An approach of using slowly crystallizing polymers to form single-polymer composites (SPCs) was investigated. The method was demonstrated using poly(ethylene terephthalate) (PET) as a model system, with which two distinct physical forms, namely, amorphous PET films and highly crystalline PET fibers can be readily obtained. Because of the large difference between the rubbery softening temperature of the amorphous phase and the melting temperature of the crystalline phase, the new process is characterized by its wide process window and enhanced manufacturability. The heating temperature and holding time were found to play a profound role in influencing the properties of the SPC. Excellent fiber-matrix interfacial adhesion was obtained at heating temperature 180 degrees C and holding time 10 s. The results also indicated that the heating rate plays a significant role in affecting the fusion and adhesion of the composite. With reduced heating rates, the fusion and adhesion properties are rapidly deteriorated.