دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و دوم شماره 3 (پیاپی 101، شهریور 1388)

Hemicellulose of Southern Yellow Pine wood spices was extracted by pressurized hot water at three different temperatures: 140°C, 155°C and 170°C. Compounding with PP (polypropylene) was performed by extrusion after preparing wood flour and sieving to determine its mesh size. The ratio of wood to polymer was 50:50 based on oven-dry weight of wood flour. All extraction treatments and control samples were compounded under two sets of conditions, without and with 2% MAPP as coupling agent. Injection molding was used to make tensile test samples (dogbone) from the pellets made by extrusion. Thermal properties of wood-plastic composites were studied by TGA and DSC while the thermal stability of pretreated wood flours, PP and MAPP were studied by TGA as well. The greater weight loss of wood materials was an indication that higher treatment temperature increases the extractability of hemicellulose. The removal of hemicellulose by extraction improves thermal stability of wood flour, especially for extraction at 170°C. Wood-plastic composites made from extracted fibers at 170°C showed the highest thermal stability. Coupling agent did not have a significant effect on thermal stability but it improved the degree of crystallinity of the composites.Surface roughness of wood fiber increased after treatment. Extraction of hemicellulose increased the degree of crystallinity but it was not significant except for samples from treated wood flour at 170°C and with MAPP.

Reaction to fire of fire-retarded rigid PUR foams and two types of metal faced rigid polyurethane foam core sandwich panel was evaluated by using cone calorimeter test method. The tests were carried out in various radiative heat fluxes from 15 to 75 kW/m2. The radiation rate effect on reaction to fire parameters, including time to ignition (TTI), peak of heat release rate (PRHR), total heat release (THR), average heat release rate (Av.RHR) and average heat of combustion (Av.EHC) was investigated. The phenomenon of char forming, when the foam is exposed to heat, leads to the formation of a protective layer on the surface of foam and hence no direct relation exists between Av.RHR and average specific mass loss rate (Av.Spec.MLR) of foam with increased radiation rate. In addition, the increased PRHR with foam density was also very smooth. The relation between TTI and heat flux was investigated for the foam and its corresponding correlation has been achieved with a specified density. Fire hazard assessment of foams and sandwitch panels was carried out by adopting Petrella and Richardson fire risk classification methods. The assessment results showed that rigid PUR foam and PUR sandwich panels may have a high contribution to bring the room to critical flashover condition, but the risk is intermediate from the viewpoint of fire endurance. The reasons of these risk levels are attributed to a very short TTI, relative high PRHR and an intermediate amount of THR. Decrease in foam density reduces heat release but it shows no significant effect on reducing flashover hazard.

The microstructure and thermal properties of the synthesized block copolymers and terpolymers via atom transfer radical polymerization (ATRP) were studied. The mole fraction of the blocks and chain length of the PVAc-b-PMA block copolymers were estimated with 1H NMR spectroscopy.Differential scanning calorimety measurement shows two different transitions at 36°C and 8°C assigned to PVAc and PMA blocks. Thermogravimetric analysis demonstrated the similar thermal degradation behavior of the block copolymers and a three-step degradation around 238, 295 and 345°C were attributed to dechlorination step and decomposition of the PVAc and PMA blocks. Study of thermal properties of PVAc-b-Poly(MA-co-MMA) copolymers indicated that glass transition temperature of second block increased from 49°C to 91°C with increase in MMA mole fraction from 0.358 to 0.733. The losses in the thermal degradation of block terpolymers were similar, and the higher thermal degredation temperature of MA unit has delayed the thermal degredation temperature of MMA in copolymer.

The effect of shear rate on dissolution of carbon dioxide in viscoelastic wheat flour matrix and cell density in a glass barrel twin screw extruder is investigated. It is found that by increasing the shear rate there will be a decrease in the required thermodynamic conditions and hence, it improves blowing agent dissolution and increases the cell density. Shear rate breaks up big bubbles and helps to better distribute the blowing agent in the matrix and hence it increases the cell density. Cell density decreases by dropping foam cooling rate. For a given cell density, a higher screw speed is needed, ifthe cooling rate is decreased.

Electrically conducting nanofiber mats of polyaniline/copolyacrylonitrile (PANI/ PAN) blend in NMP were successfully fabricated using electrospinning technique. Morphology and diameter of the nanofibers were studied by scanning electron microscope. Structural characteristics of the electrospun nanofibers were examined using differential scanning calorimetry, Fourier transform infrared spectroscopy and X-ray diffraction. Electrical of the electrospun mats was examined. The PANI/PAN blends containing up to the 30% PANI could be electrospun into the continuous fibrous structure, although pure PANI solution was not able to be electrospun into the fibrous structure. The formation of the hydrogen bonding between PAN copolymer and the PANI was predicted by the FT-IR spectra. DSC analysis showed only one Tg for the blend nanofibers, implying compatibility of the two components in the blend. X-ray diffraction showed that the crystalline peaks of PANI almost were disappeared in the PANI/PAN blends. The results of electrical conductivity measurements showed that the electrical conductivity of the blend increased with the increase in PANI content with percolation threshold of around 0.5 wt%.

Novel citric acid modified starch-carboxy methyl cellulose (CMC) -montmorillonite (MMT) bionanocomposite films were prepared by casting method and the effect of MMT on the structural and physical properties of the starch-CMC biocomposites was studied. X-ray diffraction test showed that strong polar interactions between the hydroxyl groups present in the starch and CMC chains and in the silicate layers led to intercalation of biopolymer chains into MMT layers galleries. The water vapor permeability decreased from 2.44 to 1.13×10-7 g/mhPa as the MMT percentage increased from 0 to 7%. At the level of 7% MMT, the composite films showed the highest ultimate tensile strength (27.55 MPa) and lowest strain-to-break (18.25%). The DSC tests showed that, the temperature position of the melting peak increased from 221.5°C to 243.0°C as MMT content increased from 1 to 7 %, but the glass transition decreased and gradually disappeared.

Anew and effective electrospinning method has been developed for producing aligned polymer nanofibers. The conventional electrospinning technique has been modified to fabricate nanofibers as uniaxially aligned array. The key to the success of this technique is the creation of a rotating jet by using a cylindrical collector in which the needle tip is located at its center. The unique advantage of this method among the current methods is the ability of apparatus to weave continuously nanofibers in uniaxially aligned form. Fibers produced by this method are well-aligned, with several meters in length, and can be spread over a large area. We have employed a voltage range of (6-16 kV), a collector diameter in the range of 20-50 cm and various concentrations of PAN solutions ranging from 15 wt% to 19 wt %. The electrospun nanofibers could be conveniently formed onto the surface of any thin substrate such as glass sampling plate for subsequent treatments and other applications. Therefore, the linear speed of electrospinning process is determined experimentally as a function of cylindrical collector diameter, polymer concentration and field potential difference.