The Nature of Secondary Relaxations: The Case of Poly(ethylene-<i>alt</i>-propylene) Studied by Dielectric and Deuteron NMR Spectroscopy

Abstract

Main (α-) and secondary (β-) relaxation of the linear poly(ethylene-alt-propylene) (PEP) with different molar masses are investigated by dielectric and 2H NMR spectroscopy. Regarding the α-process, time constants in the range 10–11–1 s are reported, and the correlation function is described by a Cole–Davidson function with stretching parameter β ≅ 0.34. Both methods show clear evidence of a pronounced β-process; the latter is thoroughly studied by different NMR techniques. The results are compared to toluene-d3 and toluene-d5, respectively, as reference systems. The solid-echo spectra as well as the spin–lattice relaxation display typical features of spatially highly restricted angular displacements similar to those reported in other glass-formers showing a β-process−like toluene. Above Tg all chain segments participate. As both chain and methyl group were deuterated and as the β-process is rather fast, its angular displacement growing with temperature above Tg is directly reflected in a reduction of the effective quadrupolar coupling constant; i.e., the 2H solid-state spectra get narrower upon heating. Although the time constant τβ is the same for both segments, the angular amplitude of the methyl group bearing segment is larger than that of the other chain segments, and α- and β-process merge at high temperatures. For the other segments a merging appears not to happen. In this sense, the β-process is anisotropic as is demonstrated also for toluene-d3 for which it essentially involves a rattling around the C2-axis—a finding, however, which does not explain the fact that the β-process is dielectrically active. Regarding PEP below Tg, there are indications that only about half of the methyl group bearing segments are involved in the β-process whereas the other segments fully participate. Finally, we discuss possible implications of our findings regarding the nature of the β-process and compare time constants τβ(T) as well as τα(T) of polymers like poly(methyl methacrylate), poly(propylene glycol), and poly(isoprene) among others.