Bimodal Molar Mass Distribution Research Articles

Influence of reactor operational policies on distributions of chemical composition, molar mass, sequence length and sequence frequency of multicomponent polymers obtained by free‐radical polymerization

AbstractRemarkable shifts in chemical composition, molar mass, sequence length and sequence frequency will occur in the course of free‐radical multicomponent polymerizations, if the reactivities of the comonomers are different and a simple batch reactor is used. It is shown for the terpolymerization system of methyl methacrylate/styrene/maleic anhydride that a semi‐batch reactor with appropriate regimes is suitable to obtain products with improved chemical, molecular and sequential homogeneity. However, if polymers with defined inhomogeneities like bimodal molar mass or chemical composition distributions are desired, these are also obtainable by use of appropriate operational policies, which will be illustrated for the homopolymerization of methyl methacrylate and the binary copolymerization of methyl methacrylate/maleic anhydride. Concerning the instantaneous mean chemical composition of polymer molecules, which cannot be measured directly, a new procedure is presented to determine this quantity experimentally.

Modelling the polymerization of 1,3‐butadiene in solution with a neodymium catalyst

AbstractThe batch polymerization of butadiene in hexane with an in‐situ‐activated Ziegler‐Catalyst containing neodymium versatate, ethylaluminium sesquichloride and triisobutylaluminium was studied at 45°C. The rate of polymerization was determined by online density measurements. A mathematical model has been developed which is able to describe the kinetics of polymerization at different catalyst ratios. At molar ratios of aluminium/neodymium above 40 a bimodal molar mass distribution can be observed by gel permeation chromatography which leads to the assumption of more than one active species being present in the polymerization system.

Bimodal molar‐mass distribution of cis‐1,4‐polybutadiene synthesized with nickel‐based Ziegler‐Natta catalysts

Bimodal molar‐mass distribution of cis‐1,4‐polybutadiene synthesized with nickel‐based Ziegler‐Natta catalysts

Modern methods for determining the molar mass distribution of polymers. General considerations and application to sedimentation equilibrium

A new procedure for the determination of the molar mass means Mn, Mw and Mz and of the molar mass distribution W(M) = f(M) of polymers is proposed and applied on measurements of the sedimentation equilibrium. The procedure includes the extrapolation of the experimentally determined polydisperse measurable quantity at different concentrations to zero concentration. From these values the molar mass distribution is calculated by nonlinear least squares routines of the integral equation. This method of calculation is, contrary to the inverse transformation of the integral equation, which has been used up to now, well conditioned and stable. The proposed method has been applied to sedimentation equilibrium. In this case the reduced concentration profiles (interference optic, absorption optic) or reduced concentration gradient profiles (schlieren optic) measured at different concentrations must be extrapolated to zero concentration and followed by nonlinear regression of the integral equation Uw(X) = ∫ W(M) U(X,M) dM. Measurements on polystyrenes with different polydispersities and modalities (e.g. bimodal molar mass distributions), and on the polyelectrolytes sodium polystyrene sulfonate and polyethylene imine exhibit resonable results. The result of the investigation is, that the proposed procedure is a suitable and reasonable possibility for the determination of molar mass distribution of polymers by sedimentation equilibrium. For polyelectrolytes in particular this method is better than gel permeation or size exclusion chromatography.

Linear viscoelasticity, simple and planar melt extension of linear polybutadienes with bimodal molar mass distributions

Shear oscillations, simple and planar elongations have been performed with anionically polymerized polybutadienes (PB) and their blends at room temperature. The PB components were of different molar mass averages and of narrow molar mass distributions; the blends had bimodal molar mass distributions and are represented by the weight ratio w of the high molecular component. The crossover G′(ω) = G″(ω) obtained from oscillatory measurements shows correlations with molecular parameters. For the zero shear viscosity the well-known relation η0 ∞ Mw3.4is found. The recoverable equilibrium shear compliance Je0 is nearly the same for the components; for the blends it strongly depends on w with a pronounced maximum at small w. In elongation outside the linear region strain hardening is found; its magnitude depends on Mw of the components, the composition w of the blend, the mode of elongation (simple or planar), and the elongational strain rate. The hardening revealed in the increase of the elongational viscosity above the linear viscoelastic limit increases as a function of w up to a maximum similar to Je0 such that, for both properties, the molecular processes may be the same. The elongational viscosity µ2 (from the lateral stress in planar elongation) is above the linear viscoelastic limit for bimodal and below this limit for conventional broad molar mass distributions. In general, it can be stated that with a more narrow molar mass distribution of linear polymers the elongational behavior of the melts comes closer to the linear viscoelastic limit.

Determination of bimodal molar mass distribution functions of polystyrene by photon correlation spectroscopy: numerical simulations and experimental realization in comparison with gel permeation chromatography data

Determination of bimodal molar mass distribution functions of polystyrene by photon correlation spectroscopy: numerical simulations and experimental realization in comparison with gel permeation chromatography data

Characterization of microgel‐containing polymers by the light scattering method: A new method of evaluation and its comparison with other procedures

AbstractA new method suitable for computer treatment was suggested for the evaluation of data obtained by light scattering from polymer solutions containing microgel. The method is based on the assumption of bimodal molar mass distribution and requires no additional data and/or restrictions. The accuracy of the present method was compared with that of other evaluation procedures (suggested by Lange, Gruber and Schurz, Strazielle, and Kratochvíl) using model mixtures consisting of two polymers differing by 2 to 3 orders of magnitude in molar mass. The values of molecular parameters (Mw,1, Mw, z2, (rz,12)1/2, (rz,22)1/2) obtained by the present method are in general more reliable then those calculated by means of the other evaluation procedures.