Measurement of crystallization in stretched rubber by rapid-passage NMR

Abstract

Continuous-wave rapid-passage proton NMR (30 MHz) was performed on lightly crosslinked natural cis-polyisoprene at elongations up to 720%, at temperatures between −100° and 100°C, and after delays of up to several weeks after stretch. A method of separating the spectra into a narrow and a wide component was developed and applied. The results are interpreted in terms of the effects of a glass transition and strain-induced crystallinity and are in general agreement with results by other methods. The degree of crystallinity is found to increase strongly with strain (from zero for unstretched samples to 31% at elongations of 720% at room temperature), to increase with time, reaching equilibirium most rapidly in the most highly strained samples, and to decrease approximately linearly with temperature above about 20°C.