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Polymer Testing 116 (2022) 107767
Available online 6 September 2022
0142-9418/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Preparation of isotropic tensile photosensitive resins for digital light
processing 3D printing using orthogonal thiol-ene and thiol-epoxy
dual-cured strategies
Biao Yu
a
,
b
,
c
,
**
, Jiaying Zheng
a
, Jiazhen Wu
a
, Hao Ma
d
, Xiaoqin Zhou
a
, Yonghai Hui
a
,
***
,
Fang Liu
b
, Jingwei He
b
,
*
a
School of Chemistry and Chemical Engineering, Lingnan Normal University, 524048, Zhanjiang, China
b
School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, China
c
Key Laboratory of Chean Energy Material Chemistry in Guangdong General University, 524048, Zhanjiang, China
d
College of Chemistry, Guangdong University of Petrochemical Technology, Maoming, 525000, China
ARTICLE INFO
Keywords:
3D printing
Orthogonal dual-cured strategies
Thiol-ene
Thiol-epoxy
Photosensitive resin
ABSTRACT
Poor layer interface adhesion leads to increasing anisotropic properties and weakening tensile properties of 3D
printed parts. In order to reduce anisotropic properties and enhance the tensile properties of thiol-ene light-
sensitive resins for Digital light processing (DLP) 3D printing, thiol-ene-epoxy hybrid resins containing a radical
initiator (TPO) and photobase generator (TBD-HBPh
4
/ITX) were prepared to reduce the anisotropy and enhance
the tensile properties of thiol-ene light-sensitive resins for digital light processing (DLP) 3D Printing. Real-time
Fourier transform infrared spectroscopy (RT-FTIR) indicated that the C
–
–
C bond conversion of resins exceeded
60%, and the thiol conversion was approximately 50%. The volumetric shrinkage of the thiol-ene resins
increased to an extent with thermal treatment, indicating that these resins had undergone postcuring. Dynamic
mechanical analysis showed that the glass temperature (T
g
) increased from 20.8 ◦C to 34.1 ◦C. The tensile
strength of the 3D printed part increased by approximately 500% with each printed orientation, and the degree
of anisotropy with each orientation reduced by less than 5%. It was conrmed that the superior isotropic
properties of the thiol-ene photosensitive resins for DLP 3D Printing were obtained using orthogonal thiol-ene
and thiol-epoxy dual-curing strategies.
1. Introduction
Light-cured three-dimensional printing (3D Printing) technologies
such as Stereolithography (SLA) [1], Digital light processing (DLP) [2],
and the PolyJet [3] method have gained signicant attention in the last
three decades, owing to their high printing resolution, low energy
consumption, and cost-effectiveness. Thus, numerous applications have
been produced, ranging from jewelry, electrical components, and den-
sity to tissue engineering [4,5]. The molding principle of these tech-
nologies is that the light-sensitive resins are cured with light irradiation,
layer by layer, under programming control. However, the poor layer
interface adhesion weakens the mechanical properties of the samples
printed orthogonal to the print bed, compared with the samples printed
parallel to the bed (longitudinal) [6]. Typically, vertical specimens tend
to produce parts with decreased tensile strength because there is usually
no covalent bond bridging between printed layers [7].
The anisotropic mechanical properties of light-cured 3D printed
parts are affected by the thickness of the printed layer, printed orien-
tation, exposure intensity, and exposure time. The effect of the print
raster direction on mechanical properties has been evaluated in several
studies. For instance, Sung et al. [8] investigated the mechanical prop-
erties of 3D printed photopolymers and observed poor mechanical
Abbreviations: DLP, Digital light processing; DMA, Dynamic mechanical analysis; FWHM, Full width at half-maximum; TGA, Thermogravimetric analysis; UTS,
Ultimate tensile stress; RT-FTIR, Real-time Fourier transform infrared spectroscopy.
* Corresponding author.
** Corresponding author. School of Chemistry and Chemical Engineering, Lingnan Normal University, 524048, Zhanjiang, China.
*** Corresponding author.
E-mail addresses: y.biao@lingnan.edu.cn (B. Yu), hyhai97@126.com (Y. Hui), msjwhe@scut.edu.cn (J. He).
Contents lists available at ScienceDirect
Polymer Testing
journal homepage: www.elsevier.com/locate/polytest
https://doi.org/10.1016/j.polymertesting.2022.107767
Received 15 June 2022; Received in revised form 17 August 2022; Accepted 30 August 2022
Polymer Testing 116 (2022) 107767
2
properties along with their anisotropy. The tensile toughness and
elongation-at-break are also related to the printing direction. Longitu-
dinally printed specimens exhibit tensile toughness 300% greater than
that of transversely printed specimens. Unkovskiy et al. [9] also revealed
the effect of the printed orientation, position, and postcuring on
dimensional accuracy and exural properties. They observed that the
45◦printed orientation specimens exhibited more accurate dimension
reproductions, and the 90◦oriented printed specimens exhibited supe-
rior exural strength (FS) and exural modulus (FM).
