21 research outputs found
Resolving the relaxation complexity of vitrimers: time-temperature superpositions of a time-temperature non-equivalent system
Vitrimers are polymer networks that, thanks to covalent bond exchange,
combine desirable properties of thermoplastic and thermosets, such as
flowability and insolubility. For this reason, vitrimers are considered to be
good candidates for a number of innovative applications from self-healing soft
robots to hard reprocessable materials. All these applications are related to
the unusual thermomechanical behavior of vitrimers, consequence of the
non-trivial interplay between the polymer network dynamics and the thermally
activated chemical link exchange. Here we use solid-state rheology to
investigate the properties of a recently developed epoxy-based vitrimer. The
rheological analysis demonstrates that the mechanical spectrum is composed of
two relaxation processes with distinct activation energies which are associated
with glass dynamics and covalent bond exchange, respectively. This makes the
material thermo-rheologically complex and time temperature equivalence does not
apply. Nonetheless, thanks to mechanical spectral analysis in a wide range of
stiffness, time and temperature, we are able to depict the
time-temperature-relaxation landscape in an enough precise way to account for
the two dynamical processes and recombine them to predict the mechanical moduli
in a wide (virtually unlimited) interval of frequencies, from low temperatures
(close to room temperature) to high temperatures (above the Tg)
Dicarboxylic acid-epoxy vitrimers: Influence of off-stoichiometric acid content on cure reactions and thermo-mechanical properties
International audienc
Materiaux moleculaires mesomorphes pour l'optique non lineaire
SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
Unprecedented Sequence Control and Sequence-Driven Properties in A Series of AB-Alternating Copolymers Consisting Solely of Acrylamide Units
1種類のモノマー単位で交互共重合体の合成に成功 --異なる側鎖の配列制御で液晶性を発現--. 京都大学プレスリリース. 2020-01-16.In this article, we report a method to synthesize a series of alternating copolymers that consist exclusively of acrylamide units. Crucial to realizing the unprecedented polymer synthesis is the design of a divinyl monomer that contains acrylate and acrylamide moieties connected via two activated ester bonds. This elaborate design, which is based on the reactivity ratio of the embedded vinyl groups, allows a “selective” cyclopolymerization, wherein the intramolecular and intermolecular propagation are repeated alternately under dilute conditions. The addition of an amine to the resulting cyclopolymers afforded two different acryl amide units, i.e., an amine‐substituted acryl amide and a 2‐hydroxy‐ethyl‐substituted acryl amide in alternating sequence. Using this method, we were able to furnish ten types of alternating copolymers; some of these exhibit unique properties in solution and in the bulk, which are clearly different from those of the corresponding random copolymers, and we attributed the observed differences to the alternating sequence
Control of Gelation and Network Properties of Cationically Copolymerized Mono- and Diglycidyl Ethers
The development of
low temperature curing systems has become a
major objective in thermoset technologies for both environmental and
economic reasons. The use of protic and chelating additives have recently
been underlined for the control of the cationic ring-opening polymerization
of epoxies, a curing mode that is very efficient at temperatures close
from the ambient but that can easily runaway. In this paper, we propose
to use this strategy to control the kinetics of the cationic copolymerization
of a diepoxy monomer (diglycidyl ether of bisphenol A, DGEBA) with
a monoepoxy monomer (phenyl glycidyl ether, PGE). The purpose of the
study is to tune the cross-link density (ν<sub><i>e</i></sub>) in order to control the mechanical properties of the materials.
The sol–gel transition was first investigated in details at
several frequencies by using the Fourier transform mechanical spectroscopy
method (FTMS). We found that the gel time (<i>t</i><sub><i>gel</i></sub>) and the critical conversion (α<sub><i>gel</i></sub>) can be controlled to a great extent by
promoting transfers and complexing cationic species involved in the
polymerization mechanism. The FTMS method also gives some insight
into the structure of the polymer clusters at the sol–gel transition.
The results indicate that the various additives used to control the
transition have mostly no influence on the clusters’ structure.
The properties of the fully cured networks were then investigated
via swelling and dynamic mechanical measurements. Both methods indicate
that ν<sub><i>e</i></sub> is strongly influenced by
the cross-linker content (DGEBA) but also by the additive used to
control the curing kinetics. Interestingly, the measurement of the
tensile properties at large deformations demonstrates that the resulting
system offers a series of materials with a wide range of mechanical
properties
Design of Self-Healing Supramolecular Rubbers with a Tunable Number of Chemical Cross-Links
Supramolecular rubbers incorporating a large number of physical cross-links through cooperative hydrogen bonds display high self-healing properties but limited solvent and creep resistance due to the lack of chemical cross-links. Increasing both chemical cross-links and H-bonding is therefore desirable but limited by the functionality of monomers. The present work thus devises a convergent chemical platform permitting to increase the number of chemical cross-links without changing the concentration of hydrogen-bonding groups. Starting from a single reactive prepolymer, functionalized with a defined number of hydrogen-bonding groups, a series of networks presenting different ratios of diepoxide and tetraepoxide were prepared. The curing process (controlled by 2-MI catalyst), thermomechanical behavior, and tensile properties recovery of the cured materials were investigated. Gelation state was quantified and compared to theoretical predictions. The introduction of tetrafunctional epoxide in the presence of 2-MI gave rise to gelled materials characterized by higher rigidity and strength and significantly improved creep resistance. Self-healing was observed for all materials, with 50% to 100% complete recovery in a day depending on tetraepoxide content
Gélification physique de copolymères AB alternés composés d'unités vinylphénol et maléimide: coopération entre les groupes pendants phénol et alkyle incorporés avec précision.
International audienceA series of alternating copolymers consisting of vinyl phenol and n-alkyl maleimide was synthesized via radical copolymerization of a protected styrene derivative with a functional maleimide monomer followed by the deprotection. The copolymers carrying long alkyl pendant such as C12H25-or C18H37-chains on the maleimide unit showed UCST-type thermal response in aromatic solvents and organogels were specifically formed upon cooling of the fluid solution prepared at higher temperature. Hydrogen bonding of the phenol units is crucial for the gelation and the gelation temperature and stiffness were tuneable by varying concentration, solvent and polymerization degree. Analyses by 1 H-NMR, linear rheology, WAXD, SANS and cryo-TEM gave the picture of vermicular self-assembled nano-objects formed through segregated and hydrogen-bonded packing by the precisely incorporated two units in alternating sequence
Réticulation du poly(téréphtalate de butylène) par extrusion réactive en utilisant la chimie époxy-vitrimère catalysée par le Zn(II).
International audiencePoly(butylene terephthalate) (PBT) vitrimers were prepared by reactive extrusion from industrial PBT thermoplastics using Zn(II)-catalyzed addition and transesterification chemistry. PBT thermoplastics are characterized by a high degree of crystallinity, high melting temperature and high crystallization rate, but right above their melting temperature their mechanical resistance disappear and they show a tendency to drip. We compared-OH and-COOH end-group additions on epoxies in the presence of two different catalysts, 2-methyl imidazole (2-MI) and zinc acetyl acetonate (Zn(acac) 2). With 2-MI, chain extension reactions were efficiently catalyzed in a few minutes at 270 °C but no gelation was observed. With Zn(acac) 2 ,-COOH addition and transesterification led to efficient cross-linking within a few minutes at 270 °C. Such cross-linked material combines the crystalline properties of PBT and dimensional stability above the melting temperature. PBT materials cross-linked through epoxy-vitrimer chemistry are not soluble. However, compared with radiation cross-linked PBT, vitrimer PBT is processable and can be reshaped and recycled.-