Solid-like dynamics in ultrathin films of polymeric liquids

Abstract

In this letter, we demonstrate that, at mesoscales, nonferroelectric liquid films of poly(dimethyl siloxane) exhibit significant electrostriction not present in the corresponding bulk state. Remarkably, the observed electrostrictive effect has a response time <20 μs in contrast to >5 ms recorded in conventional bulk (ferroelectric) polymers. The emergence of this fast electrostrictive strain in thin films is explained in terms of the amalgamation of two contrasting dynamic features—the influence of a highly mobile, viscous layer (at the air/film interface) on the less-mobile, but fast responding, solid-like layer at the film/substrate interface. The effect is observed for thickness below 200 nm.

Conformational rearrangements in interfacial region of polydimethylsiloxane melt films

“…Also, in thin films the presence of an interfacial "solid-like" layer for PDMS films on silica surface has been observed using quasielastic neutron scattering, 16 nuclear magnetic resonance 17 and electrostriction measurements. 18 Atomic force microscopy measurements show that the time scales for structural changes in PDMS melts at a solid surface are much longer than in the bulk, and that the formation of an immobilized layer at mica and silica surface typically takes 10 hrs for M w = 18 000 g/mol PDMS molecules. [19][20][21] However, the authors could not give a value for the layer thickness, because in atomic force microscopy the zero distance is derived from the straight part of the force curve itself and not from an independent quantity.…”

“Freezing” of Nanoconfined Fluids under an Electric Field

“…Is there an influence of the EF on the optical properties, primarily the refractive index n of the liquid, which could affect the accuracy of the calculation of the liquid film thickness? From a theoretical perspective, the variation in the refractive index Δ n due to the structural order and the orientation of induced dipoles along the EF direction can be approximated as Δ n = N A v α ρ 3 ε 0 M ( n 2 + 2 ) 2 6 n where N Av is Avogadro’s number (∼6.022 × 10 23 ), ρ ≈ 1 g/cm 3 , ε 0 is the vacuum dielectric constant, M is the molecular weight (226.46 g/mol for n -hexadecane), and α is the electronic polarizibility, which is the division of the dipole moment μ and the EF intensity E (α = μ/ E ); μ can be calculated by normalμ = 4 normalπ normalε 0 R 3 E where R is the effective radius of the molecule (for the n -hexadecane molecule, R < 3 Å , ). On the basis of eqs and , for the highest EF in the experiment, Δ n is <0.2, which will cause a variation in the liquid film thickness of not more than 1 nm.…”