Ahmad et al. [10] tested the degree of anisotropy of the photo-
polymer samples with 0◦, 45◦, and 90◦printed orientations using a
nanoindentation testing machine. The result showed that the elastic
modulus for the 0◦orientation was higher than those of the 45◦and 90◦
orientations by 35% and 390%, respectively. Conversely, Shanmuga-
sundaram [11] et al. discovered that the mechanical properties of SLA
3D printed samples were isotropic. They investigated the degree of
mechanical properties through the tensile testing of specimens built in
different orientations fabricated by SLA using statistical analysis. The
results showed that the SLA printed parts were isotropic. They also
discovered the mechanical anisotropy for DLP 3D printed samples.
Contrarily, Monzon et al. [12] tested three photopolymers with different
built directions using DLP 3D printing technology, and the results
showed that the build direction had a signicant effect on the me-
chanical properties, particularly for comparing the vertical and hori-
zontal directions.
Several strategies have been reported to reduce the mechanical
anisotropy of 3D printing technologies. For instance, Li et al. [13]
improved the toughness and interlayer bonding of SLA resins by adding
core–shell particles (CSPs). The result showed that the
elongation-at-break increased to 1303.6%. Moreover, the introduction
of CSP reduced the anisotropy in the tensile strength of printed work-
pieces from 42.1% to 0.3%. Dynamic covalent chemistry can also
improve interlayer adhesion in 3D printing. Using this principle, Yang
et al. [14] leveraged dynamic covalent chemistry based on reversible
furan-maleimide Diels–Alder linkages (DATs) to decrosslink and melt
the polymers during printing from 90 ◦C to 150 ◦C and recrosslink at a
lower temperature to their entropically favored state. This implied that
the DATs containing the printed samples exhibited isotropic mechanical
properties, including toughness, ultimate tensile strength, and
strain-at-break. Shaffer et al. [15] blended polylactic acid (PLA) with
ionizing radiation-sensitive polymers as a crosslinker. The interface
adhesion was strong and dramatically reduced the anisotropy during
fused lament fabrication (FFF) 3D printing with ionizing radiation at
various temperatures.
Thiol-ene photosensitive resins were utilized for SLA [16], DLP [17],
direct-writing printing [18], and volumetric additive manufacturing
[19], owing to their high photopolymerization reactivity, low oxygen
inhibition, low volumetric shrinkage, and low shrinkage stress. How-
ever, they present drawbacks, including low tensile stress, low tensile
modulus, and low glass transition temperature (T
g
), owing to the exi-
bility of the C–S bond, which limits their applications. Therefore, re-
searchers have used dual-cured strategies with thiol-ene and thiol-epoxy
resin systems to overcome these drawbacks and improve the mechanical
properties and glass transition temperature [20,21].
Bowman et al. [20] rst reported the preparation of
thiol-ene/thiol-epoxy hybrid polymers by combining photoinitiated
thiol-ene polymerization and a base-catalyzed thiol-epoxy reaction.
Their ndings revealed that hybrid resins with equal weight percentages
of thiol-ene and thiol-epoxy achieved the highest T
g
and lowest poly-
merization stress. Thus, two dual curable hybrid thiol-ene/thiol-epoxy
networks have been prepared using photoinitiated radical systems and
base catalysts [22]. Grauzeliene et al. [18] prepared
thiol-ene/thiol-epoxy resins based on vegetable oil for laser
direct-writing 3D micro/nanolithography. However, the dual curable
resins with these binary catalysts were unstable, and it was not condu-
cive for long-term storage because the thiol-ene Michael addition
reaction and thiol-epoxy nucleophilic ring-open addition reaction with
the catalysis of the strong base occurred in storage. Jian et al. [23] rst
prepared a photocured thiol-ene/thiol-epoxy hybrid resin system using a
free-radical photoinitiator photobase generator (TBD-HBPh
4
). The
TBD-HBPh
4,
aided by isopropyl-9H-thioxanthen-9-one (ITX) irradiated
by ultraviolet (UV) light in the range of 320–500 nm, could be cleaved to
produce a strong nucleophile base, 1,5,7-triazabicyclo[4.4.0]dec-5-ene
(TBD), which would quantitatively catalyze the thiol-epoxy reaction.
The results showed that the speed of the photoinitiated thiol-epoxy re-
action was lower than that of the photoinitiated radical thiol-ene reac-
tion. The conversions of thiol and acrylate groups photocatalyzed using
ITX/TBD-HBPh
4
could exceed 90% within 1 min. However, the epoxy
groups required at least 5 min to reach a similar conversion. Similarly,
Xia et al. [24] prepared a thiol-epoxy/thiol-Si-methacrylate hybrid
polymer using the same method. Recently, Chen et al. [25] used the
same photobase generator system (ITX/TBD-HBPh
4
) to prepare a thick
composite by delaying thiol-epoxy photopolymerization.
