M. Mihailov

Electrically conductive polymer blends comprising polyaniline

SummaryElectroactive polymer blends comprising polyaniline (PANI) as conductive constituent and poly(methyl methacrylate) (PMMA), polystyrene (PS) and methyl methacrylate-butadiene-styrene (MBS) copolymer as a thermoplastic constituent (TC) were prepared by using various techniques:in situ by oxidative polymerization of aniline in aqueous dispersions of the TC; by, coagulating of latex of TC in the acidic dispersions wherein PANI has been preliminary obtained; and by dry blending. It was shown that highest conductivity values revealedin situ prepared PANI/PMMA blends, where the intermolecular interactions between the constituents were suggested to be stronger than in the other systems studied.

On the stability of natural rubber latex acidified by acetic acid and subsequent epoxidation by peracetic acid

Abstract Natural rubber (NR) latex having up to 54% dry rubber content (drc), stabilized by 1.65-5.0 parts of nonionic surfactant [(fatty alcohol)/(ethylene oxide) condensate type] per hundred of rubber can be acidified by acetic acid to pH ca 4, retaining good aggregative stability. It was shown that this stability depends on drc, surfactant level and final pH. The stabilized latex of medium-to-high drc (25–48%) could be successfully subjected to epoxidation at moderate temperature, using preformed peracetic acid. The epoxidation proceeds rapidly and effectively, leading to pure ENR of up to 60–70 mol% epoxide content as checked by i.r. and 1 H-NMR. By swelling test and sol-gel fractionation, it was shown that the epoxidation reduces the solubility of rubber in nonpolar solvents and increases its sensitivity to polar solvents. The long lasting and highly acidic medium of reaction lead to increase in the gel content of ENR samples compared with the original NR.

Structural modifications of hexagonal crystalline polyoxymethylene. Infra-red and X-ray study

Abstract Infra-red spectra and X-ray diagrams of polyoxymethylene samples prepared in various ways, of trioxanedioxolane copolymers and of a polyoxymethylene-polydioxolane blend were obtained. With hexagonal crystalline polyoxymethylene (HCPOM), it is possible to prepare two forms, A and B, differing in their infrared spectra; with copolymers, the maximum attainable content of form A rapidly decreases with increasing content of oxyethylene units in the chain. Form A could not be prepared in the polyoxymethylene-polydioxolane blend. Infra-red spectra of the two forms, A and B, of HCPOM differ in the vibrational interactions of neighbouring chains; these interactions are large in form A and small in form B. The possible structures of the A and B forms of HCPOM are discussed.

Vibrational spectra of hexagonal crystalline polyoxymethylene

Abstract Infrared and Raman spectra of two hexagonal crystalline forms of polyoxymethylene were measured, and the characteristic infrared bands have been determined. The Raman spectra of these two forms do not differ. The vibrational spectra of both forms can be explained by the occurrence of defects in the helical structure of one of them.

Temperature changes of infra-red bands of poly(methyl methacrylate)

Abstract Studies were made of the changes with temperature of infra-red bands of poly(methyl methacrylate). of its deuterated analogues and of methyl acetate. It was found that the large changes of the maximum intensities of bands of antisymmetric vibrations of the ester methyl group are caused by rotation about the oxygen-methyl bond.

Dependence of the conformation of nascent poly(oxymethylene) on the conditions of cationic polymerization of trioxane in solution

Abstract The conformational structure of nascent poly(oxymethylene) (POM) obtained by cationic polymerization of trioxane in nitrobenzene was investigated by i.r. spectroscopy. It was found that the conformational order of this POM depends considerably on the conditions of preparation. Under conditions of simultaneous polymerization and crystallization, “A” polymer, with long sequences of monomer units in regular G conformation, is obtained. Under conditions of successive polymerization and crystallization, the formation of conformational defects in the helical POM chains is favoured. Then, depending on the supersaturation, we obtained either (a) POM of “B” form with short sequences of monomer units in G conformation, or (b) POM of a mixed type, the i.r. spectrum being describable as a superposition of “A” and “B” spectra. The results indicate that, at comparatively high catalyst concentration, the thermodynamical approach for regulation of supermolecular structure of polymers can be applied successfully for regulation of the conformational order of nascent POM also. At lower concentration of active centres in the polymerizing system, kinetic factors affect considerably the conformational structure of nascent POM.

