MASE 473: Determining Molecular Weight Using Solution Viscosity

Lab 2 Report for MATSE 473

Determining Molecular Weight using Solution Viscosity

Name: Mengfang Li

Group D Lab Partners: Michael Lorenzo, Peter Mcclure

01/24/2012

Introduction:

This lab was designed to demonstrate the method to measure the intrinsic viscosity of the prepared polystyrene. Intrinsic viscosity, which is measured from the flow time of a solution through a simple glass capillary, has considerable historical importance for establishing the very existence of polymer molecules. The aim of this lab is to estimate the molecular weight of a polystyrene sample by dilute viscometer. The viscosity average molecular weight is determined for polystyrene obtained from bulk process and solution process. The result molecular
The molecular weight is determined to be 147423 g/mol. However, the viscosity data for solution process product does not show a good fit on a straight line. While the first three data points (C1=0.0025 g/ml, C2=0.0017g/ml, C3=0.0013 g/ml) shows relatively linear behavior, the fourth data point (C4= 0.001 g/ml) has significantly deviated from the other points. This might be caused by the undissolved polystyrene which clogs up in the capillary tube and slows down the flow process. This explains why the flow time of the first concentration (C1=0.0025 g/ml) was significantly longer than the lower concentrations. As the 5ml of THF added for each run, the PS dissolves the best at the lowest concentrations. Due to the unexpected error, the intrinsic viscosity of PS (solution process) is determined as the reduced viscosity at lowest concentrations. The MW for polymer (solution process) is calculated to be 31254
This agrees with the experimental result in this lab. The bulk process yields the polymer with molecular weight about four times greater than that of solution process. The main reason causing the production of low MW polymer is that the solvent added (solution process) has made the monomer concentration much lower than the bulk process. In solution process, only soluble and low molecular weight polymers could be obtained because of chain transfer reactions with the solvents. The solvent in the solution process act as good heat conductor but also significantly lowered the monomer concentration. In contrast, bulk process enables high monomer concentration without addition of solvent. As a result, high MW polymers are easily obtained by bulk polymerization.

Conclusions/Overview:

All polymers increase the viscosity of the solvent in which they are dissolved.

The result of this experiment verifies the assumptions made at the beginning of lab. The bulk process yields the polymer with molecular weight greater than solution process mainly because of the high monomer concentrations of bulk process. The viscosity data for bulk process product shows a good linear behavior. While the viscosity data for solution process product contains errors due to undissolved PS in solution.

Comments:

In order to obtain a better viscosity data, the prepared PS should ground into powder form before