The use of pulsed NMR to follow radiation effects in long chain polymers

Abstract

Exposure of polymers to radiation can result in very small chemical changes, but very considerable modifications in physical properties such as solubility or elastic behaviour. The extent of such modifications has usually been determined by solubility measurements. We have now found that pulsed NMR techniques can, under suitable conditions, provide an alternative, more rapid, and non-destructive method of obtaining this type of information. Pulsed NMR techniques have been used to measure the proton spin-spin relaxation time, T 2, for a variety of irradiated polymers at temperatures well above their glass transitions. At these temperatures T 2 is largely determined by the long range chain motions which are greatly influenced by intermolecular couplings (crosslinks or entanglements) and network formation. Protons within a network possess a shorter T 2 than those in surrounding non-network material and this provides a means of determining the network content. This is possible for a partially crosslinked system produced, for instance, by irradiation and under favourable conditions for systems which contain a dynamic network formed by chain entanglements. Changes in molecular weight induced in polymers which undergo chain scission in preference to crosslinking on irradiation may also be followed by T 2 measurements to provide a G value for scission. This article reviews some of our pulsed NMR studies of the above changes produced by irradiation of a variety of polymers.