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Connie Roth

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Confinement and Boundary Effects on Glassy Dynamics

The study of the glass transition in confined geometries is an active field of research in polymers, small molecules, colloids, and simulations.  Confinement is seen as a method of perturbing the length scales associated with cooperativity in order to gain insight into the size, arrangement, and correlation of cooperatively rearranging regions (CRRs).  Polymer systems have been studied to the greatest extent because of the ease with which ultrathin films of known thickness can be made, and for their technological importance in applications such as microelectronics and gas separation membranes.  The large shifts in the glass transition temperature (Tg) that have been observed with decreasing film thickness has garnered a great deal of attention during the past two decades.
 

Two Simultaneous Mechanisms Causing Glass Transition Temperature Reductions in High Molecular Weight Free-Standing Polymer Films as Measured by Transmission Ellipsometry

      Justin E. Pye and Connie B. Roth, Phys. Rev. Lett. 2011, 107, 235701. [Link]

Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular weight (MW), free-standing polystyrene (PS) films over an extended temperature range.  For two different MWs, we observed two distinct reduced glass transition temperatures (Tgs), separated by up to 60 K, within single films with thicknesses h less than 70 nm.  The lower transition follows the previously seen MW dependent, linear Tg(h) behavior, while we also observed the presence of a much stronger upper transition that is MW independent and exhibits the same Tg(h) dependence as supported and low MW free-standing films.  This represents the first experimental evidence indicating that two separate mechanisms can act simultaneously on thin free-standing polymer films to propagate enhanced mobility from the free surface into the material.  The change in thermal expansion through the transitions indicate that ~90% of the film (matrix) solidifies at the upper transition with only ~10% of the material remaining mobile, freezing in at the lower transition.  Surprisingly, when we compare our results to the existing literature, and especially the low MW free-standing film data, we conclude that the upper transition encompasses the free surface region and associated gradient in dynamics.  This leaves open the question about where the small (~10%) fraction of material that has ultrafast, MW dependent dynamics resides within the film.
 

Physical Aging in Ultrathin Polystyrene Films: Evidence of a Gradient in Dynamics at the Free Surface and Its Connection to the Glass Transition Temperature Reductions

      Justin E. Pye, Kate A. Rohald, Elizabeth A. Baker, and Connie B. Roth, Macromolecules 2010, 43, 8296-8303. [Link]

We have also observed changes in physical aging in ultrathin supported films that are correlated with Tg reductions near a free surface.  Using our streamlined ellipsometry method for characterizing physical aging, an aging rate β is determined from the time-dependent decrease in the film thickness as a function of aging time following β = − d(h/h0) / d(log t).  We have measured the temperature dependence of the aging rate in ultrathin supported polystyrene (PS) films and have found reduced physical aging rates at all temperatures for film thicknesses < 100 nm.  Our analysis of these results have demonstrated that the physical aging rates in these ultrathin films are not simply shifted in correspondence with the average glass transition temperature (Tg) reductions in these films, but that the reduced physical aging rates result from a gradient in enhanced dynamics present near the free surface of these films.  The temperature dependence of the length scale characterizing the depth to which these enhanced dynamics penetrate into the film, from our analysis using a common two-layer model and a new gradient model, are in very good agreement with similar length scales characterizing the gradient in Tg dynamics, strongly suggesting that these two phenomena are related.