دو ماهنامه علوم و تکنولوژی پلیمر
سال بیست و پنجم شماره 2 (پیاپی 118، خرداد و تیر 1391)

Phenolic resins have been widely used for selective high technology applications due to their excellent ablative properties, structural integrity and thermal stability that make them appropriate for thermal insulation materials, wood products industry, coatings, moulding compounds and composite materials. Polymer layered silicate nanocomposites based on montmorillonite (MMT) have attracted a great deal of attention because of enhanced properties in mechanical, thermal, barrier and clarity properties without a significant increase in density, which is not possible with conventional fillers. Phenolic resin/montmorillonite (Cloisite 15A) nanocomposite was prepared by a combined route of solution blending and in-situ polymerization. The optimized conditions for preparation of nanocomposite were achieved by evaluation of various processing parameters (mechanical mixer, high speed disperser and high energy ultrasonic source), mixing time (0.5, 1, 3, 10, 24, 48, 72, and 96 h) and different amounts of montmorillonite (5 and 10 weight percents of montmorillonite relative to resol). X-Ray Diffractometer and thermal gravimetric analyzer were used accordingly to show the degree of nanodispersions of organomontmorillonite in polymeric matrix and the effect of nanofiller on thermal stability of nanocomposite with respect to neat resol. The results of high energy ultrasonic source show that a nanocomposite of phenolic resin with 5 wt% montmorillonite displays the best dispersion of clay layers. Thermal stability of nanocomposite was increased by 27% in comparison with neat resol

Anew model is proposed in this research to calculate the longitudinal strength of unidirectional E-glass reinforced polymer composites exposed to sulfuric acid environment, using a micromechanics model. In the proposed method, it is assumed that the residual strength of the degraded composites under acidic environment can be calculated by knowing the degraded strength properties of the constituent materials. In order to measure the properties of the degraded epoxy resins and E-glass fibers, corrosion tests are performed on them when exposed to 5% sulfuric acid for different immersion times. Acid penetration in composites is a time consuming phenomenon. Thus, before the acid reaches to inside region of composites, the degraded composites cross-section can be divided to two regions, namely intact and degraded regions. In this stage, a simple model is suggested to estimate the acid penetration depth in the degraded composites. Based on the corrosion mechanisms of glass fibers, the energy dispersive x-ray microanalysis (EDX) results of different points of composites cross-section are used to estimate the acid penetration depth in composites. Both the acid penetration depth model and micromechanics model are used to calculate the longitudinal strength of intact and degraded regions for different immersion times. Thus, the longitudinal strength of degraded composites can be calculated. Moreover, some similar unidirectional E-glass/ epoxy composites exposed to sulfuric acid for different immersion times are tested to measure the longitudinal strength of them. The theoretical results are in good agreements with those experimentally measured.

Modified fumed silica, grafted by hydroxyl-terminated polybutadiene (HTPB) via toluene diisocyanate (TDI) bridging, was incorporated in styrene butadiene rubber (SBR) as reinforcing filler. The main objective of this work was to study the effect of the modification method on final properties of the rubber compounds. As compared to unmodified nanosilica, scanning electron microscopy (SEM) represented nano dispersion of modified fumed silica particles in SBR compound. The results of mechanical uniaxial tensile test revealed the significant reinforcement property of HTPB-surface grafted nanosilica. This strong effect is attributed not only to nanodispersion of silica particles but also due to participation of HTPB double ligands in sulfur curing system of SBR and chemical linkage between rubber matrix and silica particles. The study of dynamic mechanical properties under temperature scan (linear viscoelasticity) displayed strong interaction between the rubber matrix and modified silica particles and it again reaffirmed the efficient reinforcement effect of this filler. Also, dynamic mechanical properties under strain scan (non-linear viscoelasticity) exhibited filler network formation at low filler content for this modified nanosilica. Surface modification of nanosilica by HTPB grafting intensified the mechanical reinforcement of the filler; meanwhile, it reduced the intensity of non-linear behavior by weakening the effect of filler network break down (Payne effect) relative to that of unmodified silica. Therefore, this method of silica surface modification could be a novel method in improvement of the final properties of filled SBR by nanosilica.

