Inducing hardening and healability in poly(ethylene-co-acrylic acid) via blending with complementary low molecular weight additives

Article abstract of DOI:10.1039/c8ra09597c

The design and synthesis of low molecular weight additives based on self-assembling nitroarylurea units, and their compatibility with poly(ethylene-co-acrylic acid) copolymers are reported. The self-assembly properties of the low molecular weight additive

Full text of DOI:10.1039/c8ra09597c

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Inducing hardening and healability in
poly(ethylene-co-acrylic acid) via blending with
complementary low molecular weight additives†
Cite this: RSC Adv., 2018, 8, 41445
Benjamin C. Baker,a I. German,b Gary C. Stevens,b Howard M. Colquhouna
a
*
and Wayne Hayes
The design and synthesis of low molecular weight additives based on self-assembling nitroarylurea units,
and their compatibility with poly(ethylene-co-acrylic acid) copolymers are reported. The self-assembly
properties of the low molecular weight additives have been demonstrated in a series of gelation studies.
Upon blending at low percentage weights (#5%) with poly(ethylene-co-acrylic acid) the additives were
capable of increasing the stress and strain to failure when compared to the parent copolymer. By varying
the percentage weight of the additive as well as the type of additive the mechanical properties of
poly(ethylene-co-acrylic acid) could be tailored. Finally, the healability characteristics of the blends were
improved when compared to the original polymer via the introduction of a supramolecular ‘network
within a network’.
Received 21st November 2018
Accepted 27th November 2018
DOI: 10.1039/c8ra09597c
rsc.li/rsc-advances
Introduction
Design, system, application
Ethylene-based polymers have found widespread use as
protective coatings.1 The toughness of phases arising from
polyethylene sequences and the ability to manipulate mechan-
ical properties of the co-polymer, via variations in comonomer
structure and relative content, allow access to materials suitable
for a wide range of applications.2 However, when damaged,
repair for many of these ethylene-based copolymers cannot be
realised and thus the system fails as a protection mechanism
(such as cracking within electrical cabling). The introduction of
healability within a polymeric protection system via the use of
copolymer functionalities capable of non-covalent associations
has been demonstrated successfully by Kalista et al.3,4
Several distinct approaches to the realisation of polymer-
based healable systems have been reported in the literature.5–7
The “encapsulation” and “reversible/irreversible covalent bond”
approaches offer alternative routes to healability within poly-
mer matrices.8 However, these approaches have distinct prac-
tical limitations including a limited number of break-heal
cycles (encapsulation and irreversible covalent bond
approaches), reduced toughness of the repaired system and the
need to implement external stimuli to initiate the healing
process (reversible/irreversible covalent bond approach).9 An
alternative approach whereby non-covalent sacricial bonds,
for example hydrogen bonds, are placed throughout polymeric
networks have allowed access to healability10 via bond dissoci-
ation and re-association.11 However, the inevitable compromise
between the thermal and kinetic stimuli required to initiate
healing and the ability of the system to offer viable protection
must be considered.12,13
In the light of limited petroleum feedstocks, the enhancement
of the mechanical properties of known polymeric materials and
improvement of polymer-based product lifetimes are key targets
in polymer science. This study outlines a systematic approach to
these goals whereby blends are created between commercially
available polymers and small quantities of functionalised low
molecular weight additives, specically additives which are
capable of forming stable gel networks. The bulk polymers used
in this study associate into networks via non-covalent interac-
tions and are healable because of their self-assembling char-
acter. The molecular additives were designed to bind via non-
covalent interactions with the bulk polymer and also to self-
associate in order to create an alternative network to that
found in the bulk polymer alone. Creating a ‘network within
a network’ has been shown in this study to improve both the
mechanical characteristics and healing ability of the bulk
polymer. This approach to the enhancement of properties of
existing polymer systems via blending with complementary low
molecular weight additives thus offers an alternative route to
the development of healable polymeric-based products without
the need for the synthesis of complex polymer structures.
