Chimie Moléculaire, Macromoléculaire, Matériaux

Permanently cross-linked materials have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized. Non-cross-linked polymers and those with reversible cross-links are processable, but they are soluble. We designed epoxy networks that can rearrange their topology by exchange reactions without depolymerization and showed that they are insoluble and processable. Unlike organic compounds and polymers whose viscosity varies abruptly near the glass transition, these networks show Arrhenius-like gradual viscosity variations like those of vitreous silica. Like silica, the materials can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries.

Covalently cross-linked polymers have many technological applications for their excellent properties, but they suffer from the lack of processability and adaptive properties. We report a simple, efficient method of generating adaptive cross-linked polymers via olefin metathesis. By introducing a very low level of the Grubbs' second-generation Ru metathesis catalyst, a chemically cross-linked polybutadiene network becomes malleable at room temperature while retaining its insolubility. The stress relaxation capability increases with increasing level of catalyst loading. In sharp contrast, catalyst-free control samples with identical network topology and cross-linking density do not show any adaptive properties. This chemistry should offer a possibility to combine the dimensional stability and solvent resistance of cross-linked polymers and the processability/adaptibility of thermoplastics.

Vinylogous urethane based vitrimers are polymer networks that have the intrinsic property to undergo network rearrangements, stress relaxation and viscoelastic flow, mediated by rapid addition/elimination reactions of free chain end amines. Here we show that the covalent exchange kinetics significantly can be influenced by combination with various simple additives. As anticipated, the exchange reactions on network level can be further accelerated using either Brønsted or Lewis acid additives. Remarkably, however, a strong inhibitory effect is observed when a base is added to the polymer matrix. These effects have been mechanistically rationalized, guided by low-molecular weight kinetic model experiments. Thus, vitrimer elastomer materials can be rationally designed to display a wide range of viscoelastic properties.

Self-assembly is a process in which interacting bodies are autonomously driven into ordered structures. Static structures such as crystals often form through simple energy minimization, whereas dynamic ones require continuous energy input to grow and sustain. Dynamic systems are ubiquitous in nature and biology but have proven challenging to understand and engineer. Here, we bridge the gap from static to dynamic self-assembly by introducing a model system based on ferrofluid droplets on superhydrophobic surfaces. The droplets self-assemble under a static external magnetic field into simple patterns that can be switched to complicated dynamic dissipative structures by applying a time-varying magnetic field. The transition between the static and dynamic patterns involves kinetic trapping and shows complexity that can be directly visualized.

Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory we study a simple model of autonomic self-healing of unentangled polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. We study the reaction kinetics of reversible bonds in this simple model and analyze the different stages in the self-repair process. The formation of bridges and the recovery of the material strength across the fractured interface during the healing period occur appreciably faster after shorter waiting time, during which the fractured surfaces are kept apart. We observe the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium (very low) density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess non-equilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface (formation of bridges) occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk.

Hidradenitis suppurativa is a chronic disease characterized by recurrent, painful, deep-seated, rounded nodules and abscesses of apocrine gland-bearing skin. Subsequent suppuration, sinus tracts and hypertrophic scarring are its main features. Onset is usually after puberty, although it is most common during the third decade and may persist in old age. The disease tends to be chronic and may develop to subcutaneous extension leading to indurations, sinus, and fistula having a profound impact on the quality of life. The prevalence is 1% in several studies. Axillary and inguinal involvement is more common in females; peri-anal and buttocks localizations are prevalent in males. The exact aetiology remains unknown. The primary event is a follicular occlusion with secondary inflammation, infection and destruction of the pilo-sebaceo-apocrine apparatus and extension to the adjacent sub-cutaneous tissue. Infection is common. Smoking may be a triggering factor. Obesity aggravates the discomfort. Differential diagnostic includes Crohn's disease, nodular acne and furonculosis. The main complications are arthropathy, carcinoma. Treatment depends upon the stage of the disease. Early nodular lesions may be treated by antibiotics for acute stage; long-term antibiotics, zinc salts may be useful as maintenance treatment; anti-TNF drugs have been used in severe cases; systemic steroids, estrogens, anti-androgens, retinoids have been used as options with limited success. Surgical treatment includes incision with or without drainage for limited abscesses; limited excisions are used for locally recurring draining sinuses. Total wide excision and healing with secondary intention or flaps and grafts is the only curative procedure in case of advanced disease.

