A free radical assisted strategy for preparing functionalized carbon nanotubes as a highly efficient nucleating agent for poly(L-lactide)

Abstract

In this work, we synthesized novel functionalized carbon nanotubes (CNTs) by grafting poly(ethylene glycol) methyl ether methacrylate (OEGMA) onto CNTs via free radical polymerization in which the 3-methacryloxypropyltrimethoxysilane (KH570) was used as the silane coupling agent. The resulting OEGMA grafted CNTs (CNT-OEG) were systematically characterized by Fourier transform infrared spectroscopy (FTIR), thermal analysis, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Then the obtained CNT-OEG was added to poly(L-lactide) (PLA) as a crystallization nucleation agent. The crystallization behavior of the CNT-OEG/PLA composites was investigated under isothermal and nonisothermal conditions using a differential scanning calorimeter (DSC) and polarized optical microscopy (POM). Interestingly, our results suggested that the addition of CNT-OEG improve the crystallization rate of PLA dramatically. Besides, the decoration of CNTs via free radical polymerization facilitates their distribution in the matrix. This robust method to connect reactive polymers to nanofillers including, but not limited to, graphene, clay and cellulose nanofibrils, may significantly facilitate their utilization in traditional composites or biological engineering materials.

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Introduction

Carbon nanotubes (CNTs) have been regarded as promising nanofillers in polymer nanocomposites owing to their excellent properties, such as high mechanical strength, electrical and thermal conductivity, along with high aspect ratios and small sizes.^1–3 The existing literature shows that besides the nanofillers role, CNTs can also act as efficient nucleating agents for polymers and affect their crystallization kinetics and crystalline morphology.⁴ Therefore, various semicrystalline polymers, such as poly(lactic acid) (PLA),⁵ isotactic polypropylene (iPP),⁶ poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV),⁷ and polyamide (PA),⁸ have been compounded with CNTs in order to improve their crystallization behavior. Among them, PLA has been attracting much attention because of its excellent biodegradability, biocompatibility, renewability, and mechanical properties.⁹ Unfortunately, owing to its intrinsic slow crystallization rate, PLA products are usually amorphous, especially under conventional processing conditions such as injection molding and extrusion, leading to some undesirable results such as decreased of barrier property and thermal resistance. A much number of approaches have been proposed in an endeavor to facilitating the crystallization rate of PLA. The addition of CNTs is one of the most effective methods.¹⁰ It has been reported that only 0.02 wt% multiwalled carbon nanotubes (MW-CNTs) could reduce the half-crystallization time (t_1/2) of PLA from 23 min to 5.5 min when isothermally crystallized at 115 °C, which may finally result in its improvement in physical and mechanical properties.¹¹

Recent studies indicated that the surface functional groups on CNTs had a strong influence on the crystallization of polymer nanocomposites.^12,13 CNTs with proper functional groups facilitate the dispersion and interfacial adhesion with polymer matrix which will further boost the crystallization of polymer matrix.¹⁴ Thus, surface-functionalization of CNTs with covalent grafting of organic compound or polymer were employed to improve properties of CNTs such as solubility, interfacial interactivity with a target matrix, and obtained good mechanical properties composites. There are several methods for covalent grafting of polymers, including “grafting from” and “grafting onto” methods.^15,16 The “grafting onto” method leads to low grafting density at the surface of CNTs, due to significant steric hindrances when polymeric chains diffuse to the surface of CNTs. Whilst, the advantage of “grafting from” techniques over “grafting onto” techniques is to give access to high-dense and controllable polymer-grafted CNTs. Based on merits of “grafting from” techniques, a number of composites of nanoparticles with polymers have been prepared via several polymerization methods, such as free radical polymerization, living radical polymerization, ring-opening polymerization and cationic/anionic polymerization.¹ Among them, free radical polymerization is one of the robust methods for preparation of polymer-based nanocomposites attributing to its abundance of monomer and mild reaction conditions.¹⁷ Recently, Saeid Rahimi-Razin et al.¹⁸ developed a novel method to synthesize polymerizable multiwalled carbon nanotubes through the silanization reaction of a methyl methacrylate containing silane agents with hydroxylated MWCNTs. On the basis of this work, we fabricate polymerizable CNTs using silane coupling agent which carries an silane group reserved for the reaction with CNTs and a vinyl group for subsequent free radical polymerization. However there comes another interesting issue: what kind of polymer chains should we choose to decorate the CNTs in order to improve the crystallization property of polymer matrix?

