Thiol-Ene Elastomers Derived from Biobased Phenolic Acids with Varying Functionality

Article abstract of DOI:10.1021/acs.macromol.6b01018

The synthesis and physical properties of thiol-ene elastomers derived from plant-based phenolic acids were explored. Phenolic acids of varying functionality (ranging from 2 to 4 hydroxyl and carboxyl groups per molecule) and relative placement of function

Full text of DOI:10.1021/acs.macromol.6b01018

Article
pubs.acs.org/Macromolecules
Thiol−Ene Elastomers Derived from Biobased Phenolic Acids with
Varying Functionality
Guozhen Yang,† Samantha L. Kristufek,‡ Lauren A. Link,‡ Karen L. Wooley,‡ and Megan L. Robertson*,†
†Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-4004, United States
‡Department of Chemistry, Department of Chemical Engineering, Department of Materials Science & Engineering, Texas A&M
University, College Station, Texas 77842-3012, United States
S
* Supporting Information
ABSTRACT: The synthesis and physical properties of thiol−
ene elastomers derived from plant-based phenolic acids were
explored. Phenolic acids of varying functionality (ranging from 2
to 4 hydroxyl and carboxyl groups per molecule) and relative
placement of functional groups (ortho, meta, para) were allylated
and subsequently reacted with a multifunctional thiol using a
photoinitiator. The thermal and mechanical behaviors of the
resulting elastomers were characterized. The networks derived
from difunctional allylated phenolic acids exhibited narrow glass
transitions (indicating a high degree of network homogeneity)
and glass transition temperatures (Tg) which correlated with
their cross-link density. The para placement of allyl groups on the allylated phenolic acid produced a network with the highest
cross-link density, Tg, modulus, tensile strength, and elongation at break (followed by ortho and then meta). As the functionality
of the allylated monomer increased (to 3−4 allyl groups per molecule), the cross-link density remained high yet the Tg decreased,
attributed to a lower concentration of benzene rings throughout the network structure (as all networks were prepared at the
stoichiometric ratio of allyl and thiol functional groups). The networks derived from the higher functionality allylated phenolic
acids also exhibited lower elongation at break and associated tensile strength and tensile toughness, likely due to increased
heterogeneity of the networks (indicated by higher glass transition widths compared to the networks derived from difunctional
allylated phenolic acids). All networks exhibited behavior consistent with an ideal elastomer (affine network) at low to moderate
strains, albeit with lower moduli than predicted from the monomer chemical structure. At the high end of the strain ranges
achieved, some of the networks exhibited strain hardening behavior. This work develops fundamental relationships between the
molecular structure of the phenolic acids, including number and placement of functional groups, and the physical properties of
the resulting networks.
conditions are employed, and the ability to conduct solvent-free
syntheses enhances their environmental benefit. Relatively few
studies have reported the derivation of thiol−ene elastomeric
films using biorenewable components; some notable examples
include the use of vegetable oils and their fatty acids,8−12
carbohydrates,13,14 terpenes,15 and other plant-derived mole-
cules.16,17
Here, we report on the utilization of plant-based phenolic
acids as the ene-containing components of thiol−ene networks.
Phenolic acids are plant metabolites widely distributed in
nature.18−21 They are often found in plant byproducts including
the skins and seeds of fruits and vegetables.18−21 Phenolic acids
offer many advantages as biorenewable monomers: their rigid
aromatic rings are expected to provide mechanical strength to
the resulting polymers, and the presence of multiple hydroxyl
groups and carboxyl groups leads to ease of functionalization.
INTRODUCTION
■
The development of sustainable feedstocks to replace
petroleum for the derivation of polymers has been a subject
of much recent interest in academia and industry.1,2 Thiol−ene
elastomers, which are traditionally derived from petrochemical-
based thiol- and ene-containing monomers, are attractive
materials for coatings and adhesives3 due to their ease of
fabrication, low shrinkage and stress,4 and high degree of
network homogeneity3,5−7 relative to other elastomeric
materials. The thiol−ene chemistry employed to synthesize
these elastomeric films has many advantageous features
including high conversion and yield, rapid reaction rates,
solvent-free conditions, lack of water and oxygen sensitivity,
lack of byproducts, and ability to impart spatial and temporal
control over the reaction (in the case of photoinitiation).3,5
A
diverse array of ene- and thiol-bearing molecules have been
investigated for the preparation of thiol−ene networks,3,5,6
though they are predominantly derived from petroleum
sources. Biorenewable molecules are attractive replacements
for the components of thiol−ene elastomers as mild reaction
Received: May 15, 2016
Revised: August 16, 2016
© XXXX American Chemical Society
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and thiol−ene networks (prior to exposure of the sample with UV, and
after 15 and 30 min of UV exposure, and 30 min of isothermal curing
at 150 °C). FTIR spectra are included in the Supporting Information
(Figure S7).
Through the choice of phenolic acid, the number and relative
placement of hydroxyl and carboxyl groups can be varied, which
is expected to be a convenient method of tuning the physical
properties of the resulting polymers. We have investigated five
allylated phenolic acids as components of thiol−ene networks:
difunctional molecules with varying placement of functional
groups (ortho, meta, and para) and molecules with varying
number of functional groups (ranging from 2 to 4). This work
extends an earlier publication which compared the behavior of
thiol−ene networks derived from the difunctional o- and p-
hydroxybenzoic acids;22 herein we include the m-hydroxy-
benzoic acid as well as the tri- and tetrafunctional phenolic
acids. The thermal, mechanical, and structural properties of the
thiol−ene networks were investigated. This work develops
fundamental relationships between the functionality of the
phenolic acids (number and placement of functional groups)
and the physical properties of the resulting networks, providing
new insight into the tailored design of biorenewable monomers
for thiol−ene networks. Gaining such knowledge is an
important first step toward the widespread implementation of
biobased molecules in thiol−ene network applications.
