“Rubbery Stiffening” and Rupture Behavior of Freely Standing Nanometric Thin PIB Films

Abstract

Prior bubble inflation studies have shown strong evidence of rubbery-like stiffening behaviors for several organic and inorganic thin films. The possible origin is still unclear, but recently Ngai et al. [J. Polym. Sci., Part B: Polym. Phys.201351214224] and Li and McKenna [Macromolecules20154863296336] proposed that the separation of molecular motions and dynamic fragility are closely related to the observed rubber-like stiffening behavior. In the present work, we report observations from bubble inflation measurements on nanometric thin polyisobutylene (PIB) films that test the correlations suggested by the Ngai coupling model (stiffening is related to the shape parameter describing the α-relaxation) and Li and McKenna (the rubbery stiffening correlates with the dynamic fragility). Mechanical properties and surface tension of nanometric thin PIB films were investigated through strain–stress measurements for film thicknesses ranging from 13 to 126 nm. The tests were performed at room temperature far above the glass transition temperature of PIB. In addition to stiffness and surface tension, rupture strengths were also measured. We find that the stiffness increases with decreasing film thickness and that the surface tension remains constant, independent of the film thickness. The rupture stress is found to increase with decreasing film thickness, whereas the rupture strain decreases with decreasing film thickness. Similar to the prior bubble inflation measurements in polymeric thin films, the thickness dependence of the stiffening followed a power law behavior with film thickness. The observed stiffening behavior agrees with the suggestion from Ngai et al. that the rubbery stiffening should correlate with the separation of the α- and Rouse-mode relaxations. However, unlike prior results for ultrathin polymer films, the stiffening behavior of PIB did not follow the linear relationship with dynamic fragility that was proposed by Li and McKenna.

Testing the paradigm of an ideal glass transition: Dynamics of an ultrastable polymeric glass

“…4B) is lower than the reported macroscopic value (−6.33) ( 43 ). This behavior is similar to prior observations of rubber-like stiffening seen ( 42 ) in bubble inflation measurements on ultrathin films of several polymeric materials. McKenna and coworkers ( 41 , 42 ) observed that the compliance decreased as the film thickness decreased, and they interpreted this behavior as being due to the separation of α relaxation and Rouse mode caused by nanoconfinement effects.…”

“…Thus, it is a challenge to perform traditional dynamic measurements on these small amounts of material ( 4 ). To overcome this issue, in the present work, we have used the Texas Tech University (TTU) nanobubble inflation technique ( 41 , 42 ) to measure the viscoelastic response of the ultrastable Teflon films in the deep glassy regime in order to test the temperature dependence of the dynamics in the upper bound conditions, i.e., over a range that includes T f to T g . Prior bubble inflation studies have successfully shown that viscoelastic properties can be measured with sample sizes ranging from nano- to micrograms ( 41 , 42 ).…”

“…To overcome this issue, in the present work, we have used the Texas Tech University (TTU) nanobubble inflation technique ( 41 , 42 ) to measure the viscoelastic response of the ultrastable Teflon films in the deep glassy regime in order to test the temperature dependence of the dynamics in the upper bound conditions, i.e., over a range that includes T f to T g . Prior bubble inflation studies have successfully shown that viscoelastic properties can be measured with sample sizes ranging from nano- to micrograms ( 41 , 42 ). Thus, by adopting the advantages of this technique, we are able to investigate the dynamic behavior for ultrastable polymeric films in the upper bound regime for the dynamics where the volume and enthalpy are lower than those of the glass in the equilibrium state.…”

“…McKenna and coworkers ( 41 , 42 ) observed that the compliance decreased as the film thickness decreased, and they interpreted this behavior as being due to the separation of α relaxation and Rouse mode caused by nanoconfinement effects. This is important because it is also known that nanoconfinement effects can sometimes, although not always, change the temperature dependence of the dynamics ( 41 , 42 , 44 ).…”

“…For the composite bilayer, we know the mechanical properties for nanometric thin PIB films from a recent bubble inflation study ( 42 ). In that work, the T g for PIB was −73°C and the modulus measured for a 20-nm film at 23°C was 17 MPa.…”