Well-Defined Networks from DGEBF - The Importance of Regioisomerism in Epoxy Resin Networks

Article abstract of DOI:10.1021/acs.macromol.9b01441

The previously ignored or unreported impact of regiosomerism within diglycidyl ether of bisphenol F (DGEBF) on its network properties is presented. Routes to the isomers of DGEBF were explored: high-performance liquid chromatography showed good separation of the three isomers [para-para-DGEBF (ppDGEBF), para-ortho-DGEBF (poDGEBF), and ortho-ortho-DGEBF (ooDGEBF)] with small yields; column chromatography gave good separation of pp- + po- from oo-DGEBF but pp-/po- separation was not achieved. Synthesis was optimized to crude yields of 76% for pp-; 87% for po-, and 86% for oo-. Subsequently, crosslinked networks were prepared with meta-xylylenediamine. With increasing ortho content, degradation of chemical resistance and an inherent weakening of the network was observed, that is, glass transition temperature (T_g), beta transition temperature (T_β), density, crosslink density, and the desorption diffusion coefficient decreased, whereas sorption diffusion coefficient and ultimate solvent uptake increased. This clearly shows that a subtle chemical structure change can significantly impact network performance.

Full text of DOI:10.1021/acs.macromol.9b01441

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Cite This: Macromolecules XXXX, XXX, XXX−XXX
Well-Defined Networks from DGEBFThe Importance of
Regioisomerism in Epoxy Resin Networks
Stephen T. Knox,*,†,§ Anthony Wright,‡ Colin Cameron,‡ and John Patrick Anthony Fairclough†
†Department of Mechanical Engineering, University of Sheffield, Sheffield S1 4BJ, U.K.
‡AkzoNobel, International Paint Limited, Stoneygate Lane, Gateshead NE10 0JY, U.K.
S
* Supporting Information
ABSTRACT: The previously ignored or unreported impact
of regiosomerism within diglycidyl ether of bisphenol F
(DGEBF) on its network properties is presented. Routes to
the isomers of DGEBF were explored: high-performance
liquid chromatography showed good separation of the three
isomers [para−para-DGEBF (ppDGEBF), para−ortho-
DGEBF (poDGEBF), and ortho−ortho-DGEBF (ooDGEBF)]
with small yields; column chromatography gave good
separation of pp- + po- from oo-DGEBF but pp-/po-
separation was not achieved. Synthesis was optimized to
crude yields of 76% for pp-; 87% for po-, and 86% for oo-. Subsequently, crosslinked networks were prepared with meta-
xylylenediamine. With increasing ortho content, degradation of chemical resistance and an inherent weakening of the network
was observed, that is, glass transition temperature (Tg), beta transition temperature (Tβ), density, crosslink density, and the
desorption diffusion coefficient decreased, whereas sorption diffusion coefficient and ultimate solvent uptake increased. This
clearly shows that a subtle chemical structure change can significantly impact network performance.
However, ingress of solvent molecules into the networks is
possible, which causes swelling. This swelling induces stress in
the coating and in turn damagetherefore, longevity of the
coating is related to the solvent uptake.4,5 Furthermore,
swelling leads to plasticization, a depression of the glass
transition temperature (Tg), and reduced mechanical perform-
ance.6−10 There are two relevant measures relating to solvent
uptakethe amount of uptake and the speed of that uptake.
Both the free volume and network polarity (which are both
directly related to the network structure) have been identified
as important influencing factors for chemical perform-
ance.9,11−16 To explore the impact that nuanced changes to
the network structure have upon chemical performance, it is
imperative to produce well-defined networks.
In order to form a well-defined network, ensuring a good
extent of reaction between the reactive groups is important.
When using hardeners, a key consideration is the ratio of
mixing of the reactive components, that is, the stoichiometry.
Generally, formulation for the highest crosslink densities
involves aiming for a 1:1 (stoichiometric) ratio between
reactive sites. In the case of epoxy−amine reactions, this is two
epoxy groups per primary amine, as a primary amine can react
twice before forming the final tertiary product in the network.