Monomer Synthesis. Allylation of the phenolic acids was
conducted following literature procedures.22,23 Phenolic acid (10 g)
was dissolved into 340 mL of N,N-dimethylformamide (DMF, BDH,
≥99.8%, ACS reagent) in a 1000 mL glass round-bottom flask
equipped with a rubber septum and a magnetic stirring bar. The
temperature was maintained at 0 °C using an ice bath. Potassium
carbonate (K2CO3, ≥99.0%, ACS reagent) was added to the flask. The
molar ratio of K2CO3 to phenolic acid was 2.20 to 1.00 (for 3HBA),
3.30 to 1.00 (for GenA), and 4.40 to 1.00 (for GalA). After 3 min of
stirring, allyl bromide (97%) was added dropwise with a syringe (the
molar ratio of allyl bromide to phenolic acid was 2.20 to 1.00 (for
3HBA), 3.30 to 1.00 (for GenA), and 4.40 to 1.00 (for GalA)). The
solution was stirred at room temperature for 48 h. Next, 340 mL of
distilled water was added into the solution. The solution was mixed
with an equivalent volume of ethyl acetate (BDH, ≥99.5%, ACS
grade), and a separatory funnel was used to recover the ethyl acetate
phase which contained the product. The remaining aqueous phase was
extracted two additional times with ethyl acetate (the volume of ethyl
acetate was equal to the aqueous phase volume in each extraction).
The organic phase containing the allylated phenolic acid was washed
with an equivalent volume of saturated brine and purified through
drying with magnesium sulfate (BDH, ≥99.0%, anhydrous reagent
grade) followed by distillation using a rotary evaporator to remove
ethyl acetate. DMF was removed from the allylated phenolic acid
through drying in a vacuum oven at 50 °C, until the NMR peaks
associated with DMF (7.96, 2.94, and 2.78 ppm) were not observed.
NMR and FTIR spectra obtained on SA, 4HBA, aSA, and a4HBA have
been previously reported in ref 22.
EXPERIMENTAL DETAILS
■
Materials. All chemicals were purchased from Sigma-Aldrich unless
otherwise noted below. Five phenolic acids were used in this study:
salicylic acid (SA, ≥99%, FG/Halal/Kosher), 4-hydroxybenzoic acid
(4HBA, 99%, ReagentPlus), 3-hydroxybenzoic acid (3HBA, 99%,
ReagentPlus), gentisic acid (GenA, 98%), and gallic acid (GalA, 97.5−
102.5% by titration). The chemical structures of all phenolic acids are
shown in Figure 1.
1
3-Hydroxybenzoic Acid (3HBA). H NMR (400 MHz, DMSO-d6,
ppm): δ 12.74 (broad s, 1H), 9.75 (broad s, 1H), 7.33 (d, J = 7.56 Hz,
1H), 7.29 (s, 1H), 7.24 (t, J = 7.56 Hz, 1H), 6.95 (d, J = 7.56 Hz, 1H).
13C NMR (100 MHz; DMSO-d6, ppm): δ 167.9, 157.9, 132.6, 130.1,
120.5, 120.4, 116.3.
Allyl 3-Allyloxybenzoate. (allylated 3HBA is referred to as
1
“a3HBA” in this article). H NMR (400 MHz, DMSO-d6, ppm): δ
7.54 (ddd, J = 7.79, 1.37, 1.37 Hz, 1H), 7.44 (dd, J = 2.75, 1.83 Hz,
1H), 7.42 (dd, J = 7.79, 7.79 Hz, 1H), 7.23 (ddd, J = 8.24, 2.75, 0.92
Hz, 1H), 6.06−5.96 (m, 2H), 5.40−5.34 (m, 2H), 5.24 (ddt, J = 10.53,
1.37, 1.37 Hz, 2H), 4.77 (ddd, J = 5.50, 1.37, 1.37 Hz, 2H), 4.61 (ddd,
J = 5.04, 1.37, 1.37 Hz, 2H). 13C NMR (100 MHz; DMSO-d6, ppm): δ
165.7, 158.8, 133.9, 133.1, 131.4, 130.6, 122.1, 120.6, 118.5, 118.1,
115.2, 68.9, 65.7. FTIR (ATR): 3082, 2943, 2877, 1720, 1649, 1599,
1585, 1487, 1443, 1424, 1361, 1319, 1291, 1272, 1213, 1158, 1107,
1078, 1027, 993, 983, 928, 875, 809, 754, 682, 640 cm−1.
Figure 1. Chemical structures of phenolic acids used in this study: (a)
salicylic acid (SA), (b) 3-hydroxybenzoic acid (3HBA), (c) 4-
hydroxybenzoic acid (4HBA), (d) gentisic acid (GenA), (e) gallic
acid (GalA).
1
Gentisic Acid (GenA). H NMR (400 MHz, DMSO-d6, ppm): δ
13.75 (broad s, 1H), 10.65 (broad s, 1H), 9.13 (s, 1H), 7.11 (d, J =
2.93 Hz, 1H), 6.92 (dd, J = 8.79, 2.93 Hz, 1H), 6.75 (d, J = 8.79 Hz,
1H). 13C NMR (100 MHz; DMSO-d6, ppm): δ 172.3, 154.6, 149.9,
124.3, 118.3, 115.0, 113.1.
Allyl 2,5-Bis(allyloxy)benzoate. (allylated GenA is referred to as
“aGenA” in this article). 1H NMR (400 MHz, DMSO-d6, ppm): δ 7.18
(d, J = 2.93 Hz, 1H), 7.10 (dd, J = 8.79, 2.93 Hz, 1H), 7.05 (d, J = 8.79
Hz, 1H), 6.03−5.92 (m, 3H), 5.43−5.32 (m, 3H), 5.23−5.17 (m, 3H),
4.71 (ddd, J = 5.37, 1.47, 1.47 Hz, 2H), 4.54−4.50 (m, 4H). 13C NMR
(100 MHz; DMSO-d6, ppm): δ 165.7, 152.1, 151.8, 134.2, 134.0,
133.1, 121.5, 120.3, 118.3, 118.0, 117.4, 116.8, 116.3, 70.0, 69.3, 65.5.
FTIR (ATR): 3084, 3020, 2987, 2946, 2880, 1729, 1705, 1648, 1612,
1581, 1559, 1497, 1455, 1421, 1360, 1281, 1237, 1201, 1156, 1105,
1066, 1025, 996, 926, 892, 810, 780 cm−1.
Nuclear Magnetic Resonance (NMR). The following NMR
experiments were performed on a JEOL ECA-400 instrument using
deuterated dimethyl sulfoxide (Cambridge Isotope Laboratories, Inc.,
99.9% D) as the solvent: 1H NMR (400 MHz), 13C NMR (100 MHz),
DEPT 45, 90, 135, COSY, HSQC, and HMBC. Chemical shifts were
referenced to the solvent proton resonance (2.5 ppm). Spectra
obtained from allylated SA (aSA) and allylated 4HBA (a4HBA) have
been previously reported in ref 22. Spectra obtained on 3HBA, GenA,
GalA, allylated 3HBA (a3HBA), allylated GenA (aGenA), and
allylated GalA (aGalA) are included in the Supporting Information
(Figures S1−S6).
Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra
were recorded on a Thermo Scientific Nicolet 4700 spectrometer in
transmission mode as well as using an attenuated total reflection
(ATR) stage (containing a germanium crystal). The OMNIC Series
software was used to follow selected peaks at 1.928 cm−1 resolution
using 32 scans. FTIR spectra were collected on allylated monomers
Gallic Acid (GalA). 1H NMR (400 MHz, DMSO-d6, ppm): δ 12.22
(s, 1H), 9.17 (s, 2H), 8.82 (s, 1H), 6.88 (s, 2H). 13C NMR (100 MHz;
DMSO-d6, ppm): δ 168.0, 145.9, 138.5, 120.9, 109.2.
Allyl 3,4,5-Tris(allyloxy)benzoate. (allylated GalA is referred to as
“aGalA” in this article). 1H NMR (400 MHz, DMSO-d6, ppm): δ 7.21
(s, 2H), 6.07−5.93 (m, 4H), 5.41−5.22 (m, 7H), 5.14 (ddt, J = 10.5,
1.47, 1.47 Hz, 1H), 4.75 (ddd, J = 5.37, 1.47, 1.47 Hz, 2H), 4.61 (ddd,
B
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J = 4.88, 1.47, 1.47 Hz, 4H), 4.51 (ddd, J = 5.86, 1.47, 1.47 Hz, 2H).
13C NMR (100 MHz, DMSO-d6, ppm): δ 165.4, 152.4, 141.6, 134.9,
134.0, 133.2, 125.1, 118.4, 118.0, 117.8, 108.4, 73.7, 69.9, 65.7. FTIR
(ATR): 3080, 3021, 2985, 2945, 2871, 1716, 1648, 1586, 1498, 1455,
1422, 1375, 1362, 1328, 1290, 1264, 1236, 1204, 1130, 1109, 986, 926,
863, 812, 764 cm−1.
Differential Scanning Calorimetry (DSC). The glass transition
temperature (Tg) was measured through DSC experiments conducted
using a TA Instruments Q2000 calorimeter, calibrated with an indium
standard, with a nitrogen flow rate of 50 mL/min. The cured sample
(following the protocol in the Polymer Synthesis section) was placed
in the calorimeter (using a Tzero aluminum pan), equilibrated at 40
°C, cooled to −40 C at a rate of 10 °C/min, and heated to 40 °C at a
rate of 10 °C/min. The cooling and heating scans were repeated for a
total of two measurements. The value of the Tg was determined using
the half extrapolated tangents method in the Universal Analysis
software.24 The onset and endset temperatures were also identified
with the Universal Analysis software.
Thermogravimetric Analysis (TGA). TGA experiments were
conducted with a TA Instruments Q500 analyzer. The sample was
cured following the protocol in the Polymer Synthesis section and
transferred to the analyzer. The cured sample was heated from 30 to
800 °C at a rate of 10 °C/min in an argon environment (the balance
argon purge flow was 40 mL/min and the sample purge flow was 60
mL/min).
Tensile Testing. Tensile testing was carried out with an Instron
5966 universal testing system containing a 2 kN load cell. Dogbone-
shaped testing bars (ASTM D638, bar type 5, thickness 0.4 mm) were
prepared following the procedure in the Polymer Synthesis section.
Pneumatic grips (maximum force 2 kN) were used to affix the sample
in the testing frame, at a compressed air pressure of 40 psi. The force
and change in length were measured as the sample was elongated at a
rate of 10 mm/min. Each measurement was repeated with five test
specimens that broke in the gauge region and did not contain a visible
defect at the point of fracture.
Density Measurement. The densities of phenolic acid-based
thiol−ene networks were measured in a vial containing a calcium
nitrate water solution of known concentration at room temperature.
The water content was adjusted to determine the composition range
over which the polymer transitioned from being suspended in the
solvent to sinking in the solvent. This range was narrowed until the
density could be determined to 2 significant digits after the decimal
point. The exact density of calcium nitrate solution was determined
through the quadratic equation fitting to the density−concentration
data in a previous study.25 The resulting densities were as follows: aSA-
based thiol−ene networks, 1.22 g/mL; a3HBA-based thiol−ene
networks, 1.22 g/mL; a4HBA-based thiol−ene networks, 1.22 g/mL;
aGenA-based thiol−ene networks, 1.22 g/mL; aGalA-based thiol−ene
networks, 1.22 g/mL.
Polymer Synthesis. UV curing of thiol−ene networks followed
procedures similar to those reported in ref 16 and has been previously
reported by our group for UV curing of aSA and a4HBA in ref 22. The
allylated phenolic acid was mixed with pentaerythritol tetrakis(3-
mercaptopropionate) (PETMP, >95%) (stoichiometry based on equal
molar functional groups) and 1 wt % of the photoinitiator 2,2-
dimethoxy-2-phenylacetophenone (DMPA, 99%) at room temper-
ature in a 20 mL vial (using magnetic stirring), which was covered by
aluminum foil. The mixture was placed in the following sample holders
appropriate for each characterization experiment: (a) between two
glass slides with a 0.4 mm glass spacer for TGA, DSC, DMA, and
ATR-FTIR, (b) between two NaCl windows (32 mm diameter, 3 mm
thick) with a 0.05 mm Teflon spacer for transmission-mode FTIR, and
(c) in a Teflon dogbone-shaped mold following ASTM D638 (bar
type 5, thickness 0.4 mm) for tensile testing. The sample was exposed
under continuous 365 nm UV light (4 W, Spectroline ENF-240C) for
15 min and transferred to a convection oven at 150 °C for a specified
period of time, summarized in Table 1. FTIR spectra obtained on
networks synthesized from allylated SA and allylated 4HBA were
reported in ref 22.
