All issues / Volume 1 (2007) / Issue 4 (April)
Editorial Corner. (---------------------)
Graft polymerization of glycidyl methacrylate onto the pore surface of polyacrylamide macroporous gel was implemented in DMSO-aqueous solution using diperiodatocuprate(III) complexes as an initiator. The grafting densities up to 410% were achieved. The graft polymerization was confirmed by gravimetrical methods and FTIR. The graft polymerization of polymer inside the pores of the macroporous gel resulted in increased flow resistance through the gel matrix. The distribution of grafted polymer on the gel pore surface material was studied by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). CLSM is an alternative method for studying morphology of gel surface with grafted polymer having the advantages over the SEM allowing to investigate the distribution of grafted polymer inside the hydrogel in a native hydrated state. The microscopic techniques demonstrated uneven distribution of the grafted polymer inside the gel pores as a result of initiating the graft polymerization by insoluble initiator deposited on the pore surface.
A new homologous series of thermally stable thermotropic liquid crystalline poly(azomethine-ether)s based on thiazole moiety were synthesized by solution polycondensation of 4,4`-diformyl-α,ω-diphenoxyalkanes, I–IV or 4,4`-diformyl-2,2`-dimethoxy-α,ω-diphenoxyalkanes V–VIII with the new bis(2-aminothiazole) monomer X. A model compound XI was synthesized from X with benzaldehyde and characterized by elemental and spectral analyses. The inherent viscosities of the resulting polymers were in the range 0.43–1.34 dI/g. All the poly(azomethine-ether)s were insoluble in common organic solvents but dissolved completely in concentrated H2SO4 and formic acid. The mesomorphic properties of these polymers were studied as a function of the diphenoxyalkane space length. Their thermotropic liquid crystalline properties were examined by DSC and optical polarizing microscopy and demonstrated that the resulting polymers form nematic mesophases over wide temperature ranges. The thermogravimetric analyses of those polymers were evaluated by TGA and DSC measurements and correlated to their structural units. X-ray analysis showed that polymers having some degree of crystallinity in the region 2θ = 5–60°. In addition, the morphological properties of selected examples were tested by scanning electron microscopy.
The influence of a typical prooxidative additive, cobalt stearate, on the thermal stability, degradation kinetics and lifetime of low-density polyethylene (LDPE) was investigated using non-isothermal thermogravimetric analysis (TGA) in both nitrogen and air atmosphere. The derivative thermogravimetric (DTG) curves indicate single stage and multistage decomposition process in nitrogen and air atmosphere respectively. The kinetic parameters of degradation were evaluated using the Flynn–Wall-Ozawa iso-conversion technique. The apparent activation energies for decomposition have been calculated for degradation under nitrogen atmosphere. The lifetime of LDPE (time for 5% mass loss) was estimated to be 8.2·1026 min in nitrogen and was found to decrease dramatically with increase in the concentration of cobalt stearate thereby revealing its pro-oxidative ability. Studies indicated that the service/process temperature also has a strong influence on the lifetime of all the formulations investigated. The effect of cobalt stearate on the air oven aging behavior of LDPE at two different temperatures (70°C and 100°C) was also investigated to demonstrate the pro-oxidative nature of cobalt stearate.
Thin microcellular polycarbonate (PC) sheets have been prepared by compression molding. Tensile tests were performed on microcellular PC sheets prepared under different conditions, and results show that foaming time plays the main role on the tensile property. The cellular structure prepared in this process has been shown to have a significant effect on the corresponding tensile property of microcellular PC foams. For these foams, Single Blend Model can be used to predict the effect of the relative density on the relative strength, but the experimental data of the tensile modulus is much lower than that predicted by Square-Relationship Model. The effect of cell size on the tensile property of microcellular PC foams was also investigated, and found that the cell size can affect the tensile property, which is different from the result of Kumar. In order to make the tensile property predictable, some mathematical formulas were also simulated.
The influence of adhesion to the mould wall on the released strain of a highly filled anhydride cured epoxy resin (EP), which was hardened in an aluminium mould under constrained and unconstrained condition, was investigated. The shrinkage-induced strain was measured by fibre optical sensing technique. Fibre Bragg Grating (FBG) sensors were embedded into the curing EP placed in a cylindrical mould cavity. The cure-induced strain signals were detected in both, vertical and horizontal directions, during isothermal curing at 75 °C for 1000 minutes. A huge difference in the strain signal of both directions could be detected for the different adhesion conditions. Under non-adhering condition the horizontal and vertical strain-time traces were practically identical resulting in a compressive strain at the end of about 3200 ppm, which is a proof of free or isotropic shrinking. However, under constrained condition the horizontal shrinkage in the EP was prevented due to its adhesion to the mould wall. So, the curing material shrunk preferably in vertical direction. This resulted in much higher released compressive strain signals in vertical (10430 ppm) than in horizontal (2230 ppm) direction. The constrained cured EP resins are under inner stresses. Qualitative information on the residual stress state in the molding was deduced by exploiting the birefringence of the EP.
As the main of thermosensitive polymeric materials, poly(N-isopropylacrylamide) (PNIPAM) undergoes volume changes in response to external temperature changes. While in some potential applications, a fast response rate is needed. In this article, effect of the initiator on thermosensitivity of PNIPAM hydrogels was carried out. The characterization of the resulting hydrogels was gravimetrically measured, and the result displays that with increasing the initiator dosage, much faster deswelling rate and much faster shrinking/swelling dynamic response are obtained.
Chlorination treatment of a thermoplastic styrene-butadiene-styrene rubber (SBS) with a 3 wt% solution of trichloroisocyanuric acid (TCI) in methyl ethyl ketone (MEK) introduces chlorinated and oxidized moieties on the rubber surface which increase its surface energy and produces surface microroughness. Consequently adhesion properties, evaluated by T-peel strength measurements in chlorinated SBS/solvent based-polyurethane adhesive/leather joints, are enhanced. In this study, two solvent-based polychloroprene adhesives (PCP0 and PCP30R) have been considered as an alternative to the commonly used solvent-based polyurethane adhesive (PU). A thermoreactive phenolic resin was added to one of the polychloroprene adhesive formulations (PCP30R). This tackifier resin favors chlorination of the adhesive and reinforces the interface between the chlorinated adhesive and the chlorinated rubber surface. Besides, PCP30R adhesive does not need adhesive reactivation and considerable high T-peel strength value (5.7±0.3 kN/m) was obtained. Elimination of the reactivation process implies a considerable improvement of the manufacturing process in the footwear industry.
Effects of three nucleating agents concluding talc, sodium benzoate (SB) and an ionomer (Ion., Na+) on crystallization of poly(ethylene terephthalate) (PET) were studied by differential scanning calorimetry (DSC) and polarized optical microscope (POM), the parameters of crystallization kinetics were obtained through Avrami and Ozawa equations. The fold surface free energy σe of pure PET and PET/nucleating agent blends were calculated by Hoffman-Lauritzen theory. The results indicate that the three kinds of nucleating agents increase the crystallization rate constant through promoting their nucleating effect for PET crystallization, among which SB is the best one with the same content. The crystallization mode of PET might shift from three-dimensional growth to two-dimensional growth by the addition of the nucleating agents. The values of σe of PET/nucleating agent blends are much less than that of pure PET, and PET/SB (99:1) blend has the least value of σe (18.2 mJ/m2). The conclusion based on Hoffman theory is similar to the analysis by Avrami and Ozawa equations.