High-performance, command-degradable, antibacterial Schiff base epoxy thermosets: Synthesis and properties

Article abstract of DOI:10.1039/c9ta05293c

Two diepoxides containing a two-benzene-ring-conjugated Schiff base when cured with an aromatic diamine result in high-performance thermosets combining excellent controlled degradability, stability and antibacterial properties. Epoxy resins have been wide

Full text of DOI:10.1039/c9ta05293c

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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Table of Contents Entry
Epoxy thermosets containing two-benzene-rings-conjugated Schiff base structure combined
excellent controlled degradability, stability, antibacterial property, and thermal and mechanical
properties.

Journal of Materials Chemistry A
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High-performance, Command-degradable, Antibacterial Schiff Base
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DOI: 10.1039/C9TA05293C
Epoxy Thermosets: Synthesis and Properties
Xiwei Xu,a,b Songqi Ma,*b Jiahui Wu,a Jintao Yang,a Binbo Wang,b,d Sheng Wang,b,c Qiong Li,b,c
Jie Feng,*a Shusen You,b Jin Zhub
a School of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R.
China
b Key laboratory of bio-based polymeric materials technology and application of Zhejiang province, Ningbo
Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R.
China.
c University of Chinese Academy of Sciences, Beijing 100049, P. R. China
d School of Materials Science and Engineering, Shanghai University, Shanghai 200072, P. R. China
*Corresponding authors: (Songqi Ma) E-mail: masongqi@nimte.ac.cn, Tel: 86-0574-87619806; (Jie Feng) E-
mail: fengjie@zjut.edu.cn
Abstract: Two diepoxies containing two-benzene-rings-conjugated Schiff base when cured with an aromatic
diamine result in high-performance thermosets combining excellent controlled degradability, stability and
antibacterial property. Epoxy resins have been widely used in coatings, adhesives, composites, electronic
packaging materials, etc., but they are arduous to recycle and have no antibacterial activity. In this paper, two-
benzene-rings-conjugated Schiff base diepoxy monomers were synthesized and further cross-linked with 4,4-
diaminodiphenylmethane. The chemical structures of the diepoxies as well as their thermosets were
characterized in detail. The property investigation manifested that the thermosets owned far higher glass
transition temperature of ~206 °C, tensile strength of ~122 MPa, tensile modulus of~ 2646 MPa than a
commonly-used high-performance bisphenol A epoxy resin. Moreover, they possessed excellent acidity and
temperature-controlled degradability, and superior stability toward common solvents, thermal treatment and
hygrothermal aging. Furthermore, they presented excellent coating properties (including hardness, adhesion,
and solvent resistance) and antibacterial properties with bactericidal capability of ~91%.
1. Introduction
Epoxy resins have wide application in many fields such as composites, adhesives, coatings, and electrical
packaging materials owing to their excellent mechanical properties and insulation, solvent and thermal
resistance, etc.1 However, epoxy resins are insoluble and cannot be melt and difficult to be de-cross-linked
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Journal of Materials Chemistry A
once cured because of their irreversible covalent cross-linked networks, which makes them hard to be recycled
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DOI: 10.1039/C9TA05293C
after their useful service life, resulting in serious waste of resources and environmental pollution.2-9
Currently, developing inherently degradable epoxy resin relying on introducing reversible covalent
structures into epoxy resin or curing by reversible reactions is a promising way to address the recycle issue of
epoxy resins.1, 10, 11 A series of degradable epoxy resins have been developed based on different reversible
covalent linkages such as ester bond,12-14 disulfide,15, 16 acetal linkages,10, 17, 18 Schiff base,19, 20 sulfonate ester
linkages,21 sulfite linkages,22 carbamate linkages,23 phosphorus containing structures,24, 25 carbonate linkages,26,
27 Diels–Alder addition structures,28 olefinic bonds,29 and hemiacetal/hemiketal ester linkages,30, 31 etc. Under
extra stimuli, the reversible covalent linkages in epoxy networks can be cleaved or attacked by other
compounds, corresponding to the decomposition of the epoxy networks. Although all these degradable epoxy
resins are of great scientific importance, their lower stability, thermal and mechanical properties as well as
chemical resistance relative to conventional epoxy resins limited their utilization. Thus, developing epoxy
resins combining excellent degradability and high performance is a challenge but practically significant. One
promising way is the utilization of rigid and readily controlled-degradable structures. Zhang’s group32 reported
a Diels–Alder adduct-containing epoxy resin which could be degraded through sonochemistry at around 20 °C,
the glass transition temperature (Tg) reached 97 °C. In our previous work, we exploited and synthesized
vanillin-based spiro diacetal to achieve high-performance readily degradable epoxy resins with Tg of 169 °C,
tensile modulus of 3131 MPa, tensile strength of 85 MPa.10 Adjusting the diacetal structure and the network,
Tg could be further improved to 184 °C.33 Schiff base (C=N), facilely synthesized via Schiff base reaction
between aldehyde and amino groups, is also used in the design of degradable epoxy resin.20, 34, 35 The Schiff
base conjugated by benzene ring is a rigid structure and is hopefully to design high-performance epoxy resin.
In our previous work, we introduced Schiff base into the epoxy networks through the curing reaction between
aldehyde group-containing monoepoxide and diamine.20 Beside excellent degradability in mild acidic
conditions, the epoxy thermoset possessed similar tensile strength of 81 MPa, modulus of 2112 MPa, and Tg
of 172 °C, and higher thermal stability with the degradation temperature for 5% weight loss of 323 °C and
elongation at break of 15% in comparison with a high-performance bisphenol A epoxy resin from Dow
chemical (DER331). However, the high polarity of Schiff base makes it have certain sensitivity to water, the
stability of Schiff base materials is inevitably doubted.35, 36
Bacteria are ubiquitous in our lives and threaten human health by bacterial infections. Since the discovery
of penicillin in 1928, people have conquered a multitude of bacterial infections, but the abuse of antibiotics
2

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has brought about the emergence of multidrug resistant microorganisms, which has caused more than 13
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million people worldwide to die from infectious diseases every year.37, 38 This serious issueDhOaI:s10a.1r0o3u9/sCe9dTAt0h5e293C
concern of growing researchers. Researchers in materials are trying to endow antibacterial properties to
materials. Adding heavy metal-containing nanoparticles to materials often results in highly effective
antibacterial and relatively long-lasting antimicrobial properties, because these nanoparticles can destroy the
cell membrane of bacteria.39, 40 Considering that heavy metals have certain harm to the environment and human
health, in recent years, a kind of polymer brush without heavy metal is used to reduce the adhesion of bacteria
and cooperate with antibacterial agents, which has a favorable effect.41, 42 However, researches on antibacterial
modification of high-performance resins are exceedingly rare, and antibacterial, degradable epoxy resins have
never been reported.
