Fluorescent SiO2@Tb3+(PET-TEG)3Phen Hybrids as Nucleating Additive for Enhancement of Crystallinity of PET.

A hybrid polymer of SiO2@Tb3+(poly(ethylene terephthalate)-tetraglycol)3 phenanthroline (SiO2@Tb3+(PET-TEG)3Phen) was synthesized by mixing of inorganic SiO2 nanoparticles with polymeric segments of PET-TEG, whereas PET-TEG was achieved through multi-step functionalization strategy. Tb3+ ions and β-diketonate ligand Phen were added in resulting material. The experimental results demonstrated that it was well blended with PET as a robust additive, and not only promoted the crystallinity, but also possessed excellent luminescence properties. An investigation of the mechanism revealed that the SiO2 nanoparticles functioned as a crystallization promotor; the Tb3+ acted as the fluorescent centre; and the PET-TEG segments played the role of linker and buffer, providing better compatibility of PET matrix with the inorganic component. This work demonstrated that hybrid polymers are appealing as multifunctional additives in the polymer processing and polymer luminescence field.


Introduction
Poly(ethylene terephthalate) (PET) is an important industrial material that is widely used for packaging film, fiber, and beverage bottles owing to its excellent thermophysical properties, chemical resistance, and low permeability [1]. However, its slow crystallization rate and poor crystallization [2] affect the properties and further limit its application [3] in industrial production. It is commonly accepted that addition of nucleating additives is one of the most effective protocols to improve the crystallization rate and achieve good crystallinity, which lead to the enhancement of various properties [4][5][6].
Silica nanoparticles (SiO 2 NPs) with a large surface area have been demonstrated to be nanofillers in the PET matrix, enhancing the mechanical and physical properties owing to the strong interaction between SiO 2 and the PET chain [7]. Meanwhile, the incompatibility is the big problem in achieving homogeneously dispersed SiO 2 NPs in the PET matrix. Additionally, specific properties are difficult to integrate into PET, as the bare SiO 2 lacks functional groups to anchor specific moieties. Notably, hybrid polymer is a special type of nucleating agent [8] that has the below properties of inorganic (stability and rigidity), organics (processability and flexibility), and possessing superior compatibility with the polymer matrix. Furthermore, it provides additional functions (e.g., luminescence, chemical sensing) [9] for the polymer composites [10].
To obtain PET-SiO 2 hybrids, in situ polymerization by grafting polymer chain [11] onto silica NP is more acceptable because it guarantees the homogeneity of hybridization and avoids the aggregation of silica nanoparticles [12]. In order to control the graft density, the "grafting from" method is usually adopted in the way of introducing initiating groups on the silica surface via in situ polymerization and followed by further polymerization techniques [13]. These protocols also allow luminesce properties for the PET-SiO 2 hybrids with potential applications in optical fibers, and rare earth elements are excellent candidates in which to be doped. Many researches have reported Eu, Tb, and so on doped polymers obtaining good fluorescent properties [14]. Few reports have focused on the Tb-doped PET-SiO 2 hybrids. They have great potential to fabricate highly spinnable fluorescent PET optical fiber, and their applications have been extended to the fields of coatings [15,16], membranes [17], OLED [18], and so on.
In this work, the rare-earth doped hybrid polymer of SiO 2 @Tb 3+ (PET-tetraglycol (TEG)) 3 Phen was synthesized through the multi-step polymerization process [19]. Successful coating of PET-TEG polymeric blocks and complexation of Tb 3+ were clearly identified by scanning electron microscopy (SEM) [20], X-ray photoelectron spectroscopy (XPS), and fluorescent spectrometer. The SiO 2 @Tb 3+ (PET-TEG) 3 Phen hybrids were used as nucleating reagents for PET crystallization. The differential scanning calorimeter (DSC) analysis showed that SiO 2 @Tb 3+ (PET-TEG) 3 Phen have a positive effect on enhancing the degree of crystallinity. The fluorescent experiments showed that the SiO 2 @Tb 3+ (PET-TEG) 3 Phen hybrids exhibited good luminescence properties, which give it great potential to fabricate fluorescent PET optical fiber [21].

