The Elastic Mechanical Response of Supported Thin Polymer Films

Abstract

Nanoindentation studies of the mechanical properties of sufficiently thin polymer films, supported by stiff substrates, indicate that the mechanical moduli are generally higher than those of the bulk. This enhancement of the effective modulus, in the thickness range of few hundred nanometers, is indicated to be associated with the propagation and impingement of the indentation tip induced stress field with the rigid underlying substrate; this is the so-called "substrate effect". This behavior has been rationalized completely in terms of the moduli and Poisson's ratios of the individual components, for the systems investigated thus far. Here we show that for thin supported polymer films, in general, information regarding the local chain stiffness and local vibrational constants of the polymers provides an appropriate rationalization of the overall mechanical response of polymers of differing chemical structures and polymer-substrate interactions. Our study should provide impetus for atomistic simulations that carefully account for the role of intermolecular interactions on the mechanical response of supported polymer thin films.

The Elastic Mechanical Response of Nanoscale Thin Films of Miscible Polymer/Polymer Blends

“…This is due to lack of microstructural information on different polymeric materials in the FEA studies. 17 There must exist a more reliable explanation for the elastic mechanical responses of the polymers (Figure 3), which we now discuss. Long and co-workers suggest that in the vicinity of T g , and lower temperatures, the mechanical response of a polymer is characterized by a glassy modulus and a viscous modulus.…”

“…Note that previously Chung et al showed that regardless of different input values of the elastic moduli and Poisson's ratios relevant to actual polymers, the FEA fails to account for the elastic mechanical response of supported polymer films. 17 The FEA predictions do not account for the large variations of h t (i.e., differences in the stress field propagation lengths within a film) for different polymers nor does it account for the data in Figure 3b. This is due to lack of microstructural information on different polymeric materials in the FEA studies.…”

“…3,4,44 In our previous study, we showed that the stress field propagates several hundreds of nanometers into PS, PMMA, and PC films even for indentation depths d < 3 nm. 17 Since the stress field imposed by indentation extends over much larger length scales (on the order of hundreds of nanometers) compared to the length scale of both the free surface layer and self-concentration effects 34 (local PS-rich and TMPC-rich domains on the order of nanometers), PS/TMPC blends can be considered as a homogeneous medium. It is important to note that the mechanical behavior of thin polymer films, typically in the thickness range of less than 100 nm, have also been well described in terms of dynamic heterogeneity models by Long and co-workers.…”

“…We have suggested that the propagation of stress field in polymers would be associated with ⟨u 2 ⟩ and hence the local chain stiffness, f. 17 Specifically, for a given a/h value, the magnitude of stress propagation would be larger for a polymer with a smaller value of f compared to other polymers with larger values of f, leading to a stronger degree of substrate effect. In agreement, our observations of different degree of substrate effect for PS/TMPC blends could be rationalized in terms of local vibration constant and hence local chain stiffness.…”

Elasticity patterns induced by phase-separation in polymer blend films

“…There are numerous experimental techniques enabling determination of physico-chemical properties of supported thin polymer layers, including atomic force microscopy (AFM)-based force spectroscopy [49–57], nanoindentation [46,58–60], acoustic waves measurements or buckling-based metrology [44]. In case of indentation experiments, the measured parameters are usually combination of film and substrate properties.…”