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\DOI{10.5802/crchim.374}
\datereceived{2024-11-24}
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\dateposted{2025-02-27}
\begin{document}

%\copyeditor{Reeja}

\begin{noXML}

\CDRsetmeta{articletype}{clarification}

\title{Highlights of epoxidized natural rubber: from self-vulcanizing
blends to reprocessable chemical or physical networks}

\alttitle{Des m\'{e}langes auto-vulcanisants aux r\'{e}seaux
r\'{e}versibles chimiques ou physiques : Avanc\'{e}es dans le
caoutchouc naturel \'{e}poxyd\'{e}}

\author{\firstname{Lucie} \lastname{Imbernon}\CDRorcid{0000-0003-3025-8603}}
\address{Molecular, Macromolecular Chemistry, and Materials, 
C3M, ESPCI Paris-PSL, CNRS UMR7167, PSL University, 
75005 Paris, 
France}
\email[L. Imbernon]{lucie.imbernon@carpenter.com}

\author{\firstname{Myriam} \lastname{Pire}}
\addressSameAs{1}{Molecular, Macromolecular Chemistry, and Materials, 
C3M, ESPCI Paris-PSL, CNRS UMR7167, PSL University, 
75005 Paris, 
France}
\email[M. Pire]{myriam.pire@saint-gobain.com}

\author{\firstname{Evdokia K.} \lastname{Oikonomou}\CDRorcid{0000-0003-2690-0982}}
\addressSameAs{1}{Molecular, Macromolecular Chemistry, and Materials, 
C3M, ESPCI Paris-PSL, CNRS UMR7167, PSL University, 
75005 Paris, 
France}
\email[E. K. Oikonomou]{oikonomou.evdokia@gmail.com}

\author{\firstname{Szilvia} \lastname{Karpati}\CDRorcid{0000-0001-5297-0111}}
\addressSameAs{1}{Molecular, Macromolecular Chemistry, and Materials, 
C3M, ESPCI Paris-PSL, CNRS UMR7167, PSL University, 
75005 Paris, 
France}
\email[S. Karpati]{szilvia.karpati@ens-lyon.fr}

\author{\firstname{Sylvie}\nobreakauthor\lastname{Tenc\'{e}-Girault}\CDRorcid{0000-0001-5090-2334}}
\addressSameAs{1}{Molecular, Macromolecular Chemistry, and Materials, 
C3M, ESPCI Paris-PSL, CNRS UMR7167, PSL University, 
75005 Paris, 
France}
\email[S. Tenc\'{e}-Girault]{sylvie.girault@ensam.eu}

\author{\firstname{Sophie} \lastname{Norvez}\CDRorcid{0000-0002-2478-0393}\IsCorresp}
\addressSameAs{1}{Molecular, Macromolecular Chemistry, and Materials, 
C3M, ESPCI Paris-PSL, CNRS UMR7167, PSL University, 
75005 Paris, 
France}
\email[S. Norvez]{sophie.norvez@espci.psl.eu}

\shortrunauthors

\keywords{\kwd{Epoxidized natural rubber (ENR)}
\kwd{Crosslinking by carboxylic diacids}
\kwd{Crystallizable grafts}
\kwd{Reprocessability}
\kwd{Sustainability}}

\altkeywords{\kwd{Caoutchouc naturel \'{e}poxyd\'{e}}
\kwd{R\'{e}ticulation par les diacides carboxyliques}
\kwd{Greffons cristallisables}
\kwd{Recyclage}
\kwd{Durabilit\'{e}}}

\begin{abstract}
This manuscript provides a critical review of major advancements 
we brought about
in the field of epoxidized natural rubber (ENR), highlighting
the development of ENR from self-vulcanizing blends with dicarboxylic
acids to innovative networks that are physically or chemically
reprocessable. The work includes accelerated curing using imidazole
derivatives, the introduction of thermoreversible disulfide bonds for
chemical reprocessability, and the incorporation of crystallizable
grafts for thermoplastic behavior. These advancements underscore
ENR's adaptability and environmental benefits, positioning it as a
versatile elastomer suitable for sustainable and high-performance
applications.
\end{abstract}

\begin{altabstract}
Ce manuscrit propose une revue critique des avanc\'{e}es majeures que
nous avons apport\'{e}es dans le domaine du caoutchouc naturel
\'{e}poxyd\'{e} (ENR), mettant en lumi\`{e}re le d\'{e}veloppement de
l'ENR, depuis des m\'{e}langes auto-vulcanisants avec des acides
dicarboxyliques jusqu'\`{a} des r\'{e}seaux innovants recyclables,
r\'{e}ticul\'{e}s chimiquement ou physiquement. Le travail inclut
l'acc\'{e}l\'{e}ration de la r\'{e}action d'est\'{e}rification par des
d\'{e}riv\'{e}s d'imidazole, l'introduction de liaisons disulfures
thermo-r\'{e}versibles, et l'incorporation de greffons cristallisables
pour conf\'{e}rer un comportement thermoplastique. Ces avanc\'{e}es
soulignent l'adaptabilit\'{e} et les atouts environnementaux de l'ENR,
le positionnant comme un \'{e}lastom\`{e}re polyvalent et
\'{e}co-responsable adapt\'{e} \`{a} des applications \`{a} hautes
performances.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\let\rmmu\upmu

\section{Epoxidized natural rubber as a foundation for
sustainable material}\label{sec1}
As industries increasingly seek sustainable alternatives to traditional
rubber~\cite{1,2,3}, epoxidized natural rubber (ENR) has gained attention 
for its unique dual
functionality, which enhances its potential as an eco-friendly
material. Derived from natural rubber through epoxidation, ENR combines
the elasticity and resilience of natural rubber with the added chemical
reactivity of  epoxides~\cite{4}.

The introduction of epoxides into the rubber backbone imparts polar
functionalities that broaden ENR compatibility with other polymers and
improve its resistance to oils, gases, and environmental 
degradation~\cite{5,6,7}. These polar groups also enable ENR to
interact with crosslinking agents in new ways, facilitating innovative
crosslinking strategies that can bypass traditional sulfur
vulcanization. This versatility opens the door to creating materials
that maintain or surpass the mechanical durability of standard rubber
while achieving characteristics essential for modern sustainable
design, such as recyclability and extended material lifespan.

