A study of polylactic Acid/K2SiF6:Mn4+ composite luminescent materials: design, preparation, and properties

K2SiF6:Mn4+ (KSF) has been the most representative red emitting fluoride phosphors thanks to its cheap production cost and excellent luminescence properties. Nevertheless, the photoluminescent properties of this phosphor are limited due to intrinsically poor water resistance. In this work, constructing composite luminescent materials by blending KSF with polylactic acid (PLA) polymer was developed using the melt mixing process. The tactic not only makes full use of the photoluminescent properties of KSF, but also enhances the moisture resistance ability by alleviating or suppressing hydrolysis. The photoluminescent spectra, temperature-dependence emitting spectra, scanning electron microscopy, thermogravimetric analysis, and wide-angle x-ray scattering were conducted to study the morphology, thermal stability and photoluminescent properties of the KSF@PLA composite luminescent materials. KSF was evenly distributed in PLA. Furthermore, the influence of the doping amount of KSF on the structure and properties of PLA was systematically studied and the optimal doping amount of KSF in PLA was determined to be 10%, marked as KSF@PLA 3. Finally, KSF@PLA 3 exhibited excellent moisture resistance ability and thermal stability. After soaking in deionized water for 7 days, the emission intensity of KSF@PLA 3 is almost consistent with the original emission intensity.


Introduction
As one of the promising biobased polymers, polylactic acid (PLA) has received extensive attention because of its renewable raw materials (such as corn and sugarcane) and degradable products after disposal [1][2][3].Regarding performance among biobased polymers, it is known as the most promising biobased polymer with good processing performance and mechanical strength.At present, PLA is mainly used in the fields of biology, food packaging and textiles.For example, PLA blending with bio nanoadditives, such as Tonsil ® (clay) and Aerosil ® , were developed to obtain nanocomposites for a new generation of food packaging [4].And an effective hydrocharging treatment strategy were presented to prepare biodegradable PLA melt-blown nonwovens with efficient PM 0.3 removal [5].In addition, PLA and its copolymers are used in engineering because of their excellent biocompatibility and mechanical properties [6,7].For instance, a PLA modification approach was reported to improve the performance of mixed-halide inorganic perovskites solar cells [3].Besides being an eco-friendly nontoxic polymer with features that permit use in the human body, PLA is an essential polymeric material for more eco-friendly applications [3,8].
Fluoride-based light-emitting materials, especially Mn 4+ , have been widely used in liquid crystal display, solid state lighting, and other fields due to their avoidable expensive rare earth ions and mild synthetic conditions [9,10].But the application market for phosphors is not optimistic.To further expand the space for development, the toy market caught our attention and came into our view, such as flashing finger light, light sticks and so on.Therefore, combining luminescent materials with organic polymers would be a commendable choice.The arbitrarily constructed shape according to the toy abrasive and uniformly miscibility of phosphors with organic polymers can be achieved by this strategy.As a representative of fluoride phosphors, K 2 SiF 6 :Mn 4+ (KSF) is a stable and efficient red luminescent material with high luminous efficiency, stable thermal performance, and no radiation pollution [11,12].Nevertheless, its poor moisture resistance [13,14] has currently limited its popularization.In recent decades, although the mixing strategy of organic polymers with inorganic materials already exist [15][16][17], the composite luminescent materials of KSF and PLA organic polymers have not been reported.
In this work, the KSF-doped PLA composite luminescent material was designed and fabricated using the melt mixing process to improve the moisture resistance and promote the application fields of KSF.Additionally, the influence of the doping amount of the KSF luminescent material on the structure and properties of PLA polymers was systematically studied.Considering the comprehensive properties of the prepared KSF@PLA composites, the optimal doping amount of KSF in PLA was determined to be 10%, marked as KSF@PLA 3. By comparing their photoluminescent properties and morphological characterizations, KSF@PLA 3 exhibits a superior performance.The immersion test of KSF@PLA 3 shows that the sample could maintain the emission intensity of KSF after soaking in water for 7 days and significantly enhance the moisture resistance of KSF compared with the uncompounded KSF reported in the literature.

