In situ preparation of polyaniline coated fumed and precipitated silica fillers and their composites with nitrile rubber (Investigation on structure-property relationship)

In situ synthesis of polyaniline (PAni) coated pyrogenic or fumed silica (PCFS) and precipitated silica (PCPS) were carried out by the oxidative polymerization of aniline in presence of fumed silica (FS) and precipitated silica (PS). Both uncoated and PAni coated silica fillers were characterized through scanning electron microscope (SEM), infrared spectroscopy and thermo-gravimetric analysis (TGA) to evaluate particle morphology and physico-chemical character of coated and uncoated silica particles. Semi-conducting composites made from two different types of PAni coated silica fillers with NBR exhibit different trend in the variation of electrical properties under different temperature and pressure. These differences in electrical properties of two types of composites are mainly due to physico-chemical characteristics of filler particles as well as their distribution in the polymer matrix. This type of composites may be used as semi-conducting and ESD (electrostatic discharge) material.     

An innovative approach to utilize waste silica fume from zirconia industry to prepare high performance natural rubber composites for multi-functional applications

Silica fume (SF) is silica-rich amorphous waste by-product obtained during zirconium silicate electrofusion process. The key objective of the study was to determine the efficiency of SF as a reinforcing filler in Natural Rubber (NR) compounds vis a vis the conventional filler, high abrasion furnace (HAF) black. Inter-particle distance and particle size distribution analysis from Transmission Electron Microscopy exhibited homogeneous dispersion of filler in hybrid composite (NR SF20/HAF30) with Bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT). NR composite with 20 phr SF loading improved modulus by 107%, tensile strength by 12%, and tear strength by 28% over gum NR. Hybrid composite showed 111% increase in modulus than NR SF20 composite. Theoretical modelling of Young's modulus with volume fraction of filler quite fit with Guth-Gold equation. Hybrid composite with TESPT showed 72% reduction in heat build-up compared to NR HAF50 composite. Thermal stability improved by 6 °C and rolling resistance reduced by 64% for hybrid TESPT composite compared to NR HAF50 composite. Constrained region in NR composites obtained from dynamic mechanical analysis showed improved rubber-filler interaction in hybrid TESPT composite. Hence, this work not only provides a new approach to utilize industrial waste but also provides for a high performance NR composite at low cost.     

Dielectric relaxation in water–cholesterol mixture cluster: molecular dynamics simulation

Molecular dynamics (MD) studies of the cluster composed of cholesterol (C₂₇H₄₅OH) and water molecules are presented. We have investigated several dynamical quantities of cholesterol as a function of its concentration in the mixture cluster and the temperature. The main attention was focused on the temperature and concentration dependence of the calculated total dipole moment autocorrelation function and dielectric loss of the cluster.     

Effect of inorganic phase on polymeric relaxation dynamics in PMMA/silica hybrids studied by dielectric analysis

Dielectric relaxation spectroscopy was used to investigate the effect of the inorganic phase on the polymeric relaxation dynamics in PMMA/silica hybrids synthesized in situ via sol-gel processes. It was found that the large-scale molecular motions of PMMA were influenced by the addition of silica, inducing longer mean relaxation times, more heterogeneous relaxing environments and the higher activation energy. Explanations based on hydrogen-bond interactions between two phases and a fraction of entrapped chain segments in silica networks were proposed to understand the influence of the silica.     

Molecular dynamics of an epoxy resin modified with an epoxidized poly(styrene-butadiene) linear block copolymer during cure and microphase separation processes

Molecular dynamics of diglycidyl ether of bisphenol A (DGEBA) epoxy resin modified with an epoxidized poly(styrene-b-butadiene) (SepB) linear block copolymer has been monitored during cure and microphase separation process by dielectric relaxation spectroscopy (DRS) for wide frequency and temperature ranges. Different primary and secondary relaxation processes have been analyzed for neat components and ternary mixture. Relaxational behaviour has been modelled with Havriliak-Negami, Vogel-Fulcher-Tammann and Arrhenius equations and fitting parameters and their evolution have been obtained. The retention of the epoxidized poly(butadiene) (PepB) block in the epoxy-rich phase during all the polymerization process, previously detected by our group with atomic force and transmission electron microscopies, has been confirmed by dielectric relaxation spectroscopy. The evolution of molecular dynamics during the polymerization process of the epoxy resin in the ternary system indicates a change in the trend of the main relaxation at times that agree with phase separation detected by rheology.     