Microstructure of ethylene-(vinyl acetate) copolymers prepared by emulsion copolymerization

Abstract Ethylene-(vinyl acetate) (EVA) copolymers, obtained by emulsion copolymerization at low and medium pressure (1–6 MPa), have been examined for composition, compositional heterogeneity, character of branching and distribution of comonomer units along the polymer chains. The copolymers were characterized in respect of compositional and configurational heterogeneity. The analysis of the triad populations suggests the existence of two parts in the polymer chains. The comonomer units in one part show a tendency to alternate, while the ethylene units in the other part show enhanced tendency to form blocks. Investigation of branching of EVA emulsion copolymers performed by methods insensitive to the state of the macromolecules in solution (1H- and 13C-NMR) showed that long-chain and short-chain alkyl branches were present in copolymers irrespective of composition and molecular weight. The long-chain alkyl branching is less than in the bulk copolymer obtained at high temperature and high pressure. The microstructure of EVA emulsion copolymers should be taken into account when interpreting results on solution properties of macromolecules as well as their behaviour in the solid state and in the melt.

Morphology of nascent polyoxymethylene obtained by polymerization of trioxane in nitrobenzene solution

Abstract The morphology of the crystal structure of nascent polyoxymethylene (POM) obtained by cationic polymerization of trioxane in nitrobenzene was investigated by optical and electron microscopy. It was found that the morphology depends on temperature and conversion of polymerization at constant initial monomeric concentration (1.5 mol/l). Morphological changes were observed near the equilibrium concentration of trioxane in nitrobenzene (1.5 mol/l, 35°C). At lower temperatures (10–30°C), thin lamellar crystallites, organized in spherulites, are formed by successive polymerization and crystallization. At higher temperatures (40–90°C), thick crystallites organized in oval structures are formed by simultaneous polymerization and crystallization. Peculiarities of crystal formation of POM, obtained by simultaneous and successive polymerization and crystallization, are discussed.

Polyethylene–polyethylene blends modified with an additive of isotactic polypropylene

The blends of very high molecular weight high-density polyethylene (VHMW–HDPE) or ultrahigh molecular weight high-denisity polyethylene (UHMW–HDPE) and low-density polyethylene with normal molecular weight (LDPE) in equal or prevailing concentrations of the first component have excellent mechanical properties but very high viscosity in a molten state. The present investigations are a continuation of the fact, established by us for the first time, that the addition of a third polymer, isotactic polypropylene (I-PP), at optimal low concentrations increases the melt flowability of these systems without considerable deterioration of their very good mechanical properties in the solid semicrystalline state. The comparison between the thermomechanical behavior and the sorption in organic media of the systems leads to the supposition that an uninterrupted physical network from the VHMW–HDPE has been formed.

Oxidation of natural rubber and its derivatives prepared by epoxidation and subsequent hydrobromination

Abstract The oxidations in air of epoxidized natural rubber (ENR) and its hydrobrominated product were studied and compared with the case of natural rubber (NR). By following the i.r. characteristic bands at 580, 840, 870, 1720, 1760 and 3200–3600 cm−1 during the oxidation, it was demonstrated that the hydrobrominated product of ENR is the most unstable both at room temperature and at 150°. NR oxidizes more quickly and to a greater extent than ENR. The oxidation proceeds through consumption of double bonds, leading to the formation of new oxygen-containing functional groups (hydroperoxide, hydroxyl, carbonyl, carboxyl…), changing the structure and properties of the materials. The absence of associated hydroxyl groups in the OH spectral region of hydrobrominated ENR products of low epoxide contents suggests the presence of randomly distributed epoxide groups in the polymer and indicates the random character of the epoxidation process of NR in latex.