Our research from 2009 to 2017 has explored diverse strategies to
advance ENR from basic blends to sophisticated multifunctional systems,
addressing the complex trade-offs among flexibility, toughness,
reprocessability, and environmental sustainability. These efforts have
achieved significant progress in developing ENR-based materials that
leverage ENR's unique dual functionality. These \mbox{advancements} include
self-vulcanizing ENR blends and novel crosslinking methods that utilize
chemical or physical links. Such approaches allow for the formation of
robust yet reprocessable networks, setting the foundation for
next-generation elastomers that balance mechanical strength with
environmental benefits. This manuscript traces the evolution of ENR
materials from self-crosslinking blends to crystalline grafts,
highlighting how the dual functionality of ENR can transform it into a
model for sustainable rubber development.

\section{Self-vulcanizing ENR blends with dicarboxylic
acids}\label{sec2}
The research primarily focused on blending ENR with dicarboxylic acids
like dodecanedioic acid (DA) for ``self-vulcanizable'' blends, which
cured at high temperatures (around 180~\textdegree C) without any of the
additives required for sulfur-based  crosslinking~\cite{8}. This new
ENR/DA system was designed to address the fatigue--aging trade-off in
rubbers.

Indeed, natural rubber (NR) demonstrates varying resistance to
repetitive stress (fatigue) and long-term durability (aging) depending
on the crosslinking method used, though these two properties are often
incompatible. Two primary crosslinking methods exist for natural
rubber: peroxide crosslinking and sulfur crosslinking, the latter
known as vulcanization. Vulcanization involves incorporating various
ingredients;  sulfur-only crosslinking can take several
hours, which makes it inefficient for industrial use. Instead, the reaction is
optimized to completion in a few minutes at a maximum temperature of
160~\textdegree C. To achieve this, accelerators are added, which are
classified based on their effectiveness: slow (30--60~min, such as
guanidines), moderate (15--30~min, including thiazoles and
sulfenamides), and fast (10~min, like thiurams and carbamates). To
maximize accelerator performance, activators are added, typically zinc
oxide and a fatty acid like stearic acid. Other additives, such as
antioxidants and reinforcing fillers, are also included~\cite{4}.

Adjusting the accelerator-to-sulfur ratio results in distinct
crosslinking networks, producing three vulcanization types 
(Table~\ref{tab1}). Conventional vulcanization (CV) uses a high sulfur
content, creating a network primarily composed of polysulfide bridges.
On the other hand, the efficient vulcanization (EV) \mbox{system} uses a low
sulfur content, converting polysulfide groups into di- and monosulfide
bridges. Semi-efficient vulcanization (SEV) results from intermediate
ratios, incorporating both poly- and monosulfides~\cite{4}. The chosen
vulcanization method significantly influences the rubber's final
properties, tailoring it to specific applications. For high fatigue
resistance---required for parts subjected to frequent mechanical
stress---the conventional formulation is ideal, though it has limited
aging resistance. Conversely, when thermal and oxidative resistance is
prioritized, efficient vulcanization is preferred, sacrificing some
fatigue durability  (Table~\ref{tab1}).

\begin{table*}
\caption{\label{tab1}Three types of vulcanization (CV, SEV, and EV)
determine the performance characteristics of rubbers (adapted from
reference~\cite{4}).}
\begin{tabular}{cccc}
\thead
Vulcanization type & Conventional (CV) & Semi-efficient (SEV) & Efficient (EV) \\
\endthead
Accelerator/sulfur ratio & 0.1--0.6 & 0.7--2.5 & 2.5--12 \\
Polysulfur bridges \% & 95 & 50 & 20 \\
Monosulfur bridges \% & 5 & 50 & 80 \\
Aging resistance  & Poor & Intermediate & High \\
Fatigue resistance & High & Intermediate & Poor 
\botline
\end{tabular}
\vspace*{12pt}
\end{table*}

Peroxide crosslinking, in contrast, requires no accelerators or
activators. The resulting network, formed of carbon--carbon bonds,
offers greater aging resistance but lower fatigue durability.

The fatigue--aging balance is influenced by the dissociation energy of
the bonds  involved~\cite{9}. 
\mbox{Table~\ref{tab2}} provides estimates of
these bond energies. Due to the relatively low energy of the S--S bond,
disulfide bridges break before the C--C bonds in the polymer backbone
but can subsequently re-form at new attachment points, enabling the
material to withstand considerable stress. As a result, networks with
polysulfide bridges (CV) exhibit higher resistance to repeated
mechanical stress compared to networks with monosulfide (EV) or
carbon--carbon (peroxide) bonds. In contrast, C--C--C bonds offer
greater resistance to thermal and oxidative stress, enhancing the
material's aging durability.

\begin{table*}
\caption{\label{tab2}Approximate bond energies in rubbers (data 
from~\cite{9}).}
\begin{tabular}{ccc} 
\tbody
C--C--C & $93~\mathrm{kcal}{\cdot}\mathrm{mol}^{-1}$ &
Peroxide curing: aging ${>}$ fatigue \\\xxbotline
C--S--C & 50--$60~\mathrm{kcal}{\cdot}\mathrm{mol}^{-1}$ &  
\morerows{2}{Vulcanization: fatigue ${>}$ aging} \\
C--S--S--C & $35~\mathrm{kcal}{\cdot}\mathrm{mol}^{-1}$  &  \\ 
C--(S)$_{x}$--C & $27~\mathrm{kcal}{\cdot}\mathrm{mol}^{-1}$ & 
\botline
\end{tabular}
\vspace*{4pt}
\end{table*}

With the DA-crosslinked ENR system, we aimed to shift the
fatigue--aging compromise by creating a network different from those
formed by sulfur or peroxide crosslinking. The crosslinking bridges
had to be both flexible and thermally stable, which justified the
choice of bridges formed from long aliphatic chains. Dicarboxylic acids
were selected as crosslinking agents to exploit the epoxides in ENR,
enabling stable network formation.

