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