**1. Introduction**

Fiber materials have applications in various industrial fields. One of these is compounding with bulk polymer materials to improve and strengthen their physical strength by controlling factors such as the bending elastic modulus, tensile strength, and thermal expansion coefficient. Generally, the higher the dispersion of fibers in polymer, the higher the effect on polymer.

Typical examples of fibers with industrial applications are carbon fibers, aramid fibers, and whiskers. In recent years, cellulose nanofibers [1] having a nano-sized diameter have attracted attention not only because they can be manufactured using inexhaustible and inexpensive raw materials but also because the strength of each cellulose nanofiber is 1/5th that of iron and 5 times the strength [2, 3]. Therefore, if the process cost and the method of dispersion of cellulose nanofibers in bulk materials can be improved and established, it will be used in various materials and fields, such as automobiles and home appliances.

In this chapter, we focus on the low light-scattering property due to the sufficiently small diameters as optical materials because transparency is one of the most important physical elements. Inorganic glass is a transparent material with excellent heat resistance, light resistance, and chemical resistance. Therefore, it is almost universally used as a partition plate during material conversion and energy conversion using light as an energy source. However, inorganic glass is heavy, inflexible, and fragile, and to improve impact resistance, laminated glass with an organic polymer as an interlayer film is often used. Therefore, there is a need for

**Figure 1.** *A strategic approach to highly functionalizing polymer materials through self-assembled growth into nanofibers.*

a transparent material that is light, soft, and has good processability. Transparent polymers are suitable to replace glass because organic polymers are essentially light at the elemental level and can be adjusted for hardness and flexibility. In addition, it is attractive that expansive functionality can be tailor-made at the monomer level.

One of the methods for imparting functions while taking advantage of such characteristics of organic polymers is to blend fillers. This method is particularly applicable to general-purpose polymers and is widely used for function enhancement and function conversion. Among them, nano-sized fillers are attracting particular attention in applications that require transparency because they have low light-scattering properties. However, there are several challenges in the use of nanofillers, and at present, reducing process costs and simplifying the compounding process are the main focus issues. In particular, simplification of the compounding process is a universal problem that results from the high specific surface area of nanomaterials and is thus often a barrier to development and research. This problem also exists in the cellulose nanofibers mentioned above.

This chapter outlines an approach (**Figure 1**) that utilizes nanofiber formation [4–6] using the self-assembly of small molecules as a new strategic method for functionalizing polymer materials; also, an example of its application as an optical material is introduced.