The crosslinking reaction with dicarboxylic acids exploits the
epoxy functionality of ENR to create robust bridges as depicted in 
Figure~\ref{fig1}A. This process results in a chemically crosslinked
ENR network, referred to as DA-ENR in the following. To explore the
effect of epoxidation on crosslinking, three epoxidation ratios ($m$)
were investigated: 0.1, 0.25, and 0.50, corresponding to ENR10, ENR25,
and ENR50, \mbox{respectively.} By modifying crosslinking densities and ratios
of ENR to DA, the study identified an optimal ENR/DA ratio of $p = 25$
for ENR25  (Figure~\ref{fig1}B). The ratio epoxide/diacid denoted by
$p$ is the number of epoxides for 1 diacid molecule. For ENR25, $p =
25$ corresponds to 25 epoxide units for 1 diacid, that is, 1 diacid for
100 total monomer units of the polymer backbone. This optimal ratio
produced materials with good resilience (60\%) and promising
stress--strain properties  (Figure~\ref{fig1}B).

\begin{figure*}
\vspace*{1pt}
\includegraphics{fig01}
\vspace*{3pt}
\caption{\label{fig1}(A)~Crosslinking of ENR by dodecanedioic acid (DA)
in the presence (or not) of 1,2-dimethylimidazole (DMI). The
epoxidation ratio $m$ is equal to 0.1 (ENR10), 0.25 (ENR25), or 0.50
(ENR50). The imidazole in parenthesis was not involved in the initial
experiments. (B)~Strain--stress measurements after curing ENR25 with DA
for 3~h at 180~\textdegree C in a press, at various epoxy/acid $p$ ratios
(after  reference~\cite{12}). (C)~Acceleration of ENR25/DA crosslinking
with stoichiometric amount of DMI (red curve). (D)~Effect of the DMI/DA
ratio---from 0 to 400\%---on the stress--strain properties of DA-ENR25
$(p = 25)$ after curing at 180~\textdegree C. For each ratio, the
indicated curing time was adjusted at the G$^\prime$ plateau (after
reference~\cite{10}).}
\vspace*{5pt}
\end{figure*}

The entire DA-ENR synthesis was conducted in bulk, using an internal
mixer for blending and a hot press for crosslinking, the whole 
process being performed 
without solvents. This method minimizes environmental impact by
eliminating additional reagents and waste, offering a green alternative
to traditional sulfur-based vulcanization.

\section{Accelerating crosslinking with imidazole
compounds}\label{sec3}
Nevertheless, the initial synthesis required long curing times and high
temperatures, deleterious to  material properties. A breakthrough
came with the introduction of imidazole compounds, particularly 
1,2-dimethylimidazole (DMI), as accelerators in the crosslinking 
process~\cite{10}. To ensure homogeneous dispersion of DMI, which is in
 liquid state at the operating temperature, kaolin---a chemically
inert filler---was incorporated during the compounding step in the
internal mixer. DMI enabled the curing of ENR/DA blends at
significantly lower temperatures and reduced curing times 
(Figure~\ref{fig1}C).

Mechanical testing revealed that DMI-accelerated ENR/DA blends achieved
optimal tensile properties at the stoichiometric DMI/CO$_{2}$H
ratio---one DMI molecule per CO$_{2}$H group, equivalent to 200\% DMI
per diacid  (Figure~\ref{fig1}D). For each ratio, the indicated curing
time was determined at the point where the storage modulus (G')
stabilized, indicating the completion of the crosslinking process. 
Since one DMI molecule per CO$_{2}$H group was required to activate the
crosslinking, it was hypothesized that imidazole acted as an
accelerator rather than a catalyst. The ionization of carboxylic
acid seemed to be crucial to accelerating the esterification reaction.
However, a simple acid--base reaction leading to the formation of
carboxylate groups did not explain the acceleration of the ring opening
of the epoxide. Parallel experiments using other bases like KOH or a
tertiary amine did not show any increase in the crosslinking rate. It
was hypothesized that the formation of reactive imidazolium
carboxylates would facilitate the esterification reaction between the
epoxides of ENR and the dicarboxylic acid. This hypothesis was
validated through subsequent $\mbox{}^{13}$C solid-state NMR analysis,
which revealed the ester nature of the crosslinks and confirmed the
formation of an \mbox{intermediate} \mbox{imidazolium} dicarboxylate species. These
species were found to be well dispersed within the elastomeric matrix,
contributing to the uniformity and stability of the crosslinked 
network~\cite{11}.

The shortened crosslinking time obtained with DMI not only improved
material performance but also significantly reduced the energy and time
required for curing, providing a greener alternative to traditional
sulfur vulcanization. This innovation significantly enhanced ENR's
appeal for industrial applications by opening up new possibilities for
energy-efficient production of elastomers.

An industrial version of the ENR/DA/DMI system was developed,
incorporating antioxidants and reinforcing fillers. Its fatigue and
aging properties were evaluated at the Hutchinson Research
Centre (Montargis, France) and compared to those of the ENR crosslinked with sulfur or
peroxide. This study resulted in a material with properties 
intermediate between sulfur- and peroxide-crosslinked systems 
\mbox{(Figure~\ref{fig2})}~\cite{13,14}.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Comparison of properties achieved through ENR25
crosslinking using dicarboxylic acid (DA), peroxide, or sulfur under
conventional (CV), semi-efficient (SEV), or efficient (EV) vulcanization
conditions. Inset shows ENR25 test specimens undergoing fatigue testing
at the Hutchinson Research Centre.}
\vspace*{2pt}
\end{figure}

In summary, the successful crosslinking of ENR using carboxylic
diacids, accelerated by stoichiometric proportions of DMI, has opened
new avenues for the development of advanced systems. This approach
enables ENR to be efficiently crosslinked with various functionalized
carboxylic diacids or modified with chains bearing reactive or
functional groups, such as reversible, supramolecular, or
crystallizable grafts. In the following  sections, we delve into
\mbox{further}
functionalizations: first, a diacid incorporating an
\mbox{exchangeable} S--S bond, and second, a monoacid bearing a crystallizable
tail. Since this research, the ENR/DA derivative/DMI system has
attracted significant attention in the literature, particularly for its
potential in producing eco-friendly, self-healing ENR 
materials~\cite{15,16,17,18,19,20}.