Herein, we prepared photocurable resins based on thiol-ene/thiol-
epoxy resin systems with a mixture of photoinitiator diphenyl(2,4,6-
trimethylbenzoyl)phosphine oxide (TPO) and ITX/TBD-HBPh
4
. TPO is
one of Norrish type I radical photoinitiator that is commonly selected for
DLP 3D printing because of its excellent degree of conversion, high rate
of polymerization, and stability of color. Importantly, it can be effec-
tively activated upon excitation at 405 nm (the wavelength of the light
source is widely used for commercial DLP printers). As mentioned, ITX/
TBD-HBPh
4
has been conrmed to release a strong base TBD with a pK
a
of 26.03 by effectively cleaving at 405 nm UV, which quantitatively
catalyzed the thiol-epoxy reaction with light irradiation. Therefore, TPO
and ITX/TBD-HBPh
4
were used as photosensitizers in this study. In the
rst stage, thiol-ene polymerization was initiated using TPO during the
DLP printing process, and the epoxy group in glycidyl methacrylate
(GMA) was incorporated into networks. Simultaneously, a robust
nucleophilic base, TBD, was released from TBD-HBPh
4
under UV light
during the DLP printing. Thereafter, the printed parts were heated at a
high temperature in the second stage, and the thiol groups reacted with
epoxy groups catalyzed by TBD. It was hypothesized that the interface
adhesion between the layers would be improved by reacting epoxy
groups with thiol groups. To the best of our knowledge, this is the rst
report on the use of photobase generator systems to catalyze a thiol-
epoxy reaction during light-cured 3D printing. The advantage is to
avoid the use of traditional strong bases as catalysts, which would
catalyze the thiol-epoxy reaction and destabilize the photosensitive
resin. Moreover, the orthogonal thiol-ene and thiol-epoxy dual-cured
strategies would improve the mechanical and isotropic tensile
properties.
2. Experimental section
2.1. Materials
1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD), triethylene glycol dia-
crylate (TEGDMA), and pentaerythritol tetra(3-mercaptopropionate)
(PETMP) were purchased from Beijing InnoChem Science & Technol-
ogy Co., Ltd (Beijing, China). Sodium tetraphenylborate (NaBPh
4
),
glycidyl methacrylate (GMA), pyrogallol (TPX), Diphenyl (2,4,6-trime-
thylbenzoyl)phosphine oxide (TPO), and Isopropyl-9H-thioxanthen
(ITX) were obtained from Shanghai Titan Scientic Co., Ltd
(Shanghai, China). Tricyclodecane dimethanol diacrylate (EM2204) was
obtained from Eternal Materials Co., Ltd (Zhuhai, China). The chemicals
were used without further purication.
2.2. Synthesis of TBD-HBPh
4
The synthesis of TBD-HBPh
4
was performed according to Sun’s
method [26]. TBD (1.35 g, 9.69 mmol) was dissolved in an HCl solution
(10 wt%), and the pH of the solution was adjusted to approximately 3.
B. Yu et al.
Polymer Testing 116 (2022) 107767
3
Thereafter, NaBPh
4
(3.76 g, 10.98 mmol) dissolved in 50 mL of water
was added to the solution dropwise. The mixture was stirred to obtain a
white precipitate. The crude product was washed with methanol thrice.
Finally, the product was recrystallized by a 2.5:1 (volume ratio) mixture
of methanol and dichloromethane (DCM) to obtain high-purity
TBD-HBPh
4
(3.63 g, 7.90 mmol) at a yield of 59.3%.
1
H NMR (400
MHz, DMSO‑d
6
) δ 7.22–7.14 (m, 8H), 6.92 (t, J =7.3 Hz, 8H), 6.83–6.75
(m, 4H), 3.22 (t, J =6.0 Hz, 4H), 3.14 (td, J =5.9, 2.8 Hz, 4H), 1.83 (p, J
=5.9 Hz, 4H).
2.3. Preparation of resin systems
The photosensitive resins comprised thiol monomers, ene monomers,
photoinitiators, photobase generators, and stabilizers. PETMP served as
a thiol monomer, EM2204 and TEDMA functioned as ene monomers,
TPO was used as a radical photoinitiator, and ITX and TBD-HBPH
4
were
used as photobase generator systems. The composition of the resin
systems is shown in Table 1. The chemical structures of the monomers,
photoinitiator, photobase generator, and stabilizer are shown in Fig. 1.
2.4. Degree of functional group conversion
The degree of functional group conversion (DC%) during and after
the photoinitiation of the polymerization was monitored by Fourier
transform infrared (FTIR) spectroscopy (Nicolet 6700, Thermo Fisher
Scientic, Waltham, MA, USA). The resin samples were coated on KBr
pellets to form an extremely thin lm, and the absorbance peak of un-
cured samples was obtained. Thereafter, the photopolymerization of the
sample was conducted by the irradiation of a UV source (UVP60, 405
nm, 70 mW/cm
2
, Run Duo Ke Ji, Shanghai, China) at room temperature.
