Elastic Mechanical Response of Thin Supported Star-Shaped Polymer Films

Abstract

We show evidence of thickness-dependent elastic mechanical moduli that are associated largely with the effects of architecture (topology) and the overall shape of the macromolecule. Atomic force microscopy (AFM) based nanoindentation experiments were performed on linear chain polystyrene (LPS) and star-shaped polystyrene (SPS) macromolecules of varying functionalities (number of arms, f) and molecular weights per arm M w arm . The out-of-plane elastic moduli E(h) increased with decreasing film thickness, h, for h less than a threshold film thickness, h th . For SPS with f ≤ 64 and M w arm > 9 kg/mol, the dependencies of E(h) on h were virtually identical for the linear chains. Notably, however, for SPS with f = 64 and M w arm = 9 kg/ mol (SPS-9k-64), the h th was over 50% larger than that of the other polymers. These observations are rationalized in terms of the structure of the polymer for high f and sufficiently small M w arm and not in terms of the influence of interfacial interactions.

Mechanical and viscoelastic properties of confined amorphous polymers

“…The 4‐arm star PS also exhibits a decrease in the elastic modulus for the thin films, but the elastic modulus does not begin until the film is less than ∼40 nm and the modulus only decreases by a factor of 2 when the film reaches 10 nm . This behavior is consistent with the suppression in the decrease in T g of this star PS, but differs from the nanoindentation studies by Green and coworkers where this star polymer should have mechanical properties similar to the linear PS . The mechanical properties of the centipede PS are more complex as the thick film modulus is greater than for the other architectures.…”

“…48 The work showed an increase in the elastic modulus for all polymers examined on confinement, but the stiffening of the star PS with sufficient number of arms and small enough M n of the arms was observed at larger lengthscales in comparison to the linear PS. 48 As shown in Figure 8(a), the elastic moduli of the supported PS films all increase as the film thickness is decreased. The stiffest PS for thin films is the 64 arm star with the lowest M n (9 kg/mol), while this same star polymer is the most compliant for the thickest films, even in comparison to the linear PS of a similar molecular weight.…”

“…Indentation studies of a series of star PS with varying number of arms and molecular weight of the arms illustrated that a critical value for the maximum number of arms and the minimum molecular weight where the star and linear polymers behave similarly . The work showed an increase in the elastic modulus for all polymers examined on confinement, but the stiffening of the star PS with sufficient number of arms and small enough M n of the arms was observed at larger lengthscales in comparison to the linear PS . As shown in Figure (a), the elastic moduli of the supported PS films all increase as the film thickness is decreased.…”

“…This convolution with the substrate impairs the ability to conclusively determine how nanoconfinement impacts the mechanical properties of thin polymer films. In attempts to minimize the substrate effect, some thin film studies have employed atomic force microscopes for performing the indentation . Recently, finite element computations have been applied to the interpretation of nanoindentation data of supported thin polymer films to separate substrate effects from confinement effects .…”

“…In attempts to minimize the substrate effect, some thin film studies have employed atomic force microscopes for performing the indentation. 48,49 Recently, finite element computations have been applied to the interpretation of nanoindentation data of supported thin polymer films to separate substrate effects from confinement effects. 50 When considering the measurements of T g of polymer thin films, the nanoindentation approach has an additional complication -the free surface of the film is perturbed and now in contact with the probe.…”