aDepartment of Chemistry, University of Reading, Whiteknights, Reading, RG6 6AD,
UK. E-mail: w.c.hayes@reading.ac.uk; Fax: +44 (0)118 378 6331; Tel: +44 (0)118
378 6491
bGnosys Global Ltd., 17-18 Frederick Sanger Road, The Surrey Research Park,
Guildford, Surrey, GU2 7YD, UK
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra09597c
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The self-healing mechanism of ethylene carboxylic acid
copolymers, such as poly(ethylene-co-methacrylic acid), has
been modelled as a two-stage system relying upon a supramo-
lecular rearrangement and melting of the polymeric crystalline
phase.3,4 In this model sacricial supramolecular bonds
(intermolecular hydrogen bonding between carboxylic acid
moieties) are able to dissociate at lower temperatures and
enable healability. At higher temperatures melting of the
crystalline polyolen domains enables copolymer ow and
processability. This model has been extended to the random
copolymer poly(ethylene-co-acrylic acid).3,4,13
In the present paper we report the introduction of low
molecular weight additives based upon hydrogelators14–16
featuring the nitroarylurea receptor motif into a range of
ethylene-carboxylic acid copolymers as a means of enhancing
both the mechanical properties and healability characteristics
of the bulk phase.17–23 It is known that creation of a secondary
‘so’ network within a well-dened polymer network, by
introduction of small-molecule, gel-forming additives into
copolymers,22–24 promotes healing at lower temperatures,
provided that such additives interact with the moieties within
the bulk polymer that are responsible for healing (in the
present work the carboxylic acid groups, see Fig. 1). The small-
molecule additives also strengthen the polymer matrix at
lower temperatures (ca. 20 ꢀC) via increased ordering within
the bulk (at temperatures < Tg) in agreement with studies on
precisely-dened poly(ethylene-co-acrylic acid) copolymers re-
ported by Middleton et al.25
Results and discussion
Low molecular weight additive synthesis and characterisation
Each of the additive molecules 1–6 (Fig. 2) was designed to
interact with the carboxylic acid hydrogen bonding domains
present in poly(ethylene-co-acrylic acid).
The dicarboxylic acid, sebacic acid (1), and the mono acid,
dodecanoic acid (2), were used as received. Carboxylic acids 3–5
were synthesised via a two-step process (Scheme 1), with each
respective amine-functionalised diaryl urea being formed from
the
corresponding
aryl
isocyanate
and
p-phenyl-
enediamine.10d,26 The carboxylic acids were then generated via
a ring opening reaction of the remaining amino group with
succinic anhydride.
The successful synthesis of each carboxylic acid was
conrmed by IR, 1H and 13C NMR spectroscopic analysis in
addition to mass spectrometry (see the ESI, Fig. S1–S9†). For
example, 1H NMR analysis of the carboxylic acid 3 showed that
the primary amine resonances present in the starting material
were replaced by an amide-NH resonance at 9.83 ppm (see
Fig. S1 in the ESI†). These spectroscopic data were in agreement
with the 13C NMR analysis which revealed the presence of three
different carbonyl resonances (e.g. 175.8, 169.7 and 153.8 ppm)
corresponding to the urea, amide and carboxylic acid groups
present in the product (Fig. S1†). Further proof of the successful
synthesis of 3 was evident from the IR spectrum (Fig. S3†) which
exhibited three distinct absorption bands (at 1696, 1671 and
1655 cmꢁ1, respectively) correlating to the three carbonyl
moieties, and an amide-NH stretch (at 3362 cmꢁ1). Finally mass
Fig. 1 Insertion of; (A) dicarboxylic acid additive (sebacic acid), (B) mono-carboxylic acid additive (dodecanoic acid), (C) creation of ‘network
within a network’ via functionalised carboxylic acids additives to enable secondary supramolecular interactions into poly(ethylene-co-acrylic
acid).
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Fig. 2 Low molecular weight additives showing: sebacic acid 1, dodecanoic acid 2, functionalised carboxylic acids 3–7.
spectrometric analysis showed a parent ion at m/z ¼ 395.0959 in the acid-functionalised, diaryl ureas 3–5 via ring opening of
good agreement with the calculated value. The carboxylic acid 6 succinic anhydride by 3-nitroaniline.27 The dicarboxylic acid 7
was synthesised via a similar procedure to that used to generate was synthesised using a previously reported procedure.14
Gelation studies
To probe the self-assembly capabilities of carboxylic acids 1–7,
gelation studies were initially undertaken. Hydrogels of the
carboxylic acids were generated using the glucono-d-lactone
protocol reported by Adams and co-workers.28,29 Initial gelation
studies revealed successful hydrogelation of carboxylic acids 1
and 3: indeed we nd that 3 is a supergelator, as dened else-
where,30 with a critical gelator concentration of 0.1 wt%
(Table 1).
