Patrick Navard

Reaction-induced phase separation mechanisms in modified thermosets

The reaction-induced phase separation in amorphous thermoplastic—modified epoxy systems was observed in situ using methods of different observation windows: small angle X-ray scattering (SAXS), light transmission (LT) and light scattering (LS). The transmission electron microscopy (TEM) technique was concurrently used to get direct representations of morphologies at different levels of the phase separation process. The selected systems were bisphenol-A diglycidylether cured with either 4,4′-diaminodiphenylsulfone or 4,4′-methylenebis[3-chloro,2,6-diethylaniline] in the presence of polyetherimide. The phase separation mechanisms involved were found to be greatly dependent on the initial modifier concentration and on the ratio of the phase separation rate with respect to the polymerization rate. Experimental results showed that, for modifier concentrations close to the critical fraction, the system was directly thrown into the unstable region, even at a low polymerization rate, and phase separation proceeded by spinodal demixing. On the other hand, for off-critical compositions the homogeneous solution demixed slowly via the nucleation and growth mechanism. In spite of the evolution of the phase diagram with reaction extent, the system remains in the metastable state whatever the cure temperature. The cure temperature has a strong effect on the extent of phase separation, since sooner or later vitrification of the thermoplastic-rich phase occurs and stops the evolution of morphologies. A post-cure allows the phase separation process to go further and sub-particles can be generated depending on the precure and post-cure temperatures.

Shear rheology and shear‐induced textures of a thermotropic copolyesteramide

The textures and the rheological behavior of a thermotropic copolyesteramide are characterized over five decades of shear rate. Three textures are observed, one thread‐like, one worm‐like characterized by a large density of defects and third, an ordered one characterized by a high flow orientation. The transitions from one texture to another depend on both shear rate and shear strain. Each texture corresponds to a given flow mechanism, similar to the ones found for lyotropics, and in qualitative agreement with the numerical predictions obtained from the rodlike solution model of Doi.

Deformation and breakup mechanisms of single drops during shear

The behavior of single drops of isotropic hydroxypropylcellulose solutions immersed in a polydimethylsiloxane matrix subjected to a constant shear rate was investigated using a rheo-optical technique. Our main results concern the nonstationary deformation and breakup mechanisms of drops characterized by large capillary numbers. The deformation of threads follows a pseudoaffine deformation for Ca/Cacrit larger than 2.5 and above a certain strain. The end pinching mechanism occurs at a specific scaled strain, which includes the initial drop size and the applied shear rate. Capillary instabilities develop when the thread diameter reaches a critical value, dr, inversely proportional to the applied shear rate and independent of the initial drop diameter. The time necessary for the total rupture of a thread depends on the initial drop diameter, the applied shear rate, and the critical thread diameter dr. The droplet size resulting from the final rupture of a thread is found to be half that of the critical size,...

Cellulose in NaOH–water based solvents: a review

The article is a critical review of all aspects of the dissolution of cellulose in NaOH-based aqueous solutions: from the background properties of the solvent itself, to the mechanisms of cellulose fibre swelling and dissolution, solution structure and properties and influence of additives and, finally, to the properties of various materials (fibres, films, aerogels, composites and interpenetrated networks) prepared from these solutions. A historical evolution of the research on this topic is presented. The pros and cons of NaOH-based aqueous solvent for cellulose are summarised and some prospects are suggested.

DCS Study of Smectic Phase Transitions

Abstract A previously described technique for measuring the order of phase transitionsl-3 using DSC has been applied to the liquid crystalline transitions found in TBBA. All of the values obtained were in agreement with those that have been measured by other techniques, and also with what would be anticipated from the nature of the transition. This study has shown that the technique is applicable to smectic-smectic transitions in liquid crystals.

