Abstract:

Objective In response to the national theme of low-carbon environmental protection, replacing conventional petroleum-based fibers with degradable polymer fibers has become necessary. However, it seems to be a great challenge to obtain degradable polymer fibers photochromic properties while maintaining their mechanical properties. It is therefore necessary to develop photochromic fibers with mechanical properties and discoloration effects.

Method The photochromic polylactic acid(PLA) fibers were prepared from PLA, and photochromic microcapsules by melt spinning and hot stretching processes, and their morphological, crystallographic, and thermal properties were systematically analyzed, with emphasis on the effect of photochromic microcapsules on the mechanical and reversible discoloration behavior of the fibers, so as to reveal the effect of the difference in fiber properties and their internal structure.

Results The fiber morphology structure showed that the smooth cross-sections and surfaces (Fig. 1, Fig. 2) the pure PLA fibers. As the dosage of added photochromic microcapsules increased, the fibers were found to form more and more pores and defects, leading to the deterioration of the mechanical properties of the fibers. The photochromic PLA fibers prepared in this research demonstrated a breaking strength of 3.54-4.18 cN/dtex, an elongation at break of 19.27%-27.01%, and a modulus of elasticity of 55.67-58.66 cN/dtex (Fig. 3). With the increase in dosage of photochromic microcapsules, the breaking strength and elongation at the break of the fibers illustrated a decreasing trend. Even so, when the mass fraction of microcapsules was 6%, the breaking strength and elongation at the break of the fibers were still 3.54 cN/dtex and 20.21%, which could meet the requirements of subsequent processing. Furthermore, the crystallinity of fibers with the increase in dosage of photochromic microcapsules tended to rise and then fall (Fig. 5). The crystallinity of fibers without microcapsules addition was 50.22%. The maximum crystallinity of 55.42% was reached when the mass fraction of microcapsules was 2%. With the continuous increase of photochromic microcapsules, the crystallinity decreased to 47.62%. The photochromic properties of the photochromic PLA fibers (Fig. 6-8) showed high sensitivity, excellent photobleaching (Fig. 9) and photostability (Fig. 10) with the color change completed within 1 s and returning to the original color within 50 s. The fibers' photochromic intensity varied with the microcapsules' mass fraction. The discoloration intensity of the fibers increased with the mass fraction of photochromic microcapsules, but not linearly. In addition, the fiber has excellent durability, maintaining a stable color intensity during 50 cycles of discoloration.

Conclusion Photochromic PLA fiber was successfully prepared by melt spinning technology, which has excellent mechanical properties, with a tensile breaking strength of 3.54-4.18 cN/dtex, elongation at break of 19.27%-27.01%, modulus of elasticity of 55.67-58.66 cN/dtex. Cut-in photochromic function presents high sensitivity, excellent photobleaching performance and photostability. The mechanical properties of fibers and the photochromic effect are closely related to the dispersion or aggregation state of photochromic microcapsules in the PLA matrix. When the mass fraction of microcapsules is low, their distribution in the PLA matrix is uniform, which is conducive to the orderly arrangement of PLA molecular chain segments and has a beneficial effect on the mechanical properties of fibers. When the mass fraction of microcapsules is high, the orderly arrangement of PLA molecular chain segments is hindered, which is the main factor affecting the mechanical properties of fibers. By adjusting the mass fraction of photochromic microcapsules, mutual coordination of fiber color change function was reached, leading to the possibility of achieving the mechanical properties of fibers. The fibers can be mass-produced by melt spinning, which has a broad application prospect in photochromic fabrics, anti-counterfeiting and military.

Key words: polylactic acid fiber, melt spinning, photochromic, microcapsule, discoloration mechanism, mechanical property