SUMMARY Fourteen newborn babies of different gestational ages (33 to 40 weeks) but similar postnatal age (four to 19 days) were studied during bottle‐feeding using real‐time ultrasonography, combined with respiratory monitoring. Previously undescribed tongue movements and graded changes in the temporal relationships between tongue movements, swallowing and breathing were observed among infants of differing maturity. These were most marked in the least mature infants, but were occasionally seen in term infants. The results suggest that adequate neuromuscular co‐ordination is more a function of gestational maturity than of postnatal sucking experience. The pattern of intraoral events for infants of differing maturity described in this study provides a framework for the study of feeding problems of term and preterm infants. RÉSUMÉ Développement de la coordination, de la succion, de la déglutition et de la respiration chez les nourrissons nés à terme et chez les prématurés Quatorze nouveaux‐nés d'âges differents de gestation (33 à 40 semaines) mais d'âges post‐nataux semblables (quatre à 19 jours) on*** été étudiés durant l'alimentation au biberon en utilisant une échographie en temps reel combinée avec un contrôle respiratoire. Des mouvements de langue non décrits antérieurement et des modifications graduées dans les relations temporelles entre les mouvements de langue, la déglutition et la respiration ont été observés chez les nourrissons de differentes maturités. Ils étaient au plus élevé chez les moins matures des enfants étudiés mais observés occasionnellement chez les nourrissons à terme. Les résultats suggèrent que la coordination neuro‐musculaire adéquate est plus une fonction de la maturité de gestation que de l'expérience post‐natale de succion. La distribution des évènements intra‐oraux pour les nourrissons de differentes maturités décrites dans cette étude fournit une base de travail pour l'étude des problèmes d'alimentation chez les nourrissons à terme et prématurés. ZUSAMMENFASSUNG Koordinationsentwicklung von Saugen, Schlucken und Atmen bei reif‐ und frühgeborenen Kindern Bei 14 Neugeborenen mit verschiedenem Gestationsalter (33 bis 40 Wochen), aber etwa gleichem Lebensalter (vier bis 19 Tage) wurden während der Flaschenfütterung Real‐Zeit Ultraschalluntersuchungen mit gleichzeitigen Aufzeichnungen der Atmung durchgeführt. Es wurden bisher nicht beschriebene Zungenbewegungen und abgestufte Veränderungen in der zeitlichen Relation zwischen Zungenbewegungen, Schlucken und Atmen bei Kindern mit unterschiedlichem Reifegrad festgestellt. Diese waren bei den unreifsten Kindern am ausgeprägtesten, wurden aber auch bei einigen reifen Kindern beobachtet. Die Ergebnisse zeigen, daβ eine richtige neuromuskuläre Koordination eher eine Funktion der Gestationsreife als der postnatalen Saugerfahrung ist. Die Muster intra‐oraler Abläufe bei Kindern unterschiedlicher Reife, die in dieser Studie beschrieben werden, bilden die Basis für Untersuchungen von Fütterungsproblemen bei reif‐ und frühgeborenen Kindern. RESUMEN Desarrollo de la coordinación de la succión, deglución y respiración en lactantes a término y pretérmino Catorce reciàn nacidos de diferentes edades gestacionales (33 a 40 semanas) pero de edad postnatal similar (de cuatro a 19 dias) fueron estudiados durante la toma del biberón usando una ultrasonografia a tiempo real en combinaciòn con la monitorización de la respiración. Entre los lactantes con diferente madurez se observaron movimientos de lengua no descritos previamente, así como cambios graduales en las relaciones temporales entre los movimientos de la lengua, la deglución y la respiración. Esto era marcado sobre todo en los lactantes menos maduros mientras que se observaron raramente en los lactantes a término. Los resultados sugieren que la adecuada coordinación neuromuscular es más una función de la maduración gestacional que de la experiencia succional postnatal. El esquema de los acontecimientos intraorales en lactantes de diferente madurez descritos en este estudio proporciona un marco para el estudio de los problemas de alimentación de lactantes a término y a pretérmino.

Disulfide crosslinks introduced into an ENR matrix enable the thermo-activated reprocessing of the chemically crosslinked rubber, studied in terms of stress relaxation and adhesion experiments.