\section{Thermoreversible chemical ENR networks using exchangeable
bonds}\label{sec4}
With the establishment of DA-crosslinked ENR as a versatile, functionalized elastomer,
the next major advancement was to address the recycling challenges
associated with rubber. Traditional crosslinked rubber materials are
not reprocessable due to their permanent covalent  bonds~\cite{2}. We
introduced reversible crosslinking by using disulfide bonds within the
ENR network, which can undergo bond rearrangements when heated,
allowing partial reprocessing and extension of the rubber usable 
life~\cite{21}.

Disulfide groups were introduced into the rubber using 
dithiodibutyric acid (DTDB) as a crosslinker  (Figure~\ref{fig3}A).
DTDB-cured ENR presented strong elastomeric properties due to the high
molecular weight of the rubber and efficient DA crosslinking. The
material behaves like a standard natural rubber until 100~\textdegree C
and becomes reprocessable above 150~\textdegree C owing to disulfide
rearrangements. Mechanical tests confirmed that these
disulfide-incorporated ENR materials exhibited self-healing
capabilities, retaining their mechanical integrity even after multiple
cycles of stress relaxation and reprocessing. Rheological analysis
further highlighted that these materials could be recycled at
180~\textdegree C. After grinding to a fine powder, the reprocessed
crosslinked rubber recovers most of its initial mechanical 
properties\unskip\break
(Figure~\ref{fig3}B).

\begin{figure*}
\includegraphics{fig03}
%\vspace*{3pt}
\caption{\label{fig3}(A)~Crosslinking of ENR25 with DTDB/DMI and
schematic representation of the disulfide-containing network. 
(B)~Recycling experiment. Tensile tests before (line) and after
(dashed) the recycling experiment (red: DTDB-cured; blue: DA-cured).
The image illustrates dumbbell-shaped cured rubber sample finely
grounded and then reshaped by compression molding (after
reference~\cite{21}).}
%\vspace*{5pt}
\end{figure*}

\subsection*{Perspective: comparison with sulfur vulcanization}
Since ENR can be crosslinked and recycled through disulfide bond
rearrangements, it is important to compare this method with traditional
sulfur vulcanization in terms of mechanical performance, aging
behavior, and recyclability to evaluate its environmental impact.

Sulfur vulcanization is an established industrial process, yet its
chemistry remains incompletely understood. Polysulfide rearrangements
are known to enhance rubber fatigue resistance, but this mechanism has
seen limited use in rubber recycling. Recycling via hot compression of
pulverized vulcanized rubber, without adding new rubber or additives,
has been sparsely documented, typically focusing on industrial
applications rather than on sulfur network  behavior~\cite{22,23,24}.
Historically, sulfur-vulcanized rubber stress relaxation at high
temperatures was examined by Tobolsky et~al.~\cite{25} and later by
Berry and Watson~\cite{26}, who debated the mechanisms behind this 
phenomenon~\cite{27,28}. They attributed stress relaxation to network
degradation, directly correlated with the number of elastically active
chains. Later on, Tamura and Murakami highlighted the role of oxygen in
this relaxation, noting the impact of polysulfide bonds, which can
undergo exchange causing stress  relaxation~\cite{29}.

To compare our DTDB crosslinking process with sulfur vulcanization, we
crosslinked ENR under CV conditions, which
favor polysulfide bridges due to an excess of sulfur relative to the
accelerator (sulfur/accelerator ratio of 7), enhancing fatigue
resistance. The ingredients were mixed at 40~\textdegree C for  20~min and
then cured at 150~\textdegree C under 8 tons of pressure for 30~min as
determined kinetically in the rheometer. All details of this study are
reported in  reference~\cite{30}. Key findings include the following:
\begin{enumerate}[(i)]
\item 
Crosslink density and stress relaxation: Both ENR/S and ENR/DTDB
exhibited similar crosslink densities and stress relaxation patterns,
reaching a plateau indicative of secondary reactions in both systems.
\item
Creep-recovery performance: Creep-recovery tests revealed a slight
advantage for DTDB at 150~\textdegree C, where ENR/DTDB showed linear
deformation unlike ENR/S, which displayed curve flattening due to side
reactions occurring at lower temperatures in polysulfide bonds.
\item
Recyclability: ENR25/S and ENR25/DTDB showed comparable recyclability,
retaining about 80\% in deformation and 50\% in tensile strength
post-recycling. However, ENR/S exhibited modulus loss, possibly due to
chain scission, not observed in ENR/DTDB.
\item
Aging resistance: Aging tests indicated that ENR/DTDB degraded less
than ENR/S at 150~\textdegree C, attributed to the lower reactivity of
disulfides against oxidation, unlike the polysulfides in sulfur
vulcanization, which produced degradation-promoting acids.
\end{enumerate}

In conclusion, as regards the comparative study of ENR/DTDB and ENR/S, both
materials performed similarly across tests with dynamic bonds allowing
partial recycling of samples. However, notable advantages favor
disulfide bonds. ENR/DTDB withstands temperatures up to 100~\textdegree
C with minimal residual deformation. The lower reactivity of disulfides
compared to polysulfides does not hinder thermal rearrangements but
provides better protection against thermo-oxidation. Most importantly,
DTDB eliminates the need for toxic additives like 
N-cyclohexyl-2-benzothiazolesulfenamide (CBS) and zinc oxide (ZnO),
essential in sulfur vulcanization, thus supporting the eco-friendly
benefits of crosslinking ENR with dicarboxylic acids.

\section{Thermoreversible physical ENR networks using crystallizable
grafts}\label{sec5}
ENR development progressed by incorporating crystalline grafts to form
a network with physical crosslinks. This approach involved grafting ENR
with crystallizable behenic acid (BA), leveraging ENR epoxides for
attachment through esterification accelerated using DMI.

