Papers by Keyword: Glass Transition Temperature

Abstract: We have synthesized polyester type dendrimers containing azobenzene chromophore in the core and trityl groups at the periphery using divergent growth strategy up to 3^rd generation. We analyzed dendrimer samples using NMR, HPLC, TG, DSC and UV-Vis techniques. We found out that functionalization of dendrimer periphery is not complete. Dendrimers with trityl groups at the periphery have glass transition temperatures in the range 73-87 °C.

Abstract: The polymer blend electrolytes composed of poly (ethyl methacrylate)(PEMA) and Poly (vinyl acetate)(PVAc) as host polymer and lithium perchlorate (LiClO₄) as a salt are synthesized by solvent casting technique. The polymer membranes with different wt% of PEMA and PVAc are subjected to AC impedance analysis for the investigation of ionic conductivity. The maximum ionic conductivity of 3.541 X 10^{- 5}Scm^{- 1} at 303K is reported for PEMA/PVA_C (70/30wt%) –LiClO₄ (8wt%) polymer blend electrolyte system.The complexation has been confirmed by XRD and FTIR techniques. The glass transition temperature (T_g) of the blend polymer electrolytes has been obtained from DSC measurements. The SEM micrographs show the surface morphology of the prepared samples. The electrochemical stability of the sample exhibiting high conductivity has been carried out using linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements. The potential window has been found to be-2.5 to +2.5 V. The lithium transference number evaluated using chronoamperometry technique results in a value of 0.90. The dielectric behavior of the solid polymer blend electrolytes has been analyzed as a function of frequency and temperature. The dc conductivity values obtained from the conductance spectra match the ac impedance results. The photoluminescence spectra that contain information about the local free volume of the prepared samples justify the conductivity results. The two and three dimensional images of the maximum ionic conducting sample exhibit numerous micropores.

Abstract: Epoxy/silica composites were prepared using aminopropyl triethoxysilane (APTES)-modified silica nanoparticles in the sol state. Different sizes of silica particles were synthesized and they were applied into the epoxy/silica composites with different compositions. The mechanical and thermal properties of the composites were investigated and compared with those of pristine epoxy composite. The structure and morphology of the modified silica nanoparticles and epoxy/silica composites were analyzed using field emission scanning electron microscope. The flexural modulus and tensile strength of the epoxy/silica composites were investigated by universal test machine (UTM). Also, glass transition and thermal stability were investigated using thermomechanical analyzer (TMA). Sizes of silica particles in sol state were controlled by using different concentration of the accelerator. The tensile strength of epoxy/silica composites containing 20 wt% of 30 nm silica was found to be 37.98 MPa. In addition, the glass transition temperature (T_g) decreased with increasing silica particle sizes.

Abstract: The formation and evolution of chemical-physical complex network during vulcanization in carbon black (CB) filled NR was investigated in this work. The results showed that the cross-linking density increased with increase of CB content. The variation of torque during vulcanization was attributed to crosslinks of macromolecular chains. The critical content of CB for the forming of CB network was between 30phr and 40phr (weight percentage). The CB content did not affect the glass transition temperature (T_g) obviously.

Abstract: Composite of Cadmium sulphide (CdS) nanoparticle on the surface of polystyrene- co- maleic anidride (St-co-MMA) were prepared via surfactant free emulsion polymerization. Methylmetacrylate (MMA) was used as auxiliary monomer which co-polymerized with styren (St) and provided the side for coordinating with Cd²⁺. By the coordination of Cd²⁺ ions to methyl metacrylate, decoration of the Cd²⁺ ions on the surface of copolymer were prepared successfully. With the release of S^2- ions from the thioacetamide (TAA), CdS was formed on the surface of nanorods copolymer in facile method. Fourier transform infrared spectroscopy (FT-IR) of nanocomposite was confirmed the polymerization of monomers. Structure and morphology of CdS nanoparticles have been characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD measurements suggest the cubic structure for CdS and the particles size was estimated to about 3.35 nm by applying Scherrer’s equation. The SEM analysis confirmed the nanorode structure of CdS/ (St-co-MMA) composite. The photoluminescence (PL) and UV–Vis spectroscopy revealed the quantum confinement effect in the CdS/ (St-co-MMA) nanocomposite. Using effective mass approximation (EMA) method particle sizes were calculated from the shift in optical band gap. The thermal properties of the CdS/ (MMA-co-St) were explored by thermal gravimetric analysis (TGA). The improved thermal stability of nanocomposite was attributed to the interaction of CdS nanoparticles with polymer. The CdS/ (MMA-co-St) nanocomposite exhibited a glass transition temperature around 250^◦C.

