Spectroscopic investigations on PVDF-Fe2O3 nanocomposites

Polyvinylidene fluoride-iron oxide (PVDF-Fe2O3) nanocomposites have been obtained my melt mixing of PVDF with Fe2O3 nanoparticles. The interactions between the polymeric matrix and the nanofiller have been investigated by wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy, using both red and green excitations (lasers). WAXS, FTIR, and Raman spectra confirm that all samples contain α PVDF as the major crystalline form of the polymeric matrix. Experimental data revealed small changes in the positions of X-ray lines as well as modifications of the width of X-ray lines upon loading by Fe2O3 nanoparticles. FTIR and Raman spectra are dominated by the lines of the polymeric matrix. Within the experimental errors, the positions of Raman lines are not affected by the wavelength of the incoming electromagnetic radiation, although they are sensitive to the strain of the polymeric matrix induced by addition of the nanofiller. The loading of the polymeric matrix with nanoparticles stretches the macromolecular chains, affecting their vibrational spectra (FTIR and Raman). A complex dependence of the positions of some Raman and FTIR lines on the loading with Fe2O3 is reported. The manuscript provides a detailed analysis of the effects of nanofiller on the position of WAXS, FTIR, and Raman lines

The effect of the shear-thickening transition of model colloidal spheres on the sign of N1 and on the radial pressure profile in torsional shear flows

A novel rheometer plate was used to measure radial pressure profiles during cone-and-plate and parallel-plate shearing flows of a concentrated colloidal dispersion of polymethyl methacryalate spheres suspended in dioctyl phthalate. There is a long history of using suspensions of this type as a model rheological system. The measured pressure profile can be used to calculate N1 and N2, and also provides a check on the flow field in the rheometer. At shear rates just below onset of shear thickening, our measurements show that N1 is positive as predicted by Stokesian dynamics simulations of model Brownian hard spheres, but we are unable to determine the sign of N2. After the onset of thickening, we find that in both flow geometries the pressure increases sharply with radial position. This is in striking contrast to the pressure profiles ordinarily observed for viscoelastic liquids with the exception of certain liquid crystal polymers, for which the pressure decreases with radial position. Under these conditions, the apparent values of N1 and N2 are both negative with N2 N1, as predicted by the Stokesian dynamics simulations. However, the flow in the cone-and-plate rheometer may not be viscometric after the onset of shear thickening

On the thermogravimetric analysis of polymers: Polyethylene oxide powder and nanofibers

Thermogravimetric analysis of polyethylene oxide (powder and nanofibers obtained by force spinning water or chloroform solutions of polyethylene oxide) was studied using different theoretical models such as Friedman and Flynn-Wall-Ozawa. A semiempirical approach for estimating the “sigmoid activation energy” from the thermal degradation was suggested and confirmed by the experimental data on PEO powder and nanofibers\u27 mats. The equation allowed for calculating a “sigmoid activation energy” from a single thermogram using a single heating rate without requiring any model for the actual complex set of chemical reactions involved in the thermal degradation process. For PEO (powder and nanofibers obtained from water solutions), the “sigmoid activation energy” increased as the heating rate was increased. The sigmoid activation energy for PEO mats obtained from chloroform solutions exhibited a small decrease as the heating rate was increased. Thermograms\u27 derivatives were fitted to determine the coordinates of the inflection points. The “sigmoid activation energy” was compared to the activation energy determined from the Flynn-Wall-Ozawa model. Similarities between the thermal degradation of polyethylene oxide powder and of the nanofibers obtained from water solutions were discussed. Significant differences between the sigmoid activation energies of the mats obtained from water and chloroform solutions were reported

Unexpected drop of dynamical heterogeneities in colloidal suspensions approaching the jamming transition

As the glass (in molecular fluids\cite{Donth}) or the jamming (in colloids and grains\cite{LiuNature1998}) transitions are approached, the dynamics slow down dramatically with no marked structural changes. Dynamical heterogeneity (DH) plays a crucial role: structural relaxation occurs through correlated rearrangements of particle ``blobs'' of size $\xi$\cite{WeeksScience2000,DauchotPRL2005,Glotzer,Ediger}. On approaching these transitions, $\xi$ grows in glass-formers\cite{Glotzer,Ediger}, colloids\cite{WeeksScience2000,BerthierScience2005}, and driven granular materials\cite{KeysNaturePhys2007} alike, strengthening the analogies between the glass and the jamming transitions. However, little is known yet on the behavior of DH very close to dynamical arrest. Here, we measure in colloids the maximum of a ``dynamical susceptibility'', $\chi^*$, whose growth is usually associated to that of $\xi$\cite{LacevicPRE}. $\chi^*$ initially increases with volume fraction $\phi$, as in\cite{KeysNaturePhys2007}, but strikingly drops dramatically very close to jamming. We show that this unexpected behavior results from the competition between the growth of $\xi$ and the reduced particle displacements associated with rearrangements in very dense suspensions, unveiling a richer-than-expected scenario.Comment: 1st version originally submitted to Nature Physics. See the Nature Physics website fro the final, published versio