This Review discusses the structure-property relationships in chiral molecules, macromolecules (polymers), and supramolecules (crystals, liquid crystals, or thin films) containing main-group elements. Chirality is a major property in our world, having a prominent influence on processes in biology, chemistry, and physics. Its impact in optics due to its interaction with electromagnetic waves gave rise to a multitude of effects, such as the Cotton effect and circularly polarized luminescence, making possible applications such as 3D displays and polarized sunglasses. Herein, a particular emphasis will be given to the influence of chirality on the conducting and optical properties of molecules or materials containing frontier heteroelements, particularly boron, silicon, phosphorus, and sulfur. These synergic materials are expected to become game-changers in the field of materials science by bringing new properties into the realm of reality, such as chirality-induced spin-selectivity, circularly polarized luminescence, and electrical magnetochiral anisotropy. This Review should be of interest for chemists and also physicists working in the fields of molecular and supramolecular chemistry, and molecular materials in the broadest sense.

Vitrimers are an emerging new class of permanently cross-linked polymeric materials that show a liquid behavior upon heating wherein the macroscopic deformation is controlled by the rate of internal chemical bond exchange reactions. Thus, quite uniquely among polymeric materials, flow rates and material viscosities can be enhanced or controlled by the addition of catalysts and additives. We now report a catalyst-free vitrimer system, prepared from mixing two simple components, wherein two competing bond exchange mechanisms coexist, each showing a strikingly different temperature dependence, related to the large difference in activation energy for the different exchange pathways (60 vs 130-170 kJ/mol). The low barrier process is predominant at lower temperatures, but is outcompeted by the high barrier process that becomes dominant at higher temperatures because of its much more pronounced temperature dependence. The result is an interesting and highly unusual dual viscosity profile for this new class of vitrimer materials: a very gradual decrease in viscosity at lower temperatures, intercepted by a much sharper drop in viscosity at higher temperatures. The highly counterintuitive effect where a higher barrier pathway is dominant over a much lower barrier process can be rationalized by the exchange mechanisms that involve different reactive species, but lead to the overall same exchange. We observed this unusual but highly promising behavior first for fluorinated vitrimer elastomers, aimed at high performance materials, but the effect was also shown to hold in related nonfluorinated elastomers. A new way to control and design the rheological behavior of vitrimers toward finely tuned and precisely controlled processing applications has thus been provided.

Microgel pastes and concentrated emulsions are shown to exhibit a generic slip behavior at low stresses when sheared near smooth surfaces. The magnitude of slip depends on the applied stress. Well above the yield stress, slip is negligible compared to the bulk flow. Just above the yield stress, slip becomes significant and the total deformation results from a combination of bulk flow and slip. At and below the yield stress, the bulk flow is negligible and the apparent motion is entirely due to wall slip. By directly imaging the deformation of pastes and from rheological measurements, we show that slip is characterized by universal scaling properties, which depend on solvent viscosity, bulk shear modulus, and particle size. A model based on elastohydrodynamic lubrication between the squeezed particles and the shearing surface explains these properties quantitatively.

The effect of composition and concentration of polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) copolymer triblock on final morphologies and properties of modified epoxy networks has been investigated. The DGEBA−MCDEA epoxy system, which ensures the miscibility of most of the PMMA blocks until the end of the reaction and thus the generation of a nanostructurated material, has been chosen. For low copolymer concentration (10 wt %), the network structure is found to be independent of the composition and micelles of PS and PB blocks can be undifferently observed. However, increasing copolymer amounts from 10 to 50 wt % induces a morphology change to either “spheres on spheres” or “core−shell” structure depending on the PB content in the triblock. For copolymer concentration higher than 50 wt %, the morphology strongly depends on the processing technique used, and only films prepared by solvent casting show an organization with long-range order similar to the neat block copolymer. The toughness of nanostructured epoxy networks has been evaluated. For low SBM concentration the toughness was observed to linearly vary with the PB concentration. For higher SBM concentrations (30 and 50 wt %) the “spheres on spheres” morphology is found to be more efficient than the “core−shell” one to improve the toughness of the epoxy network. But in all cases a higher toughness is obtained when the nanostructure is preserved; when a macrophase separation occurs, the toughness increase is lower.

Concentrated dispersions of soft particles are shown to exhibit a generic slip behavior near smooth surfaces. Slip results from a balance between osmotic forces and noncontact elastohydrodynamic interaction between the squeezed particles and the wall. A model is presented that predicts the slip properties and provides insight into the behavior of the bulk paste.

