Philippe Cassagnau

Grafting of polyethylene onto graphite oxide sheets: a comparison of two routes

Polyethylenes and azide-terminated polyethylenes can be converted to macroradicals and nitrenes, respectively followed by addition reactions onto the unsaturated system of graphite oxide sheets. For the first time, the addition of these macroradicals to graphite oxide sheets was compared by performing grafting reactions at 190 °C in 1,2,4-trichlorobenzene as a solvent and at 100 °C followed by a solvothermal reduction at 150 °C in a solvent mixture of 1,4-dioxane and 1,2-dichlorobenzene. Dispersion of PE coated graphite oxide in a DMF–heptane mixture was utilised to follow the introduction of polyethylene onto the GO sheet surface while the thermogravimetric analysis indicated the extent of this grafting. The grafting ratio was found to be in the 1.5 wt% range and despite this low grafting content, the amount of grafted PE was high enough to dramatically improve the affinity of GO with the heptane phase in the DMF–heptane (50/50 v/v) mixture. Polyethylene functionalised GO was imaged by scanning electron microscopy showing a significant difference in morphology between the two grafting paths. It was found that a higher level of grafting was obtained using a radical grafting reaction in the presence of benzoyl peroxide rather than the thermal cleavage of PE-N3 onto GO while a similar grafting content was obtained with the thermal cleavage of PE-N3 onto GO grafted trimethoxy(7-octen-1-yl)silane.

Dynamic behavior of crosslinked amphiphilic block copolymer nanofibers dispersed in liquid poly(ethylene oxide) below and above their glass transition temperature

Crosslinked polystyrene nanofibers were synthesized via RAFT-mediated aqueous emulsion polymerization and showed good shape stability in both the presence of solvent and at high temperature (typically above the Tg of polystyrene). The viscoelastic properties of their suspension in a low molar mass poly(ethylene glycol) matrix (PEG400) was studied as a function of the temperature, i.e., below the Tg of polystyrene at 25 °C, and above at 130 °C. Below Tg, the critical concentration ϕ* between the dilute and the semi-dilute regimes determined from the crossover of the scaling law on the zero shear viscosity for the dilute regime and the semi-dilute regime showed that a Brownian motion of the nanofibers was the dominant mechanism of relaxation according to the Doi–Edwards theory. Above Tg, the nanofibers are flexible and their Brownian dynamics did not obey the Doi–Edwards theory anymore. From the concentration dependence of the zero shear viscosity, it appeared that their dynamics obey the power laws for polymer chains in solution. Moreover, the flow activation energy at T > Tg was drastically dependent on the nanofiber concentrations whereas it was observed to be constant at T < Tg as expected from the Doi–Edwards theory. Finally, the flow activation energy became closer to the flow activation energy of polystyrene at concentrations above ϕ = 15%. Such qualitative agreement with the dynamics of flexible chains has never been observed before and might be fortuitous; it would therefore require further theoretical investigations.

Rheological and electrical properties of EVA copolymer filled with bamboo charcoal

The electrical and rheological properties of an ethylene vinyl acetate (EVA) copolymer filled with bamboo charcoal were investigated. The composites were prepared by melt process in an internal batch mixer. Size distribution analysis showed that d(50) and d(90) values of the bamboo charcoal particles are 12.7 and 40 μm, respectively, with a mean diameter of 22 μm. Scanning electron microscopy proved that the particles of bamboo charcoal present a rectangular shape. The electrical percolation threshold was observed at 0.18 volume fraction (35 wt%) of bamboo. Beyond the percolation threshold, a considerable increase in electrical properties was observed up to a limit value of 10-2 S/m. The rheological percolation was studied from different rheological models. As a result, the rheological percolation threshold was observed at 0.3 volume fraction (50 wt%) of bamboo charcoal contents. So, the electrical percolation occurs before the rheological percolation. This is principally due to the filler’s characteristics such as the specific surface area, the aspect ratio, and the surface properties. Finally, the bamboo charcoal confers high electrical properties to the EVA composite without inducing strong changes in its viscoelastic properties.

Viscosity and dynamics of nanorod (carbon nanotubes, cellulose whiskers, stiff polymers and polymer fibers) suspensions

The zero shear viscosity and the dynamic behaviors of different nanorod dispersions (carbon nanotubes (CNTs), cellulose whiskers, polymer nanofibers, crosslinked polymer nanofibers, and stiff polymers such as poly(γ-benzyl-α-l-glutamate) (PBLG)) were compared and discussed from literature data. Their Brownian dynamic behaviors have always been discussed in the frame of the Doi–Edwards theory. In agreement with this theory, the straight rigid rods (CNTs, cellulose whisker, polymer nanofibers) obey a master curve in the reduced viscosity (or rotary diffusivity) c power laws on viscosity (η0∝ φ3) and diffusivity (Dr∝ ϕ−2). On the contrary, stiff polymer chains and crosslinked polymer fibers at temperature above Tg exhibit different and two distinct dynamic behaviors. Despite their deviation from the ideal rigidity, surprisingly it can be noted that stiff polymers such as PBLG have been extremely used in the literature to verify the Doi–Edwards theory. Finally, flexible crosslinked chains at T > Tg, do not obey the Doi–Edwards theory, and their dynamics are close to the physics of polymer solutions in terms of power laws.

Morphology Development in Thermoplastic Vulcanizates (TPV)

The mechanisms of fragmentation and dispersion in molten polypropylene (PP) of several pre-crosslinked and plasticized ethylene-propylene-diene terpolymer (EPDM) networks were studied. The results highlight the importance of the gel fraction of the pre-crosslinked EPDMs on the quality of the dispersion of such networks. As a result, pre-crosslinked EPDM having a gel fraction below gEPDM=0.7 can be finely and homogeneously fragmented and dispersed in presence of PP. Nevertheless, contrarily to some theoretical model expectations, a partial fragmentation of the chemical networks was always observed even at very high crosslink density (gEPDM>0.7). It can be then admitted a collisioncoalescenceseparation type erosion mechanism of the EPDM domains. Finally, these results brought significant information on TPV morphology stabilization and their related mechanical properties.

18 – Reactive polymer processing and design of stable micro- and nanostructures

: Reactive processing is an alternative and promising method to produce micro- and nanostructured polymeric materials with controlled structure. Using this method it is possible to take advantage of the knowledge on reactive systems (polymerization, modification and blending) that have been conducted during processing. Thus, this chapter shows the potential of this technique to prepare micro- and nanostructured polymeric materials.