Polylactic Acid/Cerium Fluoride Films: Effects of Cerium Fluoride on Mechanical Properties, Crystallinity, Thermal Behavior, and Transparency

PLA is widely used in the field of disposable products for its good transparency, high strength, high modulus, and good processing performance. However, the crystallization rate and crystallinity of PLA are weak. In actual production, the PLA products that are typically obtained are amorphous with poor heat resistance, which greatly limits the application range of PLA products. Finding an effective nucleating agent to improve the transparency of PLA has been a hot topic in research. This study found that Cerium fluoride (CeF3) can effectively improve the crystallinity of PLA/CeF3 (P/F) films. When the content of CeF3 in PLA was 1 wt %, the retention ratio of visible light transmittance was 82.36%, the crystallinity was 29.8%, and the tensile strength was 59.92 MPa. Compared to pure PLA, the crystallinity of P/F1 increased by 56% and tensile strength increased by 8.76%. This study provided an alternative scheme that maintained the PLA film’s transparency and improved the crystallinity of PLA, which significantly expanded the application of PLA.

Due to their large ionic radius and unique electronic structure, rare earth elements exhibit high coordination number and strong coordination ability, especially with strong acid elements such as oxygen and sulfur [46]. In this paper, CeF 3 as the second phase was added to polylactic acid by means of coordinated interactions; cerium ions will play a special role in tuning the crystallinity and modifying the physical properties of PLA.

Preparation of PLA/CeF 3 (P/F) Blends and Films
PLA and CeF 3 were dried at 70 • C for 4 h in a vacuum oven before further processing. PLA/CeF 3 (P/F) blends were prepared by using a torque rheometer (RT01-06/02, Guangzhou, China) in mass ratios of 100/0, 99.5/0.5, 99/1, 98/2, 97/3, and 96/4; the formulation of P/F mixtures is shown in Table 1. The rotating speed was 60 rpm at 180 • C for 5 min. Then, all the samples were hot-pressed onto 0.1 mm-thick sheets at 180 • C with a pressure of 15 MPa of 2 min, and then, cooled down to room temperature; samples were cold-pressed again, and the final P/F films were obtained. A schematic illustration of the preparation of P/F films is shown in Figure 1.

Characterizations
The tensile properties of the P/F films were tested by a universal material testing machine (Instron 2365, Darmstadt, Germany) at a crosshead speed of 50 mm/min according to the ASTM D638-2008 standard. The P/F films were conditioned at room temperature in a 50% relative humidity-controlled environment for at least 24 h prior to being tested. At least five runs for each sample were measured, and the results were averaged. Then, the tensile strength, Young's modulus and elongation at break of the samples were obtained. A Fourier transform infrared spectrophotometer (Nicolet iS 50, Madison, WI, USA) was used to investigate the possible intermolecular interaction between PLA and CeF 3 . The average value was obtained from 32 scans in the standard wave-number range of 500 to 4000 cm −1 . A UV-visible-near infrared light spectrophotometer (Cary 5000, Santa Clara, CA, USA) was used to characterize the transmittance of the P/F films in the visible light region. The test wavelength range is 200-800 nm. Field emission scanning electron microscopy (FESEM) (Apreo S, Waltham, MA, USA) was used to characterize the cross-section and surface phase morphology of the P/F films. A layer of platinum was sputter-coated uniformly over all the fractured surfaces before FESEM observation. X-ray diffraction (Miniflex 600, Akishima-shi, Japan) was used to characterize the crystallinity of the P/F films. Measurements were performed over the ranges 5-70 • C, with a step of 0.05 • C and speed 10 • C/min. The thermal and crystallization behaviors of the PLA/CeF 3 blends were studied by differential scanning calorimetry (TGA/DSC 1, Zurich, Switzerland) under nitrogen atmosphere. The weight of the samples varied from 5.0 to 10.0 mg. The samples were heated from 30 • C up to 190 • C at 10 • C/min (the first heat scan) and held at 190 • C for 3 min to eliminate their previous thermal history. Following this, the samples were chilled to 30 • C at the same rate, and then, heated again from 30 • C up to 190 • C at 10 • C/min (the second heating scan). The thermal stability of the P/F films was studied by thermogravimetric analyses. Samples of about 5 mg were placed in alumina crucibles and were measured in dynamic conditions, in the temperature range from 30 to 600 • C, with a heating rate of 10 • C/min, and a 50 mL/min Ar 2 flow. Dynamic mechanical analysis was performed in a DMA instrument (DMA 1, Mettler Toledo, Zurich, Switzerland) in the tensile mode, based on the ASTM standard D4092. All samples were cut from the tensile bar specimens (40 mm × 10 mm × 0.1 mm). The temperature ranged from −20 to 100 • C, with a heating rate of 3 • C/min at an oscillating frequency of 1 Hz.

