Polyvinyl alcohol (PVOH),-(CH2-CHOH)n-, is a linear polymer which means it shows negligible amounts of branching. PVOH is soluble in water, which is unusual for synthetic polymers, but this allows PVOH to be used in a lot of commercial applications such as thickening and foaming agents. The aim of this experiment is to calculate the viscosities of cleaved and uncleaved PVOH solutions, and to calculate the fraction of head-to-head linkages in the polymer.
This will be done by using a water calibrated Ostwald viscometer held at a constant temperature. Plots of and ) vs. c for both the cleaved and uncleaved polymer were prepared and the intrinsic viscosity, η, was obtained by extrapolating linearly when concentration was equal to zero via a linear least squares regression. The head-to-head chain linkage was determined as the difference between the molecular weights of the cleaved and uncleaved polymer multiplied by the molecular weight of polyvinyl alcohol. It was found that the average value of head-to-head linkages, Δ, was -0. 00365%. It is a negative value because the cleaved PVOH has a higher molecular weight than the uncleaved. TheoryThe experiment that was performed was derived from Flory’s and Leutner’s experiment “Occurrence of Head-to-Head Arrangements of Structural Units in Polyvinyl Alcohol. ”
They found that degradation arises only from 1,2-glycol structures, and the increase in the number of molecules is a result of the degradation and that then provides a measurement of the percentage of head-to-head structures. It was also assumed that the 1,2 structures were distributed at random throughout the entire polymer, and the molecular weight distribution for the degraded polymer would represent the more probable distribution for linear polymers (Flory and Leutner, 1948). When a colligative property, such as osmotic pressure, is used, a number average molecular weight would be obtained. In our experiment, we used viscosity measurements, so our molecular weight is referred to as a viscosity average. Flory and Leutner used a common distribution in which the probability of a chain-termination reaction is constant over time, and doesn’t depend on the length of the polymer chain. The distribution they used is also function for product distribution after cleavage. Using the following distribution:P(M) = (1/Mn) e-M/Mn
It was found that the relationship between the number average molecular weight (Mn) and the viscosity average molecular weight (Mv) was found to be:Calibration of Ostwald Viscometer
An Ostwald viscometer (U-tube viscometer or capillary viscometer) is a instrument used to measure the viscosity of a liquid with a known density. [image: ]The viscometer was calibrated using water. The viscometer was placed in a large beaker equipped with a thermometer and containing room temperature water so the viscometer could stay at a constant temperature. 10 mL of water was pipetted into the side labeled as C in the picture above. The water was then drawn up to a point above the upper fiducial mark, labeled A above. The suction was released, and the flow rate was measured with a stopwatch until the water went below the lower mark, labeled as B. This process was repeated 5 times, and the average flowrate was recorded. The viscometer that was used in this experiment had a flowrate of 33. 5 seconds for water. ExperimentalTo make the polymer solutions, first a stock solution was prepared containing 4. 4263 g of dry polymer dissolved into 200 mL of distilled water. The powder was gradually sifted onto the surface and gently stirred, carefully avoiding foaming or bubbling, until the solution was homogenous. The solution was then cooled to room temperature and diluted to 250 mL using a volumetric flask.
An “initial cleaved” and an “initial uncleaved” polymer were prepared from the stock solution. The “initial cleaved” polymer was prepared by pipetting 50 mL of the stock solution quantitatively into a 250 mL Erlenmeyer flask, along with 0. 2541 g of KIO4 and 25 mL of water. The flask was then warmed up to ~70 ˚C and then cooled down to 25 ˚C in a water bath. After the polymer cooled, it was then transferred to a 100 mL volumetric flask and diluted to the mark. While the polymer was cleaving. the “initial uncleaved” polymer was prepared by pipetting 50 mL of the stock PVOH solution into a 100 mL volumetric flask. The solution was then diluted to the mark with distilled water and inverted gently to avoid bubbling or foaming. Dilutions of both the cleaved and the uncleaved polymers were prepared.
For the second dilutions, 50 mL of the initial polymers were transferred into a 100 mL volumetric flask and diluted to the mark with distilled water. For the third dilutions, 50 mL of the second dilutions were transferred into a 100 mL volumetric flask and diluted to the mark with distilled water. All of the dilutions were ran through the viscometer. The viscometer was placed into a water bath, so it would be held at a constant temperature (20. 5 ˚C). For each trial, 10 mL of the polymer you want to analyze was pipetted into the viscometer. The solution was then drawn up to a point above the upper fiducial mark using a pipette bulb. The suction was then released, and the flow rate was timed between the upper and lower marks using a stopwatch. Each polymer was tested at least 3 times or when 3 runs agreed within 1%, and the viscometer and pipette was rinsed with water and dried with acetone and air in between each run. The average flowrate for each polymer solution was recorded.
Using intrinsic viscosity, the percentage of head-to-head linkages in polyvinyl alcohol. It was found that this percentage, Δ, was -0. 00365%. Overall, the results of this experiment were accurate. In all of the trials that measured the viscosity of each polymer solution, as the solution became more dilute, the flowrate became faster; and the flowrates were within 1% of eachother. Also, as seen in graphs 1 and 2, the y-intercepts agree strongly with each other, which is what is expected to happen. Furthermore, it was expected that the plot of (1/c)(ln η/η0) vs. c would have a negative slope, which was obtained with a value of -7. 5652. One source of error in this experiment is the fact that the PVOH was assumed the density of water, which isn’t the case. The only cases in which the flowrates were the third uncleaved, and all of the cleaved polymer solutions.