The spectra were recorded every 10 s for 1 min. The DC% was calculated
from the aliphatic C
–
–
C peak at 1636 cm
−1
and thiol group peak at 2530
cm
−1
normalized against the benzene C
–
–
C bond peak at 1608 cm
−1
,
according to Eq. (1).
DC% =(1−At
A0)×100%,(1)
where A
0
and A
t
represent the absorbance peak areas of the functional
groups (C
–
–
C bond and thiol groups at 1640 and 2230 cm
−1
, respec-
tively) prior to and after irradiation for time t, respectively. DC% is the
conversion of the functional groups as a function of radiation time.
Table 1
Composition of thiol-ene-epoxy photosensitive resins.
Resins
a
EM2204
/mol
TEGDMA/mol PETMP
/mol
GMA
/mol
TPO/mol ITX/mol TBD-HBPH
4
/mol
TPX/mol
Control 0.66 0.34 0.375 0.00 0.02 0.00 0.00 0.003
Resin 1 0.63 0.32 0.375 0.05 0.02 0.01 0.00 0.003
Resin 2 0.63 0.32 0.375 0.05 0.02 0.01 0.01 0.003
a
The mole ratio of the thiol groups (–SH) and C
–
–
C bond is 0.75:1. The mass ratio of EM2204 and TEGDMA was maintained at 7:3 in the resins.
Fig. 1. Chemical structures of monomers, photoinitiator, photobase generator,
and stabilizer.
Fig. 2. Dog-bone model and 3D printed samples for tensile test printed with 0
◦, 45◦, and 90◦printed orientations (the printing direction is parallel to the platform).
B. Yu et al.
Polymer Testing 116 (2022) 107767
4
2.5. Thermal treatment and 3D printing
The 3D printing experiments were conducted on a home-built DLP
3D printer (Photo mono, Anycubic, Shenzhen, China). The curing time
of each layer was set at 20 s, and the cured thickness of each layer was
0.05 mm. For the tensile test, dog-bone samples were printed with 0◦,
45◦, and 90◦printed orientations (Fig. 2) to compare the difference
between tensile properties with and without thermal treatment. The
samples with thermal treatment were heated in an oven at 120 ◦C for 5
h.
2.6. Volumetric shrinkage
The volumetric shrinkage (VS%) was measured using the variation of
densities prior to and after polymerization, according to Eq. (2).
VS% =
ρ
polymer −
ρ
monomer
ρ
polymer
×100%,(2)
where
ρ
polymer
is the densities of the cured resin, and
ρ
monomer
is the
densities of the uncured resins.
Here,
ρ
polymer
and
ρ
monomer
were measured using a densitometer
(MDJ-300 M, Xiongfa Instrument, Xiamen, China), according to Archi-
medes’ principle. The measurement was repeated thrice.
2.7. Dynamic mechanical analysis (DMA)
Rectangular DMA samples sized 30 mm ×10 mm ×5 mm were
printed using the 3D printer. The DMA was performed using a dynamic
mechanical analyzer (Q800, TA Instruments, New Castle, USA) in the
tensile mode using a deformation of 0.1% strain, frequency of 1 Hz, force
track of 150%, and preload force of 0.01 N. Each experiment was run
from −50 to +150 ◦C with a heating rate of 3 K/min. The full width at
half-maximum (FWHM) was determined using tan δ curves. The cross-
linking density (
Ʋ
e
) was calculated using the following Eq. (3):
ϑe=ETg+50k
3RT ,(3)
where ETg+50k is the storage modulus of the rubber regions at T
g
+50 K in
MPa, R is a gas constant (8.314 J/kmol), and T is the temperature in
Kelvin. The glass transition temperature (T
g
) was determined as the peak
temperature of the tan δ curve versus temperature.
2.8. Thermogravimetric analysis (TGA)
TGA was performed using a thermogravimetric analyzer (TGA/
DSC1, Mettler-Toledo, Switzerland) from 30 ◦C to 600 ◦C with high-
purity nitrogen as purge gas at a ow rate of 50 mL/min. Approxi-
mately 10 mg samples were weighed and placed in an aluminum cru-
cible without covers.
2.9. Tensile properties
The tensile properties including ultimate tensile stress and
elongation-at-break were determined using an electronic universal
testing machine (CMT6104, MTS systems, Shenzhen, China) using a
Sintech 20/g tensile tester equipped with a 100 N load cell and tensile
testing grips. Dog-bone samples were printed using the 3D printer,
((Photon mono, Anycubic, Shenzhen, China) and these samples were
pulled under tension at 10 mm min
−1
. Seven samples were tested for
each formulation, and the reported errors for the tensile properties are
standard deviations.
2.10. Anisotropic properties
The anisotropic properties were determined by the results of
toughness obtained from the tensile tests [6]. ISO 527 type dog-bone
samples were printed using the 3D printer at different orientations
(0◦, 45◦, and 90◦), and the anisotropy was calculated using Eq. (3):
Anisotropy =(1−Toughness of 0◦(or 45◦)printed dogbones
Toughness of 90◦printed dogbones )×100%
(3)
2.11. The morphological study
The tensile fracture surface of the resins was sprayed with gold, and
microstructure was study by a scanning electron microscope (JSM-
7600F, JEOL, Japan).
