Marie-Claude Heuzey

Core−Shell Structured PEO-Chitosan Nanofibers by Coaxial Electrospinning

Core-shell structured PEO-chitosan nanofibers have been produced using a coaxial electrospinning setup. PEO and chitosan solutions, both in an aqueous acetic acid solvent, were used as the inner (core) and outer (shell) layer, respectively. Uniform-sized defect-free nanofibers of 150-190 nm diameter were produced. In addition, hollow nanofibers could be obtained subsequent to PEO washing of the membranes. The core-shell nanostructure and existence of chitosan on the shell layer were confirmed by TEM images obtained before and after washing the PEO content with water. The presence of chitosan on the surface of the composite nanofibers was further supported by XPS studies. The chitosan and PEO compositions in the nanofibrous mats were determined by TGA analysis, which were similar to their ratio in the feed solutions. The local compositional homogeneity of the membranes and the efficiency of the washing step to remove PEO were also verified by FTIR. In addition, DSC and XRD were used to characterize the crystalline structure and morphology of the co-electrospun nonwoven mats. The prepared coaxial nanofibers (hollow and solid) have several potential applications due to the presence of chitosan on their outer surfaces.

Factors influencing encapsulation behavior in composite droplet-type polymer blends

In this study the influence of the molecular weight of the dispersed phase components on encapsulation effects in the composite droplet phase was examined for high density polyethylene (HDPE)/PS/PMMA ternary blends. Three different blends composed of various PS and PMMA materials dispersed in an HDPE matrix were prepared using an internal mixer. The morphology was studied by light and electron microscopy. Current models used for predicting encapsulation effects and composite droplet formation in ternary systems (based on static interfacial tension) predict in all cases that PS will encapsulate the PMMA. However, in one case, an unexpected encapsulation of PS by PMMA was observed. It was found that arguments based on the effect of viscosity ratio or the absolute viscosity of the different dispersed phases do not explain that discrepancy. In addition, the reversal of that latter composite droplet morphology from PMMA encapsulating PS to PS encapsulating PMMA was observed upon annealing treatment. Considering all the above, a conceptual model was developed to predict encapsulation effects in composite droplet type systems based on the use of a dynamic interfacial tension (i.e. taking into account the elasticity of the polymer components). Calculations based on the dynamic interfacial tension model, using elasticities based on constant shear stress, were able to account for all of the observed encapsulation effects in this study.

Solvent‐Cast Three‐Dimensional Printing of Multifunctional Microsystems

The solvent-cast direct-write fabrication of microstructures is shown using a thermoplastic polymer solution ink. The method employs the robotically controlled microextrusion of a filament combined with a rapid solvent evaporation. Upon drying, the increased rigidity of the extruded filament enables the creation of complex freeform 3D shapes.

Enhanced dispersion of cellulose nanocrystals in melt-processed polylactide-based nanocomposites

Dispersion and distribution of cellulose nanocrystals (CNC) in a thermoplastic matrix is one of the most important issues in the development of CNC-based high performance composites. During melt processing, agglomeration of CNC is prone to occur due to poor polymer wetting on the hydrophilic CNC surface and to strong particle–particle interactions. Because of the high temperature and intensive mixing involved in melt-processing, degradation of the CNC is also possible. To avoid these problems, solvent mixing followed by solvent casting is the main processing route used in the majority of studies on polymer–CNC composites. In this work, we have explored a novel two-step process where solvent-mixing and melt-mixing were carried out sequentially to improve the overall dispersion of the CNC. The first step consisted in forming a CNC suspension into a polyethylene oxide (PEO) aqueous solution. In the second step, water was removed by freeze-drying to form a water-free well dispersed PEO/CNC mixture. The final step consisted in melt-mixing the PEO/CNC mixture into PLA for the preparation of the composites. PEO and PLA are known to be miscible in certain molecular weight and composition ranges, thus leading to a composite where the CNC particles are well dispersed into a homogeneous mixture of PLA and PEO. Two different PEO molecular weights were investigated in this study, and several formulations were compared under the same processing conditions. Direct blending of CNC and molten PLA was also carried out for comparison purposes. CNC particles tended to agglomerate during blending but the agglomerates were smaller and their number was considerably decreased when the PEO content increased in the formulation. At the highest PEO/CNC ratio, no agglomerates were observed. Thermomechanical and rheological properties of the PLA-based nanocomposites were also investigated.

Modeling fiber interactions in semiconcentrated fiber suspensions

A set of rheological equations is developed for semiconcentrated suspensions of rigid fibers in a Newtonian fluid taking into account hydrodynamic and fiber-fiber interactions. The force generated by the fiber interactions is modeled using a linear hydrodynamic friction coefficient proportional to the relative velocity at the contact point, and weighted by the probability for contacts to occur. The equation of evolution of the second-order orientation tensor, containing advection and diffusion terms due to fiber interactions, is derived to predict fiber orientation under flow. The well known fourth-order orientation tensor, related to the hydrodynamic contribution, and a newly proposed fourth-order interaction tensor are used to evaluate the total stress in the composite. A linear and a quadratic closure approximation are proposed to describe the fourth-order interaction tensor. Results are presented using the quadratic form, which is found to be more accurate than the linear one. The model is shown to describe well simple shear data of suspensions of glass fibers in a Newtonian polybutene. Moreover, fiber orientation and the average number of contacts per fiber are predicted. The newly proposed interaction coefficient varies with fiber orientation, which appears to be realistic.

