SEM/TEM Characterization of Styrene-graft-Polyethylene Copolymers Obtained Under Supercritical Conditions

MARTINI, RAQUEL EVANGELINA; YANEZ, MARIA JULIA; BARBOSA, SILVIA ELENA

Rosario, Argentina

Congreso; 10th Inter-American Congress on Electron Microscopy (CIASEM 2009) y 1st Congress of the Argentine Society of Microscopy (SAMIC 2009); 2009

Sociedad Argentina de Microscopía

In order to improve final properties of low cost polymers, like polyolefins, different methodologies are used. Typical ones are blending with other polymers (blends), adding different fillers (composites) or chemical modifications (copolymers). In copolymers, the introduction of new functional groups in a main chain can modified some properties like hydrophilicity, dyeability, permeability, antimicrobial activity, adhesion, etc. On the other hand, the use of near and supercritical fluids, as solvent aid in polymer processing and polymer chemistry is growing up. They can have gas-like diffusivities, which have important implications for reaction kinetics, as well as liquid-like densities, allowing solvation of many compounds. In this work, the copolymerization of styrene onto polyethylene (PE), using near critical n-alkanes as reaction media is proposed. The reaction is a Friedel-Crafts alkylation and the catalyst was aluminum chloride (AlCl3). By this reaction, a hydrocarbon chain can be chemically bonded to the styrene ring through an aromatic electrophilic substitution. The principal advantage of this method of reaction is that the high molecular weight PE, is solubilized by alkanes at near critical conditions, and then, the polymer chains are more available to react than in a melt blend. The final morphology of these copolymers was study by SEM and TEM. SEM technique was used on samples prepared in different ways. Initially, a general observation was performed on the reaction products melted and cryogenically fractured. This analysis permitted the identification of two kinds of particles, ones adhered and fractured with the matrix (PP) and others with typical core shell appearance. For a more accurate analysis a second type of samples were prepared subject fracture surfaces to tetrahydrofuran etching (under sonication for PS extraction) and comparing the results with original ones. It is observed an increase in the amount of copolymer at the surface in the extracted product, but not holes. This indicates that the particles are copolymer ones, not pure PS becase they could not be extracted. The mobility due to the combined effects of sonication and solvent action can also produce the coalescence of the particles, and then their size is increased. The effectiveness of the reaction was proved by SEM techniques and also was verified by C13-NMR. However, in order to an ulterior morfological analysis, TEM studies on ultramicrotomed specimens stained with ruthenium tetraoxide (RuO4) were carried out.   It can be observed the formation of concentric bands from a central amorphous region, characteristic of the PE used in this study, and corresponds to a deformation of the lamellas in spherulites. From this study  it can be inferred that the grafted PS incorporates partly into amorphous layers of polyethylene and partly into separate polystyrene regions placed alongside the stacks of PE crystalline lamellae.