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

• Published in: ACS macro letters 2020-11, Vol.9 (11), p.1521-1526
• Main Authors: Yiu, Pak Man, Yuan, Hailin, Gu, Qiao, Gao, Ping, Tsui, Ophelia K. C
• Format: Article
• Language: English
• ISSN: 2161-1653; 2161-1653
• DOI: 10.1021/acsmacrolett.0c00471

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.