دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و ششم شماره 2 (پیاپی 124، خرداد و تیر 1392)

Studies were carried out on the effect of adding different percentages of montmorillonite (3، 5، 7 and 9% of starch weight) on the physical properties of potato starch-MMT nanocomposites. Heat resistance and mechanical properties of films were measured by differential scanning calorimetry (DSC) and tensile test. Nanoparticles distribution in polymer matrix was investigated using X-ray diffraction test (XRD). For investigation of water vapor resistance of film samples، moisture sorption and water vapor permeability (WVP) were measured. The results showed that the distribution of nanoparticles in the polymer matrix was exfoliated. WVP in pure starch films was 2. 62×10-7 g/mhPa and with the addition of 9% MMT it was reduced to 1. 43×10-7 g/mhPa. With the addition of nanoclay from zero to 9%، the ultimate tensile strength of nanocomposite samples was increased from 5. 9 to 6. 63 MPa and strain-to-break was decreased from 34. 82 to 26. 83%. But the rising trend was not significant for nanocomposite samples containing low concentrations of nanoclay (0-7%). The main reasons for the enhancement of mechanical properties due to the addition of nanoclay were to establish hydrogen bonding between polymer chains and clay layers، filling the empty spaces and increase the crystalline domains. Investigation of thermal resistance of nanocomposite samples showed that they have higher thermal resistance and melting point in comparison with pure starch films. With the addition of nanoclay from zero to 9%، the melting point of film samples was increased from 218 to 232. 1°C. With the addition of nanoclay، probably the mobility of amylopectin chains decreased and crystalline domains increased. Also، with increasing nanoclay content، the glass transition temperature of nanocomposite samples was increased. This result corresponded to shrinkage in free volume and thus reduction in the polymer chains mobility in amorphous regions.

Recently، using nanoparticles in polymeric blend have been considered by many researchers a new epoch for generation of materials to meet different requirements in various industries such as car، sport، military، structure and electronic. The transesterification reaction in polyester blends during melt mixing plays an important role in the components compatibility، and the ultimate properties of the blend affected by this reaction. In this study the transesterification reaction in the blend of poly (butylene terephthalate) (PBT) /polycarbonate (PC) was studied at the presence of three commercial organic modified montmorillonite namely Cloisite 30B، Cloisite 20A and Cloisite 15A. The main difference among these nanoparticles is their surface chemical structures and initial gallery heights. Fourier transform infrared spectroscopy (FTIR) and small angel X-ray scattering (SAXS) analysis showed that tranesterification reaction was improved at the presence of Cloisite 20A and Cloisite 15A and an intercalation morphology was obtained. While in the samples containing Cloisite 30B a thermal degradation occurred and initial gallery of the nanoparticles was increased. Dynamical mechanical thermal analysis results revealed that by addition of nanoclay to polymer blend، the glass transition of polymers draw on to each other which means more compatibility has been obtained and transestrification reaction has been improved at presence of the nanoparticles. Scanning electron microscope (SEM) micrographs showed droplet-matrix morphology for PC/PBT: 70/30 ratio and co-continuous for PC/PBT: 50/50. By incorporation of nanoparticles the finer morphology was obtained in PC/PBT: 70/30 and co- continuous morphology changed to micro co-continuous in PC/PBT: 50/50.

In order to achieve dramatic improvements in the performance of rubber materials، attempts were made to develop carbon nanotube (CNT) -reinforced rubber composites. The maleic anhydride (MAH) modification of EPDM is an interesting way of compatibilizing the EPDM rubber with CNT. Novel ternary nanocomposites were prepared based on EPDM/EPDM grafted maleic anhydride (EPDM-g-MAH) blend composition with various concentrations (0-7 phr) of multi-wall carbon nanotube (MWCNT) on a two-roll mill. The effect of EPDM-g-MAH as a compatibilizer and MWCNT concentration were investigated on cure characteristics، mechanical، morphological and rheological properties of nanocomposites. The microstructure of nanocomposites has been characterized using scanning electron microscopy (SEM). At the same time the rheological behavior has been evaluated by a rubber processing analyzer (RPA). It was found that the cure time (t90) and scorch time (t5) decreased while maximum torque (MH) and minimum torque (ML) of the compatibilized composites were increased with increasing MWCNT loading which was consistent with the swelling data. It is observed that by increasing MWCNT loading the swelling index in solvent was decreased. This can be related to good interactions between carbon nanotube and EPDM matrix in presence of EPDM-g-MAH compatibilizer. The fracture surface study indicated that compatibilizer facilitated a homogenous dispersion of MWCNTs inside the matrix. On the other hand، carbon nanotubes in matrix caused roughness of the fractured surface compared with uncompatibilized samples. The mechanical properties such as tensile strength and elongation-at-break of compatibilized EPDM/MWCNT were higher than those of uncompatibilized nanocomposites. In addition، due to increasing MWCNT content the rheological properties such as storage modulus (G'') increased with respect to angular frequency while the complex viscosity decreased.

