Unlocking the Potential of Liquid Plasma Polymer Films: Characterizing Aging Effects and Their Impact on the Wrinkling Phenomenon

Here, we present the study of the intricate dynamics between the physicochemical properties of liquid propanethiol plasma polymer films (PPFs) and the formation of wrinkles in PPF/Al bilayers. The study investigates the effect of liquid PPF aging in the air before top Al layer deposition by magnetron sputtering on the wrinkling phenomenon for 4 days. Thanks to atomic force microscopy, the wrinkle dimensions were found to decrease by approximately 55% in amplitude and 66% in wavelength, correlated with an increase in the viscosity of the PPF over the aging duration (i.e., from less than 107 to 1010 Pa·s). This behavior is not linked to alterations in cross-linking degree as evidenced by time-of-flight secondary ion mass spectrometry experiments but rather to network densification driven by the inherent molecular chain mobility due to the viscous state of the PPF. X-ray photoelectron spectroscopy measurements emphasizing the absence of oxidation of the PPF over the aging duration support this, revealing a unique aging mechanism distinct from other plasma polymer families. Overall, this study offers valuable insights into the design and application of mechanically responsive PPFs involved in bilayer systems, paving the way for advancements in nanotechnology and related fields.

Flat, wrinkled, or ridged: Relaxation of an elastic film on a viscous substrate undergoing continuous compression

We conduct a finite element computational study of the dynamics of a thin elastic film bonded to a much thicker viscous substrate undergoing compression at a fixed rate. The applied compression tends to continuously increase the strain, and hence the elastic energy, of the film. In contrast to the well-studied case of a soft elastic substrate, a viscous substrate cannot store elastic energy; instead it regulates the kinetics of the various mechanisms that dissipate elastic energy of the film. Sufficiently short films remain flat because shear flow in the liquid near the ends allows rapid relaxation of the strain over the entire film length. In longer films, end-relaxation cannot relax film strain in the mid-section, which therefore buckles. Buckles initially appear as packets of approximately-sinusoidal wrinkles. With increasing strain, these packets transform into tall localized ridges separated by nearly flat regions. In all cases, the buckles cause end-relaxation to become dynamically confined to a narrow region near the ends We construct a state map identifying regions of the parameter space of strain vs film length in which the film remains flat, develops wrinkle packets, or develops localized ridges. The evolution of the film energy during continuous compression shows that ridge localization appears due to a competition between two effects: a well-spaced ridges offer a lower energy state than uniform wrinkles, but wrinkles can develop faster because they require the viscous fluid to move over shorter distances.

Observation of General Entropy-Enthalpy Compensation Effect in the Relaxation of Wrinkled Polymer Nanocomposite Films

Surface-textured polymer nanocomposite (PNC) films are utilized in many device applications, and therefore understanding the relaxation behavior of such films is important. By extending an in situ wrinkle relaxation method, we observed that the thermal stability of wrinkled PNC films, both above and below the glass transition temperature (T_g), is proportional to a film's nanoparticle (polymer grafted and bare) concentration, with a slope that changes sign at a compensation temperature (T_comp) that is determined to be in the vicinity of the film's T_g. This provides unambiguous confirmation of entropy-enthalpy compensation (EEC) as a general feature of PNC films, implying that the stability of PNC films changes from being enhanced to becoming diminished by simply passing through this characteristic temperature, a phenomenon having evident practical ramifications. We suggest EEC will also arise in films where residual stresses are associated with the film fabrication process, which is relevant to nanotech device applications.

Buckling Instabilities in Polymer Brush Surfaces via Postpolymerization Modification

We report a simple route to engineer ultrathin polymer brush surfaces with wrinkled morphologies using post-polymerization modification (PPM), where the length scale of the buckled features can be tuned from hundreds of nanometers to one micrometer using PPM reaction time. We show that partial crosslinking of the outer layer of the polymer brush under poor solvent conditions is critical to obtain wrinkled morphologies upon swelling. Characterization of the PPM kinetics and swelling behavior via ellipsometry and the through-thickness composition profile via time-of-flight secondary ion mass spectroscopy (ToF-SIMS) provided keys insight into parameters influencing the buckling behavior.

Programming Feature Size in the Thermal Wrinkling of Metal Polymer Bilayer by Modulating Substrate Viscoelasticity

We report a novel strategy for creating stress-induced self-organized wrinkles in a metal polymer bilayer with programmable periodicity (λ_S) varying over a wide range, from ∼20 μm down to ∼800 nm by modulating the viscoelasticity of the bottom polymer layer. Substrates with different viscoelasticity are prepared by precuring thin films of a thermo-curable poly dimethylsiloxane (PDMS) elastomer (Sylgard 184) for different durations (t_P) prior to deposition of the top aluminum layer by thermal evaporation. Precuring of the Sylgard 184 film for different durations leads to films with different degrees of viscoelasticity due to variation in the extent of cross-linking of the polymer matrix. The λ_S as well as the amplitude (a_S) of the wrinkles progressively decrease with an increase in the extent of elasticity of the film, manifested as an increase in the storage modulus (G'). Based on the variation in the rate of decay of λ_S with G', we identify three clearly distinguishable regimes over predominantly viscous, viscoelastic, and elastic bottom layers. While λ_S and a_S drop with an increase in G' for both the first and the third regimes, it remains nearly independent of G' for the intermediate regime. This is attributed to the difference in the mechanisms of wrinkle formation in the different regimes. We finally show that simultaneous modulation of λ_S and a_S can be used to engineer surfaces with different wettability as well as anti-reflection properties.

Photodirected Formation and Control of Wrinkles on a Thiol-ene Elastomer

Photopolymerization on a thiol-ene elastomer is a novel method to spatiotemporally control wrinkle formation gradients and complex patterns. Acrylate-rich elastomers, containing photoinitiator and photoabsorber, were strained and irradiated with UV light to generate wrinkles in as little as five seconds. The presence of the photoabsorber confines the UV light to a thin skin layer, facilitating wrinkle formation upon release of the strain. In conjunction with photomasks, wrinkles are confined and oriented with explicit boundaries defined by the irradiation pattern. Using a photoorthogonal photoiniator, we also demonstrate the ability to independently control the bulk modulus of the wrinkled elastomer. The wide array of thiol and ene monomers, photoinitiators, and photomask designs result in an alternative approach that is rapid, facile, and highly versatile, further enabling the a priori design and engineering of surface topography and shape of the material.

Surface wrinkling induced by photofluidization of low molecular azo glasses

Photoinduced surface wrinkling is demonstrated for a low molecular azo compound confined between a substrate and a thin inorganic elastic layer. The wrinkling process is investigated by time-resolved light scattering as well as by two-dimensional autocorrelation analysis of microscopic images. It is shown that the temporal evolution of the wrinkling pattern is directly controlled by the amount of photons absorbed by the sample and that there is no significant dependence of the equilibrium wavelength on irradiation intensity. Finally, the comparison of thermal and photoinduced wrinkling revealed that photoinduced wrinkles are characterized by a narrower mode distribution and less coarsening.