Rheological behavior of fiber-filled model suspensions: Effect of fiber flexibility

The rheological behavior of model suspensions consisting of a Newtonian silicone oil and fibers of different flexibilities have been investigated in steady and transient shear flows. Various fiber suspensions have been prepared to examine the effect of flexibility parameters (stiffness and aspect ratio) as well as the role of interactions in the semi-dilute and semi-concentrated regimes. The viscous and elastic properties of the fiber suspensions are shown to be strongly enhanced by fiber flexibility. With increasing flexibility, the suspensions also exhibit enhanced shear-thinning and the shear rate for the onset of shear-thinning decreases. In start-up flow and especially at low shear rate, the suspensions show large stress overshoots, and under reversal flow delayed overshoots. The magnitude of the overshoot increases as fiber flexibility gets larger. This last effect has been attributed to less orientation in the flow direction during the forward flow for the more flexible fibers. Finally, the effects...

Morphological and rheological properties of PET/clay nanocomposites

This work investigates the effects of clay chemistry and concentration on the morphology and rheology of polyethylene terephthalate (PET)/clay nanocomposites. The complex viscosity of the PET nanocomposites exhibited a more solid-like behavior, in contrast to the matrix that had a frequency-independent viscosity. In addition, at high frequencies where the behavior of the matrix should be dominant, a lower complex viscosity of the nanocomposites was observed due to PET degradation in the presence of the organoclays. The high-frequency data were used to estimate the matrix degradation using the Maron–Pierce equation. The apparent molecular weight of the PET matrix was found to decrease from 65 kg/mol for the neat PET to 30 kg/mol for a PET nanocomposite containing 8 wt% Cloisite®; 30B. The apparent yield stress in the nanocomposites was determined using the Herschel–Bulkley model. Yield stress increased with the level of exfoliation and clay concentration, from ∼0 to 166 Pa when the clay concentration increased from 2 to 8 wt%.

Innovative thermoplastic chitosan obtained by thermo-mechanical mixing with polyol plasticizers

Chitosan shows a degradation temperature lower than its melting point, which prevents its development in several applications. One way to overcome this issue is the plasticization of the carbohydrate. In this work plasticized chitosan was prepared by a thermo-mechanical kneading approach. The effects of different non-volatile polyol plasticizers (glycerol, xylitol and sorbitol) were investigated. The microstructure and morphology were determined using FTIR, XRD, TEM and SEM in order to understand the plasticization mechanism. Sorbitol, which is the highest molecular weight polyol used, resulted in plasticized chitosan with the highest thermal, mechanical and rheological properties. On the other hand, the sample plasticized with glycerol, the lowest molecular weight polyol, had the most important amorphous phase content and the lowest thermal, mechanical and rheological properties. Also, when the polyol content increased in the formulation, the plasticized chitosan was more amorphous and consequently its processability easier, while its properties decreased.

Properties of polylactide inks for solvent-cast printing of three-dimensional freeform microstructures.

Solvent-cast printing is a highly versatile microfabrication technique that can be used to construct various geometries such as filaments, towers, scaffolds, and freeform circular spirals by the robotic deposition of a polymer solution ink onto a moving stage. In this work, we have performed a comprehensive characterization of the solvent-cast printing process using polylactide (PLA) solutions by analyzing the flow behavior of the solutions, the solvent evaporation kinetics, and the effect of process-related parameters on the crystallization of the extruded filaments. Rotational rheometry at low to moderate shear rates showed a nearly Newtonian behavior of the PLA solutions, while capillary flow analysis based on process-related data indicated shear thinning at high shear rates. Solvent vaporization tests suggested that the internal diffusion of the solvent through the filaments controlled the solvent removal of the extrudates. Different kinds of three-dimensional (3D) structures including a layer-by-layer tower, nine-layer scaffold, and freeform spiral were fabricated, and a processing map was given to show the proper ranges of process-related parameters (i.e., polymer content, applied pressure, nozzle diameter, and robot velocity) for the different geometries. The results of differential scanning calorimetry revealed that slow solvent evaporation could increase the ability of PLA to complete its crystallization process during the filament drying stage. The method developed here offers a new perspective for manufacturing complex structures from polymer solutions and provides guidelines to optimize the various parameters for 3D geometry fabrication.

Interfacial and rheological properties of PLA/PBAT and PLA/PBSA blends and their morphological stability under shear flow

Blends of 75 wt. % amorphous polylactide (PLA) with 25 wt. % poly[(butylene succinate)-co-adipate] (PBSA) and poly[(butylene adipate)-co-terephthalate] were separately prepared using an internal batch mixer. The morphology and viscoelastic properties of these two blends were analyzed and compared. Annealing did not cause any pronounced morphology changes nor a subsequent modification of the viscoelastic behavior for both blends. However, applying a shear for 20 min at a rate of 0.05 s−1 induced significant droplet coalescence in both blends, although the changes in the viscoelastic response were more prominent in the PLA/PBSA blend. It was also shown that applying a shear for 10 and 20 min at a rate of 0.2 s−1 caused a slight droplet coalescence and minor changes in the viscoelastic behavior of both blends. Moreover, the Palierne model was used to calculate the interfacial tensions between the blend components. It was also utilized to estimate the droplet size after applying annealing and shearing.