3. Result and discussion
3.1. Degree of conversion
Light-cured resins for 3D printing should have high photo-
polymerization reactivity, and photosensitive resins should be cured in a
few seconds under light radiation. This study used real-time RT-FTIR to
investigate the functional group conversion. The comparison of the C
–
–
C
Fig. 3. Functional group conversion versus irradiation times. (a) C
–
–
C bond and (b) thiol conversion.
B. Yu et al.
Polymer Testing 116 (2022) 107767
5
bond (~1635 cm
−1
) and thiol (~2560 cm
−1
) conversions versus irra-
diation time among Control, Resin 1, and Resin 2 is shown in Fig. 3.
Fig. 3 shows that the C
–
–
C bond conversion could have exceeded 60%.
However, the thiol conversion was approximately 50%, and it was not as
high as expected. The chemical structure of the ene monomers plays an
important role in the reactivity of a thiol-ene reaction. Dissimilar to
other ene monomers, when (meth)acrylate reacted with the thiol
monomers during photopolymerization, they reacted with the thiol
monomers by a radical step-growth polymerization mechanism and
homopolymerized by a radical chain polymerization mechanism [27].
Bowman et al. [28] investigated the photopolymerization kinetics of
thiol-ene and thiol-acrylate systems with RT-FTIR technology. It
observed that the conversion of the acrylate functional groups was
roughly twice that of the thiol groups. The acrylate propagation kinetic
constant was 1.5 times that of the rate constant for hydrogen abstraction
from the thiol groups. Our recent study also showed that the thiol
conversion was less than ~50% when methacrylate monomers were
used as ene monomers to react with a thiol monomer [29].
The C
–
–
C bond conversion after 60 s of irradiation time for Control
(75.4 ±6.9%) was lower than that of Resin 1 (83.9 ±1.5%) and higher
than that of Resin 2 (68.4 ±5.5%). However, the thiol conversion after
60 s of irradiation time for Control (43.5 ±4.2%) was lower than those
of Resin 1 (53.5 ±1.1%) and Resin 2 (48.1 ±1.4%). The degree of
conversion was reported to be affected by monomeric structure, vis-
cosity, and functionality [30,31]. The low viscosity, high mobility, and
highly exible structure with a low T
g
were benecial for increasing the
DC% [32]. Here, the difference in the resin formulation of Control be-
tween Resin 1 and Resin 2 was that only a 5% mole fraction of the
EM2204/TEGDMA dimethacrylate monomer system was replaced by
GMA. The results showed that Resin 1 had the highest C
–
–
C bond and
thiol conversion, indicating that adding GMA might have improved the
exibility of the chain, resulting in a higher DC%. However, after adding
a photobase generator (TBD-HBPh
4
/ITX) into Resin 1, the C
–
–
C bond
and thiol conversion were reduced to an extent. This was probably due
to the poor solubility of TBD-HBPh
4
in the resins that hindered the
movement of the molecular chains.
Owing to the low concentration of epoxy groups in Resin 1 and Resin
2, it was difcult to nd the absorption peak of the epoxy groups in GMA
at 910 or 790 cm
−1
in the FTIR spectra. Thus, the epoxy group con-
version could not be calculated. However, the thiol-epoxy reaction was
conrmed to have occurred by an indirect method, such as the variation
in the T
g
during the DMA test.
3.2. Volumetric shrinkage
The volumetric shrinkage of the light-cured resins is critical for light-
curable 3D printing because it directly affects manufacturing accuracy.
During photopolymerization, the distance between monomer molecules
is reduced from van der Waals forces (~4 Å) to covalent bond distance
(~1.5 Å) [33]. As shown in Fig. 4, without thermal treatment, the
volumetric shrinkage of Control (7.40 ±0.05%) was slightly higher than
those of Resin 1 (6.97 ±0.02%) and Resin 2 (6.92 ±0.05%). However,
when the 0.05 mol ratio of the (meth)acrylate monomer system
(EM2204/TEGDMA) was replaced by GMA, the concentration of (meth)
Fig. 4. Volumetric shrinkage of Control, Resin 1, and Resin 2.
Fig. 5. Storage modulus versus temperature curves without and with thermal treatment: (a) Control, (b) Resin 1, and (c) Resin 2. Tan δ versus temperature curves
without and with thermal treatment: (d) Control, (e) Resin 1, and (f) Resin 2.
B. Yu et al.
Polymer Testing 116 (2022) 107767
6
acrylate groups also decreased. Considering that the epoxy groups in
GMA were not involved in the printing process, the concentration of
polymerizable functional groups in the resins decreased, leading to a low
volumetric shrinkage [34].
