Spatially Resolved Dynamics of Supercooled Liquids Confined in

Chapter 13

Downloaded by PENNSYLVANIA STATE UNIV on September 8, 2012 | http://pubs.acs.org Publication Date: July 17, 2004 | doi: 10.1021/bk-2004-0882.ch013

Spatially Resolved Dynamics of Supercooled Liquids Confined in Porous Glasses: Importance of the Liquid-Glass Interface Ranko Richert and Min Yang Department of Chemistry and Biochemistry, Arizona State University, Tempe, A Z 85287-1604

The dynamics of liquids is notorious to be affected by geometrical confinement, provided that sufficiently small length scales of order several nanometers are involved. Regarding the viscous regime of glass-forming materials, the effects of confinement are often characterized by a glass transition shift relative to the bulk material. We discuss several problems associated with this approach and show evidence for the important role of interfacial dynamics. In particular, we assess the spatial dependence of relaxation times within pores of 7.5 nm diameter as measured by site selective triplet state solvation dynamics experiments.

Non-crystalline materials in confined spaces and near surfaces are encountered in many situations. Lubrication, oil in rocks, water in porous materials, biological cells and membranes are only a few examples. Here, we are concerned with the behavior of glass-forming viscous materials when restricted geometrically within porous glasses characterized by length scales of several nanometers.

© 2004 American Chemical Society In Dynamics and Friction in Submicrometer Confining Systems; Braiman, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction The properties of liquids subject to geometrical restrictions at the submicron level have been addressed in a variety of experimental approaches. Surface force microscopy has revealed that the viscosity of room temperature liquids changes dramatically within a few molecular layers near atomically smooth mica surfaces (1,2). Confinement effects have also been observed for liquids in porous materials with rougher surfaces and a distribution of surface curvatures, but the effects remained less pronounced (5). For instance, Warnock has observed an increase in the viscosity of a liquid by a factor of 3 near a silica interface using picosecond experiments (4). Glass-forming materials are those which do not crystallize easily, such that the viscosity and relaxation time scale increases many orders of magnitude if the liquid is cooled below its melting point. In this supercooled regime, the time scales of molecular motions rapidly approach a value of 1 0 0 s, which marks the glass transition temperature T . The most prominent characteristics of the dynamics in the supercooled state are non-exponential relaxation patterns and pronounced deviations from an Arrhenius type temperature dependence of the transport coefficients (5). The concomitant curvature of a relaxation time constant versus temperature trace in an activation diagram is commonly described in terms of the empirical Vogel-Fulcher-Tammann (VET) law. g

\og (T/s) = A+ B/(T-T % L0

(1)

0

It implies a relaxation time divergence at a finite temperature T , typically positioned some ten Kelvins below T . Although generally accepted theory of the glass transition is available, an increasing length scale of cooperative motion is often made responsible for the slowing down of the dynamics as the temperature approaches T Originally introduced by Adam and Gibbs (6) as theory of 'cooperatively rearranging regions', this length scale £ can be viewed as the distance required by two molecules in order to relax independently. Estimates of this length based on experimental approaches have reported values of i « 3 nm. Accordingly, geometrical restrictions of supercooled liquids on the spatial scales of a few nanometers are of particular interest, because confinement and cooperativity share similar spatial dimensions. In fact, experiments on viscous liquids in porous materials have been used for determining the length scales associated with the cooperative nature of molecular motion (7). The first confinement induced shift of the glass transition has been reported for glass-forming organic molecular liquids by Jackson and McKenna using Differential Scanning Calorimetry (8). A reduction of T was observed in porous glasses, equivalent to faster dynamics i f compared with the bulk behavior 0

g

&

g

In Dynamics and Friction in Submicrometer Confining Systems; Braiman, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

183 at the same temperature. Subsequent measurements of geometrically confined materials near F arrived at both signs for the glass transition shift A F . Thus, accelerated as well as frustrated dynamics in confinement have been reported on the basis of a variety of liquids, polymers, and confining geometries with different physical and chemical surface properties (P). Various possibilities for explaining these apparent discrepancies have been discussed: Differences in the thermodynamics path used to prepare the viscous state, neglect of specific surface interactions, and disregarding the relaxation amplitudes and changes in the time dependence when comparing relaxation patterns (7). In the following, we emphasize the problems encountered when the relaxation strength and patterns are not properly taken into account. Then, we briefly discuss the technique of triplet state solvation dynamics applied to confined systems and in particular present recent results on site-selective solvation studies of 3-methylpentane in porous silica, which demonstrate how the relaxation time or viscosity depends on the distance from the liquid/glass interface. In the absence of atomically smooth conditions and specific interactions with the surface, a surface induced increase of the viscosity by more than three orders of magnitude is being observed, which emphasizes the interfacial over pure finite size effects. Consistent with typical length scales of cooperativity in viscous liquids, these frustrated dynamics disappear for distances from the pore boundary of a few nanometers.


