Iranian Polymer Journal
Year:2004 | Volume:13 | Issue:1|Papers Count:10

Effects of uniaxial pressure parallel to the current and temperature on resistance of highly dedoped polypyrrole samples were investigated. Polypyrrole samples were prepared electrochemically at 0.87 V constant potential from acetonitrile solution in presence of perchlorate anion as dopant on the gold sputtered glass as working electrode. Polypyrrole samples were dedoped chemically in NH3 for one hour. Because of high resistance of polymer samples, two-point probe technique could be applied for resistance measurements. Conductance of these highly dedoped polymers increases with pressure and temperature exponentially according to the Mott-variable range hopping theory (VRH). Change in the efficient number of states and also volume of the polymer due to the pressure can cause to increase the density of states near the Fermi level. Pressure should bring about an increase of orbital overlap, especially between the electron-rich of the neighbouring molecules. This increase of electron orbital overlap should be expected to increase the ease of inter-molecular electron transfer, by decreasing the distance and energy barriers to diffusion and tunneling and by increasing the chain-chain areas of contact, thereby increasing the probability for electron transfer. So, the number of states that electron can hop from it increases. With respect to the presence of numerous vacancies in highly dedoped polymer samples, a modified VRH model is introduced to fit the observed data based on the decreasing in polymer volume and increasing in efficient number of states that electron can hop from it.

The effect of dynamic cross-linking on process ability, mechanical properties, water absorption, aging process and morphology of rice husk powder (RHP)/polystyrene (PS)/styrene butadiene rubber (SBR) blends were studied. The blends were prepared using a Bra bender plasticoder at 170°C and rotor speed of 50 rpm. Dynamic vulcanization resulted in an increased in the tensile strength, impact strength and a decrease in percentage of elongation at break of RHP/PS/SBR composites. It was also found that dynamic cross-linking give composites with better resistance to aging process and swelling in water through better dispersion of matrices and filler/matrix adhesion.

The toxic and volatile nature of many organic solvents that are widely used in organic synthesis have  posed a serious threat to the environment. One of the recently developed methods is to use water as solvent. However, its use is limited by the low solubility of organic compounds. One of the most important strategies to overcome this limitation of water is the utilization of organic cosolvents. Thus, as new polymeric cosolvents poly (vinyl amine) (PVA) and poly (allylamine) (PAA) were prepared and used as cosolvent/catalyst systems in the regioselective ring opening of epoxide by azide ion in water at room temperature to give azidohydrine in excellent yield under mild reaction conditions. These polymeric catalysts can be used several times without any loss in their activity.

4-(4¢-tert-Butylphenyl)-1,2,4-triazolidine-3,5-dione, 4-(4¢-tert-butylphenyl)urazole (TBPTD) was prepared from 4-tert-butylbenzoic acid in 5 steps. TBPTD was reacted with n-propylisocyanate in dry N,N-dimethylformamide and the resulting bisurea derivative was obtained in high yield and was finally used as a model for polymerization reaction. The step-growth polymerization reactions of monomer TBPTD with hexamethylene diioscyanate, isophorone diioscyanate and toluene-2,4-diioscyanate were performed in N,N-dimethylacetamide solution in the presence of pyridine, triethylamine or dibutyltin dilurate as a catalyst. The step-growth reactions were performed under microwave irradiation as well as high temperature solution polymerization. The polycondensation proceeded rapidly, compared with the conventional solution polycondensation and was almost completed within 10-18 min. The resulting novel polyureas have an inherent viscosity (hinh) in a range of 0.025-0.306 dL/g in DMF or sulphuric acid at 25°C. These new polyureas were characterized by IR, 1H NMR, elemental analysis and TGA. Some physical properties and structural characterization of these novel polyureas are reported.

Biopolymers were produced by using Alcaligenes eutrophus microorganisms. The kind of substrates such as glucose, sucrose, butyric, pentanoic and propanoic acids used as carbon sources affect the accumulation and composition of biopolymers in the cell. The maximum polymer production (0.97 g/L) was obtained when 2.0 g glucose/100 mL was used as a substrate. Accumulation of polymer in the cell was 28% (wt/wt) when pentanoic acid was used as substrate. Characterization of the biopolymers was done by using different spectroscopic and thermal techniques such as FTIR, DSC, 1H NMR and GC/MS. Glass transition temperature (Tg) and melting temperature (Tm) of the polymers changed between 4.71-11°C and 147-179.8°C, respectively. Enthalpy of fusion (∆H) varied between 43.82 and 96.70 J/g, Biodegradability of PHB was tested in biological medium and in phosphate buffer solution.

Some polypyrrole free standing films or membranes doped with different electrolytes (aromatic sulphonated) were prepared from aqueous monomer solutions in order to investigate the effect of thermal treatment on their electrical conductivity. It was found that the nature of counterion has the most important effect in determining of the electrical conductivity and stability of conductivity when polymer exposed to high temperatures. However, mild heating can improve conductivity of polypyrrole conducting polymers. In order to carry out thermal studies on conductivity and measure the thermal stability of the ppy film, a variety of anions were tried. It was found that polypyrrole doped with aromatic sulphonates (mainly benzene or naphthalene sulphonates) as counterions produce flexible, and smooth films that can be readily detached from working electrode as membrane with good mechanical properties. Other counterions such as inorganics (e.g. Cl-, SO42-), aliphatic surfactants (e.g., DS), and polymeric (PVS) produced films that is either impossible to remove from working electrode or give brittle, and rough films.

The stratification behaviour of different thermoplastic acrylic copolymers with one epoxy resin and its different curing agents were studied. Xylene and methyl isobutyl ketone were used as solvents of the system. Films with different thicknesses (200, 400, and 800 micron) were applied with a film applicator. Also the effect of epoxy gel time with different curing agents was tested. The results showed that coating stratification was thickness dependent, so that in higher thicknesses and the same compositions more stratified films was obtained. In addition, reduction of gel time of epoxy composition showed untraceable effect on stratification, although long duration of curing has important effect on stratification. The reaction rate of curing agent regardless of gel time is also important. According to the obtained data, it can be concluded that stratification mechanism is not diffusion dominated and it maybe convection dominated. The effect of substrates (aluminium and glass) was investigated for the coatings, and it is found that normally glass substrate yield more stratified coatings. The IR-ATR spectra showed that low surface free energy component could not be found in the bottom layer of stratified films and most of it migrated to the top surface. Our findings to some extent support the proposed theory of minimum surface free energy of the system, but the thickness dependency, reported in our paper cannot interpreted by current theories.

Waterborne epoxy resins were prepared by reacting epoxy resins with different molecular weights (360,900,1700,3200) with diethanolamine. The resins were then neutralized with 10% by wt acetic acid and emulsified in water. The resins were applied onto steel panels by a cathodic electrodeposition procedure. The deposition behaviour of the reins was studied using the effect of voltage-time (at constant current) and current density-time (at constant voltage) variations on the determined coulombic efficiency and the throwing power.