Single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) were melt-blended with polyvinylidene fluoride (PVDF). FT-IR technique revealed that SWCNT has insignificant effect on the crystalline structure of PVDF and at 3wt% modification only 14% beta crystal is formed. This observation was accounted for by the chair structure of SWCNT. In MWCNT nancomposites it was found that at 3wt% modification 39% beta crystal is formed. Although equal in weight percentage, the number MWCNTs is less in the corresponding nanocomposites due to their multi-layer structure. This was attributed to the higher surface area and zigzag structure of the surface of MWCNT surface. This results in a typical crystal nucleation of polymer for nanotubes and promotion of beta phase formation. In agreement with FTIR measurements, the WXRD patterns of these nanocomposites did not show any significant difference compared to that of pure PVDF. In spite of smaller population of MWCNTs as compared with that of SWCNTs in nanocomposites, incorporation of MWCNTs, with the same weight percentage, augmented the rheological properties. This observation was attributed to stronger interactions between MWCNT's surface and PVDF segments.

Physical, mechanical and structural properties of poly(lactic acid) (PLA)-based films containing different amounts of nanoclay and cellulose prepared by solvent casting method were examined. Physical properties including thickness, transparency and color did not change significantly with addition of nanoparticles to the polymer matrix. X-Ray diffraction (XRD) patterns showed that pure PLA has a semi-crystalline structure and addition of nanoclay into this polymer would produce more regular structure which results in improved crystallization. It also showed that the peak is shifted to lower degrees, with greater interlayer distance of nanoclay giving an intercalated structure. Because of the nature and particle size of the MCC, it did not interact sufficiently with the polymer. Tensile strength, elastic modulus and elongation-at-break of neat PLA were 27.44 MPa, 1.84 GPa and 24.53% which with the addition of 7% of nanoclay, was changed to 40.34, 2.62 and 10.36°C, respectively. As the results of XRD, MCC were indications of no significant effect on mechanical properties, AFM images were used to evaluate the surface morphology and roughness of PLA films. Neat PLA had smoother surfaces and a lower roughness parameter (Sa). This study indicates that PLA has acceptable properties which could be used for packaging and other applications.

The transport properties of liquids and gases through polymeric materials play a very important role in some areas of industrial applications. In this study, natural rubber (NR)/CaCO3 composites were prepared by melt mixing method. By equilibrium swelling test, the transport process of toluene in the prepared natural rubber composites was investigated. The diffusion and transport of toluene through calcium carbonate-filled natural rubber composites have been studied in the temperature range 25–45°C. The diffusion of toluene through these composites was studied with special reference to the effect of filler concentration and temperature. The transport coefficients such as diffusion, permeation and sorption coefficients were estimated from the swelling data. To find out the mechanism of diffusion in prepared composites, the results of swelling studies were applied to an empirical equation. In these composites, diffusion is approximately based on Fickian diffusion mechanism and by increases in temperature; diffusion mechanism is more close to Fickian mechanism. Increase of filler content in composite would result in decreased ultimate swelling and slower diffusion rate of solvent. The diffusion rate, diffusion coefficient and the permeability increased by temperature. The study of the diffusion of toluene through filled natural rubber indicated that the concentration of filler plays an important role in the diffusion, sorption and permeation coefficients. Also interfacial interactions in NR composites were checked by dynamic-mechanical analysis. The microstructure and dispersion of calcium carbonate particles in natural rubber matrix were studied by field emission scanning electron microscopy (FE-SEM). In general, the results of swelling tests, dynamic-mechanical analysis and FE-SEM images show that the optimized value of filler in NR composites is equal to 10 phr calcium carbonate

Application of supramolecular polymer blending in preparation of materials with adjustable mechanical properties is addressed. It is anticipated that the microstructure and properties of polymer blends are drastically controlled by the inter-molecular interactions. On this basis, the effect of strongly dimerising ureidopyrimidinone end groups (UPy) with the capability of quadruple hydrogen bonding array formation on the material properties of polycaprolactone/ polyhydrofuran supramolecular polymer blends has been studied. The mechanical test results revealed that the supramolecular strategy resulted in up to 110% and 500% increase in tensile strength and Young's modulus, respectively. Observation of a single glass transition temperature in DSC traces of the blends shows that phase compatibility improved significantly upon blending.