Measurement of molecular weightof polymers

Application Note 26

Summary•		Fast, accurate and repeatable•		No sample preparation•		No hazardous solvents or high temperatures•		Simple linear calibration•		Simple and reliable technique, suitable forunskilled personnel

Application
Molecular weight (MW) is one of the crucial characteristics
of polymers. It correlates with the structural features ofmacromolecules (polymer chain length, branching andcopolymerisation transformations) which in turn affects theMethod

physical properties of materials and polymer-based products.

The NMR technique is based on measuring the NMR response

Molecular weight measurements are considered “a must”

from a whole polymer sample (powder or pellets). MW is

at various steps in polymer production, especially for quality

quantified using the linear relationship (calibration) between

control but also in research and development of new materials.

the NMR data and reference values from a set of polymer
samples with known MW.

Advantages of NMR
There are several techniques used for the determination ofCalibration

polymer molecular weight:

Ultimately, only two well-known standards are required to• Gel permeation chromatography (ASTM D5296)• Viscometry (ASTM D1601, ASTM D4603 and ASTM

calibrate the instrument as the calibration is linear. However• Light Scattering (ASTM D4001)

NMR is a comparative technique therefore cannot be more

All these methods require dissolution of the polymers

being compared; error is reduced by analysing more reference

in organic solvents and in some cases using elevatedsamples.

D1243)

initially it is recommended that the instrument is calibratedby 3-6, preferably more, standards with known molecularweight values evenly spread over the range of interest.
accurate than the reference technique against which it istemperatures, both of which introduce unwanted risks inthe work environment, significant costs for consumables andhigh energy consumption. In contrast to these wet chemistry

Measurements

methods, time-domain Nuclear Magnetic Resonance (NMR)

Polymer samples are poured into 18 mm glass NMR tubes up

does not require either the dissolution step or the use of

to a predefined mark; sample weighing is not required for thishigh temperatures.

measurement. The sample tubes are placed in a temperature-

The MQC+ benchtop Nuclear Magnetic Resonance (NMR)
analyser provides an alternative method which is quick and easy
to perform, simple to calibrate, and requires minimal sample
preparation. As such it is ideal for routine operation in either alaboratory or production environment without any requirement
for additional chemicals or specialist operator training.

controlled conditioning block at 40ºC for 20 minutes prior toanalysis. Measurement
time is 40 seconds persample.

Results
Figure 1 shows an example of a calibration formolecular weight of polyolefins with MW values
from 864 to 9360 kg/mol. As seen in this figure,
NMR gives an excellent linear correlation between
the NMR response and the reference MW-values.
The instrument repeatability was tested bymeasuring two samples five times.
Table 1 shows the repeatability test results.

Table 1. MW measurement
repeatability results
Sample ReferenceID
MW,kg/mol

Repeat

MW byNMR,kg/mol

AverageStandard
Relative
MW for 5
deviation
standard
repeat NMR
for 5 repeat
deviation formeasurements,
NMR
5 repeat NMR
kg/molmeasurements, measurements,kg/mol%

Sample186412345803
786
771
844
797

800

243Sample
28980

123458548
8660
8554
8215
8738

8543

179

2

Figure 1. Calibration for measuring
MW in polyolefin samples. Correlation
coefficient R2=0.9924, standard deviation
0.0956, variance 0.0091.

Conclusions•	NMR is very stable over the long term and rarely needs calibration
adjustment.

• NMR is insensitive to air voids between the pellets or grains of powder.
• Measurement precision is good compared to traditional methods.
• Sample measurement time is rapid.
• The NMR technique is non-destructive so the same sample may bemeasured several times before being analysed by other techniques.

• The NMR instrument may be used for measurement of other polymer
quality control characteristics.

visit www.oxinst.com/mqc for more information or email: magres@oxinst.com

5309

This publication is the copyright of Oxford Instruments plc and provides outline information only, which (unless agreed by the company inwriting) may not be used, applied or reproduced for any purpose or form part of any order or contract or regarded as the representation
relating to the products or services concerned. Oxford Instruments’ policy is one of continued improvement. The company reserves the rightto alter, without notice the specification, design or conditions of supply of any product or service. Oxford Instruments acknowledges all
trademarks and registrations. © Oxford Instruments plc, 2019. All rights reserved. Part no: MR/217/0619