Abstract: The prime objective of this study was to fabricate epoxy polymer composite modified with graphene nanoplatelets (GP) and montmorillonite nanoclay (MMT) binary filler materials. Different loading percentages of individual and binary nanofillers were incorporated into an epoxy matrix system to investigate the synergistic effect of nanofillers on composites properties. Dynamic mechanical analysis (DMA) and three point bend test were carried out to investigate the viscoelastic and flexural properties of neat epoxy and nanofillers reinforced nanocomposites. Incorporation of 3 wt. % of MMT and 0.1 wt. % of GP resulted in better flexure strength, modulus and storage modulus although there is no significance change in glass transition temperature (Tg).

Abstract: Anelastic properties of Bulk Metallic Glasses (BMG) were studied by mechanical spectroscopy using a flexural vibration apparatus. BMG’s samples, with nominal composition Cu₄₈Zr₄₃Al₉ and Cu₅₄Zr₄₀Al₆, were produced by skull push-pull casting technique in rectangular cavity cooper mold. In both samples, the differential scanning calorimeter patterns have evidenced the presence of amorphous structure, although the X-ray diffraction for Cu₄₈Zr₄₃Al₉ composition has shown a heterogeneous microstructure embedded in the amorphous matrix. Anelastic relaxation spectra were obtained using an acoustic elastometer system with vibration frequency in the kilohertz bandwidth, a heating rate of 1 K/min, vacuum greater than 10^-5 mBar in the temperature range of 300 K to 620 K. In the flexural apparatus, an acoustic elastometer system, the internal friction (energy loss) and the elastic modulus were obtained by free decay of vibrations and by the squared of the oscilation frequency, respectively. Internal friction spectra were not reproducible among the measurements, which may imply atomic rearrangement in the samples due to consecutive heating. Normalized elastic modulus data showed distinct behavior from the first to the other measurements evidencing irreversible microstructural alterations in the samples possibly associated with mechanical relaxation due to the motion of atoms or clusters in the glassy state.

Abstract: In this study various compositions in the phosphate based glass (PBG) system of (50-x)P₂O₅-40Ca-(5+x)Na-5TiO₂ and (50-x)P₂O₅-40Ca-(5+x)Na-5Fe₂O₃, where x= 5 and 10 were investigated for glass transition temperature (T_g) via thermo mechanical analyser (TMA) and differential scanning calorimetry (DSC). The amorphous nature of the glasses was confirmed via XRD. The T_g measured via DSC was consistently higher by 19°C-29°C compared to TMA and was due to the thermal history and the heating rate of the samples. The T_g increased with increasing phosphate content in both glass systems. The T_g for Ti containing PBG was found to be in the range of 453°C-500°C whilst T_g for Fe containing PBG was in the range of 449°C-494°C. Consistently higher T_g for the Ti containing glass series compared to the Fe containing glasses may be attributed to the smaller ionic radius and therefore higher field strength of Ti⁴⁺.

Abstract: The dispersion and exfoliation of graphene oxides in polymer matrix remains a challenge for graphene oxides based epoxy nanocomposites fabrication. In the present paper, we reported a simple and facile solvent exchange technique to successfully transfer graphene oxides (GOs) from aqueous solution to ethanol. In addition, we found that GO dispersion in epoxy resins was affected by the curing agents. Good dispersion of GOs in epoxy resin together with enhanced thermal and mechanical properties were observed when epoxy was cured with aliphatic curing agents. For aromatic curing agent, high loading of GOs leaded to GOs aggregation, but well dispersed GOs was observed at low loading of GOs. Especially, a 12 °C increase of glass transition temperature of the epoxy resin was observed with only 0.1 wt% GOs was added to the epoxy resin.

Abstract: Rice husk ash (RHA) is a product from the burning of the rice husk and it become a significant material for glass preparation due to high amount of amorphous silica that can be produced. From the recent studies, the highest amount of silica is observed at 550°C at 6 hours which is 99.36% by XRF detection. Nevertheless, the amount of silica decreasing proportionally if the temperature profile is higher than 550°C and the time taken is more than 6 hours. Quaternary glasses are prepared using melt-quenching method using the highest amount of silica from RHA combined with Bismuth Oxide (Bi₂O₃), Boron Oxide (B₂O₃), and Zinc Oxide (ZnO). The ratio of quaternary glass that used are 30%SiO₂: 20%B₂O₃: xZnO : (50-x)%Bi₂O₃ (% mole) ; where x= 10,20,30,40. The physical properties and glass transition temperature (Tg) profiles are determined using standard measurement instrument. Moreover, the density measured will enhance the verification of Oxygen Packing Density for prepared quaternary glasses.