Novel biphasic structured composite prepared by in situ silica filling in natural rubber latex

The sol‐gel reaction of tetraethoxysilane in natural rubber (NR) latex was conducted to produce in situ silica‐filled NR latex, followed by adding sulfur cross‐linking reagents to the latex in a liquid state. The latex was cast and subjected to sulfur curing to result in a unique morphology in the NR composite of a flexible film form. The contents of in situ silica filling were controlled up to 35 parts per one hundred rubber by weight. The silica was locally dispersed around rubber particles to give a filler network. This characteristic morphology brought about the composite of good dynamic mechanical properties. Synchrotron X‐ray absorption near‐edge structure spectroscopy suggested that the sulfidic linkages of the sulfur cross‐linked composites were polysulfidic, S_x (x ≥ 2), and a fraction of shorter polysulfidic linkages became larger with the increase of in situ silica. The present observations will be of use for developing a novel in situ silica‐filled NR composite prepared in NR latex via liquid‐phase soft processing. Copyright © 2011 John Wiley & Sons, Ltd.     

Structure and dynamics of MG rubber studied by dynamic mechanical analysis and solid‐state NMR

Methyl‐methacrylate‐grafted natural rubber was prepared by free radical polymerization of methyl methacrylate in natural rubber latex, and their structure and dynamics were investigated by dynamic mechanical analysis and solid‐state nuclear magnetic resonance (NMR). Samples were prepared by chemical initiation and high‐energy radiation. The changes of glass transition temperature and tan δ max with different total poly(methyl methacrylate (PMMA) content are reported. The effect of the change in composition in copolymers on tan δ peak width, tan δ max, and area under the tan δ curve are used to understand the miscibility and damping properties. Solid‐state ¹³C‐NMR measurements were carried out to determine several relaxation time parameters, such as rotating frame and laboratory frame proton and carbon relaxation times. Cross polarization times and carbon relaxation times were interpreted based on the changes in the molecular motion. Proton relaxation times were interpreted based on the heterogeneity of the matrix. Results confirmed phase separation and a presence of an interfacial region. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1141–1153, 1999     

Studies on accelerated sulphur vulcanization of natural rubber using 1-phenyl-2, 4-dithiobiuret/tertiary butyl benzothiazole sulphenamide

The use of binary accelerators has gained tremendous importance since it increases the production rate of the article made of that elastomer. The authors have analyzed the performance of a novel binary accelerator system in the sulphur vulcanization of natural rubber (NR). The vulcanizates resulting from the binary accelerated process obtained good mechanical properties, ageing and swelling resistance. Network characterization of the mixes was done using swelling measurements, stress-strain analysis etc. The chain entanglement density was measured using dynamic mechanical analysis. However the performance is found to be dependent on the relative proportion of mono, di and polysulphidic linkages in the material. The result of the study points out that the proposed system can be active in NR regardless of the vulcanization recipe and temperature. The performance of the new binary system in filled vulcanizates is also studied. Based on the processing, mechanical and chemical characterization an optimum concentration is suggested for the new system.     

Influence of reaction parameters on the structure of in situ rubber/silica compounds synthesized via sol–gel reaction

In situ silica was synthesized in three non‐vulcanized rubber matrices, namely natural rubber, styrene‐butadiene rubber, and EPDM (ethylene‐propylene diene ter‐polymer), using the sol–gel method with tetra‐ethoxysilane (TEOS) as silica precursor and hexylamine as catalyst. The effect of the reaction parameters such as the amount of TEOS, the reaction time (15–120 min), and the type of rubber was explored. Transmission electron microscopy was used to study the gradient in silica content and particle size over the sample thickness. The diffusion gradient of TEOS and catalyst solution in the rubber matrix responsible for the gradient was studied with Fick's law. An excellent dispersion of silica was obtained for all rubbers, even for the very non‐polar EPDM, without the use of any additives to improve the dispersion. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 967–978     

Local modes of molecular mobility in comb-shaped liquid crystalline ionomers containing alkaline metal ions

Molecular mobility of the comb-shaped copolymer poly(4-[6-(acryloyloxy)hexyloxy]benzoic acid-co-butyl acrylate) and Li- and Rb-containing LC ionomers based on this polymer was studied at low temperatures by the methods of dielectric spectroscopy and thermally stimulated depolarization current. The above copolymers are shown to experience the γ₂, γ₁, and β-processes, which are related by the reorientation of the end СООН groups, by the mobility of a spacer, and by the reorientation of mesogenic fragments (dimers of oxybenzoic acids) with respect to a long axis, respectively. The formation of multiplet structures in ionomers leads to a suppression of the intensity of the β-process that relates with a partial breakdown of hydrogen-bonded dimmers and with a decrease of their molecular mobility.     

