Polymer dynamics under cylindrical confinement featuring a locally repulsive surface: A quasielastic neutron scattering study.

We investigated the effect of intermediate cylindrical confinement with locally repulsive walls on the segmental and entanglement dynamics of a polymer melt by quasielastic neutron scattering. As a reference, the corresponding polymer melt was measured under identical conditions. The locally repulsive confinement was realized by hydrophilic anodic alumina nanopores with a diameter of 20 nm. The end-to-end distance of the hydrophobic infiltrated polyethylene-alt-propylene was close to this diameter. In the case of hard wall repulsion with negligible local attraction, several simulations predicted an acceleration of segmental dynamics close to the wall. Other than in attractive or neutral systems, where the segmental dynamics is slowed down, we found that the segmental dynamics in the nanopores is identical to the local mobility in the bulk. Even under very careful scrutiny, we could not find any acceleration of the surface-near segmental motion. On the larger time scale, the neutron spin-echo experiment showed that the Rouse relaxation was not altered by confinement effects. Also the entanglement dynamics was not affected. Thus at moderate confinement conditions, facilitated by locally repulsive walls, the dynamics remains as in the bulk melt, a result that is not so clear from simulations.

The role of the binding salt sodium salicylate in semidilute ionic cetylpyridinium chloride micellar solutions: a rheological and scattering study.

The micellar system based on cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) in brine solution is investigated on both macro- and micro-length scales through rheology and scattering measurements. The linear viscoelasticity of the system and its structural parameters are explored by systematically changing the amount of NaSal over an extremely wide range of concentrations, thus producing salt-to-surfactant molar ratios from zero to about 8.5. As a result, the well-known non-monotonic behaviour of the zero-shear rate viscosity as a function of salinity can be connected to micellar morphological changes, whose driving force is represented by the simultaneous binding and screening actions of NaSal. The viscosity behaviour can be seen as a direct consequence of consecutive lengthening/shortening of the contour length, where the micelles attempt to minimize the electrostatic charge density on their surface. Along similar lines, the scattering measurements of the semidilute solutions show that the local stiffness of the micellar chain changes with increasing salt content influencing the elasticity of the resulting network. Within this general view, the branching of the micelles can be seen as a side effect attributable to the main character of the play, namely, the binding salt NaSal, whereas the overall dynamics of the system is driven by the considerable changes in the entanglement density of the micellar network.

Nanoscale Motion of Soft Nanoparticles in Unentangled and Entangled Polymer Matrices.

We have studied the motion of polyhedral oligomeric silsesquioxane (POSS) nanoparticles modified with poly(ethylene glycol) (PEG) arms immersed in PEG matrices of different molecular weight. Employing neutron spin echo spectroscopy in combination with pulsed field gradient (PFG) NMR we found the following. (i) For entangled matrices the center of mass mean square displacement (MSD) of the PEG-POSS particles is subdiffusive following a t^{0.56} power law. (ii) The diffusion coefficient as well as the crossover to Fickian diffusion is independent of the matrix molecular weight and takes place as soon as the center of mass has moved a distance corresponding to the particle radius-this holds also for unentangled hosts. (iii) For the entangled matrices Rubinstein's scaling theory is validated; however, the numbers indicate that beyond Rouse friction the entanglement constraints appear to strongly increase the effective friction even on the nanoparticle length scale imposing a caveat on the interpretation of microrheological experiments. (iv) The oligomer decorated PEG-POSS particles exhibit the dynamics of a Gaussian star with an internal viscosity that rises with an increase of the host molecular weight.

Molecular View on Supramolecular Chain and Association Dynamics.

The chain and association dynamics of supramolecular polymer ensembles decisively determines their properties. Using neutron spin echo (NSE) spectroscopy we present molecular insight into the space and time evolution of this dynamics. Studying a well characterized ensemble of linearly associating telechelic poly(ethylene glycol) melts carrying triple H-bonding end groups, we show that H-bond breaking significantly impacts the mode spectrum of the associates. The breaking affects the mode contributions and not the relaxation times as was assumed previously. NSE spectra directly reveal the so far intangible H-bond lifetimes in the supramolecular melt and demonstrate that for both the microscopic and the macroscopic dynamics of the supramolecular ensemble the instantaneous average of the M_{w} distribution governs the system response at least as long as the Rouse picture applies.

Molecular scale dynamics of large ring polymers.

We present neutron scattering data on the structure and dynamics of melts from polyethylene oxide rings with molecular weights up to ten times the entanglement mass of the linear counterpart. The data reveal a very compact conformation displaying a structure approaching a mass fractal, as hypothesized by recent simulation work. The dynamics is characterized by a fast Rouse relaxation of subunits (loops) and a slower dynamics displaying a lattice animal-like loop displacement. The loop size is an intrinsic property of the ring architecture and is independent of molecular weight. This is the first experimental observation of the space-time evolution of segmental motion in ring polymers illustrating the dynamic consequences of their topology that is unique among all polymeric systems of any other known architecture.

Effect of nanoconfinement on polymer dynamics: surface layers and interphases.

We present neutron spin echo experiments that address the much debated topic of dynamic phenomena in polymer melts that are induced by interacting with a confining surface. We find an anchored surface layer that internally is highly mobile and not glassy as heavily promoted in the literature. The polymer dynamics in confinement is, rather, determined by two phases, one fully equal to the bulk polymer and another that is partly anchored at the surface. By strong topological interaction, this phase confines further chains with no direct contact to the surface. These form the often invoked interphase, where the full chain relaxation is impeded through the interaction with the anchored chains.

Direct observation of confined single chain dynamics by neutron scattering.

Neutron spin echo has revealed the single chain dynamic structure factor of entangled polymer chains confined in cylindrical nanopores with chain dimensions either much larger or smaller than the lateral pore sizes. In both situations, a slowing down of the dynamics with respect to the bulk behavior is only observed at intermediate times. The results at long times provide a direct microscopic measurement of the entanglement distance under confinement. They constitute the first experimental microscopic evidence of the dilution of the total entanglement density in a polymer melt under strong confinement, a phenomenon that so far was hypothesized on the basis of various macroscopic observations.

Exploring the surface permeability of nanoporous particles by pulsed field gradient NMR.

A new method to determine the surface permeability of nanoporous particles is proposed. It is based on the comparison of experimental data on tracer exchange and intracrystalline molecular mean square displacements as obtained by the PFG NMR tracer desorption technique with the corresponding solutions of the diffusion equation via dynamical Monte Carlo simulations. The method is found to be particularly sensitive in the "intermediate" regime, when the influence of intracrystalline diffusion and surface resistances of the nanoporous crystal on molecular transport are comparable and the conventional method fails. As an example, the surface permeabilities of two samples of zeolite NaCaA with different crystal sizes are determined with methane, as a probe molecule, at room temperature.