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Block Copolymer Nanoﬁ bers and Nanotubes

Guojun Liu

67

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

2

2.1

Introduction

Block copolymer nanoﬁ bers in this chapter refer to cross - linked cylindrical struc-

tures that are made from block copolymers with diameters below 100 nm and

lengths up to hundreds of micrometers. Figure 2.1 depicts the structures of

nanoﬁ bers prepared from an A - B diblock and an A - B - C triblock copolymer, respec-

tively. In the case of diblock nanoﬁ bers, either the core or the corona [1, 2] can be

cross - linked. Once dried, nanoﬁ bers with cross - linked coronas may not re - disperse

readily in solvents, the same as for block copolymer nanospheres with cross - linked

shells [3] . Hence, our discussion in this chapter will be mainly on diblock nano-

ﬁ bers with cross - linked cores. For nanoﬁ bers with cross - linked cores, the soluble

corona chains stretch into the solvent phase helping disperse the ﬁ bers, and the

cross - linked core provides the structural stability. Although the nanoﬁ bers are

depicted in Figure 2.1 as being rigid and straight, in reality, they can bend or

contain kinks.

Three scenarios can be differentiated for triblock nanoﬁ bers. In scenario one,

the corona block is cross - linked. Again because of dispersibility considerations we

have not prepared and studied such ﬁ bers. Our focus so far has been on scenario

two where the middle layer of the nanoﬁ ber is cross - linked. In scenario three, the

innermost block is cross - linked. The structure of such a nanoﬁ ber bears close

resemblance to block copolymer cylindrical brushes [4] when the inner most block

is short relative to the other blocks. Block copolymer cylindrical brushes can be

obtained by polymerizing a diblock macromer. There have been a number of

reports on the properties and applications of block copolymer cylindrical brushes

[4] . While this chapter will not go beyond triblock nanoﬁ bers, except for one case

when nanoﬁ bers of a tetrablock copolymer are discussed, the methodologies devel-

oped for di - and triblock copolymer nanoﬁ ber preparations should apply equally

well to a preparation of nanoﬁ bers from tetra - and pentablock copolymers and to

more complex copolymers.

68 2 Block Copolymer Nanoﬁ bers and Nanotubes

In principle, nanotubes can be prepared by cross - linking tubular micelles [5 – 8]

of diblock copolymers. The focus of discussion here is, however, on nanotubes

derived from triblock copolymers. Two possible conﬁ gurations for such nanotubes

are depicted in Figure 2.2 .

In case one, the tubular core is void. Such a nanotube is derived from a triblock

nanoﬁ ber with a cross - linked intermediate layer by degrading the innermost block

[9] . In case two, the tubular core is lined by a polymer. In this instance, a nanotube

is derived from a triblock nanoﬁ ber with a cross - linked intermediate layer by cleav-

ing pendant groups off the core block [10] .

The preparation of block copolymer nanoﬁ ber [11] and nanotube [9] structures

were only reported for the ﬁ rst time a few years ago. Over the years, there

have been reports on the use of block copolymer nanoﬁ bers and nanotubes as

vehicles for drug delivery [12] , as scaffolds for cell growth [13, 14] , as precursors

for ceramic magnetic nanowires [15, 16] and as precursors for carbon nanoﬁ b-

ers [17, 18] , etc. While ﬁ nding novel applications for such structures is of para-

mount importance, the emphasis of this chapter will be on research undertaken

in my group aimed at achieving a fundamental understanding of the physical

and chemical properties of these materials. In Section 2.2 , the preparation of

block copolymer nanoﬁ bers and nanotubes will be described and the solution

properties of the nanoﬁ bers and nanotubes will be discussed in Section 2.3 .

The different reaction patterns of nanoﬁ bers and nanotubes will be examined

in Section 2.4 . In Section 2.5 , some conclusions will be drawn and my perspec-

tives on where block copolymer nanoﬁ ber and nanotubes research is going will

be presented.

Figure 2.1 Structural illustration of a diblock nanoﬁ ber (top)

and a triblock nanoﬁ ber (bottom).

Figure 2.2 Two conﬁ gurations for a nanotube.

2.2 Preparation 69

2.2

Preparation

2.2.1

Nanoﬁ ber Preparation

Nanoﬁ bers can be prepared from chemically processing either a block - segregated

copolymer solid [19] or block copolymer cylindrical micelles [20] formed in a block -

selective solvent. Figure 2.3 depicts the processes involved to obtain nanoﬁ bers

from a block - segregated diblock solid.

The ﬁ rst step involves casting a ﬁ lm from a diblock with an appropriate com-

position so that the minority block segregates from the majority block forming

hexagonally packed cylinders. This is followed by cross - linking the cylindrical

domains. The cross - linked cylinders are then levitated from the ﬁ lm or separated

from one another by stirring the ﬁ lm in a solvent that solubilizes the uncross -

linked diblock.