Mechanical Properties
As shown in Figure 2a, with the increase in CeF 3 content, the tensile strength and elongation at break of the P/F films increased at first and then decreased. When the content of CeF 3 was 1 wt %, the tensile strength of the P/F1 film was the highest, reaching from 55.09 to 59.92 MPa, which was 8.76% higher than that of pure PLA. When the content of CeF 3 was 2 wt %, the elongation at break of the P/F2 film was the highest, reaching 2.53%, which was 78% higher than that of pure PLA. The tensile stress-strain curves of P/F films are shown in Figure 2b. When the content of CeF 3 was 2 wt %, the overall effect of strengthening and toughening was better. This showed that CeF 3 improved the ductibility ability of P/F film in the range of 0.5% to 2%.
As shown in Figure 3, compared with pure PLA (P/F0), the spectral peak of P/F4 had several new characteristic peaks in the range of 500~700 cm −1 . For example, these characteristic peaks (532, 543, 571, 578 and 598 cm −1 ) of P/F4 represent the coordination interactions that exist between cerium and oxygen atoms in PLA. In addition, new characteristic peaks also appeared in 673 and 693 cm −1 . These two peaks were assigned to the vibration modes from the Ce-O band.
In general, the main characteristic peaks of PLA were retained, which proved that CeF 3 does not change the main structure of PLA, but had a certain coordination effect and coupling relationship. The coordination effect and coupling relationship were conducive to enhancing the mechanical properties of PLA, which was also verified by the mechanical property test results in Figure 2.

UV-Vis Transmittance
In order to study the effect of CeF 3 on the visible light transmittance of P/F films, the UV-vis spectral transmittance of P/F films was measured. The calculation formula of visible light transmittance is the following relationship (Equation (1)): where T v % is the average visible light transmittance, T w760 % is the visible light transmittance at 760 nm, and T w390 % is the visible light transmittance at 390 nm. Figure 4 shows that the visible light transmittance of P/F films decreased slightly with the increase in the addition of CeF 3 and the specific data, as shown in Table 2.  When the content of CeF 3 in PLA was 1 wt %, the visible light transmittance retention ratio of the P/F1 film was 82.36%. Pure PLA film and P/F1 film samples are shown in Figure 5.  Figure 6 shows the cross-section and surface morphology of P/F films. The FESEM images show that CeF 3 were well dispersed in the P/F films, and there were few cavities in the brittle cross-section, which indicated that the interface between CeF 3 and PLA was good. This may be due to the formation of various coordination modes between cerium and oxygen in the hydroxyl and carboxyl groups of PLA, and the formation of layered, network polymers or infinite chain structure, which improved the compatibility of different components. It also indicated that the addition of CeF 3 could tune the strength and toughness of PLA, which was consistent with the test results of mechanical properties.

X-ray Diffraction (XRD)
In order to analyze the effect of CeF 3 on the crystal structure of PLA, the annealed P/F films were analyzed by XRD. Figure 7 shows the XRD patterns of P/F films annealed for 2 h at 110 • C. There was a diffuse hump peak of P/F0 without annealing between 5 and 25 • C, indicating that the P/F0 was in the amorphous state. When P/F0 was annealed for 2 h, only a weak crystallization peak appeared. However, when PLA was added to CeF 3 , an obvious crystallization peak appeared. With the increase in CeF 3 content, the crystallization peak intensity increased first and then decreased slightly, which proved that a certain amount of CeF 3 was helpful to promote the crystallization of PLA.

Differential Scanning Calorimetry (DSC)
DSC analysis was carried out to investigate the melting and crystallization behaviors of the P/F films. The DSC second heating curves of P/F films are shown in Figure 8, which displayed three main transitions successively: a glass transition, a cold crystallization exotherm, and a melting endotherm. The measured values of the phase transition parameters are summarized in Table 3. With the increase in CeF 3 content, the glass transition temperature (T g ) of PLA hardly changed, suggesting that CeF 3 did not change the glass transition temperature of PLA. However, the addition of CeF 3 significantly decreased the cold crystallization temperature (T cc ) of PLA. When the CeF 3 content increased from 0.5 wt % to 4 wt %, the T cc of P/F films decreased from 116.25 to 103.99 • C gradually and the ∆H c gradually decreased from 10.97 to 1.87 J/g, indicating the addition of CeF 3 could accelerate the crystallization of PLA. The degree of crystallization of the sample was evaluated from the heat evolved during crystallization by the following relationship (Equation (2)): where X c % is the degree of crystallinity of the samples, ∆H m is the heat of fusion of the PLA in the blend, ∆H c is the enthalpy of cold crystallization of the PLA in the blend, ∆H 0 m ∆ is the heat of fusion for 100% crystalline PLA (93.1 J/g) [34], and W PLA is the weight fraction of PLA in the blend.  When the content of CeF 3 in PLA was 1 wt %, the crystallinity was 29.8%, which was 56% higher than that of pure PLA. Other P/F films' crystallinity is shown in Table 3. The crystallization ability increased likely because of the coordination interaction between PLA and CeF 3 or heterogeneous nucleation in PLA, which might result in the ordered segmental arrangement of PLA chains. A schematic diagram of the interaction between CeF 3 and PLA is shown in Figure 9.