The Preparation of Tb 3+ /PET-TEG/Phen
Firstly, the low molecular weight PET (M n = 11,500) was synthesized via condensation polymerization, according to our previous report [22]. Then, phenol and tetrachloroethane (mass ratio 1:1) was mixed with the above PET in a round bottom flask. The mixture was kept in an oil bath of 70 • C for 2 h with continuously stirring. Then, 5 mL ethylene glycol was added dropwise, followed by 30 mg Sb 2 O 3 and 16 mL TEG. Afterwards, the temperature was raised to 110 • C, and kept for 2.5 h to form PET-TEG. To obtain Tb 3+ /PET-TEG/Phen, an amount of TbCl 3 (0.02 mol/L) and Phen (0.02 mol/L) were mixed with PET-TEG, and reacted at 70 • C for 6 h. (2)

The Preparation of SiO2-TEG
Firstly, SiO2 was treated with sulfoxide chloride as follows: 1.5 g of SiO2 nanoparticles was dispersed in benzene to form a homogeneous suspension, after which 12.5 mL sulfoxide chloride was added and the mixture was kept at 65 °C for 5 h. The suspension was centrifuged and the precipitate was washed twice with benzene. Then, the solid was dried in an oven (100 °C). Next, 0.5 g of the pretreated SiO2 NPs was dispersed in 20 mL toluene, and mixed with 10 mL tetraethylene glycol for reaction at 70 °C for 5 h with continuously stirring under an N2 atmosphere. Finally, the product was washed twice with toluene and dried in an oven.

The Preparation of SiO2@Tb 3+ (PET-TEG)3Phen
Briefly, 1.4 g of Tb 3+ /PET-TEG/Phen was put into a mixed solution of phenol and tetrachloroethane (mass ratio 1:1) in a round bottom flask, which was placed in an oil bath (70 °C), and 5 mL of ethylene glycol was added. After 1 h, the reaction temperature was raised to 100 °C, following by the addition of 30 mg of Sb2O3 and 0.4 g SiO2-TEG, and the reaction was kept for another 2 h. Afterwards, the solid product was separated and washed twice with acetone and ethanol, respectively. The chemical reaction principle is the same as (1).

The Preparation of SiO 2 -TEG
Firstly, SiO 2 was treated with sulfoxide chloride as follows: 1.5 g of SiO 2 nanoparticles was dispersed in benzene to form a homogeneous suspension, after which 12.5 mL sulfoxide chloride was added and the mixture was kept at 65 • C for 5 h. The suspension was centrifuged and the precipitate was washed twice with benzene. Then, the solid was dried in an oven (100 • C). Next, 0.5 g of the pre-treated SiO 2 NPs was dispersed in 20 mL toluene, and mixed with 10 mL tetraethylene glycol for reaction at 70 • C for 5 h with continuously stirring under an N 2 atmosphere. Finally, the product was washed twice with toluene and dried in an oven. (1) (2)

The Preparation of SiO2-TEG
Firstly, SiO2 was treated with sulfoxide chloride as follows: 1.5 g of SiO2 nanoparticles was dispersed in benzene to form a homogeneous suspension, after which 12.5 mL sulfoxide chloride was added and the mixture was kept at 65 °C for 5 h. The suspension was centrifuged and the precipitate was washed twice with benzene. Then, the solid was dried in an oven (100 °C). Next, 0.5 g of the pretreated SiO2 NPs was dispersed in 20 mL toluene, and mixed with 10 mL tetraethylene glycol for reaction at 70 °C for 5 h with continuously stirring under an N2 atmosphere. Finally, the product was washed twice with toluene and dried in an oven.

The Preparation of SiO2@Tb 3+ (PET-TEG)3Phen
Briefly, 1.4 g of Tb 3+ /PET-TEG/Phen was put into a mixed solution of phenol and tetrachloroethane (mass ratio 1:1) in a round bottom flask, which was placed in an oil bath (70 °C), and 5 mL of ethylene glycol was added. After 1 h, the reaction temperature was raised to 100 °C, following by the addition of 30 mg of Sb2O3 and 0.4 g SiO2-TEG, and the reaction was kept for another 2 h. Afterwards, the solid product was separated and washed twice with acetone and ethanol, respectively. The chemical reaction principle is the same as (1).