Table 1. Composition and Curing Protocol of Thiol−Ene
Networks
allylated
molar ratio of reagents
(allylated phenolic acid:
PETMP:photoinitiator)
UV exposure time/
isothermal postcuring time
at 150 °C
phenolic
a
acid
aSA
1.0:0.50:0.018
1.0:0.50:0.018
1.0:0.50:0.018
1.0:0.75:0.025
1.0:1.0:0.032
15 min/10 min
15 min/10 min
15 min/10 min
15 min/20 min
15 min/20 min
a3HBA
a4HBA
aGenA
aGalA
a
The phenolic acids were allylated and subsequently cured with
pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and a
photoinitiator, as described in the Experimental Details.
Network Synthesized from A3HBA. FTIR (ATR): 2922, 2852,
1737, 1716, 1600, 1584, 1488, 1469, 1443, 1387, 1352, 1320, 1275,
1225, 1140, 1106, 1074, 1033, 997, 937, 886, 808, 756, 683 cm−1.
Network Synthesized from AGenA. FTIR (ATR): 2917, 2849,
1736, 1609, 1580, 1541, 1498, 1467, 1423, 1388, 1353, 1284, 1239,
1202, 1144, 1031, 930, 817, 784, 766, 746 cm−1.
Network Synthesized from AGalA. FTIR (ATR): 2916, 2849,
1737, 1647, 1586, 1499, 1469, 1429, 1387, 1355, 1332, 1288, 1207,
1145, 1114, 1052, 1015, 931, 871, 843, 766 cm−1.
Dynamic Mechanical Analysis (DMA). The dynamic mechanical
behavior of cured thiol−ene films (following the protocol in the
Polymer Synthesis section) was probed using a Q800 dynamic
mechanical analyzer (TA Instruments) with a nitrogen environment.
Specimens of 0.4 mm thickness were cut with a razor blade to have the
following dimensions: 10 mm × 5 mm × 0.4 mm (length × width ×
thickness).
Four experiments and analyses were conducted: (1) isothermal
strain sweeps were conducted at desired temperatures and using a
frequency of 1 Hz to locate the range of strains in the linear
viscoelastic region; (2) isothermal frequency sweeps were conducted
from 0.1 to 10 Hz at desired temperatures, using a strain within the
linear viscoelastic region; (3) time−temperature superposition of the
data was performed at −10, −5, 0, 5, 10, 15, 20, 30, and 40 °C; and (4)
temperature ramps were conducted at a constant strain and frequency.
In the case of time−temperature superposition, the master curve was
prepared using the TA Instruments Rheology Advantage Data Analysis
software with 30 °C chosen as the reference temperature.
RESULTS AND DISCUSSION
■
Synthesis of Thiol−Ene Networks Derived from
Phenolic Acids. Five phenolic acids were explored as
components of thiol−ene networks: SA, 3HBA, and 4HBA
(difunctional phenolic acids with ortho, meta, and para
placements of functional groups, respectively); GenA (a
trifunctional phenolic acid); and GalA (a tetrafunctional
phenolic acid). Allylation of the phenolic acids was conducted
following procedures previously reported by our group,22 also
following a prior literature study.23 Figure 2 shows the allylated
versions of each phenolic acid.
NMR was used to monitor the progress of the allylation
reaction (Figure 3 and Figures S1−S6; data obtained on aSA
and a4HBA were discussed in ref 22), which are agreement
with prior literature.26−31 The H NMR spectra of all allylated
1
phenolic acids show the disappearance of peaks located in the
range of 8−14 ppm associated with the carboxyl and hydroxyl
groups in the phenolic acids. The peaks located in the region of
4−6 ppm correspond to the allyl groups in the allylated
phenolic acids. The ratios of the peak area associated with the
CH2−O protons on the allyl group to the peak area associated
with the aromatic protons are very close to the theoretical
predictions (Table 2). The conversions of the allylation
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Figure 2. Chemical structures of allylated phenolic acids used to
prepare thiol−ene networks: (a) aSA, (b) a3HBA, (c) a4HBA, (d)
aGenA, and (e) aGalA.
reactions were quite high, 97−100% (Table 2), calculated using
1
the integrals of peaks associated with aromatic protons of H
NMR data obtained on the allylated phenolic acids prior to
extraction (reported in Figures S2, S4, and S6). The yields of
allylated phenolic acids were around 80% (Table 2). The
relative number and placement of functional groups on the
phenolic acids did not have a significant impact on the allylation
reaction, as evidenced by similar conversions and yields in all
five phenolic acids (Table 2 and ref 22).
The photoinitiated thiol−ene reaction between the allylated
phenolic acids and the tetrafunctional thiol PETMP (Scheme 1
and Scheme S1), which follows the well-established step-
growth radical mechanism,3,5 was monitored through FTIR.
Allyl ethers generally exhibit high reactivities in the thiol−ene
reaction,3 and thiol−allyl ether systems exhibit high prop-
agation rates relative to rates of chain transfer.32
FTIR spectra obtained upon UV curing of the allylated
phenolic acids with the multifunctional thiol PETMP are shown
in Figure S7 for a3HBA, aGenA, and aGalA and were
previously reported in ref 22 for aSA and a4HBA (assignment
of vibrational modes is summarized in Table S1). In all spectra,
conversion of the thiol was monitored through disappearance
of the peak located at 2570 cm−1 (S−H stretching), quantified
in Tables S2−S6, while network formation was observed
through an increase in the intensity of the peak located at 2950
cm−1 (alkane C−H stretching). Conversion of allyl groups on
the allylated phenolic acids was monitored by decreases in the
following peak intensities over time: 932 and 996 cm−1 (olefinic
C−H bending), 1647 cm−1 (CC stretching), and 3080
cm−1 (olefinic C−H stretching). We could not determine
whether the peaks associated with the allyl groups disappeared
completely, as they were all located in the vicinity of
neighboring peaks (Figure S7). The peak located at 932 cm−1
may also include contributions from C−O stretching of ester
groups present in both the phenolic acids and PETMP.33,34 As
increasing the UV exposure time beyond 15 min did not result
in any appreciable differences in the FTIR spectra or
conversion (Tables S2−S6), 15 min was chosen as the UV
exposure time for all allylated phenolic acids (Table 1).
FTIR and differential scanning calorimetry (DSC) were
employed to identify the isothermal annealing time for the
thiol−ene networks following UV exposure to achieve the
highest possible conversion of functional groups. Thiol−ene
networks were prepared through 15 min of exposure to UV
followed by isothermal curing in a convection oven at 150 °C
(the reaction conversion increased after the 150 °C postcure,
detailed in Tables S2−S6). In Figures S8 and S9, the maximum
1
Figure 3. H NMR data obtained from (a) a3HBA, (b) aGenA, and
(c) aGalA. Additional spectra are included in Figures S1−S6. Spectra
obtained from aSA and a4HBA were previously reported in ref 22.
