Dynamic
covalent bonds in a polymer network lead to plasticity, reshapability,
and potential recyclability at elevated temperatures in combination
with solvent-resistance and better dimensional stability at lower
temperatures. Here we report a simple one-step procedure for the catalyst-free
preparation and intramolecularly catalyzed stress-relaxation of dynamic
polyester networks. The procedure is based on the coupling of branched
OH-end functional polyesters (functionality ≥ 3) by pyromellitic
dianhydride (PMDA) or 2,5-bis(methoxy-carbonyl) benzenesulfonic acid
resulting in ester linkages with, respectively, a COOH or a SO3H group in a position ortho to the ester
bond. This approach leads to an efficient external catalyst-free dynamic
polyester network, in which the topology rearrangements occur via
a dissociative mechanism involving anhydrides. The SO3H-containing
network is particularly interesting, as it shows the fastest stress
relaxation and does not suffer from unwanted additional transesterification
reactions, as was observed in the COOH-containing network.
Controlled grafting of polyaniline from the surface of carbon nano dots using ‘grafting from’ strategy is reported. The nano structured polyaniline coated carbon nano dots produced show excellent electrochemical performance.
This work was funded by the Dutch Research Council (NWO), Project No. 731.016.202. We are grateful to Annelore Aerts for her help in mechanical testing of the samples. We thank Dr. Ruth Cardinaels for fruitful discussions.
The diamide–imide
equilibrium was successfully exploited
for the synthesis of dynamic covalent polymer networks in which a
dissociative bond exchange mechanism leads to high processibility
at temperatures above ≈110 °C. Dynamic covalent networks
bridge the gap between thermosets and thermoplastic polymers. At the
operating temperature, when the network is fixed, dynamic covalent
networks are elastic solids, while at high temperatures, chemical
exchange reactions turn the network into a processible viscoelastic
material. Upon heating a dissociative network, the viscosity may also
decrease due to a shift of the chemical equilibrium; in such materials,
the balance between processibility and excessive flow is important.
In this study, a network is prepared that upon heating to above ≈110
°C dissociates to a significant extent due to a shift in the
amide–imide equilibrium of a bisimide, pyromellitic diimide,
in combination with poly(tetrahydrofuran) diamines. At room temperature,
the resulting materials are elastic rubbers with a tensile modulus
of 2–10 MPa, and they become predominantly viscous above a
temperature of approximately 110 °C, which is dependent on the
stoichiometry of the components. The diamide–imide equilibrium
was studied in model reactions with NMR, and variable temperature
infrared (IR) spectroscopy was used to investigate the temperature
dependence of the equilibrium in the network. The temperature-dependent
mechanical properties of the networks were found to be fully reversible,
and the material could be reprocessed several times without loss of
properties such as modulus or strain at break. The high processibility
of these networks at elevated temperatures creates opportunities in
additive manufacturing applications such as selective laser sintering.
Bond exchange via
neighboring group-assisted reactions in dynamic
covalent networks results in efficient mechanical relaxation. In Nature,
the high reactivity of RNA toward nucleophilic substitution is largely
attributed to the formation of a cyclic phosphate ester intermediate
via neighboring group participation. We took inspiration from RNA
to develop a dynamic covalent network based on β-hydroxyl-mediated
transesterifications of hydroxyethyl phosphate triesters. A simple
one-step synthetic strategy provided a network containing phosphate
triesters with a pendant hydroxyethyl group.
31
P solid-state
NMR demonstrated that a cyclic phosphate triester is an intermediate
in transesterification, leading to dissociative network rearrangement.
Significant viscous flow at 60–100 °C makes the material
suitable for fast processing via extrusion and compression molding.
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