science
Vitrimers – the miracle polymer materials combining
the properties of glass and plastic?
Anna JUROWSKA*, Kamil JUROWSKI – Faculty of Chemistry, Jagiellonian University in Kraków,
Poland
Please cite as: CHEMIK 2015, 69, 7, 389–394
Introduction
A class of polymers that are formed in a typically irreversible
bonding process and are materials that can no longer flow or dissolve,
rendering them practical for both high-temperature and solventintensive applications are known as cross-linked thermosets. This
class of polymers cannot be reprocessed by traditional means such as
melting process. Numerous methods involving thermally reversible
physical cross-links (e.g. Diels−Alder cycloadducts), have been
utilized to create reprocess able cross-linked polymers [1 – 2]. But
these reactions do not keep the number of cross-links in the network
structure, hence the materials exhibit a loss in structural integrity
upon heating. Do polymer materials combining the properties of glass
and plastic exist?
What are vitrimers?
Vitrimers are a new class of polymer materials that was invented
and first described by Leibler and co-workers [3]; these materials are
characterized by highly desirable properties, combining malleability
and repairability at high temperatures with insolubility [4].
Polymers of this class are very different in comparison
to thermosets and thermoplastics. It is well known that the thermosets
(or elastomers), made of permanently cross linked polymers, are
insoluble and cannot be reshaped once synthesized (cross linked).
On the other hand, thermoplastics made of polymer chains can be
easily reshaped at will, but are soluble [5].
Vitrimers are very sophisticated, due to their composition – they
consist of a covalent organic network that can rearrange its topology
via reversible exchange reactions that preserve the total number of
network bonds and the average functionality of the nodes. That means
that vitrimers are made of atoms that are covalently bonded to form
a network and the design principle is based on the reversible network
topology freezing. Through bond exchange reactions the network
topology is able to change, the material relaxes stresses and flows even
though the total number of bonds remains constant in time and does
not fluctuate [6 – 7]. Stresses in a deformed vitrimer can relax due
to the network topology rearrangements, resulting in the deformation
becoming permanent.
Furthermore, according to the mentioned mechanism, vitrimers
can flow under mechanical stress. When the temperature decreases
(cooling), the exchange reactions slow down and the network topology
appears to be fixed on experimental time scales. Hence, a vitrimer
behaves like an elastic thermoset (elastomer). On the other hand,
after heating, the exchange reactions become faster and the viscosity
decreases, causing the vitrimer to become malleable. It is very
important that the temperature of this reversible glass transition can
be tuned with the aid of a catalyst that controls the exchange reaction
rate and the activation energy [4].
Corresponding author:
Anna JUROWSKA - M.Sc., e-mail: [email protected]
392 •
Transesterification reactions as a clue of vitrimers properties
Polyester epoxy resins, that incorporated a Zn2+ transesterification
catalyst (e.g. zinc acetate; Zn(ac)2), were the first example explaining
the intrinsic properties of vitrimers [2]. Sophisticated, interchain
transesterification occurs upon heating, and when a stress is applied
to the material, the bonds rearrange such that the stress is completely
dissipated. Due to the fact that transesterification is an isodesmic
reaction, the materials do not exhibit a loss in structural integrity,
even at elevated temperatures in solvents. Figure 1 represents an
example of schematic view of a network with exchange processes
that preserve the total number of links and the average functionality
of cross-links, and an example of topological rearrangements via
exchange reactions preserving the network integrity; the middle
image shows that the exchange does not require depolymerization
in the intermediate step.
Fig. 1. (A) Exchange process via transesterification in hydroxyester networks and (B) schematic view of a network with exchange
processes that preserve the total number of links and average
functionality of cross-links
The properties of vitrimers vs properties of thermoplastics
It is well known that when an amorphous polymer melt is
cooled down, it undergoes a glass transition. If the temperature is
in the vicinity of the glass transition temperature Tg, the polymer is
characterized by immediately hardens and also its viscosity increases
by several orders of the magnitude according to a Williams-LandelFerry (WLF) law. Figure 2 (A) shows V−T characteristics of
a thermoplastic polymer [8 – 9].
For this kind of polymers, at T under than Tg – Amorphous I, the
physical state is conventionally achieved to as glass, at T above than Tg
(Amorphous II) as liquid. Figure 2 (B) shows that above Tg, the viscosity
of a thermoplastic polymer is characterized by WLF power law with
the temperature. On the other hand, at Figure 2 (C) V−T characteristics of a vitrimer is presented, where it is possible to observe two glass
transitions: 1) the classical Tg and 2) Tv, a glass transition that reflects
topology freezing upon cooling., on the Figure 2 (D) the viscosity follows according to Arrhenius law in the vicinity of Tv [10].