D
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1
varying (3HBA is the m-hydroxybenzoic acid, complementing
SA and 4HBA, which are respectively o- and p-hydroxybenzoic
acids). The biorenewable contents of the aGenA and aGalA
networks (derived from tri- and tetrafunctional allylated
phenolic acids) are calculated to be 23 and 20 wt %,
respectively (GenA is one of the most commonly found
aromatic acids in plants,38 in both edible and nonedible
components, and GalA is found in plant sources such as bark,
leaves, and nuts39). We conducted similar calculations for prior
literature studies which report thiol−ene networks derived from
biorenewable components and found that the biorenewable
content in prior studies on thiol−ene networks ranged from 20
to 58 wt %.8−10,12−16 One strategy to increase the total
biorenewable content in our materials would be to employ a
multifunctional thiol derived from a renewable resources,10,17 in
addition to the allylated phenolic acids.
Physical Properties of Thiol−Ene Networks Derived
from Phenolic Acids of Varying Functionality. Dynamic
mechanical analysis (DMA) was used to explore the dynamic
moduli of the thiol−ene networks. The strain and frequency
dependencies of the storage (E′) and loss (E″) moduli are
shown in Figures S10 and S11 at selected temperatures. Time−
temperature superposition was applied to the frequency-
dependent E′ and E″. Reduced moduli (E′r and E″r) were
first obtained by multiplying each modulus by a vertical shift
factor, bT:40,41
Table 2. H NMR Characterization of Phenolic Acid
Allylation
phenolic
acid
conv peak area of CH2O on allyl group:peak area of yield
a
(%)
aromatic protons
(%)
3HBA
GenA
GalA
99.4
97.5
100
4.12:4.00 (4:4)
6.06:3.00 (6:3)
8.10:2.00 (8:2)
77
78
80
a
Theoretical ratio is given in parentheses.
absorbance of various FTIR peaks and Tg are plotted as
functions of the isothermal annealing time. For the networks
derived from difunctional allylated phenolic acids (aSA, a3HBA,
and a4HBA), the conversion and Tg did not change upon
increasing the isothermal annealing time beyond 10 min, and
therefore an isothermal annealing time of 10 min was chosen.
In the case of networks derived from multifunctional allylated
phenolic acids (aGenA and aGalA), a slightly longer isothermal
annealing time of 20 min was chosen (to ensure the maximum
possible conversion of functional groups, which did not change
for annealing times beyond 20 min). The UV and isothermal
curing protocols are summarized in Table 1. The final
conversion of all networks following UV curing and isothermal
annealing is reported in Table S7; the aSA network exhibited
the highest conversion (97%), followed by the a3HBA, a4HBA,
and aGenA networks (88%), with the lowest conversion
achieved by the aGalA network (83%). The allyl ester and allyl
ether moieties are anticipated to exhibit slightly different
reactivities (increasing the electron density of the ene
component increases the thiol−ene reaction rate;3 the allyl
ether has a higher electron density than the allyl ester35), and
therefore it is more likely that unreacted allyl groups are
associated with an ester rather than an ether. Increasing the UV
exposure time and isothermal annealing time did not increase
the conversion of any of the thiol−ene networks reported here.
The inability to achieve 100% conversion in these networks is
likely due to trapped functional groups (allyl and thiol) in the
network at relatively high conversion.
We quantified the biorenewable content in the thiol−ene
networks by calculating the wt % of phenolic acid in the final
material. The biorenewable content (i.e., the contribution of
the phenolic acids, which can be derived from plant sources) of
the aSA and a4HBA networks, both derived from difunctional
allylated phenolic acids, is calculated to be 29 wt % (SA is found
in various fruits and vegetables,36 including their waste
products, and 4HBA is found in coconut husks37). We do
not consider a3HBA to be a biorenewable chemical, as there is
not a significant plant-based source for 3HBA to our
knowledge. Rather, we have included this chemical in this
study to highlight observed trends in the physical properties of
the thiol−ene networks derived from difunctional phenolic
acids, in which the relative placement of functional groups is
T
T
0
bT =
(1)
E′r = bTE′
(2)
E″r = bTE″
(3)
where the reference temperature, T0, was taken to be 30 °C.
The reduced moduli were then shifted horizontally by applying
a horizontal shift factor, aT, at each temperature:
ωr = aTω
(4)
where ωr are the reduced frequencies. At each temperature, aT
was identified as that required to produce a smooth and
continuous master curve. The master curves containing the
shifted data are shown in Figure 4.
The Williams−Landel−Ferry (WLF) equation is well-
established for describing the temperature dependence of aT
for polymers at temperatures above or in the vicinity of the
Tg:42
−C1(T − T )
0
log aT =
C2 + (T − T )
(5)
0
The WLF equation fit to the shift factor as a function of
temperature is shown in Figure 5. In all networks, the data are
consistent with the WLF equation. C1 is an empirical
a
Scheme 1. Photoinitated Thiol−Ene Reaction between a3HBA and the Tetrafunctional Thiol PETMP
a
Equivalent thiol−ene reactions for the other allylated phenolic acids (aSA, a4HBA, aGenA, and aGalA) are shown in Scheme S1.
E
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parameter; however, C2 can be described as C2 = fr/αf, where fr
is the fractional free volume of the material at the reference
temperature and αf is the coefficient of thermal expansion of
the free volume (αf ∼ αl − αg, where αl and αg are the
coefficients of expansion of the liquid and glassy states,
respectively). Significant differences are observed in the C2
values for all five networks, possibly indicating differences in the
values of fr and αf for these materials.
The cross-link density (υc) of each thiol−ene network was
calculated from E′ in the rubbery plateau region of the plot of
E′ vs ω (Figure S11), using the theory of rubber elasticity:43
E′
3RT
υ =
c
(6)
where R is the gas constant. The resulting cross-link densities
are summarized in Tables S8−S12, in which measurements
were obtained on multiple specimens over multiple independ-
ently prepared specimens for each type of thiol−ene network.
Figure 6a shows the resulting average values of the cross-link
densities for the thiol−ene networks of varying functionality.