The functionalised dicarboxylic acid 7 has already been re-
ported to be a supergelator,14 but carboxylic acid 2 was not able
to form stable hydrogels which implies that bifunctionality is
required to promote supramolecular network growth. Of the
previously unreported functionalised carboxylic acids studied
Table 1 Gelation properties of 1–7 where; G ¼ gel (withstanding the
vial inversion test for > 1 hour)a
Molecule
Gelation state
CGC [mM]
wt%
1
2
3
4
5
6
7
G
GP
G
P
P
P
98.8b
—
2.7
—
—
—
2.0
—
0.1
—
—
—
G
0.9
0.03
a
b
GP ¼ Gelatinous precipitate, P ¼ precipitate. At a concentration of
297.2 mM (6.0 wt%) molecule 1 can also behave as a thermogelator.
Scheme 1 Generic synthesis of gelator molecules 3–5.
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Table 2 Mechanical properties of films formed of blends of pEAA15 and carboxylic acids 1–7 in 1 and 5 wt% with respect to plasticizer (film
dimensions averaging 40 ꢂ 10 ꢂ 1 mm, true strain rate of 0.2 sꢁ1
)
wt%
additive
Tensile strength
(MPa)
Fracture stress
(MPa)
Uniform strain
(%)
Strain to fracture
(%)
Energy absorbed
(MPa)
Young's
modulus (MPa)
Film system
pEAA15
pEAA15/1
2.20
2.83
3.30
1.86
0.95
2.28
2.66
1.81
1.85
2.04
0.36
1.75
0.59
1.90
1.95
2.75
3.12
1.49
0.46
2.09
1.95
1.37
1.67
1.72
0.26
1.54
0.32
1.78
7.30
3.26
9.54
9.09
62.80
8.00
9.21
12.90
5.52
16.26
5.72
13.14
16.10
12.91
9.84
3.59
10.38
9.58
69.50
10.66
17.14
17.62
6.13
18.37
6.49
16.44
20.40
14.88
0.26
0.11
0.39
0.13
0.34
0.26
0.41
0.27
0.08
0.31
0.02
0.23
0.09
0.22
117.47
220.49
123.78
23.72
0.98
117.48
146.21
54.45
58.42
52.03
24.22
18.53
6.57
1
5
1
5
1
5
1
5
1
5
1
5
1
pEAA15/2
pEAA15/3
pEAA15/4
pEAA15/5
pEAA15/6
pEAA15/7
53.04
(3–6), only the system with the nitro substituent in the meta
position with respect to the urea bond (3) formed stable gels (as
shown by the vial inversion test and rheological analysis; see
ESI, Fig. S10†). Rheological analysis undertaken on hydrogels of
3 revealed an increased maximum storage modulus (400 kPa)
with respect to gelator 7 (294 kPa: the concentration of both
gelators was 20 mM, see ESI, Fig. S11†).14 The failure of
carboxylic acids 4 and 5 to yield stable gels highlights the
importance of the nitro moiety, and its substitution-position on
the aromatic ring, in the formation of complementary hydrogen
bonding networks. It is proposed that meta-substitution of the
nitro moiety allows favourable self-assembly towards gelation
(as is demonstrated in analogues of 3–6 reported in previous
studies),14–16 whereas para substitution leads to crystallisation
and ultimately to precipitation from solution. Furthermore,
when the urea moiety was absent, as in the case of the carboxylic
acid 6, no bril growth and hence no gelation was observed. The
differences in the assembly of the hydrogelators 3 and 7 were
evident from studies on the dye absorption capabilities of each
gelator. Whilst gels of compound 7 absorbed aromatic dyes
such as methylene blue from aqueous solution (as previously
reported), the carboxylic acid 3 did not demonstrate such ability
(see ESI, Fig. S12†).14–16
Tensile properties of pEAA15 blends
Mechanical analysis was carried out on blends of pEAA15 and
carboxylic acids 1–7 to assess the impact of low molecular
weight additives at loading levels of 1 and 5 wt%. Studies were
undertaken on lms (average dimensions 40 ꢂ 10 ꢂ 1 mm)
using a tensometer, with a true strain rate of 0.2 sꢁ1. Each
sample was analysed ve times and the average stress–strain
prole was recorded (see Table 2 and ESI Fig. S14–S20†).
