79 research outputs found
Phase-separation in ion-containing mixtures in electric fields
When a liquid mixture is subjected to external electric fields, ionic
screening leads to field gradients. We point out that if the mixture is
initially in the homogeneous phase, this screening can bring about a robust
phase-separation transition with two main features: (i) the phase separation is
expected to occur in any electrode geometry, and (ii) the voltage required is
typically of the order of 1 V and even less. We discuss several applications of
the effect relevant to the field of microfluidics, focusing on the creation of
a nanometer-scale lubrication layer in the phase-separation process.Comment: 5 pages, one figur
Phase separation and self-assembly in vitrimers: hierarchical morphology of molten and semi-crystalline polyethylene/dioxaborolane maleimide systems
Vitrimers - a class of polymer networks which are covalently crosslinked and
insoluble like thermosets, but flow when heated like thermoplastics - contain
dynamic links and/or crosslinks that undergo an associative exchange reaction.
These dynamic crosslinks enable vitrimers to have interesting
mechanical/rheological behavior, self-healing, adhesive, and shape memory
properties. We demonstrate that vitrimers can self-assemble into complex meso-
and nanostructures when crosslinks and backbone monomers strongly interact.
Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane
maleimide as the crosslinkable moiety were studied in both the molten and
semi-crystalline states. We observed that PE vitrimers macroscopically phase
separated into dioxaborolane maleimide rich and poor regions, and characterized
the extent of phase separation by optical transmission measurements. This phase
separation can explain the relatively low insoluble fractions and overall
crystallinities of PE vitrimers. Using synchrotron-sourced small-angle X-ray
scattering (SAXS), we discovered that PE vitrimers and their linear precursors
micro-phase separated into hierarchical nanostructures. Fitting of the SAXS
patterns to a scattering model strongly suggests that the nanostructures -
which persist in both the melt and amorphous fraction of the semi-crystalline
state - may be described as dioxaborolane maleimide rich aggregates packed in a
mass fractal arrangement. These findings of hierarchical meso- and
nanostructures point out that incompatibility effects between network
components and resulting self-assembly must be considered for understanding
behavior and the rational design of vitrimer materials
Electrostatic Cancellation of Gravity Effects in Liquid Mixtures
We point out that a spatially-varying electric field can be used to cancel
the effect of gravity in liquid mixtures by coupling to the different
components' permittivities. Cancellation occurs if the system under
consideration is small enough. For a simple ``wedge'' electrode geometry we
show that the required system size and voltage are practical, easily realizable
in the Lab. Thus this setup might be a simple alternative to more expensive or
hazardous options such as the space-shuttle, drop-tower, or magnetic levitation
experiments.Comment: 1.5 pages, one figure. Accepted to PRE brief report
Dynamic covalent chemistry in polymer networks : a mechanistic perspective
The incorporation of dynamic covalent linkages within and between polymer chains brings new properties to classical thermosetting polymer formulations, in particular in terms of thermal responses, processing options and intrinsic recycling abilities. Thus, in recent years, there has been a rapidly growing interest in the design and synthesis of monomers and cross-linkers that can be used as robust but at the same time reactive organic building blocks for dynamic polymer networks. In this perspective, a selection of such chemistries is highlighted, with a particular focus on the reaction mechanisms of molecular network rearrangements, and on how various mechanistic profiles can be related to the mechanical and physicochemical properties of polymer materials, in particular in relation with vitrimers, the recently defined third category of polymer materials. The recent advances in this area are not only expected to help direct promising emerging polymer applications, but also point towards the need for a better fundamental understanding of chemical reactivity within a macromolecular context
Chemical control of the viscoelastic properties of vinylogous urethane vitrimers
International audienc
Etude de l'autoréparation d'élastomères supramoléculaires
National audienceDes travaux récents ont permis de développer un nouveau type d'élastomère constitué d'un réseau de molécules associées entre elles via des liaisons hydrogènes faibles et réversibles. Cet élastomère supramoléculaire présente un pouvoir autoréparant remarquable : les surfaces formées par pressage ou moulage sont peu autoadhésives ; en revanche, une pièce rompue peut retrouver ses propriétés mécaniques d'origine en mettant simplement en contact les surfaces de fracture. Nous montrons ici au moyen d'une expérience de type « tack » que l'autoréparation de surfaces de fracture et l'autoadhésion de surfaces mises en forme mettent en jeu des mécanismes distincts impliquant des énergies d'adhésion et des échelles de temps très différentes. En particulier, des traitements thermiques révèlent que la désactivation de ce pouvoir autoréparant est fortement accélérée par un recuit des surfaces endommagées. Ces résultats suggèrent l'existence de mécanismes de reconstruction des surfaces gouvernés par la mobilité moléculaire
On Sorption of Gases and Liquids in Glassy Polymers
この論文は国立情報学研究所の電子図書館事業により電子化されました
Control of reactions and network structures of epoxy thermosets
International audienc
Orienting Ion-Containing Block Copolymers Using Ac Electric Fields
ABSTRACT: We consider orientation mechanisms for block copolymers in an electric field. Theoretical and experimental studies have shown that nonuniformity of the dielectric constant gives rise to a preferred orientation of the melt with respect to the applied field. We show that the presence of ions, as found in anionically prepared copolymers, may increase the alignment effect markedly. Time-varying (ac) and static (dc) fields are considered within a unified framework. We find that orientation of block copolymers can in principle be achieved without a dielectric contrast if there is a mobility contrast. The presence of ions is especially important at small field frequencies, as is in most experiments. Unlike the no-ions case, it is found that orienting forces depend on the polymer chain lengths. The mobile-ions mechanism suggested here can be used to reduce the magnitude of orienting fields as well as to discriminate between block copolymers of different lengths
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