Critical Molar Mass Research Articles

Viscoelastic properties of poly(dibutyl itaconate) melts

AbstractComplex dynamic viscosities η* of poly(dibutyl itaconate) melts were determined for nonuniform polymers with viscosity‐average molecular weights varying from 8000 to 327000 in the temperature range from 323 to 463 K and the frequency range from 0,1 to 100 rad/s. By applying the time and temperature superposition principle, master curves for the selected reference temperature T0 = 383 K were obtained, permitting the estimation of the Newtonian viscosity in terms of molecular weights. The temperature dependence of viscous flow was analysed by verification of the linearity of the aT shift factor, interpreted via the Williams‐Landel‐Ferry equation. Deviations from linearity were observed only for very low molecular weights M and at the highest temperatures. The activation energy of viscous flow was calculated by applying the concept of absolute reaction rates and the concept of free volume. Good agreement was observed. The linearized η* vs. M dependence changes slope at a critical molar mass Mc = 69000 with exponents of M α = 1,05 below and β = 2,92 above the Mc value in the function η* = f(M). The rheological properties of poly(dibutyl itaconate) are discussed in terms of substituent size and compared to those of other related vinyl polymers.

Studies of cyclic and linear poly(dimethylsiloxanes): 27. Bulk viscosities above the critical molar mass for entanglement

Low-shear bulk viscosity measurements are presented for sharp (narrow distribution) cyclic and linear poly(dimethylsiloxane) (PDMS) fractions in the number-average molar mass range 5 × 10 2 to 3.2 × 10 4 g mol −1. The results allow the first estimate to be made of the critical molar mass for entanglement, M c, for PDMS rings. This value does not differ significantly from that of the linear polymer. The viscosity-molar mass relationships are reported here for ring and chain samples above and below M c.

Bestimmung der Molmassenverteilung sowie der Stabilit�tsgrenze von Polyacrylamiden unter Benutzung einer kombinierten Ausschlu�chromatographie/Kleinwinkel-Laser-Streulichtphotometer Anlage

The molecular weight distribution including the calculation ofMn, Mw andMz of polyacrylamide in aqueous 0.1 M sodium sulphate-solution has been determined by simultaneously and continuously monitoring the effluent from a size-exclusion chromatographic (SEC) column filled with porous glasses with a low-angle-laser-light-scattering (LALLS) photometer. The recorder traces of the scattering intensity and the refractive index difference have been digitalized in about 400 steps corresponding to 200μl fractions. Each fraction has been assumed to be monodisperse. This method gives absolute data rather than relative data obtained from any calibration method. The investigations clearly show a flow-rate dependent degradation of samples with relatively low molar masses (Mw =500 000 g/mol atMw/Mn=2.5) at typical flow rates Q≥1 ml/min. A discussion of some problems encountered using aqueous SEC and a way to determine the critical molar mass, below which no degradation could be observed, based on elongational flow theories is given.