After heating at 120 ◦C for 5 h, an increasing tendency in the volu-
metric shrinkage of the resins was observed. The volumetric shrinkage
increased by 0.86% from 7.40% to 8.26% for Control, by 0.31% from
6.97% to 7.28% for Resin 1, and by 1.14% from 7.09% to 8.25% for
Resin 2. This demonstrated that these resins had undergone postcuring
with the thermal treatment. The volumetric shrinkage of Resin 2
exceeded that of Resin 1. It was conrmed that TBD was released from
TBD-HBPh
4
during the light-cured 3D printing, and it catalyzed the
thiol-epoxy nucleophilic addition reaction or thiol-acrylate Michael
addition [14]. The increase in the volumetric shrinkage of Control was
less than that of Resin 2 and higher than that of Resin 1. This was
because the (meth)acrylate groups were self-initiated during the poly-
merization at 120 ◦C without any redox or radical initiators [34,35].
3.3. Dynamic mechanical analysis
The viscoelastic properties of Resin 1, Control, and Resin 2 were
determined by DMA. The storage modulus and tan δ curves are shown in
Fig. 5. The data obtained from DMA such as FWHM, T
g
, and
Ʋ
e
are
summarized in Table 2.
The breadth of the tan δ peak is related to the degree of in-
homogeneity of the distribution of comonomers and crosslinking points
in the network structure [36] and can be measured by an FWHM of the
tan δ peak (FWHM). The smaller the FWHM, the more the homogeneity
of the polymer network [37]. As shown in Table 2, the FWHM of the
three cured resins prior to heating was below 20 ◦C ranging from 18.3 ◦C
to 19.5 ◦C. This suggested that their polymer networks were uniform and
homogeneous. This is because the thiol-ene based resins exhibited a
more homogeneous polymer network than those of the (meth)acryl-
ate-based or epoxy-based resins during photopolymerization because of
their step radical polymerization mechanism [38]. After heating at
120 ◦C for 5 h, the FWHM of Control reduced from 18.3 ◦C to 16.4 ◦C,
indicating that its crosslink density distribution became more homoge-
neous. Contrarily, the FWHM values for Resin 1 and Resin 2 slightly
increased. The increasing trends of FWHM value of Resin 1 and Resin 2
with thermal treatment suggested that the polymeric network become
more heterogeneity. Because self-polymerization of epoxy groups or
thiol-epoxy reaction would form a phase-separated structure in poly-
meric network, it may lead to the crosslinking density distribution
became more heterogeneity.
Here, the T
g
was determined by the peak temperature of tan δ, which
is usually related to the crosslink density of the polymer network [39].
As shown in Table 2, the T
g
of Control was 32.3 ◦C, which was 11 ◦C
higher than those of Resin 1 and Resin 2 without thermal treatment. As
mentioned, the difference in the monomer systems in Control between
Resin 1 and Resin 2 was that a 5% mole fraction of the dimethacrylate
systems, EM2204/TEGDMA, in Control was substituted with GMA,
which was equivalent to a monoacrylate monomer. The epoxy groups in
GMA did not participate in the polymerization reaction during photo-
polymerization. The lower functionality of GMA, compared with those
of TEGDMA and EM2204 might have increased the free volume and
reduced the hindrance of molecular motion [40]. This could have led to
a reduction in the T
g
of resin Resin 1 and Resin 2.
Interestingly, the T
g
of Resin 2 increased by 13.3 ◦C from 20.8 ◦C to
34.1 ◦C, demonstrating that the thiol-epoxy reaction had occurred in
Resin 2 with thermal treatment. This suggested that the Brønsted-basic
TBD was released from the photobase generator during the vat photo-
polymerization [6,7]. A similar increasing trend of the rubber modulus,
E
r
’
, was observed, correlating with the calculated results of
Ʋ
e
, according
to Flory’s idea elasticity theory [41]. The T
g
and mechanical properties
of the thiol-ene resin would be improved by adding epoxy monomers as
comonomers [42] because the interpenetrated hybrid thiol-ene/epoxy
network was obtained via UV-induced polymerization and thermally
cured polymerization [43]. It was observed that there was only one
narrow peak of tan δ for Resin 2 after heating for 5 h at 120 ◦C, which
indicated that the polymer network was homogeneous and that no
phase-separated structure was formed [44].
Table 2
Thermal properties including FWHM, glass temperature transition (T
g
), storage modulus (E’), crosslinking density (
Ʋ
e
), and T
d5%
and T
max
of the resins.
Resins Thermal treatment FWHM (
o
C) T
g
a
(
o
C tan δ peak) E’
Tg+50k
(MPa)
Ʋ
e
(mol ×cm
-3
×10
−3
) T
d5%
T
max
Control Without 18.3 32.3 4.2 0.47 323 422
With 16.4 33.4 5.1 0.57 318 423
Resin 1 Without 19.1 20.9 4.4 0.51 307 403
With 20.8 22.7 5.2 0.60 322 376/424
Resin 2 Without 19.5 20.8 4.3 0.50 279 405
With 20.2 34.1 6.9 0.77 260 387
Fig. 6. TG and DTG curves of 3D printed samples.