Thus, in this paper, for the first time, we reported high-peformance epoxy thermosets combining excellent
degradability and antibacterial property. Two Schiff base diphenols were synthesized through the reaction of
p-hydroxybenzaldehyde (or vanillin) and p-aminophenol, then the two diphenols were epoxidized to
synthesize two diepoxies (PBE and VBE) containing Schiff base structure. In order to achieve the high
performance of the epoxy resin and address the stability issue of Schiff base, two benzene rings were
conjugated to Schiff base when synthesizing monomers…However, when epoxy resin is cured by an amine-
based curing agent, exchange reaction of Schiff base should occur, which might break the stable two-benzene-
rings-conjugated Schiff base.20, 35 Therefore, an aromatic dimaine, 4,4-diaminodiphenylmethane, was used as
the curing agent in this work, Schiff base kept conjugated by two benzene rings even if the exchange reaction
of Schiff base occurred during the curing process. This design greatly increased the stability of the Schiff base
epoxy resin. Materials containing Schiff base were used in fungicides, however, they were often combined
with metal ions.43, 44 Here, the antibacterial effect of Schiff base without metal ions was investigated by
comparing the antibacterial properties of Schiff base epoxy resins with commercial epoxy resin. Lignin and its
derivatives are commonly used in the preparation of antimicrobial materials.45-48 We speculate that it may be
related to methoxy group rich in lignin. Here, the antibacterial properties of epoxy resin synthesized by p-
hydroxybenzaldehyde and methoxy group-containing vanillin were compared to investigate the effect of
methoxy group on antibacterial properties. Bisphenol A epoxy resin accounts for more than 90% of the epoxy
precursors in the world, thus, a high-performance bisphenol A epoxy resin (DER331) was investigated as the
control. The chemical structures of PBE, VBE and DER331 are illustrated in Fig. 1
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Journal of Materials Chemistry A
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DOI: 10.1039/C9TA05293C
Fig. 1 Chemical structures of PBE, VBE and DER331
2. Experimental section
2.1 Materials
4,4-Diaminodiphenylmethane
(DDM),
vanillin,
p-hydroxybenzaldehyde,
p-aminophenol
and
tetrabutylammonium bromide were purchased from Aladdin-reagent Co., China. Epoxy resin (DGEBA, trade
name DER331, epoxide equivalent 182~192 g/eq) was supplied by DOW Chemical Company.
Epichlorohydrin (ECH), N,N-dimethylformamide (DMF), petroleum ether (boiling range: 60~90 °C), ethanol
(purity ≥ 99.5%) and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd., China.
2.2 Synthesis of Schiff base diepoxy monomers
Schiff base can be synthesized by simple dehydration-condensation reaction between amine and aldehyde.
Here we used a simple one-step reaction between p-aminophenol and p-hydroxybenzaldehyde to synthesize a
Schiff base diphenol, and then epoxidized the diphenol with epichlorohydrin. 10.9 g (0.1 mol) of p-
aminophenol, 12.2 g (0.1 mol) of p-hydroxybenzaldehyde and 200 mL of ethanol were poured into a 500 mL
three-necked flask equipped with a reflux condenser and a magnetic stirrer, and reacted at 50 °C for 1 h. After
the reaction was completed, the mixture was added dropwise to 200 mL petroleum ether and filtered to give a
pale yellow product, 4-((4-hydroxybenzylidene)amino)phenol (PBP, 16.7 g), with a yield of approximately
78.4 %. Then, 10.7 g (0.05 mol) of PBP, 46.3 g (0.5 mol) of epichlorohydrin and 1.07 g (10 wt.% of PBP)
tetrabutylammonium bromide (TBAB, catalyst) were added to a 250 mL three-neck flask equipped with a
magnetic stirrer and a reflux condenser, reacted at 80 °C for 2 h, then the mixture was placed in an ice bath
and kept below 5 °C. 3 g (0.075mol) of 40 wt% sodium hydroxide (NaOH) aqueous solution was added
dropwise to the mixture within 5 min, reacted below 5 °C for 5 h, and washed three times with water to obtain
a yellow solution. The solution was added dropwise to petroleum ether to give a white powder (PBE, 10.8 g)
with a yield of approximately 66.2%. VBE, a white powder, with a yield of approximately 74.9% was obtained
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by the same synthetic method for PBE by using vanillin instead of p-hydroxybenzaldehyde. The synthetic
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DOI: 10.1039/C9TA05293C
routes are illustrated in Scheme 1.
Scheme 1 Synthetic routes of Schiff base diepoxy monomers PBE and VBE.
1H NMR (400 MHz, DMSO-d6, ppm, PBE) δ = 8.54 (s, 1H), 7.86 (d, J = 8.7 Hz, 2H), 7.24 (d, J = 8.8 Hz,
2H), 7.08 (d, J = 8.7 Hz, 2H), 6.99 (d, J = 8.8 Hz, 2H), 4.38 (ddd, J = 31.8, 11.4, 2.5 Hz, 2H), 3.88 (ddd, J =
26.8, 11.4, 6.6 Hz, 2H), 3.41 – 3.33 (m, 2H), 2.86 (dd, J = 9.6, 5.1 Hz, 2H), 2.77 – 2.67 (m, 2H).
13C NMR (400 MHz, DMSO-d6, ppm, PBE) δ = 161.05 (s, 5C), 158.28 (s, 10C), 157.06 (s, 4C), 145.27 (s,
5C), 130.62 (s, 22C), 129.98 (s, 5C), 122.68 (s, 23C), 115.53 (s, 23C), 115.23 (s, 23C), 69.62 (d, J = 5.7 Hz,
21C), 50.13 (d, J = 15.2 Hz, 21C), 44.23 (s, 20C).