The cross-link densities (and also plateau moduli) of the thiol−
ene networks prepared from the difunctional aSA and a3HBA
(ortho and meta placements of allyl groups, respectively) were
quite similar to one another. A significant increase in cross-link
density was observed for the thiol−ene network prepared from
the difuctional a4HBA (para placement of allyl groups). We
previously proposed that the lower cross-link density of the aSA
network as compared to the a4HBA network is due to the
neighboring allyl groups found on aSA; the structure of the aSA
network contains bulky aromatic groups intruded into network,
which impacts the networks architecture and reduces the cross-
link density.22 Surprisingly, we see that the ortho and meta
placements of the allyl groups (aSA vs a3HBA) result in very
similar cross-link densities, which are lower than that of the
network with para placements of allyl groups (using a4HBA).
Upon increasing the functionality (number of functional
groups, fallyl) to 3 and 4, the cross-link density remained fairly
consistent with that of the a4HBA network (compare values of
the a4HBA, aGenA, and aGalA networks in Figure 6a). We
note that although conversion of functional groups was high in
all the networks (in the range of 82−97% in our study; Tables
S2−S7), they did not achieve complete conversion; however,
the trends observed in the cross-link density are not explained
by minor differences in functional group conversion (Table
S20).
The glass transition temperatures (Tg) of thiol−ene networks
(prepared following the protocol in Table 1) were explored
through differential scanning calorimetry (DSC). Measure-
ments obtained on multiple independently prepared specimens
are summarized in Tables S13−S15. The Tg values measured
upon the first and second heating scans were comparable to
one another, within the error of the measurement. The average
and standard deviation values obtained during the first heating
scan are provided in Figure 6b and Table S16. The Tg’s of all
networks were lower than room temperature, as is often
observed in thiol−ene networks, which contain flexible
thioether linkages.44 First, we will compare the networks
derived from difunctional allylated phenolic acids (aSA, a3HBA,
and a4HBA). The a4HBA network exhibited the highest Tg,
attributed to the high cross-link density of this network (Figure
6c). Though the aSA and a3HBA networks had similar cross-
link densities, the Tg of the aSA network was significantly
higher than that of the a3HBA network (Figure 6c). Differences
Figure 4. Master curves for the reduced (a) storage modulus (E′r) and
(b) loss modulus (E″r) of thiol−ene networks derived from aSA
▲
▼
(green ), a3HBA (purple ◀), a4HBA (yellow ), aGenA (dark
■
●
blue ), and aGalA (red ). Reference temperature for time−
temperature superposition was 30 °C. Data obtained on aSA and
a4HBA networks were previously reported in ref 22.
in the aSA and a3HBA network Tg’s may possibly be explained
by differences in steric hindrance resulting from the relative
placements of the allyl groups around the aromatic ring in the
allylated monomer (ortho vs meta positions). Next, we examine
the effect of increasing the functionality of the allylated
phenolic acid on the network Tg. The aGenA and aGalA
networks, derived from the allylated phenolic acids with the
highest functionalities, exhibited high cross-link densities
(comparable to the a4HBA network), yet low Tg values
(comparable to the a3HBA network; Figure 6c). However,
these networks were all prepared at stoichiometric composi-
tions (i.e., equal concentrations of allyl and thiol groups; Table
1). As the functionality of the allylated phenolic acid increased,
the molar concentration of allylated phenolic acid used to
prepare the network (required for stoichiometric balance with
the multifunctional thiol PETMP) thus decreased. We
hypothesize that the presence of the aromatic rings on the
allylated phenolic acids increases the glass transition of the
networks. Thus, decreasing the molar concentration of aromatic
rings throughout the network (through decreasing the
F
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concentration of allylated monomer) resulted in reduction of
the Tg.
Figure 5. Temperature-dependent shift factor, aT, for thiol−ene
▲
networks derived from aSA (green ), a3HBA (purple ◀), a4HBA
■
▼
●
(yellow ), aGenA (dark blue ), and aGalA (red ). Curves
indicate the fit of the WLF equation to the data (for aSA, green curve:
C1 = 9.64 and C2 = 82.47 K; for a3HBA, purple curve: C1 = 6.599 and
C2 = 63.93 K; for a4HBA, yellow curve: C1 = 5.85 and C2 = 53.15 K;
for aGenA, solid dark blue curve: C1 = 16.27 and C2 = 90.35 K; for
aGalA, solid red curve: C1 = 10.69 and C2 = 72.18 K). Data obtained
on aSA and a4HBA networks were previously reported in ref 22.
Thiol−ene networks typically exhibit sharp glass transitions,
indicating a high degree of network homogeneity.3,5−7 We
observed similarly sharp transitions in the DSC and DMA data,
shown in Figures S12 and S13. This is in contrast with the
behavior observed in free radical and sulfur cross-linked
networks with greater levels of inhomogeneity.45−49 The aSA,
a3HBA, and a4HBA networks exhibited glass transitions with
similarly narrow widths (Table 3), indicating comparable
degrees of homogeneity in the networks derived from
difunctional allylated phenolic acids, regardless of the place-
ment of functional groups (ortho, meta, para). As the
functionality of the allylated phenolic acid was increased to 3
(aGenA) and 4 (aGalA), the Tg width increased, presumably
indicating the formation of less homogeneous networks.
Similarly, two peaks were observed in the DMA data for
aGalA and aGenA networks (Figure S12), providing further
evidence of increased heterogeneity in those networks.
Tensile testing was employed to probe the mechanical
behavior of the thiol−ene networks. Tensile experiments were
conducted on multiple specimens for each network type
(reported in Figure 7a and Figures S14−S16); the average
values of relevant parameters are shown in Table S17. The data
in Figure 7a were fit to the ideal elastomer (affine network)
model, described by43
Figure 6. (a) Cross-link density (υc) and (b) glass transition
temperature (Tg) of thiol−ene networks derived from aSA (green
⎛
⎜
⎝
⎞
⎟
⎠
E
3
1
σ =
λ −
λ2
■
▲
▼
(7)
), a3HBA (purple ◀), a4HBA (yellow ), aGenA (dark blue ),
●
and aGalA (red ), where fallyl is the functionality (number of allyl
groups) of the allylated phenolic acid used to prepare the thiol−ene
network. o, m, and p designations on the plot indicate networks
derived from the o-, m-, and p-hydroxybenzoic acids (fallyl = 2),
respectively. In (c) Tg is plotted as a function of υc. The standard
deviations on these plots indicate error characterized through multiple
where λ is the stretch ratio (λ = 1 + ε, where ε is the strain) and
E is the tensile modulus (Young’s modulus). Figure 7b shows a
plot of the tensile data, formatted to highlight consistency with
the ideal elastomer model. The data are consistent with the
ideal elastomer model and low to moderate strains for all five
G
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Figure 6. continued
measurements obtained on multiple independently prepared speci-
mens. All data are summarized in Tables S8−S16.