A high performance passenger tire tread compound was optimized for its mechanical، dynamical and thermal properties. A reference compound was based on a blend of SBR and BR، sulfur and other ingredients without accelerator، carbon black and aromatic oil. The effects of CBS/TMTD and TBBS/TMTD as accelerator systems were studied with different quantities and the best accelerator system was chosen. Then، the blends of N330 and N550 carbon blacks were added in different quantities and the properties of these samples were studied to determine the best carbon black blend. Finally، the effect of different quantities of aromatic oil was investigated and the optimized quantity of aromatic oil and the final properties of tire tread compound were defined. The mechanical and dynamical tests were carried out on appropriate samples to determine tensile strength، elongation-at-break، fatigue-to-failure، abrasion resistance، hardness، resilience، dynamical-mechanical properties and temperature rise due to the heat build-up. The results showed that the compound containing 0. 8 phr CBS، 0. 7 phr TMTD، 40 phr N330،20 phr N550 and 15 phr aromatic oils demonstrated the best properties.

Polypropylene/starch nanocomposites compatibilized with PP-g-MA or EVA، with values 0، 3 and 5 wt% of modified clay (Cloisite 30B) were prepared by melt intercalation technique and the mechanical properties، morphology and degradation of nanocomposites were investigated. Tensile test results showed that in the presence of 5 wt% nanoclay، values of tensile strength، elastic modulus and elongation-at-break are 15. 5 MPa، 10. 2 MPa and 4. 2% for PP-g-MA compatibilized blends and 10. 0 MPa، 7. 0 MPa and 19. 4% for EVA compatibilized blends، respectively. Also، the presence of 5 wt% nanoclay increased 9. 1 % of tensile strength; 70 % of elastic modulus and decreased 49% of elongation-at-break for PP-g-MA compatibilized blends and increased 40. 8% of tensile strength; 27. 3% of elastic modulus and 49% of elongation-at-break for EVA compatibilized blends. The reason for these properties improvement could be proper dispersion and physical network consisting of silicate layers in the polymer matrix. SAXS patterns and TEM images confirmed that the prepared nanocomposites were exfoliated. SEM microscopic images show droplet morphology that is indicative of incompatibility of two polymers. Oxidative degradation of samples exposed to UV light was studied using FTIR spectroscopy. The weight loss percentage of MA-5، EVA-3 and EVA-5 specimens after 120 days of exposure in activated sludge resulted in 20. 7، 28. 4 and 37. 9%، respectively. These results indicate that biodegradation of EVA compatibilized blends is much higher with increasing nanoclay which has improved the biodegradablility of the blends.

One of the most important factors that affect tire friction is surface roughness، which determines the size of the real contact area، real pressure distribution on the contact interface، and scales of mechanical engagement between viscoelastic rubber and a rough substrate. The need to predict coefficient of friction (COF) for rubber on rough surfaces for applications such as traction of tires on the road surfaces led to some physical models such as Heinrich-Kluppel’s model. The current study examines the applicability of the Heinrich-Kluppel model، using different viscoelastic representations، in numerical simulations of COF for rubber، and its agreement with the experimental results. For this purpose، roughness characteristics of the surfaces and viscoelastic properties of rubber were measured by fractal analysis and dynamic-mechanical-thermal analysis (DMTA)، respectively. These data were employed in the numerical code to simulate COF for a rubber sample. The model was also modified by replacing the Zener viscoelastic representation in the original model with the generalized Maxwell viscoelastic representation. On the other hand، COF for rubber was measured on the same rough surface (different sand-papers) by an in-house friction tester، and results were compared with the numerical results. It was shown that computer simulation could predict the load and speed dependence of rubber friction very well. The application of the generalized Maxwell model improved agreement between the numerical and experimental results for high sliding speeds where the Zener viscoelastic model failed to predict the right trend in variation of COF with speed. This speed range was matched with the sliding velocities in the footprint of tire under rolling conditions.

Nowadays، blending of natural and synthetic polymers has attracted a considerable interest in tissue engineering for reaching desirable properties. In this study، blend nanofibrous scaffolds were electrospun from poly (caprolactone) /chitosan/poly (vinyl alcohol) blend solutions in 2:1: 1. 5 mass ratio. SEM results showed that uniform and bead-less nanofibers with 136±21nm average diameter were obtained from electrospun poly (caprolactone) /chitosan/poly (vinyl alcohol) blend solutions. Tensile strength test and contact angle measurement demonstrated that addition of PCL led to higher mechanical and physical properties of the resulting web. As a result، addition of PCL in the blend supported the web physical integration in aqueous media of body. Fibroblast cells were cultured on nanofibrous webs to investigate web biocompatibility. The biological results showed appropriate biological properties for the produced nanofibrous webs. Tissue engineered scaffolds were prepared by seeding mesenchymal stem cells on half of the scaffolds. In vivo studies were carried out to investigate the effect of scaffolds on healing of excisional cutting wounds created on the back of the rats. From pathological studies، wounds treated with scaffolds (both acellular and cell-seeded ones) showed better healing compared with control specimens. The results showed that wounds acquired complete healing by the scaffolds within 10-day of post operation، while control wounds achieved healing after 15 days of post surgery operation. It is notable that، cell-seeded-scaffold-treated-wounds showed best healing process due to stem cells ability for collagen regeneration.