It was found that blending the carboxylic acids 1 and 2 with
pEAA15 had
a signicant impact upon the polymer's
mechanical properties (Table 2). The diacid 1 was found to
increase the tensile strength of the bulk phase at low weight
concentrations (#5 wt%) (see Table 2 and Fig. 3). Blending of
1 wt% of the diacid 1 was thus found to afford a stronger but
more brittle material than pure pEAA15 as reected in the
increased tensile strength and Young's modulus, though with
decreased uniform strain and energy absorbed. At 1 wt%
incorporation, compound 1 is thus simply an anti-plasticizer
for pEAA15. However, blends comprising 5 wt% of 1 with
showed signicant increases in both the strain-to-fracture and
fracture stress compared with pEAA15 itself, as well as
increased tensile strength and Young's modulus. This
combination of increased stiffness, toughness, and elasticity
of the material at 5 wt% incorporation of dicarboxylic acid 1
indicates that a change in the degree of ordering of the system
has occurred (see Fig. 3).17,31
Initial blending procedures
To investigate the potential of network formation within an
existing polymer network, lms were cast successfully from
blends of the carboxylic acids 1–7 and poly(ethylene-co-acrylic
Strain
acid) (15 wt% acrylic acid: pEAA15). Blends were obtained via In contrast to the increase in stiffness resulting from incorpo-
dissolution of both polymer and low molecular weight carbox- ration of dicarboxylic acid 1 with pEAA15, the monocarboxylic
ylic acid in dimethylformamide (DMF) and removal of solvent acid 2 acted as a plasticizer when blended at sufficient loading
under high vacuum. In the case of blends of pEAA15 with the levels. Thus, at a loading of 5 wt% of 2, a 70-fold increase in
diacid 1 it was found that phase separation occurred at an strain-to-fracture was thus observed as well as a decrease in
additive loading of 10 wt% as determined by DSC analysis (see both the tensile strength and Young's modulus. It is proposed
ESI, Fig. S13†). In the light of this observation, only those blends that such plasticization results from the inability of the mono-
containing 1 wt% and 5 wt% of 1 in pEAA15 were investigated in carboxylic acid 2 to effectively self-assemble (beyond hydrogen
more detail.
bonded dimerisation), preventing the formation of effective
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reinforcing networks within the polymer blends,17,30 (see gela-
tion studies, Table 1).
Each blend demonstrated plasticization as evidenced by the
observation of lower Tg, and Tm values with respect to pEAA15
Blending of the urea-functionalised carboxylic acid 3 with (Table 3). Interestingly a slight increase in Tg was observed in
pEAA15 afforded an increase in the material's tensile strength the blends containing higher additive loadings (5 wt%) in
as well as its uniform strain (Table 2 and Fig. 3). The increase in comparison to the blends featuring 1 wt% of the carboxylic acid
tensile strength was less pronounced than in the blends with additives 1 and 3. This trend was, however, not observed in the
the dicarboxylic acid 1, but in contrast the increase in uniform case of additive 2. It was noted that transition temperatures
strain was greater. It is proposed that these changes in observed for random copolymer pEAA15 were in good agree-
mechanical properties are a result of the introduction of ment with previously-reported value.32 Percentage crystallinity
a weaker secondary interaction in the self-assembly motif of the (calculated from comparison with the enthalpy of melting of
bisarylnitro urea moiety in 3 (Fig. 1C).
