From Monomers to Self-Assembled Monolayers: The Evolution of Molecular Mobility with Structural Confinements

The effect of structural constriction on molecular mobility is investigated by broadband dielectric spectroscopy (BDS) within three types of molecular arrangements: monomers, oligomers and self-assembled monolayers (SAMs). While disordered monomers exhibit a variety of cooperative and local relaxation processes, the constrained nanodomains of oligomers and highly ordered structure of monolayers exhibit much hindered local molecular fluctuations. Particularly, in SAMs, motions of the silane headgroups are totally prevented whereas the polar endgroups forming the monolayer canopy show only one cooperative relaxation process. This latter molecular fluctuation is, for the first time, observed independently from other overlapping dielectric signals. Numerous electrostatic interactions among those dipolar endgroups are responsible for the strong cooperativity and heterogeneity of the canopy relaxation process. Our data analyses also revealed that the bulkiness of dipolar endgroups can disrupt the organization of the monolayer canopy thus increasing their ability to fluctuate as temperature is increased

Molecular Flexibility of Self-Assembled Systems: Effects of Building Block Polarity

The self-assembly of molecules is considered as one of the most promising approach to conceive sophisticated and dynamic materials with a molecular-level control of structure, morphology and property. However, recent advancements highlight the need to ameliorate the understanding of molecular flexibility within supramolecules. Rod-shaped molecules were selected for this study. Composition of molecules were varied according to the number and location of dipoles. Three groups of building blocks were chosen: nonpolar one with no strong dipole, monofunctional one containing one dipole at one end, and bifunctional one composed of one dipole at each end. Self-assembly processes of these molecules were then carefully investigated and compared. The absence of strong dipoles within molecules was found to confer structural flexibility to the final supramolecules. Within aggregates, molecules are highly mobile and able to undergo several structural reconfigurations. In contrast, more stable supramolecules are prepared when molecules contain one or more polar extremities. Despite this constrained environment, molecular segments can locally move, thus revealing a ubiquitous degree of freedom for molecular motions. This research work aims at highlighting the flexibility of self-assembled systems, and also bring to light the potential of local molecular motions as an encouraging way to functionalize constrained supramolecules.L'auto-assemblage moléculaire est désormais considéré comme l'une des approches les plus prometteuses pour la conception de matériaux à nanostructures complexes. Cependant, les récents progrès effectués ont aussi amené la nécessité d'améliorer la compréhension des mécanismes régissant la flexibilité des molécules. Il a ainsi été décidé d'étudier l'effet de la composition des briques moléculaires sur leur processus d'assemblage, et la labilité structurale des systèmes assemblés. De manière à pouvoir comparer rigoureusement les résultats expérimentaux, un seule morphologie de briques moléculaires, en forme de "bâtonnet", a été choisie et trois groupes distincts de molécules ont été sélectionnés : non-polaires, qui ne possèdent pas de dipôle important, monofonctionelles, lesquelles possèdent une terminaison polaire et une seconde non-polaire, et bifonctionelles, constituées d'un groupe polaire à chaque extrémité séparés par une chaine non-polaire Ainsi, l'influence des groupements dipolaires sur la labilité de la nanostructure finale du matériau a pu être explorée. Cette étude permet ainsi de mettre en exergue la remarquable diversité des flexibilités structurales qui peuvent être rencontrées dans les systèmes auto-assemblés. De plus, elle dévoile le potentiel des mouvements moléculaires locaux en tant qu'approche encourageante pour fonctionnaliser des structures auto-assemblées supposées inertes ou contraintes

Combining Flash DSC, DSC and broadband dielectric spectroscopy to determine fragility

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Reducing the Gap between the Activation Energy Measured in the Liquid and the Glassy States by Adding a Plasticizer to Polylactide