The Preparation of PET/SiO2@Tb 3+ (PET-TEG)3Phen
The prepared SiO2@Tb 3+ (PET-TEG)3Phen was used as additives to obtain an SiO2/Tb-hybridized PET composite. In detail, a different amount of SiO2@Tb 3+ (PET-TEG)3Phen was blended with 2.4. The Preparation of SiO 2 @Tb 3+ (PET-TEG) 3 Phen Briefly, 1.4 g of Tb 3+ /PET-TEG/Phen was put into a mixed solution of phenol and tetrachloroethane (mass ratio 1:1) in a round bottom flask, which was placed in an oil bath (70 • C), and 5 mL of ethylene glycol was added. After 1 h, the reaction temperature was raised to 100 • C, following by the addition of 30 mg of Sb 2 O 3 and 0.4 g SiO 2 -TEG, and the reaction was kept for another 2 h. Afterwards, the solid product was separated and washed twice with acetone and ethanol, respectively. The chemical reaction principle is the same as (1).

The Preparation of PET/SiO 2 @Tb 3+ (PET-TEG) 3 Phen
The prepared SiO 2 @Tb 3+ (PET-TEG) 3 Phen was used as additives to obtain an SiO 2 /Tb-hybridized PET composite. In detail, a different amount of SiO 2 @Tb 3+ (PET-TEG) 3 Phen was blended with commercial PET slices using a micro-blender via the melt blending method. For the different amount of additives, the PET hybrids are named as 0%, 1%, 2%, and 3%.

Characterization
The molecular weight of PET-TEG was characterized by gel permeation chromatography (GPC, HLC-8320GPC, Kyoto, Japan). The column temperature was set at 25 • C, and the column pressure was 31 MPa. The mobile phase was DMF, and the elution time was 1 h. The elements of the prepared additive and PET hybrids were analyzed by X-ray photoelectron spectroscopy (XPS, D8 Advance, Bruker, Cologne, Germany). The excitation source was Mg Kα X-ray, the power was 300 W, and the C1s spectrum (284.8 eV) was used as calibration. The functional groups were analysed by Fourier transform infrared spectroscopy (FTIR, MAGNA-IR 5700, Nicolet, Inc., Palo Alto, CA, USA). The morphology and composition were characterized by transmission electron microscope (TEM, JEM-F200, Kyoto, Japan) and SEM-EDS (Energy Dispersive Spectrometer) (GENESIS XM, Waltham, MA, USA). The fluorescence properties were measured by fluorescence spectrometer (Spectro Xepos, Cologne, Germany) and microspectro phometer (20/30PV, Craic, Inc., Palo Alto, CA, USA). The excitation wavelength was set as 331 nm. The crystallization performance of PET hybrids was characterized by differential scanning calorimeter (DSC, DSC214 Polyma, Cologne, Germany). The temperature range was set from 25 to 280 • C. The ramp rate was 10 • C/min.

Molecular Weight Determination of PET-TEG
As illustrated in Scheme 1, the PET-TEG is a key matrix to integrate SiO 2 and Tb for the subsequent hybridization with PET slices. It is crucial to determine the molecular weight of PET-TEG in order to hybridize SiO 2 and Tb with precisely controlled content. As measured by gel permeation chromatography (GPC) [23], the molecular weight distribution curve of PET-TEG shows a single peak, indicating the distribution is narrow. The Mw fraction below 95% located between 17,000 and 70,000 ( Figure S1). It can be seen from Table 1 that the LMPET (low molecular weight) sample presented a degree of polydispersity of around 2.5, and the PET-TEG sample presented a degree of polydispersity of around 1.2. In addition, the molecular weights (M w and M n ) [24] show a slight variation from the PET sample to PET-TEG; according to statistics, even small differences in molecular weight values can cause considerable differences in PET properties.
Polymers 2020, 12, x FOR PEER REVIEW 4 of 12 commercial PET slices using a micro-blender via the melt blending method. For the different amount of additives, the PET hybrids are named as 0%, 1%, 2%, and 3%.