Experimental Materials
The applied PLA (Grade 4032D, NatureWorks LLC, Minnetonka, MN, USA) was dried overnight in a vacuum before use, of which the 1 HNMR spectra were shown in figure S1.The red phosphor K 2 SiF 6 :Mn 4+ was supplied by Ganzhou Zhonglan Rare Earth Materials Technology Co., Ltd.(Ganzhou, China).The ethanol and CH 2 Cl 2 were bought from Admas LLC (Shanghai, China).

Sample preparation
For the spin coating technology, a 5 g quantity of PLA was dissolved in 10 ml of CH 2 Cl 2 , and pour certain amounts of KSF into the solution, stirring constantly.Then drip the mixture to the rotary coater to obtain KSF@PLA composites (S-KSF@PLA).For melt mixing process, a 5 g quantity of PLA was dissolved in 10 ml of CH 2 Cl 2 .Subsequently, certain amounts (1, 5, 10, and 20 wt%) of KSF were mixed in the solution and also during the melt mixing process with melting temperature at 180 °C.The composites were prepared and labeled KSF@PLA 1 (1 wt%), KSF@PLA 2 (5 wt%), KSF@PLA 3 (10 wt%), and KSF@PLA 4 (20 wt%).The schematic diagram for the sample preparation procedures was shown in figure 1.

Characterizations
The fluorescence spectra studied the luminescence properties of the phosphors and composites on a Shimadzu RF6000 spectrofluorometer.Furthermore, 1 HNMR was measured on a Bruker ARX 400 NMR spectrometer, with chloroform-das the solvent.Scanning electron microscopy (SEM) micrographs were taken using a JEOL FESEM 6700F electron microscope.Wide-angle x-ray scattering (WAXS) measurements were executed on a Rigaku Nanopix-SP.Thermogravimetric analysis (TGA) profiles were obtained by heating the samples from 30 °C to 800 °C (TGA550, TA Instruments-Waters LLC, New Castle, DE).A Nikon D3500 camera was used to
The KSF@PLA composites with different KSF mass doping ratios prepared by the spin coating method were marked 1 wt%, 5 wt%, 10 wt%, and 20 wt% as S-KSF@PLA 1-4, respectively.Excited with a 455 nm excitation light, the photographs of the S-KSF@PLA 1-4 composites are depicted in figure S2, and the corresponding 2D map of the excitation and emission spectra of these samples are shown in figures S3-7.Obviously, the KSF particles are shown to be unevenly distributed in PLA, especially at reduced doping concentrations.Apart from the emission intensity, the positions and shapes of excitation and emission peaks are the same as the KSF.The emission spectra with different KSF doping amounts are provided by figure S8.Under 455 nm excitation, the emission intensity gradually increases, attributed to more KSF doped into PLA with increasing the doping amounts of KSF.
Subsequently, the melt mixing process was successfully applied to prepare a well-mixed KSF@PLA composite sample.Figure 3(a) gives the satisfactory result of the selection prepared using melt mixing in daylight and with ultraviolet (UV) light.Comparing the models designed using both methods, the KSF particles are well distributed in the polymer matrix designed using melt mixing.With an increased KSF doping concentration, the dispersion of inorganic particles in the polymer matrix tends to be uniform.Figure 3(b) shows the corresponding emission spectra of the KSF@PLA composites.Under 455 nm excitation, besides the different luminescence intensities, the peak positions and shapes of the emission spectra of the KSF@PLA composites with different KSF doping amounts are the same.These composites mainly exhibit red fluorescence emission, and the prominent emission peaks are centered at 613, 630, and 650 nm.The emission peaks in the KSF@PLA composites are consistent with those of Mn 4+ in KSF, indicating that the composite material does not change the crystal phase of KSF and that their emission peak reaches the maximum value around 630 nm, attributed to the spin-forbidden characteristic transition of Mn 4+ from 2 E g -4 A 2g .The emission intensity for KSF@PLA composites with the same quality of PLA can be gradually enhanced as increasing KSF doping content.
Diffracted intensity distribution of KSF@PLA composites Figure 4 shows the diffracted intensity distribution of KSF@PLA composites with different doping ratios.It shows from the 1D WAXS profiles of KSF that its characteristic diffraction peaks appear at 2θ of 19.6°and 32.2°( Figure S9) [20].The diffraction peaks of these KSF@PLA composites also appeared at the same position, and with increasing KSF doping content, the intensity of the characteristic peaks gradually increased.This result shows that with the increased KSF content, the force of inorganic particles on the PLA molecules increases, hindering intermolecular interaction and enhancing crystallization properties.Under melt processing, PLA is in an amorphous state, and no crystalline diffraction peaks appear [21].
In the composite material, the crystal diffraction peak intensity of KSF is much higher than that of the organic crystal diffraction peak, so the signal intensity change of KSF is mainly observed in figure 4(b).Combined with figure 4(a), which shows the middle and 2D waxs diffraction images, different degrees of KSF particle aggregation are evident in the composite material.Based on the above XRD results, when the doping ratio is 10 wt%, the dispersion degree of inorganic particles in the composite material is optimal.