The extent of reaction of the reactive groups can be monitored
using near-infrared (NIR) spectroscopy.17
INTRODUCTION
■
Epoxy resins are used in a wide variety of applications, from
coatings to composites. These applications subject the epoxy
resin to a wide range of chemical and environmental assailants.
Composite structures such as wind turbines and aircraft
structures are exposed to weathering, encapsulation coatings
may be exposed to challenging chemical environments, high
temperatures, or both. This study determines the effect of
regioisomerism within diglycidyl ether of bisphenol F
(DGEBF) on the chemical performance, in this case the
resistance to methanol (selected as a model penetrant), as an
indicator of chemical performance, which is an indicator of the
barrier this network would form.
Epoxy resin polymer networks are produced on the reaction
of multifunctional epoxide molecules with (a) themselves in
the presence of a catalyst or (b) another multifunctional
reactive component (or “hardener”), such as an anhydride or
amine. Bisphenol-based epoxy monomers are commonly used,
typically diglycidyl ether of bisphenol A or F (DGEBA or
DGEBF). Epoxy networks have a high crosslink density
contributing to increased chemical performance and strength/
stiffness. These properties can be tuned by adjusting the
structure of the network.1−3
Chemical performance describes the resistance of a material
to chemical attackmost commonly by solvent, although
resistance to oxidation and flammability are also worthy of
consideration. Epoxy networks provide good chemical
performance and thus protection from a range of solvents as
crosslinking means that they are insoluble in any solvent.
Received: July 10, 2019
Revised: August 9, 2019
© XXXX American Chemical Society
A
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ortho−ortho-bisphenol F (Tokyo Chemical Industry UK Ltd.);
epichlorohydrin (Alfa Aesar); crystal violet solution (0.5% solution
in glacial acetic acid), meta-xylylenediamine (MXDA), and
tetrabutylammonium bromide (Sigma-Aldrich Co. Ltd.); tetraethy-
lammonium bromide, acetic acid, and potassium hydroxide (Fisher
Scientific); and perchloric acid (0.1 mol L−1 solution in acetic acid)
(VWR International).
Preparatory High-Performance Liquid Chromatography. A
Varian ProStar high-performance liquid chromatography (HPLC)
system was used, with two 210 pumps and a 320 UV detector. The
autosampler used was a 410 model, with a 1000 μL injection loop.
The column (19 × 250 mm) was a Waters XBridge Prep C18 5 μm
OBD. The detector was operated at 280 nm, and the system was run
at a flow rate of 17.0 mL min−1.
The chemical structure of the component parts greatly
influences the structure and hence properties of the formed
network, such as the glass transition temperature of the
network. Two commonly used amines are 3,3′- and 4,4′-
diaminodiphenylsulfonewith the difference in their chemical
structure being the position of the amine groups around the
aromatic rings. Upon stoichiometric cure with DGEBA, they
exhibit a 40 °C difference in Tg (4,4′ greater than 3,3′), despite
this subtle chemical change.18 Variation in Tg for amines has
also been shown using the regioisomers of phenylene
diamine.19
All bisphenol-based epoxy monomers show chain extension:
oligomeric forms of the monomer are present in the resin
mixture resulting from the synthetic method. Additionally,
DGEBF has been shown to contain further complexity in its
composition. Nuclear magnetic resonance (NMR) spectrosco-
py shows that DGEBF contains both para- and ortho-
substitution on the aromatic rings contained within its
structure, leading to the presence of the three isomers, pp,
po, and oo (Figure 1). Prior to this work, it was not known
how this molecular complexity is likely to impact on any
subsequent network relative to an individual isomer of
DGEBF.
NMR Spectroscopy. All NMR spectroscopy was completed using
a Bruker AVANCE 400 MHz spectrometer. Chemical shifts are given
relative to the lock solvent.
