Anisotropy of the Segment Mobility versus Self- and Pair-Correlation Functions in Polymer Melts under Mesoscopic Confinement

Abstract

Techniques typically used for studies of polymer dynamics such as NMR relaxometry, quasielastic neutron scattering, multiple-quantum build-up NMR, dielectric relaxation spectroscopy, and mechanical relaxation are specified and commonly classified in terms of correlation functions. Two categories of correlation functions are identified with respect to their specific ability to describe translational fluctuations on the one hand, and molecular reorientations on the other. The first category is of the dynamic-structure factor type reflecting the absolute or relative displacement behavior of particles. This type of function is in contrast to the second category, namely correlation functions of spherical harmonics of different orders characterizing rotational diffusion of molecules or molecular groups. In polymers, rotational diffusion tends to be strongly anisotropic. It is elucidated that the long-time tail of the correlation decay is particularly indicative for model characteristic features. The representation of experimental results by correlation functions instead of methodspecific technical terms permits unambiguous comparisons and interpretations based on different techniques. This is demonstrated in the context of a problem of particularly topical interest, namely polymer melts confined in nanoscopic porous matrices. Methods probing correlation functions of spherical harmonics are shown to be sensitive to rotational chain dynamics severely modified under geometrical confinement, the socalled corset effect. On the other hand, correlation functions of the dynamical structure factor type characterizing translational fluctuations reveal little influence of such constraints in the experimentally accessible time/space window. In order to support this classification, the potentially competitive influence of wall adsorption effects is discussed in addition. Criteria permitting to rule out this sort of retardation mechanism under appropriate conditions are specified.

Molecular Dynamics in Polymers

“…In this context, it is most important to distinguish the role of adsorption at the pore walls as a potential source of phenomena with a similar appearance as the corset effect as discussed in Ref. [106]. Finally, in Ref.…”

Dynamics of ionic liquids in bulk and in confinement by means of1H NMR relaxometry – BMIM-OcSO4in an SiO2matrix as an example

“…Nevertheless, in most cases, research is directed towards water [1][2][3] (because of its relevance to biology) and polymers. [4][5][6] The physical and chemical properties of confined molecules are very different than in bulk. Studies concerned with phase transition behavior show that geometrical restrictions lead to a shift of the glass transition temperature, melting and freezing points; [7][8][9] for instance water confined in nanotubes shows a liquid-like behavior at temperatures as low as about 50 K. 7 Such studies are often accompanied by molecular dynamics simulations attempting to explain the observed phenomena at the molecular level.…”

Structure and dynamics of guest molecules confined in a mesoporous silica matrix: Complementary NMR and PDF characterisation

“…Even when the averaging cannot be characterised directly, the inuence of the uctuating interaction tensors on the different NMR relaxation times is correlated to the characteristics of the dynamics. [54][55][56] As a consequence NMR is increasingly applied to study dynamics in conned porous environments [57][58][59][60][61][62][63][64][65] mostly by measuring the effect on quadrupolar interactions from isotopically deuterated molecules. In this study, we use the 13 C Chemical Shi Anisotropy (CSA) and 23 Na quadrupolar couplings to probe rotational diffusion of the complex inside the nanopores.…”