# Determining Molecular Weight Using Solution Viscosity

Topics: Polymer, Viscosity, Viscometer Pages: 6 (1278 words) Published: July 24, 2012
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 weight values were compared to evaluate the advantage and disadvantage of bulk and solution process.

Dilute solution viscometry of polystyrene was utilized to find the intrinsic viscosity of the polymer, the measure of how much a polymer can increase the viscosity of a solvent (tetrahydrofuran, THF) at a certain temperature (25°C). The Mark Houwink equation, =KMa illustrates the relationship between intrinsic viscosity [ and molecular weight [M]. This relationship was investigated in this lab and was used to find the molecular weight of the PS sample using the given Mark Houwink K and a values.

Relative viscosity is defined to be . Since most polymer solution and pure solvent have density approximately equal. The relative viscosity is a simple time ratio:

Specific viscosity is the fractional change in viscosity upon addition of polymer:

The intrinsic viscosity can be determined from extrapolating sp/c and c-1ln(rel) vs. c to c=0. A typical graph is shown below [2]:

[, the intrinsic viscosity[1].

More importantly for our purposes is the scaling relationship between [] and molecular weight: =KMa (Mark-Houwink Relationship)

Operational Procedure:

The Ubbelohde capillary viscometer is sketched below[1]:Capillary Capillary
A: Plug while drawing fluid into capillary
A: Plug while drawing fluid into capillary
D: Timing lines
D: Timing lines
C: Pressure equilibration arm
C: Pressure equilibration arm
Little bulb, whose volume = V.
Q = V/tflow
Little bulb, whose volume = V.
Q = V/tflow
Big Bulb/Reservoir
Big Bulb/Reservoir
B
B

For pure THF flow time measurement, 10ml of THF was added to the Ubbelohde viscometer. The efflux time of pure THF was determined using the viscometer at room temperature. 0.025g of PS was added to viscometer in order to set an initial concentration and the efflux time was determined. Subsequently, 5ml of the THF solvent were added to the viscometer to make 4 concentrations: C1= 0.0025 g/ml

C2= 0.0017 g/ml
C3= 0.0013 g/ml
C4= 0.001 g/ml

The flow time for each concentration was determined three times and the average flow time was obtained by taking the average of the three values. The flow time is defined as the time which fluid level passes between the timing lines labeled on Ubbelohde viscometer.

The intrinsic viscosity and weight average molecular weight were determined using the method illustrated in the previous (introduction) section. Mark-Houwink parameters were (298K) K=3.9 * 10-2 ml/g and a= 0.58 [3].

Results:

PS (Bulk Process):

Table 1: Dilute Viscosity Data for PS (Bulk Process)
| Efflux Time (sec)| rel| sp| C (g/ml)| sp/c | c-1ln(rel) | Pure| 123.3567|  |  |  |  |  |
0.025 g PS +10ml| 131.6867| 1.0675| 0.0675| 0.0025| 27.0110| 26.1381| 0.025 g PS +15ml| 130.0200| 1.0540| 0.0540| 0.0017| 32.4099| 31.5649| 0.025 g PS +20ml| 128.4500| 1.0413| 0.0413| 0.0013| 33.0314| 32.3677| 0.025 g PS +25ml| 127.5300| 1.0338| 0.0338| 0.0010| 33.8312| 33.2715|

Figure 1. plot for intrinsic viscosity determination (Bulk Process)

The intrinsic viscosity [is...