Wavefront kinetics of plasma oxidation of polydimethylsiloxane: limits for sub-μm wrinkling

Abstract

We investigate the surface plasma oxidation of polydimethylsiloxane (PDMS) elastomers and its implication for the morphologies attainable by wrinkling of glassy-elastomer 'bilayers'. The kinetics of glassy skin formation is found to follow a logarithmic dependence with plasma exposure time t and, for various plasma intensities I, the relevant control variable is shown to be dose (≡I × t). We model the mechanism and kinetics of glassy film formation by plasma oxidation with a frontal propagation coarse-grained model, describing the spatio-temporal evolution of a conversion order parameter (ϕ) orthogonal to the film surface. The model is validated by X-ray reflectivity experiments, which confirm the logarithmic growth and quantify the initial growth of a transient, incomplete, skin layer during the early stage of plasma exposure. Three regimes are identified as (I) induction, (II) formation and (III) propagation with a combination of X-ray and wrinkling experiments. The simultaneous increase in thickness and skin mechanical modulus is found to be responsible for an unexpected minimum wavelength λmin attainable, which depends on critical strain εc and is ultimately limited by mechanical failure of the elastomer (λmin ≃ 140 nm is demonstrated at ε = 200%). We conclude by establishing a 1D surface morphology diagram, in terms of wavelength λ and amplitude A, limitations and capabilities for producing highly ordered (sub-)micropatterns over macroscopic areas using plasma oxidised PDMS under uniaxial strain.

Wrinkling Measurement of the Mechanical Properties of Drying Salt Thin Films

“…Air plasma oxidation is carried out using a Harrick Plasma PDC-002, with air pressure P = 1 mbar, power p = 7 W, and exposure times t = 2, 20 and 40 min. Plasma exposure causes the surface CH 3 groups to be converted into OH groups 38,39,40,41 , resulting in the formation of a glassy-like layer of defined thickness, which becomes thicker as the oxidation time increases 4,6 . The oxidised PDMS is thus hydrophilic, permitting full wetting of the aqueous solution and planar film formation.…”

“…Eox is independently determined 4,6 in the absence of a top film. The property of interest,  E f , can thus be obtained, according to:…”

“…However, while PDMS substrates readily form laminates with floated or evaporated films 8,17,36,37 , with good interfacial bonding, solution deposition requires surface wetting and thus modification of the otherwise hydrophobic elastomer. We oxidise the PDMS coupons by plasma air plasma exposure or UV ozonolysis 3,4,6 . To minimise crack formation, surface oxidation takes place under strain, employing a uniaxial strain stage and stretching the coupon by ε ≈ 20%.…”

“…Spontaneous surface wrinkling of laminate films can occur as a result of mechanical stress 3,4,5,6 , expansion and contraction associated thermal deposition, or during solvent evaporation and shrinkage 7,8,9,10,11 , and is often undesirable 7,8,11,12,13,14,15 . The analysis of wrinkling topographies has also been shown to provide a powerful tool for the mechanical characterisation of thin and ultra-thin polymeric, composite and metallic supported thin films 2,7,16,17,18 .…”

Controlled fabrication of nanoscale wrinkle structure by fluorocarbon plasma for highly transparent triboelectric nanogenerator

“…To date, most studies involving wrinkling to create ordered structures rely on plasma or ultraviolet-ozonolysis (UVO) oxidation and metal deposition to create the stiff skin. Although these approaches are quite practical, the material properties of the substrate during oxidation can significantly alter the typical morphologies and even block the formation of the wrinkle structure (that is, the process does not have universality to different materials), whereas the deposition of the metal layer would greatly decrease the transparency of the substrate 21,22 . All of these drawbacks limit the practical application of this method in specific occasions that require high transparency and other materials.…”