crystalline poly[ethylene])32 showed little difference between the
The importance of the presence and substitution-position of pure copolymer and blends of the copolymer and the bifunc-
the nitro functionality (as well as a second aromatic ring and tionalised carboxylic acids 1 and 3 (Table 3). Only those samples
urea moiety) in reinforcing the properties of the bulk polymeric blended with the monofunctionalised acid 2 showed any slight
phase was conrmed by analysis of blends of carboxylic acids 4– decrease in percentage crystallinity. It is therefore concluded
6 with pEAA15. In these studies it was found that these additives that the small molecule additives do little to decrease the overall
acted solely as plasticizers, lowering the tensile strength of the crystallinity of the copolymer, suggesting that the interactions
polymer and increasing the uniform strain (Table 2). Thus, are indeed carboxylic acid polymer-carboxylic acid additive
blending the carboxylic acids 4–6 did not have the same bene- specic.25
cial effects on mechanical properties as found in the blends of
Additional DSC studies were undertaken whereby two indi-
pEAA15 with 3. The importance of the interactions between the vidual heal/cool cycles (on the same sample) were separated by
ꢀ
carboxylic acid moieties of copolymer and additive was also a 24 hour gap (20 C) (ESI, Fig. S25–S28†). This gap was intro-
demonstrated via analysis of blends of the gelator 7 with duced to probe the ability of the blends to relax into more
pEAA15. At a loading of just 1 wt%, additive 7 was found to thermally stable states over a period of 24 hours. Each of the
enhance the properties of the copolymer substantially with blends was found to exhibit identical thermal characteristics to
respect to both tensile strength and Young's modulus (Table 2). those observed 24 hours previously (ESI, Fig. S25–S28†). Several
lower thermal transitions (ca. 7–10 ꢀC), not observed in the
second heat/cool cycles (see Table 3) were apparent in both the
rst scans of the individual heal cool cycles separated by 24
hours. Interestingly such lower thermal transitions evident in
the DSC thermograms of polymer blends of pEAA15/1-3
(recorded from the post relaxation analysis) correlate well
with those observed in well-dened poly(ethylene-co-acrylic
acid) block copolymers.25
Differential scanning calorimetric studies
In order to characterise the thermal transitions of the blends
DSC analysis was conducted. Studies were focused on the
blends that demonstrated modied mechanical properties
(Table 2) when compared to the bulk polymer, in accordance
with the model shown in Fig. 1. The transition temperatures
were determined by the application of two successive heat/cool
cycles, the rst cycle to remove thermal/processing histories
Healing studies
and the second to identify the transitions (e.g. Tg and Tm) (see Healing studies were undertaken on pEAA15 and its blends
ESI Fig. S21–S24†). using mechanical property recovery as the key indicator. It was
Fig. 3 Stress–strain curves (average of five analyses plotted) for pEAA15 (black), pEAA15/1 (5 wt%) (blue dashed), pEAA15/3 (5 wt%) (blue solid).
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Table 3 Thermal properties of formed of blends of pEAA15 and
additives 1–3 in 1 and 5 wt% with respect to plasticizer after the second
heat/cool cycle (heating rate 15 ꢀC minꢁ1, cooling rate 5 ꢀC minꢁ1) with
percent crystallinity (c) determined by assuming a melt enthalpy of 293
J gꢁ1 for 100% crystalline poly(ethylene)32
acid 3 were shown to dramatically increase the thermal heal-
ability of pEAA15 (for example a 28% increase in recovery of
tensile strength with respect to the pure copolymer, Table 4).
Although the blend featuring carboxylic acid 2 exhibited
a greater degree of healing with respect to energy absorbed and
Young's modulus recovery there was a drop in the mechanical
properties when compared to pure pEAA15. This blend also
showed decreases in the thermal transition temperatures which
in turn implies simple plasticisation of the bulk polymer phase.
Notably, the thermal healing studies revealed the ability of
the carboxylic acid 3 blended with pEAA15 to heal with greater
efficiency (with respect to each mechanical property recorded,
Table 4) than the blend formed with the dicarboxylic acid 1.
These data suggest that the insertion of a secondary supramo-
lecular functionality (in the form of the diaryl urea of 3, see
Fig. 1), which is capable of self-assembly to form an alternative
network, actually increases the healability of the bulk phase
without disrupting its mechanical performance.
An additional healing study on these blends involved the
application of pressure to the cut lms. Films were prepared (40
ꢂ 10 ꢂ 1 mm) and cut with a scalpel before the cut edges were
overlapped (1 mm lengthways) and then subjected to a pressure
of 0.98 MPa overnight. The lms containing the carboxylic acid
3 additive healed most successfully under this regime using
tensile strength recovery as the key indicating factor (see entries
c in Table 4). Interestingly the plasticized blends of pEAA15/2
exhibited some limited recovery when this healing method
was used, in terms of recovering both the tensile strength and
energy absorbed to break.
wt%
Film system
additive
Tg
Tm
Tc
c
pEAA15
pEAA15/1
ꢁ12
ꢁ14
ꢁ13
ꢁ15
ꢁ15
ꢁ16
ꢁ15
84
82
82
82
81
82
83
73
71
71
72
71
72
72
7
7
7
5
4
7
6
1
5
1
5
1
5
pEAA15/2
pEAA15/3
decided to limit these studies to the blends that possessed
improved properties (with respect to tensile strength and
uniform strain) when compared to the bulk polymer. For this
reason, blends of carboxylic acids 1 and 3 with pEAA15 at an
additive loading of 5 wt% and 2 with pEAA15 (1 wt%, selected as
a control with limited weakening effects) were studied (Table 4
and ESI Tables S1–S3†).