Characterization
The molecular weight of PET-TEG was characterized by gel permeation chromatography (GPC, HLC-8320GPC, Kyoto, Japan). The column temperature was set at 25 °C, and the column pressure was 31 MPa. The mobile phase was DMF, and the elution time was 1 h. The elements of the prepared additive and PET hybrids were analyzed by X-ray photoelectron spectroscopy (XPS, D8 Advance, Bruker, Cologne, Germany). The excitation source was Mg Kα X-ray, the power was 300 W, and the C1s spectrum (284.8 eV) was used as calibration. The functional groups were analysed by Fourier transform infrared spectroscopy (FTIR, MAGNA-IR 5700, Nicolet, Inc., Palo Alto, CA, USA). The morphology and composition were characterized by transmission electron microscope (TEM, JEM-F200, Kyoto, Japan) and SEM-EDS (Energy Dispersive Spectrometer) (GENESIS XM, Waltham, MA, USA). The fluorescence properties were measured by fluorescence spectrometer (Spectro Xepos, Cologne, Germany) and microspectro phometer (20/30PV, Craic, Inc., Palo Alto, CA, USA). The excitation wavelength was set as 331 nm. The crystallization performance of PET hybrids was characterized by differential scanning calorimeter (DSC, DSC214 Polyma, Cologne, Germany). The temperature range was set from 25 to 280 °C. The ramp rate was 10 °C/min.

Molecular Weight Determination of PET-TEG
As illustrated in Scheme 1, the PET-TEG is a key matrix to integrate SiO2 and Tb for the subsequent hybridization with PET slices. It is crucial to determine the molecular weight of PET-TEG in order to hybridize SiO2 and Tb with precisely controlled content. As measured by gel permeation chromatography (GPC) [23], the molecular weight distribution curve of PET-TEG shows a single peak, indicating the distribution is narrow. The Mw fraction below 95% located between 17,000 and 70,000 ( Figure S1). It can be seen from Table 1 that the LMPET (low molecular weight) sample presented a degree of polydispersity of around 2.5, and the PET-TEG sample presented a degree of polydispersity of around 1.2. In addition, the molecular weights (Mw and Mn) [24] show a slight variation from the PET sample to PET-TEG; according to statistics, even small differences in molecular weight values can cause considerable differences in PET properties. Scheme 1. Illustration of the synthetic route for SiO2@Tb 3+ (poly(ethylene terephthalate)-tetraglycol)3 phenanthroline (SiO2@Tb 3+ (PET-TEG)3Phen).

Structure Characterization
The synthesis of SiO 2 @Tb 3+ (PET-TEG) 3 Phen required multiple steps, starting from SiO 2 and PET, and in each step, the obtained samples were characterized by FTIR. (Figure 1) The bands at 794 cm −1 and 1058 cm −1 are the characteristic stretching vibration absorption peaks of Si-O-Si [25,26], which can be found in both SiO 2 and SiO 2 -TEG. In SiO 2 -TEG, the C-H (1122 cm −1 ) and C-O (1248 cm −1 ) overlapped with that of Si-O-C and Si-O-Si, which cannot be clearly distinguished. In PET-TEG, the symmetrical vibration absorption peak (721 cm −1 ) [27] can be assigned to the C-O-C group of the PET skeleton. The characteristic absorption peak at 1713 cm −1 is ascribed to the C=O of polyester [28]. Compared with PET-TEG, the band at 1650 cm −1 of Tb 3+ (PET-TEG) 3 Phen shows a new absorption peak of the conjugated carbonyl; it can be assigned to the incorporation of the Tb-complex [29], in which Tb coordinated with nitrogen and oxygen atoms, as shown in Scheme 1. In SiO 2 @Tb 3+ (PET-TEG) 3 Phen, the presence of the bands at 1058 cm −1 and 1650 cm −1 confirmed the existence of SiO 2 and Tb 3+ , respectively, indicating the synthetic protocol was well conducted and the final samples were successfully prepared.

