Glass Transition in Sub-nanometer Confinement
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ABSTRACT Broadband dielectric spectroscopy (10- 2 Hz - 10aHz) is employed to study the molecular dynamics of low-molecular-weight glassforming liquids being confined to nanopores. For the H-bond forming liquid propylene glycol being confined to (uncoated and silanized) nanopores (pore size: 2.5 nm, 5.0 nm and 7.5 nm) a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores the relaxation time distribution is broadened on the long term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores resulting in a relaxation rate which is slightly faster compared to the bulk liquid. For the H-bonded liquid ethylene glycol (EG) embedded in zeolites of different pore size and topology one observes a sharp transition from a single-molecule dynamics to that of a liquid depending on the coordination number of the confined molecules. While EG in silicalite (showing a single molecule relaxation) has four neighboring molecules, EG in zeolite beta or AIPO 4-5 has a coordination number of five and behaves like a bulk liquid.
INTRODUCTION The nature of glassforming liquids is still not well understood and is an interesting subject of worldwide research and discussion [1-5]. One important question is the length scale on which cooperative motions of a liquid take place [6-16]. The existence of a length scale can be proved by measuring the molecular dynamics of guest molecules being confined to nanoporous environment. Nanoporous sol-gel glasses with pore diameters of 2.5, 5.0 and 7.5nm have shwvon to be a favourable host system for doing this [14-18]. Broadband dielectric spectroscopy. covering a dynamics range of more than 11 dccades, is an excellent. tool for such studies. Modern chemistry enables to realize one single isolated molecule in one zeolitic cage by use of the structure-directed synthesis. Zeolites have well-defined pore dianeters below 1 nm. Varying the size and the topology of the zeolitic host system allows to study the transition from the dynamics of a single isolated molecule to that of a bulk liquid.
115 Mat. Res. Soc. Symp. Proc. Vol. 543 ©1999 Materials Research Society
EXPERIMENT Controlled porous glass from Geltech Inc. with nominal pore sizes of 2.5, 5.0 and 7.5 nm and a narrow pore size distribution is used. After evacuating the disc shaped glasses (diametei 10 mm, thickness 0.2mm) to 10- mbar at 570 K for 24 h to remove volatile impurities they are filled from the liquid phase at 70'C with propylene glycol (PC). Some of the samples are chemically treated inside the closed vacuum chamber using hexamethyldisilazane (Aldrich Chemical Company) for 30 minutes at 700C and again
evacuated for 3 h at 70 °C. This treatment replaces the -OH groups on the glass surface with trimethylsilyl groups. The replacement of the -OH groups is checked by means of FTIR spectroscopy. Both sides of the sample disks axe covered with aluminum foil (thickness 800 nm) to en
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