Fig.1 Structure of Poly(p-phenylene benzobisoxazole) (PBO)
Enlarge Image

Fig.2 Method for preparing PBO nanofibers
Enlarge Image

Fig.3 TEM image of PBO nanofibers and an electron diffraction image of the circled area (lower left)
Enlarge Image

Fig. 3 Photo of the PBO nanofiber mat
Enlarge Image

Intellectual Property and Enterprise

Preparation of rigid polymer nanofibers by new crystallization method

Tetsuya Uchida
Graduate School of Natural Science and Technology
Okayama University

 Poly(p-phenylene benzobisoxazole) (PBO) (Fig. 1) is a rigid polymer in which the molecular chains are unable to fold. PBO fibers have excellent physical properties, including high strength, a high elastic modulus, and high thermal stability.

 However, nanofibers with diameters of less than 100 nm and aspect ratios higher than 100 have attracted attention recently because of potential applications in many areas, including high efficiency filters, organic electronics, electromagnetic shield materials, compact batteries, and high-performance polymer composite. Methods for the preparation of nanofibers include conjugated melt spinning, melt-blowing, and electro spinning, with the lattermost being the most versatile.
 However, these methods require not only specialized equipment but also high voltages. Furthermore, using conventional nanofiber preparation methods for a rigid polymer such as PBO is difficult because it is insoluble in organic solvents, and soluble only in concentrated sulfuric acid and other similar acids.
 We previously reported that single crystals with different forms can be produced by crystallizing PBO from dilute solutions in concentrated sulfuric acid. Here, we have used this result to develop new method for preparing PBO nanofibers with an average diameter of approximately 50 nm (Fig.3; Table1) via crystallization from a dilute solution in sulfuric acid (Fig.2).

Table 1 Properties of the PBO nanofibers

Diameter 53 ± 22 nm
Length 6.7 ± 1.1 µm
Specific surface area 88.4 g/m2

 Furthermore, we prepared a PBO nanofiber mat by a simple method using filtration and a vacuum heating press. The PBO nanofiber mat (Fig.3) exhibited excellent mechanical properties such as high thermal stability and high porosity. Notably, the thermal diffusivity was anisotropic, with the magnitude in the in-plane direction of the film greater than that in the thickness direction (Table 2).

Table 2 Physical Properties of the PBO nanofiber mat

Physical property
Density 1.10 ± 0.07 g/cm3
Porosity 28.7% ± 4.2 %
Elastic modulus 1.48 ± 0.14 GPa
Thermal resistance:
5% weight loss 610 °C
10% weight loss 650 °C
Specific surface area 42.6 g/m2
Thermal diffusivity:
In-plane direction
5.36 ± 0.38×10-6 m2/s
Thickness direction 0.29 ± 0.04×10−6 m2/s

 PBO nanofibers: High performance nano-filler for polymer composite (high thermal stability, high thermal conductivity, electrically non-conductive, high elastic modulus, high strength)
 PBO nanofiber mats: highly thermally stabile nanofiber filter, highly thermally conductive nanofiber sheet.

★Contact: Mototaka Senda, Ph.D.,
 US & EU Representative of Intellectual Property Office,
 Okayama University, 2450 Peralta Blvd. #119, Fremont,
 CA 94536, USA
 Email: takasenda@okayama-u.ac.jp