1H NMR (400 MHz, DMSO-d6, ppm, VBE) : δ = 8.52 (s, 1H), 7.55 (s, 1H), 7.40 (d, J = 8.2 Hz, 1H), 7.25
(d, J = 8.8 Hz, 2H), 7.09 (d, J = 8.3 Hz, 1H), 6.99 (d, J = 8.8 Hz, 2H), 4.36 (ddd, J = 16.7, 11.4, 2.6 Hz, 2H),
4.00 – 3.72 (m, 5H), 3.43 – 3.32 (m, 2H), 2.86 (dd, J = 9.2, 4.5 Hz, 2H), 2.77 – 2.65 (m, 2H).
13C NMR (400 MHz, DMSO-d6, ppm, VBE) δ = 158.59 (s, 5C), 157.06 (s, 3C), 150.87 (s, 3C), 149.58 (s,
3C), 145.24 (s, 3C), 130.24 (s, 3C), 123.87 (s, 5C), 122.67 (s, 11C), 115.53 (s, 12C), 113.09 (s, 6C), 110.09
(s, 6C), 70.27 (s, 5C), 69.65 (s, 5C), 55.94 (s, 6C), 50.14 (d, J = 12.8 Hz, 10C), 44.26 (d, J = 6.7 Hz, 10C).
2.4 Preparation of the Cured Epoxy Resins
3.55 g (0.01 mol) of PBE (or 3.25 g (0.01 mol) of VBE) and 0.99 g (0.005mol) of DDM were added in 10 mL
of DMF, and heated to 155 °C to quicken the dissolution of PBE (or VBE) in DMF. After being completely
dissolved, the pre-reaction was carried out for 10 min to prevent the precipitation of the epoxy monomer
and DDM in the next step. The solution was transferred to a tinplate and then reacted at 155 °C until the
solution was completely evaporated to give pre-cured products. The pre-cured films were carefully removed
from the tinplate, and cut into specimens with dimensions of 40 mm (length) × 10 mm (width) × 0.25 mm
(thickness) for tensile tests, 20 mm (length) × 5 mm (width) × 0.25 mm (thickness) for dynamic mechanical
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analyses. The solution was coated on a butanone cleaned tinplate, pre-cured according to the pre-curing method
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described above to obtained specimens with thickness of around 100 μm for coatiDnOgI: 1p0e.1r0f3o9r/Cm9TaAn0c5e293C
determination. The solution was also transferred to a tinplate to get specimens with dimensions of 20 mm
(length) × 20 mm (width) × 0.5 (thickness) mm for antibacterial properties examination. The pre-cured
products were successively post-cured at 120 °C for 2 h, 150 °C for 2 h, 180 °C for 1 h, and 200 °C for 1 h in
a vacuum oven. The cured epoxy resin was named as PBE-DDM (or VBE-DDM).
2.5 Characterization
1H NMR and 13C NMR spectra were measured through an AVANCE III Bruker NMR spectrometer (Bruker,
Switzerland) with DMSO-d6 as solvent. The infrared spectra (FT IR) were recorded on a Micro-FTIR Cary660
spectrometer (Agilent, America) and the absorbance mode was used. Time-of-flight mass spectrometry (TOF-
MS) was performed by dissolving the samples in dichloromethane and measuring them on a Bruker Autoflex
III MALDI-TOF-MS mass spectrometer in positive mode. Thermogravimetric analysis (TGA) was examined
by a Mettler-Toledo TGA/DSC1 thermogravimetric analyzer (Mettler Toledo, Switzerland). Approximately 5
mg of the samples were placed in nitrogen atmosphere, and the temperature was raised from 50 °C to 800 °C
at a heating rate of 10 °C min-1. Differential scanning calorimetry (DSC) was tested via a Mettler-Toledo Star
1 apparatus to investigate the glass transition temperatures (Tgs) of the cured samples. The cured samples
(approximately 5 mg) were heated from 50 °C to 250 °C at a heating rate of 20 °C min−1 and held at 250 °C
for 3 min to remove the thermal history, then cooled to 50 °C at a cooling rate of 20 °C min-1, and finally
heated to 250 °C at a heating rate of 20 °C min-1. The second heating curves were used to obtain the Tgs of the
cured samples. Dynamic thermomechanical analysis (DMA) was performed on a TA Instrument (DMA Q800,
America) in tension mode with dimensions of around 20 mm (length) × 5 mm (width) × 0.5 mm (thickness)
to measure the dynamic mechanical properties of the cured samples. The cured samples were heated to 250 °C
from 0 °C at a heating rate of 5 °C min−1 under a frequency of 1 Hz. The tensile properties of the cured samples
were measured using an Instron 5567 Electric Universal Testing Machine (Instron, America) at room
temperature. The cured samples with dimensions of 40 mm (length) × 10 mm (width) × 0.25 mm (thickness)
were measured at a cross-head speed of 2 mm min−1 with a gauge length of 20 mm. The pencil hardness of the
coatings was measured based on ASTM D 3363-00. The adhesion of the coatings on tinplate was evaluated
using crosshatch adhesion method according to ASTM D 3359-09. The solvent resistance of the coatings was
determined by the double rub method according to the ASTM D 5402-06 modified by using cheesecloth
wrapped around a hammerhead. Chemical degradation was determined by immersing the cured epoxy resins
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in the corresponding solutions at 50 °C or 23 °C. The samples were weighed after placing in a vacuum oven
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at 80 °C for 2 h, and recorded as initial weight. Degradation times refer to the times when tDhOeI:s1a0m.10p3l9e/Cs9wTAe0r5e293C
completely dissolved in the solution. The samples that could not be completely dissolved were transferred to
a vacuum oven and dried at 80 °C for 8 h, and weighed as the final weights. The final weight of the fully
degraded sample is zero. The degradation rate is calculated based on the starting weight, the final weight, the
degradation time and the volume of the acidic solutions used for degradation. For swelling rate measurement:
the samples were immersed in THF, methanol, DMF, ethanol, acetone and water at 23 °C or 50 °C for 24 h.
m0 is the initial dried mass of the samples, m1 is their mass after swelling; the swelling rate is calculated by
(m1- m0)/m0. The antibacterial activity of the surfaces of PBE-DDM, VBE-DDM and DER331-DDM was
investigated by Escherichia coli ((E. coli, Gram-negative). E. coli was cultured on Luria-Bertanni (LB, OXOID)
agar plate for 14~16 h at 37 °C, then a single colony was picked to the 40 mL LB at 37 °C. E. coli was incubated
for approximately 12 h under shaking at 200 rpms and adjusted to an appropriate optical density of∼0.1 (E.
coli) by dilution for reserving. The samples were soaked in 70% alcohol for 10 min to kill the bacteria on the
surface, then the dead bacteria were removed with phosphate-buffered solution (PBS). All processed samples
were placed in the solution containing E. coli and then held at 37 °C for 24 h. Bacteria that were not adhered
to the surface of the samples were washed three times with PBS. LIVE/DEAD BacLight kit (Thermo Fisher
Scientific Inc., NY) was used to stain the samples for 10 min, then the stained samples were washed by PBS
to remove the excessive dye. Finally, bacteria on the surface of samples were observed by Axio Observer A1
fluorescence microscope.