Titration for Epoxide Equivalent Weight. Tetraethylammo-
nium bromide (40 g, 0.19 mol) was dried for approx. 1 h at 100 °C. It
was then dissolved with warming in acetic acid (450 cm3) and allowed
to cool. Eight drops of 0.5% crystal violet solution in acetic acid were
added, and the mixture neutralized to an emerald green color using
0.1 mol dm−3 perchloric acid in acetic acid (approx. 5 cm3). Samples
of DER 354 (approx. 0.1 g) were dissolved in this mixture and then
titrated with 0.1 mol dm−3 perchloric acid in acetic acid to the
emerald green end-point. The epoxide equivalent weight (EEW) was
then calculated from eq 1, where m is mass of the sample, T is the
titre, and [PA] is the concentration of perchloric acid.
m
EEW =
T × [PA]
(1)
Column Chromatography of DER 354. The low molecular
weight components, that is the isomers of diglycidyl ether bisphenol
F, present in DER 354 (11 g) were separated by column
chromatography, with a column volume of approximately 1400 cm3.
The stationary phase was 0.6 Å silica for flash chromatography. The
mobile phase was 35:65 ethyl acetate/hexane. Two main fractions
were obtained [F1 (average mass obtained 1.05 g), F2 (average mass
obtained 5.45 g)].
Optimization of Epoxidation of Bisphenol A. Route A:
bisphenol A (2.279 g, 9.98 mmol) was dissolved in epichlorohydrin (8
mL) and stirred for 6 h at room temperature. To this, potassium
hydroxide (0.7 g, 12.5 mmol) in water (6 mL), pentane (10 mL), and
diethyl ether (10 mL) were added. The mixture was stirred for a
further 12 h. The organic layer was separated and washed with water
and dried over magnesium sulfate. The solvent and epichlorohydrin
were removed under vacuum to give a yellowish resin (2.90 g, 8.52
mmol, 85.4%). Route B: ground potassium hydroxide (1.50 g, 26.7
mmol) and tetrabutylammonium bromide (0.32 g, 0.993 mmol) was
added with stirring to bisphenol A (2.00 g, 8.76 mmol) dissolved in
epichlorohydrin (10 mL). This solution was then stirred overnight.
Water was added to the solution, and the organic layer separated. The
aqueous layer was extracted with diethyl ether twice and ethyl acetate
once. The four organic layers were combined and washed three times
with water, before drying over magnesium sulfate. Finally, the solvent
and epichlorohydrin were removed under vacuum to give a colorless
resin (2.63 g, 7.73 mmol, 89%).
Synthesis of Isomers. Ground potassium hydroxide (approx. 6 g,
108 mmol) and tetrabutylammonium bromide (approx. 1.2 g, 3.62
mmol) were added with stirring to an isomer of bisphenol F (approx.
7 g, 35 mmol) dissolved in epichlorohydrin (40 mL). This solution
was then stirred for 3 to 6 days. Water was added to the solution, and
the organic layer separated. The aqueous layer was extracted with
diethyl ether once and ethyl acetate twice. The four organic layers
were combined and washed three times with water, before drying over
magnesium sulfate. Finally, the solvent and epichlorohydrin were
removed under vacuum to give crude product (average yields 93, 87,
and 86% for pp, po, and oo, respectively). The po- and oo-DGEBF
products required column chromatography (see below for details) to
be performed to obtain a product of good purity. The ppDGEBF was
Figure 1. Three isomers of DGEBF, para−para (ppDGEBF), para−
ortho (poDGEBF), and ortho−ortho (ooDGEBF).
A wealth of literature exists for the study of the properties
and behavior of bisphenol F-based epoxy resins, although
discussion of any regioisomerism is in most cases neglected
and in some cases the structure is described as a single
isomer.5,10,16,18,20−23 As these studies use a range of different
DGEBF-based resin mixtures and do not discuss the isomeric
distribution, there is a risk that changes in properties attributed
to another factor may be the result of resins of different
compositions. This work seeks to understand the differences
between networks consisting of the different regioisomers.
Furthermore, the production of networks with individual
isomers will provide insight into the relative impacts of
changing geometry and mass between crosslinks in epoxy
networks (because they are changed, where polarity remains
virtually unchanged). Network properties will be studied using
a range of thermal/physical measures (thermal transitions,
density, and crosslink density) and solvent uptake as a probe
into the free volume and packing of the polymer.
