1. 1
Improving Performance of Photopolymer Resins Through
Oxygen Desensitization
Kenneth Ainslie
U.S. Department of Energy Office of Science, Science Undergraduate Laboratory Internship
(SULI)
California Polytechnic State University, San Luis Obispo
Lawrence Berkeley National Laboratory
Berkeley, California
August 5, 2016
Prepared in partial fulfillment of the requirements of the U.S. Department of Energy Office of
Science, Science Undergraduate Laboratory Internship (SULI) under the direction of Raymond
Weitekamp and Aditya Balasubramanian in the Material Science Division at Lawrence Berkeley
National Laboratory

2. 2
INTRODUCTION
Photo-functional resins and photopolymers are crucial components in 3D lithographic
fabrication. Resins are the final building material for 3D printing systems such as direct light
projection (DLP) and stereolithography (SLA).1
Though current resins in commercial use have
low oxophilicity, they lack the chemical and mechanical integrity to be functional components of
a mechanical or thermal system. Commonly used 3D printing resins form highly cross-linked
brittle structures utilizing a free radical mechanism. Once this reaction initiates radical formation,
it is no longer sensitive to oxygen and successfully undergoes chain growth polymerization.2
The
resulting polymeric structures are brittle and only suitable for prototyping rather than direct mass
manufacturing of a product.
Production of higher toughness, 3D printable components requires a different mechanism
of chain growth and cross-linking. One such way of photopolymerization utilizes olefin
metathesis reactions. The polymerization method studied in this work is photolithographic-based
olefin metathesis polymerization (PLOMP).3
This new photo-functional process operates by UV
excitation of ruthenium based Grubb’s catalyst (FIG. 1). The catalyst initiates polymerization of
cyclic olefin monomers (e.g., dicyclopentadiene (DCPD)) to form cyclic olefin polymers
(COPs). COPs are a class of cross-linked, tough, heat resistant, and chemically inert thermoset
polymers typically used in specialty resins.4
However, the metallic center in the catalyst is
readily oxidized in ambient conditions, and once this occurs the catalyst is exhausted and the
polymer can no longer polymerize.5,6
ABSTRACT
Photolithographic-based olefin metathesis polymerization (PLOMP) has shown promise in the
additive manufacturing industry due to its ability to produce polymers with exceptional
mechanical behavior and functional group tolerances [Note: the resulting polymer has the
exceptional mechanical behavior, not the polymerization process]. The major drawback of
polymer resins used in PLOMP is catalyst decomposition due to oxygen in ambient
environments. In this study, the impact of visco enhancers and antioxidants as oxygen-
protective species on the ambient resin performance was analyzed. Raman spectroscopy
analysis indicates that increasing the viscosity by 30x improves the ambient life span of the
material by more than two orders of magnitude. The introduction of an antioxidant improved
performance at low loadings of 0.1 - 0.2 wt. % 4-methoxyphenol (MEHQ) by 9x.
FIG. 1 – PLOMP mechanism. The catalyst performs a photo-activated ring-opening metathesis polymerization (ROMP)
when excited by UV light. The reaction above is what builds the poly(DCPD) chains while cross-linking occurs through the
thermal energy generated during PLOMP.

