Crystallization and hydrolytic degradation behaviors of poly(l -lactide) induced by carbon nanofibers with different surface modifications

Polymer Degradation and Stability 170 (2019) 109014

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Crystallization and hydrolytic degradation behaviors of poly(L-lactide) induced by carbon nanofibers with different surface modifications Xin-zheng Jin a, Xu Yu a, Cheng Yang a, Xiao-dong Qi a, Yan-zhou Lei b, Yong Wang a, * a School of Materials Science & Engineering, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu, 610031, China b Analytical and Testing Center, Southwest Jiaotong University, Chengdu, 610031, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 August 2019 Received in revised form 6 October 2019 Accepted 20 October 2019 Available online 25 October 2019

Interfacial interaction exhibits great role in determining the microstructure evolution and performance of the polymer composites. In most cases, it is expected to enhance the degree of the interfacial interaction as much as possible. In this work, carbon nanofibers (CNFs) were modified through two different methods, i.e. carboxyl functionalization in compounded H2SO4/HNO3 (3:1, vol/vol) to obtain the f-CNFs and then further reaction with poly (ethylene glycol) (PEG) to obtain the PEG-grafted CNFs (g-CNFs). The results showed that the grafting ratio of functional groups on f-CNFs was about 13.09 wt%, while after being grafted by PEG, the grafting ratio was further enhanced up to 50.95 wt%. Largely enhanced dispersion stability in aqueous was achieved for f-CNFs and g-CNFs. Surface modification also improved the dispersion states of CNFs in the poly (L-lactic acid) (PLLA) composites, and stronger interfacial adhesion was achieved in the PLLA/f-CNF and PLLA/g-CNF composites. Studying on crystallization behaviors of the composites showed that raw CNFs (r-CNFs) and f-CNFs exhibited good nucleation effects on PLLA crystallization with nucleation activation energies of
12.49 and
11.81 kJ/mol, respectively, while the nucleation effect of the g-CNFs was very weak and the nucleation activation energy was
5.45 kJ/mol. However, compared with the PLLA/f-CNF composite, the PLLA/g-CNF composite exhibited largely enhanced growth rate of spherulites during the isothermal crystallization process. Researches about the hydrolytic degradation behaviors of the PLLA composites showed that incorporating f-CNFs accelerated the hydrolytic degradation while it was greatly suppressed by g-CNFs. For example, the PLLA/geCNFe2 sample showed much lower hydrolytic degradation rate (0.0134%/h) compared with the PLLA/reCNFe2 (0.0521%/h) and PLLA/feCNFe2 (0.086%/h) samples. This work confirms that appropriate interfacial interaction is more favorable for the realization of the nucleation effect of CNFs and the acceleration of the hydrolytic degradation of the PLLA matrix, while strong interfacial interaction achieved through grafting polymer chains not only suppresses the nucleation effect but also enhances the hydrolytic degradation resistance of the PLLA matrix. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Poly(L-lactic acid) Carbon nanofibers Interfacial interaction Crystallization Hydrolytic degradation

1. Introduction Poly (L-lactide) (PLLA) is a biodegradable and biocompatible polymer, and it attracts dramatically increasing concerns in recent years due to its excellent comprehensive properties. First, PLLA can be fabricated from the completely natural resources, such as corn and sugarcane [1]. Second, PLLA can be processed into various articles due to its good processing flowability and it is now widely used in many fields ranging from commodity products, consumer

* Corresponding author. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.polymdegradstab.2019.109014 0141-3910/© 2019 Elsevier Ltd. All rights reserved.

electronics to medical devices [2]. Third, PLLA can be completely degraded into water and carbon dioxide with the aid of enzyme and microbial population [3]. In the acidic and/or alkaline condition, PLLA also exhibits hydrolytic degradation and the hydrolytic degradation rate can be easily tailored [3,4]. This is very significant since the wide application of PLLA may dramatically reduce the risk of waste plastic on ecosystem. PLLA is a semicrystalline polymer, however, its crystallization ability is much lower than those of the common polyolefins and other polyesters [5,6]. The common processing method usually results in the amorphous PLLA because of the relatively high cooling rate. Compared with the crystalline PLLA, the amorphous PLLA usually exhibits low heat resistance, low

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modulus and strength, and high hydrolytic degradation rate. Therefore, from a viewpoint of engineering application, enhancing crystallization ability of PLLA is of great significance to achieve promising mechanical and thermal properties [7]. However, from a viewpoint of reclamation or reducing environmental pollution, reducing the crystallinity of PLLA is favorable for achieving high degradation rate [8e10]. Incorporating nanofillers is demonstrated as an efficient way to improve the crystallization ability of PLLA [11]. To date, various nanofillers, including the zero-dimensional particles such as silica (SiO2) [12,13] and titanium dioxide (TiO2) [14], the one-dimensional nanofillers such as carbon nanotubes (CNTs) [15], cellulose nanocrystals (CNCs) [16e18] and carbon fibers (CFs) [19], etc, and the two-dimensional nanofillers such as clay [20], graphene oxide (GO) [21,22], and layered double hydroxide (LDH) [23], etc, have been incorporated into PLLA to promotes the crystallization of PLLA. The crystallization behaviors of the PLLA in the composites are influenced by many factors, such as the type, content and aspect ratio of nanofillers, the interfacial interaction between nanofillers and the PLLA matrix, and the crystallization conditions. Among these factors, interfacial interaction exhibits the most important role in tailoring the crystallization ability of the PLLA matrix, which can be achieved through the different mechanisms. First, the good interfacial interaction can promote the homogeneous dispersion of nanofillers in the composites, which provides more nucleation sites for the PLLA crystallization, leading to larger nucleation density and higher crystallization rate [13,24]. Second, it is suggested that good interfacial interaction is more favorable for the nucleation of PLLA crystallites through adsorbing PLLA molecular chains on the surface of the nanofillers, which is favorable for the reduction of the crystallization activation energy [17]. Specifically, if the nanofillers have the large surfaces, they can provide the substrate for the ordering stacking of PLLA molecular chains, further facilitating the epitaxial crystallization of the PLLA matrix [25]. There are several types of interfacial interactions in polymer composites, such as the electrostatic interaction [26], p-p interaction [27,28], hydrogen bonding interaction [28,29] and covalent bonding interaction [30], etc. In the PLLA composites, constructing hydrogen bonding interaction between nanofillers and PLLA molecular chains is the most widely used method. Since most of nanofillers have the inert surface feature and few functional groups, the surface modification of nanofillers becomes very important [31,32]. In this condition, the type and content of functional groups, which are usually dependent upon the modification methods, affect the degree of the interfacial interaction of the composites and then affect the nucleation effect of the nanofillers on polymer crystallization. For example, Liang Y et al. [24] researched the effects of the surface chemical groups (hydroxyl groups, carboxyl groups and fluorine atoms) of CNTs on the crystallization of PLLA. They found that carboxyl groups (eCOOH) exhibited higher nucleation efficiency compared with the hydroxyl groups (-OH) and fluorine atom (-F). They also found that the steric hindrance of surface chemical groups affected the nucleation efficiency of CNTs. Surface chemical groups with larger bulk might impede the nucleation of PLLA due to the restricted mobility of molecular chains. Zhao Y et al. [33] and Li YL et al. [34] found that carboxy-functionalized CNTs exhibited better nucleation effect compared with the pristine CNTs on PLLA crystallization. However, Xu Z et al. [32] found that increasing the content of carboxyl groups on the sidewalls of CNTs resulted in lower nucleation rate. Grafting polymers on the surface of nanofillers are also demonstrated as an efficient way to intensify the interfacial interaction of the composites, especially the grafted polymer is identical to the matrix [35e37] or it is miscible with the polymer matrix. In

