Experiment 6: Effect of Cross-linking on a Solution's Viscosity

The purpose of the experiment is to determine the effect of cross-linking has on the viscosity of the polymer solution. In the experiment, a hotplate was used to heat 50 mL of tap water in a 200 mL beaker to boiling. It took nearly two hours to fully dissolve 2 grams of PVA [poly(vinyl alcohol)] in the water by sprinkling minor amounts bit by bit. 10 drops of 0.1% methyl red indicator was added to PVA solution for the cross-linking part. 0.4 grams of borax was dissolved into 10 mL of hot water. 3.0 mL of the dissolved borax was added to the cooled PVA solution in 1.0 mL aliquots. The cross-linking consisted of the addition of sulfuric acid and sodium hydroxide. 3.0 M H2SO4 was added to the mixture until it became faint red. 2.0 mL of 1.0 M NaOH was added to the mixture to neutralize the H2SO4. Originally, it was planned to measure viscosity quantitatively with a start and end position of the mixture. It was planned to time how long it takes for the ball bearing to move through the start and finish markers on the beaker. However, as the experiment carried on, it was visible from the difference in composition of the mixture that viscosity had increased and a qualitative measurement of viscosity was best. The ball bearing was only able to pass through the least viscous times of the mixture: after the total addition of PVA and after the addition of H2SO4. The viscosity of the mixture increased after every addition of Borax and at the addition of NaOH. The addition of the acid resulted in the lowering of viscosity whereas the addition of base returned the solution back to its higher viscosity. Possible errors in experiment could arise from measuring the wrong amounts of the solution or even overheating the PVA when dissolving. Another error is not allowing the PVA to dissolve at minor amounts.

Materials

• Hotplate
• Tap water
• PVA [poly(vinyl alcohol)]
• Beakers
• Stirrer rod
• 0.1% Methyl Red Indicator
• Borax