Polymer Dynamics in Nanochannels of Porous Silicon: A Neutron Spin Echo Study

Abstract

Neutron spin echo spectra of poly(ethylene oxide) (PEO) melts confined in porous silicon (the mean pore diameter: 13 nm) were recorded at Q = 0.05, 0.08, and 0.11 Å -1 and successfully analyzed in terms of a two-state model, where chains adsorbed to the pore walls exhibit much slower internal dynamics than in the bulk and their centers-of-mass do not move, while free chains have bulklike internal dynamics and diffuse within an infinite cylinder in the center of the pore. The radius of this cylinder was found to be 1.4 nm for PEO 3 kg/mol (3k); this corresponds to the thickness of the adsorbed layer to be approximately equal to the Flory radius. As opposed to PEO 10k, for which details on the center of mass diffusion could not be discerned, for PEO 3k the diffusion rate along the pore was found to be smaller than that in the radial direction.

Reduced-mobility layers with high internal mobility in poly(ethylene oxide)–silica nanocomposites

“…Interestingly, in a closely related earlier NSE study of PEO3k confined to cylindrical SiO 2 -covered pores in silicon of about 13 nm diameter, the results were compatible with a prominent layer of about 5 nm thickness featuring either slowed-down or even negligible Rouse dynamics. 12 This may be due to the curvature effect mentioned above and stresses the complementarity of neutron-scattering and NMR results as well as the potential of joint analyses for future studies on such samples with increased immobilized fractions.…”

“…They are specific for polymers displaying attractive interactions with either the bare or the surface-modified filler. Bare oxide fillers such as silica and alumina, displaying surface hydroxy groups, interact favorably with polymers capable of acting as hydrogen-bond acceptor, such as poly(dimethyl siloxane), PDMS, [8][9][10] poly(ethylene oxide), PEO, [11][12][13][14][15] poly(2-vinyl pyridine), P2VP, 16,17 or different acrylate polymers. 5,[18][19][20][21][22] The amount of dynamically modified polymer, and with this the nominal thickness of an assumed-to-be contiguous reduced-mobility layer, can be estimated on the basis of results from different techniques.…”

“…URL: www.physik.uni-halle.de/nmr system and the same method. [12][13][14] In this context DSC, 10,16,19 dielectric spectroscopy, 23,24 NMR spectroscopy, 13,18,20,21 and static 22 as well as dynamic 9,11,12,14,15 neutron scattering techniques have been used. Apart from purely structural (=density) information obtained by static scattering, 22 most other techniques are sensitive to dynamic properties.…”

Recent progress on polymer dynamics by neutron scattering: From simple polymers to complex materials

“…Mixtures of deuterated and protonated chains matching the scattering length density of the porous matrix were infiltrated. We first describe the experiments on PEO confined in porous silicon by Kusmin et al 46 Unentangled and weakly entangled melts were investigated, consisting of chains of molecular weights of 3 and 10 Kg/mol, respectively, infiltrated in pores of 13 nm diameter. In porous silicon, the pores run strictly perpendicular to the wafer surface, allowing selectively probing molecular motions along the pore and in the radial direction, testing for a dynamic anisotropy.…”

“…Confinement is in fact the focus and common denominator of a variety of recent works. Confined polymer dynamics at different levels have been characterized by QENS in systems with different confining agents, for example, polymers infiltrated in nano-pores (one-dimensional, 1D), [46][47][48][49][50] intercalated in layered structures (2D), 51 adsorbed on clays, 52 surrounded by the vitrified component in a thermodynamically miscible polymer blend, 45 or confined by the neighboring microphase in a diblock copolymer. 53 Finally, hydrated polymers and polymer solutions-in particular, microgels-have also recently been investigated by QENS.…”