Klaus H. Ebert

Fractionation of polymers by gel permeation chromatography: An experimental and theoretical approach

Summary The mechanism of gel permeation chromatography has been studied in a series of experiments performed on separation matrices of controlled pore glass with known pore structure and narrow pore size distribution, and using dextran fractions with accurately determined molecular weights and very narrow molecular weight distribution. Molecular weight calibration of the columns and effects of experimental variables on the shape of the elution peak and the peak broadening function were investigated, and the distribution and transport of polymer molecules between the mobile phase and the stationary phase within the pores of the separation material were discussed. An improved stochastic model for molecular weight calibration of separation matrices with cylindrical pores is developed as a function of the pore diameter.

Size Exclusion Chromatography (SEC) of Complex Polymers - Generalized Analytical Corrections for Imperfect Resolution

Abstract Herein is reported generalized analytical solutions which permit correction for imperfect resolution when the molecular weight calibration curve is nonlinear and the variance of single-species chromatograms changes significantly with molecular size of the polymer solute. Two kinds of generalized analytical solutions have been obtained. One is a solution of Tungs integral equation for the corrected chromatogram or the molecular weight distribution and the other is a solution for the corrected molecular weight averages of the whole polymer. Also discussed is the use of local corrections for imperfect resolution across the chromatogram with detectors such as the low angle laser light scattering spectrophotometer (LALLS) when used with micro and macropackings.

Effects of mechanical stress on the reactivity of polymers: activation of acid hydrolysis of dextran by ultrasound

SummaryThe influence of ultrasonic irradiation on the kinetics and mechanism of acid hydrolysis of dextran is studied. The increase in the rate constants of the combined hydrolytic and ultrasonic degradation was investigated as a function of the molecular weight of dextran. For molecular weights higher than the limiting value for ultrasonic degradation the rate constants are proportional to the molecular weight raised to the power of 4/3; for molecular weights below the limiting value the rate constants are proportional to the molecular weight raised to the power of 5/6. The effects are discussed in connection with stretching forces which act on the polymer molecules as a result of ultrasonic cavitation.

Free radical polymerization of p-methyl styrene

Abstract An experimental investigation is reported of the free-radical synthesis kinetics of poly( p -methyl styrene) in cyclohexane at low monomer conversion in the temperature range 50°–70°C using AIBN initiator. The p -methyl styrene monomer contained greater than 97% para isomer. Isothermal conversion/time curves were obtained using glass ampoule reactors and gravimetry and molecular weight distribution and weight-average molecular weights were measured by s.e.c. and low angle laser light scattering photometry ( LALLSP ). Transfer to small molecules (monomer, cyclohexane, —) was found to be small or negligible with virtually all of the polymer chains formed by termination by combination. Predicted and measured weight-average molecular weights are in good agreement for polymer synthesized at low conversions.

Oligomer formation in the chemical degradation of dextran

SummaryThe production of oligomers — to a degree of polymerization 5 — during acid hydrolization of polymer dextrans was estimated quantitatively by a combined aqueous SEC and adsorption separation. It was found that the molar concentrations of the oligomers were higher for those with lower DP. This experimental result is in agreement with a degradation model, in which the molecular weight decreases with the power of 2/3 versus time, which has been proposed earlier.

Modelling of Polymer Gel Formation and Gel Reactions with Monte Carlo Methods for 3-dimensional Networks

The thermal treatment of poly-(p-methyl-styrene) was investigated experimentally. In contrast to polystyrene, gels are formed and decomposed again, depending on temperature. Based on the experimental results a reaction model for polymer decomposition, polymer gel formation and polymer gel degradation was developed. A Monte Carlo simulation program using methods from percolation theory is presented which covers thermal gel formation, gel formation caused by radiation, gel formation during polymerization, thermal, mechanical and ultrasonic decomposition of polymer gels and highly branched polymers.