3 RESULTS AND DISCUSSION
3.1 Preparation and chemical characterization of PBE, VBE, PBE-DDM and VBE-DDM
The chemical structures of PBE, VBE, PBE-DDM and VBE-DDM were characterized by 1H NMR, 13C NMR,
FT IR and TOF-MS. As shown in the Fig. 2, the chemical shift and integral area of the peaks in the 1H NMR
spectra and peak numbers in the 13C NMR spectra are perfectly consistent with the respective chemical
structures of PBE and VBE, which is also suggestive of the high purity of the epoxy monomers. From the
TOF-MS spectra (Fig. S1, Fig. S2), it can also be seen that the molecular weights of PBE and VBE are both
identical with the theoretical values. These results are indicative of the successful synthesis of PBE and VBE.
As shown in the FT IR spectra (Fig. S3), the peak at 910 cm-1 belongs to the epoxy groups of PBE and VBE,
while this peak disappeared in the FT IR spectra of PBE-DDM and VBE-DDM, indicating that the epoxy
group was completely reacted. The appearance of the hydroxyl peak at 3350 cm-1 also indicates the ring-
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opening reaction of the epoxy group with the amino group. The peak of Schiff base at 1620 cm-1 existed in the
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DOI: 10.1039/C9TA05293C
FT IR spectra of PBE-DDM and VBE-DDM, indicating that Schiff base structure was successfully introduced
into the epoxy networks.
Fig. 2 1H NMR spectra of a) PBE and b) VBE; 13C NMR spectra of c) PBE and d) VBE.
During the curing reaction of PBE (or VBE) with DDM, as shown in Scheme 2, three kinds of repeating
units containing Schiff base structure should be generated. The repeating unit a was produced by the ring-
opening reaction of the epoxy groups of PBE (or VBE) and the amino groups of DDM, which is a typical
curing process for epoxy/amine systems.49 When epoxy resin is cured by an amine-based curing agent,
exchange reaction (transimination) of Schiff base should occur, because Schiff base can react with amine to
produce a new Schiff base and a new amine under heating.20, 35 When one amino group of the DDM proceeded
transamination with one Schiff base structure, and another amino group of the DDM reacted with the epoxy
groups, it would produce the repeating unit b. When both amino groups of the DDM proceeded transamination
with two Schiff base structures, it would produce repeating unit c. Obviously, all the Schiff bases in three
repeating units are conjugated with two benzene rings, which was beneficial for achieving the high
performance of the epoxy resin and addressing the stability issue of Schiff base.
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DOI: 10.1039/C9TA05293C
Scheme 2 Preparation route and three repeating units containing Schiff base structure of PBE-DDM (or
VBE-DDM).
3.2 Thermal and mechanical properties of the cross-linked epoxy resins
Glass transition temperature (Tg) controls the highest end-use temperature for thermosets, thus the Tgs of the
Schiff base epoxy resins were examined by DSC and DMA. The DSC curves and DMA curves are shown in
Fig. 3 and Fig. 4, and their data are summarized in Table 1. As can be seen, both PBE-DDM and VBE-DDM
exhibited higher Tgs than DER331-DDM, and the Tg of PBE-DDM was higher than that of VBE-DDM. Tg of
thermosetting polymer has a close tie with its cross-link density and rigidity of the segmental chain.14, 50-52 The
higher cross-link density and rigidity of the segmental chain are, the higher Tg is. In order to compare the cross-
link density of the cured epoxy resins, the average molecular weight between cross-link points (Mc) which
reflects the theoretical cross-link density as well as the actual cross-link density were calculated through
equations (1) 51, 53, 54 and (2) 55-57, respectively.
푒푝표푥푦·푀푒푝표푥푦 + 푛퐷퐷푀·푀퐷퐷푀
푀푐 = 푛
(1)
푛퐷퐷푀
퐸′ = 3휈푒푅푇
(2)
where n and M represent the molarity and molar mass of the epoxies (PBE and VBE) and curing agent (DDM)
in the epoxy formulations. E’ is the storage modulus of the thermosets in the rubbery plateau region at Tg + 60
°C, R is the gas constant, and T is the absolute temperature. The calculated Mc and νe values are summarized
in Table 1. The higher Mc means the lower theoretical cross-link density for thermosets. Obviously, both Mc
and νe values indicate the cross-link density order of PBE-DDM > VBE-DDM > DER331-DDM. The Schiff
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base epoxy resins presented higher rigidity than DER331-DDM due to their two-benzene-rings-conjugated
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Schiff base structures. Thus, the higher Tgs of the two Schiff base epoxy resins than DER331 are from their
higher cross-link density and rigidity; PBE-DDM’s higher Tg than VBE-DDM is mainly because of its higher
cross-link density. The higher Tgs of the two Schiff base epoxy resins than DER331-DDM implies that the
highest end-use temperature was increased compared with DER331-DDM, which should extend the range of
applications.
Fig. 3 DSC curves of PBE-DDM and VBE-DDM.
Fig. 4 DMA curves of PBE-DDM and VBE-DDM.
