Responsive Micellar Films of Amphiphilic Block Copolymer Micelles: Control on Micelle Opening and Closing

Abstract

We reported the deliberate control on the micelle opening and closing of amphiphilic polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) micellar films by exposing them to selective solvents. We first treated the micellar films with polar solvents including ethanol and water (pH = 4, 8, and 12) that have different affinities to P2VP. We observed opening of the micelles in all the cases. Both the size of opened pores and the opening rate are dependent on the solvency of different solvents for P2VP. We then explored the closing behavior of the opened micelles using solvents having different affinities to PS. We found that the opened micelles were recovered to their initial closed micelle forms. The recovery was accompanied by a slow micelle disassociation process which gradually reduced the micelle size. The rates of the micelle closing and disassociation are also dependent on the solvency of different solvents for PS.

An Emerging Pore‐Making Strategy: Confined Swelling‐Induced Pore Generation in Block Copolymer Materials

“…2011, 23,[2134][2135][2136][2137][2138][2139][2140][2141][2142][2143][2144][2145][2146][2147][2148] demonstrated that an increase in the pH value of the water in which BCP fi lms were incubated led to faster pore forming and larger pore sizes, while it exerted a completely opposite effect in the case of PS-b -P2VP. [ 40 ] …”

“…The pore formation can be terminated simply by withdrawing the fi lms from the solvent, thus freezing intermediate opening pore morphologies with various pore sizes and surface topographies. Figure 2 shows four intermediate porous morphologies obtained by incubating PS-b -P2VP micellar fi lms in water (pH = 8) for different periods of time ranging from 30 s to 18 h. [ 40 ] In addition, it was found that in the same BCP system the pore openness of the BCP fi lm treated in water (pH = 8) for 18 h was similar to that of the BCP fi lm treated in ethanol for only 30 s, mainly because ethanol has a stronger interaction to P2VP chains than water (pH = 8), while the difference between the affi nity of ethanol and water to PS chains into which the swelling solvents penetrate plays a minor role. Since the micelle cores forming blocks are often weak polyelectrolytes, for instance, P2VP, P4VP, and PAA, some solution properties, such as pH values and ionic strength, greatly infl uence the kinetics of the pore-forming process, facilitating more controls in fi nely tuning the sizes and morphologies of the resultant pores.…”

“…The scan size of these AFM images is 1 μ m × 1 μ m. Reproduced with permission. [ 40 ] Copyright 2010, American Chemical Society.…”

“…Aqueous solutions were used extensively during fi lm fl oating and transferring, and these may induce a morphology reconstruction to the exposed bottom surface, since it was previously demonstrated that water, even in the a weak basic or neutral state, is capable of triggering the rupture of PS coronas to form pores by the swelling of P2VP blocks. [ 40 ] On the other hand, it is reasonable to assume that the rupture of micelle coronas cannot take place from the bottom side, which contacts the substrate, because of the tight confi nement of the rigid substrate that can not be deformed, and the accumulated pressure from swelling can only be released from the weakest points, i.e., the cap of the micelle coronas. Even for BCP fi lms prepared from neutral solvents in which no micelle forms and the swellable domains are directly exposed to the air and substrate sides without the cushion of the nonswollen blocks in micellar fi lms, the presence of a thin polymer layer underneath the pores produced by swelling in PS-b -P4VP fi lms was demonstrated.…”

“…For example, for a PS -b-P2VP polymer with a PS molecular weight of 102 000 and a P2VP molecular weight of 97 000, the average diameter of the dry micelles prepared from o -xylene is ≈ 53 nm and the thickness of the PS corona can be estimated to be ≈ 5 nm. [ 40 ] With the continuous diffusion of ethanol molecules into micelle cores, P2VP keeps swelling, driving cores to expand. However, since P2VP cores are confi ned in the glassy PS corona, the expansion of the P2VP cores is prohibited.…”