دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و یکم شماره 5 (پیاپی 97، آذر و دی 1387)

The mechanism and the parameters affecting the matrix-fibril morphology in melt spun polyblend fibers are discussed. The properties of polymeric blends depend considerably on their final morphologies. Blending of two miscible polymers, normally gives rise to formation of a single-phase morphology while blending of two immiscible polymers can create a two or three (in case of using compatibilizer) phase morphology. Disperse phase in matrix phase could be formed in different shapes with playing important parts in the final properties of the blends. Though, formation of multiphase morphology may result in deterioration of physical properties, by choosing proper components and polymer blend contents, these properties can be improved. In immiscible polymeric blends depending on polymer processing methods various morphologies can take shape. However, in melt spinning process due to intensive extensional flow at take-up region the most convenient conditions for creation of matrix-fibril morphology can be achieved. According to various reports, extensional fields are acting more effectively than shear fields to shape the matrix-fibril morphology. In addition, extensional flows can reduce fibril's diameter while enhancing fibrils lengths evenness. However, reducing the ratio of disperse phase to matrix phase viscosity (ηd/ηm) increases the probability of fibrils formation, in such a way, that even in the fields with zero extension force still fibrils formation phenomenon can occur. The finest fibril's diameter in viscosities ratio of around 1 (ηd/ηm=1) was observed. Using compatibilizing agents also can reduce fibril's diameter.On the effect of viscoelasticity parameter of two-component system some researchers have reported that; when matrix phase elasticity is higher than the disperse phase, the morphology of the fibrils still remain unchanged. Changing the blend ratio of the two-phase polymers may also have effect in the formation of the number of fibrils and their diameters.

The effects of rubber mixing conditions, such as the changes in material loading procedure and rotor speed on die swell behavior were studied with the aid of an experimental small extruder. Also the changes were examined on Mooney viscosity of the compounds, simultaneously. It is shown that these changes in mixing condition considerably affect the Mooney viscosity of the compounds rather than die swell. Two control parameters of Mooney viscosity and die swell, i.e. "rubber breakdown" and "filler dispersion in mixing process" were identified and it may be concluded that the role of thesecond agent on die swell behavior is not very significant. Finally, by feeding the final compounds in an industrial extruder, it is observed that these processing variables on die swell and extrusion behavior of compound play little effect compared to the formulation variables. The experimental die swell parameter, however could predict the extrusion behavior much better than Mooney viscosity parameter.

Tricalcium phosphate as hydroxyapatite is used as a suspension stabilizer in styrene polymerization process. Particle size of TCP plays an essential role in the particles’ size distribution and geometrical form of polystyrene products. As the particle size of TCP is reduced, there will be much better chance to engulf the styrene particles. The higher the number of TCP particles surrounding each styrene particle, the lesser will be their tendency to form a large particle after collision. Therefore, there will be higher percentages of spherical polystyrene with small particle size and narrower size distribution in the product. Experimental results have indicated that the addition of sodium hexametaphosphate (SHMP) to the reaction mixture of lime and phosphoric acid, after drying the product by spray dryer, lead to decrease the size of TCP particles from ca. 5 μm (without SHMP) to ca. 1.5 μm (with SHMP). In this study, the role of TCP containing SHMP as polymer suspension stabilizer and consequently the beads size of polystyrene is investigated in laboratory scale. The results show that despite addition of SHMP to the reaction mixture of lime and phosphoric acid decreases the TCP particles size and the mean bead size of the product of polystyrene become larger than the product prepared by TCP without SHMP.

Suspension polymerization processes are significantly important in polymer bead formation. One of the important factors for bead size control in these processes is variation in surface tension. In this research, the prediction of optimum concentration of a surface active agent was investigated on the basis of the experimental surface tension diagrams. The experimental data show that the desired surface tension for a specific size distribution of expandable polystyrene beads is 64.5 mN/m. In spite of non-linear dependence of surface tension on concentration, the expandable polystyrene bead size distribution histograms indicate that the predicted concentration of sodium alkylbenzenesulfonate by the experimental model match its found optimum value.

In order to investigate the role of multi-wall carbon nanotubes (MWCNTs) on fracture mechanism of epoxy nanocomposites, a series of tensile standard specimens reinforced with different carbon nanotube contents (0, 0.3, 0.6 and 1 wt%) were produced. The fracture surfaces of the produced nanocomposites were evaluated using scanning electron microscope (SEM). The results show that the surface fracture of epoxy nanocomposites comprised of three regions, i.e. mirror, transition and final propagation zones. The extension of all zones depends strongly on curing agent as well asMWCNTs content. The mirror zone is disappeared as curing agent and MWCNTs content increases, while the transition zone depends on the nucleation rate of secondary microcrack. The pattern of final propagation zone becomes coarser as MWCNTs are added to epoxy system.

Ethylene was homopolymerized over Ziegler-Natta catalyst and the homopolymerization was modeled using moment equations. Mechanism was modeled according to five different reaction centers of catalyst. For each center, there are different reaction rate coefficients; therefore the final product of each center would be expected to be different. Modeling results showed good conformity to the experimental results. According to the results obtained, the molecular weight distribution of each active center follows a Schultz-Flory distribution. However, the molecular weight distribution ofpolymer produced is much broader than a Schultz-Flory distribution. Besides, the order of polymerization with regards to monomer concentration is different for each center and it is higher than unity. Moreover, the catalyst active centers deteriorate in the presence of hydrogen and consequently catalyst yield drops. Nevertheless, polymerization kinetics is not affected much by hydrogen. Hydrogen also reduces polymer molecular weight since it is a strong transfer agent in olefin polymerizations. Notwithstanding, it does not affect polydispersity index. Finally, by increasing the cocatalyst concentration the activity of active centers is not changed, while it lessens the molecular weight as a transfer agent.