Three independent studies were undertaken on lms of the
blends (40 ꢂ 10 ꢂ 1 mm) to ascertain the degree of healability.
For the thermal studies, lms of blends were cast from solution
(DMF) and cut into two pieces using a scalpel before being
placed in contact without overlap of the cut edges. The lms
ꢀ
were then held at 60 C for 2 hours (entries a in Table 4) and
50 ꢀC for 8 hours (entries b in Table 4). These temperatures were
ꢀ
Further healing studies were conducted with poly(ethylene-
co-acrylic acid) having 5% (pEAA5) and 20% (pEAA20) acrylic
acid content, blended with the additives 1 and 3. DSC analysis
of the blends of pEAA5 with carboxylic acid 1 revealed phase
chosen as they are at least 15 C lower than any of the endo-
therms recorded in the DSC analysis.
It was found that samples of pure pEAA15 demonstrated only
limited healability (Table 4). Blends of pEAA15 and carboxylic
Table 4 Percentage healing of films of the blends based on tensile strength, energy absorbed and Young's modulus recoverya
Energy
Young's
Tensile strength recovery (%)
absorbed recovery (%)
modulus recovery (%)
Film system
wt% blended
a
b
c
a
b
c
a
b
c
pEAA15
—
5
1
5.5
2.7
30.6
33.1
17.7
—
38.7
42.1
45.5
41.0
10.8
73.3
0.8
0.3
10.0
3.2
3.1
—
23.1
5.1
23.1
25.6
5.4
4.8
12.1
—
75.8
12.3
22.4
17.6
78.4
28.3
pEAA15/1
pEAA15/2
pEAA15/3
10.6
97.9
12.7
5
34.1
a
(a) Healing at 50 ꢀC 8 hours, (b) healing at 60 ꢀC 2 hours and (c) healing under pressure (0.98 MPa) 8 hours. The stress–strain data are reported in
the ESI, see Fig. S29–S39.
Table 5 Mechanical properties of films formed from blends of pEAA20 and carboxylic acids 1 and 3 at additive loadings of 10 wt% (film
dimensions averaging 40 ꢂ 10 ꢂ 1 mm, true strain rate of 0.2 sꢁ1
)
wt%
additive
Tensile strength
(MPa)
Fracture stress
(MPa)
Uniform strain
(%)
Strain to fracture
(%)
Energy absorbed
(MPa)
Young's
modulus (MPa)
Film system
pEAA20
pEAA20/1
pEAA20/3
4.86
4.23
6.30
4.47
4.11
5.78
64.7
122.9
98.4
72.1
129.8
103.1
2.84
4.85
4.90
26.79
40.24
33.76
10
10
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Fig. 4 Stress strain curves (average of five analyses plotted) showing pEAA20 and pEAA20/3 before (solid purple) and after (dashed purple) after
healing under pressure (0.98 MPa) (8 hours). *Note that pEAA20 failed to heal under similar pressure conditions.
separation (observed via melt point of the additive) at additive additives with the nitro moiety in the meta position provide the
loadings of 5 wt% in contrast to those of pEAA20 which are most benecial additive properties to the resulting blends.
homogeneous at 10 wt% (see ESI Fig. S40–S41†). This trend is in
It is proposed that the introduction of functionalised small
agreement with the proposal that the small molecule additives molecule additives to reinforce and promote healability could
interact with the carboxylic acid moieties present in the poly- be developed for a range of copolymers. Variations in the
meric backbone.
moieties responsible for the formation of so ‘gelator type’
To probe potential system hardening and healability, lms networks would enable control over structural stability whilst
of the blends formed between pEAA20 and carboxylic acids 1 variations in the moieties responsible for additive–polymeric
and 3 (each at an additive loading of 10 wt%) were subjected to interactions would enable compatibility. However, the compo-
tensile testing (ESI Fig. S42–S45†). An increase in strength and sition of the copolymer is also crucial: it must feature comple-
elastomeric response was realised in both of these blends (see mentary residues to permit binding with the low molecular
Table 5 and Fig. 4).
