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.
Dielectric spectroscopy and temperature-modulated differential scanning calorimetry are employed to study the molecular dynamics of oligomeric poly(propylene glycol) (PPG) melts of different molecular weights confined to nanoporous glasses (pore sizes 2.5, 5.0, 7.5 and 20 nm). Moreover, the results obtained for the polymers are compared with those for the corresponding monomer. The experimental results are discussed in the framework of the interplay of confinement and adsorption effects. For large pore sizes (> 5 nm) a speeding up of the molecular dynamics is observed (the confinement effect), whereas for small pore sizes (< 5 nm) a slowing down of the segmental fluctuations is found (the adsorption effect). In addition, a minimal length scale for the glass transition of 1.6 nm is estimated for PPG confined in nanoporous glasses. This supports the idea that the molecular motions responsible for the glassy dynamics must be describable by a characteristic length scale.
Polarized Raman scattering investigations are carried out to investigate the conformations of the macromolecules inside the pores. These experiments show that the confined polymer chains are locally stretched. This effect increases with decreasing pore size.
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 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.