Several hypotheses guided this work. (i)~If BA retained its crystalline
structure within the ENR matrix, the grafted BA domains could form a
physical network with reversible crosslinking points. These crystalline
regions would act similarly to those in thermoplastic elastomers, where
crystallites or 
\mbox{physical} bonds provide elasticity while enabling
thermal re-formation. (ii)~ENR crosslinked with dicarboxylic acids might
also develop strain-induced crystallization (SIC) similar to natural
rubber. (iii)~ENR might retain such an intrinsic SIC property alongside
the BA crystallites, creating a dual crystallization mechanism. This
dual crystallization would enable BA-grafted ENR to transition between
an elastic, rubbery state at ambient temperatures and a plastic,
meltable state above the BA melting point. Such a design would allow
the material to benefit from both flexibility at lower temperatures and
reprocessability at higher temperatures.

\subsection{Examining SIC in
DA-crosslinked ENR}\label{sec5.1}
SIC is a unique property of natural rubber, contributing to its high
resistance to crack propagation and making it valuable in demanding
applications like automotive tires. Our research demonstrated that ENR
crosslinked with DA retained the ability to crystallize under strain,
using X-ray scattering techniques in situ during cyclic 
stretching~\cite{31}.

Figure~\ref{fig4} displays the stress--strain curves for the chemical
network DA-ENR25 crosslinked at a  $p = 50$ rate (50 epoxides or 200
ENR25 monomers per diacid molecule). The experiment was carried out
at 20~\textdegree C over three cycles until rupture at the
third cycle. Representative wide-angle X-ray scattering (WAXS) patterns revealed the
structural evolution during the second tensile cycle. At the initial
state $(\lambda = 1)$, the pattern exhibited an isotropic  amorphous 
halo. As tensile strain increased  $(\lambda = 4.6)$, the scattering
intensity became concentrated along the equator, indicating the
alignment of ENR chains in the direction of the applied force 
(Figure~\ref{fig4}A). Upon further stretching beyond $\lambda = 5$,
characteristic crystalline peaks appeared, signifying the formation of
strain-induced crystals. Detailed peak indexation at  $\lambda\approx
7$ is provided in  Figure~\ref{fig4}B. These strain-induced crystals
subsequently melted as the sample relaxed, returning to its original
shape. This second mechanical cycle exhibited a slight shift compared
to the first, attributed to the Mullins effect. However, after the
second unloading, the sample fully recovered its shape, with the
unloading curve eventually aligning with the loading curve, indicating
excellent elasticity of the  chemical network. The final cycle,
extended until break, aligned perfectly with the second  loading.

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig04}
\vspace*{-2pt}
\caption{\label{fig4}WAXS experiments in DA-ENR25  $(p = 50)$ over
three traction cycles (red: first cycle; green: second cycle; blue:
third cycle extended to rupture).  (A)~Patterns observed during the
second loading and unloading phases, with the traction direction
oriented vertically. Strain rate: 1.25~min$^{-1}$. (B)~Close-up of the
pattern at $\lambda\approx 7$, highlighting the anisotropic amorphous
halo of extended ENR chains and peak assignment of the SIC crystals
within an orthorhombic lattice.} 
\vspace*{-2pt} 
\end{figure*}

The results showed that the DA-ENR networks exhibited 
SIC behavior comparable to 
\mbox{sulfur-vulcanized}
natural rubbers. The lattice {parameters,} calculated in the orthorhombic
system  (Table~\ref{tab3}), are very close to those reported in the
case of sulfur-crosslinked  ENR25~\cite{32}. They are slightly higher
than those reported for S-cured natural  rubber~\cite{33}, resulting
in a larger cell volume, which is likely due to the incorporation of
the oxiranes of ENR within the structure of the strain-induced 
crystals~\cite{31}. The high fatigue resistance typical of
sulfur-vulcanized rubber is traditionally associated with its SIC
capability. Experiments demonstrated that ENR crosslinked with DA
retained the ability to crystallize under strain in accordance with
the good fatigue resistance behavior observed in this 
material~\cite{14}. Moreover, the DA-based crosslinking did not
interfere with the lattice structure or crystallization onset, allowing
for structural integrity of the material under repetitive stress. These
findings confirmed the potential for DA-crosslinked ENR to serve in
applications demanding longevity and mechanical robustness,
establishing it as a sustainable alternative for conventional
sulfur-crosslinked natural rubbers.

\begin{table*}
\caption{\label{tab3}Comparison of SIC lattice parameters in the
orthorhombic system for DA-ENR25 and BA-g-ENR25. The crosslinking rate
in DA-ENR25 corresponds to an epoxy/diacid ratio of  $p= 25$.}
\begin{tabular}{cccccc}
\thead
 & $T$ & $a$~(nm) & $b$~(nm) & $c$~(nm) & Volume (nm$^{3}$) \\
\endthead
DA-crosslinked ENR25 & 20~\textdegree C & 1.25 & 0.95 & 0.82 & 0.97 \\ 
BA-grafted ENR25 & 10~\textdegree C & 1.22 & 0.93 & 0.84 & 0.95
\botline
\end{tabular}
\end{table*}

\subsection{Integrating crystalline grafts for physical crosslinking in
ENR}\label{sec5.2}
Detailed results of this  section are available in reference~\cite{30}.

\subsubsection{Synthesis of BA-grafted ENR25}
The reaction between ENR and BA followed the same mechanism as the DA
crosslinking of ENR: an acid group reacts with an oxirane to form an
ester bond  (Figure~\ref{fig5}A). DMI was added in stoichiometric
proportions relative to  acid functionalities to accelerate
esterification, with the BA content set at one acid molecule per two
epoxides. This high concentration of grafting molecules was needed to
make crystallization observable. The resulting grafted ENR is referred
to as BA-g-ENR in the following.