Reinforced silica–epoxy vitrimer nanocomposites have been made in a solvent-free, easily processable, and economical way with filler contents up to 40 wt %. Increasing silica content leads to higher modulus materials in both glassy and rubbery regions. These nanocomposites are insoluble like permanent cross-linked networks but can completely relax stresses by thermoactivated exchange reactions that rearrange the network topology. Furthermore, the surface functionalization of particles improves the dispersion of fillers and accelerates the relaxation process. Similar results were obtained with industrial precipitated silica, which would allow vitrimer nanocomposites to be produced on an industrial scale.

We propose a strategy to obtain through a facile one-pot synthesis a large variety of supramolecular materials that can behave as differently as associating low-viscosity liquids, semicrystalline or amorphous thermoplastics, viscoelastic melts or rubbers. Such versatility is achieved thanks to simultaneous synthesis of branched backbones and grafting of associating units. This contrasts with usual synthetic pathways that rely on grafting functional groups on preprepared backbones. We use oligocondensation of fatty di- and triacids with diethylenetriamine and finely tune the molecular weight and degree of branching by end-capping some acid groups before condensation by reaction with aminoethylimidazolidone. Supramolecular assembly is formed thanks to complementary and self-complementary associations of amide, imidazolidone, and dialkylurea groups, and the stoichiometry directly controls the mesoscopic structure and properties.

BACKGROUND AND OBJECTIVE: Many acne grading methods exist; however, there is no agreed-upon standard. Our objective was to create and validate a reproducible acne assessment scale for rating the severity of juvenile facial acne suitable for use in France and Europe. METHODS: The scale we created described the different types of acne lesions in a manner similar to global assessment scales used in clinical trials. The scale was then validated by seven expert dermatologists in the field of acne [the Global Evaluation Acne (GEA) group] first on 34 photographic cases of Caucasian acne patients and second by clinical examination of 22 acne patients. RESULTS: There was good agreement in Investigators' assessments of acne both on photographs and patients (R = 0.8057; P < 0.0001, and R = 0.8437; P = 0.0015). CONCLUSION: The GEA Scale is a global scale validated both on photographs and acne patients which can be used either in clinical research or by the dermatologist in his office.

Polystyrene-block-polybutadiene-block-poly[(methyl methacrylate)-stat-(methacrylic acid)] (SB(MA)) block copolymers incorporating acid-reactive functionalities in the last block have been synthesized and studied as modifiers for epoxy thermosets based on the diglycidyl ether of bisphenol A (DGEBA). Different techniques including differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM) have been used to demonstrate the effectiveness of the reaction-induced modification compared to that with the nonreactive or slowly reacting polystyrene-block-polybutadiene-block-poly[(methyl methacrylate)-stat-(tert-butyl methacrylate)] SB(MT) triblock copolymer. Morphological characteristics revealed by TEM indicate that SB(MT) and SB(MA) are both miscible with the epoxy prepolymer. The kinetics of grafting, network formation, and possibly phase separation were quantified from FT-IR, DSC, and cloud point investigations of DGEBA/DDS (4,4‘-diaminodiphenyl sulfone) as an epoxy−thermoset model system in the presence of poly[(methyl methacrylate)-stat-(methacrylic acid)] (HT121) or the block copolymers. The cure of the thermoset/block copolymer system has been explored using six different curing processes: 2-phenylimidazole (2-PI), alone or in the presence of methyltetrahydrophthalic anhydride (MTHPA) as comonomer, accelerated dicyandiamide (DICY), and three different diamines as comonomers without accelerator: 4,4‘-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA), 4,4‘-methylenedianiline (MDA), and DDS. The use of reactive block copolymers instead of nonreactive ones permits a better control of morphology. The materials' performances are analyzed in terms of transparency, glass transition temperature, Tg, and linear elastic mechanics at break (critical intensity factor, KIC).

Poly(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 disappears 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-methylimidazole (2-MI) and zinc acetylacetonate (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.

The local dynamics and the nonlinear rheology of soft colloidal pastes are shown to exhibit a remarkable universal behavior in terms of a unique microscopic time scale. This variable is associated with structural relaxation under the combined action of local frictional forces and elastic driving forces. These results establish a link between the local dynamics of pastes and their nonlinear flow behavior and provide a unified description of paste rheology.

Vitrimers are a class of polymers that flow when heated but are insoluble. Vitrimers are made of covalent networks with dynamic links and/or cross-links that undergo an associative exchange reaction. These dynamic cross-links 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 cross-links and backbone monomers strongly interact. Vitrimers featuring polyethylene (PE) as the backbone and dioxaborolane maleimide as the cross-linkable moiety were studied in both the molten and semicrystalline 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 microphase 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 semicrystalline 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.