Well dispersed fractal aggregates as filler in polymer-silica nanocomposites: long range effects in rheology

We are presenting a new method of processing polystyrene-silica nanocomposites, which results in a very well-defined dispersion of small primary aggregates (assembly of 15 nanoparticles of 10 nm diameter) in the matrix. The process is based on a high boiling point solvent, in which the nanoparticles are well dispersed, and controlled evaporation. The filler's fine network structure is determined over a wide range of sizes, using a combination of Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy (TEM). The mechanical response of the nanocomposite material is investigated both for small (ARES oscillatory shear and Dynamical Mechanical Analysis) and large deformations (uniaxial traction), as a function of the concentration of the particles. We can investigate the structure-property correlations for the two main reinforcement effects: the filler network contribution, and a filler-polymer matrix effect. Above a silica volume fraction threshold, we see a divergence of the modulus correlated to the build up of a connected network. Below the threshold, we obtain a new additional elastic contribution of much longer terminal time than the matrix. Since aggregates are separated by at least 60 nm, this new filler-matrix contribution cannot be described solely with the concept of glassy layer (2nm)

Local influence of boundary conditions on a confined supercooled colloidal liquid

We study confined colloidal suspensions as a model system which approximates the behavior of confined small molecule glass-formers. Dense colloidal suspensions become glassier when confined between parallel glass plates. We use confocal microscopy to study the motion of confined colloidal particles. In particular, we examine the influence particles stuck to the glass plates have on nearby free particles. Confinement appears to be the primary influence slowing free particle motion, and proximity to stuck particles causes a secondary reduction in the mobility of free particles. Overall, particle mobility is fairly constant across the width of the sample chamber, but a strong asymmetry in boundary conditions results in a slight gradient of particle mobility.Comment: For conference proceedings, "Dynamics in Confinement", Grenoble, March 201

Temperature- and thickness-dependent elastic moduli of polymer thin films

The mechanical properties of polymer ultrathin films are usually different from those of their counterparts in bulk. Understanding the effect of thickness on the mechanical properties of these films is crucial for their applications. However, it is a great challenge to measure their elastic modulus experimentally with in situ heating. In this study, a thermodynamic model for temperature- (T) and thickness (h)-dependent elastic moduli of polymer thin films Ef(T,h) is developed with verification by the reported experimental data on polystyrene (PS) thin films. For the PS thin films on a passivated substrate, Ef(T,h) decreases with the decreasing film thickness, when h is less than 60 nm at ambient temperature. However, the onset thickness (h*), at which thickness Ef(T,h) deviates from the bulk value, can be modulated by T. h* becomes larger at higher T because of the depression of the quenching depth, which determines the thickness of the surface layer δ

The Physics of the Colloidal Glass Transition

As one increases the concentration of a colloidal suspension, the system exhibits a dramatic increase in viscosity. Structurally, the system resembles a liquid, yet motions within the suspension are slow enough that it can be considered essentially frozen. This kinetic arrest is the colloidal glass transition. For several decades, colloids have served as a valuable model system for understanding the glass transition in molecular systems. The spatial and temporal scales involved allow these systems to be studied by a wide variety of experimental techniques. The focus of this review is the current state of understanding of the colloidal glass transition. A brief introduction is given to important experimental techniques used to study the glass transition in colloids. We describe features of colloidal systems near and in glassy states, including tremendous increases in viscosity and relaxation times, dynamical heterogeneity, and ageing, among others. We also compare and contrast the glass transition in colloids to that in molecular liquids. Other glassy systems are briefly discussed, as well as recently developed synthesis techniques that will keep these systems rich with interesting physics for years to come.Comment: 56 pages, 18 figures, Revie

Unveiling thermal transitions of polymers in subnanometre pores

The thermal transitions of confined polymers are important for the application of polymers in molecular scale devices and advanced nanotechnology. However, thermal transitions of ultrathin polymer assemblies confined in subnanometre spaces are poorly understood. In this study, we show that incorporation of polyethylene glycol (PEG) into nanochannels of porous coordination polymers (PCPs) enabled observation of thermal transitions of the chain assemblies by differential scanning calorimetry. The pore size and surface functionality of PCPs can be tailored to study the transition behaviour of confined polymers. The transition temperature of PEG in PCPs was determined by manipulating the pore size and the pore–polymer interactions. It is also striking that the transition temperature of the confined PEG decreased as the molecular weight of PEG increased