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_<i>e</i>) 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>_<i>gel</i>) and the critical conversion (α_<i>gel</i>) 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_<i>e</i> 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.-