Block Copolymer Nanotubes by Melt‐infiltration of Nanoporous Aluminum Oxide

Abstract

The use of shape-defi ning hard templates containing arrays of cylindrical nanopores, such as self-ordered nanoporous anodic aluminum oxide (AAO), [ 1 , 2 ] is a well-established synthetic approach to one-dimensional (1D) nanostructures. [ 3 ] However, generating specifi c functionalities requires control over mesoscopic structure formation processes occurring in the confi ned geometry of nanorods or nanotube walls, such as crystallization and phase separation. [ 4 ] The self-assembly of block copolymers (BCPs) in nanopores having hard confi ning pore walls has been considered as an attractive access to 1D nanostructures exhibiting mesoscopic fi ne structures as a second hierarchical structure level. Their nanoscopic domain structures could be converted into polymeric scaffolds containing mesoporous structures by post-infi ltration process steps including selective degradation of one of the blocks [ 5 ] or selective swelling of one of the components. [ 6 , 7 ] Since nanotubes can be exploited as nanoscopic containers, pipelines or separation media, it is highly desirable to fabricate tubular nanostructures, the walls of which consist of microphase-separated BCPs. Hence, potential functionalities of nanotubes and of BCPs could be combined. Thus, for example, nanotubes with walls composed of multiple concentric layers having different properties could be accessible in this way. Catalytic systems, micro reactorscharge storage systems, pipelines or sensors consisting of hallow/tubular nanostructures with multilayered walls may show superior properties as compared to device architectures based on corresponding soilid rodlike nanostructures. [ 8 ] BCP nanotubes inside nanoporous hard templates were produced previously by solution-infi ltration of BCPs dissolved in organic solvents. [ 9 , 10 ] In this case, the adsorption kinetics depends on a multitude of parameters including temperature and thermodynamic quality of the solvent. Most importantly, the evaporation of the solvent required for solidifi cation of the BCP is a non-equilibrium process diffi cult to control in a satisfactory manner, which involves additional migration of polymer from the supernatant solution into the pores, separation into solventrich and polymer-rich phases, occurrence of hydro dynamic instabilities and surface reconstruction processes, as well as uncontrolled development of ill-defi ned gradient structures. In contrast, the melt infi ltration of block copolymers has always yielded solid nanorods via a capillary fi lling mechanism. [11][12][13][14] Here we demonstrate that during melt infi ltration of microphase-separated BCPs having a fl uorinated block into AAO hard templates precursor fi lms form, the solidifi cation of which yields exclusively tubular BCP nanostructures. Terminal fl ow of the BCPs is not a prerequisite for precursor fi lm formation if the surface energy of the newly formed BCP/air interface is low enough, thus providing access to tubular BCP nanostructures with walls having mesoscopic fi ne structures such as concentri...

Melt Infiltration: an Emerging Technique for the Preparation of Novel Functional Nanostructured Materials

“…Many different morphologies were obtained by varying the molecular weights of the blocks and the aspect ratio of the template. Pulamagatta et al were able to prepare block‐copolymer nanotubes by microphase separation of the fluorinated polymer block with lower a surface tension to the polymer/air interface …”

Dynamic study of polystyrene-block-poly(4-vinylpyridine) copolymer in bulk and confined in cylindrical nanopores

“…[9,10] Moreover, both Russell et al and Steinhart et al have used these templates to confine block copolymers (BCPs) to investigate the influence of cylindrical confinement on the morphology of BCPs where the pore diameter in size is similar to the characteristic period of the BCP microdomain morphology. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Müller et al studied the crystallization behavior in polystyrene-block-polyethylene (PS-b-PE) within the alumina membrane [29], and Russell et al studied the capillary flow of a microphase-separated block copolymer into nanoscopic pores, by means of BDS. [30,31] The well-defined nanometric geometries of AAO templates in the nanoscale make these platforms interesting systems to investigate the influence of geometrical confinement on dynamics not only of the homopolymers but of block copolymers too.…”

Novel Three‐Dimensional Nanoporous Alumina as a Template for Hierarchical TiO2 Nanotube Arrays

“…Porous anodic aluminium oxide (AAO), made by electrochemical anodization of aluminium, has been extensively investigated in the past two decades for its widespread applications in the field of nanoscience and nanotechnology. Besides its use as photonic crystals,1, 2 sensors,3 bio‐separators4 and superhydrophobic supports,5, 6 porous AAO is very likely the most commonly used hard template, which allows fabrication of a broad spectrum of 1D nanostructures 7–20. In general, the anodization of aluminium has to be carried out under relatively mild conditions with a chemically or electrochemically polished mirror‐finished Al surface.…”