(162a) Dramatic Effects of Nanoscale Confinement On Glass Transition Temperature (Tg) In Polymers: Perturbing the Tg by 50-100 K In Supported and Free-Standing Films and by the Presence of Neighboring Layers of Other Polymers

Using the temperature dependence of fluorescence intensity intrinsic to a polymer of interest or from dyes labeled at trace levels to a polymer, we show that Tg can be altered from bulk response by as much as 50-100 K via nanoconfinement in substrate-supported and freely standing films.  Similarly large perturbations to polymer Tg can be observed in nanoconfined multilayer films of different polymers or blends with nanospheres of one polymer dispersed in a matrix of a second polymer. 

In particular, we shall describe how supported homopolymer films may exhibit positive or negative deviations from bulk Tg response depending on the relative strengths of the effects of the polymer-air interfaces, which typically reduce Tg, and attractive interactions at polymer-substrate interfaces, which increase Tg.  We shall also show how the Tg perturbations in free-standing homopolymer films (without a substrate) can be understood, at least in part, from the free-surface effects characterized in substrate-supported films.  We shall further demonstrate how the perturbations that occur at interfaces lead to a gradient of Tg response some tens to hundreds of nanometers into the film interior.  Additionally, we shall describe how fragility (as a glass former) may be important in the strength of the Tg-confinement effect, both in terms of the Tg change exhibited at a particular thickness as well as the thickness at which the average film Tg deviates from bulk response.

We shall also denote behavior that has resulted in new questions related to the fundamental nature of glass-forming materials.  We have found via multilayer film studies that an ultrathin layer of one polymer species can have its glass transition behavior strongly modified by or even “slaved” to that of neighboring domains of other polymers.  Similar results have been obtained in dilute blends of an A-B diblock copolymer in a B homopolymer, leading to nanospheres of the A block being dispersed in B-phase matrix.  This indicates that multilayer films and nanodispersions of different, immiscible polymers can yield properties that are not possible with conventional polymer blends.