M. Narkis

Polymerization of aniline in the presence of DBSA in an aqueous dispersion

Abstract This paper describes a unique process of polymerization in an aqueous medium of an anilinium–dodecyl benzene sulfonic acid (DBSA) complex obtained by reacting aniline with DBSA prior to polymerization. The typical properties of the complex, appearing as fine needles, have been characterized and its polymerization behavior in the presence of DBSA upon addition of ammonium peroxydisulfate in an aqueous medium was investigated by visual color inspection, UV-VIS spectroscopy and pH measurements. The polymerization process was interrupted at different stages and polyaniline (PANI) powders were obtained by precipitation with methanol. The powders morphology was investigated using scanning electron microscopy (SEM) and their electrical conductivity was measured on compression molded strips. UV-VIS and pH measurements show that the average oxidation state of the formed PANI chains vary during polymerization from doped pernigraniline to doped emeraldine and correlate with the dispersions color. In the beginning of the polymerization course, the SEM studies show agglomerates consisting of spherical PANI particles. As polymerization proceeds, the voids among these particles are filled, forming a smooth surface of the PANI agglomerates. Simultaneously, the conductivity of the PANI powders increases with the polymerization time.

Sensors for liquids based on conductive immiscible polymer blends

Abstract The present paper discusses the application of conductive immiscible polymer blends as sensor materials for detection of organic liquid solvents and of their vapors. Immiscible polymer blends of high impact polystyrene (HIPS), ethylene vinyl acetate copolymer (EVA) and carbon black (CB), and compounds of EVA/CB have been used to produce a series of electrically conductive filaments by a capillary rheometer process. In these immiscible blends, HIPS serves as a matrix and EVA as the semi-crystalline dispersed phase. The enhancement of conductivity in these blends is due to the attraction of CB to EVA, giving rise to conductive networks. The dc electrical resistivity of extruded filaments, produced at different shear levels, is found to be sensitive to various organic liquid solvents. The shear rate, at which the filaments are produced, has an important effect on the HIPS/EVA/CB filaments sensitivity. The compositions studied were close to the double-percolation structure believed to perform best as sensor materials. The HIPS/EVA interface seems to play an important role in the sensing process. In some cases, liquid contact/drying cycling of filaments indicates stabilization of the sensitivity change, making the sensing process reversible. Liquid transport principles are an important basis for interpretation of the sensing behavior of immiscible blend-based filaments in contact with liquids.

Polyaniline–DBSA/polymer blends prepared via aqueous dispersions

Abstract Stable polyaniline–dodecyl benzene sulfonic acid (PANI–DBSA) aqueous dispersions were obtained by a unique method of aniline polymerization in the presence of DBSA, through an anilinium–DBSA complex appearing as solid needle-like particles, in an aqueous medium. The average size of the PANI primary particles, determined by small angle X-ray scattering (SAXS), is 18.7 nm. These primary particles form aggregates, which further cluster into ∼50 μm agglomerates. PANI–DBSA/polymer blends were obtained by mixing an aqueous PANI–DBSA dispersion with an aqueous emulsion of the matrix polymer, followed by water evaporation. These blends exhibit electrical conductivity already at a very low PANI–DBSA content (0.5 wt.%). The conductivity level of the various blends depends on the PANI content, on the surfactant present in the polymer matrix emulsion, and it is practically independent of the polymer matrix nature. Thus, a similar structuring mechanism prevails in these blends, irrespective of the polymer matrix (contrary to solution and melt blends). The PANI–DBSA particles strongly segregate within the polymer matrix, already in the combined aqueous dispersion, and upon drying, a very fine conductive network is formed. This strong segregation tendency leads to a conductive network formation already at low PANI–DBSA contents, thus generating the conductive blends.

Structure and tensile behavior of irradiation-and peroxide-crosslinked polyethylenes

Abstract Three grades of polyethylene differing mainly in their molecular weight were irradiation and peroxide crosslinked. Their gel content, degree of swelling, melting temperature, degree of crystallinity, and tensile properties were determined. The irradiation and peroxide crosslinking of the same polymers makes it possible to properly compare the effects of the two crosslinking methods. Upon irradiation the competition between crosslinking and chain scission reactions determines the level of the critical dose required to form the first gel and the magnitude of maximum attainable gel content. Crosslinking causes trapping of entanglements which then contribute to the effective network density determined by solvent swelling. crystalline polyethylene enhances the degree of crystallinity and crystal perfection while subsequent crystallization from the melt is hindered by the presence of crosslinks. The homogeneity of the crosslinked network, or distribution of crosslinks, depends on the crosslinking metho...

Electrically conductive composites based on epoxy resin with polyaniline-DBSA fillers

A conductive epoxy-anhydride system containing polyaniline (PANI)-dodecylbenzenesulfonic acid (DBSA) has been developed and characterized. Two forms of PANI-DBSA, powder and paste (containing excess DBSA), have shown that excess DBSA in the paste contributes to improved dispersion of PANI-DBSA in the resin, and thus a lower percolation threshold is found. Excess DBSA, however, retards the curing reaction of epoxy/hardener system, but this deficiency can be remedied by using higher accelerator concentrations. Similar trends were found by incorporation of PANI-DBSA coated mica particles, however, the PANI-DBSA engulfed mica particles result in a much lower percolation threshold compared to the PANI-DBSA powder or paste. SEM observation provides useful information for understanding the conductivity behavior of the conductive epoxy systems. Significantly different morphologies are observed for the PANI-DBSA powder and paste dispersed in the epoxy matrix.