Recently, some reports have been published, which results show that plasticizers such as poly(ethylene glycol) (PEG), glucose monoesters and partial fatty acid esters could be used to improve the flexibility of PLA.^19,20 Among them PEG is polar, water-soluble, good biocompatible and miscible with PLA, so it has been wildly used in PLA matrix in recent decades.^21,22 Moreover, PEG chains can increase the polymer chain mobility, which leads to an enhancement in the crystallization of PLA via a reduction in the energy required for the chain folding process during crystallization.^23,24 Yang et al. has mentioned that the crystallization behavior of PLA could be influenced by the addition of PEG.²⁵ In PEG/PLA blend systems, PEG promoted the spherulite growth rate but depressed the nucleation density of PLA. It has been reported that the existence of PEG not only facilitated the growth rates of transcrystallinity but also improved the preferential orientation of PLA chains.²⁶ Based on their works, we choose OEGMA, containing PEG chains as well as a vinyl group, as a candidate to decorate CNTs.

In our previous research²⁷ we proposed a method to synthesize a type of nanoparticle with the functionalized nanofiller in the polymer matrix by using glycidyl methacrylate (GMA), which carries an epoxy group reserved for the reaction with nanotitania and a vinyl group for subsequent RAFT polymerization. In this study, we explored a strategy to prepare CNT/PLA nanocomposites with significantly modified crystallization behavior using CNT-OEG. The nucleation ability of CNTs was improved efficiently by grafting PLA-miscible polymer chains onto the CNTs surfaces. Simultaneously, PEG side chains of OEGMA can act as a plasticizer and improve the chain mobility of PLA during the process of spherulite growth. The results show us an efficient way to improve the dispersion and interfacial interactions between CNT and semicrystalline polymers through a simple and general free radical polymerization reaction.

Experimental section

Materials

Commercially available PLA comprising 2% DLA (trade name 4032D) was manufactured by Nature Works LLC (USA). The weight-average molecular weight and number-average molecular weight of PLA were 2.23 × 10⁵ and 1.06 × 10⁵ g mol⁻¹, respectively. The CNTs and hydroxyl CNTs (CNT-OH) with ∼30 μm length and 20–30 nm diameter were purchased from Chengdu Organic Chemicals Co Ltd, the Chinese Academy of Sciences R&D center for Carbon Nanotubes. The weight ratio of –OH group in CNT-OH is about 1.76 wt%. 3-Methacryloxypropyltrimethoxysilane (KH570) and OEGMA (the average number molecular weight (M_n) of OEGMA is 475) were acquired from J&K Scientific Co. (Guangdong, China). All other reagents were purchased from Kelong Chemical Co. (Chengdu, China) and used as received.

Synthesis of KH570-modified CNT (CNT-KH)

The fabrication of CNT-OEG mainly contains two procedures (as shown in Fig. 1). (I) The introduction of double bonds to CNT via KH570. (II) The attachment of OEGMA to CNT-KH via free radical polymerization. The detailed synthesis process of CNT-KH was as follows: CNT-OH (100 mg) were dispersed in 100 mL deionized water and the suspension was treated with ultrasound for 30 min, yielding completely exfoliated CNT-OH suspension. Then, KH570 was added into the as-prepared liquid and the reaction solution was adjusted to a pH value of 4.5. The hydrolysis of KH570 and condensation of CNT-OH were carried out simultaneously at 60 °C for 12 h to produce the KH570 functionalized CNTs solution. The methoxy groups of KH570 hydrolyze readily in aqueous solvents to form silanol groups, and self-condensation of the silanol to insoluble polysiloxane was likely to occur, greatly hindering the condensation between the silanol groups and the hydroxyl groups on the CNT-OH. Furthermore, the experimental condition (pH 4.5) facilitated the hydrolysis reaction while restrained the self-condensation reaction to ensure that a majority of silanol groups sufficiently condensed with hydroxyl groups on the CNT-OH.^28,29 The KH570 modified CNTs (CNT-KH) were obtained by centrifugation and redispersion in water for 4 times. The deposit was dried to constant weight at 60 °C in vacuum for 24 h.

