Processing Induced Nonequilibrium Behavior of Polyvinylpyrrolidone Nanofilms Revealed by Dewetting

Abstract

Polyvinylpyrrolidone (PVP) nanofilms prepared by spin-coating have vast applications in biological and microdevice fields. However, detailed knowledge of processing induced nonequilibrium behavior of PVP nanofilms and solutions for minimizing residual stresses toward high-quality films has still been lacking. In the present study, we first explored the rapid film formation process via statistics on nascent holes. Next, by employing dewetting as a major probe, we revealed that many processing conditions, particularly previously overlooked variables like the atmosphere, substrates, and immersion time, were correlated substantially with the degree of nonequilibrium of nanofilms. Proper aging temperature and time were demonstrated essential for releasing residual stresses and achieving more equilibrium nanofilms. This work offered abundant experimental evidence in the building relationship between the processing and nonequilibrium nature of polymer nanofilms, which were crucial for their preparation and application.

Nascent Holes on Spin-Coated Polymer Nanofilms: Effect of Processing and Solvents

“…de Gennes et al suggested that severe tensile stress was prevailing in nanofilms thicker than 70 nm if there was a pre-solidified “crust” layer formed upon fast evaporation. ,, High surface roughness at the nanometer scale was manifestation of surficial stress within “crust.” Here, smooth film with steep cracks is observed in Figure A, indicating the significant stress accumulated across the whole film depth upon evaporation, other than that on simply surficial layer. Meanwhile, non-adhesive interface and modulus disparity between film and hydrophobic substrate allowed relaxation of stress by lateral shrinking, thus resulting in the formation of cracks . Similar cracks (5 μm in width) induced by in-plane tensile stresses were reported by Reiter et al after quenching annealed films at temperature approaching T g .…”

“…In spin coating, modifying spin speed is a convenient and effective way to control the final film thickness, , morphologies of depositing films, ,, and the degree of equilibrium of nanofilms. Figure A demonstrates that film thickness decreased with a power law relation with increasing spin speed.…”

“…Here, pure solvent was first deposited onto the substrate for various retention times and then removed by fast spin. Afterward, polymer solution was dropped onto the pure-solvent-treated substrate and spin instantly; (2) spin coating: after immersion, polymer nanofilm was spin-coated on top of PDMS substrates for 60 s at various spin speed (ω = 3000–9000 rounds per minute, rpm) in closed conditions to achieve more equilibrium films . After spin coating, nascent holes and other morphological features appeared on nanofilms.…”

“…Nascent holes, that is, ring structures or pinholes, appear after the spin-coating process, particularly severe when the substrate was poor wetting. Therefore, variation in even a single factor, such as preparation atmosphere, solvent, spin speed, immersion time (the amount of time the solution stays on the substrate), and so forth, may finally result in a dramatic change in the structure of the film. ,,, Our previous work demonstrated the morphological changes in nascent holes induced by film thickness, in atmosphere during spin-coating, and in interfacial properties. Moreover, the study on the nascent holes provided us a unique tool to reveal formation of nanofilms at various conditions.…”

Assessing Polymer-Surface Adhesion with a Polymer Collection

“…Recent trends of integration and portability generate growing demand for miniaturized devices, which brings great opportunity to this field. However, the uncertainty of the polymer-surface adhesion slows down this evolution. , On the one hand, spontaneous dewetting phenomena of polymers on substrates make it hard to generate polymer microstructures with defined morphologies (e.g., size, thickness, and shape). − On the other hand, mismatched polymer–substrate pairs lead to poor adhesion, having a negative impact on device stability. Therefore, it is necessary to get a better control of polymer-surface adhesion.…”