Structure Characterization
The synthesis of SiO2@Tb 3+ (PET-TEG)3Phen required multiple steps, starting from SiO2 and PET, and in each step, the obtained samples were characterized by FTIR. (Figure 1) The bands at 794 cm −1 and 1058 cm −1 are the characteristic stretching vibration absorption peaks of Si-O-Si [25,26], which can be found in both SiO2 and SiO2-TEG. In SiO2-TEG, the C-H (1122 cm −1 ) and C-O (1248 cm −1 ) overlapped with that of Si-O-C and Si-O-Si, which cannot be clearly distinguished. In PET-TEG, the symmetrical vibration absorption peak (721 cm −1 ) [27] can be assigned to the C-O-C group of the PET skeleton. The characteristic absorption peak at 1713 cm −1 is ascribed to the C=O of polyester [28]. Compared with PET-TEG, the band at 1650 cm −1 of Tb 3+ (PET-TEG)3Phen shows a new absorption peak of the conjugated carbonyl; it can be assigned to the incorporation of the Tb-complex [29], in which Tb coordinated with nitrogen and oxygen atoms, as shown in Scheme 1. In SiO2@Tb 3+ (PET-TEG)3Phen, the presence of the bands at 1058 cm −1 and 1650 cm −1 confirmed the existence of SiO2 and Tb 3+ , respectively, indicating the synthetic protocol was well conducted and the final samples were successfully prepared. To obtain bond information of the samples, XPS was used to analyse the C-bond and the results are shown in Figure 2. In the deconvolution spectrum of C1s in SiO2-TEG, the peaks at 284.4 and 286.1 eV can be assigned to C-C and C-O-C, respectively [29], verifying the grafting of TEG moieties To obtain bond information of the samples, XPS was used to analyse the C-bond and the results are shown in Figure 2. In the deconvolution spectrum of C1s in SiO 2 -TEG, the peaks at 284.4 and 286.1 eV can be assigned to C-C and C-O-C, respectively [29], verifying the grafting of TEG moieties onto SiO 2 . Further, the increased C content (12.74%), compared with SiO 2 , can also prove this (Table 2). In the sample of SiO 2 @Tb 3+ (PET-TEG) 3 Phen, the C=C peak was deconvoluted at 284.5 eV, which can be from the benzene ring of Phen [30]. The peak that appeared at 288.6 eV can be assigned to the O=C=O, indicating the presence of PET. Element content analysis (Table 2) also proved the presence of Tb (0.79%, atomic ratio) and N (comes from Phen). These results demonstrated the successful synthesis of SiO 2 @Tb 3+ (PET-TEG) 3 Phen.
Polymers 2020, 12, x FOR PEER REVIEW 6 of 12 onto SiO2. Further, the increased C content (12.74%), compared with SiO2, can also prove this ( Table  2). In the sample of SiO2@Tb 3+ (PET-TEG)3Phen, the C=C peak was deconvoluted at 284.5 eV, which can be from the benzene ring of Phen [30]. The peak that appeared at 288.6 eV can be assigned to the O=C=O, indicating the presence of PET. Element content analysis (Table 2) also proved the presence of Tb (0.79%, atomic ratio) and N (comes from Phen). These results demonstrated the successful synthesis of SiO2@Tb 3+ (PET-TEG)3Phen.

Characterization of Fluorescence Properties
The fluorescence properties of the synthesized Tb 3+ /PET-TEG/Phen and SiO2@Tb 3+ (PET-TEG)3Phen were measured by fluorescence spectrometer, and the results are shown in Figure 3. The excitation wavelength was determined at 331 nm ( Figure 3b). In Figure 3a, the fluorescence intensity of SiO2@Tb 3+ (PET-TEG)3Phen is much weaker compared with Tb 3+ /PET-TEG/Phen. It is believed that the Tb 3+ /PET-TEG/Phen was grafted onto the SiO2 surface through crosslink. Thus, the SiO2 play the role of collectors, which accumulated surrounding Tb 3+ and increased the local concentration of PET-TEG segments. In this case, much of the unfolded PET-TEG segments would lead to the decrease in fluorescence intensity. To verify this hypothesis, Tb 3+ /PET-TEG/Phen with different concentrations was prepared in DMF from 0.002 mol/L to 0.01 mol/L to investigate the fluorescence behaviour. It can be seen that there are four fluorescence emission peaks [31], with the strongest fluorescence emission peak being the 5 D4-7 F5 transition. With the increasing concentration, the fluorescence intensity increased. However, the intensity reached the maximum at the concentration of 0.006 mol/L, where a further increase in concentration leads to fluorescence fading. Limited solubility of PET-TEG segments in DMF induced the excess of folded PET-TEG segments, which caused Tb 3+ fluorescence quenching. These results also implied the potential usage of SiO2@Tb 3+ (PET-TEG)3Phen not only in crystallization enhancement, but also in fluorescent polymers.