B. Yu et al.
Polymer Testing 116 (2022) 107767
7
3.4. Thermogravimetry analysis
The thermal stability of the resins was characterized by TGA under
nitrogen. The TG curves are shown in Fig. 6. Table 2 shows the 5%
weight loss temperature (T
d5%
) and temperature of the maximum
weight loss rate (T
max
). The T
d5%
of the resins exceeded 260 ◦C, indi-
cating the good thermal stability of the resins. However, compared with
Control and Resin 1, the T
d5%
of Resin 2 was lower with and without
thermal treatment. This suggested that the presence of Resin 2 nega-
tively affected the thermal stability of the resins. In addition, the T
max
of
Fig. 7. Ultimate tensile stress (UTS) of the 3D printed samples with and without thermal treatment: (a) Control, (b) Resin 1, and (c) Resin 2. Tensile stress–strain
curves of the 3D printed samples with and without thermal treatment: (d) Control, (e) Resin 1, and (f) Resin 2.
Fig. 8. Toughness of 3D printed samples: (a) Control, (b) Resin 1, and (c) Resin 2. Anisotropy of 3D printed samples with different printed orientations: (d) Control,
(b) Resin 1, and (c) Resin 2.
B. Yu et al.
Polymer Testing 116 (2022) 107767
8
Resin 1 and Resin 2 were lower than that of Control without or with
thermal treatment (Table 2). A possible reason was that TBD, which was
released from Resin 2 by photocleavage and thermal cleavage, acceler-
ated the thermal decomposition of the ester bond in the thiol and (meth)
acrylate monomers [45,46].
3.5. Tensile properties
The effect of the printed orientation and thermal treatment on tensile
properties was investigated by tensile testing. Fig. 7 shows the ultimate
tensile stress (UTS) plots and tensile stress–strain curves of the 3D
printed samples with and without thermal treatment. As shown in
Fig. 7a, the printed orientation greatly affected the tensile properties of
Control. It was observed that the sample printed at 90◦was the stron-
gest. The reason could be explained that the interface adhesion between
layers may be more robust than that printed with 0◦and 45◦printed
orientation. In addition, the samples printed at 90◦have highest number
of layers which were parallel to the direction of load during the tensile
test that leads to greater tensile stress [47].
Prior to the thermal treatment, the UTS of the printed orientation
samples was in a trend of 90◦>45◦>0◦. This indicated that their tensile
properties were anisotropic with different printed orientations. After
heating at 120 ◦C for 5 h, the UTS of Control printed in all three printed
orientations increased to an extent, and a similar increasing trend for
UTS with an increase in the printed orientation was observed. Inter-
estingly, the UTS of the 90◦oriented printed samples increased
approximately 200% from ~2 to ~6 MPa, considerably greater than
those of the 0◦and 45◦samples. (Fig. 7a). As shown in Fig. 7b, the tensile
stress of Resin 1 was lower than that of Control with the same printed
orientation, indicating that the substitute of 0.05 mol% dimethacrylate
monomer systems, EM2204/TEGDMA, with Resin 1 negatively affected
the tensile properties. As mentioned, owing to the epoxy groups in the
monomer, GMA had no contribution to crosslink during the photo-
polymerization. Thus, Resin 1 was equivalent to a monofunctional
monomer. The low functionality of GMA might have led to a low
macroscopic cross-density, resulting in a loose polymer network, leading
to a reduction in tensile stress [29]. The tensile stress with all (0◦, 45◦,
and 90◦) printed orientations of Resin 2 was comparable to that of Resin
1 without thermal treatment.
After heating at 120 ◦C for 5 h, the tensile stress of Resin 1 had a
slight variation. However, the tensile stress of Resin 2 increased
approximately 500% from ~1 to ~5 MPa with all printed orientations
(Fig. 7c). The tensile strain-at-break also increased from ~40% to 200%.
Moreover, the UTS of all orientations became nearly the same. It
conrmed our hypothesis that the interfacial adhesion had been
improved by the thiol-epoxy, which was catalyzed by the TBD released
from Resin 2 during the thermal treatment [23,25]. In addition, an
interpenetrating network was formed by the thiol-ene and thiol-epoxy
reactions, which improved the tensile properties [38].
3.6. Anisotropic properties
The anisotropic properties were characterized through the tensile
toughness [6] of the samples printed in different directions, and the
tensile toughness of the 3D printed samples was calculated from the
integral area under the stress–strain curves. The effects of printed di-
rection and thermal treatment on the toughness and anisotropy were
investigated. As shown in Fig. 8a, the tensile toughness of Control
without thermal treatment increased with an increase in the printed
angle (90◦>45◦>0◦). Correspondingly, the anisotropy of Control
reduced with an increase in the printed angle (Fig. 8c). The anisotropy of
the 0◦printed orientation was 67.1%, and that of the 45◦printed sam-
ples was 21.5%. After thermal treatments, because the tensile toughness
of the 90◦printed samples signicantly increased, correlating with their
UTS (Fig. 7a), and their anisotropy increased to 88.9% (0◦printed
orientation) and 81.2% (45◦printed orientation). Compared with Con-
trol, the tensile toughness of Resin 1 changed to a degree without and
with thermal treatment (Fig. 8b). The degree of anisotropy with the
0◦and 45◦printed orientation was reduced from 59.3% to 13.7% and
from 58.1% to 32.7%, respectively (Fig. 8e). The tensile toughness and
degree of anisotropy of Resin 2 with the same printed orientations were
similar to those of Resin 1 without thermal treatment (Fig. 8c and f).