Fig. 1 Schematic illumination of the fabrication of the CNT-OEG via free radical polymerization.

Synthesis of CNT-OEG via free radical polymerization

The CNT-KH obtained above was dissolved into dioxane with ultrasound for 30 min, then 5 g OEGMA, 0.02 g AIBN and 0.01 g divinylbenzene (DVB) were added into the solution as illustrated in Fig. 1. Incorporation of a very small amount of DVB led to the emergence of branched and starlike chains without gelation, effectively increasing grafting rate of OEGMA onto the CNTs.^30,31 The mixture was reacted at 70 °C for 12 h under N₂ atmosphere, centrifugation and redispersion in water and ethanol for 3 times. Finally, the resulting product was dried at 40 °C under vacuum for further use. The obtained product was CNT-OEG.

Preparation of PLA/CNT-OEG nanocomposites

Solution coagulation method was utilized to guarantee the good distribution of CNT nanofillers in PLA. Taking PLA/CNT-OEG (100 : 0.1 wt/wt) as an example, the detailed procedure was as follows: 0.01 g of CNT-OEG was added to 100 mL of ethanol (C₂H₅OH), then the mixture was subjected to ultrasound for 60 min to obtain a uniform dispersion. 10 g of PLA was completely dissolved in 100 mL of dichloromethane (CH₂Cl₂), subsequently. By pouring the predispersed C₂H₅OH/CNT-OEG suspension into the CH₂Cl₂/PLA hybrid, coagulated material precipitated continuously. The PLA/CNT-OEG coagulates were then transferred to blowing dryer, left overnight at 55 °C, and dried in a vacuum oven for 24 h at 80 °C to remove residual solvent. PLA/CNT and PLA/CNT-OH were also prepared using the same method.

Characterization

TEM images were taken on a JEOL-100CX transmission electron microscope (JEOL, Japan) to examine the morphology of the CNT-OH and CNT-OEG samples. The samples were prepared by one drop casting on a lacy copper grid followed by evaporation of the solvent at room temperature. XPS experiments were carried out on an XSAM800 (Kratos Company, UK) with Al Kα radiation (hν = 1486.6 eV). In order to determine the successful fabrication of CNT-OEG, FTIR spectra were recorded with Nicolet 6700 spectrophotometer (Thermal Scientific, USA) within the range 400–4000 cm⁻¹ using a resolution of 0.5 cm⁻¹. All spectra were baseline corrected. Thermal gravimetric analysis (TGA) was carried out under a nitrogen atmosphere on a Netzsch TG 209 F1 apparatus using a heating rate of 10 °C min⁻¹ from 40 to 800 °C. Gel permeation chromatography (GPC) analysis was performed at 40 °C on a HLC-8320GPC system (Dong Chao corporation, Japan), equipped with two columns (Column Super HM-H, 6.0 mm × 15 cm), and a differential refractive-index detector. POM observation was performed on an Olympus BX51 polarizing optical microscopy (Olympus Co., Tokyo, Japan) equipped with a Micro Publisher 3.3 RTV CCD. The temperature was controlled by a Linkam CSS-450 high temperature optical stage. The samples were first heated to 190 °C at a rate of 30 °C min⁻¹ and held at this temperature for 5 min to eliminate thermal history. Then cooled to 130 °C at cooling rates of 30 °C min⁻¹ and held for 30 min. DSC measurements were carried out in a TA-Q200 DSC (TA Instruments, USA) under a nitrogen flow, and calibrated by indium as the standard. For nonisothermal crystallization, samples were first heated to 190 °C at a rate of 10 °C min⁻¹ and held at 190 °C for 3 min to erase thermal history. Then, they were cooled to 40 °C at cooling rates of 5 °C min⁻¹ and reheated at a rate of 10 °C min⁻¹. For isothermal crystallization, the samples were also first annealed at 190 °C for 3 min to eliminate any thermal history and then quenched at a rate of 30 °C min⁻¹ to the desired isothermal crystallization temperatures (T_c) (120 °C, 125 °C and 130 °C) for 30 min. The degree of crystallinity (X_r) during heating progress is calculated using the equation: X_r = 100 × (ΔH_cc + ΔH_m)/ΔH₀, in which the heat of melting of perfectly crystalline PLA (ΔH₀) was 93.0 J g⁻¹. ΔH_cc was the cold crystalline enthalpy; ΔH_m was the melt enthalpy.³²