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}(A)~Reaction scheme of ENR25 with BA at an
acid-to-epoxy ratio of 1:2. (B)~FTIR spectra of the ENR25/BA/DMI blend
before (green) and after (red) grafting, zoomed in the carbonyl area.
(C)~DSC traces of BA-g-ENR25 at 
$10~\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$. (D)~Dynamic
mechanical analysis of a BA-g-ENR25 sample at
$3~\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$.}
\end{figure*}

BA was grafted onto ENR25 through a reactive extrusion process at
180~\textdegree C for 25~min. The reaction was deemed complete once the
torque measurement stabilized. This grafting method is entirely
solvent-free, offering substantial industrial and environmental
benefits. Although ENR grafting is achievable within the extruder, ENR
crosslinking was not feasible due to the adhesion of crosslinked
samples in the extruder. For crosslinking, the mixing and curing steps were
separated, with curing conducted under a hot press. This two-step
approach also remained solvent-free and maintained scalability for
industrial applications.

\subsubsection{Spectroscopic analysis (FTIR and
$\mbox{}^{1}$NMR)} 
FTIR spectroscopy provided evidence of successful BA grafting through
ester linkages, as new peaks associated with ester groups appeared
after reactive extrusion (in particular, an IR broad band at 
1732~cm$^{-1}$ characteristic of the ester carbonyl signal, 
Figure~\ref{fig5}B) alongside a decrease in the intensity of peaks
corresponding to carboxylic acids in BA.  $\mbox{}^{1}$H NMR
spectroscopy further confirmed successful grafting by showing
diminished signals for epoxy protons, indicative of ring opening of
oxiranes (not shown). After 25~min of grafting at 180~\textdegree C,
approximately half of the epoxy functions were consumed. Since BA was
introduced at a 1:2 ratio into the epoxides, this indicates that the
reaction reaches 
\mbox{completion} within about 25~min. To ensure complete
grafting of BA onto ENR, the grafting time was fixed at 30~min.

\subsubsection{Thermal properties and differential scanning calorimetry
(DSC)}
DSC analysis provided insight into the melting and crystallization
behavior of BA-g-ENR for an acid:epoxy ratio of 1:2. Free BA exhibits
a melting point around 78~\textdegree C; however, when grafted onto
ENR, the BA crystal's melting signal shifted to approximately
39~\textdegree C and the corresponding crystallization peak to
27~\textdegree C  (Figure~\ref{fig5}C). This decrease in melting
temperature suggested that the grafted BA crystalline domains were
structurally constrained by their attachment to the rubber matrix. Even
though modified, these melting and crystallization transitions provided
a temperature range over which the material maintained elastic
properties as determined using dynamic mechanical analysis (DMA).

\begin{figure*}
{\vspace*{-3pt}}
\includegraphics{fig06}
{\vspace*{-3pt}}
\caption{\label{fig6}Microscopic characterization of BA-grafted ENR25,
with BA constituting 37 wt\% of the total formulation.  (A)~TEM image
of the physical network stained with RuO$_{4}$. Crystalline lamellae
appear in white.  (B)~SAXS measurements comparing DA-crosslinked ENR25
(chemical network) and BA-grafted ENR25 (physical network). As the DA-ENR
sample (black curve) includes kaolin used during chemical network
synthesis, the BA-g-ENR sample (purple curve) was similarly
loaded with kaolin for comparison.}
{\vspace*{-3pt}}
\end{figure*}

\subsubsection{Dynamic mechanical analysis}
The temperature evolution of the storage modulus $(E^{\prime})$ and 
loss modulus $(E^{\prime\prime})$ was measured in a film tension
configuration. Samples with a thickness of 1.5~mm were subjected to a
temperature ramp of
$3~\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$ from
${-}$100~\textdegree C to ${+}$100~\textdegree C at 1~Hz under 0.1\% strain.
The DMA revealed a broad peak in the tan $\delta$ curve corresponding
to the first drop in $E^{\prime}$, identifying the glass transition
temperature $(T_{\mathrm{g}})$ of ENR25 around ${-}$15~\textdegree C 
(Figure~\ref{fig5}D). Below this $T_{\mathrm{g}}$, the high $E^{\prime}$ value
indicated a robust network structure reinforced by crystalline
crosslinks. A second sharper drop in $E^{\prime}$ was observed around
50~\textdegree C corresponding to the melting of BA crystals. Between
these transitions, $E^{\prime}$ decreased in the range of values
typical of elastomers, showing significant temperature dependence near
room temperature. This analysis highlights the dual nature of
BA-grafted ENR: it behaves as an elastomer at ambient temperature and
exhibits thermoplastic properties above 50~\textdegree C.

The functional range of this thermoplastic elastomer extends from its
glass transition temperature near ${-}$15~\textdegree C to the BA crystal's
melting temperature of approximately 40--50~\textdegree C. Within this
range, the modulus exhibits strong temperature dependence, likely
aligning with the degree of BA crystallinity. Mechanical and structural
analyses of the grafted material were conducted at 10~\textdegree C to
leverage the enhanced crystallinity of the material at this
temperature.

\subsubsection{Microstructure analysis by transmission electron
microscopy (TEM)}
Blend morphologies were observed using a transmission electron
microscope operated \mbox{under} a \mbox{voltage} acceleration of 80~kV. Thin 
sections (60--100~nm) were cut at ${-}$80~\textdegree C using a
cryo-ultramicrotome and stained by exposing the thin  sections to
ruthenium tetroxide (RuO$_{4}$) vapors for 2~min at room
temperature. TEM images confirmed the formation of crystalline lamellae
within the BA-g-ENR  (Figure~\ref{fig6}A). These lamellae, visible as
white rows in the stained samples, formed a periodic array throughout
the ENR matrix, with an interlamellar spacing estimated to be
approximately 12~nm. The regularity of these lamellae underscored the
uniformity of the grafting process, with BA long aliphatic chains
crystallizing consistently across the ENR matrix. These organized
lamellar structures contributed to the material's enhanced mechanical
properties at lower temperatures, where the crystalline domains
restricted polymer chain mobility as crosslinks would do in a physical
network. 

\subsubsection{Small-angle X-ray diffraction (SAXS)} 
SAXS experiments were conducted at room temperature in transmission
mode using the Cu K$\rmalpha$ radiation from an X-ray generator
operating at 40~kV  and 25~mA. The SAXS profiles of BA-grafted ENR25
were compared to those of DA-crosslinked ENR25 and unmodified ENR25 as
shown in  Figure~\ref{fig6}B. The spectra are displayed in the Kratky
representation, effective for assessing lamellar structures. DA-ENR25
contains kaolin as used in the synthesis of the chemical network. For
comparison, a BA-g-ENR25 sample was also loaded with kaolin. 

