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

Abstract

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.

Load-bearing entanglements in polymer glasses

“…2C ). The measured moduli for polystyrene are comparable to literature values of bulk polystyrene specimens ( 32 , 33 ) and experimental values of thin films ( 16 , 34 , 35 ). The maximum stress σ Max for blends with ϕ > 0.80 is also approximately constant ( Fig.…”

Enhanced Free Surface Mobility Facilitates the Release of Free-Volume Holes in Thin-Film Polymer Glasses

“…The ability of facilitating hole diffusion by the free surface can be related to the nature of enhanced segmental dynamics near surface. ,− Because of the lack of neighboring molecules, which alleviates the degree of molecular crowding and caging, − segments at surface regions can rearrange readily even at T < T g,bulk to create favorable pathways for volume holes to pass through the film and release to the vapor phase. This also causes the acceleration of segmental relaxation in thin polymer films because of the void-induced structural relaxations . The surface-promoted hole release in thin-film polymer glasses seems to be in line with the argument of reduced density and increased hole size at the glass surface − and the phenomenon of acceleration of “bubble-releasing relaxation” near the surface in vapor-deposited colloidal glasses …”

Nanoconfinement Controls Mechanical Properties of Elastomeric Thin Films

“…Although measurements on E have been reported, the results are contradictory. Typical results in rubbery 12,16,18,19,21 and glassy 7,14,15,22 states are summarized in Figure 4C. Glassy films mainly present a reduced E as thickness is below 20 nm, whereas some reports claim a neutral trend or an increase.…”