Table 3. Homogeneity of Thiol−Ene Networks Derived
a
from Allylated Phenolic Acids
allylated phenolic
acid
Tg width (°C) from
Tg width (°C) from
b
c
d
fallyl
DMA
DSC
e
aSA
2 (o)
2 (m)
2 (p)
3
9.6
10.7
10.0
19.4
17.1
5.8 0.9
6.1 0.3
6.3 0.5
15.7 0.8
a3HBA
e
a4HBA
aGenA
aGalA
4
14.5 0.9
a
b
Samples were prepared following the protocol in Table 1. fallyl is the
functionality (number of allyl groups) of the allylated phenolic acid
c
used to prepare the thiol−ene network. Full width at half-maximum
of peak observed in tan δ as a function of temperature (Figure S12).
d
e
Difference of the onset and endset temperatures (Figure S13). Data
obtained on SA and 4HBA networks were previously reported in ref
22.
types of thiol−ene networks. At higher strains, in the vicinity of
the elongation at break, some of the networks (a4HBA, aGenA,
and aGalA) exhibited a positive deviation from the ideal
network model (at a given strain, stress was higher than
predicted).
A Mooney−Rivlin plot is traditionally used to identify
nonideal behavior in networks. The Mooney−Rivlin model is
defined as50,51
2C2
λ
σ
λ −
= 2C1 +
1
λ2
(8)
where C1 and C2 are empirical constants. In the case of an ideal
network, C2 is equal to 0. The tensile data are plotted in the
Mooney−Rivlin format, as shown in Figure 7c. The behaviors
of both the SA and 3HBA networks were consistent with the
ideal elastomer model, resulting in a Mooney−Rivlin coefficient
C2 of close to 0 over the entire strain range, for all specimens
that were tested (Table S18). The other networks (a4HBA,
aGalA, and aGenA) were also consistent with the ideal
elastomer model (and C2 was close to 0) at low to moderate
strains. As the strain was increased, the Mooney−Rivlin plots
for the a4HBA, aGalA, and aGenA networks exhibited a
negative slope (and negative value of C2 reported in Table
S18). Networks traditionally exhibit strain softening at low to
moderate strains, observed as a positive slope in the Mooney−
Rivlin plot, attributed to trapped entanglements and described
by models that account for topological constraints.52−54 The
tensile behavior of the thiol−ene networks examined here is not
consistent with trapped entanglements but is similar to that
reported previously for spatially homogeneous gels55,56 and
end-linked poly(dimethylsiloxane) networks.57 At higher strain
values, the deviations from the ideal network model observed in
Figure 7 for the a4HBA, aGalA, and aGenA networks (showing
a negative slope in Figure 7c) are indicative of strain hardening
of the networks. Strain hardening is usually observed under two
conditions: (1) strain-induced crystallization of the network58
or (2) deviation from Gaussian strand conformations in the
network59 (both typically observed at much higher strain values
than were achieved in Figure 7).
Figure 7. (a) Representative data showing tensile stress (σ) as a
function of strain (ε); (b) tensile data plotted to highlight consistency
with the ideal elastomer model (eq 7), where λ is the stretch ratio; and
(c) tensile data plotted in the Mooney−Rivlin format (following eq 8).
In all figures the solid curves indicate data obtained from thiol−ene
networks derived from aSA (green), a3HBA (purple), a4HBA
(yellow), aGenA (dark blue), and aGalA (red). The dashed black
curves indicate the fit of the ideal elastomer model eq 7 to the data.
Data were obtained on multiple independent specimens of each
H
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high degree of homogeneity of the network. The agreement
with the ideal elastomer model (and fit with the Mooney−
Rivlin equation with C2 close to 0) implies a lack of
heterogeneities such as dangling ends and trapped entangle-
ments in these networks.47,62 However, there is one important
inconsistency observed in fitting the model to the data. The
molecular weight between cross-links (Mc) can be readily
calculated from the cross-link density (Mc = ρ/υc) (in the range
of 800−1100 g/mol, Table S19). If we consider a perfect
network (without any defects), then we can calculate the
molecular weight between junctions in the network using the
chemical structure of the network (in the range of 200−500 g/
mol, Table S20 and Figure S17). The experimental Mc is
significantly higher than the predicted Mc for all networks
(differing by factors of 2 and 4 for networks derived from
difunctional [aSA, a3HBA, a4HBA] and multifunctional
[aGenA, aGalA] allylated phenolic acids, respectively).
Similarly, the experimental cross-link densities of all networks
are lower than the theoretical predictions (Tables S19 and
Figure 7. continued
sample type, and all tensile data are shown in Figures S14−S16 (for
a3HBA, aGenA, and aGalA). Data obtained on aSA and a4HBA
networks were previously reported in ref 22. In (c), the parameters
extracted from the fit of eq 8 to the data are shown in Table S18 and
reported in ref 22. Data obtained at 1/λ > 0.95 were not included in
(c), based on uncertainties in the tensile testing measurements at these
strain values (ε < 0.05).60
The ideal elastomer model (of an affine network) makes the
following assumptions regarding the structure of the network:
all elastic chains in the network have the same length, all cross-
link junctions have the same functionality, the network is
homogeneous, and each effective elastic chain obeys Gaussian
statistics.43,61 Thiol−ene chemistry traditionally results in
networks of high conversion of functional groups (in the
range of 82−97% in our study; Tables S2−S7) and sharp glass
transitions (Table 3), generally taken to be an indication of the
Figure 8. (a) Tensile strength, (b) tensile modulus, (c) % elongation at break, and (d) tensile toughness for thiol−ene networks derived from aSA
■
▲
▼
●
(green ), a3HBA (purple ◀), a4HBA (yellow ), aGenA (dark blue ) and aGalA (red ), where fallyl is the functionality (number of allyl
groups) of the allylated phenolic acid used to prepare the thiol−ene network. o, m, and p designations on the plot indicate networks derived from the
o-, m-, and p-hydroxybenzoic acids (fallyl = 2), respectively. The standard deviations on these plots indicate error characterized through multiple
measurements obtained on multiple independently prepared specimens (Table S17). Data obtained on aSA and a4HBA networks were previously
reported in ref 22. The tensile parameters plotted as a function of the cross-link density are shown in Figure S18.
