DUAL SCALE ROUGHNESS DRIVEN PERFECTLY HYDROPHOBIC SURFACES PREPARED BY ELECTROSPRAYING A POLYMER IN GOOD SOLVENT-POOR SOLVENT SYSTEMS
Materials Science & Engineering, Ph.D. Dissertation, 2012
Prof. Dr. Yusuf Z. Menceloğlu (Thesis supervisor)
Prof. Dr. Canan Atılgan
Prof. Dr. Yaşar Gürbüz
Assoc. Prof. Dr. Mustafa M. Demir (İzmir Institute of Technology)
Assoc. Prof. Dr. Mehmet Yıldız
Date & Time: July 20th, 2012 – 13:00
Place: FENS L027
Keywords: Perfectly hydrophobic, superhydrophobic, dual scale roughness, nano scale roughness, Lotus effect, contact angle hysteresis, electrospraying, electrospinning, fluoropolymer, phase separation, binary solvent system, dissipative particle dynamics
A facile method to produce perfectly hydrophobic surfaces (advancing and receding angles both 180o, having no affinity for water) via electrospraying is demonstrated. When a copolymer of styrene and a perfluoroalkyl acrylate monomer was electrosprayed in good solvents, surfaces composed of micron size beads were formed and fairly low threshold water sliding angles could be achieved. Addition of high boiling point poor solvents to the solutions resulted nanoscale roughness on the beads. However, even the nanoscale roughness dominated topographies achieved by this method exhibited contact angle hysteresis although deducted to be relatively small. On the other hand, when the electrospraying process parameters were set such that micron size hills of nanoscopically rough beads were formed, 0o sliding angles, implying zero contact angle hysteresis, were measured. Videos of droplets recorded and the adhesive forces measured during a contact and release experiment revealed that these dual scale rough surfaces were indeed perfectly hydrophobic. Application of the method with other binary good solvent - poor solvent systems also resulted in perfect hydrophobicity. Overall results showed how the differences in surface topology affected the wettability of surfaces within a very narrow range between perfect and extreme hydrophobicity (advancing and receding angles both close to 180o).
In order to interpret the formation of different surface topographies achieved by electrospraying the copolymer in good, poor and binary solvent systems, dissipative particle dynamics simulations and dynamic light scattering analysis were performed. Simulations in the good solvent revealed relatively homogenous solutions at all concentrations, whereas the polymer was observed to phase separate even at low concentrations in the poor solvent. Light scattering experiments yielded useful information about the hydrodynamics of the real chains in the corresponding solvent systems in the dilute regime. It was found that the polymer forms stable segregates in the poor solvent due to the weak interaction with the solvent. It was concluded that formation of smooth bead morphologies is due to homogenous drying of the polymer from the good solvent. On the other hand, polymer segregates lead to nanoscopic features in the regions where the solidification occurs in the poor solvent environment.