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Helical Polymer - Based Supramolecular Films

Akihiro Ohira , Michiya Fujiki , and Masashi Kunitake

159

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

ISBN: 978-3-527-31794-3

5

5.1

Introduction

Nature ’ s elegant bottom - up fabrication of hierarchical superhelical assemblies

over a wide range of scales has inspired the development of functional advanced

materials from efﬁ cient approaches [1] .

In natural systems, hierarchical helical architectures, such as the double helix

of DNA, the triple helix of collagen, and the α - helical coiled - coils of myosin and

keratin on the molecular level, are ubiquitous [2] . They also occur frequently

in artiﬁ cial systems, and it is well known that amphiphilic compounds, such as

phospholipids, amino acids, nucleic acid derivatives, and carbohydrate com-

pounds, fabricate a variety of helical superstructures including tubes, disks,

bundles, ﬁ bers, and cholesteric liquid crystals [1, 3] . Until recently, extensive

studies have been conducted into the helical supramolecular assemblies of organic

molecular materials [1, 3 – 5] and, indeed, recent progress in the areas of polymer

and supramolecular sciences has led us to construct highly ordered and well -

deﬁ ned helical architectures for synthetic helical polymers [6] . Akagi et al. reported

the synthesis of hierarchical helical polyacetylene ﬁ ber under an asymmetric reac-

tion ﬁ eld consisting of a chiral nematic ( N

*

) liquid crystal [7] . Nolte et al. demon-

strated that the formation of various helical superstructures of amphiphilic block

copolymer containing a polystyrene tail and a charged helical polyisocyanide

head group derived from isocyano - l - alanine - l - alanine and isocyano - l - alanine - l -

histidine [8] . Yashima et al. succeeded in the construction of both two - dimensional

( 2 - D ) and three - dimensional ( 3 - D ) smectic self - assemblies composed of helical

polyisocyanides [9] . The same group also created a one - dimensional ( 1 - D ) helical

supramolecular architecture of double - stranded helical polyacetylenes, which can

form 2 - D, self - assembled structures on the substrate [10] . Today, the control of

such highly ordered and hierarchical helical assemblies of synthetic helical poly-

mers is a major challenge in polymer and supramolecular sciences, especially

from the point of view of developing the materials sciences. Generally, the driving

forces in the formation of helical assembled structures are varied, including a

160 5 Helical Polymer-Based Supramolecular Films

chiral interaction and a polar interaction between solute – solvent and solute – solute,

such as hydrogen bonding, electrostatic interactions, coordination bonding, and

π - π stacking. If a simple system were to organize a hierarchical helical structure

only by weak intermolecular interactions such as van der Waals interactions, it

might be helpful to understand the chiral interactions between the molecules. In

this chapter, we will review several helical polymer - based supramolecular assem-

blies in order to provide a better understanding of the driving forces in the forma-

tion of these helical assemblies. First, we describe helical polymer - based 1 - D and

2 - D architectures from the perspective of the construction of supramolecular

organization with helical polymers. Next, we describe helical polymer - based func-

tional ﬁ lms, focusing on the functionalities, such as switch, memory, transfer and

ampliﬁ cation, that are derived from the intrinsic chirality of helical polymers. Two

different types of well - known helical polymer were chosen. A poly( γ - l - glutamate) -

derivative polypeptide and a synthetic helical polysilane polymer were selected

because these are well studied and possess ideal helical conformation, thus allow-

ing us to investigate both the intramolecular and intermolecular interactions.

Studies of the crystal structure of poly( γ - l - glutamate) derivatives have focused

primarily on the packing structure of the α - helices. Kaufman et al. initially reported

that long n - alkyl side chains of polymers based on acrylic and methacrylic acids

can pack into parafﬁ n - like crystallites [11] . Watanabe et al. discovered the thermo-

tropic liquid crystal nature of these polymers and investigated the molecular

packing and thermotropic behaviors for a series of poly( γ - l - glutamate)s with n -

alkyl side chains, ranging from amyl to octadecyl [12] .

