Thermal properties of radiolytically synthesized PVA/Ag nanocomposites

The radiolytic method was used to synthesize two types of nanocomposites with silver, PVA/Ag by film casting and PVA hydrogel/Ag nanocomposites. This method is particularly suitable for generating metal nanoparticles in solution. The radiolytic species (solvated electrons and secondary radicals) exhibit strong reducing properties such that metal ions are reduced at each encounter. Metal atoms then tend to grow into larger clusters. It was found that solid or swollen polymers are able to stabilize small crystallites against spontaneous growth via aggregation. Using differential scanning calorimetry (DSC), the melting behavior and kinetics of the PVA/Ag nanocomposites were investigated and compared to those of pure PVA. The melting as well as crystallization behavior of polymers is crucial because it governs the thermal properties, impact resistance and stress strain properties. Understanding the melting behavior is significant not only to tailor the properties of nanocomposites but to investigate the interactions between the constituents. The DSC curves of pure PVA and prepared nanocomposites show only one melting peak between 175 and 230 degrees C, indicating that the melting behavior of these two systems are analogous. In both cases, with increasing heating rate, the melting peak shifts to a higher temperature, but with increasing Ag content the peak melting temperature is lower. When specimens are heated at high heating rate, the motion of PVA molecular chains cannot follow the heating temperature on time due to the influence of heat hysteresis, which leads to a higher peak melting temperature. When Ag nanoparticles are added they increase the heat transfer among the PVA molecular chains decreasing the melting temperature. The Ag content is a major factor affecting the degree of crystallinity. It was observed that at low nanofiller content, up to the 0.5 wt%, the degree of crystallinity of the nanocomposites increased, while at a higher content the crystallization was retarded. The half time of melting is non-linearly dependent on the amount of nanofiller. In the range from 0.25 to 1 wt% Ag it slightly increases, because at a low Ag content the nanoparticles act as a heterogeneous nucleation agent during the crystallization process. For large amounts of nanofiller, the half time of melting is markedly higher than for pure PVA. At a higher Ag content, the nanoparticles act as a barrier that restricts the thermal motion of PVA molecular chains and the half time of complete melting increases. The significantly lower melting activation energy of the nanocomposites with high amount of nanofiller compared to pure PVA, calculated by the Kissinger method, indicated that nanoparticles reduced the heat barrier for the melting process.5th Young Researchers Seminar, Dec 25-26, 2006, Belgrade, Serbi

Non-isothermal dehydration of equilibrium swollen radiolytically sinthesized Fe3O4 – PVA ferrogel nanocomposite

In this study, the Fe3O4 - PVA ferrogel nanocomposite was synthesized by gamma irradiation. Obtained ferrogel had greater swelling capacity and activation energy of dehydratation as measured by thermogravimetric analysis under non-isothermal conditionsPhysical chemistry 2008 : 9th international conference on fundamental and applied aspects of physical chemistry; Belgrade (Serbia); 24-28 September 200

Radiolytic synthesis and characterization of PVA/Au nanocomposites: The influence of pH values

Nanocomposites which consist of polymer matrix and nanoparticles of noble metal are a very important class of nanomaterials. The optical properties of these materials are strongly dependent on the nanoparticles size as well as on the synthesized procedure of nanocomposites preparation. In this work the Au nanoparticles were synthesized by the simple radiolytic procedure, using steady state gamma irradiation, under different pH values. PVA/Au nanocomposite films were obtained by solvent evaporation from Au colloids. UV-Vis absorption spectra of Au colloids show absorption in the range of 450 to 650 nm with the maximum around 520 nm, which is the typical plasmon band of Au nanoparticles. Absorption spectra of PVA/Au nanocomposites are red shifted compared to corresponding primary Au colloids. This red shift is not result of any change in particle size distribution and can be explained by the change of dielectric property of surrounding medium. IR spectra indicated anchoring the OH groups of PVA molecule at the cluster surface. This effect limits the growing of the metal clusters and confirmed that the PVA is a god matrix for in situ synthesis of Au nanoparticles. The size of Au nanoparticles is dependent on the experimental conditions. Obtained results indicated that the pH value of solution played an important role. The values of absorption maximum were not dependent on the concentration of Au3+ cations, but show strong dependence on the pH value of the initial solution. The increase of pH value reduced the mean diameters of nanoparticles. The lowest value of lambda(max) was obtained for colloid with pH 7, which theoretically corresponds to the nanoparticles with minimum dimension. These results are in agreement with the XRD measurement of PVA/Au nanocomposite films and theoretical calculations. According to obtained results of radiolytic synthesis of Au nanoparticles in PVA solution can be conclude that it is possible to control the particle size by tuning the pH value of solution.6th Conference of Young Researchers, Dec 24-26, 2007, Belgrade, Serbi

Swelling behavior of Ag/PVA hydrogel nanocomposite synthesized by γ-irradiation

In this study, the Ag/PVA hydrogel nanocomposite was synthesized using γirradiation. Incorporated Ag NPs were less than 10 nm in diameter with face centered cubic (fcc) crystal structure. Incorporation of Ag NPs into PVA hydrogel significantly affects the characteristic parameters of swelling process

Mass spectrometric investigation of silver clusters

Silver clusters were produced by two different synthesis procedures, related with different reduction species. PVA (polyvinyl alcohol) was successfully utilized as a source to reduce silver (Ag) metal ions without using any additional reducing agents to obtain Ag clusters. Silver clusters with atoms numbering 5-29 are registered through mass spectrometry using MALDI TOF and MALDI TOF/TOF techniques. Analysis showed that clusters with magic numbers are the largest, while those with an odd number of atoms are larger than clusters with an even number of atoms. Stability of these molecules, magic number actually, is in relation with their electronic configuration