In the purple SAXS spectrum of BA-g-ENR25 with kaolin, a broad peak
appears around 0.075~\AA{}$^{-1}$ corresponding to distances of
approximately 80 \AA{}. This peak is also present in the DA-crosslinked
ENR25 network (black curve) but is absent in the spectra of pure ENR25
(gray curve) and kaolin-free physical network (blue curve). Therefore,
the peak at 0.075~\AA{}$^{-1}$ was attributed to the filler. Notably,
the kaolin-free physical network displays a distinct peak around
0.045~\AA{}$^{-1}$ corresponding to distances of approximately
140~\AA{}. This peak is also visible as a shoulder in the spectrum of
the kaolin-containing physical network. This distance aligns with the
12~nm interlamellar spacing observed in TEM images, confirming that
this peak is due to BA crystalline lamellae.

\subsubsection{In situ X-ray scattering experiments during cyclic
stretching}
WAXS experiments were conducted at 10~\textdegree C during loading and
unloading cycles of BA-g-ENR25 samples, with three successive cycles
analyzed. The final cycle was extended to rupture. All experiments were
performed at a constant speed of 
$25~\mathrm{mm}{\cdot}\mathrm{min}^{-1}$ corresponding to a strain
rate of  $1.25~\mathrm{min}^{-1}$. Figure~\ref{fig7} displays the
mechanical curves for each cycle along with the X-ray patterns recorded
during the initial loading and unloading phases.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Stress--strain curves recorded at  10~\textdegree C
for three successive stretching cycles of a BA-g-ENR25 sample (red:
first cycle; green: second cycle; blue: third cycle extended to
rupture). Filled and empty squares represent loading and unloading,
respectively, with a strain rate of 1.25~min$^{-1}$. The in situ 2D
X-ray patterns captured along the stress--strain curves during the first
cycle correspond to successive elongations $\lambda$.}
\vspace*{-1pt}
\end{figure}

\paragraph*{Initial loading cycle}
The first loading cycle
(red curve in  Figure~\ref{fig7}) deviates from
typical elastomeric behavior, displaying characteristics of a
semi-crystalline material. The sample shows poor elasticity, with a
substantial residual strain of approximately 300\%, indicating limited
recovery.  Young's modulus, calculated from the initial slope of the
stress--strain plot, is notably high at around 5.5~MPa. In comparison,
DA-ENR chemical networks present lower Young's moduli, typically around
1.5~MPa or less. The increase of modulus in case of BA-g-ENR25 is likely
attributed to the presence of graft crystals at 10~\textdegree C.
\looseness=-1

Before stretching $(\lambda = 1)$, the 2D X-ray pattern 
(Figure~\ref{fig7}a) reveals some crystallization alongside the
amorphous halo previously observed in the DA-ENR chemical network 
($\lambda = 1$ in  Figure~\ref{fig4}). A secondary ring, attributed
to the BA crystals, appears near the halo but at a higher Bragg angle.
The crystalline ratio of BA-g-ENR25 calculated from this $(\lambda =
1)$ ring at 10~\textdegree C is 12\%.

Upon stretching, this outer ring aligns along the meridian (stretching
axis), indicating the orientation of BA crystals, interpreted as
clusters of aligned C21 chains. A minimum intensity is observed along
the equator, perpendicular to the stretching axis 
\mbox{(Figure~\ref{fig7}b)}. Beyond $\lambda = 2$, the orientation along the
meridian varies with increasing stretch 
(\mbox{Figure~\ref{fig7}c},~d) as highlighted by dashed oval lines in  
\mbox{Figure~\ref{fig8}}.
At the onset of stretching, the short graft BA chains  orient
perpendicularly to the stretching direction  (Figure~\ref{fig8}A),
resulting in concentrated intensity along the meridian. With increased
stretching, these short chains  begin to tilt 
(Figure~\ref{fig8}B).

\begin{figure}
\vspace*{-1pt}
\includegraphics{fig08}
\vspace*{-2pt}
\caption{\label{fig8}Schematic model illustrating BA crystal
orientation during the stretching phase as observed in the 2D patterns
of: (A)~Figure~\ref{fig7}b and  (B)~Figure~\ref{fig7}d. Dashed oval lines
highlight the evolution of signal intensity upon stretching. The dashed
line denoted as Eq represents the equator, perpendicular to the
stretching axis. In (C), the (120) and (201) characteristic peaks from
SIC superimpose onto the existing signal from the BA crystals as
highlighted by white arrows.}
\vspace*{-5pt}
\end{figure}

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig09}
\caption{\label{fig9}2D X-ray patterns captured at  10~\textdegree C
along the stress--strain curves during  (A)~the second stretching cycle
and  (B)~the third cycle, which is extended to rupture. Filled and
empty squares represent loading and unloading, respectively, with a
strain rate of 1.25~min$^{-1}$.}
{\vspace*{2pt}}
\end{figure*}

For elongation ratios beyond $\lambda = 5$, peaks associated with
SIC of ENR superimpose 
\mbox{(Figure~\ref{fig7}e)}. The characteristic (120) and (201) SIC peaks are
marked by arrows in  Figure~\ref{fig8}C. Upon unloading, these
strain-induced crystals dissipate while the primary signal from BA
crystals gradually reorients along the meridian 
(Figure~\ref{fig7}f--h). 

At the end of the first loading cycle, the
BA crystals remain oriented although their tilt has been lost.

\paragraph*{Subsequent cycles} 
The 2D X-ray patterns for the second cycle and final stretching are
shown in  
\mbox{Figure~\ref{fig9}.} After the second unloading, the material
exhibits low hysteresis with a weak residual strain, which dissipates
within 5~min. The subsequent cycles align closely with the second
cycle, showing very similar 2D patterns. During stretching, the signal
intensity, initially concentrated along the meridian, disorients
towards the equator, and SIC appears as the
loading curve upturns. Since the third loading cycle is extended to
rupture, the SIC signal intensifies here. Upon unloading, the SIC
signal dissipates while the BA crystal orientation returns to the
meridian.