EXPERIMENTAL SECTION
■
Materials. The following materials were used as supplied: Dow
Epoxy Resin 354 (DER 354 (the reaction product of an isomeric
mixture of bisphenol F and epichlorohydrin)) (Olin Corporation);
bisphenol A, para−para-bisphenol F, para−ortho-bisphenol F, and
B
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an off-white waxy solid (1H NMR (C6D6, 400 MHz): δ 2.13 (dd, 1H,
J = 5.2, 2.4 Hz), 2.25 (dd, 1H, J = 5.2, 4.0 Hz), 2.86−2.92 (m, 1H),
3.45 (dd, 1H, J = 11.2, 5.6 Hz), 3.65 (dd, 1H, J = 11.2, 3.2 Hz), 3.74
(s, 1H), 6.76−6.81 (m, 2H), and 6.95−7.04 (m, 2H)), residual
internal C6D5H, the poDGEBF was a yellowish liquid, which slowly
solidified to an off-white waxy solid, and ooDGEBF was a white solid.
Column Chromatography of Crude po- and oo-DGEBF of
Isomers. Crude poDGEBF/ooDGEBF was purified by a standard
column chromatography procedure with a column volume of
approximately 1400 cm3. The stationary phase was 40−63 μm silica
for flash chromatography. The mobile phase was 35:65 ethyl acetate/
hexane. The total poDGEBF obtained was 10.9290 g (35.0 mmol,
total yield: 62%) (1H NMR (C6D6, 400 MHz): δ 2.11−2.18 (m, 2H),
2.23−2.29 (m, 2H), 2.82−2.91 (m, 2H), 3.39−3.49 (m, 2H), 3.61−
3.67 (m, 2H), 4.00 (s, 2H), 6.54 (d, 1H, J = 8.4 Hz) 6.76−6.82 (m,
2H), 6.84−6.89 (m, 1H), 7.03−7.10 (m, 2H), and 7.12−7.18 (m,
overlap with residual solvent), and ooDGEBF obtained was 10.6628 g
(34.1 mmol, total yield: 52%) (1H NMR (C6D6, 400 MHz): δ 2.14
(dd, 1H, J = 5.2, 2.4 Hz), 2.25 (dd, 1H, J = 5.2, 4.0 Hz), 2.83−2.88
(m, 1H), 3.45 (ddd, 1H, J = 11.2, 5.6, 2.0 Hz), 3.65 (dd, 1H, J = 11.2,
3.2 Hz), 4.26 (s, 1H), 6.56 (d, 1H, J = 8.0 Hz), 6.87 (t, 1H, J = 7.4
Hz), 7.06 (td, 1H, J = 11.1, 4.0 Hz), and 7.23 (dd, 1H, J = 7.2, 1.2
Hz).
Glass Slide Preparation. Hydrogen peroxide was added with
care and gentle stirring to sulfuric acid in a 1:3 ratio to produce the
“piranha solution”. Slides (as supplied) were placed in the “piranha
solution” and left for 15 min before rinsing thoroughly with deionized
water. The treated slides were placed in a rack and dried at 50 °C in
an oven and stored there until use.
NIR Spectroscopy. NIR spectroscopy was performed on an
Ocean Optics NIRQuest 2500. Spectra were sampled using an
integration time of 10 ms and taking 100 scans to average. Sample
cells were prepared using a PTFE spacer between glass slides, fastened
together using a small amount of epoxy resin. For the molar extinction
coefficient determination with varied path length, different thicknesses
(approx. 0.55, 0.6, 0.8,, 0.85 mm) of PTFE were used to make the
cells. For degree of cure measurements, cells with a path length of
0.55−0.6 mm were used, and cure completed in a heated cell, using
the same cure schedule as that described in the subsequent section.
Spectra were obtained throughout the cure.