After thermal treatments, the trend of the tensile toughness of Resin 2
with each printed orientation was similar to their UTS. Importantly, the
degree of anisotropy of the 0◦and 45◦printed orientation were reduced
from 53.1% to 4.2% and 25.9%–1.9%, respectively (f). It exhibited su-
perior isotropic properties in the printed parts with thermal treatment.
As discussed, the reason was that the interface adhesion was enhanced
Fig. 9. Representative SEM images (45
◦printed orientation) of the tensile fracture surface of Control (a and d), Resin 1 (b and e) and Resin 2 (c and f) without and
with thermal treatment (x 200).
B. Yu et al.
Polymer Testing 116 (2022) 107767
9
by a thiol-epoxy reaction catalyzed by TBD between the layers. There-
fore, the hypothesis of this research is acceptable.
3.7. The morphological study
To further judge the homogeneity of polymeric network and inves-
tigate the fracture mechanism, the tensile fracture surface of Control,
Resin 1 and Resin 2 was examined by scanning electron microscope
(SEM). The fracture surfaces of the three resins without and with ther-
mal treatment were smooth (Fig. 9), indicating that the fracture mech-
anism was brittle fracture. It was consistent with tensile stress-strain
curves (Fig. 7). Different from Control, microphase-separated mor-
phologies that were shown as particles well distributed in the matrix
were observed in Resin 1(Fig. 9b and e) and Resin 2 (Fig. 9c and f),
suggesting that the matrix formed a microphase separation structure by
adding epoxy monomer GMA. It may be attributable to the great dif-
ference in solubility parameters of epoxy monomers (GMA) with (meth)
acrylate monomers (EM2204 and TEGDMA) and thiol monomers
(PETMP) [48].
When the morphological surface was further magnied to 5000
times (Fig. 10), the microphase was shown as islands in Resin 1 (Fig. 10b
and e) and Resin 2 (Fig. 10c and f). The size of the microphase shown as
island become bigger in Resin 1 (Fig. 10e) after thermal treatment, it
may be attributed to that the self-polymerization of epoxy groups pro-
moted phase separation. Conversely, the size of microphase shown as
the islands in Resin 2 (Fig. 10f) become smaller and more uniformly
after thermal treatment, indicating that the thiol-epoxy reaction cata-
lyzed by TBD during heating induced phase separation. The possible
reason was that dispersed domain size would decrease with increasing
cross-linking density since the introduction of cross-links would kineti-
cally restrict the growth of the domain size [44].
4. Conclusion
We successfully prepared isotropic tensile photosensitive resins for
DLP 3D Printing using orthogonal thiol-ene and thiol-epoxy dual-cured
strategies. GMA, as a crosslink, was added into thiol-ene light-sensitive
resins containing TPO and TBD-HBPh
4
/ITX for 3D printing. During the
molding process, the thiol-ene resins were photopolymerized by a step
polymerization mechanism. TBD was released from TBD-HBPh
4
by light-
cleaving. The interface adhesion between layers was enhanced by the
thiol-epoxy reaction catalyzed using TBD. Thus, the tensile strength and
toughness of the 3D printed parts with each orientation improved
approximately by 500% after heating for 5 h at 120 ◦C. Importantly, the
degree of anisotropy was reduced to less than 5% with each printed
orientation.
Author statement
Biao Yu: Writing – original draft, conceptualisation, Methodology,
Investigation.
Jiaying Zheng: Methodology, Investigation.
Jiazhen Wu: Investigation.
Hao Ma: Investigation.
Xiaoqin Zhou: Methodology.
Yonghai Hui: Methodology, Writing – review & editing.
Fang Liu: Writing – review & editing.
Jingwei He: Writing – review & editing, supervision.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgements
This research was supported by the foundation of Nature Science
Foundation of China. (No. 21805125, 21902070 and 81970974), Nature
Science of Lingnan Normal University (No. ZL1609, ZL1905), The
Guangdong Basic and Applied Basic Research Foundation
(No.2021A1515010165), The Special Fund of Science and Technology
Planning Project of Maoming City (No.2020KJZX023) and the Open
Project of Key Laboratory of Chean Energy Material Chemistry in
Fig. 10. Representative SEM images (45
◦printed orientation) of tensile fracture surface of Control (a and d), Resin 1 (b and e) and Resin 2 (c and f) without and with
thermal treatment (x 5000).
B. Yu et al.
Polymer Testing 116 (2022) 107767
10
Guangdong General University (CEMC2022018). We also thank Dr.
Long Qingwu (Shunde Polytechnic, Foshan, 528333, China) for giving
help of SEM study.
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