this condition, the molecular chains of the matrix and the grafted polymer can mutually diffuse and entangle, and the interface between nanofillers and polymer matrix becomes inconspicuous. Yoon JT et al. [38] fabricated a series of PLLA-grafted-CNTs with various lengths of PLLA molecular chains, in which the content of grafted PLLA was varied from 14.55 wt% to 42.58 wt%. They found that the dispersion of nanofillers and the mechanical properties of the composites were greatly dependent upon the PLLA chain length of the PLLA-grafted-CNTs. Namely, longer PLLA chain length exhibited better dispersion of CNTs and better mechanical properties. However, the crystallization of PLLA matrix was not reported in their work. Xu JZ et al. [21] synthesized poly (ethylene glycol) (PEG)-grafted-GO with grafting ratio of about 12.5 wt% and they found that the presence of the flexible PEG chains on GO surface not only enhanced the nucleation ability of GO but also served as a chain mobility promoter and boosted the crystal growth rate of PLLA. It is worth noting that in their work, the grafting ratio of PEG was relatively low and the size of GO was large and therefore, most of the GO surface could be approached by the PLLA molecular chains. So, an interesting question is proposed that whether the nucleation effect of the nanofillers is still present at high grafting ratio or not. Recently, Liu HL et al. [7] reported that grafting poly (Dlactide) (PDLA) on the surface of nanofillers promoted the formation of stereocomplex structure at the interface, which greatly accelerated the crystallization of the PLLA matrix. The grafting ratio of PDLA was as high as 80.5 wt%. In this condition, the nucleation effect was actually attributed to the formation of the large number of stereocomplex crystallites on the surface of CNTs rather than to the CNTs themselves. As one of the most important performances, hydrolytic degradation ability and its tailoring mechanisms are always being considered in designing the new PLLA-based materials. The hydrolytic degradation behavior of PLLA is influenced by many factors [39e44], including the external factors relating to environmental temperature and solution pH value, and the internal microstructure factors relating to the molecular weight, crystalline structure, and the presence of other components such as polymer, plasticizer, and nanofillers, etc. Incorporating nanofillers into PLLA usually brings a large number of interfaces and the concentration increase of the polar groups at the interface region, which result in the accelerated hydrolytic degradation rate of PLLA at the interface region [45]. However, if nanofillers exhibit the nucleation effect on PLLA crystallization and promote the increase of the crystallinity, the hydrolytic degradation rate may be suppressed to a certain extent [39,46]. This indicates that nanofillers exhibit dual effects in the PLLA matrix, and the hydrolytic degradation ability of the PLLA composites is a result of the competition between the positive effect originated from the interface promoting hydrolytic degradation and the negative effect originated from the crystalline structure suppressing hydrolytic degradation. Similarly, it is interesting to ask what will happen if the nanofiller surface is covered by the high density of grafting molecular chains? In this work, to clarify the above-mentioned doubts, high content of PEG is grafted on the surface of the carbon nanofibers (CNFs), which have larger diameter and low aspect ratio compared with the CNTs [47e49]. For comparison, the carboxylfunctionalized CNFs are also fabricated. The effects of the three kinds of CNFs, including the raw CNFs (r-CNFs) as prepared, carboxyl-functionalized CNFs (f-CNF) and PEG-grafted CNFs (gCNFs), on the crystallization and hydrolytic degradation behaviors of the PLLA matrix are comparatively investigated, and it is expected to provide new insight about the role of interfacial interaction in tailoring the crystallization and performances of the PLLA composites.

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2. Experimental part 2.1. Materials PLLA with a trademark of 2003D was purchased from NatureWorks (USA). It had a very few D-isomer and the content was about 4.3% and therefore, all the results obtained in this research could be related to the L-isomer rather than the D-isomer of PLLA. The other important parameters about PLLA included a weight-average molecular weight (M w ) of 2.53  105 g/mol, a melt flow rate of 5.6 g/ 10 min (190  C, 2.16 kg), and a density of 1.25 g/cm3. CNFs with a purity of about 99.9% were obtained from Beijing Xinjincheng Electronics Co., Ltd (Beijing, P.R. China). The outer diameter of CNFs was about 150e200 nm and the length of a single CNF was about 10e20 mm. PEG with Mw of 400 g/mol was purchased from Chengdu Kelong Chemical Reagent Co., Ltd (Chengdu, P.R. China). The other chemical reagents, such as H2SO4 (98%), HNO3 (65e68%) and dichloromethane (CH2Cl2) (analytical purity) were also purchased from Chengdu Kelong Chemical Reagent Co., Ltd (Chengdu, P.R. China).