Network Preparation for Solvent Sorption/Desorption
Studies. Formulation (DER 354). DER 354 was added to MXDA
in a 1:1 stoichiometric ratio of epoxide to amine functional groups,
and then mixed by hand, using a stirring rod. The resulting mixture
was covered using a parafilm (to limit any interaction with oxygen or
carbon dioxide or evaporation of the amine) and held, with further
mixing (initially every 5 min, then less regularly), at room
temperature. After 180 min, the mixture was applied to glass slides
(see below).
Formulation (po/ppDGEBF). Epoxy resin was added to MXDA in a
1:1 stoichiometric ratio of epoxide to amine functional groups and
then mixed by hand, using a stirring rod. The resulting mixture was
covered using a parafilm (to limit any interaction with oxygen or
carbon dioxide or evaporation of the amine) and held, with further
mixing (initially every 5 min, then less regularly), at 33 °C (to prevent
recrystallization). Upon any clouding, indicating recrystallization, the
mixture was heated with a heat gun until meltedand then returned
immediately to 33 °C. After 75 min, the mixture was applied to glass
slides (see below).
Formulation (ooDGEBF). ooDGEBF was added to MXDA in a 1:1
stoichiometric ratio of epoxide to amine functional groups and then
mixed by hand, using a stirring rod. The resulting mixture was covered
using parafilm and held, with further mixing (initially every 5 min,
then less regularly), at a higher temperature of 40 °C (here,
recrystallization presented a much greater issue). Upon excessive
clouding, the mixture was heated with a heat gun followed by
immediate cooling back to 40 °C. After 20 min, the mixture was
applied to glass slides (see below).
drawdown cube (Sheen Instruments). The slides were then placed in
an oven and cured under a nitrogen atmosphere. Oxygen-free
nitrogen, obtained from BOC, was purged through the oven at
approx. 5 cm3 min−1. The cure schedule was an initial temperature of
60 °C, with a ramp of 1 °C min−1 to 160 °C, where the oven was held
for 3 h. Upon completion of the 3 h, the samples were removed from
the oven and allowed to cool from 160 °C. All samples were uniform,
colorless, and clear films. Any samples not immediately analyzed were
placed in a desiccator over phosphorus pentoxide to prevent moisture
uptake.
Dynamic Mechanical Analysis. Dynamic mechanical analysis
(DMA) was performed on a PerkinElmer DMA8000, using the single
cantilever mode, heated at a rate of 3 °C min−1. Three beams were
prepared for each sample formulation, approximately 10 mm wide and
1.6 mm thick, and the dynamic response to a sinusoidal force applied
at a frequency of 1 s−1 was recorded. The Tg was taken as the
temperature at the highest intensity of the peak in the tan δ trace from
a Lorentzian fit of the peak using OriginLab 2017. For the β-transition
measurements, the sample chamber was cooled to approximately
−192 °C, before the measurement was performed. The area of the
peak was obtained using a fitted cubic baseline and integrating the
area beneath the curve. Crosslink density, ν, is calculated from eq 2,
where E is the rubbery plateau storage modulus (taken as the storage
modulus at Tg + 40 K), R is the gas constant, and T is temperature.24
E
3RT
ν =
(2)
Helium Pycnometry. Helium pycnometry was performed on a
Micrometrics AccuPyc 1330, using approximately 0.4 g of sample in a
sample cell of 1 cm3 and a standard of known mass and volume for
calibration.
Methanol Sorption/Desorption Measurements. Coated glass
slides were placed in methanol, upright and separate using Coplin jars,
and weighed at time intervals ranging from 16 h to 90 days (see the
Supporting Information). Intervals between measurements were
increased as time increased due to the slowing of mass uptake.
Upon reaching a plateau in mass, these samples were removed from
the solvent and placed in an oven at 40 °C, and weighed at time
intervals until a reasonable plateau was obtained (a return to the
original dry mass was only possible upon heating to 120 °C under
reduced pressure).
RESULTS AND DISCUSSION
■
In approaching an isolated epoxy resin isomer, there are two
main approaches: (a) separate from an industrially available
mixture of the isomers or (b) synthesize from individual pure
starting materials. First, separation from DER 354 was
considered. NMR spectroscopy shows DER 354 consists of a
3.5:3.2:1 pp/po/oo ratio, determined as per the method
described by Domke.25 This is not however the ratio of the
three simplest molecules, rather the ratio of the substitution.