Polysilane can be prepared by an easy synthetic route, and exhibits unique elec-

tronic and optical properties that are attributed to the delocalized σ electrons along

the Si – Si main chain [13] . Much research has been conducted on the behavior of

polysilane in solution in order to elucidate polymer conformation [14] . Consequen-

tially, it has been found that the optical and electronic properties of polysilane may

change easily with the side chain structure, solvent, or temperature. Optically

active polysilanes with various chiral substituents in the side chains adopt helical

conformations with a preferential screw - sense [14] . Furthermore, it has been

found that helical polysilanes exhibit many interesting characteristics both in solu-

tion and in solid states [15] . The relationship between the global conformation of

a polymer chain and the physical properties in dilute solution has already been

established from spectroscopic evidence in helical polysilanes [16] .

Research into the supramolecular assembly based on a helical polymer is

expected to contribute signiﬁ cantly to developments within the life sciences and

materials sciences. In the ﬁ eld of life sciences, it is important to clarify how the

existence of the helical conﬁ guration is related to human vital activity. In the ﬁ eld

of materials sciences, a helical conformation is used for functional materials, for

example in electronic and/or ionic conductors, using the directivity with a well -

deﬁ ned helical conformation, molecular sensing and sensor systems using the

regularity of the helical conformation. Helical and superhelical assemblies are

ubiquitous in nature, as exempliﬁ ed in biological polymers such as DNA and

structural proteins, wherein the information stored in the molecular building

5.2 Helical Polymer-Based 1-D and 2-D Architectures 161

blocks is utilized for supramolecular organization into higher - order helical archi-

tectures at different hierarchical levels, from nanoscopic to macroscopic lengths.

Inspired by this elegant spontaneous fabrication into higher - order helical archi-

tectures at different hierarchical levels in nature, the self - organization of synthetic

organic molecules has been widely exploited to generate novel functional supramo-

lecular architectures. However, in order to achieve this system, it is necessary to

design a programmed molecule which can self - organize spontaneously into

higher - ordered supramolecular structures with controlled morphology, dimen-

sion, and size. As the helical polymer possesses a programmed and well - organized

structure, a supramolecular assembly based on a helical polymer would be promis-

ing for controlling the hierarchy in multi - scale so as to create a natural system.

5.2

Helical Polymer - Based 1 - D and 2 - D Architectures

Today, the precise control of topology at the single polymer chain level (1 - D),

especially for helical polymers, is a “ hot issue, ” because understanding the 1 - D

change of topology is the ultimate goal for the development of polymer - based

nanomaterials, and will take advantage of the solubility, ﬂ exibility, stability, and

electronic properties of these materials [17] . The 2 - D, self - organized microscopic

and/or mesoscopic structures composed of helical polymers are also of consider-

able interest both for advancing the fundamental understanding of structural

properties, such as helicity and rigidity, and for developing potential applications,

including liquid crystals and molecular sensing and sensory systems at the mon-

olayer level [18] . In addition to the structural properties, understanding the interac-

tions that operate between the molecules is also crucial for self - organization. The

results of this research will be useful for the construction of hierarchical 3 - D bulk

phases.

In general, the supramolecular 1 - D and 2 - D architectures composed of helical

polymers can be visualized by using scanning probe microscopy ( SPM ) tech-

niques, such as scanning tunnel microscopy ( STM ) and atomic force microscopy

( AFM ). These not only provide the opportunity for direct observations on atomic

ﬂ at substrates with submolecular resolution, but also reveal the structural informa-

tion of individual polymer chains, whereas most spectroscopic characterizations

have provided averaged data for multiple molecules or aggregates. Observations

of biopolymers and synthetic polymers [19 – 22] in the isolated or aggregated state

using SPM (mainly AFM) have been conducted in order to analyze molecular

characteristics such as chain length, end - to - end length, polymer diameters, and

chain rigidity [23, 24] . In fact, AFM has proved very advantageous for observing

helical characteristics, such as helicity, helical pitch, and helix reversal [9] .