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compounding processing as shown in the following. A certain number of CNFs and PLLA were added into dichloromethane (CH2Cl2) in which the weight/volume ratio of PLLA/CH2Cl2 was maintained at 1/10, and the solution was stirred at 35  C for 4 h with the aid of ultrasonication to ensure the homogeneous dispersion of CNFs and the complete dissolution of PLLA. After that, the solution was poured into a culture dish and then statically placed for 48 h to ensure the complete removal of CH2Cl2. The composite was then compression-molded at a melt temperature of 195  C and a pressure of 5 MPa to obtain the sample slice with a thickness of 0.5 mm. The compression-molded sample was quenched in ice/water mixture to obtain the amorphous PLLA matrix. In this work, three kinds of composites including PLLA/rCNF, PLLA/f-CNF and PLLA/g-CNF were prepared through the completely same preparation procedures as mentioned above. The sample notation was defined as PLLA/reCNFex (or PLLA/feCNFex, PLLA/geCNFex), where x represented the weight fraction of CNFs in the composites. Here, the weight fractions of CNFs in the composites were varied from 0.5 to 2.0 wt%. 2.4. Microstructure characterizations

2.2. Functionalization of CNFs Schematic 1 shows the surface modification methods of CNFs applied in this work. The oxidation modification of CNFs was carried out through the following procedures. First, about 200 mg raw CNFs (r-CNFs) were added into a three-necked flask that already contained 300 mL H2SO4/HNO3 (3:1, vol/vol) mixed acids. After that, the solution was ultrasonically treated at a power of 150 W for 2 h. Subsequently, the temperature of the solution was heated from room temperature to 60  C and then magnetically stirred for 6 h. The solution was then poured into 1700 mL deionized water, then the diluted solution was filtered. The modified CNFs were carefully washed using fresh deionized water until the pH value of the filtrate was 7. Finally, the modified CNFs were dried at 60  C for 24 h in a vacuum oven to obtain the oxidation-functionalized CNFs (fCNFs). The PEG-grafted CNFs (g-CNFs) were prepared according to the following procedures. First, 150 mg f-CNFs were added into a threenecked flask, then 50 mL PEG was poured into the three-necked flask. The mixture was heated to 110  C using an oil bath and magnetically stirred for 30 min to ensure the homogeneous mixing of f-CNFs and PEG. Then, 10 mL H2SO4 was added into the mixture. After being further magnetically stirred at 110  C for 24 h, the mixture was poured into the deionized water. The diluted solution was then filtered, washed and further filtered for several times to ensure the complete removal of the residual PEG and H2SO4. Finally, the g-CNFs were further dried at 60  C for 24 h in the vacuum oven. 2.3. Preparation of PLLA/CNF composites Before sample preparation, PLLA was dried in the oven set at 40  C for 24 h. The composites were prepared through solution-

The dispersion states of the three kinds of CNFs in aqueous solution was characterized using UVevis spectroscope UV-2600 (SHIMADZU, Japan). CNFs were dispersed into aqueous and then ultrasonically treated at 150 W for 30 min. After that, the solution was periodically taken out and then detected. The measurements were carried out at a wavelength range of 190e900 nm. The surface modification of CNFs was characterized using a Fourier transform infrared spectroscope (FTIR) Nicolet 6700 (Thermo Fisher Scientific, USA). The measurements were carried out in a transmission mode, and the scanning range was set at 4004000 cm
1 with a resolution of 2 cm
1. A thermogravimetric analysis (TGA) Q500 (TA Instrument, USA) was used to further detect the content of the functional groups or the grafting ratio of CNFs. Before measurement, the CNF powder was further dried at 60  C for 12 h in the vacuum oven. After that, 8 mg CNF powder was placed in a crucible and then heated from 50  C to 800  C at a heating rate of 10  C/min. The measurements were carried out in nitrogen atmosphere. A Raman microscope LabRAM HR Evolution (HORIBA Scientific, France) was used to further characterize CNFs. During the measurements, the incident laser light had a wavelength of 532 nm, the power was 1% and the wavenumber range was 100e3500 cm
1. A transmission electron microscope (TEM) JEM 2100F (JEOL, Japan) was used to characterize the morphologies of the CNFs. The characterization was carried out at an operating voltage of 200 kV. The dispersion states of CNFs in the PLLA/CNF composites were characterized through an optical microscope (OM) and a scanning electron microscope (SEM) at different magnifications. During the OM characterization process, the sample slice with a thickness of about 20 mm was prepared through compression molding processing that was carried out at a melt temperature of 190  C and a

Schematic 1. Carboxyl functionalization of CNFs and the grafting of PEG on functionalized CNFs.

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pressure of 5 MPa, and the characterization was conducted on a polarized optical microscope (POM) DM2700P (Leica, Germany). During the SEM characterization process, the sample prepared through compression molding processing was cryogenically fractured in liquid nitrogen, and then the fractured surface was coated with a thin layer of gold. The SEM characterization was conducted on a Fei Inspect (FEI, the Netherlands) at the operating voltage of 5 kV. Furthermore, the surface morphologies of the hydrolyzed samples were also characterized through using SEM. The nonisothermal crystallization and melting behaviors of samples were investigated using a differential scanning calorimeter (DSC) STA449C Jupiter (Netzsch, Germany). During the measurement, sample of about 8 mg was quickly heated to 200  C and maintained at 200  C for 3 min to erase any thermal history, then the sample was cooled down to room temperature (23  C) at a cooling rate of 1  C/min. After that, the sample was further heated to 200  C at a heating rate of 5  C/min. The second heating process was then used to analyze the melting behaviors of the samples. The measurements were carried out in nitrogen atmosphere. The degree of crystallinity (Xc ) can be calculated according to the following relation:

Xc ¼

DHm
DHcc  100% DHom  4

(1)