The oligomeric structures (e.g., Figure 2) in the resin mixture
also contribute to this ratio. If we consider the ways in which
the original three structures can combine (regioisomeric
motifs), when considering our reactant on a molecular level,
we can say that there are at least 12 different “monomers” from
which the network is built if n ≤ 1.
Figure 2. An example of a chain-extended oligomer of DGEBF, while
only considering para-substitution. Each phenylene ring can be either
ortho- or para-substituted when considering the diversity of possible
isomers for a particular degree of chain extension (i.e., value of n).
Application to Slides. The mixtures described above were drawn-
down onto prepared glass slides held at 30 °C, using a 400 μm slot,
C
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Ponten showed that HPLC can be used to separate the
isomers of DGEBF.26 With an adjusted method, we have also
attained separation. Figure 3 shows the initial analytical HPLC
difficult to eliminate all chain extension, but as the starting
material only contains the single regioisomer, the number of
possible molecules is substantially reduced (from 12 to 2).
Route B was shown to produce the epoxide molecule in an
89% yield, in a good degree of purity.
This method was then applied to the individual bisphenol F
isomers, with crude yields of 76, 87, and 86% for pp-, po-, and
oo-DGEBF, respectively. NMR spectroscopy showed the
spectra in general agreement with the targeted products
(with some disparities in the oo-, especially in the aromatic
region). The HPLC trace of ppDGEBF shows a single isomer
in the monomeric region and, as expected, a substantially
simpler oligomeric region, with only a single regioisomeric
motif within the molecules (i.e., pp−pp/pp−pp−pp rather
than pp−po or pp−po−op). Titration for EEW showed some
chain extension, but lower than DER 354, with an average n of
0.05. Similar results were obtained for the po isomerthe
increased diversity in the oligomeric region is due to different
modes of addition of two po units (po−op/op−op/op−po).
However, the EEW was found to be 170.6 g mol−1
corresponding to an n of 0.11. Furthermore, the obtained
ooDGEBF showed an unexpectedly diverse oligomeric region
in the HPLC chromatogram and an even higher EEW (181.6 g
mol−1n = 0.20). In order to produce comparable networks,
column chromatography was performed on both the oo and po
isomers.
Figure 3. HPLC chromatogram for DER 354 using a 50:50
acetonitrile/water isocratic solvent system. The major peaks are
labeled with the responsible component of a DGEBF-based mixture.
The region at 23−36 min illustrates the large number of different
higher molecular weight “monomers”.
data, showing three pronounced peaks at 10−15 min and a
more complex region at 25 min. Preparatory HPLC yielded
samples of the three monomeric forms confirmed by NMR
spectroscopy. The region at 23−26 min illustrates the number
of different oligomeric species present. The number of distinct
peaks shows the number of different oligomeric species
present. Despite successful separation, the cost is very high
with only milligram quantities isolated and HPLC is therefore
unsuitable for network preparation.
Column chromatography gave much purer compounds for
both of the isomers. Using a mobile phase of 45:55 ethyl
acetate/n-hexane, quantities above 10 g were obtained (Table
1). The variation in total yield shows the relative impurity of
A more widely used chromatographic method is column
chromatography. Thin layer chromatography showed separa-
tion between two main spots, with one being much bolder than
the other (Rf values of 0.42 and 0.49, respectively). The faster
eluting compound (fainter spot) was isolated by column
chromatography and found to be the oo isomer. The major
spot was shown to be a mixture of the pp and po isomers.
Within this eluted fraction, it was shown that initially the
fraction was richer in po, becoming richer in pp in progressing
through the fraction. Both NMR spectroscopy and titration to
obtain EEW (the mass per mole of epoxide groups) show that
the higher oligomers have been removed. The quantities
isolated were larger, with the column scaled to allow loading of
approx. 11 g of DER354. However, because of the small
proportion of ooDGEBF in DER 354, less than 2 g was
isolated per column. Hence, chromatographic methods offer an
expensive route to very high purity isomers (HPLC) or a
simpler route to a partly purified reactant mixture (column).
