Anisotropic mechanical properties of aligned polystyrene-block- polydimethylsiloxane thin films

Abstract

Mechanical properties of multicomponent polymers such as block copolymers (BCP) are critical to their utilization in thin film applications, yet the effect of block alignment of even the simplest BCPs on thin film modulus has not been thoroughly explored. Polystyrene-block-polydimethylsiloxane (PS-b-PDMS) thin films with cylindrical PDMS domains (fPDMS = 0.23) are aligned by cold zone annealing-soft shear mode (CZA-SS). Increasing the maximum temperature of the thermal zone and/or decreasing the CZA-SS velocity improves the alignment of the PS-b-PDMS domains as determined by atomic force microscopy (AFM) and small-angle neutron scattering (SANS). The maximum Hermans orientation factor (S) for this system using the CZA-SS conditions examined is S ≈ 0.85, yielding anisotropic mechanical properties as determined by surface wrinkling. While the in-plane modulus perpendicular to PDMS cylinder axis is nearly invariant of S, the modulus parallel to the cylinder axis is increased by a maximum of 31% for S ≈ 0.85, compared to its unaligned state (S ≈ 0). These results demonstrate the highly anisotropic response of the glassy matrix strength to alignment of internal rubbery cylindrical channels. This anisotropic structure-property coupling is consistent with expectations associated with highly aligned composites, but this CZA-SS methodology enables facile examination of the influence of extent of alignment on physical properties that has not been quantitatively investigated in the past. © 2013 American Chemical Society.