Morphology and structure
Figure 5 shows the SEM images of pure KSF and the cross-section of KSF@PLA composites.When the doping concentration is low, such as 1 wt%, there are no noticeable KSF particles in the composites, as shown in figure 5(b).Tiny KSF particles can be observed on the surface of KSF@PLA 2 and KSF@PLA 3 composites (figures 5(c) and (d)).These particles were few, did not cause damage to the PLA matrix, and partially changed the surface bonding of pure PLA.Nevertheless, when the doping concentration is 20 wt%, KSF particles agglomerate in the composite matrix.With the increased KSF content, some agglomeration appeared on the PLA surface, reducing the composites' mechanical properties.Particularly, the surface of some KSF particles became smooth (figure 5(e)) in comparison with pure KSF particles (figure 5(a)).This could be due to the sample preparation method for PLA melt processing, which made the original surface sharp.The element distribution of KSF@PLA composites was analyzed using SEM-EDS mapping (Figure S10).The results show that KSF particles are distributed in the PLA, and the constituent elements F, Si, K, and Mn are evenly distributed.Consequently, under the appropriate KSF doping ratio, a composite material with good dispersibility can be prepared using simple melt processing.
To investigate whether there is any interaction between KSF and PLA, the infrared (IR) spectra were used (figure 6).Based on the data, we noted the stretching vibration peaks of Si-F bonds at 741, 641, and 482 cm −1 in the IR spectrum of KSF [20].In the IR spectrum of polylactic acid (PLA), the blue lines indicate that the absorption peak at 1756 cm −1 corresponds to the stretching vibration of C=O, while the peaks at 1189 cm −1 and 1131 cm −1 represent the stretching vibrations of C-O, confirming the presence of ester groups.Additionally, peaks between 2900 and 3000 cm −1 are attributed to the stretching vibration of C-H, the peak at 1455 cm −1 corresponds to the deformation vibration of C-H, and the peak at 1382 cm −1 is a characteristic peak of methyl, further confirming the structure of polylactic acid [22,23].Importantly, the IR spectrum of the KSF@PLA composite shows characteristic peaks of both components, represented by dashed lines in the figure 6.This indicates that there may be interactions between KSF and PLA in the composite, providing important clues for a more in-depth understanding of the properties of the composite material.