Thermogravimetric Analyses (TGA)
The thermal stability of PLA is critical, as this property is considered as the limiting factor for processing as well as for end-use applications [50]. Figure 10 shows the TGA thermograms of the P/F films, and Table 4 presents a summary of thermal performance, i.e., the initial, 5% mass loss, maximum mass loss and final residue at 500 • C. The mass was barely lost before 100 • C, proving that the moisture in PLA was removed by drying before further processing. The TGA curve was a flat period before 250 • C, then dropped at 280-380 • C suddenly, and then, tended to be stable. It was a typical one-step degradation reaction [51]. After 500 • C, P/F almost decomposed completely.  The initial thermal stability of the P/F film was like that of pure PLA. The TGA data of the P/F films are shown in Table 4. When the amount of CeF 3 was 2 wt %, the initial degradation temperature of P/F2 films was 330.8 • C, and the maximum degradation temperature was 364.2 • C, which was 15.1 and 10.1 • C higher than that of pure PLA, respectively. When the amount of CeF 3 was more than 2%, the initial decomposition temperature of the P/F1 film was lower than that of pure PLA, but the maximum degradation temperature was still higher than that of pure PLA. This was likely due to the effect of CeF 3 on the interaction between PLA chains. When at a lower temperature, the movement ability of PLA molecular chains was limited and the effect of CeF 3 on the interaction between PLA molecular chains was dominant. With the temperature increase, the movement ability of PLA molecular chains intensified, and the heterogeneous CeF 3 particles promoted the crystallization of PLA. Consequently, the DSC test results were confirmed, resulting in the improvement of the maximum degradation temperature of PLA. In general, CeF 3 was beneficial to improving the thermal stability of PLA.

Dynamic Mechanical Analysis (DMA)
DMA was used to analyze the miscibility and modulus changes of the P/F films. Figure 11a,b show the dependence of loss factor (tan δ) on the temperature for PLA mixtures with different contents of CeF 3 . Only one tan δ peak was observed for P/F films, while pure PLA had a T g of 65 • C. Figure 11a,b show that the incorporation of CeF 3 resulted in a small change in the glass transition temperature of PLA. The tan δ peaks of all the P/F films (near 65 • C) were between that of pure PLA. As shown in Figure 11b, the storage modulus (E ) of pure P/F films gradually decreased with the increasing CeF 3 component, and the storage modulus (E ) of the P/F2 films was 1000 MPa, which was 63.6% lower than that of pure PLA. This suggested that P/F films showed a lower storage modulus than pure PLA from −20 to 60 • C, which showed that CeF 3 had increased the flexibility of PLA, which was consistent with the test results of mechanical properties.

Conclusions
In this paper, P/F films were prepared, and the effects of different amounts of CeF 3 on mechanical properties, visible light transmittance, thermal stability and crystallinity were studied. The results are as follows: The tensile strength and elongation at break of the P/F films increased first and then decreased with the increase in CeF 3 content. The tensile strength of P/F1 was 59.92 MPa and the elongation at break of P/F2 was 2.53%, which was 8.76% and 78% higher than that of pure PLA, respectively. UV-vis transmittance analysis showed that CeF 3 had little effect on the transparency of P/F films. When the content of CeF 3 in PLA was 1 wt %, the visible light transmittance retention ratio of the P/F0 film was 82.36%. DSC analysis showed that CeF 3 promoted the crystallization of PLA; compared to pure PLA, the crystallinity of P/F1 increased by 56%. TG analysis showed that the initial degradation temperature of the P/F2 films was 330.8 • C, and the maximum degradation temperature was 364.2 • C, which was 15.1 and 10.1 • C higher than that of pure PLA, respectively. DMA analysis showed that the addition of CeF 3 could reduce the storage modulus of P/F effectively; the storage modulus (E ) of the P/F2 films was 1000 MPa, which was 63.6% lower than that of pure PLA, indicating that CeF 3 had a toughening effect on the PLA film. As a result, CeF 3 can improve the performance of PLA without affecting transparency, which had a potential application value in the field of food packaging.