(180ac) Tuning the Tg's of Polymers by 100 K: Equivalence of Confinement Effects in Multilayer Films and Dilute Polymer Blends

The glass transition of a polymer occurs when, upon cooling, chains are no longer able to adopt equilibrium conformations on the experimental time scale. This transition from a disordered, rubbery or liquid equilibrium state to a glassy, non-equilibrium state is accompanied by major changes in properties such as viscosity and modulus.  Near the glass transition temperature (T_g), relaxation requires the cooperative motion of tens to hundreds of chain segments within a local volume of ~1-4 nm in length scale.¹  Despite decades of research, a fundamental understanding of T_g remains as one of the great challenges in condensed matter physics.²

Studies of polymers confined in one or more dimensions at the nanoscale have revealed large changes in properties such as T_g relative to bulk behavior.^3-8  In thin polymer films, a reduction in T_g is observed in the absence of attractive polymer/substrate interactions and attributed to the influence of the free surface.  Ellison and Torkelson³ demonstrated via a multilayer/fluorescence method that T_g is reduced by 32 °C relative to bulk in a 14-nm-thick fluorescently labeled polystyrene (PS) layer atop a bulk unlabeled PS layer.³  Additional studies of these 14-nm-thick labeled PS layers indicated that the T_g of ultrathin PS top layers can be tuned depending on the polymer underlayer species in bilayer films.  In contrast to the Ellison and Torkelson work,³ when placed atop a PMMA bulk layer, a 14-nm-thick PS layer exhibits a T_g that is within 2 °C of the bulk value.⁴  Confinement effects on T_g have also been observed in multilayer films made via forced assembly coextrusion where each layer is some number of nanometers thick, with the overall film being some microns in thickness.⁶

Here, we demonstrate via fluorescence methods that the T_g an ultrathin PS layer can be tuned by a ~100 °C range when placed atop a bulk layer of a different, immiscible polymer (leaving a free surface on the PS film) and by nearly as broad a range when placed between two bulk neighboring domains of an immiscible polymer in a trilayer film.  The coupling of T_g behavior between immiscible polymers is observed even for PS layers with thicknesses more than a factor of ten greater than the PS radius of gyration (~6 nm) or the length scale of cooperativity near T_g (~1-4 nm). Similar strong tunability of PS T_g is observed when PS nanospheres are present at very dilute levels (3 vol %) in a matrix of a second, immiscible polymer.

We draw a connection between the studies of bilayer and trilayer films of immiscible polymers and our other studies of immiscible and miscible polymer blends near the limit of infinite dilution of PS, where each PS chain is dispersed at a molecular level within the matrix of a second polymer.  Using a novel approach involving intrinsic PS fluorescence,⁹ we show that very dilute levels of PS (0.10 vol % and less) report PS T_g values that are strongly perturbed from bulk response by the matrix of the second polymer.  More importantly, the measured T_g value for the very dilute PS chains that are well-dispersed in a given second polymer species is within experimental error identical to that obtained in trilayer films containing an 11-14 nm thick PS layer located between two bulk layers of the same second polymer.  These results underscore the major role of neighboring polymer domains in influencing the T_g response of both an infinitely dilute blend species and ultrathin layers in multilyer films and indicate that the T_g perturbations to individual polymer species in polymer blends and T_g-confinement effects in multilayer films have the same physical origin.

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