Our literature search revealed that the ﬁ rst report on block copolymer nanoﬁ ber

synthesis appeared in 1996 [11] . In that report, block - segregated solids of two

polystyrene - block - poly(2 - cinnamoyloxyethyl methacrylatc), or PS - PCEMA, samples

were used as the precursor. The two PS - PCEMA samples used had 1250 and 160

units of PS and PCEMA for polymer 1 and 780 and 110 for polymer 2, respectively.

Thus, the volume fraction of PCEMA was ≈ 26% in both samples. After ﬁ lm forma-

tion from a diblock by evaporating a toluene solution slowly, the ﬁ lm was annealed

at 90 ° C for days to ensure clean phase segregation between PS and PCEMA. This

ﬁ lm was then sectioned by ultra - microtoming to yield thin slices for transmission

electron microscopic (TEM) examination. Shown in the left panel of Figure 2.4 is

Figure 2.3 Schematic illustration of the steps involved in the

preparation of diblock copolymer nanoﬁ bers from a block -

segregated solid.

70 2 Block Copolymer Nanoﬁ bers and Nanotubes

a TEM image of a thin section for polymer 1 or PS

1250

- PCEMA

160

, where the sub-

scripts denote the numbers of styrene and CEMA units, respectively (Scheme 2.1 ).

It can be seen that the OsO

4

- stained dark PCEMA phase consist of hexagonally

packed cylinders dispersed in the PS matrix and that the cylinders slant, aligned

along a diagonal direction of the image. A similar PCEMA block segregation

pattern was found for polymer 2. Such ﬁ lms were then irradiated by UV light to

cross - link the PCEMA cylindrical domains. Stirring the irradiated samples in THF

helped levitate the cross - linked PCEMA cylinders from the ﬁ lms to yield solvent -

dispersible nanoﬁ bers. In the right panel, a TEM image of the resultant nanoﬁ bers

is shown. It can be observed that the ﬁ bers in this sample are still entangled.

The groups working with Wiesner [21] , Ishizu [22] , Muller [23] and my group

[24] , have also used chemical rather than photochemical methods to cross - link the

cylindrical domains of block copolymer solids to prepare nanoﬁ bers. To prepare

nanoﬁ bers from polystyrene - block - polyisoprene (PS - PI), we utilized a diblock that

had 220 styrene and 140 isoprene units, respectively, which corresponded to a

volume fraction of ≈ 30% for PI. The cylindrical domains were then cross - linked

by exposing the ﬁ lm to sulfur monochloride (Scheme 2.2 ).

Isolated nanoﬁ bers were obtained by separating the cross - linked cylindrical

domains in THF. The left panel of Figure 2.5 shows a TEM image of such

nanoﬁ bers.

Figure 2.4 Left: Thin - section TEM image of a solid PS

1250

-

PCEMA

160

sample. The dark PCEMA cylinders are slanted

diagonally (from bottom right to top left). Right: TEM image

of the PS

1250

- PCEMA

160

nanoﬁ bers on a carbon - coated copper

grid aspirated from THF.

Scheme 2.1

2.2 Preparation 71

A drawback with chemical cross - linking of block - segregated solids is the long

diffusion time required for the cross - linker to penetrate the ﬁ lm. Insufﬁ cient reac-

tion time leads to non - uniform cross - linking with higher degrees of cross - linking

found close to the surfaces. Such non - uniform cross - linking can occur with photo-

cross - linking for the short penetration distance of the light. These complications

can be avoided by preparing nanoﬁ bers starting from block copolymer cylindrical

micelles formed in a block - selective solvent.

Figure 2.6 depicts processes involved in preparation of diblock nanoﬁ bers start-

ing from cylindrical micelles. Firstly, this requires the preparation of a diblock

with an appropriate composition. Then, a selective solvent has to be found that

solubilizes only one block of the diblock copolymer. In such a block - selective

solvent, the insoluble blocks of different chains aggregate to form a cylindrical

core stabilized by chains of the soluble block. Nanoﬁ bers are obtained by cross -

linking the core chains.

A report on the preparation of block copolymer nanoﬁ bers from the cross -

linking of cylindrical micelles ﬁ rst appeared in 1997 [25] . The cylindrical micelles

were prepared also from PS - PCEMA in reﬂ uxing cyclopentane, which solubilized

PS and not PCEMA. Nanoﬁ bers were obtained after the photocross - linking of the

Scheme 2.2

Figure 2.5 Left: TEM image of PS - PI nanoﬁ bers prepared from

the cross - linking of block - segregated PS

220

- PI

140

solid. The

length of the white box is 4300 nm. Right: TEM image of PS - PI

nanoﬁ bers prepared from the cross - linking of PS

130

- PI

370

cylindrical micelles.