Irradiation effects on polycaprolactone

Abstract The structure and some physical properties of γ-irradiated polycaprolactone (PCL), a semi-crystalline linear saturated polyester, were studied as function of the irradiation dose level. The critical dose level for gel formation is 26 Mrad and above this irradiation dose the number of scission events is similar to the number of crosslinking events. G.p.c. results show that the initial rather narrow molecular weight distribution gradually widens with increasing dose in the pre-gelation region. A significant difference between first and second d.s.c. scans of irradiated PCL is shown and explained. Scission and crosslinking reactions associated with the irradiation process occur preferentially in the non-ordered regions. Small irradiation doses, 2–5 Mrad, are shown to have a dramatic effect on the tensile elongation at break by converting ductile PCL samples into brittle materials.

Segregated structures in carbon black-containing immiscible polymer blends: HIPS/LLDPE systems

The structure/electrical resistivity relationship in CB-loaded immiscible HIPS/LLDPE blends was studied. Effects of CB content and location, dispersed polymer phase size and shape, dispersed phase viscosity, and processing procedures were examined. The elongated dispersed phase in CB-containing blends is essential for promoting conductivity in formulations prepared by melt mixing and compression molding. However, the same formulations proved highly resistive when injection-molded, due to orientation and excessive shearing.

Morphology of ternary immiscible polymer blends

Abstract Ternary blends consisting of thermoplastic and thermotropic immiscible polymers were studied. Both thermodynamic and kinetic considerations were found to affect their multiphase structure. Thermodynamics is expressed by means of spreading coefficients, whereas the kinetic effect is driven by the dispersed phase viscosity ratio. Some morphologies could be predicted, when both effects acted cooperatively. However, in cases where the effects were opposing, kinetics hindered the development of the expected structure; interpenetration between the two minor phases, rather than engulfing or separately dispersed morphology, took place. In cases where two relatively polar phases were dispersed in a nonpolar matrix (e.g., nylon and polycarbonate in polypropylene), the interaction between the two dispersed minor phases always existed due to their low interfacial tension. Spreading of one minor phase over another, rather than penetration, is the dominating mechanism of encapsulation in polymer blends, contra...

Polyaniline synthesis: influence of powder morphology on conductivity of solution cast blends with polystyrene

Abstract Synthesis of polyaniline (PANI) was performed under different conditions followed by dedoping, redoping with dodecyl benzene sulfonic acid (DBSA) and then blending with PS. The morphologies of the as-polymerized, doped and blended PANI were studied. The main polymerization stages seem to include: PANI oligomers assembling into nuclei, nuclei growing into primary particles (10 nm), primary particles assembling into aggregates (≈0.5 μm) and aggregates assembling into agglomerates (≈10 μm). The morphology of the as-polymerized PANI was found to be strongly related to the rate of oxidant addition, synthesis duration and synthesis temperature. This morphology dominates the effects of DBSA doping and dispersing the resulting PANI–DBSA in the matrix polymer. A fine PANI–DBSA powder with weakly bound aggregates is likely to disperse well in a solvent and hence promote the formation of the desired fine-network morphology and yield a low percolation threshold and high conductivity. Synthesis at a high oxidant addition rate, an excess of oxidant, a relatively high polymerization temperature and a short synthesis duration should diminish the tendency to form dense complex structures. These dense structures prevent efficient DBSA doping, deaggregation and the desired fine-network dispersion of PANI–DBSA in the blends.

On the "curiosity" of electrically conductive melt processed doped-polyaniline/polymer blends versus carbon-black/polymer compounds

Carbon black/polymer conductive compounds have been known and commercially used for many years, and their scientific background is quite well established and documented. In contrast, polyaniline/polymer blends (PANI/polymer) processible via dry (solvent-free) melt-shaping methods are still relatively unknown, insufficiently understood, and only a single commercial PANI/polymer blend for dry melt processing is presently commercially available (PANI/PVC, Zipperling, Germany). In this communication, a mechanism of PANI structuring in dry melt-processed PANI/polymer systems is suggested. In addition, the differences between these two conductive “fillers” (carbon black and PANI) in melt blending and processing, and the rules governing their mode of dispersion in the solidified polymer matrix, which determines the blends conductivity levels, are discussed. In future papers, detailed experimental evidence, supported by molecular modeling calculations, for the PANI/polymer systems, will be presented to support the ideas expressed in the present communication further. Conducting carbon blacks (CB) often consist of elongated aggregates (low aspect ratios) composed of very small (nanometric) primary particles sintered together. Upon melt blending with a polymer and processing, the CB may undergo deagglomeration, aggregate erosion and fracturing, and reagglomeration, resulting in either a uniform or more often a nonuniform distribution of the black particles [1, 2]. The level of the particle distribution nonuniformity varies and, as a rule, higher nonuniformity levels result in higher conductivity levels owing to the formation of conducting paths [3, 4]. For example, nonuniform distributions are formed in semicrystalline polymers, where carbon black particles are selectively located within the amorphous regions, and in polymers having low affinity to the surface of the carbon black particles [5]. Thus, in semicrystalline polymers and particularly in relatively nonpolar and low surface tension polymers, represented, for example, by polyethylene and polypropylene (PP), the tiny carbon black particles tend to segregate and even percolate, by forming conducting networks at extremely low content of the CB particles, e.g. 3 wt% Ketjenblack EC in PP [6], as in Fig. 1(a). Other parameters, such as melt-blending conditions including shear level and shear history, are less important within the practical acceptable regions of blending regarding the CB structuring and conductivity levels obtained. More uniform distributions of carbon black particles are obtained in amorphous polar polymers having higher surface tensions, similar to that of CB (~50 dyne/cm). Thus, by dispersing carbon black (Ketjenblack EC) particles in a soft, amorphous and polar random co-polyamide 6/6.9 (=poly[HN– (CH2)5 – CO] – co – [HN – (CH2)6 – NH – CO – (CH2)7 – CO]) [7,8], percolation has not been realized up to the