International audienceThe kinetic fragility of a glass-forming liquid is an important parameter to describe its molecular mobility. In most polymers, the kinetic fragility index obtained from the glassy state by thermally stimulated depolarization current is lower than the one determined in the liquid-like state by dielectric relaxation spectroscopy, as shown in this work for neat polylactide (PLA). When PLA is plasticized to different extents, the fragility calculated in the liquid-like state progressively decreases, until approaching the value of fragility calculated from the glass, which on the other hand remains constant with plasticization. Using the cooperative rearranging region (CRR) concept, it is shown that the decrease of the fragility in the liquid-like state is concomitant with a decrease of the cooperativity length. By splitting the fragility calculated in the liquid, in two contributions: volume and energetic, respectively, dependent and independent on cooperativity, we observed that the slope of the fragility plot in the glass is equivalent to the energetic contribution of the fragility in the liquid. It is then deduced that the difference between the slopes of the relaxation time dependence calculated in both glass and liquid is an indicator of the cooperative character of the segmental relaxation when transiting from liquid to glass. As the main structural consequence of plasticization lies in the decrease of interchain weak bonds, it is assumed that these bonds drive the size of the CRR. In contrast, the dynamics in the glass are independent on plasticization structural effects

From a Three-Phase Model to a Continuous Description of Molecular Mobility in Semicrystalline Poly(hydroxybutyrate-co-hydroxyvalerate)

In most cases, a three-phase model consisting of crystalline domains surrounded by rigid amorphous areas dispersed in the mobile amorphous phase is required and eventually sufficient to accurately describe the microstructure of semicrystalline polymers. This work shows that the microstructure developed by poly(hydroxybutyrate-co-hydroxyvalerate) by cold crystallization is better described by a complex two-phase model in which the boundaries of the crystalline domains and the surrounding amorphous environment form a \u201ccontinuum of mobility\u201d. This concept can be extended to a wide range of semicrystalline polymers. When the crystalline and the amorphous phases are strongly coupled, the rigid and mobile amorphous fractions are hardly fractionated, and the whole noncrystalline phase should be rather depicted as continuum with a broad distribution of the relaxation times associated with the glass transition. Such a depiction of the amorphous phase allows taking into account any modification of the mobility landscape with time, which can be evidenced as the progressive spreading of the relaxation functions. From a practical point of view, the rigidification of the \u201ccontinuum of mobility\u201d upon storage in conditions of cold crystallization could be considered as a cause of the progressive embrittlement sometimes observed in semicrystalline materials during physical aging and would be explained by a redistribution of the relaxation times translated in terms of relaxation temperatures

Reducing the Gap between the Activation Energy Measured in the Liquid and the Glassy States by Adding a Plasticizer to Polylactide

The kinetic fragility of a glass-forming liquid is an important parameter to describe its molecular mobility. In most polymers, the kinetic fragility index obtained from the glassy state by thermally stimulated depolarization current is lower than the one determined in the liquid-like state by dielectric relaxation spectroscopy, as shown in this work for neat polylactide (PLA). When PLA is plasticized to different extents, the fragility calculated in the liquid-like state progressively decreases, until approaching the value of fragility calculated from the glass, which on the other hand remains constant with plasticization. Using the cooperative rearranging region (CRR) concept, it is shown that the decrease of the fragility in the liquid-like state is concomitant with a decrease of the cooperativity length. By splitting the fragility calculated in the liquid, in two contributions: volume and energetic, respectively, dependent and independent on cooperativity, we observed that the slope of the fragility plot in the glass is equivalent to the energetic contribution of the fragility in the liquid. It is then deduced that the difference between the slopes of the relaxation time dependence calculated in both glass and liquid is an indicator of the cooperative character of the segmental relaxation when transiting from liquid to glass. As the main structural consequence of plasticization lies in the decrease of interchain weak bonds, it is assumed that these bonds drive the size of the CRR. In contrast, the dynamics in the glass are independent on plasticization structural effects

Thermally stimulated depolarization currents and dielectric spectroscopy analysis of plasticized amorphous polylactide

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Thermally stimulated depolarization currents and dielectric spectroscopy analysis of plasticized amorphous polylactide

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Thermal growth of organic supramolecular crystals with screw dislocations

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