I
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S20). We hypothesize that primary loop formation63−65 may be
significant in these networks, thereby increasing Mc and
decreasing both υc and the modulus.
given strain was higher than that predicted for an ideal
elastomer.
ASSOCIATED CONTENT
* Supporting Information
The tensile properties of the thiol−ene networks were
characterized using measurements conducted on multiple test
bars prepared from multiple independently prepared specimens
(Figure 8 and Table S17). In the networks prepared from
difunctional allylated phenolic acids, the a4HBA network (para
position) exhibited the highest tensile modulus (Figure 8) and
associated cross-link density (Figure 6), following by the aSA
(ortho) and then a3HBA (meta) networks. The a4HBA
network also exhibited the highest elongation at break, tensile
strength, and tensile toughness compared to the a3HBA and
aSA networks (Figure 8). We previously attributed differences
in the mechanical properties of the aSA and a4HBA networks
to microscopic changes in the network which impact alignment,
rotation, displacement, and stretching of bonds, which are likely
to be impacted by the placement of the allyl groups around the
ring (ortho, meta, para).22 Here we can conclude that both meta
and ortho placements are less favorable arrangements of
functional groups as compared to the para placement. In the
case of networks derived from multifunctional phenolic acids
(aGenA and aGalA), the high tensile modulus and low
elongation at break and toughness may be attributed to two
factors: the high cross-link density (Figures 6) and increased
heterogeneity (evidenced by the higher glass transition width in
Table 3) of the networks.
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.6b01018.
NMR spectra obtained on the phenolic acids and
1
allylated phenolic acids, including H NMR, 13C NMR,
DEPT 45, 90, 135, COSY, HSQC, and HMBC (Figures
S1−S6); synthetic schemes for photoinitiated thiol−ene
reaction between allylated phenolic acids and PETMP
(Scheme S1); FTIR spectra obtained before and after
curing (Figure S7); assignment of FTIR vibrational
modes (Table S1); thiol−ene reaction conversion
quantified through FTIR (Tables S2−S7); FTIR peak
intensities and glass transition temperatures as functions
of isothermal postcure time (Figures S8 and S9); storage
and loss moduli as a function of strain (Figure S10) and
frequency (Figure S11) determined through DMA;
cross-link density determined from DMA using repeat
measurements on various specimens (Tables S8−S12);
glass transition temperatures determined through DSC
using repeat measurements on various specimens (Tables
S13−S15); average and standard deviation of glass
transition and thermal degradation temperatures (Table
S16); tan δ vs temperature measured through DMA
(Figure S12) and raw DSC data (Figure S13) showing
width of the glass transition; stress−strain curves for each
specimen measured and included in the average values of
the tensile parameters reported (Figures S14−S16);
average and standard deviations of tensile properties
(Table S17); parameters extracted from fit of Mooney−
Rivlin equation to the tensile data (Table S18);
calculation of molecular weight between cross-links
using the DMA modulus and theoretical predictions
(Table S19 and Figure S17); and tensile parameters as
functions of the cross-link density (Figure S18) (PDF)
CONCLUSIONS
■
Biobased phenolic acids (found in plant sources) were allylated
and subsequently reacted with a multifunctional thiol (in a
photoinitiated reaction) to form networks. A series of phenolic
acids were explored in which the relative number (2−4) and
placement (ortho, meta, para) of functional groups was varied.
The networks derived from difunctional allylated phenolic acids
exhibited narrow glass transitions (indicating a high degree of
network homogeneity) and glass transition temperatures (Tg)
which correlated with their cross-link density. The para
placement of allyl groups on the allylated phenolic acid
produced a network with the highest cross-link density, Tg,
modulus, tensile strength, and elongation at break (followed by
ortho and then meta). These variations in thermal and
mechanical behavior were attributed to differences in the
cross-link densities of the networks as well as microscopic
changes in the network upon deformation, which are likely
impacted by the placement of functional groups around the
aromatic ring. As the functionality of the allylated monomer
increased (to 3−4 allyl groups per molecule), the cross-link
density remained high yet the Tg decreased, attributed to a
lower concentration of aromatic rings throughout the network
structure (as all networks were prepared at the stoichiometric
ratio of allyl and thiol functional groups). The networks derived
from the higher functionality allylated phenolic acids also
exhibited lower elongation at break and associated tensile
strength and tensile toughness, likely due to increased
heterogeneity of the networks (indicated by higher glass
transition widths compared to the networks derived from
difunctional allylated phenolic acids). All networks exhibited
behavior consistent with an ideal elastomer (affine network) at
low to moderate strains, albeit with lower moduli than
predicted from the monomer chemical structure. At the high
end of the strain ranges achieved, some of the networks
exhibited strain hardening behavior, in which the stress at a
AUTHOR INFORMATION
Corresponding Author
*E-mail mlrobertson@uh.edu; Tel 713-743-2748 (M.L.R.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support from the National
Science Foundation (CMMI-1334838 (M.L.R.) and CHE-
1410272 (K.L.W.)), the Norman Hackerman Advanced
Research Program of the Texas Higher Education Coordinating
Board (003652-0022-2013, M.L.R.), the University of Houston
(M.L.R.), and the Welch Foundation (A-0001, K.L.W.). The
authors thank Ramanan Krishnamoorti, Brian Rohde, and
Daehak Kim (University of Houston) for access, training, and
data analysis on the FTIR and TGA instruments. We thank
Hiruy Tesefay (University of Houston) for assistance with
monomer synthesis and purification. We appreciate the
assistance of Charles Anderson for access and training in the
University of Houston Department of Chemistry Nuclear
Magnetic Resonance Facility. We appreciate the contribution of
Haleh Ardebili and Mejdi Kammoun (University of Houston)
J
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́ ́ ́
(19) Manach, C.; Scalbert, A.; Morand, C.; Remesy, C.; Jimenez, L.
for access and training on the DMA instrument. We thank
Bradley D. Olsen and Rui Wang (Massachusetts Institute of
Technology) for helpful discussions regarding primary loop
formation in networks. We appreciate the advice and assistance
of Shu Wang, Avantika Singh, and Vivek Yadav (University of
Houston).
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