72 2 Block Copolymer Nanoﬁ bers and Nanotubes

PCEMA cores. Aside from photocross - linking, the groups working with Bates [26] ,

Discher [27] , Manner [2] , Wooley [28] and Stupp [13] , in addition to my group [29,

30] , have reported on the chemical cross - linking of cylindrical micelles to prepare

nanoﬁ bers. For example, recently we prepared nanoﬁ bers by cross - linking PS - PI

cylindrical micelles formed in a block - selective solvent, N , N - dimethyl acetamide,

for PS [24, 31] . For cylindrical micelle formation with PI as the core, the PI weight

fraction should be relatively high. We followed the recipe of Price [32] and used a

sample consisting of 130 styrene units and 370 isoprene units. The cylindrical

ﬁ bers were cross - linked using S

2

Cl

2

. The right - hand panel of Figure 2.5 shows a

TEM image of such nanoﬁ bers. Compared with ﬁ bers prepared from the solid -

state syntheses described above, the ﬁ bers in the right - hand panel of Figure 2.5

are shorter. A more quantitative analysis of the length distributions revealed that

the ﬁ bers from the solution preparation approach are more monodisperse. Fur-

thermore, the preparation yield can be high, approaching 100%.

2.2.2

Nanotube Preparation

Nanotubes have been prepared by my group mainly via the derivatization of tri-

block nanoﬁ bers. The ﬁ rst block copolymer nanotubes were prepared from PI

130

-

PCEMA

130

- P t BA

800

[9] , where P t BA denotes poly( tert - butyl acrylate). This involved

ﬁ rstly dispersing the triblock in methanol. As only the P t BA block was soluble in

methanol, the triblock self - assembled into cylindrical micelles consisting of a PI

core encapsulated in an insoluble PCEMA intermediate layer and a P t BA corona.

After photocross - linking the PCEMA intermediate layer, the PI core block was

Figure 2.6 Schematic illustration of the steps involved in the

preparation of diblock copolymer nanoﬁ bers from cylindrical

micelles formed in a block - selective solvent.

2.2 Preparation 73

degraded by ozonolysis to yield nanotubes. The removal of the PI block was dem-

onstrated by infrared absorption and TEM analyses. More importantly, Rhodam-

ine B could be loaded into the tubular core. Figure 2.7 shows a TEM image of

such nanotubes stained by OsO

4

. The center of each tube appears lighter than the

PCEMA intermediate layer because the PI block was decomposed.

To facilitate the incorporation of inorganic species into the tubular core, nano-

tubes containing PAA - lined cores were more desirable. Figure 2.8 depicts the steps

involved in the preparation of PS - PCEMA - PAA nanotubes from PS

690

- PCEMA

170

-

P t BA

200

via a solid - state precursor approach [10] .

Step 1 (A → B in Figure 2.8 ) involved casting ﬁ lms from the triblock containing

concentric P t BA and PCEMA core - shell cylinders dispersed in the matrix of PS.

This required the PS volume fraction to be ≈ 70% [33] and was achieved by mixing

some PS homopolymers (hPS) with the triblock to increase the PS volume fraction.

Figure 2.7 TEM image of PCEMA - P t BA nanotubes where the PI core has been degraded.

Figure 2.8 Schematic illustration of processes involved in

preparing PS - PCEMA - PAA nanotubes.

74 2 Block Copolymer Nanoﬁ bers and Nanotubes

The left - hand panel of Figure 2.9 shows a TEM image of a thin section of an hPS/

PS - PCEMA - P t BA solid. On the right - hand side of this image we can see numerous

concentric light and dark ellipses with short stems. These represent projections

of cylinders with PCEMA shells and P t BA cores aligned slightly off the normal

direction of the image. The PCEMA shells appear darker, because OsO

4

stained

the PCEMA selectively. The diameter of the P t BA core is ≈ 20 nm. On the left of

this image we see cylinders lying in the plane of the picture. Thus, the orientation

of the cylindrical domains varied from grain to grain, in the micrometer size range,

because we did not take special measures to effect their macroscopic alignment.

In step 2 (B → C, in Figure 2.8 ), the block - segregated copolymer ﬁ lm was irradi-

ated with UV light to cross - link the PCEMA shell cylinder. The cross - linked

cylinders were levitated from the ﬁ lm by stirring in THF (C → D). PS - PCEMA

nanotubes containing PAA - lined tubular cores were prepared by hydrolyzing the

P t BA block in methylene chloride and triﬂ uoroacetic acid (D → E). The right - hand

panel of Figure 2.9 shows a TEM image of the intestine - like nanotubes. The

stained PCEMA layer does not have a uniform diameter across the nanotube

length because of its uneven collapse during solvent evaporation. The presence of

PAA groups inside the tubular core was demonstrated by our ability to carry out

various aqueous reactions inside the tubular core, as will be discussed later.

2.3

Solution Properties

Figure 2.10 shows a comparison between the structures of a PS - PI nanoﬁ ber and

a poly( n - hexyl isocyanate), PHIC, chain. In a PHIC chain, the backbone is made

of imide units joined linearly and the hairs are the hexyl groups. Their counter-

parts in a PS - PI nanoﬁ ber are the cross - linked PI cylinder and PS chains, respec-

Figure 2.9 Left: TEM image of a thin section of the PS

690

-

PCEMA

170

- P t BA

200

solid. Right: TEM image of the PS - PCEMA -

PAA nanotubes.

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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