Where DH om represents the fusion enthalpy of completely crystalline PLLA with 100% crystallinity, and 4 is the weight fraction (wt%) of the PLLA matrix in the composites. Here, the value of DH om is 93 J/ g according to the literature [50]. Furthermore, the nonisothermal crystallization behaviors of the samples were also studied at different cooling rates (0.5, 1, 1.5 and 2  C/min). The isothermal crystallization morphologies of the samples were characterized using the POM as mentioned above. First, the sample was heated to melt completely, and then it was quickly pressed to obtain the sample slice with a thickness of about 20 mm. Subsequently, the sample slice was transferred to a hot stage LNP95 (Linkam Instrument, UK) set at 200  C and maintained at this temperature for 3 min to erase thermal history. After that, the sample was cooled down to the isothermal crystallization temperature (130 or 132  C) at a cooling rate of 50  C/min, and the crystalline morphology evolution with increasing isothermal crystallization time was recorded via taken images.

2.5. Hydrolytic degradation measurements The hydrolytic degradation measurements were carried out to declare the effects of different surface modification methods of CNFs on the performance of the PLLA composites. The sample was prepared through the compression molding method as mentioned above. The hydrolytic degradation measurements were carried out at 37  C in alkaline solution (NaOH/H2O, pH ¼ 13). The weight change of the sample during the hydrolytic degradation process was monitored through periodically taking out the hydrolyzed sample from the alkaline solution, washing and free-drying at
18  C for 24 h and weighing the weight of the sample, successively. The hydrolytic degradation medium was maintained invariant during the whole measurement process. To quantitatively describe the hydrolytic degradation behaviors of the samples, a residual weight fraction (F) and a hydrolytic degradation rate (R, %/h) were calculated according to the following equations:

F¼

wt  100% w0

(2)

R¼

1
F t

(3)

Where w0 was the initial weight of the sample while wt was the residual weight of the sample obtained after being hydrolyzed for time t (h). 3. Results and discussion 3.1. Functionalization of CNFs The functionalization of CNFs was firstly evaluated through investigating the dispersion stability in water. Fig. 1a shows the evolution of the UVevis spectra of the different solutions with increasing time, and the corresponding changes of solution appearances that were recorded by camera. As shown in Fig. 1a, the rCNFs exhibit very poor dispersion and the solution exhibits very low absorption intensity, even if the solution was ultrasonically treated and immediately measured. f-CNFs show relatively homogeneous dispersion in water and the solution exhibits apparent absorption intensity as shown in Fig. 1b. However, the absorption intensity gradually decreases with increasing the standing time, which indicates that the dispersion of f-CNFs in water is unstable and most of f-CNFs precipitate with increasing the standing time. Interestingly, the highest absorption intensity is achieved for the gCNFs (seen in Fig. 1c). Specifically, the absorption intensity maintains nearly invariant with increasing the standing time. The changes of solution appearances as shown in Fig. 1d further confirm that r-CNFs have poor dispersion stability in water, while the dispersion stability of CNFs is improved to a certain extent when they were treated by strong acid. Largely enhanced dispersion stability is achieved when the f-CNFs were further treated by PEG. PEG is a well-known water-soluble polymer. Therefore, it can be deduced that the good dispersion stability of the g-CNFs in water is mainly attributed to the presence of PEG. Thus, the following work is carried out to confirm the successful grafting of PEG on CNF surface. Fig. 2a shows the FTIR spectra of the three kinds of CNFs. Compared with the r-CNFs, f-CNFs exhibit a characteristic absorption band at wavenumber of 1727 cm
1, which can be attributed to the stretching vibration of C]O, indicating the presence of carboxyl groups on the f-CNFs after being functionalized using H2SO4/HNO3. The g-CNFs exhibit several characteristic absorption bands at 2923 cm
1 (asymmetric stretching vibration of eCH2) and 2854 cm
1 (symmetrical stretching vibration of eCH2), 1733 cm
1 (stretching vibration of eC]O in ester groups), 1635 cm
1 (stretching vibration of eC]C-), 1458 cm
1 (bending vibration of eCH2), 1163 cm
1 (asymmetric stretching vibration of -C-O-C- in ester groups), 1113 (asymmetric stretch vibration of eC-O-C- in ether groups), 1062 cm
1 (stretch vibration of eC-O-C- in ether groups) and 1034 cm
1 (stretching vibration of -C-O-C- in ester groups) [51]. The presence of these characteristic absorption bands clearly confirms the successful grafting of PEG on the CNF surface. Fig. 2b shows the TGA curves of the three kinds of CNFs. As expected, r-CNFs show nearly no weight loss even the temperature is increased up to 800  C, confirming the inert surface feature of rCNFs. Apparently enhanced weight loss is achieved for the f-CNFs and the weight loss is enhanced up to 13.46 wt% at 800  C, which clearly confirms the introduction of a large number of oxygencontaining groups on the f-CNFs after being treated by strong acid and in this condition, the grafting ratio is about 13.09 wt%. Interestingly, dramatically increased weight loss is achieved for gCNFs at about 300e400  C, which can be attributed to the pyrolysis of the grafted PEG chains on the g-CNF surface. According to the

X.-z. Jin et al. / Polymer Degradation and Stability 170 (2019) 109014

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Fig. 1. Evolutions of the UVevis spectra of (a) r-CNFs, (b) f-CNFs and (c) g-CNFs in aqueous with increasing the standing time. The aqueous solution was ultrasonically treated for 30 min. (d) Photos showing the dispersion states of the three kinds of CNFs in water after being ultrasonically treated and statically placed for 2 days.