Results and discussion

Characterization of CNT-OEG

The morphology of CNT-OH and CNT-OEG was examined by TEM. Fig. 2 shows their TEM images. An enlarged diameter of CNT-OEG samples compared to CNT-OH which may results from the covered poly(OEGMA) (POEGMA) layers was observed clearly.³³ As a silane coupling agent, KH570 can chemically link to CNT-OH via reactive groups (Si–O–CH₃) react with the hydroxy groups of CNT-OH. Then the KH570 bonded to the CNT surface was copolymerized with OEGMA via a radical polymerization to form CNT-OEG. Similar results have been reported by Tang et al.³⁴ They claimed that the enlarged diameter of modified CNT samples is consistent with the chemical-linked polymer on the surface of CNTs.

Fig. 2 TEM images of the CNT-OH (a and a₁) and CNT-OEG (b and b₁).

XPS was employed to further determine the presence of POEGMA moieties on the surfaces of the CNT-OEG. As shown in Fig. 3, the peaks at 530.6, 258.5, 102.5 and 156.5 eV in the full spectrum of CNT-OEG are assigned to O, C, Si 2p and Si 2s elements, respectively. Compared with CNT-OH, the appearance of Si 2p band in the spectrum of CNT-OEG originates from the covalent attachment of OEGMA on the edges of CNT-OEG.^18,35 The change in C, O and Si contents calculated from the XPS results further confirms the introduction of KH570 and OEGMA atoms, as listed in Table 1. These results also demonstrated that OEGMA moieties were successfully anchored onto the surface of CNTs.

Fig. 3 XPS spectra for CNT-OH (a) and CNT-OEG (b).

Table 1 Summary of the element composition of the CNT-OH and CNT-OEG

Element (atom%)	C	O	Si
CNT-OH	91.9	8.1	—
CNT-OEG	86.9	11.8	1.4

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Fig. 4 shows the FTIR spectra of the CNT-OH (curve (a)), the CNT-KH (curve (b)) and the CNT-OEG (curve (c)). The strong absorption peaks at 1620 cm⁻¹ is assigned to C=C stretching mode of sp² network of CNT basal plane which can be seen in all these three samples.³⁶ The broad absorption at 3300–3400 cm⁻¹ is the stretching vibration of the –OH groups on the CNT-OH surface.³⁷ Compared with the FTIR spectra of CNT-OH, the FTIR spectra of CNT-KH and CNT-OEG show the characteristic absorption of CNT-OH at 1620 cm⁻¹, and the increasing intensity of absorption at 3300–3400 cm⁻¹, stemming from the reason that there have much more –OH groups in both KH570 and POEGMA. With comparison of FTIR spectra of CNT-KH, CNT-OEG reveals that the intensities of the bands at 1711 cm⁻¹ (stretching vibration of C=O),³⁸ 2860 cm^−l and 2930 cm⁻¹ (symmetric and asymmetric vibration of –CH₂) have increased significantly after the free radical polymerization. Moreover, the appearance of band at 1091 cm^−l (Si–O–C/Si–O–Si) provided more evidence for this successful chemical functionalization.³⁹ The FTIR spectrum confirms that the POEGMA molecules are covalently bound to the CNT-OH, as represented in Fig. 1.

Fig. 4 FTIR spectra of CNT-OH (a), CNT-KH (b), and CNT-OEG (c).

Further evidence for the fabrication of CNT-OEG can be ascertained from TGA measurement. Fig. 5 shows that the thermal degradation of CNT-OH, CNT-KH and CNT-OEG occurs in stages during heating from 40 °C to 800 °C under N₂ atmosphere. As shown in Fig. 5a, 5.2% of mass loss is resulted from pyrolysis of the unstable functionalized –OH groups in CNT-OH. Obviously, the major mass loss of CNT-OEG occurs between 300–400 °C, which is mainly corresponded to the pyrolysis of the grafted POEGMA on CNTs. Taking into account the residue of CNT-OEG and CNT-KH at 800 °C, the grafting ratio of POEGMA is estimated as 9 wt%. Therefore, the grafting ratio by this method is efficient to change the characterization of the CNTs as well as their nanocomposites.