Characterization of Fluorescence Properties
The fluorescence properties of the synthesized Tb 3+ /PET-TEG/Phen and SiO 2 @Tb 3+ (PET-TEG) 3 Phen were measured by fluorescence spectrometer, and the results are shown in Figure 3. The excitation wavelength was determined at 331 nm ( Figure 3b). In Figure 3a, the fluorescence intensity of SiO 2 @Tb 3+ (PET-TEG) 3 Phen is much weaker compared with Tb 3+ /PET-TEG/Phen. It is believed that the Tb 3+ /PET-TEG/Phen was grafted onto the SiO 2 surface through crosslink. Thus, the SiO 2 play the role of collectors, which accumulated surrounding Tb 3+ and increased the local concentration of PET-TEG segments. In this case, much of the unfolded PET-TEG segments would lead to the decrease in fluorescence intensity. To verify this hypothesis, Tb 3+ /PET-TEG/Phen with different concentrations was prepared in DMF from 0.002 mol/L to 0.01 mol/L to investigate the fluorescence behaviour. It can be seen that there are four fluorescence emission peaks [31], with the strongest fluorescence emission peak being the 5 D 4 -7 F 5 transition. With the increasing concentration, the fluorescence intensity increased. However, the intensity reached the maximum at the concentration of 0.006 mol/L, where a further increase in concentration leads to fluorescence fading. Limited solubility of PET-TEG segments in DMF induced the excess of folded PET-TEG segments, which caused Tb 3+ fluorescence quenching. These results also implied the potential usage of SiO 2 @Tb 3+ (PET-TEG) 3 Phen not only in crystallization enhancement, but also in fluorescent polymers.   Figure 4 showed the TEM images and the elements content of SiO2@Tb 3+ (PET-TEG)3Phen. The SiO2 is monodisperse with an average size of around 150 nm. The nanoparticles are discrete without connection, indicating a clean surface (Figure 4a). For comparison, the SiO2-TEG exhibited a coating outside the SiO2 surface and crosslinked with each other (Figure 4b). This solidly proved that the TEG-moieties successfully grafted onto SiO2. As shown in Figure 4c, the SiO2 sphere turned to be more irregular, indicating the Tb 3+ /PET-TEG/Phen moieties had been grafted. SEM-EDS and elemental analysis demonstrated the presence of C, O, Si, and Tb (Figure 4d, Figure S4), and further proved the successful synthesis of SiO2@Tb 3+ (PET-TEG)3Phen. The content of Si and Tb was 2.02 and 0.01 wt %, respectively, corresponding to SiO2 and TbCl3 (Table 3), which is in accordance with the experimental operation of SiO2-PET-TEG and Tb 3+ /PET-TEG/Phen.   Figure 4 showed the TEM images and the elements content of SiO 2 @Tb 3+ (PET-TEG) 3 Phen. The SiO 2 is monodisperse with an average size of around 150 nm. The nanoparticles are discrete without connection, indicating a clean surface (Figure 4a). For comparison, the SiO 2 -TEG exhibited a coating outside the SiO 2 surface and crosslinked with each other (Figure 4b). This solidly proved that the TEG-moieties successfully grafted onto SiO 2 . As shown in Figure 4c, the SiO 2 sphere turned to be more irregular, indicating the Tb 3+ /PET-TEG/Phen moieties had been grafted. SEM-EDS and elemental analysis demonstrated the presence of C, O, Si, and Tb (Figure 4d, Figure S4), and further proved the successful synthesis of SiO 2 @Tb 3+ (PET-TEG) 3 Phen. The content of Si and Tb was 2.02 and 0.01 wt %, respectively, corresponding to SiO 2 and TbCl 3 (Table 3), which is in accordance with the experimental operation of SiO 2 -PET-TEG and Tb 3+ /PET-TEG/Phen.  Figure 4 showed the TEM images and the elements content of SiO2@Tb 3+ (PET-TEG)3Phen. The SiO2 is monodisperse with an average size of around 150 nm. The nanoparticles are discrete without connection, indicating a clean surface (Figure 4a). For comparison, the SiO2-TEG exhibited a coating outside the SiO2 surface and crosslinked with each other (Figure 4b). This solidly proved that the TEG-moieties successfully grafted onto SiO2. As shown in Figure 4c, the SiO2 sphere turned to be more irregular, indicating the Tb 3+ /PET-TEG/Phen moieties had been grafted. SEM-EDS and elemental analysis demonstrated the presence of C, O, Si, and Tb (Figure 4d, Figure S4), and further proved the successful synthesis of SiO2@Tb 3+ (PET-TEG)3Phen. The content of Si and Tb was 2.02 and 0.01 wt %, respectively, corresponding to SiO2 and TbCl3 (Table 3), which is in accordance with the experimental operation of SiO2-PET-TEG and Tb 3+ /PET-TEG/Phen.   Table 3. Content of elemental microanalysis (SEM-EDS) of SiO 2 @Tb 3+ (PET-TEG) 3 Phen.  Figure 5 shows the high-magnification field emission scanning electron microscopy (FESEM) images; from the figure, we can see that the nano-SiO 2 aggregates more seriously in PET, the size of the agglomerates is not uniform, and the average size is more than 250 nm. After modification, its dispersion in PET increases and agglomeration decreases [32]. In addition, TEG-PET is grafted on the surface of SiO 2 , making the nucleating agent more compatible with the PET matrix, and the size of the part of the nucleating agent dispersed in PET is 100-150 nm.