Fig. 2. (a) FTIR spectra and (b) TGA curves of the r-CNFs, f-CNFs and g-CNFs, and (c) the comparison of Raman spectra between r-CNFs and f-CNFs.

residual weight at 800  C, it is then deduced that the grafting ratio of PEG is about 50.95 wt%. Furthermore, the surface modification of CNFs was further

detected through Raman measurements. As shown in Fig. 2c, the rCNFs exhibit three characteristic absorption bands at 1377 cm
1 (Dband), 1572 cm
1 (G-band) and 2691 cm
1 (G0 -band), and the

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X.-z. Jin et al. / Polymer Degradation and Stability 170 (2019) 109014

intensity ratio of I1377 =I1572 is about 0.369. The f-CNFs exhibit intense absorption bands and the ratio of I1377 =I1572 is increased up to 0.788 on the one hand. On the other hand, a should peak is also observed at 1600-1635 cm
1, which can be attributed to the D0 band. The increase of the I1377 =I1572 ratio and the presence of D0 band clearly show the presence of many defects in the graphite-like materials [47,52,53]. Furthermore, D0 -band is also the indication of a certain oxygenated functionalization [54]. Here, the Raman spectrum of g-CNFs was not obtained possibly due to that the presence of the PEG molecular chains, which greatly influence the detecting of g-CNFs. The surface morphologies of the CNFs were characterized through using TEM, and the typical images are shown in Fig. 3 rCNFs exhibit the relative smooth surface with a dense carbon layer as the shell (Fig. 3a) [49]. After being functionalized by strong acid, the diameter of the CNFs is reduced and the dense carbon layer is removed (Fig. 3b). The g-CNFs show the typical ‘core-shell’-like structure with PEG as the shell while CNFs as the core (Fig. 3c). Specifically, from Fig. 3c one can see that most of the CNF surfaces are covered by PEG and the exposed CNFs become very small. This further confirms the successful grafting of PEG on the CNF surface.

3.2. Dispersion of CNFs in the PLLA composites The dispersion states of CNFs in the PLLA matrix were investigated through different characterization methods. Fig. 4a, b and 4c show the OM images of the representative PLLA/reCNFe1, PLLA/ feCNFe1 and PLLA/geCNFe1 composites, respectively. r-CNFs exhibit poor dispersion in the matrix, and some large agglomerates with size up to 20 mm can be observed. However, largely improved dispersion is achieved for the f-CNFs and g-CNFs. Although the agglomerates of f-CNFs or g-CNFs can be still observed, the size of the agglomerates are greatly decreased. The dispersion states and interfacial adhesion states of CNFs in the composites can be clearly seen in the SEM images. From Fig. 4d and g one can see that some rCNFs aggregate together and form the agglomerates in the PLLA/ reCNFe1 on the one hand. On the other hand, it is clearly seen that many r-CNFs are pulled out from the PLLA matrix and the extracted r-CNFs have the smooth surface. Furthermore, the clear interfaces between r-CNFs and PLLA matrix are differentiated at higher magnification (Fig. 4g). This indicates the poor interfacial adhesion between r-CNFs and the PLLA matrix, which also confirms the poor interfacial interaction in the composite. In the PLLA/feCNFe1

Fig. 3. TEM images showing the morphologies of the (a) r-CNFs, (b) f-CNFs and (c) g-CNFs.

Fig. 4. (aec) TOM and (def) images showing the dispersion states of r-CNFs (a, d, g), f-CNFs (b, e, h) and g-CNFs (c, f, i) in the composites, and (gei) SEM images obtained at higher magnifications.

X.-z. Jin et al. / Polymer Degradation and Stability 170 (2019) 109014

composite (Fig. 4e and h), besides the largely improved dispersion of f-CNFs, most of f-CNFs tightly imbed in the PLLA matrix, which indicates the good interfacial adhesion between f-CNFs and PLLA matrix. In other words, the carboxyl functionalization of CNFs improves the interfacial interaction of the composite. Apparently different morphologies are observed in the PLLA/geCNFe1 composite (Fig. 4f and i). First, few g-CNFs are observed from the SEM image, which indicates the excellent interfacial adhesion in the composite. Second, many spherical particles are observed in the image. Taking into account the composition of the composite, it is believed that the spherical particles are mainly related to the presence of PEG on the g-CNFs surface. In view of thermodynamics, PEG exhibits good miscibility with PLLA [55] and it is widely used as the plasticizer of the PLLA [56]. However, once PEG is grafted on the surface of CNFs, the mobility of PEG molecular chains is greatly decreased, which possibly leads to the phase separation at the interface region between g-CNFs and PLLA matrix. 3.3. Crystallization behaviors of the composites The nonisothermal crystallization and melting behaviors of samples were comparatively investigated through using DSC, and the results are shown in Fig. 5 and Table 1. From Fig. 5a one can see that the pure PLLA sample exhibits nearly no exothermic phenomenon during the cooling process. The PLLA/reCNFe1 and PLLA/ feCNFe1 samples exhibit intense exothermic peaks, and the crystallization temperatures (Tc ,  C) are at 105.0 and 104.9  C while the melt-crystallization enthalpy (DHc , J/g) are 13.12 and 17.93 J/g, respectively. This indicates that the crystallization ability of the PLLA matrix is enhanced by incorporating r-CNFs or f-CNFs. In other words, both r-CNFs and f-CNFs show the heterogeneous nucleation effects on crystallization of PLLA. Interestingly, the PLLA/geCNFe1 sample shows the similar cooling curve to that of the pure PLLA sample, which indicates that g-CNFs do not show the apparent heterogeneous nucleation effect during the cooling process. The DSC heating curves are illustrated in Fig. 5b. There are several transitions in the heating curve, including the glass transition (Tg ,