Fig. 5 TGA curves of CNT-OH (a) CNT-KH (b) and CNT-OEG (c).

The existing literatures show that polymerization both in solution and on the surface of CNT-OEG was not differ significantly, the molecular weights and polydispersity index (PDI) of the POEMGA formed both in solution and on the surface of CNT-OEG were almost same.⁴⁰ Herein, the molecular weight of the POEMGA on the surface of CNT-OEG could be estimated by the molecular weight of the POEGMA formed in solution. Fig. 6, shows the GPC trace of the POEMGA recovered in solution. The M_n of POEGMA is about 9190 and the PDI is 1.48.

Fig. 6 GPC traces of free POEGMA recovered from polymerization solution. The M_n of POEGMA is 9190. The PDI = 1.48.

Crystallization kinetics of PLA/CNT-OEG nanocomposites

The isothermal crystallization performances of PLA/CNT-OEG composites were firstly investigated by POM with the filler weight ratio of 0.1 wt%. The PLA/CNT, PLA/CNT-OH composites with the same weight ratio were also prepared for comparison. As shown in Fig. 7, compared to the PLA/CNT, the observation of remarkably decreased density of spherulites after adding CNT-OH can be illustrated by the weak interfacial interaction and the steric effect between the PLA and CNT-OH.⁴¹ What arouse our interests is that the density of spherulites rises considerably for PLA/CNT-OEG. Fujisawa et al. results show us that enhancing dispersion, crystallization kinetics, and interfacial interaction within PLA matrix can be obtained when cellulose nanofibrils are decorated with PEG chains.^42,43 Therefore, we believe that the dispersion of CNT-OEG is improved and some attractive interactions are likely to be formed between the PEG side chain of POEGMA on the surface of CNT-OEG and PLA matrix. It was these enhancing dispersion and attractive interactions that enhanced the crystallization kinetics of PLA effectively.

Fig. 7 POM of PLA and PLA/CNT nanocomposites during isothermally crystallizing at 130 °C: (a) PLA/CNT, (b) PLA/CNT-OH, (c) PLA/CNT-OEG.

Fig. 8A shows the relative crystallinity (X(t)) derived from the DSC isothermal crystallized as a function of the crystallization time for the PLA nanocomposites isothermally crystallized at 120 °C. All these curves have similar sigmoid shapes, moreover, the corresponding crystallization time for PLA/CNT-OH is much shorter than that of PLA/CNT. What arouse our interest is that the time to complete the crystallization of PLA is markedly reduced by the incorporation of CNT-OEG. It is obvious that the incorporation of the POEGMA chains enhance the isothermal crystallization of PLA remarkably when compared with untreated CNTs. The half crystallization time (t_1/2) derived from the DSC isothermal crystallized at different temperatures are displayed in Fig. 8B. The crystallization rate (1/t_1/2) of the PLA/CNT-OEG is vitally boosted at each temperature. For instance, the t_1/2 of PLA/CNT, PLA/CNT-OH is 14.6 min, 10.3 min at 125 °C respectively. With addition of 0.1 wt% CNT-OEG, an unprecedented 5.5 min reduce of t_1/2 can be obtained. As temperature increases ranging from 120 °C to 125 °C, the t_1/2 increases as well for PLA/CNT and PLA/CNT-OH composites. It is acknowledged that the formation of polymeric spherulite is essentially dependent on nucleation and crystal growth. It results from a compromise between nucleation and growth of crystals: crystal nucleation is favored at low temperature when molecular mobility is low, whereas crystal growth is favored at high temperature when viscosity is low.^10,44,45 Beyond 110 °C, the nucleation is blocked due to the low viscosity of polymer chains, so the t_1/2 increases linearly. In contrast to PLA/CNT and PLA/CNT-OH, the PLA/CNT-OEG sample exhibits limited change with temperatures. It stems from the superb nucleation ability of CNT-OEG. Thus we can conclude that effect of the access crystallization ability of PLA is CNT-OEG > CNT-OH > CNT.

