An isotactic polypropylene (i-PP) microfiber was continuously produced by using a carbon dioxide (CO 2 ) laser-thinning apparatus developed in our laboratory. The CO 2 laser-thinning apparatus could wind up the obtained microfiber in the range of 100 m min Ϫ1 to 2500 m min Ϫ1 . The diameter of the microfiber decreased and its birefringence increased with increasing winding speed. When the microfiber obtained by irradiating the CO 2 laser operated at a power density of 31.8 W cm Ϫ2 to the original fiber supplied at 0.30 m min Ϫ1 was wound at 1,387 m min Ϫ1 , the obtained microfiber had a diameter of 3.5 m and a birefringence of 25 ϫ 10 Ϫ3 . The draw ratio calculated from the supplying and the winding speeds was 4,623-fold. The SEM photographs showed that the obtained microfibers had a smooth surface without a surface roughened by a laser-ablation and were uniform in diameter. The wide-angle X-ray diffraction photographs of the microfibers wound at 848 and 1,387 m min
Ϫ1showed the existence of the oriented crystallites.
An isotactic polypropylene (i-PP) microfiber was obtained by irradiating a carbon dioxide laser to previously drawn fibers. To prepare the thinner i-PP microfiber, it is necessary to previously draw original i-PP fibers under an applied tension of 7.8 MPa at a drawing temperature of 140°C. The drawn fiber was heated under an applied tension of 0.3 MPa using the laser operated at a power density of 39.6 W cm Ϫ2 . The thinnest i-PP microfiber obtained under optimum conditions had a diameter of 1.8 m and a birefringence of 30 ϫ 10 Ϫ3 . Its draw ratio estimated from the diameter reached 51,630. It is so far impossible to achieve such a high draw ratio by any drawing. The wide-angle X-ray diffraction photograph of the microfiber shows the existence of the oriented crystallites. Laser-heating allows easier fabrication of microfibers compared with the conventional technology such as the conjugate spinning.
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