(324b) Fragility Plays a Key Role in Determining the Magnitude of the Tg-Confinement Effect in Polymer Films

Polymer films with one or more free surfaces and no substantial attractive substrate interaction report a reduced glass transition temperature, Tg, upon confinement by various thermodynamics or pseudo-thermodynamic experimental methods including the following: ellipsometry, dielectric relaxation spectroscopy, differential scanning calorimetry, fluorescence spectroscopy, Brillouin light scattering, x-ray reflectivity, and neutron reflectivity.  Despite the many theories and models which seek to explain why Tg decreases upon confinement in supported films with no substantial substrate interaction, rarely has the question been considered regarding why the Tg-confinement effect is so dependent on polymer species. 

To understand what aspect of polymer structure or dynamics contribute to the strength of Tg-confinement effects, we consider the well-documented reduced surface Tg.  If the surface is a region where the requirements for cooperative segmental relaxation are reduced, and thus Tg is reduced, polymers which relax in more cooperatively should experience a greater free-surface effect.  The ability of a polymer species to undergo cooperative relaxation is connected to the fragility (m), which is a parameter that reflects the structure and intermediate range order of a glass former.  According to Angell (J. Non-Cryst. Solids, 1985, 73, 1-17) who first defined fragility and classified glasses as strong or fragile, “The temperature dependence of the average relaxation time as well as the detailed relaxation function seem to be closely connected with the nature of the intermediate range order.”  Materials with higher fragilities possess less intermediate range order while stronger glass formers are typically characterized by an open network structure.  Strong glasses possess a nearly Arrhenius temperature dependence of alpha-relaxation times when heated above Tg while fragile glass formers exhibit a sharp, non-Arrhenius decrease in structural relaxation times.  This non-Arrhenius behavior reflects the presence of cooperative behavior as fragile glasses require a larger number of neighboring units to participate in structural relaxation.  Thus, fragile glass formers may experience a greater modification to Tg by the presence of a free surface in a film.  We hypothesize that more fragile glass formers will experience larger Tg-confinement effect in the absence of substantial polymer-substrate interactions.

We combine some of our group’s past, published research data as well as new experimental results of Tg-confinement effects in seven polymer film systems: polystyrene (PS), PS with 2 wt% dioctyl phthalate (DOP), PS with 4 wt% DOP, poly(4-methyl styrene) (P4MS), polycarbonate (PC), polysulfone (PSF), and poly(vinyl chloride) (PVC).  All Tg measurements were performed by fluorescence so that issues associated with comparing results from different techniques are circumvented.  We also present measurements of fragility using a single technique, a differential scanning calorimetry based approach, first outlined by Angell, and investigate the connection between fragility and confinement effects.  We restrict discussion to single-layer films with no substantial polymer-substrate interaction.  This geometry represents the simplest case of a confined polymer glass former and is an appropriate starting point for understanding the role of polymer structure and properties on the strength of confinement effects.  For confined films of linear chains with a free surface and no substantial interactions, we observe a one-to-one correlation between higher fragility and larger Tg reductions upon confinement.  In particular, the magnitude of the Tg-confinement effect increases in the following order: PS + 4 wt% DOP < PS + 2 wt% DOP < PS < P4MS ~ PC < PSF < PVC.  Fragility values increase in the following order: PS + 4 wt% DOP < PS + 2 wt% DOP < PS < P4MS ~ PC < PSF < PVC.

The fact that the steepest reduction in Tg with confinement occurred with PVC, which is the most fragile known polymeric glass former, makes evident that the magnitude of the Tg-confinement is not correlated with increasing Tg, because PVC has the lowest Tg of any of the polymer systems tested.  As an example of the extreme strength of its Tg-confinement effect, Tg is reduced from its bulk value by more than 40 K in a 47-nm-thick film.  Polysulfone also exhibit very strong confinement effects, albeit weaker than PVC, with a 24-nm-thick films reporting a Tg reduced from bulk response by 53 K.  At the opposite extreme, PS + 4 wt% DOP exhibits not Tg-confinement effect in a film of 13 to 14 nm thickness.

Our study demonstrates that fragility is a key variable which is a reflection of the local medium range order in a polymer glass which determines the susceptibility of a glass former to perturbations (in this case induced by confinement).  Thus, a picture for Tg-confinement effects emerges in terms of the molecular level packing of the polymer chains which are in turn related to the fundamental vibration time scale of dynamics.  Fragility can explain the magnitude of Tg-confinement effects in neat polymer as well as plasticized, antiplasticized, or solvent sorbed films provided there are no attractive interactions with the supporting substrate and the polymers are linear chains.  Routes for further testing of the fragility hypothesis will be suggested and caution will be issued for cases where additional factors may influence the Tg-confinement effect.