Formation of Microporous Films via Pattern Photo-Polymerization Induced Phase Separation
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Formation of Microporous Films via Pattern PhotoPolymerization Induced Phase Separation Scott Meng, Domasius Nwabunma, and Thein Kyu* Institute of Polymer Engineering The University of Akron, Akron, OH 44325
We describe a method of fabricating microporous films through pattern photopolymerization induced phase separation in a mixture of solvent/monomer using multi-wave mixing, i.e., four-wave mixing through interference of two horizontal and two vertical waves. The simulation on the microporous forming process was undertaken in the framework of the time-dependent Ginzburg-Landau (TDGL) Model B equations coupled with the reaction kinetic equation of photopolymerization. In the total free energy description, Flory-Huggins free energy of mixing was combined with the elastic free energy of the network. The calculated results showed that the microporous size, shape, and spacing of the micropores depend on the angle of interference and the reaction rate controlled by the incident UV intensity.
*Corresponding author: [email protected]
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Introduction: Microporous polymeric films have been widely used as membrane separators in the manufacture of battery, filters, fuel cells and medical devices.1-3 Various methods of fabricating microporous membranes have been reported;1-14 one of the typical methods is to mix a polymer with solvent and/or with chemically treated particles, extrude the mixture through a slit die, stretch the sheet, and finally remove the residual solvent. Another methodology is to generate microporous pattern via phase separation of polymer blends and solutions during evaporation. A recent approach is to photo-etch the polymer by UV radiation and then shape the structurally inhomogeneous microporous elements. However, these methods have limited control over the structure homogeneity such as uniformity in pore size and distribution that is needed to improve performance and reliability. Recently, a novel methodology has been developed to fabricate electrically focusable liquid crystal microlens arrays through pattern photo-polymerization induced phase separation (PPIPS) in liquid crystal (LC)/UV reactive monomer mixtures based on four-wave mixing, i.e. the constructive and destructive inference of two vertical waves and two horizontal waves.16 To account for the spatio-temporal emergence of the LC domains, time-dependent Ginzburg-Landau (TDGL) model has been introduced in which the concentration and orientational order parameters are coupled along with the FloryHuggins free energy for mixing, the Maier-Saupe free energy for nematic ordering, and the free energy due to the network elasticity. The simulation showed that the periodic pattern thus formed in two-dimension resembles a checkerboard or droplet arrays. A similar technique has also been practiced in fabricating the periodic pattern of photonic CC6.6.2
crystals.15 Although the Maier-Saupe free energy plays an important role in describing the orientational order within the LC rich domains, we felt that it may be inconsequential to the pattern fo
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