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.