Elements
Polymers 2020, 12, x FOR PEER REVIEW 8 of 12 Figure 5 shows the high-magnification field emission scanning electron microscopy (FESEM) images; from the figure, we can see that the nano-SiO2 aggregates more seriously in PET, the size of the agglomerates is not uniform, and the average size is more than 250 nm. After modification, its dispersion in PET increases and agglomeration decreases [32]. In addition, TEG-PET is grafted on the surface of SiO2, making the nucleating agent more compatible with the PET matrix, and the size of the part of the nucleating agent dispersed in PET is 100-150 nm.

Crystallization Behaviour of PET/SiO2@Tb 3+ (PET-TEG)3 Phen Hybrid
The synthesized SiO2@Tb 3+ (PET-TEG)3Phen was used as an additive to blend with PET, obtaining SiO2/Tb-PET hybrids. The crystallization of PET samples was measured with differential scanning calorimeter (DSC). The effect of SiO2@Tb 3+ (PET-TEG)3Phen on the crystallization of PET can be characterized by the cold-crystallization temperature (Tcc) and melt-crystallization temperature (Tmc) of PET samples, where lower Tcc or higher Tmc corresponds to the higher crystallization rate. Figure 6 shows the crystallization behavior of pristine PET, as well as that with different amounts of additives, namely 1%, 2%, and 3% of SiO2@Tb 3+ (PET-TEG)3Phen. Obviously, the Tmc of PET increased greatly with the addition of SiO2@PET-TEG-PET (2%) and SiO2@Tb 3+ (PET-TEG)3Phen (2%) ( Figure S3a). According to the calculated crystallinity (Table 4), it can be found that, compared with SiO2, the addition of Tb 3+ in the SiO2@Tb 3+ (PET-TEG)3Phen did not contribute too much in the crystallization of PET hybrids, indicating that the SiO2 played the significantly effect to enhance the crystallinity.
The presence of the benzene ring in the main chain structure of the polymer molecule made it unfavorable for molecule chain movement, which led to a low crystallization rate and poor crystallinity [33,34]. With the addition of SiO2@Tb 3+ (PET-TEG)3Phen as additives, the polymer segments that grafted on the surface of SiO2 afford SiO2 possess excellent compatibility with the PET matrix and resulted in a good dispersibility [35]. Taking advantage of that, the internal structure of PET polymer was changed and the chain mobility was improved. Thus, the SiO2@Tb 3+ (PET-TEG)3Phen act as a nucleating agent, and promoted the crystallization rate and crystallinity of PET. Most importantly, the incorporated Tb 3+ affords the PET hybrid excellent fluorescent properties with light emission at 550 nm, which can be processed to be fluorescent fiber and film in the fields of textile and capacitors [36].