Thermal properties of radiolytically synthesized PVA/Ag nanocomposites

The radiolytic method was used to synthesize two types of nanocomposites with silver, PVA/Ag by film casting and PVA hydrogel/Ag nanocomposites. This method is particularly suitable for generating metal nanoparticles in solution. The radiolytic species (solvated electrons and secondary radicals) exhibit strong reducing properties such that metal ions are reduced at each encounter. Metal atoms then tend to grow into larger clusters. It was found that solid or swollen polymers are able to stabilize small crystallites against spontaneous growth via aggregation. Using differential scanning calorimetry (DSC), the melting behavior and kinetics of the PVA/Ag nanocomposites were investigated and compared to those of pure PVA. The melting as well as crystallization behavior of polymers is crucial because it governs the thermal properties, impact resistance and stress strain properties. Understanding the melting behavior is significant not only to tailor the properties of nanocomposites but to investigate the interactions between the constituents. The DSC curves of pure PVA and prepared nanocomposites show only one melting peak between 175 and 230°C, indicating that the melting behavior of these two systems are analogous. In both cases, with increasing heating rate, the melting peak shifts to a higher temperature, but with increasing Ag content the peak melting temperature is lower. When specimens are heated at high heating rate, the motion of PVA molecular chains cannot follow the heating temperature on time due to the influence of heat hysteresis, which leads to a higher peak melting temperature. When Ag nanoparticles are added they increase the heat transfer among the PVA molecular chains decreasing the melting temperature. The Ag content is a major factor affecting the degree of crystallinity. It was observed that at low nanofiller content, up to the 0.5 wt%, the degree of crystallinity of the nanocomposites increased, while at a higher content the crystallization was retarded. The half time of melting is non-linearly dependent on the amount of nanofiller. In the range from 0.25 to 1 wt% Ag it slightly increases, because at a low Ag content the nanoparticles act as a heterogeneous nucleation agent during the crystallization process. For large amounts of nanofiller, the half time of melting is markedly higher than for pure PVA. At a higher Ag content, the nanoparticles act as a barrier that restricts the thermal motion of PVA molecular chains and the half time of complete melting increases. The significantly lower melting activation energy of the nanocomposites with high amount of nanofiller compared to pure PVA, calculated by the Kissinger method, indicated that nanoparticles reduced the heat barrier for the melting process.

Thermal behaviour of Co(II), Ni(II), Cu(II), Zn(II), Hg(II) and Pd(II) complexes with isatin-beta-thiosemicarbazone

The isatin-beta-thiosemicarbazone (ITC) complexes of Co(II), Ni(II), Cu(II), Zn(II), Hg(II) and Pd(II) were prepared and characterized by elemental analysis, as well as molar conductivity, magnetic susceptibility, FTIR, UV-Vis and H-1 NMR spectroscopic methods. The complexes were also studied for its thermal stability. They all behaviour as anhydrous complexes and its thermolysis passes through the stages of deamination (517-547 K) and complete thermal decomposition (619-735 K).32nd Workshop on Heterogeneous Equilibria between Phases (JEEP 2006), Mar 30-31, 2006, Univ Rouen, Rouen, Franc

Fabrication of Ag-PVA hydrogel nanocomposite by gamma-irradiation

The radiolytic formation of Ag nanoparticles in crosslinked PVA hydrogel was investigated. The reduction of Ag+ ions was performed using strongly reducing species such as hydrated electrons, propan-2-ol and PVA radicals. Ag+ ions were efficiently reduced in swollen PVA matrix by PVA radicals. Thermal and thermooxidative properties of radiolytically obtained nanocomposites were affected by the content of nanofiller as well as by different routes of preparation.Microsymposium on Polymer Gels and Networks, Jul, 2005, Inst Macromol Chem Acad Sci, Prague, Czech Republi

Dynamic thermogravimetric degradation of gamma radiolytically synthesized Ag-PVA nanocomposites

The degradation kinetics of gamma radiolytically synthesized Ag-PVA nanocomposites was investigated by thermogravimetric method under dynamic conditions (30-600 degrees C) in an inert atmosphere. Thermogravimetric analysis showed that thermal degradation of composites was a two-stage process for the lower amount of nanofiller and single-stage for the higher amount of nanotiller. The Vyazovkin model-free kinetics method was applied to calculate the activation energy (E-a) of the degradation process as a function of conversion and temperature. At a given degradation temperature, PVA as a host in nanocomposite presents lower reaction velocity, while its E-a is higher than that of pure PVA. (c) 2007 Elsevier B. V. All rights reserved

Radiolytic synthesis and characterization of Ag-PVA nanocomposites

The Ag-PVA nanocomposites with different contents of inorganic phase were prepared by reduction of Ag+ ions in aqueous PVA solution by gamma irradiation followed by solvent evaporation. Optical properties of the colloidal solutions and the nanocomposite films were investigated using UV-vis spectroscopy. Structural characterization of the Ag nanoparticles was performed by TEM and XRD. Interaction of the Ag nanoparticles with polymer matrix and the heat resistance of the nanocomposites were followed by IR spectroscopy and DSC analysis. IR spectra indicated that Ag nanofiller interact with PVA chain over OH groups. The changes of heat resistance upon the increase of the content of inorganic phase are correlated to the adsorption of polymer chains on the surface of Ag nanoparticles. (c) 2007 Elsevier Ltd. All rights reserved