The height of DSC phase transition peaks

A description of phase transitions as recorded by differential scanning calorimetry (DSC) is given. A new numberN is defined asN=h′/h, whereh is the height of a transition peak for a mass of samplem and a heating rate †p andh′ is the height of the same peak for a mass2m or a heating rate2†p.N is theoretically derived in the case of isothermal and nonisothermal first order phase transitions and of a second order phase transition. The equivalence of mass and heating rate is proved. An example of the possible use ofN is given.ZusammenfassungEine Beschreibung von Phasenübergängen, registriert durch DSC, wird gegeben. Eine neue ZahlN wird eingeführt und alsN=h′/h definiert, wobeih die Höhe des Übergangspeaks für die Probenmassem und die Aufheizgeschwindigkeit†p undh′ die Höhe des gleichen Peaks für die Masse2m oder die Aufheizgeschwindigkeit2†p sind.N ist im Falle isothermer und nichtisothermer Übergänge erster Ordnung und eines Phasenübergangs zweiter Ordnung theoretisch ableitbar. Die Äquivalenz von Masse und Aufheizgeschwindigkeit wurde überprüft. Ein Beispiel für eine mögliche Anwendung vonN wird angegeben.РезюмеПриведено описание ф азовых переходов, установленных метод ом ДСК. Новый параметрN определяет ся выражениемN=h′/h, гдеh-высота пика переход а для образца с массойm и скорости н агрева †n, аh′-высота т ого же самого образца с масс ой 2m и скорости нагрева 2†n. В еличинаN теоретичес ки выведена в случае изо термических и неизотермических фа зовых переходов перв ого порядка и фазового перехода вт орого порядка. Доказана экв ивалентность массы и скорости нагрева. Приведен при мер возможного использования парам етраN.

Small‐angle light scattering of polymer liquid crystals under shear flow

We presented some experimental results on flow small‐angle light scattering (‘‘flow SALS’’) of lyotropic polymer liquid crystals which involved real‐time, in situ, and simultaneous measurements of mechanical properties and small‐angle light scattering (SALS) of a complex structured fluid. The flow‐SALS studies were conducted under steady‐state Couette flow and during relaxation process occurring after the cessation of the steady‐state shear flow. We found the following universal behavior on the structure and properties for two kinds of the liquid crystal systems, i.e., poly(γ‐benzyl L‐glutamate) in m‐cresol and hydroxypropyl cellulose in water. (i) Upon increasing shear rate γ, the ‘‘polydomain’’ structure gradually tends to be transformed to the essentially ‘‘monodomain’’ texture through annihilation of the disclinations. On the top of this general tendency, we found more specific characteristic of the fluid systems with γ, (ii) at γ

Shear induced textures of thermotropic acetoxypropylcellulose

Acetoxypropylcellulose is a thermotropic cellulose derivative which is cholesteric from below room temperature up to 170 °C. Its textures during and after shear flow were studied by optical microscopy and small-angle light scattering (SALS) light scattering. The main features are similar to those occurring in hydroxypropylcellulose solutions, i.e., a polydomain texture evolving into a striated texture, as seen by optical microscopy. Elliptical or else four lobes with a streak are seen in the SALS patterns. Defects appearing as short dark lines not aligned with the flow direction are shown to be responsible for the existence of the four lobes of the SALS patterns. In addition these defects seem to be located where the band texture develops during relaxation.

Collision-induced dispersion of agglomerate suspensions in a shear flow

Agglomerates suspended in a polymer fluid have been known to disperse in a flow through two mechanisms-rupture and erosion. Using silica agglomerates, it is shown here that a third mechanism can occur, i.e., detachment of fragments due to agglomerate collision. This mechanism requires a much lower overall stress than erosion (and rupture, which occurs at even larger stresses than erosion). The fragment concentration produced by collision at a given time is proportional to the square of the applied shear rate.

Swelling kinetics of a polyelectrolyte gel in water and salt solutions. coexistence of swollen and collapsed phases

The swelling of polyelectrolyte gels in water and salt solutions is associated with the presence of up to three coexisting phases: a dry phase, a swollen phase, and a collapsed one due to the binding of metal ions to polyions. Swelling in pure water proceeds in two ways: a transition from the dry to swollen state, whose kinetics scales with the square of the initial gel size, and a further swelling that scales linearly with the initial gel size. We show that there exist various swelling paths depending on the gel interaction with metal ions and their concentration.