3D Printing of Structures
3D models were prepared using SolidWorks (Dassault Systèmes SE, Vélizy-Villacoublay, France) computer-aided design (CAD) software. Prior to 3D printing, the models were imported into the computer-aided manufacturing (CAM) software, Repetier (Hot-World GmbH & Co. KG, Willich in North Rhine-Westphalia, Germany), for slicing into discrete layers. The resulting toolpath control code (gcode) was modified using custom MATLAB (MathWorks, Natick, MA, USA) scripts in order to redefine position axis such that the extrusion head would move downwards on the Z-axis, and along the Y-axis for horizontal prints for each layer. Structures were subsequently printed through a 0.58 mm diameter nozzle onto glass substrates at a printing speed of 10 mm/s and extrusion pressure of 75 kPa. UV irradiation (66 mW/cm2, 15 seconds) was applied every alternating layer. The conductive paths were printed through a 0.41 mm diameter nozzle at a printing speed of 10 mm/s and extrusion pressure of 190 kPa.
3. Results and Discussion
3.1    Mechanical and Material Characterization
DMA of thermally cured samples over the range of 30 - 170°C revealed that the two-stage resin has a Tg of 94.4, 121.2, 114.8, 97.4, and 91.5 °C for P, PE30, PE50, PE70, and E resin compositions respectively, indicated by the peaks in tanδ curves (dashed lines) (Figure 2a). Importantly, the materials exhibit a single distinct Tg despite the varying concentrations of monomers in the photo and thermal resin constituents having different Tg values. This suggests that the photo and thermal cure networks are highly compatible due to epoxide groups presented in the reactive dilutant GMA component of the photopolymer resin, allowing for the epoxy resin component to form an interpenetrating network with the TMPTA photopolymer network with no macroscale phase separation between the two components. As photo resin concentration increases, the tanδ curves begin to exhibit less and less distinct peaks. As a result, the tanδ peak for P is comparably small in height, but very broad. This indicates there is a longer property transition compared to all other conditions, which can account for the P condition’s deviation from the upward trend in Tgas photo resin concentration increases. Furthermore, based on the storage modulus behavior of P, it can be surmised this phenomenon is due to the highly crosslinked nature of the photo cure network as it has triacrylate monomers and therefore has more crosslinking points. This high crosslink concentration inhibits movement of the interpenetrating polymer network, resulting in the observed increasing trend in Tg versus photo resin concentration.
The results of the tensile tests for each resin composition are presented in Figure 2b. The solely photopolymer resin (P) exhibits highly rigid, albeit low strength, behavior as evidenced by the comparably large Young’s modulus versus other conditions (2.21 GPa) coupled with inferior tensile strength and fracture strain. However, increasing epoxy resin concentration resulted in an increase in strength and ductility of the cured resins. Indeed, the addition of epoxy resin – even in the lowest concentration PE30 – affords a 170% increase in tensile strength versus the solely photopolymerized resin (23.6 ± 2.1 versus 63.5 ± 3.5 MPa). Moreover, tensile strength of PE70 is comparable to that of the solely epoxy resin (81.4 ± 5.7 versus 79.5 ± 4.8 MPa), indicating that the epoxy network becomes the primary contributor to strength of the material despite it only comprising of 70 wt% epoxy. The increasing influence of the epoxy network over the rigid photo-cure network also manifests in a 0.18 – 0.32 GPa drop in Young’s modulus versus P as epoxy concentration increases. Furthermore, increasing epoxy resin concentrations leads to a linear increase in fracture strain from 1.4 for P to 6.6% PE70 (R2 = 0.93). However, despite PE70 and E having comparable tensile strengths, there is a relatively large discrepancy in fracture strain increase from PE70 to E (Δε = 2.7%). This implies that, despite the strength of the epoxy network, the highly crosslinked – and therefore rigid – photo-cure acrylate network impedes deformation of the interpenetrating two-stage network.