Hybrids consisting of a microporous film and polymeric microspheres were fabricated via a simple method without a special apparatus. Highly ordered microporous polymer films with honeycomb structure were fabricated by a dissipative process utilizing amphiphilic poly(acrylic acid)- block-polystyrene, which was synthesized by atom transfer radical polymerization followed by an acid-catalyzed ester cleavage reaction. In order to embed the microsphere efficiently, the dried microporous films should be soaked in methanol to alter the surface functionality and to improve the wettability of the film surface. The introduction of amino functionality to polystyrene microspheres by seeded polymerization of N,N-dimethylaminoethyl methacrylate drastically improved the embedding efficiency. The effect of open pore size was also investigated.
Highly ordered microporous polymer films with honeycomb structure have been fabricated by a simple non- template method utilizing various types of block copolymers [
Various applications of honeycomb films have been considered such as separation membranes [
In this paper, a microporous film and polymeric microspheres hybrids were fabricated via a simple method without a special apparatus. Poly(acrylic acid)-block-polystyrene was used to fabricate a microporous film.
Block copolymers are synthesized via atom-transfer-radical polymerization as a previously reported method [
A 50-mL three necked flask equipped with a stirrer bar, a nitrogen inlet, and a rubber septum was charged with poly(t-BA) (0.801 g, 0.380 mmol), CuBr (54 mg, 0.380 mmol) under nitrogen. After degas process as mentioned above, styrene (St) (19.75 g, 0.19 mol) (500 equivalent to the macroinitiator) and PMDETA (0.33 g, 0.380 mmol) was added. Polymerization was carried out at 90˚C for 24 h. Similar procedures were utilized for purification. Yield: 42%, DP = 285 (for styrene), Mn = 3.1 × 104, Mw/Mn = 1.7.
A 50-mL three necked flask equipped with a stirrer bar, a nitrogen inlet, and a condenser was charged with 3.0 g of poly(t-BA)-block-poly(St), p-toluene sulfonic acid monohydrate (0.33 g, 1.72 mmol), and 3.0 mL of dioxane under nitrogen atmosphere. The reaction mixture was refluxed for 10 h to eliminate t-butyl group. After cooling the solution was poured into excess amount of water to precipitate the product (2.8 g).
Uniform sized polystyrene microsphere (PSt) with a diameter of 1.7 μm was synthesized via dispersion polymerization in aqueous ethanol as previously reported [
Carbon disulfide solution (0.1 - 0.5 wt%, 50 μl) was drop-cast on a cleaned glass slide over an area of ca. 1.1 cm2 (circle with a diameter of 1.2 cm) in a custom designed flow-hood (640 cm3) [
Microspheres (PSt or PS-DM) (1.0 g) were dispersed in 15 mL of distilled water. Microporous film on a glass slide was immersed in the slurry for 30 min with gentle stirring. After washing with water, the hybrid film was dried in vacuo.
Resulting polymers were characterized with 1H-NMR (ECX 300, JEOL, Japan), IR (FT/IR-4100, JASCO, Japan), and GPC [
Successful preparation of PSt-DM was confirmed by solubility change and IR spectroscopy. PSt-DM showed partial solubility in chloroform due to the presence of PDM (insoluble in chloroform) and crosslinking with EDMA.
As described in a previous report [
After drying a microporous film in vacuo for 24 h, the film was immersed in the slurry of microspheres for 30 min. In this process, however, almost no particles were embedded although the open diameter of pores (ca. 3 μm) was larger than the diameter of microsphere as shown in
During the formation of pores, it is considered that water droplets were stabilized by the carboxyl groups. Collapse and coalescence of droplets were prevented by this stabilization. On other words, it is speculated that pore walls are covered with carboxyl groups just after the evaporation of water. However, it is considered that carboxyl groups go inside the film when it is exposed to air in order to lower the surface tension. As Kajiyama reported, the mobility of film surface is much higher than that of bulk [
In order to put back the carboxyl groups to the surface, microporous films were soaked in methanol. Once the surface is rich in hydrophilic carboxyl groups, it is expected that wettability is improved, and electrostatic interaction is enhanced.
As mentioned in Section 3.2, the open pore size of microporous film can be controlled. Finally we investigated the relationship between the ratio of pores containing microsphere(s) and the open pore size. As the pore size increases, the ratio of pore containing microsphere increased as shown in
Honeycomb films were successfully fabricated from the amphiphilic block copolymer, poly(acrylic acid)-block- polystyrene) prepared via atom transfer radical polymerization followed by the acid catalyzed elimination reaction. After drying, the surface of the film was hydrophobic since carboxyl groups went inside the film in order to minimize the surface energy, and hybridization did not occur by immersing the film into slurry of polymer particles. By soaking the film in methanol, the wettability increased, and hybrid films were successfully obtained, where microspheres were embedded in the pore. The embedding efficiency of microspheres modified with amino groups was much higher than that of conventional polystyrene microspheres. Electrostatic interaction plays an important role for the hybridization. With the increase of open pore size, multi-numbers of microspheres were embedded in a single pore. It is noteworthy that our process utilizes no special apparatus for the fabrication of hybrids. It is expected that periodic arrangements of aggregates of microspheres can be fabricated using various combinations of pore size of microporous films and diameter of microspheres.