Segmental and chain dynamics in nanometric layers of poly(cis-1,4-isoprene) as studied by broadband dielectric spectroscopy and temperature-modulated calorimetry

Abstract

Segmental and chain dynamics in nanometric (7–400 nm) layers of poly (cis-1,4-isoprene) (PI) are analyzed by Broadband Dielectric Spectroscopy (BDS) and temperature-modulated AC calorimetry. While for the segmental mode, taking place at the length scale of 2–3 polymer segments and corresponding to the dynamic glass transition, no dependence on the layer thickness and molecular weight is found, the normal mode, reflecting fluctuations of the end-to-end vector of the chain, shows pronounced effects: (i) it strongly varies in its relaxation strength with the layer thickness; (ii) for polymers having a molecular weight M_w comparable to M*, the critical molecular weight marking the onset of reptation dynamics, the mean spectral position does not change with the thickness, (iii) in contrast, polymers with M_w > M* are found to be severely influenced in their relaxation strength and the mean spectral position of the normal mode relaxation, and (iv) it is proven that the concentration of the polymer solution out of which the layers are prepared by spincoating has a hitherto unrecognized impact on the chain dynamics in (one-dimensional) nanometric confinement. These results prove that the dynamic glass transition in thin layers of PI is not influenced by nanometric confinement, while the chain dynamics are altered in a manifold of ways due to interactions with the surface of the underlying substrate.