Integrating ultrafast and stochastic dynamics studies of Brownian motion in molecular systems and colloidal particles

Ultrafast spectroscopy and stochastic dynamics studies of chemical dynamics in solution with high resolution in both space and time have been undertaken for many years, but it is still challenging to connect fundamental knowledge obtained from stroboscopic approaches at ultrashort timescales and small length scales with that obtained by directly measuring individual particle motion at longer timescales. Therefore, it is interesting, conceptually and experimentally, to understand the similarities and differences between these two approaches to the study of chemical dynamics in condensed phase systems. We discuss recent advances in the understanding of the transition from ballistic to diffusive motion and chemical reaction rate theories and describe the significance of the findings in relation to the study of thermally activated processes at multiple time and length scales.     

Molecular dynamics simulations of a nucleoside analogue of 1,4-dihydro-4-oxoquinoline-3-carboxylic acid synthesized as a potential antiviral agent: Conformational studies in vacuum and in water

Conformational analysis of nucleosides may have direct applications to the structure–activity relationship (SAR) studies and in the design of new drug candidates. Although conformational analysis may be accessed in many different ways, in this work it was performed using molecular dynamics (MD) simulation in order to study the dynamic behavior of a nucleoside derivative of 1,4-dihydro-4-oxoquinoline-3-carboxylic acid, synthesized by our group as a potential antiviral agent. The MD simulation was carried out during 10 ns in vacuum and in a box of water at two different temperatures (i.e., 300 and 600 K) using the AMBER force field. The in vacuum MD simulation results are in agreement with the crystallographic structure and with the DFT calculations of the nucleoside, revealing the anti conformer as the more stable one. The simulation in water, however, shows that both conformers may exist at 300 K, the temperature of the in vivo and in vitro assays, revealing that both the syn and anti conformers should be considered in a MD simulation study of the inhibitor–enzyme complex. Simulations are also in agreement with the NOE experiment, which shows that the anti conformer is the preferential one in DMSO-d6 solution at 298 K.     

Temperature effects on the structure and dynamics of liquid dimethyl sulfoxide: A molecular dynamics study

The molecular dynamics (MD) simulation technique has been employed to investigate the thermodynamic properties and transport coefficients of the neat liquid dimethyl sulfoxide (DMSO). The fluid has been studied at temperatures in the range 298–353 K and at a pressure equal to 1 atm. The simulations employed a nine-site potential model, which is presented for the first time here, and all the available non-polarizable models. The performance of each model is tested using the same statistical mechanical ensemble and simulation method under the same conditions, revealing its weaknesses and strengths. Thermodynamic properties, microscopic structure and dynamic properties, such as transport coefficients, rotational and single-dipole correlation times have been calculated and compared with available experimental results. Estimations of transport coefficients from various theoretical and empirical models are tested against experimental and MD results. Translational and rotational dynamics suggest the existence of the cage effect and agree with the Stokes–Einstein–Debye relation. The dipole relaxation times calculated are discussed in terms of simple and useful approximations, such as the Glarum–Powles and Fatuzzo–Mason models.     

Reinforcement effect of carbon nanofillers in an epoxy resin system: Rheology,molecular dynamics,and mechanical studies

The reinforcing effect of carbon nanoparticles in an epoxy resin has been estimated with different approaches based on rheology, molecular dynamics (evaluated by differential scanning calorimetry, dielectric relaxation spectroscopy, and thermally stimulated depolarization current), and dynamic mechanical analysis. Carbon particles aggregate as the volume increases and form a fractal structure in the matrix polymer. The dispersion microstructure has been characterized by its viscoelastic properties and relaxation time spectrum. The scaling of the storage modulus and yield stress with the volume fraction of carbon shows two distinct exponents and has thus been used to determine the critical carbon volume fraction of the network formation (Φ*) for the carbon/epoxy dispersions. At nanofiller concentrations greater than Φ*, the overall mobility of the polymer chains is restricted in both dispersions and solid nanocomposites. Therefore, (1) the relaxation spectrum of the dispersions is strongly shifted toward longer times, (2) the glass‐transition temperature is increased and (3) the relaxation strength of both the secondary (β) and primary (α) relaxations increases in the nanocomposites, with respect to the pure polymer matrix. The dispersion microstructure, consisting of fractal flocs and formed above Φ*, is proposed to play the main role in the reinforcement of nanocomposites. Moreover, the network structure and the interface polymer layer (bond layer), surrounding nanoparticles, increases the relaxation strength and slows the cooperative α relaxation, and this results in an improvement of the mechanical properties. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 522–533, 2005