QC of polymers using NMR Relaxometry on the benchtop
Approaches to the quality control of polymers have developed dramatically over the past two decades. Manufacturers require faster, cheaper and more reproducible testing methods to assure the quality of their products and improve the economics of polymer manufacturing. This has resulted in the replacement of many old fashioned methods, particularly those involving wet chemistry, difficult and slow protocols, and methods which use hazardous or environmentally undesirable materials, such as many solvents. NMR relaxometry is one of the techniques that have become well established in the QA/QC laboratories of the polymer industry. In this paper, the technology of NMR relaxometry will be discussed with examples shown of how the technique can be used in place of the older methods around which many test standards were originally written.
NMR relaxometry systems make measurements on the mobility of components in a material, distinguishing between liquid, amorphous and ordered solid (e.g. crystalline) components.
We will explore examples where this easy-to-use relaxometry technique has been applied to make a number of measurements where chemical extraction can be avoided, and which may be performed in a basic benchtop environment by operators requiring no knowledge of how an NMR system works.
An example of this is the measurement of xylene solubles in polypropylene. Polypropylene is a semi-crystalline polymer with domains of both relatively rigid and flexible material. The rigid domains are largely crystalline while the more flexible domains are substantially amorphous. The crystalline regions are predominantly formed from long chain polypropylene molecules which neatly fold and stack to form crystalline polymer.
However, there will also be shorter, lower molecular weight species present, and some branched molecules. These do not stack and fit in with the crystalline segments, and tend to congregate together between the crystalline regions to form an am ...