Formation mechanism of asymmetric porous polymer films by photoinduced phase separation in the presence of solvent

Abstract

Photoinduced phase separation in the presence of a nonreactive volatile solvent provides a simple, light-tunable, and costeffective route for fabrications of porous polymer films. Here, we show that the short-time (<10 s) ultraviolet irradiation of a photoreactive solution film promotes particular asymmetries in the composition and porosity across the thickness. Confocal Raman spectroscopy revealed that the compositional asymmetry is maximum at a certain critical light intensity. We discuss the film formation mechanism based on the light-driven diffusion of the solvent and monomer.

Reaction-driven solvent transport in UV-curable phase-separating coatings

“…Our preliminary measurements showed that the conversion reached 0.6 under continuous photo-irradiation for 20 s (not shown here), and reached 0.9 in methyl isobutyl ketone. 27 This implies that the relatively low conversion in the present study is due to the limited amounts of active radicals generated during the short photo-irradiation time. On the other hand, the increase in light intensity increased the absorbance of 1363 cm -1 associated with the symmetric stretching of CH 3 bonds in MEK molecules.…”

“…From a practical viewpoint, the accumulation of solvent molecules at the bottom surface is sometimes undesirable because it may promote a decrease in evaporation rates, trapping skinning of the solvent, 51 curling 27 and/or delamination due to drying-induced stresses driven by the difference in strain between the top and bottom surfaces. Thus, it is important to understand (i) the physical origin of solvent localization and (ii) the relaxation mechanism of a nonuniform concentration profile to avoid the drying defects.…”

“…The photoinitiator (Irg819) and the acrylate monomer (Aronix-M9050) used in this work are commercially available and applicable to practical reactive systems that require relatively high glass transition temperatures after curing. 43 The relative amount of initiator, i.e., the mass ratio of initiator to monomer was chosen not to exceed the solubility limit of the initiator but to be high enough to achieve monomer conversions higher than 20% when the sample was exposed to UV light for 1 s. We have previously utilized 1,3 a-alkyl amino phenone (Irgacure 379 EG; BASF) as initiator and explored formation mechanism of submicron-scale pores 27 and nonuniform concentration distributions 28 in M9050 solution systems. We here chose Irg819 as one of the common initiators that have been used to examine photo-polymerization reaction kinetics.…”

“…This nonequilibrium curing process in the presence of a solvent, herein referred to as frontal photoinduced phase separation (fPIPS), has received considerable attention as a promising route for the fabrication of battery separators and precision filters as it enables the creation of light-tunable, submicronscale pores in cured films by subsequent evaporation from solvent-rich domains. 27,28 Viklund et al 29 prepared molded macroporous polymer monolith utilizing monomer mixtures of trimethylolpropane trimethyacrylate and 2,3-epoxypropyl methacrylate and porogenic solvents of toluene and 2,2,4-trimethylpentane. Yu et al 30 examined the photo polymerization of a mixture of ethylene dimethacrylate, butyl meth-acrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in a mixture of 1-propanol, 1,4-butanediol, and water.…”

“…However, the underlying diffusion mechanisms remain poorly understood. According to Yoshihara, 27 UV exposure from the top surface led to solvent enrichment at the bottom of photocuring coatings in the absence of evaporation, demonstrating the light-driven, anomalous solvent transport. Despite the progress in physical modeling of reaction-induced phase separation [36][37][38] as well as experimental investigations for thermally-cured phase-separating epoxy systems, [39][40][41] to the best of our knowledge, few studies have attempted to explain the reasons why solvent molecules diffuse during the curing of coatings, and how the coupling between photo-reactions, stress development, and phase separation impact the diffusion behavior of the solvent.…”