Table 1 Glass transition temperature (Tg), average molecular weight between cross-link points (Mc), storage
modulus at Tg + 60 °C (E′) and cross-link density (ve) of PBE-DDM, VBE-DDM and DER331-DDM
Tg (°C)
Mc
E′
ve
Sample
DSC DMA (g mol-1) (MPa) (g mol-1)
PBE-DDM
VBE-DDM
206
181
204
196
848
908
15.59 1163.18
14.82 1123.01
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DER331-DDM51 166
174
944
10.75
852.11
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DOI: 10.1039/C9TA05293C
Beside the high Tgs, the mechanical properties of PBE-DDM and VBE-DDM were also pretty high. As
shown in Fig. 5 and Table 2, the tensile modulus and strength of these two epoxy resins were higher than
DER331-DDM (the commercial high-performance bisphenol A epoxy resin), which was due to their rigid two-
benzene-rings-conjugated Schiff base structure, Schiff base related hydrogen bonding20 and higher cross-link
density.50 PBE-DDM possessed more rigid structure and higher cross-link density than VBE-DDM,
corresponding to its higher modulus and strength. Owing to the highly cross-linked three-dimensional structure,
both Schiff base epoxy resins presented low elongation at break but still comparable to DER331-DDM. In
general, the lower cross-link density of cross-linked polymers corresponds to a larger space for motion, which
should result in the better stretchability. However, in the two thermosets PBE-DDM and VBE-DDM, the
segmental chain between the cross-link points is not the same. The methoxy group in VBE-DDM might reduce
the mobility of segmental chain, resulting in its lower toughness (elongation at break) than PBE-DDM.
Fig. 5 Representative tensile stress-strain curves of PBE-DDM and VBE-DDM.
Table 2 Tensile properties of PBE-DDM, VBE-DDM and DER331-DDM
Young’s modulus Tensile strength Elongation at
Sample
(MPa)
(MPa)
122±9
break (%)
8.7±0.3
PBE-DDM
VBE-DDM
2646±101
2196±89
93±5
76±6
6.3±0.2
7.4±0.5
DER331-DDM51
1893±140
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3.3 Degradation of the Cross-linked epoxy resins
Covalent cross-linked networks containing Schiff base tend to have certain degradation properties.58-60 Because
Schiff base can degrade into amino and aldehyde groups under acidic conditions,20,35 Schiff base thermosets
would degrade into amino and aldehyde groups containing oligomers, as shown in Fig. 6.
Fig. 6 Degradation mechanism of Schiff base epoxy thermosets.
The degradation process of Schiff base is affected by temperature,61 Therefore, the degradation property of
Schiff base-containing polymer will have a certain temperature dependence. This feature provided the
possibility of designing polymer with temperature-controlled degradability. As shown in Fig. 7, the ratio of
water to tetrahydrofuran (THF) exhibited a certain effect on the degradation of PBE-DDM and VBE-DDM.
Because water was one of the reactants for the degradation reaction, increasing water content could promote
the degradation reaction. However, if water content was too high, it would reduce the wettability toward the
samples and the solubility of degraded products, leading to an adverse effect on the degradation, which is
similar to the result in our previous work.10 According to the experimental results, the most favorable ratio of
water/THF for degradation of the two Schiff base epoxy resins is 2:8. As expected, the degradation of PBE-
DDM and VBE-DDM had very evidently temperature dependence. As an example, PBE-DDM exhibited huge
different degradation rate in 0.1 M H2SO4 solution (water/THF (v:v) = 2:8) at 50 °C and 23 °C. Degradation
could proceed smoothly at 50 °C with a fast rate, while almost no degradation occurred at 23 °C. As can be
seen from the digital photographs in Fig. 7, after being degraded at 50 °C for 12 h, PBE-DDM degraded and
completely dissolved in 0.1 M H2SO4 solutions; but after being degraded at 23 °C for 12 h, the sample remained
almost intact and the solution was colorless. This demonstrates that the two Schiff base epoxy resins possess
excellent chemical stability toward acidic solutions at room temperature, and controlled degradation can be
12

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Page 14 of 22
achieved by simply increasing the temperature under mild acidic conditions.
View Article Online
DOI: 10.1039/C9TA05293C
Fig. 7 The degradation rate of a) PBE-DDM and b) VBE-DDM in 0.1 M H2SO4 solution with different solvent
ratios at different temperature (around 20 mg sample in 10 mL solution) and the corresponding digital
photographs in 0.1 M H2SO4 solution (water/THF (v:v) = 2:8).
Beside temperature dependence, the degradation of the two Schiff base epoxy resins also presented acidity
dependence. Degradation process of Schiff base is an acid-catalyzed reversible process62, and the effect of
hydrogen ion concentration on the degradation process of Schiff base was extremely obvious (Fig. S5). It can
be observed from Fig. 8 that as the acidity (acid concentration and strength) increased, the degradation rate of
the samples was accelerated. Both PBE-DDM and VBE-DDM degraded rapidly in 0.1M H2SO4 solution, while
the degradation rate was greatly reduced by decreasing the acid concentration to 0.01M or by replacing H2SO4
with weaker acids (HCl, H3PO4 and CH3COOH). As shown in Table S1, the degradation rates of PBE-DDM
and VBE-DDM in 0.1M H2SO4 solution (water/THF (v:v) = 2:8) were 0.117 and 0.191 mg mL-1 h-1, respectively,
while decreased to 0.021 and 0.023 mg mL-1 h-1 in 0.01M H2SO4 solution (water/THF (v:v) = 2:8), respectively,
and reduced to 0.0097 and 0.023 mg mL-1 h-1 in 0.1 M CH3COOH solution (water/THF (v:v) = 2:8) , respectively.
This feature can ensure the excellent degradability of the samples in relatively mild acidic conditions and the
superior stability in weaker acidic conditions with low acid concentration or weak acids.
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Journal of Materials Chemistry A
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DOI: 10.1039/C9TA05293C
Fig. 8 The degradation rate of a) PBE-DDM and b) VBE-DDM in different acid solution with different acid
concentration at 50 °C (around 20 mg sample in 10 mL solution, water/THF (v/v) = 2:8).
The solvent system required for polymer degradation has a great relationship with its application prospects.
As shown in Fig. 9 and Table S1. The degradation rate of PBE-DDM and VBE-DDM in 0.1 M H2SO4 solution
with different main solvents at 50 °C varied widely. The two samples both exhibited high degradation rate in
THF and methanol systems. Since THF and methanol can be prepared from biomass sources, this method of
degradation in THF or methanol based solution is in line with the requirements of sustainable development.
For the common solvents (ethanol, acetone, and water) systems, the degradation rates of PBE-DDM and VBE-
DDM were quite slow, which infers that these Schiff base epoxy resins are stable in daily use and have bright
application prospects. The effect of different solvents on the degradation rate of Schiff base epoxy resins are
mainly related to the swelling rate of the solvents toward the samples (Table S2). As the swelling rate increased,
the solvent entered the epoxy network more easily, speeding up the degradation rate of the samples.
Fig. 9 The degradation rate of a) PBE-DDM and b) VBE-DDM in 0.1 M H2SO4 solution with different main
solvents at 50 °C (about 20 mg sample in 10 mL solution, water : main solvent (v/v) = 2:8).