Crystallization Behaviour of PET/SiO 2 @Tb 3+ (PET-TEG) 3 Phen Hybrid
The synthesized SiO 2 @Tb 3+ (PET-TEG) 3 Phen was used as an additive to blend with PET, obtaining SiO 2 /Tb-PET hybrids. The crystallization of PET samples was measured with differential scanning calorimeter (DSC). The effect of SiO 2 @Tb 3+ (PET-TEG) 3 Phen on the crystallization of PET can be characterized by the cold-crystallization temperature (T cc ) and melt-crystallization temperature (Tmc) of PET samples, where lower T cc or higher T mc corresponds to the higher crystallization rate. Figure 6 shows the crystallization behavior of pristine PET, as well as that with different amounts of additives, namely 1%, 2%, and 3% of SiO 2 @Tb 3+ (PET-TEG) 3 Phen. Obviously, the T mc of PET increased greatly with the addition of SiO 2 @PET-TEG-PET (2%) and SiO 2 @Tb 3+ (PET-TEG) 3 Phen (2%) ( Figure S3a). According to the calculated crystallinity (Table 4), it can be found that, compared with SiO 2 , the addition of Tb 3+ in the SiO 2 @Tb 3+ (PET-TEG) 3 Phen did not contribute too much in the crystallization of PET hybrids, indicating that the SiO 2 played the significantly effect to enhance the crystallinity.    The presence of the benzene ring in the main chain structure of the polymer molecule made it unfavorable for molecule chain movement, which led to a low crystallization rate and poor crystallinity [33,34]. With the addition of SiO 2 @Tb 3+ (PET-TEG) 3 Phen as additives, the polymer segments that grafted on the surface of SiO 2 afford SiO 2 possess excellent compatibility with the PET matrix and resulted in a good dispersibility [35]. Taking advantage of that, the internal structure of PET polymer was changed and the chain mobility was improved. Thus, the SiO 2 @Tb 3+ (PET-TEG) 3 Phen act as a nucleating agent, and promoted the crystallization rate and crystallinity of PET. Most importantly, the incorporated Tb 3+ affords the PET hybrid excellent fluorescent properties with light emission at 550 nm, which can be processed to be fluorescent fiber and film in the fields of textile and capacitors [36].

Effects on Environment
With the wide spread application, PET waste disposal poses a serious problem to maintaining a clean environment [37]. However, it should be pointed out that PET does not create a direct hazard to the environment, but because of its substantial fraction by volume in the waste stream and its high resistance to the atmospheric and biological agents, it is seen as a noxious material.
To ease the threat to environment, PET recycling seems to be the best way for environmental remediation. As one of the world's most recyclable polymer [38], PET with functional ester groups can be cleaved by some reagents, such as water (hydrolysis), alcohols (alcoholysis), acids (acidolysis), glycols (glycolysis), and amines (aminolysis). The polymer backbone is degraded into monomer units (i.e., depolymerisation) or randomly ruptured into larger chain fragments (i.e., random chain scission) with associated formation of gaseous products. Therefore, monomers, petroleum liquids, and gases can be yielded through chemical recycling, and furthermore, monomers are purified by distillation and drying and used for manufacture of polymers.

Conclusions
In summary, inorganic SiO 2 nanoparticles and rare earth Tb 3+ ions were effectively linked with PET-TEG polymeric segments to form a hybrid polymer through a systematic strategy. Fluorescence characterization showed good fluorescence properties of the hybrid polymer and the quench mechanism was investigated. As an additive to PET, the SiO 2 @Tb 3+ (PET-TEG) 3 Phen hybrid polymer was well blended with PET matrix to improve the T c and T m by 5.7% and 7.3%, respectively, with respect to pure PET. The improved crystallinity was ascribed to the portion of SiO 2 nanoparticles, which function as nucleation sites. The Tb 3+ acted as the fluorescent centre, and the PET-TEG segments played the role of linker and buffer to contribute better dispersibility of SiO 2 nanoparticles in the PET matrix. Overall, this work paves a way for the synthesis of multifunctional polymer hybrids to meet the needs in industrial and biomedical fields such as display screens and drug delivery indicator.