Proton NMR Characterization of Room-Temperature Aging in Isotactic Polypropylene and Ethylene/Octene Copolymers
David L. VanderHart, Chad R. Snyder, R G. Alamo
In this paper, we utilize the relatively high sensitivity of proton NMR to revisit a well recognized problem of ambient-temperature aging in a couple of semicrystalline polymers, isotactic polypropylene (iPP) and 2 ethylene/octene (EO) copolymers, all of whose non-crystalline (NC) regions have glass transition temperatures, Tg, substantially lower than ambient. Bloch-decay proton spectra are used to monitor, via the generation of difference spectra pertaining to different aging times, changes in mobility. For iPP, both lineshape and relaxation changes are consistent with secondary crystallization, distinct from the thickening of primary crystals, being the primary source of change. Significant reductions in mobility are also seen for EO copolymers; however, the notion of crystallinity is greatly compromised by the non-crystallizing branches in these materials that contain 9 and 15 mole % octene.The difference spectra yield minimum fractions of chains whose mobility is modified by aging. Over a 4 day aging period, the fraction of such residues varies between 1.8 % and 4% of the total number of residues. Furthermore, there is a suggestion in the data, albeit not proof, that the regions where motions are strongly modified are not very homogeneously distributed throughout the non-crystalline regions. For EO copolymers, aging followed melt crystallization; for iPP, aging was followed after thermal cyclings at least 70C below the melting temperature.
, Snyder, C.
and Alamo, R.
Proton NMR Characterization of Room-Temperature Aging in Isotactic Polypropylene and Ethylene/Octene Copolymers, ACS PMSE Preprint, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=852118
(Accessed December 7, 2023)