Abstract:

Objective Flexible thin film display devices have inherent challenges in reduced stability and comfort. Luminescent fibers have excellent flexibility and deformability and thus are suitable for the preparation of wearable display fabrics. In order to further address the challenges in the preparation of luminescent fibers, coating luminescent materials on yarns using conjugated electrostatic spinning technology was explored.

Method Polyvinylidene fluoride (PVDF) spinning solution was prepared by using a solvent system of N,N-dimethylformamide and acetone. The spinning solution mixed with inorganic luminous particles was prepared by adding different mass ratios of luminous particles into polyvinylidene fluoride spinning solution. Conductive silver-plated polyamide was used as the core yarn and PVDF/ZnS:Cu²⁺ functional layer was wrapped on silver-plated polyamide using the electrostatic spinning core yarn technology as the skin layer with a micro-nano fiber yarn machine. The properties of electroluminescent yarn were tested and characterized, and the influence of different mass ratio of luminous particles on spinnability and properties of electroluminescent yarns was discussed.

Results The inorganic luminescent particles were well entangled on the surface of silver-plated polyamide on electroluminescent yarns with six mass ratios. The average diameters of the polyvinylidene fluoride micro- nanofibers for the six mass ratio yarns (1:1,1:1.5,1:2,1:3,1:4,1:5) were 0.51, 0.50, 0.67, 0.42, 0.41, 0.61 μm, respectively. The average diameter of the ZnS:Cu²⁺ luminous particles was 20.07 μm. The entangled luminous particles on the yarn increased with increasing mass ratio of luminous particles. The fluorescence microscope images showed that the inorganic luminous particles were uniformly distributed on the yarns, and the average fluorescence intensities of the yarns with six different luminous particle mass ratios of 1:1, 1:1.5, 1:2, 1:3, 1:4 and 1:5 were 108.372, 117.935, 137.347, 139.865, 148.301, 147.013, respectively. The FT-IR analysis revealed no shift or change in the peaks after addition of luminous particles, and the β-phase content of the six core yarns was 81.11%, 77.73%, 72.59%, 63.87%, 58.27%, and 74.04%, respectively. The enthalpies of fusion of the six core yarns were 23.53, 13.92, 10.18, 6.22, 3.45 and 5.19 J/g. Mechanical tests showed that the breaking strengths of the PVDF/silver-plated polyamide core yarns and the six PVDF/ZnS:Cu²⁺/silver-plated polyamide core yarns were 7.318、11.075、11.891、13.959、15.324、16.718、12.584 MPa, respectively, and the elongation at break was 54.474%, 49.660%, 49.112%, 48.769%, 48.223%, 46.588%, and 48.278%, respectively. The bending stiffness of the six yarns were 0.001, 0.011, 0.009, 0.024, 0.026 and 0.001 cN·cm², and the bending hysteresis moment was 0.003,0.012,0.016,0.026,0.026,0.003 cN·cm. The luminous intensities of the six yarns were 0.79, 1.88, 2.36, 2.67, 3.30, and 2.36 cd/m², respectively, with less than 10% variation in 4 h. CIE color coordinates of luminescent yarns were (0.184, 0.359), (0.181, 0.357), (0.184, 0.382), (0.185, 0.393), (0.188, 0.394), and (0.181, 0.383), respectively. The PVDF/ZnS:Cu²⁺/silver-plated polyamide core yarns could achieve multiple and up to 26 cm long luminescent displays.

Conclusion PVDF/ZnS:Cu²⁺/silver-plated polyamide electroluminescent yarn was successfully prepared by one-step electrostatic spinning of core-spun yarn. As the mass ratio of ZnS:Cu²⁺ increased, the luminous particles coated on the core-spun yarn was increased which were uniformly coated on the core-spun yarn, and the ZnS:Cu²⁺ luminous particles were coated on the silver-plated polyamide by the physical entanglement of the PVDF micro-nano fibers. At the mass ratio of PVDF to ZnS:Cu²⁺ of 1:4, the maximum number of luminous particles was encapsulated in the core yarn, which reached the encapsulation threshold of the electrostatically spun PVDF/ ZnS:Cu²⁺ system. PVDF/ZnS:Cu²⁺/silver-plated polyamide electroluminescent yarns have good flexibility and weavability, and they can be be applied in dark, dim daylight and non-direct light scenes. Multiple and long electroluminescent yarns can be excited. PVDF/ZnS:Cu²⁺/silver-plated polyamide electroluminescent yarns have good application prospects in preparing wearable electronic displays.

Key words: electrospinning, polyvinylidene fluoride, silver-plated polyamide, core-spun yarn, electroluminescence property, flexible electronic display device, intelligent textile