Gary C. Tepper

The aim of this study was to investigate the nucleation, growth, and surface deposition of poly(2,2,2-trifluoroethyl methacrylate) [poly(TFEMA)] from the one-phase, cloud point, and two-phase regions of a supercritical CO₂–toluene solvent. A ternary mixture of 20 wt% toluene + 79 wt% scCO₂ + 1 wt% poly(TFEMA) at 40.0 °C was exposed to a fluorine-doped tin oxide (FTO) surface for 30 min at pressures placing the solution in (i) a one-phase region (15.86 MPa), (ii) the cloud point (12.37 MPa), and (iii) a two-phase region (8.96 MPa). Using the Altunin–Gadetskii–Haar–Gallagher–Kell (AG–HGK) equation of state (EOS), the corresponding CO₂ densities are 793.9, 729.2, and 477.8 kg m⁻³. Scanning electron microscopy (SEM) and particle-size analysis (sample sizes N = 852–1177) show particle-size distributions (PSDs) that are well described by the following lognormal form: the mean diameter increases monotonically with a decrease in pressure (1.767 μm → 2.605 μm → 2.863 μm), while dispersion tightens slightly near the cloud point (coefficient of variation, CV: ≈0.47 → 0.44) and then broadens strongly in the two-phase region (CV ≈ 1.02). Morphologies transition from sparse, compact islands (one-phase) to agglomerated, necked spheres (cloud point) and finally hierarchical populations containing hollow/pitted large particles (two-phase). These outcomes are consistent with a phase-state-controlled shift in nucleation pathways, as follows: from heterogeneous surface nucleation in the one-phase regime to homogeneous nucleation with agglomeration at the cloud point, and to homogeneous nucleation with coalescence and solvent capture in the two-phase regime. The results provide a mechanistic basis and practical design rules for pressure-programmable control of fluoropolymer coatings prepared from scCO₂/aromatic-cosolvent systems.

The nucleation, growth, and surface deposition of poly(2,2,2-trifluoroethyl methacrylate) [poly(TFEMA)] were investigated across three thermodynamic regimes of a supercritical CO₂–toluene system. A ternary mixture of 20 wt% toluene + 79 wt% scCO₂ + 1 wt% poly(TFEMA) at 40 °C was exposed to a fluorine-doped tin oxide (FTO) surface for 30 min at pressures placing the solution in (i) a one-phase region (15.86 MPa), (ii) the cloud point (12.37 MPa), and (iii) a two-phase region (8.96 MPa). Using the Altunin–Gadetskii–Haar–Gallagher–Kell (AG–HGK) equation of state (EOS), the corresponding CO₂ densities are 793.86, 729.15, and 477.83 kg m⁻3. Scanning electron microscopy (SEM) and particle-size analysis (sample sizes N = 852–1177) show particle-size distributions (PSDs) that are well described by a lognormal form: the mean diameter increases monotonically with decreasing pressure (1.767 µm → 2.605 µm → 2.863 µm), while dispersion tightens slightly near the cloud point (coefficient of variation, CV: ≈0.47 → 0.44) and then broadens strongly in the two-phase region (CV ≈ 1.02). Morphologies transition from sparse, compact islands (one-phase) to agglomerated, necked spheres (cloud point) and finally to hierarchical populations containing hollow/pitted large particles (two-phase). These outcomes are consistent with a phase-state-controlled shift in nucleation pathways: from heterogeneous surface nucleation in the one-phase regime to homogeneous nucleation with agglomeration at the cloud point, and to homogeneous nucleation with coalescence and solvent capture in the two-phase regime. The results provide a mechanistic basis and practical design rules for pressure-programmable control of fluoropolymer coatings prepared from scCO₂/aromatic-cosolvent systems.

Supercritical CO₂ (scCO₂) is a versatile solvent for polymer processing; however, many partially fluorinated polymers exhibit limited solubility in neat scCO₂. Organic cosolvents such as toluene can enhance polymer–solvent interactions, thereby improving solubility. The cloud point behavior of poly(2,2,2-trifluoroethyl methacrylate) (poly(TFEMA)) at 3 wt% concentration in scCO₂–toluene binary mixtures was investigated over a temperature range of 31.5–50 °C and toluene contents of 0–20 wt%. Solvent mixture densities were estimated using the Altuin–Gadetskii–Haar–Gallagher–Kell (AG–HGK) equation of state for CO₂ and the Tait equation for toluene. For all compositions, the cloud point pressure was observed to increase linearly with temperature. The cloud point pressure decreased monotonically with increasing toluene concentration and at the highest concentration of 20 wt% was reduced by approximately 40% in comparison to neat scCO₂. The addition of toluene lowered the solvent density, but the increase in solvent–solute molecular interactions resulted in the observed decrease in cloud point pressure. Toluene is shown to be an effective cosolvent for dissolving poly(TFEMA) in scCO₂, offering a promising approach to lowering operating pressures in fluoropolymer processing. Our results provide valuable phase behavior data for designing scCO₂-based extraction, impregnation, and particle formation processes involving poly(TFEMA).

A new process is reported for the incorporation of a fluoropolymer into a solid perovskite film. Poly(trifluoroethyl methacrylate) [CH₂C(CH₃)(CO₂CH₂CF₃)]_n was delivered to methylammonium lead iodide (CH₃NH₃PbI₃) perovskite films by crystallizing the film in supercritical carbon dioxide/ethanol containing the dissolved fluoropolymer. The surface was characterized before and after fluoropolymer exposure using scanning electron microscopy, Raman spectroscopy, and contact angle measurements. The results indicate that the fluoropolymer was incorporated into the perovskite film during the supercritical fluid crystallization process. The incorporation of a hydrophobic fluoropolymer into perovskite has the potential to improve resistance to environmental degradation.

A new process is reported for the incorporation of a fluoropolymer into a solid perovskite film. Poly(trifluoroethyl methacrylate) [CH2C(CH3)(CO2CH2CF3)]n was delivered to methylammonium lead iodide (CH3NH3PbI3) perovskite films by crystallizing the film in supercritical carbon dioxide/ethanol containing the dissolved fluoropolymer. The surface was characterized before and after fluoropolymer exposure using scanning electron microscopy, Raman spectroscopy and contact angle measurements. The results indicate that the fluoropolymer was incorporated into the perovskite film during the supercritical fluid crystallization process.

Performance degradation under environmental conditions currently limits the practical utility of perovskite-based solar cells. The moisture stability of CH₃NH₃PbI₃ perovskite films and solar cells was measured during exposure to three different levels of relative humidity. The films were crystallized at two different temperatures with and without simultaneous exposure to supercritical carbon dioxide. The film crystallinity, optical absorption, and device photoconversion efficiency was measured over time for three relative humidity levels and both crystallization methods. It was determined that film crystallization in supercritical CO₂ resulted in significant improvement in moisture stability for films processed at 50 °C, but negligible improvement in stability for films processed at 100 °C.

Hydrophobic materials with tunable wettability were developed by electrospinning aligned polystyrene (PS) fibers onto the surface of a unimorph composite piezoelectric substrate. An electric field was used to modify the curvature of the substrate resulting in a corresponding change in the morphology of the electrospun coating. Contact angle measurements were performed on droplets deposited onto the surface before and after application of the electric field. The water droplet contact angle was observed to change in response to the applied voltage. Contact angle measurements were performed as a function of surface fiber density and suggest that the change in contact angle is caused by a transition from Wenzel to Cassie–Baxter wetting. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41592.