3. 3
The desired outcome of the project is to desensitize PLOMP resins to oxygen in ambient
atmospheres, thereby enhancing the scope of applications for PLOMP resins in manufacturing.
In this study, the extreme oxygen sensitivity of the resin was addressed using chemical and
physical modifications to the resin system. The resin was modified in two different ways:
increasing viscosity (physical barrier) and implementation of antioxidants (chemical radical
scavengers). Increased viscosity was achieved through the addition of long chain entangled
polymers. The increase in viscosity slows the diffusion of oxygen through DCPD and ultimately
reduces the number of interactions between the catalyst and oxygen. Antioxidants react with
oxygen radicals to make non-reactive product. To test the efficacy of these modifications, a
series of experiments were conducted that measured the amount of time the resin could be
exposed to ambient conditions before the PLOMP resins no longer polymerized.
MATERIALS AND METHODS
Preparation of Resist Samples: Resins with latent catalyst were prepared by first synthesizing the
modified Grubb’s ruthenium catalyst according to Weitekamp3
under inert N2 conditions. 0.25
wt/v. % of the antioxidant 4-methoxyphenol (MEHQ) was added to degassed DCPD monomer
resin and mixed until homogenous.3
The resin was then pumped into a quasi-inert environment
of low purity N2, and latent catalyst in chloroform was pipetted into the resin mixture and mixed
thoroughly to make the resin photosensitive. The resist was shielded from light during the entire
process. 2 ml of resin were pipetted into 4 ml vials while in the quasi-inert atmosphere so that
each vial could be opened at a specific time during tests. The vial opened at time zero served as
the control during the viscosity series and air exposure studies.
Antioxidant Studies: Four different types of antioxidants were purchased from Sigma-Aldrich.
The antioxidants included were 4-methoxyphenol (MEHQ), 4,4′-methylenebis(2,6-di-tert-
butylphenol) (Ethanox ® 702), Tris(2,4-di-tert-butylphenyl) phosphite (Irgafos®
168), and l,3,5-
trimethyl-2,4,6-tris (3,5-di-tert- butyl-4-hydroxybenzyl) benzene (Ethanox®
330). The basis for
selecting these complexes can be found in discussion. Various antioxidant loadings were added
to DCPD for testing (Table I).
Table I – Antioxidant Loadings
Antioxidant Weight/Volume %
MEHQ 0.25 (Control)
702 0.3, 3.0a
330 0.3, 3.0a
168 0.3 ,3.0a
a
from patent: Giardello, Michael A. et al. “Polyolefin compositions having variable density and methods for their
production and use.” U.S. Patent 6,525,125.25 February 2003.
Visco enhancer Studies: Polystyrene-block-polyisoprene-block-polystyrene (PSPIPS) was
chosen as the visco enhancer. PSPIPS loadings ranged from 0 to 10 wt/v. % and were exposed to
air for 0, 2, and 60 minute increments (t=0 was irradiated by UV light within a glove box). The
loading that performed the best was examined under longer air exposure times of up to 4 hours.

4. 4
All visco enhancer solutions included 0.25 wt/v. % MEHQ for comparison to the current
formulation.
Viscosity Measurement: The viscosity of each type of resist was measured using the Brookfield
DV1 Viscometer in order to quantify the increase in viscosity with respect to 100% DCPD
monomer resin. Sample volumes were 6.7 ml following Brookfield guidelines for spindle SC-18
at 10-100 rpm for 1 minute. Each sample was tested thrice (n = 3) and viscosity measurements
were averaged.
UV Exposure: Two fixed plates were aligned as described by Vuorio7
to ensure a constant
distance between the sample and the light source. A 405 nm UV LED was used for irradiation
(FIG. 2). 300 μl of resin was pipetted into a 400 μl well, and 2 wells were exposed (n = 2) to UV
light for each time period. The 300 μl resin volume was used to prevent over-filling of the well,
which would lead to a curved resin surface of varying distance from the light source. To reduce
the possibility of resistive heating by the UV light source, a low power intensity of 22 mW/cm2
was used. This power intensity is the same value used in the DLP printer.8
The LED light source
was calibrated to 22 mW/cm2
± 2.0 mW/cm2
using Thorlabs’ digital optical power meter
console. Four measurements were made within a 1 cm2
area of the well location. A calibration of
the LED was done each day that an exposure test was conducted in order to account for day-to-
day LED power variation.
Prior to UV irradiation, the resin was exposed to air for 0, 2, or 60 minute periods. Resins
that showed greater stability were exposed to air for extended times of 2-4 hours. The vial of
resin opened at time zero remained within a glove box and was irradiated by UV light under the
flow of N2 (15 psi) in a 300 liter glove box. Oxygen levels in the glove boxes ranged from 100-
180 ppm during tests. The irradiance time for each sample was 4 minutes for a total energy input
of 5280 mJ. To prevent further PLOMP reactions, the well plates were placed into a dark box
immediately after exposure and covered with aluminum foil.
FIG. 2 – Three UV LEDs were fixed to the top aluminum plate, while the bottom plate held the PLOMP samples at
a constant distance from the bulbs to ensure repeatable irradiance levels.
Raman Spectroscopy: Raman spectroscopy was used to quantitatively measure the monomer
conversion percent (α) to a cross-linked polymer. The fraction of monomer converted was
calculated as the ratio of the sum of the areas under the peak norbornene and cyclopentene olefin
peaks to the area under the aliphatic cis and trans olefin peaks (FIG. 3). A total of ten α
Well plate
LED

5. 5
measurements were made for each type of resin. Each well was measured once at five different
depths from the surface of the sample. The lowest measurement was 500 μm form the surface.
FIG. 3 – Raman spectra of a characteristic PLOMP sample that has partial monomer conversion (55%). The higher
the intensity of the pDCPD peaks the more conversion there was during PLOMP and post processing.
RESULTS
Influence of Antioxidants
MEHQ proved to be the most effective oxygen scavenger of the 4 antioxidants analyzed
within a quasi-inert N2 atmosphere (FIG. 4). All antioxidant comparison tests were done in a
quasi-inert N2 atmosphere with low levels of oxygen (< 200 ppm) present.
FIG. 4 – MEHQ antioxidant performs the best among the 4 antioxidants studied in protecting the catalyst
from oxygen in solution and in air.
160 ± 21 ppm O2
pDCPD
DCPD