In this section, as examples of 1 - D structures, such as rods and circles, we

describe the changes in the 1 - D architectures of isolated helical polysilanes with

different chain lengths on the surfaces. Additionally, we describe a mesoscopic

2 - D superhelical assembly of polysilane formed by homochiral intermolecular

162 5 Helical Polymer-Based Supramolecular Films

interactions. The observation of the associated superstructure of the synthetic

helical polymers was signiﬁ cant, especially for a wide range of scales, and also

provided a better understanding of the mechanism of formation of the highly

ordered and hierarchical architectures. We also present details of the formation of

2 - D epitaxial adlayers of poly( γ - l - glutamate)s as examples of 2 - D helical, polymer -

assembled structures. It should be noted here that not only the observation of both

1 - D and 2 - D architectures but also the direct visualization of intramolecular and

intermolecular interactions existing between helical polymers were clearly dem-

onstrated by AFM, with molecular - level resolution.

5.2.1

Formation of Various 1 - D Architectures of Helical Polysilanes on Surfaces

The topology of an isolated helical polysilane chain bearing a ﬂ uoroalkyl side group

(Scheme 5.1 , polymer 1 ) changes from a rod - like to a circular structure on a mica

surface, with the main chain length determined by means of AFM [25] . Polymer

1 is an ideal model molecule with different lengths; it is a 1 - D, semi - ﬂ exible semi-

conductor that has exhibited unique properties that have contributed to intramo-

lecular CF/Si interactions [26] .

5.2.1.1 Direct Visualization of 1 - D Rod, Semi - Circle and Circle Structures by AFM

Figure 5.1 exempliﬁ es typical AFM images of 1 on mica surfaces. In these images,

three different forms of topology – namely, rod, semi - circle, and circle struc-

tures – coexisted and can be clearly distinguished. Figures 5.1 c, d, and e show AFM

images of rod, semi - circle, and circle structures of isolated 1 , respectively, in a

relatively narrow area. The average height of both the rod and circle structures was

approximately 0.86 nm and was almost uniform, which corresponds to the real

height of adsorbates.

Scheme 5.1 Chemical structure of ﬂ uoroalkyl - containing helical polysilane 1 .

5.2 Helical Polymer-Based 1-D and 2-D Architectures 163

Figure 5.1 Typical atomic force microscopy (AFM) images of

isolated polysilane 1 chains on mica surfaces in (a) large area,

(b) cross - sectional proﬁ le of line a – b in (a), (c) rod - like

structure, (d) rod - like and semi - circle structures, and

(e) rod - like, semi - circle, and circle structures.

All of the observed rod, semi - circle, and circle structures were not aggregated

forms but isolated single polymer chains. The distribution of the average number

of contour lengths ( L

n

) is shown in Figure 5.2 .

In the region of shorter lengths in Figure 5.2 (region I), only rod - like structures

were observed (see Figure 5.1 c), and this was essentially consistent with the results

that 1 possesses a stable, semi - rigid, helical conformation by the intramolecular

CF/Si interaction in the solution system [26] . However, it should be emphasized

here that the chain topology eventually transformed from rod to circle structures

as the main chain length increased on the surfaces. In particular, semi - circle

structures were observed in the very narrow region of medium length (400 ∼ 500 nm)

in Figure 5.2 (region II), thereby showing that the topological switching between

the rod and circle structures is attributable to the discontinuous transition phe-

nomenon. The distribution of the end - to - end distance also exhibited a discrete

decrease as contour length increased, as shown in Figure 5.3 .

Figure 5.4 shows a high - resolution AFM image of the higher - order helical con-

formation in a circle structure. The observed helical pitch in the AFM images was

determined to be approximately 44.2 ± 4.2 nm, although it was much longer than

that expected from the 7

3

helical structure model (0.45 nm). This might be attrib-

uted not only to a tip broadening effect but also to an intramolecular CF/Si interac-

tion, leading to a long - period macromolecular right - handed helicity [27] . Except

for the limitation of the AFM spatial resolution, and because such helical confor-

mations cannot be seen within the rod and semi - circle structures, signiﬁ cant long

164 5 Helical Polymer-Based Supramolecular Films

Figure 5.3 Distribution of the end - to - end distance of 1

obtained from AFM images as a function of contour length.

The numbers I, II, and III correspond to the rod, semi - circle,

and circle structures, respectively.

Figure 5.2 Contour length distribution of 1 estimated from

AFM images. Regions I, II, and III indicate the existence of

rod - like, semi - circle, and circle structures, respectively.

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

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