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

Elastic moduli, E , 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, h , from 8 to 130 nm. As γ̇ increases, E increases initially, then approaches the bulk value, E bulk , when γ̇ exceeds a characteristic value (≡ τ -1 ) that decreases with increasing h . The noted variation of E with γ̇ shows that stress relaxation occurs in the films during measurement when γ̇τ ≪ 1, while the noted variation of τ -1 with h shows that thinner films relax faster. Consequently, E decreases with decreasing h if γ̇ is small, but displays independence of h if γ̇ is large. Visually, the crossover takes place at around γ̇ = 0.0015 s -1 , where at γ̇τ > 1 for all films.