As the chromatographic approach did not yield an effective
route to individual pp and po, the synthetic route was
considered. Rather costly individual isomers of bisphenol F
were available from TCIUK. Therefore, in order to ensure that
the reaction was efficient and high-yielding, an optimization of
the synthetic route was performed using bisphenol A. The two
routes identified from the literature differ in the nature of the
reaction mixtureroute A, from Wengert et al., involving a
biphasic aqueous/organic mixture stirred vigorously, and route
B from Zhou et al. was an organic/solid mixture.27,28
Table 1. Yields of oo- and po-DGEBF Obtained from
Column Chromatography Over Two Runs of Approximately
Equal Loadings per Isomer
mass loaded/g
mass obtained/g
ooDGEBF
poDGEBF
21.62
15.24
12.89 (60%)
10.93 (72%)
the crude ooDGEBF, as expected from the HPLC and NMR
data. For the purified products, n was found to be 0.04 for the
po and 0.07 for the oo. Furthermore, the HPLC and NMR
data showed good agreement with expected spectrawith
significantly reduced complexity in the higher MW region. The
1H NMR spectra and HPLC chromatograms of the final
products are shown in the Supporting InformationFigures
S1 & S2.
In order to compare the performance of the various isomers,
they were cured in a stoichiometric ratio with MXDA. The
industrial DGEBF-based resin, DER 354 was also cured using
the same hardener and method to provide a comparison.
Degree of cure measured by NIR spectroscopy showed
complete or near-complete reaction for all samples: 100.0%
for the pp, po, and DER 354 and 97.3% for the oo (this slight
reduction for the oo network, however, as our forthcoming
publication shows, has little impact on network properties) and
therefore the key factor in any observed property change will
be driven by the chemistry changes. It is worth noting that
further to the evidence given by NIR spectroscopy, the curing
networks were held for 3 h at 160 °C, well above the ultimate
Tg for each of the networks (ultimate Tgs are between 107 and
117 °Cas shown in Figure 4), supporting that complete/
near-complete reaction has been achieved.
Following synthesis, the quality of the product was analyzed
by NMR spectroscopy. Route A showed problems with
incomplete conversion, unwanted side-products, and chain
extension. However, route B was shown to be much more
efficient. This process was then optimized for the ratio of
reactants, adjusting both TBAB and PTC concentrations. It is
D
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Figure 4. Physical properties of networks produced using MXDA as the amine monomer and para−para- (pp), para−ortho- (po), and ortho−ortho-
(oo)-DGEBF or Dow Epoxy Resin 354 (DER 354) as the epoxide monomer. (a) Glass transition temperature (Tg), the β-transition temperature
(Tβ) as determined by DMA and (b) crosslink density as determined by DMA (calculated from eq 2see Experimental Section) and density as
determined by helium pycnometry. Error bars show standard error for three samples for Tg and crosslink density, standard error for two samples for
Tβ, and standard deviation of ten measurements of a single sample for density.
Networks produced with the isolated isomers and DER 354
show clear differences in thermal/physical properties. Figure 4
shows the glass transition temperature (Tg), beta-transition
temperature (Tβ), bulk density, and crosslink density for the
networks. Considering the isomers in isolation, decreases in Tg,
Tβ, density (only for oo), and crosslink density are observed as
the ortho-content increases. As might be expected, for a
mixture of isomers, DER 354 shows intermediate properties for
Tg, Tβ, density, and crosslink density, relative to the two
extremes represented by pp and oo. Generally, the results
indicate the production of a better packed network as the
ortho-content is decreased. β-Transition measurements are
indicative that there is a molecular motion present for the pp
and po networks that is significantly reduced for the oo
shown by a much-reduced peak area for the oo network
(Figure 5). This may facilitate the enhanced packing (higher
Figure 6. Chemical performance in methanol of networks produced
using MXDA as the amine monomer and para−para- (pp), para−
ortho- (po), and ortho−ortho- (oo)-DGEBF or Dow Epoxy Resin 354
(DER 354) as the epoxide monomer. Shown are the ultimate uptake
(by original mass) of each network and the diffusion coefficients of
sorption (Dsor) and desorption (Ddes, at 40 °C). Error bars show
standard error of the three samples.