Fig. 8 (A) Plots of X(t) versus the crystallization time for the composites crystallized isothermally at 120 °C and (B) the plot of t_1/2 of the composites crystallized isothermally versus temperature: (a) PLA/CNT, (b) PLA/CNT-OH, (c) PLA/CNT-OEG.

Fig. 9A shows the cooling DSC curves of PLA composites after eliminating their thermal history. The crystallization peak temperature (T_p) of PLA/CNT-OH and PLA/CNT are faintly detectable compared to PLA/CNT-OEG, confirming the strong nucleating ability of CNT-OEG.⁴⁶ Compared with T_p and crystallization enthalpy (H_c) of PLA/CNT and PLA/CNT-OH, the increasing of T_p and H_c of PLA/CNT-OEG indicates that the functional groups affects the nucleation ability of CNT. The DSC traces in the second heating run (Fig. 9B) evidences a strong cold crystallization peak of PLA/CNT and PLA/CNT-OH. On the contrary, the cold crystallization peak of nanocomposites PLA/CNT-OEG almost disappeared. The result suggesting that the strong interactions at the polymer–filler interface promotes the crystallization process of PLA. Similar conclusions have been reported by Mariano Pracella et al.⁴⁷ for composites containing cellulose nanofibres and PVAc. The X_r of PLA nanocomposites are listed in Table 2. The X_r of PLA/CNT and PLA/CNT-OH are 9.2% and 11.8% respectively, while the X_r of PLA/CNT-OEG is 32.8%. In PLA/CNT, the CNTs were suggested to provide the templates for PLA chains to landscape. In PLA/CNT-OEG, the presence of POEGMA may facilitate the dispersion of CNT-OEG in matrix. Thus more nuclei spots were obtained. Furthermore, POEGMA chains can also act as crystal accelerator, so the growth of crystal is boosted. The synergistic effect of nucleation and growth of CNT-OEG on the PLA crystallization give rise to the enhancement on the overall crystallization kinetics of PLA as shown in DSC results. Furthermore, the high crystallinity of PLA/CNT-OEG nanocomposites will be benefit to improve the properties of PLA products, thus widening their application fields.⁴⁸

Fig. 9 Nonisothermal DSC scans of PLA nanocomposites at a constant cooling rate of 5 °C min⁻¹ (A), and its heating curves at a constant heating rate of 10 °C min⁻¹ (B): (a) PLA/CNT, (b) PLA/CNT-OH, (c) PLA/CNT-OEG.

Table 2 The cold crystallization peak (T_cc), exothermic heat of cold crystallization (H_cc), melting temperature (T_m), endothermic heat of melting (H_m) and crystallinity (X_r) of PLA nanocomposites obtained from the heating scan

Samples	Tcc	Hcc	Tm	Hm	Xr (%)
PLA/CNT	117.0	24.2	165.0	32.8	9.2
PLA/CNT-OH	116.1	21.7	167.3	32.7	11.8
PLA/CNT-OEG	102.1	8.0	168.5	38.5	32.8

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Conclusion

In the present work, we have demonstrated a simple way to fabricated CNT-OEG via free radical polymerization by using silane coupling agents. The structure of CNT-OEG was confirmed by FTIR, TEM, XPS and TGA. Effects of CNT-OEG on the crystallization of PLA were studied through DSC and POM. The addition of CNT-OEG to PLA matrix significantly increased the crystallization rate of PLA. The POEGMA chains immobilized on the surface of the CNT may accelerate nucleation rate of PLA. Therefore the CNT-OEG is expected to be used as an effective nucleating agent for semicrystalline biopolymers. It is worth stressing that this chemical methodology to graft polymers on the surface of CNTs explored herein is simple and has the potential to allow better integration of CNTs into multicomponent systems. The results shown here could have wide implications since they demonstrate that it is possible to use a simple and general chemical reaction to connect reactive polymers like PS, PMMA, PVA, etc. and CNTs with very different but complementary properties.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant 51173121), the Innovation Team Program of Science & Technology Department of Sichuan Province (Grant 2013TD0013), the Doctoral Program of the Ministry of Education of China (20130181130012). Prof. Zhong-Ming Li and his group are cordially thanked for their assistance with the DSC and POM results.

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