No best answer

1. First of all, in terms of molecular structure, the molecular chain structure of the rubber material must be relatively soft. This is actually easy to understand. For nylon, the molecular chain structure actually meets this requirement.
2. The rubber material cannot have strong intermolecular forces (hydrogen bond, crystallization, etc.). For example, polyethylene (PE) should be very flexible only from the point of view of molecular structure, but because of its high crystallinity, PE can only be made into plastic products in daily life. Nylon is difficult to make rubber, and I think this is the main reason. The molecular chain structure of nylon is relatively regular and will have a greater degree of crystallinity. It is difficult to make rubber.
In other respects, the requirements for molecular weight and molecular weight distribution of the same kind of fiber and rubber are different, and the requirements for chain segment regularity are also different. For example, polypropylene with a very simple structure can be made into plastic or rubber.