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Page 16 of 22
3.4 Stability of the cross-linked epoxy resins
View Article Online
DOI: 10.1039/C9TA05293C
As mentioned above, the Schiff base epoxy resins are relatively stable toward acidic solutions at room
temperature, or with weaker acidity or with common solvents with low swelling rate toward the resins. In
addition, the chemical stability of the Schiff base epoxy resins was also investigated by immersing samples in
water/THF (v/v=2/8) or 0.1 M NaOH aqueous solution at 50 °C for 48 h. As shown in Table S3, the weight
loss of the two Schiff base epoxy resins after experiments was extremely low from 0.386~0.829 %, which
means that they were stable in neutral and basic solutions.
Moreover, the hygrothermal aging resistance of the Schiff base epoxy resins was investigated. High
humidity and high temperature environment may accelerate materials’ aging, thus hygrothermal aging
resistance is an important part of evaluating the stability of thermosets, especially the hydrolysable thermosets.
Thus, the two Schiff base epoxy resins were examined by Xenon test chamber at a humidity of 90% and a
temperature of 60 °C. As can be seen in Fig. 10, the Tgs of PBE-DDM and VBE-DDM are extremely stable
during the 12 days’ hygrothermal aging process, which is indicative of their excellent hygrothermal stability.
Fig. 10 DSC curves of a) PBE-DDM and b) VBE-DDM after a series of days at 90 % humidity, 60 °C.
Furthermore, the thermal degradation behavior of PBE-DDM and VBE-DDM was characterized by TGA to
examine their thermal stability. Fig. 11a exhibits non-isothermal degradation curves of both samples under
nitrogen atmosphere, and the data are summarized in Table 3. The thermal stability of polymers is often indicated
by the statistic heat-resistant index (Ts) which is calculated through an equation (3) with Td5% (degradation
temperature of 5% weight loss) and Td30% (degradation temperature of 30% weight loss).63, 64
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Journal of Materials Chemistry A
[
(
)
]
푇푠 = 0.49 푇푑5% + 0.6 푇푑30% ― 푇푑5%
(3)
View Article Online
As shown in Table 3, PBE-DDM exhibited higher Td5% and Ts than VBE-DDM, which meanDsOthI:a1t0.t1h0e39t/hCe9rTmA0a52l93C
stability of PBE-DDM is higher than that of VBE-DDM. This is probably due to the lower temperature
cleavage of methoxy groups and lower aromatic content and cross-link density of VBE-DDM than those of
PBE-DDM.50, 65 PBE-DDM and VBE-DDM both exhibited relatively lower thermal stability than DER331-
DDM, which suggests that the Schiff base structure thermally degraded at relatively lower temperature. While
PBE-DDM and VBE-DDM both kept stable before 330 °C, which is still sufficient for the most applications
of epoxy resins. Interestingly, both PBE-DDM and VBE-DDM presented excellent char formation ability and
had much higher char residue at 700 °C (R700) than DER331-DDM, which was mainly due to the abundant
Schiff base structures in the two resins.50, 66 Take PBE-DDM as an example, the char residue of PBE-DDM at
700 °C reached 45.6%, and the shape of the PBE was well maintained during the carbon formation process
(Fig. 11b and Fig. 11c). This exciting property made these Schiff base epoxy resins possible to be as the
precursors for specific-shape carbon materials.
Fig. 11 a) TGA curves of PBE-DDM and VBE-DDM; digital photos of PBE-DDM b) before and c) after
carbonization in argon at 800 °C for 2 h.
Table 3 The thermal stability of PBE-DDM, VBE-DDM and DER331-DDM
Sample
Td5% (°C) Td30% (°C) R700 (%) Ts (°C)
PBE-DDM
VBE-DDM
346
336
381
399
375
403
45.6
40.8
13.9
185.1
176.1
193.2
DER331-DDM51
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Page 18 of 22
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DOI: 10.1039/C9TA05293C
3.3 Coating Properties and Antibacterial Properties of the Cross-linked Epoxy Resins
Coatings, as a major application of epoxy resins, play a paramountly important role in protecting and
beautifying the substrate materials. Thus, the coatings based on the Schiff base epoxy resins were prepared
and the coating properties including pencil hardness, solvent resistance and adhesion of the cross-linked epoxy
resins were investigated, and the data are summarized in Table 4. The pencil hardness of thermosets-based
coatings increases with the increase of the rigid structure and cross-link density of the thermosets.14, 52 PBE-
DDM possesses a more rigid Schiff base structure than the DER331-DDM molecular structure, and higher
cross-link density compared to VBE-DDM, corresponding to the highest pencil hardness up to 8H. Although
they are quite rigid, both PBE-DDM and VBE-DDM have favorable adhesion. This might be due to the high
content of the polar groups (Schiff base structures and the hydroxyl groups produced during the curing process)
in their networks, which were in favor of forming hydrogen bonding with the substrate. In addition, all of the
samples exhibited excellent solvent resistance with MEK double rubs above 400.
Table 4 Coating properties of PBE-DDM, VBE-DDM and DER331-DDM
Pencil
Hardness
8H
Crosshatch Adhesion
MEK
double rubs
＞400
Sample
(5B = Best)
PBE-DDM
VBE-DDM
5B
3B
5B
6H
＞400
DER331-DDM
6H
＞400
Gram-negative bacteria (E. coli) is a kind of bacteria that is extremely common in our life, so it was used
to test the bactericidal effect of the surfaces of the coatings. For contact antibacterial experiments, the surfaces
of the coatings were cocultured with bacteria solution for a prespecified time. The bactericidal capability were
determined by live/dead cell stain and observation through fluorescent microscope. Fig. 12 presents the typical
fluorescent images of attached bacteria on VBE-DDM, PBE-DDM and DER331-DDM. DER331-DDM based
coatings exhibited exceedingly low bactericidal capability with killing rate of 3.82%. The only difference
between the chemical structures of DER331 and PBE is the Schiff base structure (C=N) in PBE, so the
excellent bactericidal capability (78%) of PBE-DDM was presumably due to the excellent antibacterial
properties of Schiff base. Compared with PBE-DDM, VBE-DDM presented higher bactericidal capability with
killing rate of 91%, which suggests that the methoxy group in VBE might also have a certain contribution to
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Journal of Materials Chemistry A
the bactericidal capability. These results indicate that these Schiff base epoxy resins have great potential to be
View Article Online
DOI: 10.1039/C9TA05293C
used as high-performance antibacterial coatings.