6. 6
0.25 wt/v. % MEHQ improves the performance of the PLOMP resins by ~ 8 times
compared to the control. 3.0 wt./v. % Ethanox®
702 marginally improves resin resistance to
oxygen by 1.3 times. The other antioxidants tested performed worse than the control indicating
they hindered PLOMP through chemical pathways. Since MEHQ performed the best out of the
antioxidants tested, a concentration series was performed to identify optimal MEHQ loading
(FIG. 5). Loadings between 0.05 wt/v. % and 0.4 wt/v. % MEHQ were tested.
FIG. 5 – Monomer conversion percentages from MEHQ concentration series show a maximum around 0.1 wt/v. %
MEHQ.
Raman spectra showed a higher α at lower concentrations of MEHQ. The optimal
concentration of MEHQ is likely to be between 0.1 and 0.2 wt/v. %, with α decreasing at lower
concentrations. This improved the resin conversion, within a low purity N2 atmosphere, by 9
times compared to a control in the quasi-inert testing environment.
Influence of viscosity
An increase in air stability with increase in viscosity is indicated by the lack of change in
α with extended periods of air exposure. α remains constant after 1 hour air exposure with 10
wt/v. % PSPIPS loading (FIG. 6) while the pure DCPD sample exhibits a change from 58% ±
1% to 26% ± 2% monomeric conversion. The 10 wt/v. % PSPIPS resin was tested at up to 4
hours of air exposure with relatively little change in α (25% ± 2% to 16% ± 2%) compared to
standard DCPD resins (54% ± 2% to 12% ± 2%) (FIG. 7). The 10% PSPIPS resin also solidified
at a lower value of α (40%) than observed in the pure DCPD resin. This resulted in a soft
elastomer that was observed to return to its original shape after deformation by hand.
156 ± 32 ppm O2

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FIG. 6 – Increasing viscosity decreases the maximum possible monomer conversion, likely due to the PSPIPS
slowing down the interaction between the catalyst and DCPD. All samples contain 0.25 wt/v. % MEHQ.
FIG. 7 – 10 wt/v. % PSPIPS resin maintains a relatively steady level of monomer conversion over 4 hours in an
ambient atmosphere.
To reveal the extent of improvement in resin oxygen-desensitization with increased
viscosity, the negative chemical influence of PSPIPS on inhibiting PLOMP reaction was
decoupled from the improvement in viscosity. This was accomplished by normalizing α to the
monomer conversion control (αₒ) (FIG. 8). We used time t80 at α / αₒ = 0.8αₒ as an arbitrary
figure of merit to measure the improvement due to viscosity. The high viscosity resins have t80 >
200 min compared to standard PLOMP resins that had t80 < 2 min. Thus, an increase in viscosity
from 5-150 cP increases the air stability of the resin by two orders of magnitude.
0 2.5 5 7.5 10
PSPIPS wt/v. %
Control in low purity N
2
(145 ± 26 ppm)
1 hour in air
0.25 wt.% MEHQ
10 wt.% PSPIPS,
0.25 wt.% MEHQ

8. 8
FIG. 8 – The MEHQ control samples show a drastic non-linear decrease in monomer conversion over just 1 hour of
exposure to air while the higher viscosity resin decays more linearly with a smaller slope.
DISCUSSION
Our findings indicate that increasing viscosity helps mitigate the degradation of the resin
likely by lowering the diffusivity of oxygen into the resin. Furthermore, this study demonstrates
that the antioxidant MEHQ was the most effective at scavenging oxygen radicals out of the four
antioxidants examined. A trend of slower α decay by increasing visco enhancer concentration
displayed varying degrees of stability at higher viscosities (> 70 cP). Lower loadings of MEHQ
(0.1-0.2 wt/v. %) appear to be optimal for PLOMP processes.
The antioxidants prevent interaction between the catalyst and oxygen through either
hindering a primary or secondary mechanism of oxidation (FIG. 9). Hindered phenols are
primary antioxidants, meaning they are oxygen radical scavengers. Oxygen radicals react with
the phenols, which inhibit oxidation by chain terminating reactions. The three primary
antioxidants in this study were Ethanox®
702, Ethanox®
330, and MEHQ. Both the Ethanox
antioxidants have been used in thermal curing processes to prevent polymer degradation,
especially in DCPD based resins.9
Organophosphorous compounds such as phosphites are
secondary antioxidants. Secondary antioxidants decompose hydroperoxides into non-radical,
non-reactive, and thermally stable products. The secondary antioxidant Irgafos®
168 was chosen
for oxidation reactions that would occur during post-bake steps following initial UV curing.10
0.8αₒ

