Quasielastic neutron scattering of poly(methyl phenyl siloxane) in the bulk and under severe confinement

Abstract

Quasielastic neutron scattering was utilized to investigate the influence of confinement on polymer dynamics. Poly(methyl phenyl siloxane) chains were studied in the bulk as well as severely confined within the approximately 1-2 nm interlayer spacing of intercalated polymer/layered organosilicate nanohybrids. The temperature dependence of the energy resolved elastic scattering measurements for the homopolymer and the nanocomposites exhibit two distinct relaxation steps: one due to the methyl group rotation and one that corresponds to the phenyl ring flip and the segmental motion. Quasielastic incoherent measurements show that the very local process of methyl rotation is insensitive to the polymer glass transition temperature and exhibits a wave-vector independent relaxation time and a low activation energy, whereas it is not affected at all by the confinement. At temperatures just above the calorimetric glass transition temperature, the observed motion is the phenyl ring motion, whereas the segmental motion is clearly identified for temperatures about 60 K higher than the glass transition temperature. For the nanohybrid, the segmental motion is found to be strongly coupled to the motion of the surfactant chains for temperatures above the calorimetric glass transition temperature of the bulk polymer. However, the mean square displacement data show that the segmental motion in confinement is faster than that of the bulk polymer even after the contribution of the surfactant chains is taken into consideration.

Inelastic neutron scattering for investigating the dynamics of confined glass-forming liquids

“…As an example we show in Fig. 2 the elastic scans [20] for PMPS in bulk and confined in a nanocomposite of PMPS intercalated in organically modified silicate [29]. Bulk PMPS (M w = 2600 g mol À1 ; T g = 223 K) shows a clear decrease of the elastic scattering below T g , which is due to the fact that the methyl group dynamics becomes faster than the time of $4ns corresponding to the experimental resolution.…”

“…Here the scale factor can not be interpreted quantitatively, because the silicate host contains protonated surfactants, the strong scattering of which was corrected, but not rigorously. It should be noted, however, that the quasielastic spectra of the pure silicate do not show any relaxation process at temperatures around 100 K [20], that is the surfactant molecules do not exhibit any relaxation at these low temperatures as long as there is no polymer present in the galleries. It seems also that the step related to the methyl group rotation could be somewhat broader.…”

“…2 examples for elastic scans on poly (methyl phenyl siloxane), PMPS (M w = 2600 g mol À1 , M w /M n = 1.20, T g = 223 K), on a silicate matrix and on a nanocomposite containing PMPS. The upper panel for PMPS reveals two steps at the higher Q-values [20]. The first step at low temperatures is due to rotations of the methyl group attached to both polymer chains and which become fast enough for time resolution of the instrument [21,22].…”

“…3). Elastic temperature scans on bulk PMPS (M w = 2600 g mol À1 , T g = 223 K), the silicate matrix (which contains a surfactant) and the intercalated nanocomposite (25% PMPS), measured on IN16 [20]. All data are normalized to the lowest temperature at each Q-value.…”

Quasielastic neutron scattering in soft matter

“…This finding provides a microscopic explanation of the changes in bulk dynamics resolved with mechanical measurements. QENS has also been used to investigate the polymer dynamics in intercalated polymer/layered silicate nanohybrids [84,85]. The observed effect of confinement depends on the system considered.…”