Goal and Objectives
Introduction
Experimental
Calculations
Discussion
Pre-lab Quiz

Goal

The goal of this lab is for you, the enthusiastic lab student to learn and understand how to estimate the molecular weight of an unknown polymer sample by dilute solution viscometry.

Objectives

1. Learn proper use of the Cannon-Ubbelohde viscometer.
2. Determine viscosity average molecular weight (M_v) of your polymer.

Introduction

For this lab's introductory information, visit these fun-filled Macrogalleria pages:

Dilute Solution Viscosity
Molecular Weight

It is imperative that you read these pages before you perform the lab. Questions in the pre-lab quiz will test your knowledge of this material.

The overall procedure presented in this lab is generic and may be used for most polymer - solvent systems. The polymer - solvent - concentration combination, however, will determine how viscous the solvent and solution are and which size viscometer to use.

Experimental

1. Using a 100 ml volumetric flask, prepare 1 wt. % solutions of your polymer in an appropriate solvent.

2. Using 50 ml volumetric, prepare dilutions of 0.75, 0.50, and 0.25 wt %. (Dilutions can also be done in the viscometer. Caution, limit of viscometer reservoir is ~20 ml.)

3. By using an Ubbelohde viscometer, determine the flow time for solvent (deionized water). NOTE: THE SOLVENT FLOW TIME MUST BE AT LEAST 100 SECONDS. IF NOT, CHANGE VISCOMETER SIZE.

Cannon-Ubbelohde Viscometer Instructions:

a. Clean viscometer with suitable solvent and blow dry, clean, filtered air through viscometer to remove traces of solvent.

b. Introduce sample into viscometer by pipetting ~10 ml into Resevoir J via Tube C. (Click here to see a picture of what your hands should be doing)

c. Place viscometer in constant-temperature bath (25.0^oC). Allow 10 minutes for viscometer to thermally equilibrate.

d. Apply suction to Tube A while holding finger over Tube B and bring sample above Mark E into Bulb D. (picture)

e. Efflux time is measured by allowing the sample to flow freely through Bulb F. Start stopwatch when the meniscus reaches mark E and and stop when it reaches mark G.(picture) (picture)

f. Record efflux time in seconds to hundredths (i.e. 102.59 sec) and repeat.

4. Repeat efflux time measurement (steps d and e above) at least three times. The readings should agree to within 0.1% of the average flow time. If flow time variation is noted, repeat efflux time measurement six times and take the average of three readings within 0.1% of each other.

Calculations

1. Make a table of concentration (g/100ml), efflux time (sec), h/h_o, h_sp/c, and (ln h_r)/c.

2. Plot h_sp/c vs. c and ln h_r/c vs. c on the same plot. Determine y-intercept(s).

3. Calculate molecular weight of unknown from Mark-Houwink equation, using constants found in literature.

Discussion

1. Discuss the effect of a variation in temperature on viscosity from a molecular viewpoint.

2. Discuss the differences in molecular size and shape in a good solvent, a poor solvent, and a q solvent from a molecular viewpoint.

Pre-lab Quiz