weight additive. Furthermore the crystalline and amorphous
Studies on the healability of blends of pEAA20 focused on structures of the copolymer must allow self-assembly of the low
a comparison of pEAA20 with the best performing blend (see molecular weight additive if benecial properties (such as
Table 5). The blend pEAA20/3 (10 wt%) was the most successful recovery of strength on healing) are to be realised.
under pressurised healing conditions (Table 4) using the heal-
ing conditions described above. Under these conditions
Experimental
recovery of the properties was not observed in pEAA20 whilst the
blends of pEAA20 and 3 (10 wt%) exhibited a degree of heal-
Chemicals and solvents were purchased from Sigma Aldrich
ability in terms of tensile strength (55% recovery), energy
and used as purchased, unless otherwise specied. THF was
absorbed to break (17% recovery) and Young's modulus (54%
distilled from sodium and benzophenone under inert condi-
recovery) (see Fig. 4).
tions prior to use. NMR spectra were obtained using Bruker
Nanobay 400 and Bruker DPX 400 instruments (operating at 400
MHz and 100 MHz for 1H NMR and 13C NMR spectroscopy,
respectively). NMR samples were prepared in DMSO-d6 or
Conclusions
CD3OD and dissolution was achieved with slight heating and
It has been demonstrated that blending functionalised molec- sonication (5–10 minutes). Mass spectra were obtained on
ular additives with copolymers of ethylene and acrylic acid can a Thermo-Fisher Scientic Orbitrap XL LCMS (operating in
serve the dual purpose of both reinforcement and increased electrospray mode) – samples were prepared in DMSO for direct
healability via the creation of a supramolecular ‘network within injection (1 mg mLꢁ1). A Perkin Elmer 100 FT-IR (diamond ATR
a network’. This was realised by generating a so ‘gelator type’ sampling attachment) was employed for IR spectroscopic
network phase within a polymeric network. This scenario is only analysis. Samples for IR characterisation were in powder form.
possible when the small molecular additives are able to interact UV spectra were recorded using a Varian Cary 300 Bio or a Per-
by hydrogen bonding with the residues within the copolymer kinElmer Lambda 25 UV/Vis spectrometer. Sample solutions
structure that are responsible for supramolecular bonding and were analysed in quartz cuvettes with a 5.0 mm path length and
network formation. Manipulation of the mechanical properties were baseline corrected with respect to a blank cell with the
of the bulk polymer phase has been demonstrated by varying appropriate solvent.
the additive concentrations as well as through changes in the
The pH-stimulated hydrogels of 1 and 3 were produced via
structural composition of the additive. It was noted that those dissolution of the gelator in NaOH(aq.) (0.5 mL, 0.1 M) followed
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by addition of glucono-d-lactone (0.5 mL, 0.2 M).26 The solutions 146.4, 140.9, 134.3, 134.0, 125.1, 119.5, 119.1, 117.4, 30.9,
were then le for 2 hours to acidify and gel. The critical gelation 28.8 ppm; MS (ESI) m/z [M + H+] calculated for C17H17O6N4
concentration (CGC) of 3 was determined in a 2 mL screw top 373.1143, found 373.1142.
¼
glass vial. Minimum gelator mass was determined to the near-
(5) 4-Oxo-4-((4-(3-phenylureido)phenyl)amino)butanoic acid;
est 1 mg, then varied every 0.2 mg to obtain increased accuracy a white powder, (0.09 g, 73%) Tdec 242 ꢀC; IR (ATR)/cmꢁ1 3314,
of the CGC value. Dye uptake measurements were carried out 3270, 3030, 1696, 1638, 1601, 1562, 1444, 1404, 1301, 1226,
via extraction of 0.5 mL sample from dye/gelator mixtures, 1183, 1054, 902, 799, 735, 619; 1H NMR (400 MHz, DMSO-d6) ¼
ltering through sterile syringe lters (0.45 mm Minisart® 12.13 (s, 1H), 9.87 (s, 1H), 8.62 (s, 1H), 8.57 (s, 1H), 7.50 (m, 4H),
syringe lter).