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a b
c d
FIG. 9 – Antioxidant complexes (a) 4-Methoxyphenol (MEHQ). (b) 4,4′-Methylenebis(2,6-di-tert-butylphenol)
(Ethanox ® 702). (c)Tris(2,4-di-tert-butylphenyl) phosphite (Irgafos® 168. (d) l,3,5-trimethyl-2,4,6-tris (3,5-di-tert-
butyl-4-hydroxybenzyl) benzene (Ethanox® 330).
Surprisingly, none of the three alternate antioxidants that we chose for this study
outperformed MEHQ. The Ethanox®
702 was patented as an antioxidant for DCPD but still
substantially underperformed compared to MEHQ. The Ethanox®
330 and Irgafos®
168
antioxidants seemed to perform worse than the control sample. A possible explanation for the
poor performance is the alternate antioxidants were patented for use in thermal polymerization
processes where a catalyst would not necessarily be present. This could mean that all three
possibly reacted with the PLOMP catalyst in a negative manner.
Higher viscosity PLOMP resins perform better under ambient conditions over longer
periods of time. Due to its high molecular weight (> 30,000), long chain structure, ability to
dissolve high loadings (up to 20 wt/v. %) in DCPD, and low reactivity with the catalyst, PSPIPS
was chosen as the visco enhancer. From Figure 6 there seems to be a significant change in slope
between 5 wt/v. % and 7.5 wt/v. % PSPIPS (38-75 cP) where the higher loading of PSPIPS
significantly slows down PLOMP leading to a lower α. Looking at Figure 8, the higher viscosity
resin decays much slower suggesting oxygen is diffusing slower. This observation is supported
by one-dimensional diffusion models of oxygen diffusing into DCPD that predicts oxygen
diffusion at extended time scales (in comparison with the time scale of our study) (Eqn. 1).
Combining Stokes-Einstein equation with the generalized form of Fick’s 2nd
Law shows a linear
relationship between diffusion time and viscosity.
𝑡 =
𝑁𝐴6𝜋𝑎
4𝑅𝑇
𝐿2
𝜂
(1)

10. 10
where
𝑁𝐴6𝜋𝑎
4𝑅𝑇
𝜂 = 𝐷−1
D = diffusivity
η = viscosity
a = radius of diffusing molecule
NA = Avogadro’s Number
R = universal gas constant
T = temperature
L = diffusion distance
t = time to diffuse a distance L
A working theory on how the PSPIPS increases viscosity of the resin is by entangling
long chains together which impede the diffusion of oxygen. Entangling could occur through
homogenizing DCPD and PSPIPS together where the individual PSPIPS chains entangle with
each other through mixing (FIG. 10).
FIG. 10 - PSPIPS entangled with itself and isolating DCPD monomer and catalyst from oxygen. Increased amount
of PSPIPS should increase viscosity more by further entangling itself. Photo credit to Wang.11
Variations in α from sample to sample could be due to slight changes in temperature,
which would have a substantial impact on the PLOMP process in both approaches. Another
source of error could be due to variations in the power output of the UV LEDs, which degrade
over time and decrease irradiance.
CONCLUSIONS
A combination of higher viscosities and radical-scavenging antioxidants improves PLOMP
performance in ambient conditions over extended periods of time. Replacing PSPIPS with cross-
linkable moieties will increase the initial monomer conversion of high viscosity resins. These are
general methods that can be used in many photopolymer liquids sensitive to gaseous impurities.
By implementing these methods, the resins may retain their functionality over longer periods of
time in ambient conditions.

11. 11
ACKNOWLEDGMENTS
This work was supported in part by the U.S. Department of Energy, Office of Science, Office of
Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate
Laboratory Internship (SULI) program. Thank you to the polySpectra team: Aditya
Balasubramanian, Coleman Rainey, Corinne Allen, and Raymond Weitekamp. Also thank you to
Jean Lee for the use of her time and resources in this study. Thank you to the Molecular Foundry
staff especially Teresa Chen, Tevye Kuykendall, and Tracy Mattox. This work was also
supported by Cyclotron Road and Lawrence Berkeley National Laboratory.
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