7

 C), cold-crystallization (T ,  C) and meting of crystallites (T ,  C). cc m The first transition at temperatures of about 56e66  C is the result of the glass transition overlapping relaxation peak. The pure PLLA and PLLA/geCNFe1 samples show the apparent cold crystallization at 119.6 and 116.1  C, respectively, while the cold crystallization becomes inconspicuous in the PLLA/reCNFe1 and PLLA/feCNFe1 samples. This is contrarily different from the melt-crystallization behaviors as observed in Fig. 5a. Since most of the PLLA matrix in the PLLA/reCNFe1 and PLLA/feCNFe1 samples already crystallizes during the cooling process via melt-crystallization mechanism, few PLLA continuously crystallizes during the subsequent heating process via cold-crystallization mechanism and therefore, the exothermic peak becomes inconspicuous. Although Tcc of the PLLA/ geCNFe1 is slightly lower than that of the pure PLLA sample, the two samples show the similar cold-crystallization enthalpies (DHcc , J/g), indicating that the two samples show the similar cold crystallization behaviors. In other words, the effect of g-CNFs on PLLA crystallization is still inconspicuous. Furthermore, all the samples show the similar Tm at about 147.1e149.0  C, which indicates the similar lamellae thickness and crystal form in all the samples. Furthermore, from Table 1 it can be seen that the PLLA/reCNFe1 and PLLA/ feCNFe1 samples show Xc of 18.5% and 20.5%, respectively, while the pure PLLA and PLLA/geCNFe1 samples show Xc of 1.9% and 2.1%, respectively. This clearly indicates that grafting PEG on the surface of CNFs greatly suppresses the heterogeneous nucleation effect of CNFs on PLLA crystallization. The nonisothermal crystallization behaviors of the samples at different cooling rates (0.5, 1, 1.5 and 2  C/min) were also investigated using DSC, and the results are shown in Fig. S1. According to the Kissinger method and the other related literatures [57e62]:

! d ln

4 T 2p

DEa   ¼
R d T1p

(4)

Where R is the gas constant, the nucleation activation energy (DEa ,

Fig. 5. DSC cooling (a) and heating (b) curves of the pure PLLA and the different PLLA-based composites as indicated.

Table 1 Thermal parameters of the representative samples obtained through DSC measurements. Samples

Tc ( C)

DHc (J/g)

Tg ( C)

Tcc ( C)

DHcc (J/g)

Tm ( C)

DHm (J/g)

Xc (%)

PLLA PLLA/reCNFe1 PLLA/feCNFe1 PLLA/geCNFe1

90.8 105.0 105.0 99.3

0.92 13.12 17.93 1.45

60.7 60.9 60.8 60.7

120.0 116.2 106.1 116.1

16.96 3.83 2.44 18.47

149 147.7 147.4 148.7

18.69 21.06 21.50 20.40

1.9 18.5 20.5 2.1

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kJ/mol) can be calculated if the cooling rate (4,  C/min) and crystallization peak temperature (Tp ) are determined. The results shown in Fig. S1 indicate that DEa of the PLLA/geCNFe1 sample is about
5.45 kJ/mol, which is larger than that of the PLLA/feCNFe1 (
11.81 kJ/mol) and PLLA/reCNFe1 (
12.49 kJ/mol). This indicates that grafting PEG on the surface of CNFs is unfavorable for the nucleation of PLLA crystallites. The effects of surface modification on crystallization behaviors of samples can be further confirmed through isothermal crystallization studies as shown in Fig. 6 and Fig. 7. After being isothermally crystallized at 130  C for 45 min, pure PLLA sample shows the sporadic spherulites with average diameter of about 40 mm (Fig. 6a), confirming the weak nucleation ability of pure PLLA. Largely enhanced nucleation densities are achieved for the PLLA/ reCNFe1 (Fig. 6b) and PLLA/feCNFe1 (Fig. 6c) samples and in this condition, it is very difficult to differentiate the spherulites one by one. This further confirms the excellent heterogeneous nucleation effects of r-CNFs and f-CNFs on PLLA crystallization. Interestingly, the PLLA/geCNFe1 sample (Fig. 6d) shows the similar crystalline morphology to that of the pure PLLA sample, although the former sample shows slightly enhanced nucleation density compared with the latter sample. Obviously, the results obtained through isothermal crystallization process agree well with the nonisothermal crystallization process as shown in Fig. 5. Namely, rCNFs exhibit good nucleation effect on PLLA crystallization, and the carboxyl-functionalization does not affect the nucleation effect of CNFs apparently. However, the heterogeneous nucleation effect of CNFs is dramatically reduced if PEG is grafted onto the surface of CNFs. PEG has been widely used as the plasticizer of PLLA, and incorporating PEG usually promotes the enhancement of mobility of the PLLA molecular chains, which is favorable for the growth of spherulites during the crystallization process. For better understanding the effects of surface modification methods of CNFs on the crystallization behaviors of the PLLA matrix, the evolution of

crystalline morphologies during the isothermal crystallization process was monitored, and the typical morphologies at different crystallization time are shown in Fig. 7. Here, to clearly record. The growth of spherulites, the isothermal crystallization temperature was further increased to 132  C. Furthermore, the variations of spherulite diameters (D, mm) of samples with increasing crystallization time are also illustrated in Fig. 7e. The slope of the curve D versus crystallization time represents the growth rate (k, mm/min) of spherulites. As shown in Fig. 7, at the same crystallization time, pure PLLA sample shows the smallest spherulite diameter and the slope of the curve is also the smallest one (0.639 mm/min). While the PLLA/geCNFe1 sample shows the largest spherulite diameter and the biggest slope (1.897 mm/min). Although the nucleation effect of g-CNFs is worse than those of the r-CNFs and f-CNFs, PLLA/g-CNFs-1 composite show higher growth rate of PLLA spherulites compared with the PLLA/reCNFe1 and PLLA/feCNFe1 composites. This indicates that the grafted PEG molecular chains still exhibit the role of plasticizer and enhance the mobility of the PLLA matrix and finally, promote the growth of spherulites during the crystallization process. The similar phenomenon has also been reported by Xu JZ et al. [21]. In their work, the PEG molecular chains that were grafted onto GO surface enhanced the growth rate of PLLA spherulites. The PLLA/feCNFe1 composite shows smaller growth rate of spherulites compared with the PLLA/reCNFe1 composite, which is possibly attributed to the more homogeneous dispersion of f-CNFs in the former composites exhibiting more apparent steric hindrance effect on growth of PLLA spherulites. According to the above results, more visualized schematic representations have been proposed in Fig. 8 to explain the roles of fCNFs and g-CNFs in nucleating the crystallization of PLLA. As mentioned above, f-CNFs have many carboxyl groups on the surface, and these carboxyl groups may interact with the functional groups of the PLLA molecular chains through hydrogen bonding interaction, which promotes the local ordering or conformational

Fig. 6. POM images showing the isothermal crystallization morphologies of the representative samples. The isothermal crystallization was carried out at 130  C for 45 min. (a) Pure PLLA, (b) PLLA/reCNFe1, (c) PLLA/feCNFe1, and (d) PLLA/geCNFe1.