7.36 (m, 2H), 7.26 (m, 2H), 6.95 (m, 1H) ppm; 13C NMR (100
Thermogravimetric analysis employed a TA Instruments MHz, DMSO-d6) ¼ 173.9, 169.6, 152.5, 139.8, 128.7, 121.7, 119.5,
TGA Q50 attached to a TGA heat exchanger, platinum cruꢁci1ble 119.0, 118.6, 118.1, 30.9, 28.9 ppm; MS (ESI) m/z [M + H+]
ꢀ
and an aluminium TA-Tzero pan (heating rate 15 C min to calculated for C17H18O4N3 ¼ 328.1292, found 328.1290.
500 ꢀC). Differential scanning calorimetry analysis employed
(6) 4-((3-Nitrophenyl)amino)-4-oxobutanoic acid; 3-nitroani-
a TA DSC Q2000 with TA Refrigerated Cooling System 90 line (0.1 g, 0.72 mmol) was dissolved in dry THF (50 mL). To the
ꢁ1
ꢀ
(aluminum TAꢀ-Tzero pans and lids) (heating rate 15 C min
,
solution succinic anhydride was added (0.064 g, 0.72 mmol) and
the solution stirred under reux for 24 hours. The product was
cooling rate 5 C minꢁ1).
The strengths of lms were determined by texture analysis precipitated into HCl(aq) (1.0 M, 200 mL), the precipitate
(Texture Analyser, Stable Microsystems). Analysis was conduct- collected by ltration and washed with H2O (2 ꢂ 25 mL) to yield
ed with A/TG tensile grips (screw-initiated vice clamp, knurled- the title compound as a yellow powder (0.142 g, 83%) Tdec
jaw faces 35 mm ꢂ 35 mm) at a true strain rate of 0.2 sꢁ1, using 239 ꢀC; IR (ATR)/cmꢁ1 3260, 3198, 3105, 2863, 2567, 1694, 1674,
a trigger force of 0.01 N. The strength of the lm was taken as 1610, 1553, 1523, 1432, 1403, 1340, 1256, 1176, 1083, 951, 806,
the maximum force applied before fracture which is seen 732, 670; 1H NMR (400 MHz, DMSO-d6) ¼ 10.47 (s, 1H), 8.63 (s,
graphically as a sharp drop in recorded force. All lms tested 1H), 7.87 (m, 2H), 7.58 (m, 1H), 2.60 (t, 2H, J ¼ 6.8 Hz), 2.52 (t,
were prepared by solvent casting (DMF) to facilitate lm 2H, J ¼ 6.8 Hz) ppm; 13C NMR (100 MHz, DMSO-d6) ¼ 173.7,
homogeneity. It was later ascertained that tensile properties 170.9, 147.9, 140.3, 130.1, 124.8, 117.5, 112.9, 31.0, 28.5 ppm;
were equivalent for melt and solvent cast lms of pEAA15/1. MS (ESI) m/z [M + H+] calculated for C10H11O5N2 ¼ 239.0668,
Films were cut into a dog-bone shape before testing, giving found 239.0670.
a strain-measurement region with dimensions averaging 40 ꢂ
10 ꢂ 1 mm.
Conflicts of interest
Carboxylic acids 1 and 2 were used as supplied. The small
molecule additives 3–5 were synthesised according to the There are no conicts to declare.
following procedure. The appropriate aromatic bis amine, 1-(4-
aminophenyl)-3-(3-nitrophenyl)urea (to give 3) or 1-(4-
aminophenyl)-3-(4-nitrophenyl)urea (to give 4) (0.1 g, 0.36
Acknowledgements
mmol) or 1-(4-aminophenyl)-3-phenylurea (to give 5) (0.08 g, The authors would like to acknowledge nancial support from
0.36 mmol) was dissolved in dry THF (50 mL). To this solution the University of Reading and Gnosys Global Ltd (PhD
succinic anhydride (0.032 g, 0.36 mmol) was added directly and studentship for BCB). In addition, the University of Reading
the solution was then stirred under reux for 24 hours. The (EPSRC-Doctoral Training Grant) is acknowledged for providing
product was precipitated into HCl(aq) (1.0 M, 200 mL), collected access to instrumentation in the Chemical Analysis Facility.
via ltration at the pump and then washed with H2O (2 ꢂ 25
ꢀ
mL) before drying in vacuo (80 C, 2 hours) to afford:-
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ꢀ
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RSC Adv., 2018, 8, 41445–41453 | 41453