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- Confinement (4)
- Glass transition (4)
- Neutron scattering (3)
- Boson peak (2)
- Discotic liquid crystals (2)
- Poly(methyl phenyl siloxane) (2)
- Alpha relaxation (1)
- Broadband dielectric spectroscopy (1)
- Cooperativity length (1)
- Density of states (1)
Organisationseinheit der BAM
The glassy dynamics of poly(propylene glycol) (PPG) and poly(methyl phenyl siloxane) (PMPS) confined to nanoporous glasses (pore sizes 2.520 nm) investigated by dielectric spectroscopy, temperature modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature modulated DSC agree quantitatively indicating that both experiments sense the glass transition.
For PPG the glassy dynamics in nanopores is determined by a counterbalance of an adsorption and a confinement effect where the temperature dependence of the relaxation times obeys the Vogel/Fulcher/Tammann (VFT-) equation. The former effect results from an interaction of the confined macromolecules with the internal surfaces which in general slows down the molecular dynamics. A confinement effect leads to an acceleration of the segmental dynamics compared to the bulk state and points to an inherent length scale on which the glassy dynamics takes place. The step of the specific heat capacity Deltacp at the glass transition vanishes at a finite length scale of 1.8 nm. This result supports further the conception that a characteristic length scale is relevant for glassy dynamics.
For PMPS down to a pore size of 7.5 nm the temperature dependence of the relaxation times follows the VFT-dependence and a confinement effect is observed like for PPG. At a pore size of 5 nm this changes to an Arrhenius-like behavior with a low activation energy. At the same pore size Deltacp vanishes for PMPS. This points to a dramatic change in the character of molecular motions responsible for glassy dynamics and supports further the relevance of a characteristic length scale on which it takes place.
Quasielastic neutron scattering experiments on PMPS reveal that the microscopic dynamics characterized by the mean square displacement depends on confinement above the glass transition. The diffusive character of the relevant molecular motions seems to disappear at a length scale of about 1.6 nm.
The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.520 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.
In this article we present inelastic neutron scattering (INS) experiments on different systems of confined glass-formers. The aim of these experiments is to study the influence of spatial restriction on the microscopic dynamics related to the glass transition. Such results could be helpful for the detection of a currently speculated cooperativity length of the glass transition. The glass-forming component is either a molecular liquid or a polymer. The confining matrices are `hard' (silica glass, silicon) or `soft' (microemulsion droplets). For some experiments the confining structure could be spatially oriented. Except for the soft confinement the naïvely expected acceleration effect could only be found at low temperatures where INS experiments are difficult because of the long relaxation times. A clear effect of confinement could be observed for the glass-typical low energy vibrations (boson peak). This effect seems to be completely different for soft and hard confinement. Surprisingly, the experiments on oriented nanopores did not show any signs of an anisotropy of the dynamics.
In this article, results on the microscopic dynamics of a low-molecular glass former and a polymer confined in nanoporous silica obtained by inelastic neutron scattering are presented. By combining time-of-flight spectroscopy and backscattering spectroscopy we are able to cover a large dynamical range, from the low-frequency vibrations to the relaxation. The most prominent effect was observed on the boson peak in the vibrational spectrum. A strong reduction of the modes at lowest frequencies could be observed in both cases. The main effect on the relaxation is a broadening for the salol system. For the polymeric system a difference of the confinement influence on the methyl-group rotation and the relaxation can be detected from a first qualitative interpretation of the data.
Inelastic neutron scattering for investigating the dynamics of confined glass-forming liquids
(2005)
Inelastic neutron scattering was employed over recent years to investigate the influence of spatial confinement on the dynamics of glass-forming systems. We review the common phenomena observed by neutron scattering in such different confining hosts like porous glasses, molecular sieves, clays or free standing polymer films, which impose a spatial limitation to the motion of small organic molecules, oligomers or polymers. Near the glass transition temperature the mean squared displacements of the confined molecules show clear deviations from the bulk behavior. The observed increase or decrease of the mean squared displacements confirms the high relevance of the interface interaction near walls of confining media without excluding additional real confinement effects. We show a new comparison of the mean squared displacement for PDMS and PMPS in bulk and in different type of restricting geometries, which evidence a weak influence of the restricting geometry on the local methyl group motion, but a strong influence on the glass transition dynamics, if wall interactions are taken into account. Strong wall interaction is also supported by the intermediate scattering function, measured either by combining neutron backscattering and time-of-flight experiments to cover 3 decades in time from ns to ps or by neutron spin echo, which reveal above Tg an increasing elastic fraction with decreasing pore size and a slowing down of the dynamics. Furthermore we show that a reduction of modes below the Boson peak frequency is a more general feature of confined glass-forming systems.
The glassy dynamics of poly(propylene glycol) (PPG) and poly(dimethyl siloxane) (PDMS) confined to a nanoporous host system revealed by dielectric spectroscopy, temperature-modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature-modulated DSC agree quantitatively indicating that both experiments sense the glass transition. For PPG the segmental dynamics is determined by a counterbalance of adsorption and confinement effect. The former results form an interaction of the confined macromolecules with the internal surfaces. A confinement effect originates from an inherent length scale on which the underlying molecular motions take place. The increment of the specific-heat capacity at the glass transition vanishes at a finite length scale of 1.8 nm. Both results support the conception that a characteristic length scale is relevant for glassy dynamics. For PDMS only a confinement effect is observed which is much stronger than that for PPG. Down to a pore size of 7.5 nm, the temperature dependence of the relaxation times follows the Vogel-Fulcher-Tammann dependence. At a pore size of 5 nm this changes to an Arrhenius-like behaviour with a low activation energy. At the same pore size vanishes for PDMS. Quasielastic neutron scattering experiments reveal that also the diffusive character of the relevant molecular motions --found to be characteristic above the glass transition-- seems to disappear at this length scale. These results gives further strong support that the glass transition has to be characterised by an inherent length scale of the relevant molecular motions.
The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DK), temperature modulated DSC (TMDSC) and neutron scattering (NS). DK and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. At a pore size of 5 nm the temperature dependence of the relaxations times changes from a Vogel-Fulcher-Tammann like behavior to an Arrhenius one. At the same pore size Dcp vanishes. These results give strong support that the glass transition has to be characterized by an inherent length scale of the relevant molecular motions. Quasielastic neutron scattering experiments reveal a strong change even in the microscopic dynamic.
The effect of a nanometer confinement on the vibrational
and molecular dynamics of a liquid crystals E7 was studied by dielectric
spectroscopy (DS) and neutron scattering (NS). E7 undergoes a
glass transition phenomenon besides the nematic to isotropic phase
transition. As confining host a molecular sieve of the MCM-41 type
is used having cylindrical pores with a radius of 1.25 nm. The results
obtained by both methods are compared in detail.