It is well known, that polymers due to the all of other organic
compounds can be defined as fragile glass-formers [11]. However,
silica (the archetype of glass), undergo a very gradual Arrhenius-like
viscosity change near Tg [12]. Due to this fact, silica and few other
similar inorganic compounds are called strong glass-formers. What is
nr 7/2015 • tom 69
100 MPa, the materials appears to be an elastic solid, depending on
the cross-link density. In the analogy for a classical glass transition, the
value of the topology freezing transition temperature Tv is correlated
with the cooling rate. Thermal expansion measurements on vitrimers
are evidence of the rate dependence of the transition at Tv and glasslike nature [10].
It must be emphasized that, the freezing topology transition
cannot be considered as a viscous-to-elastic gel transition. The
connectivity of the molecules and number of bonds increase, when
approaching the gelation transition. At the gel point, the system is
characterized by a broad distribution of linked clusters with one of
them being “infinite” and percolating through the sample – the system
becomes elastic. For vitrimers, the network is always “infinite” and
connectivity does not change. The network flows above Tv, and the
material is insoluble at all temperatures, the number of bonds remains
constant. After further cooling (under Tv), vitrimers undergo another,
much more immediately transition from an elastic to a hard solid,
a glass with modulus of about 1 GPa. It is important to notice, that
this transition from an elastic solid to a hard glass is analogous to Tg
according to classical thermosets and elastomers [13].
Applications of vitrimers
The described properties make vitrimers excellent candidates
for applications in, e.g., the aviation, automotive, electronic, and
sporting goods industries. Moreover, they have been shown to be
useful in a variety of practical applications, including adhesives and
liquid-crystalline elastomers [14], and hold tremendous promise for
a range of advanced material technologies. Vitrimers have also been
produced utilizing olefin metathesis and those materials exhibited
healing properties at room temperature [15, 16].
Conclusions
Thermoplastics can be processed in a molten state, but often have
poor heat or chemical resistance, while thermosets are more resistant,
but cannot be re-shaped or recycled. Like glass, vitrimers remain solid
but malleable within a broad temperature range. Vitrimers are crosslinked polymers with flow dynamics dependent on the transient nature
of their network structure.
The authors wish to draw attention to the fact that this article results due
to the participation in the course of “Polymer Chemistry” for Doctoral Candidates
at the Jagiellonian University in Kraków (Faculty of Chemistry) – the lecturer:
prof. Stanisław Penczek). Additionally this subject was presented by authors
at the 57th Meeting of The Polish Chemical Society and The Society of Engineers and
Technicians of Chemical Industry, which was in Częstochowa, September 16th, 2014
on special session dedicated to Prof. Stanisław Penczek.
This article was made by the support of Anna Jurowska M.Sc. and Kamil
Jurowski M.Sc. scholarship by the Marian Smoluchowski Kraków Research
Consortium “Matter-Energy-Future” granted the status of a Leading National
Research Centre (KNOW).
Literarure
Fig. 2. Characteristics of a thermoplastic polymer (A-B) and
characteristics of a vitrimer (C-D)
On the other hand, in vitrimers, the exchange reactions are
thermally activated – Figure 2 (C-D). Under cooling, the relaxation
time and viscosity, controlled by exchange reactions rate, decrease
slowly according to the Arrhenius law. At some temperature Tv, the
mechanical relaxation time (controlled by the exchange reaction rate)
is longer than the experimental time scale and on this time scale, the
network topology is frozen. Between the elastic modulus 1 MPa and
nr 7/2015 • tom 69
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• 393
science
more it is possible to glass blowing or easy shaping by local heating
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*Anna Jurowska – Ph.D. student of Coordination Chemistry Group
(Department of Inorganic Chemistry, Faculty of Chemistry, Jagiellonian
University in Kraków, Poland). She is the author of 6 scientific papers
(in that 2 in polish language). She has participated in many national and
international conferences. Her research interests focus on: coordination
chemistry and chemistry of new molecular materials.
e-mail: [email protected], phone: +48 12 663 22 23
Kamil Jurowski – Ph.D. student of Toxicology and Pharmacology
Analysis Group (Department of Analytical Chemistry, Faculty of Chemistry,
Jagiellonian University in Kraków, Poland). He is the author of 8 scientific
papers (in that 2 in polish language) and 2 monographs. He is the author
of 6 presentations on international conferences and 5 presentations
on national conferences. His research interests focus on: (bio)medical
chemistry, clinical toxicology, clinical biochemistry, lipidomics, genomics,
integromics, proteomics, metallomics.
e-mail: [email protected], phone: +48 12 663 56 03
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