Hybrid carbon nanofiber (Cnf) products (e.g., mats, yarns, webs, etc.) and methods of fabricating the same are provided. The hybrid Cnf products are flexible and lightweight and have high thermal conductivity. An electrospinning process can be used to fabricate the hybrid Cnf products and can include preparation of an electrospinning solution, electrospinning, and carbonization (e.g., under a vacuum condition).

[Problem] To provide: a carbon fiber precursor fiber that can efficiently produce a carbon fiber at a low cost which is excellent in mechanical strengths even without an infusibilization treatment; a carbon fiber; and a method for producing the carbon fiber.

[Solution] A carbon fiber precursor fiber of the present invention includes a polymer containing a constituent unit represented by General Formula (1) below:

##STR00001## where in the General Formula (1), X and Y each independently represent a divalent substituent, a single bond, or a structure forming a fused ring by sharing one side of two adjacent rings, and the divalent substituent is selected from the group consisting of —O—, —S—, —OSO—, —NH—, —CO—, —CH.sub.2—, and —CH(CH.sub.3).sub.2—.

[Problem] To provide: a carbon fiber precursor fiber that can efficiently produce a carbon fiber at a low cost which is excellent in mechanical strengths even without an infusibilization treatment; a carbon fiber; and a method for producing the carbon fiber.

[Solution] A carbon fiber precursor fiber of the present invention includes a polymer containing a constituent unit represented by General Formula (1) below:

##STR00001## where in the General Formula (1), X and Y each independently represent a divalent substituent, a single bond, or a structure forming a fused ring by sharing one side of two adjacent rings, and the divalent substituent is selected from the group consisting of —O—, —S—, —OSO—, —NH—, —CO—, —CH.sub.2—, and —CH(CH.sub.3).sub.2—.

To provide a carbon fiber precursor fiber that can efficiently produce a carbon fiber excellent in mechanical strength without an infusibilization treatment; a carbon fiber; and a method for producing the carbon fiber. The carbon fiber precursor fiber includes a polymer represented by General Formula (1) below: ##STR00001## where in the General Formula (1), Ar.sub.1 represents an aryl group expressed by any one of Structural Formulas (1) to (5) below, and Ar.sub.2 represents an aryl group expressed by Structural Formula (6) or (7) below: ##STR00002##

[Problem] To provide: a carbon fiber precursor fiber that can efficiently produce a carbon fiber at a low cost which is excellent in mechanical strengths even without an infusibilization treatment; a carbon fiber; and a method for producing the carbon fiber.

[Solution] A carbon fiber precursor fiber of the present invention includes a polymer containing a constituent unit represented by General Formula (1) below:

##STR00001## where in the General Formula (1), X and Y each independently represent a divalent substituent, a single bond, or a structure forming a fused ring by sharing one side of two adjacent rings, and the divalent substituent is selected from the group consisting of O, S, OSO, NH, CO, CH.sub.2, and CH(CH.sub.3).sub.2.

[Problem] To provide: a carbon fiber precursor fiber that can efficiently produce a carbon fiber at a low cost which is excellent in mechanical strengths even without an infusibilization treatment; a carbon fiber; and a method for producing the carbon fiber.

[Solution] A carbon fiber precursor fiber of the present invention includes a polymer containing a constituent unit represented by General Formula (1) below:

##STR00001## where in the General Formula (1), X and Y each independently represent a divalent substituent, a single bond, or a structure forming a fused ring by sharing one side of two adjacent rings, and the divalent substituent is selected from the group consisting of O, S, OSO, NH, CO, CH.sub.2, and CH(CH.sub.3).sub.2.