g

g

Relevance of Relaxation Amplitudes Casting the impact of geometrical confinement on the dynamics into a glass transition shift AT is not always appropriate. In the context of restricted geometries, a common observation is a bifurcation of the relaxation into a slow and fast component, often associated with a distinct surface layer of highly reduced molecular mobility. Whenever the relaxation pattern or the observed correlation functions changes qualitatively, the differences between bulk and confined dynamics are only poorly represented by A F or by a change in time scale. Less obvious are the problems encountered when the shape of the relaxation trace is preserved to a good approximation but the amplitude is altered significantly (7). Such a situation is found frequently in dielectric relaxation studies of liquids and polymers confined to porous glasses. Dielectric techniques have been applied to a number of porous materials, and the interpretation of the data is not entirely straightforward (10,11). One of the problems is that the dielectric results refer to a mixture of two different materials, the liquid (filler) under study and the confining material (matrix), e.g. porous glass. Unfortunately, the two distinct dielectric contributions from the filler and matrix do not simply add to the signal measured for the composite g

g



184 material. Instead, theories of the electrostatics of heterogeneous dielectrics like those of Maxwell, Wagner, and Sillars have to be involved, unless the dielectric behavior of filler and matrix are similar (72). A prerequisite for successfully retrieving the dielectric function of the filler from the measurement of the composite is the knowledge of the filler volume fraction, the dielectric properties of the matrix material, and details regarding the confining geometry. The source of ambiguity discussed in the following is not related to these electrostatic mixing effects, i.e., we now assume that they are negligible or have been solved. There are numerous examples of relaxation data obtained for liquids confined to porous media where the amplitude of the relaxation is significantly smaller than the bulk counterpart. Some examples clearly show that relaxation strength is not really lost, but shifted far away from the time or frequency range of the bulk relaxation (75). This is the situation shown schematically in Figure 1.

-

2

-

1

0

1

"og>/s" ) 1

2

-

2

-

1

0

1

2

log (t/s) 10

Figure I. (a) Dielectric loss for a bulk and confined situation, assumed to separate the response into a contribution near the bulk peak frequency and a much slower one. Apparently, the relaxation under confinement is faster than in the bulk, (b) The same situation of (a) after transformation to the time domain. Comparing the bulk and confined results on an absolute amplitude scale indicates that there is no faster component in the confined case.

In Figure l a the dielectric loss e"{co) is calculated for an exponential bulk relaxation with amplitude a = 1 and time constant T i = 1 s (solid curve). The confined case is assumed a superposition of two exponentials with equal bulk

bu

k


185 weights, but differing in their time scale, T = 0.5 s and T J = 3000 s (dashed curve). Without knowledge of the intensities, one would compare the loss peak positions after normalizing the peak heights, which leads to the amplitude a = 1 and time constant T = 0.5 s for the normalized fast component in confinement (dotted curve). Although such data are easily interpreted as a confinement induced acceleration of the dynamics, comparing the results on the correct ordinate scales demonstrates that no faster component exists in the confined case. Figure lb reflects the identical situation but calculated as time domain signals. That the short time relaxation remained unchanged is particularly clear in the time domain representation, where the solid and dashed curves coincide for short times. Intentionally, the above example has been selected such that the slope at time t = 0 remains constant, using FAST

S

o w

mTm


FAST

(2)

The example discussed here clearly shows that disregarding relaxation amplitudes can lead to erroneous interpretations of the relaxation time changes.

Triplet State Solvation Dynamics Solvation dynamics experiments employing the long lived excited triplet states are particularly useful for assessing the dynamical behavior of molecules in the supercooled state close to the glass transition at F (14). The method is based upon the electronic excitation of a chromophore as dopant whose permanent dipole moment changes from the ground state value JUQ to that of the excited state ju . The S

Spatially Resolved Dynamics of Supercooled Liquids Confined in

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