Self-Assembled Block Copolymer Nanotubes

Polyferrocenylsilane Block Copolymers Nanotubes and Nanowires through Self-Assembly... 

Despite many reports on nanotube formation from phospholipids, glycoUpids, peptidic amphiphiles, and other small-molecule surfactants and theoretical studies of this subject [14], reports on nanotube formation from the self-assembly of block copolymers are rare. There have been no theoretical treatments examining their formation or properties. It is not even known if block copolymer nanotubes are thermodynamically stable entities or kineti-cally controlled association products when formed in block-selective solvents. 

There have been only few reports on nanotube formation from the self-assembly in block-selective solvents of copolymers consisting of only coiled blocks. Since nature abhors vacuum, the spontaneous formation of tubular structures from block copolymers in bulk has not been reported and is probably impossible. While the direct preparation of block copolymer nanotubes by self-assembly has been so far difficult, it has been relatively easy to prepare cylindrical nanoaggregates or micelles from ABC triblock copolymers in selective solvents for A only. In such aggregates or micelles, the A block comprises the corona and the C and B blocks comprise the core/shell cylinders. In bulk at the right triblock copolymer composition, the different blocks of an ABC triblock copolymer segregate predictably into C and B core/shell cylinders dispersed in the A matrix [50,51], if the interfacial tension between the A and C blocks are comparable to that between the A and B blocks and that... 

Block copolymer nanotubes can be prepared either directly from block copolymer self-assembly in block-selective solvents or from the chemical processing of ABC triblock copolymer nanofibers. There has been only one report on the formation of self-assembled nanotubes from coil-coil AB diblocks in block selective solvents, and it occurred for a sample with a very low weight fraction of the soluble block. Nanotubes were formed from coil-coil-coil ABA triblock copolymers at much higher weight fractions for the soluble A blocks. Still, lower soluble block weight fractions were required for nanotube than for vesicle formation. It remains to be seen if these trends can be generalized to other block copolymers containing purely coil blocks. 

The self-assembly of crystalline-coil and rod-coil diblock copolymers in block-selective solvents presented quite some surprises. Crystalline-coil diblocks formed tubular nanoaggregates in block-selective solvents for the coil blocks at coil to crystalUne block repeat unit number ratios substantially larger than 1, e.g., 12 and 18 for the PFS-PDMS diblock copolymers. This made the block copolymer nanotubes much easier to access. It again remains to seen if such a trend can be generalized to other diblock copolymers. Thus, much remains to be done to establish the best experimental conditions for formation of self-assembled nanotubes. Theories need to be developed to understand the formation and property of self-assembled block copolymer nanotubes. 

Cylindrical nanoaggregates can be prepared from ABC triblock copolymers in solvents selective for one of the end blocks, e.g.. A, over a much wider copolymer composition window than nanotubes. They are much easier to access than the nanotubes. Such aggregates consist of an A corona, an insoluble B shell, and an insoluble C core. With a proper copolymer design, one can then crosslink the B shell and sculpt away (either fully or partially) the C core to yield nanotubes. An ABC triblock copolymer undergoes self-assembly also... 

Like small amphiphile nanotubes [14], block copolymer nanotubes should have potential applications in controlled deUvery and release [97,98], in encapsulation [99], and in nanoelectronics [100] etc. Although there have been reports on laboratory use of block copolymer nanofibers as vehicles for drug delivery [101], as scaffolds for cell growth [88,102], as precursors for ceramic magnetic nanowires [103,104], and as precursors for carbon nanofibers [105,106[, practical applications of block copolymer nanotubes have not been reported. This is probably due to the difficulty in making such structures. Their preparation from the self-assembly of block copolymers and interests from industry will likely change the scenery of nanotube appUca-tions in the futme. 

Liu, GJ. (2008) Block copolymer nanotubes derived from self-assembly. Advances in Polymer Science 220,29-64. 

Block Copolymer Nanotubes Derived from Self-Assembly G. Liu... 

Abstract Polyferrocenyldimethyl-silane (PFS) diblock copolymers with polyisoprene (PFS-PI) or with polydimethylsiloxane (PFS-PDMS) self-assemble in simple alkane solvents to form what appear by TEM to be dense flexible cylinders (nanowires) or nanotube-like structures. Typical widths are on the order of 20 to 30 nm, with variable lengths often greater than 10 un. The structures that form, and the dimensions of the tube-like structures or wires, depend upon the composition of the polymers and the lengths of the blocks. Light scattering experiments show that the PFS-PDMS (block ratio 1 12) solutions aged... 

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