Preparation Of Cholesteric Liquid Crystal Textiles And Their Thermochromic Performance

As a thermochromic materials,cholesteric liquid crystals have the advantages of reversible and repeatable discoloration,fast response and tunable full color,which have great potential in the fields of camouflage,detection,prevention and control when utilized to smart textiles.However,cholesteric liquid crystals have the ability to flow and have no affinity with fibers,so the combination of cholesteric liquid crystals and textile materials should be obtained through encapsulation methods.Moreover,due to the soft texture and uneven surface of textile materials,cholesteric liquid crystal on textile materials cannot meet the requirements of planar display for cholesteric liquid crystal,which results in the unobtrusive color of cholesteric liquid crystal textiles and hinders the application of cholesteric liquid crystal textile materials.Therefore,it is important to construct cholesteric liquid crystals textiles with non-planar substrate by encapsulation methods,and study the coloration and discoloration performance of cholesteric liquid crystals in non-planar form,thus improving the color brightness of cholesteric liquid crystals textile and expanding the application of cholesteric liquid crystals in textile.Based on this,the effect of the ratio of cholesteric liquid crystal mixture on coloration and discoloration performance of cholesteric liquid crystal was investigated,and the suitable cholesteric liquid crystal mixture(CLC)for textiles was selected.Then three kinds of encapsulation methods were used to construct cholesteric liquid crystal textiles with non-planar structure,and the effect of the configuration characteristics on the coloration characteristics and temperature response performance of cholesteric liquid crystal textiles were analyzed.The color performance of cholesteric liquid crystals was improved according to both the chromogenic principle of cholesteric liquid crystals and the configuration characteristics of cholesteric liquid crystals textiles,and thermochromic textiles with uniform and bright color and rich color levels were fabricated.The results are as follows:(1)CLC was prepared by binary or ternary mixing of cholesteryl nonanoate(CN),cholesteryl oleyl carbonate(COC)and cholesteryl chloride(CC).The relationships between the ratio of the CLC and its clearing point,color range,color sensitivity and color levels were studied.HSV color model was utilized to analyze the color performance of the CLC.The results showed that COC/CN and COC/CC mixtures demonstrated a linear relationship between the clearing point and ratio of CLC,and the color range of CLC was expanded with the increasing mass fraction of higher melting point component shifting the initial color to lower temperature.COC/CN mixture with the ratio of 1:1 exhibited color across full visible spectrum from red ?yellow?green?blue?purple?colorless within the temperature range(31.9-34.4?),and the temperature resolution can reach 0.1?,which was suitable for room temperature color-changing fabrics.CC was added to COC/CN mixture as a third component in the COC/CN/CC mixture,whose clearing point did not change dramatically with addition of CC.Moreover,color of the COC/CN/CC mixture changed from green to red to colorless with the increase of temperature when the ratio of COC:CN:CC was 1:1:0.6,which showed an opposite color performance compared with the COC/CN mixture.(2)CLC composed of COC,CN and CC with a ratio of 1:1:0.6 was encapsulated in the fiber to prepare PVP/CLC fibers by electrospinning technology.In order to eliminate the angle dependence of the Bragg reflection,CLC was first emulsified using mixed emulsifiers Tween 20 and Span 80 to obtain CLC dispersions,which transformed the shape of CLC to space symmetric structure.The CLC mixture was better dispersed and stabilized when the Tween 20: Span 80 was 6:4,emulsifier amount at 1.0%,dispersing speed at 5000 r/min for 60 min.The particle size of the CLC dispersion was 928 nm and the CLC dispersion maintained a selective reflection features and reversible thermochromic property.The CLC droplets in the CLC dispersion were spherical and had a smooth surface.PVP/CLC fibers prepared with CLC dispersion had spindle-shape beads and exhibited uniform color,angle-independent and reversible thermochromic properties.(3)Coaxial coating method was used to encapsulate CLC with COC:CN:CC ratio of 1:1:0.6 on the outside of black nylon monofilaments,and the liquid crystal clad fiber(LCC fiber)with three-layer concentric cylindrical structure and the polymer dispersed liquid crystal clad fiber(PDLCC fiber)with two-layer concentric cylindrical structure were designed and prepared respectively.The results showed that the director of CLC in LCC fiber aligned along the core and polymer sheath surfaces and twisted radially,which led to a short wavelength shift of reflectance spectrum compared with CLC in planar texture and the angle?dependent color only along the fiber axis.Moreover,the geometry of the center fiber also affected the color performance,and the LCC fiber obtained from the flat center fiber has a higher reflectance due to its better flatness.PDLCC fiber had a porous cladding with a diameter of 10 ?m CLC droplets dispersed in the PVP,whose reflectance spectrum also shifted to short wavelength while angle-independent property was achieved.The LCC fiber and PDLCC fiber showed obvious color,visible to naked eyes.The temperature response characteristics of LCC fiber and PDLCC fiber were the same as those of planar CLC,and the color changed from green to red to colorless with the increase of temperature.(4)Cholesteric liquid crystal microcapsules(CLCM)were prepared by encapsulating liquid crystal mixture(CLC)with a 1:1 ratio of COC and CN as core and gelatin and acacia as shell via complex coacervation method.The results showed that CLCM with obvious core-shell structure was spherical and had good sealing and solvent resistance.Thermochromic property of the CLCM was closely related to its particle size,and CLCM in the range of 3-30 ?m had bright color while CLCM less than 3 ?m exhibited poor color.CLCM showed excellent thermochromic property producing color progressively moving from red ?yellow?green?blue?purple?colorless as the temperature was raised,which was consistent with the discoloration performance of CLC under planar texture.When utilized to cotton fabric coating,flat and smooth substrate was more conducive to the color of CLCM coating.The peak reflection was maximized when the CLCM coating beyond 3 layers,and the reflectivity of the coated cotton fabric cannot be improved by keeping increasing the layer thickness while the color became white.In order to further improve the color brilliance of the coated cotton fabric,left handed liquid crystal microcapsule(LH CLCM)and right handed liquid crystal microcapsule(RH CLCM)with the same color temperature range and color were chosen for coating.The results showed that the reflectivity of the coated fabric can be effectively improved from 32% to 60%,thus brightly coloured cotton fabric was obtained.