Strain Rate and Thickness Dependences of Elastic Modulus of Free-Standing Polymer Nanometer Films.

Elastic moduli, , of free-standing polystyrene (PS) single-layers and polystyrene-polydimethylsiloxane (PS-PDMS) bilayers are measured by uniaxial tensile testing at room temperature under different strain rates, γ̇, and for PS thicknesses, , from 8 to 130 nm. As γ̇ increases, increases initially, then approaches the bulk value, , when γ̇ exceeds a characteristic value (≡ τ) that decreases with increasing . The noted variation of with γ̇ shows that stress relaxation occurs in the films during measurement when γ̇τ ≪ 1, while the noted variation of τ with shows that thinner films relax faster. Consequently, decreases with decreasing if γ̇ is small, but displays independence of if γ̇ is large. Visually, the crossover takes place at around γ̇ = 0.0015 s, where at γ̇τ > 1 for all films.