(quantified by the sorption diffusion coefficient, Dsor), and it
had the highest value for the desorption diffusion coefficient
(Ddes). The oo network had by far the highest uptake and Dsor
and the lowest Ddes. As with thermal/physical properties, the
po network showed intermediate properties relative to the
other two regioisomers. Similarly, the industrial mixture, DER
354, showed intermediate properties. When the chemical
performance is considered alongside the thermal/physical
properties, there is strong evidence that regioisomerism leads
to a significant shift in network structure and performance.
This is not to suggest any knowledge of longer-range structure,
rather that the change in molecular structure of the monomer
gives rise to notable differences between the networks formed
and therefore an inherent difference in the molecular structure
of the respective networks.
Figure 5. DMA measurement of tan δ vs temperature, showing the β-
transition for the networks produced using MXDA as the amine
monomer and para−para- (pp), para−ortho- (po), and ortho−ortho-
(oo)-DGEBF or Dow Epoxy Resin 354 (DER 354) as the epoxide
monomer. Inset: the Tβ and area of the peak associated with the β-
transition for each of the networks. Error bars show standard error of
two samples.
The combined measurements suggest that the effectiveness
of network packing is influenced by the amount of ortho-/para-
substitution. They suggest that a well-packed network is
formed by ppDGEBFbecause it has the highest density and
crosslink density and lowest ultimate uptake. By contrast, the
properties obtained for the ooDGEBF network suggest that the
ortho moieties inhibit packing with a much-reduced density
and increased ultimate uptake. The reduction in properties for
the po network relative to the pp is supportive of the view that
ortho-substitution is inhibitive to packing. Although it is not
strongly correlated with the density measurement, an increase
density) that is achieved for these networks. The beta
transition for DER 354 appears to be a convolution of the
three isomerically pure networks, both in terms of temperature
and shape (particularly the shoulder observed in the oo trace at
approx. −10 °C).
The networks produced were also shown to vary with
respect to their chemical performanceresults from methanol
sorption are shown in Figure 6. The pp network was shown to
uptake the least amount of methanol, at the slowest rate
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in ultimate uptake suggests greater free volume and a reduced
crosslink density.
and MXDA at 160 °C; and details of conversion
calculation from NIR spectroscopy (PDF)
Considering the DER 354, which has an ortho-content of
0.375, as has already been discussed, there is similarity with the
po in the observed propertiescrosslink density, ultimate
uptake, and Ddes measurements are similar within error, and
the variation in density is limited. Generally, these results
further support the finding that ortho-substitution is a major
influence in network performance (and therefore structure),
although the disparity in the ortho-content between po and
DER 354 does also show that other factors will impact upon
these properties. For example, the degree of chain extension in
DER 354 is much higher than for the purer networks.
Figure 7 shows the varied methanol sorption profiles of the
three isomerswhat is immediately clear is the sigmoidal
AUTHOR INFORMATION
Corresponding Author
*E-mail: s.knox@leeds.ac.uk.
ORCID
Stephen T. Knox: 0000-0001-5276-0085
John Patrick Anthony Fairclough: 0000-0002-1675-5219
■
Present Address
§School of Chemical and Process Engineering, University of
Leeds, Leeds, LS2 9JT.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
Funding for S.T.K. from EP/L016281/1 EPSRC Centre for
Doctoral Training in Polymers, Soft Matter and Colloids.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank AkzoNobel and the EPSRC
for funding via the CDT in Polymers, Soft Matter and
Colloids; and Dr. Joel Foreman and Roderick Ramsdale-
Capper for DMA access.
Figure 7. Methanol sorption profiles of DGEBF/MXDA networks
made with the different isomers of DGEBF [para−para- (pp), para−
ortho- (po), and ortho−ortho (oo)]. The individual data points are
shown joined with straight lines.
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■
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.9b01441.
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