Molecular Soft-Interface Science

Intermolecular forces are classified into three categories; (1) purely electrostatic Coulomb force, (2) polarization force, and (3) quantum mechanical force. The interaction between charges, permanent dipoles, etc. belongs to the first category. The polarization force is the interaction arises from the dipole moments induced in atoms and molecules by the electric field of charges and permanent dipoles.

Polymer synthesis techniques are very important to create and design soft interfaces. In particular, the strict control of polymer architecture is an important issue. Homopolymers, random copolymers, and block copolymers with well-defined molecular weight, structure, and narrow molecular weight distribution can be prepared via living or controlled/living polymerization techniques.

The properties of the soft materials are much affected by the interactions between the building blocks of the composed molecules. The molecules exhibit the self-assembling properties, and form the macroscopic higher ordered structure of soft materials. At the same time, the physical properties of the molecules are able to be designed in order to control the soft materials properties. In this section, the methods of soft materials preparations are described based on the molecular self-assembling properties. The nanostructure of the soft materials is controlled by the bottom-up technologies based on the self-assembling properties, and is in contrast to the top-down technologies of inorganic materials. The detailed methods and examples of the soft materials are described, and the molecules of special properties are also described.

A molecule that is a coupled oscillator of atoms is vibrated by irradiation of light. When the vibration is delayed in terms of phase after that of light, the oscillator absorbs light; otherwise, the light passes through the matter although the oscillation frequency matches the frequency of the light. This phenomenon can be monitored by infrared (IR) spectroscopy. Since the light absorption is correlated with the polarity of the matter, IR spectroscopy reveals anisotropic permittivity, which depicts molecular orientation. On the other hand, the molecular vibration is also excited by irradiating a high-energy photon to generate a complex vibration that is a product of normal-mode functions, which emits Raman scattering comprising of two steps: the dipole excitation by irradiating light and the emission from the dipole. The first step is done via the Raman tensor, which reflects the symmetry of polarizability defined at the molecular coordinate. By analyzing these physical processes, details of the chemical structure and molecular interactions in a thin film are revealed.

Sum frequency generation (SFG) vibrational spectroscopy has been used to study molecular structures at surfaces and interfaces [1‐7]. To obtain SFG signals, centrosymmetry of the system must be broken. This condition can be satisfied only at interfaces, leading to an SFG signal that is highly interface specific.

X-ray photoelectron spectroscopy (XPS), which is one of the most surface-sensitive spectroscopy, provides much information of which kind of atoms is existed at the surface of materials. In addition, chemical states of the atoms can be also analyzed.

Using scattering techniques, we can obtain information of nanostructure of soft matters by measuring the angular dependence (or time fluctuation in dynamic method) of scattered lights, with laser, X-ray, or neutron as an incident beam. For polymers, micelles, and colloids in solution or dispersion, their size and shape can be evaluated, and also occasionally higher order structure such as a spatial distribution of these solutes in solution can be estimated for concentrated systems. The typical optical arrangement of a scattering technique is shown in this chapter. Reflection of X-ray or neutron provides us information of nanostructure on the surfaces and interfaces also in nanometer scale. It is very useful for structural studies of polymer thin films, monolayers, and LB films.

This chapter covers the basic principles of reflectivity and examples of X-ray reflectivity (XR), neutron reflectivity (NR), and grazing incidence X-ray diffraction (GIXD). Reflectivity has been applied to surface and interface structure analysis, and the NR and XR are used complementary because the contrast between two given elements is different for X-rays and for neutrons. GIXD can be used as surface sensitive crystalline structure analysis.

Scanning electron microscopy (SEM), an important member of the electron microscopy family, is a versatile instrument widely used in various fields such as nanotechnology, biology, and the life sciences for imaging of micro- and nanostructure morphology and characterizations of chemical composition of various materials.

Electron microscopy is a versatile scientific technique used in the investigation and characterization of materials science, biology, and life science. The principle of electron microscopy is similar to optical microscopy but uses electrons to illuminate and magnify specimens instead of light.

This chapter focuses on the SPM techniques that can be useful for the study of soft interface sciences. Different from the optical and electron microscopes, the SPM has no requirements on light source and lens, and therefore, the resolution has no limitation due to the wavelength of light and optical aberration. One of the most impressive features of SPM is versatility of detecting conditions: there is no requirement of vacuum condition, which is conventionally necessary for the electron microscopic techniques, and SPM works not only at an ambient condition but also in solution and at elevated temperatures. Therefore, SPM is applicable to characterize the surface features of soft materials including topography, friction, electron density of states, interaction force, and so on. This chapter provides brief overviews of SPM and illustrates its application for the study of soft materials.

High-performance soft interfaces were designed by tethering various polyelectrolyte brushes with anionic, cationic, and zwitterionic functional groups on substrate surfaces using a surface-initiated controlled radical polymerization technique. Ion-containing polymer brushes afforded superhydrophilic surfaces inducing antifouling properties in water. Repeatable adhesion systems without organic solvents were achieved through oppositely charged polyelectrolyte brushes by controlling the electrostatic attractive interaction between the brushes. Environmentally friendly water lubrication systems were also demonstrated by the high-density ion-containing polymer brushes.

The methodology of the evaluation of surface materials, surface modification layers, and sensing methods for bio/chemical molecules detection is described. In this chapter, the most important, widely used, highly sensitive, and user-friendly sensing methods for surface evaluation technology are introduced. Surface plasmon resonance measurement (SPR), quartz crystal microbalance (QCM), electrochemical measurement (cyclic voltammetry (CV) and others), the field-effect transistor (FET) method, and other methods for evaluating molecular affinities and detecting limited target molecules are taken up and role of soft interface for bio- and chemical sensors is reviewed.

Protein adsorption is the first phenomenon that occurs when synthetic materials are exposed to a living organism. The uncontrolled (nonspecific) protein adsorption becomes a trigger for unfavorable foreign body reactions to the materials from a host. Suppression of nonspecific protein adsorption is quite important to prepare synthetic materials for biomedical applications. One of the most robust approaches is zwitterionic phosphorylcholine immobilization by a mimicking of biomembrane processes. The phosphatidylcholine surface of the biomembrane provides an inert surface for biological reactions of proteins and glycoproteins to occur smoothly on the membrane. This fact provides very significant information for the development of nonprotein-fouling surfaces. In this chapter, reliable methodologies for the creation of nonprotein-fouling and hemocompatible surfaces are described with a focus on 2-methacryloyloxyethyl phosphorylcholine (MPC).

Our bodies are composed of numerous different cell types. One fertilized ovum divides until maturation, and then organs are formed from these matured cells. Stem cells, which are present even in mature tissues, such as bone marrow and fat tissue, supply tissues and organs with progenitor and mature cells. Recently, stem cell-based tissue engineering has been investigated as a new therapeutic strategy (Langer and Vacanti in Science 260:920–926, [1]) in which stem cell differentiation and proliferation are controlled to regenerate the structure and function of an organ based on cell engineering and biomaterial science techniques.