P971: Effect of ionic strength on the configuration of a polymer in aqueous solution
An experiment for the physical chemistry laboratory has been developed in which students determine the configuration of poly(vinylpyrrolidone), or PVP, in both water and an aqueous solution of sodium sulfate. Student groups begin the laboratory by crafting a hypothesis about the effect of ionic strength on the configuration of a polymer in aqueous solution. Next, they plan a procedure to measure flow times of solutions of selected concentrations of different molecular weights of PVP using an Ubbelohde dilution viscometer. These flow times are then measured and subsequently used to calculate relative and specific viscosities. From plots involving these viscosities versus concentration, the intrinsic viscosity for each polymer sample is determined. An additional plot of log [intrinsic viscosity] versus log [molecular weight] yields the Mark-Houwink parameters “a” and “K”. Student groups make comparisons between these two values and those found in the literature, in addition to determining the shape of PVP in water based on the “a” parameter. A further extension is planned to obtain the weight average molecular weight of each PVP sample using matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI TOF MS), thereby introducing an additional method of data analysis. For larger laboratory sections various student groups could be assigned other solvents for PVP. Another possibility would be to assign different polymers, particularly biopolymers if a number of biochemistry majors are enrolled in the course.