Thermal behavior and stability
The TGA curves of pure KSF, PLA, and KSF@PLA composites with varying doping ratios are presented in figures 7(a) and (b).The results demonstrate that an increase in the KSF content leads to a gradual rise in the starting temperature of decomposition, the increase is significant.Furthermore, the major weight loss ending temperatures of the KSF@PLA composites with four different doping ratios are basically around 375 °C.However, it is noteworthy that the addition of KSF enhances the thermal stability of PLA when the doping ratio is below 10 wt%.Conversely, when the doping ratio reaches 20 wt%, the thermal stability of the composite is similar to that of pure KSF.This finding can be attributed to the accumulation of KSF in the composite at high doping concentrations, which affects its thermodynamic properties.Based on the TGA results, the optimal performance of the composite is achieved when the doping ratio is 10 wt%.These findings have significant implications for the design and development of KSF@PLA composites with desirable thermal properties [24].
Figure 8(a) displays the temperature-dependence emission spectra of KSF@PLA 3.With the rise of temperature, there is no variation observed in the spectra shapes and the peak positions of KSF@PLA 3. At the same time, only a small attenuation occurs in the emission intensity at temperature above 393 K.The specific numerical changes at different temperature can be seen from the normalized intensity of KSF@PLA 3, provided in figure 8(b).The sample exhibits excellent thermal quenching behavior, which can retain its emission intensity almost unchanged at the temperature range of 298-393 K.When the temperature increases up to 473 K, the luminescence intensity of the sample maintains 97.898% of its initial value at 298 K.The results of the temperature-dependent emission spectra are basically consistent with the commercial KSF phosphor reported in literature, indicating that the excellent thermal stability can be achieved in the KSF@PLA 3 sample [25].

Moisture resistance of KSF
It is well known that the moisture resistance of KSF is poor in its application fields.[11] Therefore, focusing on the moisture resistance of KSF@PLA composites is of great significance.Among other tests, the immersion test is usually considered the most direct and effective method for testing the moisture resistance of a sample.Compared with that immersed for 1 day, no recognizable change in the emission spectra of KSF@PLA composites can be observed after soaking in water for 7 days (figure 9(a)).Figure 9(b) depict the pictures of  KSF@PLA 3 under natural and UV light when immersed in deionized water for 0-7 days.Unsurprisingly, KSF@PLA 3 is white in natural light.A bright red light can be observed under UV light, demonstrating that KSF has been successfully doped into the PLA polymers.After soaking in deionized water for 7 days, the bright redlight emission is still observable in KSF@PLA 3 under the light of the UV lamp.As shown in figure 9(c), the emission intensity of KSF@PLA 3 after soaking in water for 7 days is almost consistent with the original emission intensity.It reveals that the brilliant performance of KSF can be maintained by doping it in PLA polymers.Based on the reported literature, after soaking in water for 5-30 min, the body color of KSF changes from bright yellow to brown-black under natural light and from bright red to faint red under UV light [14,20,26].Hence, the moisture resistance of KSF can be significantly improved after compounding KSF and PLA.

Conclusions
In summary, KSF-doped PLA (KSF@PLA) composite luminescent materials were successfully designed and fabricated using the melt mixing process, which takes full advantage of the luminous properties and improves the moisture resistance of KSF.The influence of the doping amount of KSF on the structure and properties of PLA polymers indicates that when the doping amount of KSF in PLA was 10%, labelled as KSF@PLA 3, good distributable composite luminescent materials with KSF can be obtained.The KSF@PLA 3 not only demonstrates good moisture resistance and high luminous efficiency, but also stable thermal performance.The temperature-dependence emission spectra show that the emission intensity of KSF@PLA 3 almost unchanged at the temperature range of 298-393 K.When the temperature increases up to 473 K, the luminescence intensity of the sample maintains 97.898% of its initial value at 298 K.Moreover, the immersion test of KSF@PLA 3 illustrates that the sample could maintain the emission intensity of KSF after soaking in water for 7 days.This study can provide insight into the preparation and application of biodegradable inorganic/organic luminescent materials.

Figure 2 .
Figure 2. Photoluminescent properties of pure KSF: (a) 2D map of the emission and excitation spectra conducted in a solid state, (b) excitation spectra with Em 635 nm, and (c) emission spectra in Ex 455 nm.

Figure 3 .
Figure 3. Photoluminescent properties of KSF@PLA composites: (a) Images of a KSF@PLA composites bulk with a 365 nm UV light turned on and off.(b) Emission spectra of KSF@PLA composites with different phosphor content (from low to high: KSF@PLA 1→4).