Fig. 12 a-c) Fluorescence microscopy images of E. coli on coatings based on a) VBE-DDM, b) PBE-DDM,
and c) DER331-DDM; d) Bacteria concentration and killing rate of PBE-DDM, VBE-DDM, and DER331-
DDM toward E. coli.
4. Conclusions
Two high-performance Schiff base epoxy thermosets combining excellent controlled degradability and
antibacterial properties were successfully prepared. The high cross-link density of the thermosets as well as
the high rigidity from the two-benzene-rings-conjugated Schiff base contributed the high performance with
the Tg of ~206 °C, tensile modulus of ~2646 MPa and tensile strength of ~122 MPa. The stability of the Schiff
base was greatly improved by the conjugation with two benzene rings, which made the thermosets acidity and
temperature-controlled degradable under mild acidic conditions. Owing to the conjugated Schiff base structure,
both epoxy thermosets presented high char residue at 700 °C up to 45.6% and the shape of the samples could
be maintained during the carbon formation process, which made these thermosets possible for preparation of
specific-shape carbon materials. On account of the Schiff base structure, the two epoxy resins both exhibited
excellent antibacterial properties which could further improved by the methoxy group on the benzene ring.
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Meanwhile, the epoxy thermosets exhibited outstanding coating properties with pencil hardness of ~8H,
View Article Online
adhesion of ~5B (best), MEK double rubs of >400, which meant that these thermosets have gDreOaI:t10p.o10t3e9n/tCi9aTlAf0o5r293C
antibacterial coating application. This work provides a promising method to achieve high-performance
command-degradable thermosets with excellent antibacterial properties. In the future, more thermosets
containing aromatic rings-conjugated Schiff base would be synthesized and the structure–property
relationships as well as their applications would be systematically investigated.
Acknowledgements
The authors thank the financial support from the National Natural Science Foundation of China (No. 51773216
and 51603221) and Youth Innovation Promotion Association, CAS (No. 2018335).
Notes and references
1 S. Ma and D. C. Webster, Prog. Polym. Sci., 2018, 76, 65-110.
2 P. R. Christensen, A. M. Scheuermann, K. E. Loeffler and B. A. Helms, Nat. Chem., 2019, 11, 442-448.
3 D. Montarnal, M. Capelot, F. Tournilhac and L. Leibler, Science, 2011, 334, 965-968.
4 C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and M. Poliakoff, Science, 2012, 337, 695.
5 Z. Zou, C. Zhu, Y. Li, X. Lei, W. Zhang and J. Xiao, Sci. Adv., 2018, 4, eaaq0508.
6 H. Sardon and A. P. Dove, Science, 2018, 360, 380.
7 T. E. Long, Science, 2014, 344, 706-707.
8 J. M. Garcia, G. O. Jones, K. Virwani, B. D. McCloskey, D. J. Boday, G. M. ter Huurne, H. W. Horn, D. J. Coady, A. M.
Bintaleb, A. M. S. Alabdulrahman, F. Alsewailem, H. A. A. Almegren and J. L. Hedrick, Science, 2014, 344, 732-735.
9 P. Chakma and D. Konkolewicz, Angew. Chem. Int. Ed., 2019, DOI: https://doi.org/10.1002/anie.201813525.
10 S. Ma, J. Wei, Z. Jia, T. Yu, W. Yuan, Q. Li, S. Wang, S. You, R. Liu and J. Zhu, J. Mater. Chem. A, 2019, 7, 1233-1243.
11 A.-C. Albertsson and M. Hakkarainen, Science, 2017, 358, 872-873.
12 M. Hayashi, R. Yano and A. Takasu, Polym. Chem., 2019, 10, 2047-2056.
13 Y. Liu, Z. Tang, Y. Chen, C. Zhang and B. Guo, ACS Appl. Mater. Inter., 2018, 10, 2992-3001.
14 S. Ma, D. C. Webster and F. Jabeen, Macromolecules, 2016, 49, 3780-3788.
15 A. Ruiz de Luzuriaga, Mater. Horiz., 2016, 3, 241.
16 A. Ruiz de Luzuriaga, J. M. Matxain, F. Ruiperez, R. Martin, J. M. Asua, G. Cabanero and I. Odriozola, J. Mater. Chem.
C, 2016, 4, 6220-6223.
17 T. Hashimoto, H. Meiji, M. Urushisaki, T. Sakaguchi, K. Kawabe, C. Tsuchida and K. Kondo, J. Polym. Sci., Part A: Polym.
Chem., 2012, 50, 3674-3681.
18 A. Yamaguchi, T. Hashimoto, Y. Kakichi, M. Urushisaki, T. Sakaguchi, K. Kawabe, K. Kondo and H. Iyo, J. Polym. Sci.,
Part A: Polym. Chem., 2015, 53, 1052-1059.
19 M. Harada, J. Ando, M. Yamaki and M. Ochi, J. Appl. Polym. Sci., 2014, 132, 41291-41297.
20 S. Wang, S. Ma, Q. Li, X. Xu, B. Wang, W. Yuan, S. Zhou, S. You and J. Zhu, Green Chem., 2019, 21, 1484-1497.
21 Y.-D. Shin, A. Kawaue, H. Okamura and M. Shirai, React. Funct.Polym., 2004, 61, 293-302.
22 L. Zhao, Y. Liu, Z. Wang, J. Li, W. Liu and Z. Chen, Polym. Degrad. . Stab. 2013, 98, 2125-2130.
23 L. Wang and C. P. Wong, J. Polym. Sci., Part A: Polym. Chem., 2000, 37, 2991-3001.
19

Page 21 of 22
Journal of Materials Chemistry A
24 W. Liu, Z. Wang, L. Xiong and L. Zhao, Polymer, 2010, 51, 4776-4783.
25 Z. Chen, L. Zhao and Z. Wang, Polymer, 2013, 54, 5182-5187.
View Article Online
DOI: 10.1039/C9TA05293C
26 L. Wang, H. Li and C. P. Wong, J. Polym. Sci., Part A: Polym. Chem., 2000, 38, 3771-3782.
27 G. Huh, K.-O. Kwon, S.-H. Cha, S.-W. Yoon, M. Y. Lee and J.-C. Lee, J. Appl. Polym. Sci., 2009, 114, 2093-2100.
28 K. Luo, T. Xie and J. Rzayev, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 4992-4997.
29 J. R. GRIFFITH, ACS Symp. Ser., 1979, 114, 259-262.
30 L. González, X. Ramis, J. M. Salla, A. Mantecón and A. Serra, Polym. Degrad. Stab., 2007, 92, 596-604.
31 L. González, F. Ferrando, X. Ramis, J. M. Salla, A. Mantecón and A. Serra, Prog. Org. Coat., 2009, 65, 175-181.
32 Y. Min, S. Huang, Y. Wang, Z. Zhang, B. Du, X. Zhang and Z. Fan, Macromolecules, 2015, 48, 316-322.
33 W. Yuan, S. Ma, S. Wang, Q. Li, B. Wang, X. Xu, K. Huang, J. Chen, S. You and J. Zhu, Eur. Polym. J., 2019, 117, 200-
207.
