Application Note 19 Determination of the Degree of Polymerization of a Polymethacrylate by means of Pulsed NMR S.A. Jones Bruker Analytische Messtechnik GmbH, 7512 Rheinstetten 4, West-Germany H.J. Stronks Bruker Spectrospin (Canada) Ltd., 555 Steeles A ve. East, Milton, Ontario, Canada, L9T 1Y6
The Bruker Minispec is a small, highly automated and easy to use pulsed NMR spectrometer that is particularly well-suited to routine quality or process-control analysis in a variety of industries. The instrument is microprocessor controlled and can be operated by personnel with only minimal instruction. The principal areas of application of the minispec are the food and chemical industries. Of particular importance in the latter field is the polymer industry. The development of polymers, large macromolecules formed by the repetitive combination of smaller, simpler molecules (monomers), has been one of the most significant and far reaching aspects of industrial development in this century. Polymers are complex systems, the manufacture of which must be carefully monitored if the finished product is to have the desired properties. Fast methods of analysis which offer rapid feedback of results, such as spectroscopic techniques, are therefore of great utility in the manufacture of polymer materials. Earlier minispec application notes have described the use of the minispec to measure the rate of polymerization of styrene (number 16). This report describes the use of the minispec to measure the degree of polymerization of a polymethacrylate. Method It is possible to use the minispec to monitor a polymerization reaction because of the fact that the polymer and monomer have different relaxation times. Relaxation times are important NMR parameters that govern the decay of the NMR signal following an initial pulse of exciting radiation. By using the so-called ‘spin-echo’ technique it is possible to isolate the signal due to the monomer, which is proportional to the amount of monomer present, and hence to follow the course of the reaction. The general procedure employed when using the minispec for measurements of this kind is as follows: The samples are weighed into sample tubes and thermostatted (in this case at 40°C). The instrument is calibrated with at least three standard samples measured by some other technique; the weight and degree of polymerization of each sample is entered and the calibration line is calculated by regression analysis. The calibration parameters are stored in the microprocessor. Unknown samples can then be measured against the calibration curve. The sample weight is entered, the instrument is triggered automatically when the sample is inserted and the result is calculated, displayed and, if desired, printed out.
Bruker Optik GmbH Silberstreifen D-76287 Rheinstetten Germany Tel. +49 721 5161-141 Fax +49 721 5161-237 e-mail:
[email protected] www.bruker.de last change: 06/24/02
Bruker Optics Inc. 2635 North Crescent Ridge Drive The Woodlands/Houston TX.77381 USA Tel. +1 281 292 2447 Fax +1 281 292 2474 e-mail:
[email protected] www.theminispec.com
Bruker Optics Ltd. 555 Steeles Avenue East Milton, Ontario L9T 1Y6 Canada Tel. 905 876 4641 Fax 905 876 4421 e-mail:
[email protected] www.theminispec.com
Instrument Parameters RTA 13 EDM 321A 46 2 2s
Probehead Program Attenuation Enhancement Relaxation Delays Mode Bandwidth Duration 01 Duration 02
Diode Low 1 ms 1ms The calibration graph is shown in Fig. 1 (on the left). The linear correlation coefficient is 0.995, indicating excellent linearity. The samples were then measured against the calibration curve. The total measurement time for the weighed sample was only about 10 seconds.
1.24
Signal / Wt 1.18
1.12
Linear correlation: 0.995 Intercept: 1.700 Slope: -0.0131 35.00
39.50
% Polymer
44.00
Figure 1
Table 1 shows a comparison between the given percentage polymerization values and those measured with the minispec. As can be seen, the agreement is very good. Conclusion
% Polymer (given)
% Polymer (minispec)
35.0 36.0
35.0 35.7
37.0 38.0
37.5 38.0
39.0 40.0 41.0
38.8 39.8 41.2
42.0 43.0
42.0 43.0
44.0
44.0
Table 1 Bruker Optik GmbH Silberstreifen D-76287 Rheinstetten Germany Tel. +49 721 5161-141 Fax +49 721 5161-237 e-mail:
[email protected] www.bruker.de last change: 06/24/02
Bruker Optics Inc. 2635 North Crescent Ridge Drive The Woodlands/Houston TX.77381 USA Tel. +1 281 292 2447 Fax +1 281 292 2474 e-mail:
[email protected] www.theminispec.com
Bruker Optics Ltd. 555 Steeles Avenue East Milton, Ontario L9T 1Y6 Canada Tel. 905 876 4641 Fax 905 876 4421 e-mail:
[email protected] www.theminispec.com
The measurements described in this report show that pulsed NMR using the BRUKER minispec is an excellent method for monitoring the course of polymerisation reactions. The main advantages of this technique in comparison with conventional methods of analysis are the speed and simplicity with which the results are obtained.
