Ultrathin Polymer Films: Rubbery Stiffening, Fragility, and T_g Reduction

Abstract

The mechanical and viscoelastic responses of poly(ethyl methacrylate) (PEMA) ultrathin films over thicknesses ranging from 21 to 112 nm have been studied using a nanobubble inflation method. The stress−strain response of the PEMA thin films shows a rubbery stiffening as the film thickness decreases, and the present results are combined with prior measurements of the rubbery stiffening index S for multiple polymers. We find that S is linearly correlated with the dynamic fragility index m. The results are also consistent with expectation from Ngai et al.'s [J. Polym. Sci., Part B: Polym. Phys. 2013, 51 (3)] recent proposition based on his coupling model but do not seem to correlate with the molecular composite concept proposed by Page et al. [Nano Lett. 2014, 14 (5)] as a means to explain the dependence of the rubbery stiffening on chemical structure of the polymer. In addition, we observe a reduction in glass transition temperature of as much as 15.7 K for PEMA films of 21 nm thickness.

Glass Transition Phenomenon for Conjugated Polymers

“…The T g of thin films, defined by a simple nanoconfined physical model, has been characterized by different types of measurements, including dynamic, thermodynamic, and pseudothermodynamic (see subsequently for the definitions). In addition, it can also be obtained by investigating the thin film rheological behavior …”

Probing the rheological properties of supported thin polystyrene films by investigating the growth dynamics of wetting ridges

“…For instance, with the behavior of chain entanglement, the nanobubble inflation experiment showed a rubbery stiffening for the free-standing thin films, 34,35,37 which may indicate that the macromolecule chains in the suspended thin film are more entangled than those in the bulk material, according to the theory of rubbery elasticity (i.e. For instance, with the behavior of chain entanglement, the nanobubble inflation experiment showed a rubbery stiffening for the free-standing thin films, 34,35,37 which may indicate that the macromolecule chains in the suspended thin film are more entangled than those in the bulk material, according to the theory of rubbery elasticity (i.e.…”

Mechanical and viscoelastic properties of confined amorphous polymers

“…For thin film T g , some of the early work discussing thickness‐dependent T g illustrated that T g could be increased or decreased for the same polymer depending on the interactions with the substrate . In the absence of a substrate (bubble), bubble inflation rheometry indicates a strong correlation between polymer fragility and the stiffening of the rubbery polymer on confinement . Due to the challenges in separating substrate interactions and fragility effects, additional work, in particular from simulations where the distribution of mechanical properties can be obtained, is necessary to understand how fragility of the polymer impacts its mechanical properties.…”

“…Earlier work from Tsui and coworkers using AFM adhesion force measurements showed evidence of stiffening in thin films of rubbery poly(t‐butyl acrylate), which is also consistent with the bubble inflation measurements. The stiffening of rubbery polymers under confinement as determined from bubble inflation rheometry appears to scale directly with the bulk fragility of the polymer with higher fragility leading to greater stiffening as shown in Figure . In this case, the stiffening index, S , is determined from the slope of log biaxial rubbery compliance versus log thickness and provides a single parameter measure of how much the rubbery modulus increases on nanoconfinement.…”