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9

regulation of PLLA molecular chains on the surface of the f-CNFs (Fig. 8a). During the crystallization process, the locally ordered PLLA chain segments act as the nucleus and then promote the further crystallization of PLLA molecular chains on the surface of f-CNFs [63]. However, in the PLLA/g-CNF composites (Fig. 8b), the surface of CNFs is covered by PEG molecular chains and in this condition, it is difficult for the migration of PLLA molecular chains to the surface of g-CNFs and forming the stable and local ordering stacking. Consequently, the PLLA molecular chains are excluded from g-CNFs surface and the nucleation effect of g-CNFs on PLLA crystallization is greatly weakened.

3.4. Hydrolytic degradation behaviors

Fig. 7. POM images showing the evolution of crystalline morphologies of the pure (a) pure PLLA, (b) PLLA/reCNFe1, (c) PLLA/feCNFe1, and (d) PLLA/geCNFe1 samples with increasing crystallization time: (a, b, c, d) 10 min, (a1, b1, c1, d1) 50 min, (a2, b2, c2, d2) 90 min, and (a3, b3, c3, d3) 120 min. (e) Variation of the spherulite diameters of PLLA in different samples as indicated in the graph with increasing crystallization time. The isothermal crystallization occurred at 132  C.

Hydrolytic degradation ability is one of the important parameters that are widely used to evaluate the applicability of the PLLAbased articles in real engineering application. Besides the crystalline structure of the PLLA matrix, the degree of the interfacial interaction also exhibits great role in tailoring the hydrolytic degradation behaviors of the composites. Here, to eliminate the effect of crystalline structure on the hydrolytic degradation behaviors of the composite samples, all the samples were quenched in ice/water mixture to obtain the completely amorphous PLLA matrix. The crystalline structures of samples are confirmed through wide angle X-ray diffraction (WAXD) measurement as shown in Fig. S2 (Supporting Information). Fig. 9 shows the variations of residual weight fraction (F) with increasing hydrolytic degradation time (t). For all the composite samples, F gradually decreases with increasing t. The similar phenomenon has already been widely reported elsewhere [45,64e66]. However, the three kinds of composite samples exhibit the different variation trends in F. As shown in Fig. 9a, incorporating r-CNFs leads to smaller F, especially at high r-CNF content. This indicates that the presence of r-CNFs facilitates the hydrolytic degradation of the PLLA matrix. The mechanism is mainly attributed to the interfaces between r-CNFs and PLLA, which provides channels for water penetration. However, the degree of the hydrolytic degradation of the PLLA/r-CNF is small. For example, after being hydrolyzed for 192 h, the PLLA/reCNFe2 sample shows F of 64.4%, which is only 13.0% smaller than that of the pure PLLA sample (74%). Largely decreased F is obtained for the PLLA/f-CNF composite samples (Fig. 9b). The more the f-CNFs in the composite sample, the smaller the F is. This indicates that higher fCNFs content leads to better hydrolytic degradation ability of the PLLA/f-CNF composite samples. For example, at t of 192 h, the PLLA/

Fig. 8. Schematic representations showing the different roles of (a) f-CNFs and (b) g-CNFs in nucleating the crystallization of the PLLA matrix.

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Fig. 9. Variation of residual weight fraction of samples as indicated in the graphs versus the hydrolytic degradation time. (a) PLLA/reCNFex, (b) PLLA/feCNFex, and (c) PLLA/ geCNFex.

feCNFe0.5 composite sample shows F of 55.9%. While in the PLLA/ feCNFe1 and PLLA/feCNFe2 sample, F is further decreased to 48.4% and 39.4%, respectively. This clearly indicates that incorporating the carboxyl-functionalized CNFs greatly enhances the hydrolytic degradation ability of the PLLA composites. Very interestingly, F exhibits the totally different variation trends in the PLLA/g-CNF composite samples as shown in Fig. 9c. Apparently enhanced F is achieved and F tends to increase with increasing gCNF content. For example, at t of 192 h, the PLLA/geCNFe0.5 composite sample shows F of 85.5%, while at g-CNF content of 2 wt %, F is further increased to 88.9%. This clearly indicates that the presence of g-CNFs greatly suppresses the hydrolytic degradation of the PLLA matrix. For better understanding the interfacial interaction on the hydrolytic degradation behaviors of the PLLA composites, the hydrolytic degradation rate (R, %/h) is calculated, and then the variation of R versus CNF content is illustrated in Fig. 10. The PLLA/r-CNF composite samples shows gradually increased R with increasing r-CNF content, further confirming that higher r-CNF content leads to better hydrolytic degradation ability of the composites. For example, the PLLA/reCNFe2 composite sample shows R of 0.0521%/h, which is apparently higher than the 0.0326%/h of the PLLA/reCNFe0.5 composite sample. As expected, apparently enhanced R is achieved for the PLLA/f-CNF composite samples. The PLLA/feCNFe0.5 composite sample shows R of 0.0594%/h, which is even higher than that of the PLLA/reCNFe2 composite sample. At f-

Fig. 10. Comparison of the hydrolytic degradation rates of the composites.

