Structure and dynamics of polyrotaxane and slide-ring materials
Graphical abstract
Introduction
Supramolecular chemistry has developed functional polymers with novel molecular architectures: supramolecular polymers [1], [2]. Supramolecular polymers are defined as polymeric assemblies of molecular monomers through non-covalent interactions, such as hydrogen bonding, van der Waals force, topologically interlocking connection, and so on. In recent years, supramolecular polymers with topologically interlocked structure have attracted a great attention [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], because their components can move with topological restrictions. A typical example is polyrotaxane (PR) in which a number of cyclic molecules are threaded onto a linear polymer and are trapped by capping the chain with bulky end groups (Fig. 1) [1]. The cyclic molecules in PR can slide and rotate on the polymer chain, and this additional kinetic freedom has been utilized to produce functional materials with novel dynamic properties.
Many attractive functional materials based on PR have been reported. Covering the axial polymer with the cyclic molecule resulted in unique design of nanomaterials, such as molecular tubes formed by cross-linking adjacent cyclic molecules in a single PR [16] and insulated molecular wires incorporating conductive polymers [17]. Sliding and rotational motion of the cyclic molecules on the chain also enables various molecular devices with unique dynamical properties: molecular shuttles [5], drug delivery systems [18], multivalent ligand systems [19], [20], energy transfer systems [21], and crystalline polyrotaxane with movable mesogenic side chains [22]. Furthermore, PR has been applied to a novel kind of polymer network known as “slide-ring (SR) material” (Fig. 2) [13], [23]. SR materials are prepared by cross-linking cyclic molecules on different polyrotaxanes. The cross-links, which comprise two cyclic molecules, can slide along the polymer chains and thereby behave as pulleys to equalize the internal stress in the SR materials. The sliding motion of movable cross-links results in remarkable mechanical properties significantly different from those of conventional polymeric materials with fixed junctions [13], [14], [15], [23], [24], [25], [26]. The mechanical properties of slide-ring gels are similar to those of biomaterials such as mammalian skin, vessel, and tissues. The slide-ring gels can be used as materials for soft contact lens, artificial skin and vessel and the scratch-resist properties of the slide-ring elastomer without solvent have recently been applied to top coating on a mobile phone.
Despite the development of the applications to nanomaterials, basic physical properties of PR, such as conformation and molecular dynamics, have not fully been studied so far. For example, the sliding motion of the cyclic molecules in PR has not been directly observed, and the time scale of the sliding motion is known less. To evaluate the sliding speed in various PRs allow us to control the switching speed from ON state to OFF state in the molecular shuttles, the mechanical properties of the SR materials, and so on. Structure and dynamics of PR, especially the sliding motion, should be revealed to understand potential functions of PR and develop its application to polymeric materials.
In addition, the physical properties of PR are quite interesting topics in polymer physics. The cross-linking in SR materials is regarded as a real example of the slip-link model for the chain-entanglement effect [27], which was previously proposed theoretically. The experimental and theoretical investigations on the dynamics of PR and SR materials may inspire new developments in polymer physics.
In this article, we review recent studies conducted to investigate structure and dynamics of PR and SR materials. Section 2 describes the findings about the conformation of PR in dilute and semi-dilute solutions by sophisticated small-angle neutron scattering (SANS) experiments: (1) the conformation of the axial polymer chain in PR, (2) the distribution of the cyclic molecules on the axial polymer chain, and (3) mechanically interlocked structure between the axial polymer and cyclic molecules. Section 3 introduces the molecular dynamics of PR. Dynamic light scattering (DLS) studies prove that the cyclic molecules in PR can slide on the axial polymer. In addition, neutron spin echo (NSE) technique successfully determines the diffusion constant of PR in nm and ns scale. Section 4 summarizes SANS and small-angle X-ray scattering (SAXS) results of SR gels with microscopic homogeneity due to the mobility of cross-links.
Section snippets
Conformation
The molecular structure of PR, composed of α-cyclodextrin (CD) and poly(ethylene glycol) (PEG) [28], placed on a substrate was directly visualized by scanning tunneling microscopy (STM) [29], [30] and atomic force microscopy (AFM) [31]. On the other hand, the conformation of the PR in solution, which is of considerable importance in many application including slide-ring gels, has been investigated by small-angle neutron scattering (SANS) [32], [33], [34], [35], [36]. SANS is a very powerful
Dynamics of polyrotaxane in solution
The most unique feature of PR is the molecular dynamics such as the sliding and rotation of the cyclic molecules on the axial polymer chain, because the dynamics is quite important to understand the functions of polymeric materials using PR. Thus far, some groups have reported nuclear magnetic resonance (NMR) studies on the dynamics of PR. Ceccato et al. performed 13C NMR experiments on PR in DMSO-d6 and estimated the rotational correlation time of CD in PR [61]. From the diffusion-ordered 2D
Deformation mechanism of slide-ring materials
We have developed a novel type of polymer gel called “slide-ring gel (SR gel)” in which polymer chains are connected by figure-of-eight cross-links [23], as shown in Fig. 2. The double-looped cross-links in SR gels can move along polymer chains to minimize the local stress in the gels just like pulleys. In order to confirm the pulley effect, we studied the static structure of the SR gels under uniaxial deformation by means of the small-angle neutron scattering (SANS) [65], [72], [73] and X-ray
Conclusions
Polyrotaxane (PR), consisting of α-cyclodextrins (CDs) and poly(ethylene glycol) (PEG), is a typical supramolecular polymer with mechanically interlocked structure and has been applied to various functional nanomaterials including ‘slide-ring (SR) material’ which is a cross-linked PR network with movable cross-links. Scattering techniques are very powerful tools to investigate the physical properties of PR and SR materials. We conducted SANS studies to determine the static structure of PR in
Acknowledgment
The authors are grateful to Dr. Yasushi Okumura, Mr. Changming Zhao, Dr. Masatoshi Kidowaki, Dr. Jun Araki, Dr. Yasuhiro Sakai, Dr. Hideaki Yokoyama, Dr. Naoki Masui, Dr. Toshiyuki Kataoka, Mr. Yusuke Domon, and Ms. Rumiko Kasahara for their continuous support during the course of this study. The author also gratefully acknowledges the support of Dr. Hitoshi Endo, Dr. Michihiro Nagao, Dr. Noboru Osaka, Dr. Takeshi Karino, Dr. Satoshi Okabe, and Dr. Mitsuhiro Shibayama in the SANS and NSE
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