Composition and rheology of polyamide-6 obtained by using bi- and tri-functional coupling agents

A statistical approach is developed, based on a Monte Carlo method, in order to determine the statistical composition of a polyamide-6 sample composed of caprolactam (an AB-type monomer) and of a di-acid (A2 type) or a tri-acid (A3 type) as coupling agents. For this composition, the linear rheological behavior of these systems is predicted using a tube-based theory. This allows us to show that while coupling agents of type A2 can be seen as flow improver, the effect of branching agents of type A3 , depending on the synthesis recipe and the conversion level, can lead either to an increase or to a decrease of the viscosity. By adding specific amount of these agents, we also show that it is possible to obtain materials with the same zero-shear viscosity but with different shear thinning behavior. Furthermore, the polydispersity of linear samples of the same average number molecular weight, M n , is discussed in function of the amount of A2 monomers they contain. Ranging from 2 to 1.5, this difference in polydispersity is expected to have a significant influence on the processing behavior of such materials.     

A numerical study of various rheological polydispersity measures

Model calculations were performed in order to investigate the sensitivity of various rheological polydispersity parameters for variations in the moments of the molar mass distribution (MMD) of linear polymers. Molar mass distributions were generated with the Gaussian and the Generalised exponential distribution functions, using a fixed weight average molar mass M _w and variable M _w /M _n and M _z /M _n . Assuming linear entangled polymeric chains, the linear viscoelastic properties were predicted by calculating the stress relaxation modulus of the consecutive monodisperse fractions with the BSW relaxation time spectrum and blending these curves with the double reptation blending rule. BSW relaxation parameters appropriate for polypropylene were used.  It was found that both the zero-shear viscosity and the so-called cross-over frequency, at which and are equal, depend mostly on M _w but also significantly on both M _w /M _n and M _z /M _w . By contrast, the steady-state compliance depends mainly on M _z /M _w , its functional dependence on moments of the MMD being best described by the Ferry equation.  None of the polydispersity parameters PI (from the modulus cross-over), MODSEP (the modulus separation) or PDR (from the shape of the flow curve), as introduced in literature depends solely on the polydispersity M _w /M _n . PI is the most sensitive indicator for this purpose. Finally, the parameters ER ( at a fixed low value of , MODSEP en DRI (from the shape of the flow curve) are shown to be good indicators for the weight (M _z /M _w ) of the high molar mass tail of the molar mass distribution. Received: 5 May 1998 Accepted: 30 July 1998     

A broad-band dielectric study of poly(vinyl chloride): Effect of molar mass and processing conditions on space-charge mechanisms

The dielectric properties of poly(vinyl-chloride) (PVC) were investigated as a function of frequency and temperature. Attention was focused on the low frequency dielectric properties. Samples with different molar masses were studied. Two dipolar retardation processes were observed, one due to a local motion in the polymeric chain and the other at the glass transition. In compression-molded samples of high molar mass an additional low-frequency space-charge polarization mechanism was found at high temperatures. The effect was most pronounced at the highest molar masses. The space-charge mechanism was absent in samples of well-kneaded material. This macroscopic polarization process in compression molded samples is probably due to discontinuities in the poorly gelified material because of a residual grain particle structure. The detected effects of air inclusions in a PVC matrix can be described by the Maxwell-Wagner theory. The magnitude of the space-charge mechanism is an indication of the effectiveness of the processing method used in destroying the grain structure. © 1994 John Wiley & Sons, Inc.     

Electrochemical reduction of the lanthanide ions: Part I. First reduction step of ytterbium(III) in acidic 1 M NaClO4 solution

The reduction of Yb(III) to Yb(II) in 1 M NaClO₄ in the pH range 1.9–6.6 was studied by d c. polarography, cyclic voltammetry and electrode impedance measurements as a function of frequency and electrode potential. It results that the d.c. reversible reduction is followed by a homogeneous chemical reaction and is accompanied, by an irreversible process which is attributed to a lowering of the overpotential of the reduction of the hydrogen ions. Values of the rate constant and transfer coefficient pertaining to the charge transfer step were deter mined.     

An interlayer model for the complex dielectric constant of composites

The complex dielectric constant of a composite with an interlayer was studied as a function of the volume fractions and the properties of the filler, the interlayer, and the matrix. The theoretical approach is analogous to the calculation of the shear modulus, the bulk modulus, and the termal expansivity of particulate filled polymers using the interlayer model (IM).An analytical expression describing the influence of an interlayer on the generalized dielectric constant of the composite as a function of the volume fraction and interlayer properties is derived.In the case of a composite with non-conductive constituents, the equations for static and oscillatory electric fields are similar. When conductive constituents are present, the complex dielectric constants have to be replaced by the generalized complex dielectric constants.For a composite of non-conductive materials, without interlayer, the obtained relation reduces to the classical Rayleigh equation. In the case of a composite with conductive constituents, also without interlayer, the complete solution of Wagner's theory is found. Special attention has been paid to the case of a water interlayer in a glass-bead filled non-conductive matrix material.