CNF content of 2 wt%, R is further enhanced up to 0.086%/h. Conversely, the PLLA/g-CNF composite samples show largely reduced R compared with the PLLA/f-CNF and even PLLA/r-CNF composite samples. Specifically, higher g-CNF content leads to smaller R. For example, the PLLA/geCNFe2 composite sample

X.-z. Jin et al. / Polymer Degradation and Stability 170 (2019) 109014

11

Fig. 11. SEM images showing the surface morphologies of the (a, d) PLLA/r-CNFs-1, (b, e) PLLA/feCNFe1, and (c, f) PLLA/geCNFe1 composite samples obtained after being hydrolyzed for 96 h. Images were obtained at different magnifications as indicated.

shows R of only 0.0134%/h. The variations of R with increasing CNF content further confirm that carboxyl functionalization of CNFs promotes the hydrolytic degradation of PLLA composites while grafting PEG onto CNFs suppresses the hydrolytic degradation of PLLA composites. The surface morphologies of the representative hydrolyzed samples were also characterized using SEM, and the results are shown in Fig. 11. After being hydrolyzed, CNFs are exposed and they can be clearly seen on the surface of the hydrolyzed PLLA/reCNFe1 and PLLA/feCNFe1 samples on the one hand. On the other hand, one can see that more serious erosion occurs at the interface region between PLLA matrix and r-CNFs (or f-CNFs), especially in the PLLA/ feCNFe1 sample. However, apparently different surface morphology is achieved for the hydrolyzed PLLA/geCNFe1 sample. First, the surface is smoother than those of the other two samples. Second, fewer g-CNFs are observed on the surface. This confirms the low degree of hydrolytic degradation in the PLLA/geCNFe1 sample and in this condition, the hydrolytic degradation possibly mainly occurs in the PLLA bulk rather than at the interfacial region. For better understanding the hydrolytic degradation mechanisms of the PLLA/f-CNF and PLLA/g-CNF composite samples, more

visualized schematic representations are proposed in Fig. 12. In the PLLA/f-CNF composites (Fig. 12a), due to the presence of the carboxyl groups on the surface of f-CNFs, the functional groups on the PLLA molecular chains are adsorbed on the f-CNF surface and consequently, the concentration of functional groups at the interface region is greatly increased compared with the bulk matrix. During the hydrolytic degradation process in the alkaline solution, the large number of functional groups at the interface region adsorb water molecules and then promote the hydrolytic degradation of the PLLA. However, in the PLLA/g-CNF composites (Fig. 12b), PEG molecular chains are tightly anchored on the CNF surface through covalent bond, which is apparently different from the hydrogen bonding interaction in the PLLA/f-CNF composites. On the one hand, the concentration of functional groups at the interface region is diluted by PEG molecular chains. On the other hand, although PEG is miscible with PLLA, the mobility of the PEG molecular chains is greatly reduced since the ends of the molecular chains are adhered on the CNF surface and consequently, the interface region is mainly occupied by PEG rather than by PLLA, which leads to the microphase separation at the interface region. Similarly, although PEG is water-soluble, the grafted PEG cannot be

Fig. 12. Schematic representations showing the different hydrolytic degradation mechanisms of the (a) PLLA/f-CNF and (b) PLLA/g-CNF composites.

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X.-z. Jin et al. / Polymer Degradation and Stability 170 (2019) 109014

dissolved by water due to presence of the covalent bond interaction between PEG and CNFs, which is apparently different from the randomly untrammeled PEG molecular chins [67]. As a consequence, the hydrolytic degradation of PLLA at the interface region is greatly suppressed by PEG. This clearly indicates that the strong interfacial interaction is favorable for enhancing the hydrolytic degradation resistance of the PLLA-based composites. The above results show that the nucleation effect of nanofillers on PLLA crystallization and the hydrolytic degradation behaviors of the composites are greatly dependent upon the surface modification of the nanofillers. Surface modification of CNFs achieved through using strong acid provides appropriate interfacial interaction, which not only endows the CNFs with good nucleation effect on PLLA crystallization but also greatly promotes the hydrolytic degradation of the PLLA matrix. While grafting PEG onto CNFs provides the strong interfacial interaction with the PLLA matrix and consequently, the nucleation effect of CNFs is greatly weakened and the hydrolytic degradation resistance is also enhanced. The next work will be focused on further tailoring the degree of interfacial interaction through changing the grafting density and the chain length of PEG, and then investigate the microstructure and the performance changes of the PLLA composites. Furthermore, the correlation between interfacial interaction, crystallization and hydrolytic degradation behaviors will be further investigated. 4. Concluding remarks In summary, the effect of surface modification of CNFs on crystallization and hydrolytic degradation behaviors of PLLA have been comparatively investigated. The main results are listed as follows. (1) Carboxyl-functionalized CNFs (f-CNFs) and PEG-grafted CNFs (g-CNFs) have been successfully prepared, and they exhibit the grafting ratios of 13.09 wt% and 50.95 wt%, respectively. (2) After surface modification, the dispersion stability of CNFs in the aqueous is greatly improved, especially when PEG is grafted on the CNF surface. The three kinds of CNFs (r-CNFs, f-CNFs and g-CNFs) exhibit different dispersion states in the PLLA composites due to the different degrees of interfacial interaction to the PLLA matrix, which can be confirmed through the morphological characterizations. (3) R-CNFs exhibit good nucleation effect on PLLA crystallization, and the nucleation effect is not apparently influenced by carboxyl functionalization modification. However, the nucleation effect is greatly suppressed by grafting PEG on the CNF surface. This can be further confirmed through the variations of DEa . The PLLA/reCNFe1, PLLA/feCNFe1 and PLLA/ geCNFe1 samples show DEa of
12.49,
11.81 and
5.45 kJ/ mol, respectively. Furthermore, the grafted PEG molecular chains are favorable for the growth of PLLA spherulites during the isothermal crystallization process. (4) Hydrolytic degradation behavior measurements show that incorporating f-CNFs promotes the hydrolytic degradation of the PLLA matrix while the presence of g-CNFs greatly suppresses the hydrolytic degradation of the PLLA matrix. The PLLA/geCNFe2 sample shows R of only 0.0134%/h, which is much smaller than those of the PLLA/reCNFe2 (0.0521%/h) and PLLA/feCNFe2 (0.086%/h) samples. (5) The above results clearly show that appropriate interfacial interaction is favorable for the realization of the nucleation effect of CNFs and the acceleration of the hydrolytic degradation of the PLLA matrix while strong interfacial interaction not only weakens the nucleation effect of CNFs but also suppresses the hydrolytic degradation of the PLLA matrix.

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