Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is shown to give inaccurate results for the analysis of low molecular weight ethoxylate polymers. It is demonstrated that when the degree of ethoxylation is low (n < 9), MALDI-TOFMS produces substantially higher values for the number-average molecular weight (M n ) than obtained with more classical methods such as NMR spectrometry and a wet chemical method to determine the hydroxyl number. Apparently, this is the result of discrimination of the former technique against lower molecular weight ethoxylates in a polymer distribution. The results presented in this paper demonstrate this discrimination through the analysis of a series of ethoxylates with progressively lower M n values. It was also found that by derivatization of the polymer with phthalic anhydride, to produce the carboxylate derivative, the discrimination is reduced and data obtained with MALDI-TOFMS match more precisely the results obtained with the other methods.
A high-frequency torsional rod apparatus (HFTRA) has been developed for measurements of moderately high-frequency viscoelastic properties of dilute, low viscosity polymer solutions. It employs a long cylindrically shaped resonating element driven by an X-cut (torsional mode) quartz piezoelectric crystal. Up to 11 different normal modes are employed, resulting in 11 discrete frequencies, ranging from approximately 20 to 500 kHz. Measurements are made in free decay; a key feature of the apparatus is the method for high precision measurements of damping coefficients and eigenfrequencies. The instrument is suitable for liquids with O.OO3P < Iv*1 < 5P and v' 3 1.337j'. Qpically, 7' has a relative uncertainty of ?2% or less, with the relative uncertainty for 4 being about 4% for liquids with # greater than O.OlP. Values of 7' obtained in the HFTRA for low viscosity liquids with small g show excellent agreement with the known steady flow viscosities. Measurements of the viscoelastic properties of polymer solutions employing the HFTRA and two other instruments capable of higher precision measurements show excellent agreement, demonstrating that the HFTRA is sufficiently precise for dilute-solution viscoelasticity studies; the working frequency range is such that most of the polymer relaxation time spectrum can be probed for many polymer/solvent combinations.
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