Overall, this combined mechanical and structural analysis demonstrates
that a pre-stretching step breaks and orients BA crystals, aligning
grafts \mbox{perpendicularly} to the ENR chains. During subsequent stretching,
these crystals reversibly tilt, returning to their original orientation
upon unloading, but do not dissipate. Consequently, BA crystals act as
physical crosslinks providing elastomeric properties to the ENR.

\paragraph*{Analysis of the SIC signal}
The lattice parameters for SIC crystals calculated within the
orthorhombic system are presented in  Table~\ref{tab3} alongside those
for ENR25 crosslinked with DA  (Section~\ref{sec5.1}). The parameters
for DA-crosslinked ENR25 were derived from measurements at
20~\textdegree C while those for BA-grafted ENR were obtained at
10~\textdegree C due to insufficient sample crystallinity at
20~\textdegree C (9\%, compared to 12\% at 10~\textdegree C), which
would not provide reliable data. Nevertheless, the calculated lattice
parameters closely align with those observed for the chemical network,
indicating that the crystallized grafts do not interfere with
SIC behavior of the\break rubber.

\section{Comparison between chemical and physical networks}\label{sec6}
Stretching cycles for the DA-ENR25 chemical network were recorded at
10~\textdegree C and plotted along with the BA-g-ENR25 third cycle
until rupture as shown in  Figure~\ref{fig10}. For comparison, the BA-g-ENR
curve was reset to $\lambda = 1$ after recalculating the strain at
the end of the first cycle of pre-stretching, which retained a 300\%
strain following the breaking and orientation of the BA crystals. Two
chemical networks with different crosslinking densities were
considered corresponding to epoxy/diacid ratios of  $p = 25$ and $p=
50$. The latter exhibited a lower crosslinking density. The second
cycle for DA-ENR25 demonstrated excellent reversibility (green curves),
attesting to the elastomeric character of both chemical networks at
10~\textdegree C.\looseness=-1

\begin{figure}
\includegraphics{fig10}
\caption{\label{fig10}Comparison of mechanical properties for
chemically crosslinked DA-ENR25 and physically crosslinked BA-g-ENR25.
Two crosslinking densities were considered corresponding to
epoxy/diacid ratios  $p = 25$ and $p = 50$. Green curves: second
loading cycle. Blue curves: third loading cycle until rupture. All
experiments were conducted at 10~\textdegree C with a strain rate of 
1.25~min$^{-1}$.}
\end{figure}

The physical network cycle closely aligns with that of the chemical
network, displaying stress--strain \mbox{behavior} at deformations below 
$\lambda= 5$, which falls between the two crosslinking densities.
However, the physically crosslinked rubber exhibits superior ultimate
mechanical properties. WAXS patterns indicate that both chemical and
physical networks undergo SIC. In the
BA-grafted ENR, however, two distinct reversible physical networks
coexist: one formed by SIC within the rubber
matrix and another arising from physically connected crystallizable
grafts. This dual-network structure not only enhances the mechanical
properties compared to the chemical network alone but also imparts
reprocessability to the physical network, offering a significant
functional advantage. \looseness=-1

\section{Concluding remarks}\label{sec7}
ENR25 was efficiently grafted with BA through a solvent-free reactive
extrusion process, resulting in self-organized crystallite lamellae
observable via electron microscopy and SAXS. Mechanical tests revealed
that after a single pre-stretching cycle, the resulting material
exhibited elastomeric behavior, with crosslinking solely through BA
crystals. During \mbox{pre-stretching,} grafts oriented into a stable
configuration, enabling further cycles and yielding a thermoplastic
elastomer exhibiting SIC properties. This
combination provided enhanced tensile strength and flexibility,
outperforming chemically crosslinked versions. Our physically
crosslinked ENR, using crystallizable BA grafts, exhibits the
outstanding mechanical properties of natural rubber, such as tear and
crack propagation resistance due to SIC, alongside the reprocessability
of thermoplastic elastomers. With enhanced toughness at low
temperatures, BA-g-ENR is well suited for applications demanding
flexibility and strength. The remeltable BA crystalline domains make
this modified ENR a sustainable choice for recyclable rubber products
that maintain structural integrity upon remolding.

Although this approach marks a significant step in sustainable elastomer
development, a limitation remains: the melting temperature of BA-g-ENR
around 40~\textdegree C is slightly too low for consistent
room-temperature applications. A potential solution involves creating a
doubly crosslinked ENR by introducing chemical reprocessability
alongside physical crosslinking. For instance, crosslinking BA-g-ENR
with DTDB, which contains exchangeable disulfide groups, could produce
a network stable at temperatures above 40~\textdegree C while retaining
elasticity. This would allow the material to maintain its unique
properties over a wider temperature range while enhancing
recyclability.

To conclude, ENR exemplifies sustainable rubber
development. Our research from 2009 to 2017 transformed ENR from simple
self-vulcanizing blends to advanced multifunctional materials. Each
stage deepened our understanding of how ENR can \mbox{combine} traditional
rubber properties with modern demands for sustainability, durability,
and reprocessability. Through the integration of chemical or physical
crosslinking, ENR has evolved into a versatile material well-suited
for applications ranging from tires to flexible electronics, where both
resilience and reduced environmental impact are essential. This
progress has paved the way for ENR to lead innovations in
eco-friendly rubber solutions.

{\vspace*{-2pt}}

\section*{Declaration of interests}

{\vspace*{-2pt}}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and have
declared no affiliations other than their research organizations.

{\vspace*{-2pt}}

\section*{Acknowledgments}

{\vspace*{-2pt}}

The authors warmly thank Pierre-Antoine Albouy from LPS in Paris-Saclay
for in situ X-ray measurements under stretching, C\'{e}dric
Lorthioir for conducting  solid-state NMR analyses, Beno\^{i}t Le
Rossignol from the Hutchinson Research Centre (Montargis, France) for
fruitful collaboration, and Ilias Iliopoulos and Ludwik Leibler for their
continuous encouragement and support.

{\vspace*{-2pt}}

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