34 Q. Yu, X. Peng, Y. Wang, H. Geng, A. Xu, X. Zhang, W. Xu and D. Ye, Eur. Polym. J., 2019, 117, 55-63.
35 S. Zhao and M. M. Abu-Omar, Macromolecules, 2018, 51, 9816-9824.
36 S. Dhers, G. Vantomme and L. Avérous, Green Chem., 2019.
37 K. E. Boehle, J. Gilliand, C. R. Wheeldon, A. Holder, J. A. Adkins, B. J. Geiss, E. P. Ryan and C. S. Henry, Angew. Chem.
Int. Ed., 2017, 56, 6886-6890.
38 J. Song and J. Jang, Adv. Colloid Interfac., 2014, 203, 37-50.
39 M. Moritz and M. Geszke-Moritz, Chem. Eng. J., 2013, 228, 596-613.
40 Y. Lu, Y. Mei, M. Drechsler and M. Ballauff, Angew. Chem. Int. Ed., 2006, 45, 813-816.
41 B. Wu, L. Zhang, L. Huang, S. Xiao, Y. Yang, M. Zhong and J. Yang, Langmuir, 2017, 33, 7160-7168.
42 J. Wu, D. Zhang, L. Zhang, B. Wu, S. Xiao, F. Chen, P. Fan, M. Zhong, J. Tan and Y. Chu, Prog. Org. Coat., 2019, 134,
153-161.
43 A. Naz, S. Arun, S. S. Narvi, M. S. Alam, A. Singh, P. Bhartiya and P. K. Dutta, Int. J. Biol. Macromol., 2018, 110, 215-
226.
44 K. Mahmood, W. Hashmi, H. Ismail, B. Mirza, B. Twamley, Z. Akhter, I. Rozas and R. J. Baker, Polyhedron, 2019, 157,
326-334.
45 J. Dai, S. Ma, L. Zhu, S. Wang, L. Yang, Z. Song, X. Liu and J. Zhu, Polymer, 2017, 108, 215-222.
46 W. Yang, E. Fortunati, F. Dominici, G. Giovanale, A. Mazzaglia, G. M. Balestra, J. M. Kenny and D. Puglia, Eur. Polym.
J., 2016, 79, 1-12.
47 D. Zmejkoski, D. Spasojević, I. Orlovska, N. Kozyrovska, M. Soković, J. Glamočlija, S. Dmitrović, B. Matović, N. Tasić
and V. Maksimović, Int. J. Biol. Macromol., 2018, 118, 494-503.
48 D. Xiao, W. Ding, J. Zhang, Y. Ge, Z. Wu and Z. Li, Chem. Eng. J., 2019, 358, 310-320.
49 C. Gioia, G. Lo Re, M. Lawoko and L. Berglund, J. Am. Chem.Soc., 2018, 140, 4054-4061.
50 S. Wang, S. Ma, Q. Li, W. Yuan, B. Wang and J. Zhu, Macromolecules, 2018, 51, 8001-8012.
51 S. Wang, S. Ma, C. Xu, Y. Liu, J. Dai, Z. Wang, X. Liu, J. Chen, X. Shen, J. Wei and J. Zhu, Macromolecules, 2017, 50,
1892-1901.
52 S. Ma and D. C. Webster, Macromolecules, 2015, 48, 7127-7137.
53 T. Xie and I. A. Rousseau, Polymer, 2009, 50, 1852-1856.
54 S. You, S. Ma, J. Dai, Z. Jia, X. Liu and J. Zhu, ACS Sustain. Chem. Eng., 2017, 5, 4683-4689.
55 J. Scanlan, J. Polym. Sci., 1960, 43, 501-508.
56 L. W. Hill, Prog. Org. Coat., 1997, 31, 235-243.
57 P. Li, S. Ma, J. Dai, X. Liu, Y. Jiang, S. Wang, J. Wei, J. Chen and J. Zhu, ACS Sustain. Chem. Eng., 2017, 5, 1228-1236.
58 P. Taynton, H. Ni, C. Zhu, K. Yu, S. Loob, Y. Jin, H. J. Qi and W. Zhang, Adv. Mater., 2016, 28, 2904-2909.
59 P. Taynton, K. Yu, R. K. Shoemaker, Y. Jin, H. J. Qi and W. Zhang, Adv. Mater., 2014, 26, 3938-3942.
60 H. Li, J. Bai, Z. Shi and J. Yin, Polymer, 2016, 85, 106-113.
20

Journal of Materials Chemistry A
Page 22 of 22
61 R. Mo, J. Hu, H. Huang, X. Sheng and X. Zhang, J. Mater. Chem. A, 2019, 7, 3031-3038.
62 E. H. Cordes and W. P. Jencks, J. Am. Chem.Soc., 1962, 84, 832-837.
View Article Online
DOI: 10.1039/C9TA05293C
63 H. H. Horowitz and G. Metzger, Anal. Chem., 1963, 35, 1464-1468.
64 S. Ma, X. Liu, L. Fan, Y. Jiang, L. Cao, Z. Tang and J. Zhu, Chemsuschem, 2014, 7, 555-562.
65 J. R. Mauck, S. K. Yadav, J. M. Sadler, J. J. La Scala, G. R. Palmese, K. M. Schmalbach and J. F. Stanzione, Macromol.
Chem. Phys,, 2017, 218.
66 H. Liu, K. Xu, H. Cai, J. Su, X. Liu, Z. Fu and M. Chen, Polym. Adv. Technol., 2012, 23, 114-121.
21