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Morphology, Thermal, and Crystallization Properties of Poly(butylene succinate)-Grafted Nanocrystalline Cellulose by Polymerization In Situ.

INTRODUCTION

Sustainable polymers derived from renewable resources have attracted increasing consideration as they are not only petroleum independent but also environmentally friendly [1-3]. The use of sustainable polymers provides a benign method for reducing material dependence on petroleum resources and to reduce plastic-related pollution caused by the use of petroleum-based polymers [4]. Cellulose is the most abundant polysaccharide, being a polymer found in all plants, some animals, and even in some primitive lifeforms, such as a mushrooms, microbes, amoeba, and seaweed [5]. It is the most common organic matter in nature and has abundant intermolecular and intramolecular hydrogen bonding, resulting in a fibrous, rigid, stable polymer. Consequently, cellulose has attracted significant interest as a remarkable reinforcement for biocomposites [6]. Nanocrystalline Cellulose (NCC), procured by cellulose acidolysis, has many attractive and excellent properties, such as nanoscale dimensions and large specific surface area as well as admirable mechanical strength and modulus, which have attracted great consideration for use in polymer nanocomposites [7-9]. In addition, NCC is biodegradable and renewable, which makes it a perfect agent for enhancing biobased materials and biodegradable polyesters, such as poly(butylene succinate) (PBS) [10-12], and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [13]. and poly(lactic acid) (PLA) [14,15]. The most noted dicarboxylic acid-derived polymer is PBS, which exhibits brilliant mechanical capabilities, processibility, and biodegradability [16]. Commercially effective PBS is manufactured by a condensation reaction of succinic acid and 1,4-butanediol [17-21]. producing a semicrystalline polymer with a glass-state transition temperature of -37[degrees]C and melting point of 114[degrees]C [16,22].

The ultimate properties of polymer nanocomposites have been noted to depend on the interspersion state of the nanoparticles [23]. Great diffusion of nanoparticles with very petty sum affords the substrate polymer various modified properties, such as reinforced crystallinity degree, enhanced mechanical properties, and improved rheological performance [24,25]. NCC might be a significant selection for the purpose of reinforcing PBS thermal stability because, as mentioned earlier, it is a promising nanoscale filler used for enhancing many biobased polyesters. To further the interspersion of NCC and increase its adhesion to polymers, some NCC surface alteration methods have been examined, including that application of polyurethanc [26], acetylation [5], and waterborne epoxy [27]. In addition, dissolution solidification has been used in the fabrication of original NCC-enhanced PBS nanocomposites [11]. The above approaches could effectively further enhance NCC dispersion in biobased polymers, but the resulting adhesion enhancement has been meagre, which is partly because of the finite compatibility of these additives with the polymeric matrix [8]. In contrast, polymerization in situ has been used to provide polymer-grafted NCC nanocomposites and to further the compatibility of NCC and polymer matrices [28]. The compounds 1,4-butanediol, succinic anhydride, and NCC were used in this study to successfully synthesize poly(butylene succinate)-grafted NCC (PBS-g-NCC) nanocomposites via polymerization in situ, and subsequent analyses clearly showed that this approach increased the thermal stability and degree of crystallinity of the product.

In the past, there have been many studies that used poly(butylene succinate)-grafted NCC to reinforce other materials but not PBS, such that there has been no independent research into the thermal stability and crystallinity of PBS-g-NCC. Thus, in this study, the morphology, thermal stability and crystallinity was examined in synthesized PBS-g-NCC nanocomposites containing NCC in different mass ratios. Physical property studies will continue in following articles.

EXPERIMENTAL

Materials

Succinic anhydride ([C.sub.4][H.sub.4][O.sub.3], AR) at the purity 98.0% was supplied by Shanghai Titan Scientific Co., Ltd. (Shanghai, China). The 1,4-butanediol at 99.0% purity ([C.sub.4][H.sub.10][O.sub.2], AR) were produced by Chengdu Kelong Chemical Reagent Factory (Chengdu, China). Titanium butoxide (Ti[(OBu).sub.4], CP), microcrystalline cellulose (MCC, AR), and concentrated sulfuric acid ([H.sub.2]S[O.sub.4], AR) were manufactured by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Synthesis of PBS-G-NCC

Preparation of NCC. First, 12.5 g of MCC was reacted with 64 wt% [H.sub.2]S[O.sub.4] (200 mL) and the mixture rapidly stirred at 50[degrees]C for 4 h, at which point a fivefold distilled water dilution was then applied to stop the reaction. The suspension was centrifuged to isolate the suspended crystals, which were then washed in distilled water, centrifuged, and separated again. This process was repeated five times for each sample. Finally, each sample was dialyzed against distilled water for several days until the pH reached 6.0-7.0. The product was freeze-dried for 12 h and stored in dry conditions until use.

Synthesis of Short Chain PBS. Syntheses were implemented with equimolar amounts of 1,4-butanediol (10 g) and succinic anhydride. In turn, NCC, succinic anhydride, and 1,4-butanediol were added to a three-necked flask and ultrasonically dispersed at 900 W power for 10 min. An enclosed three-necked flask, without [N.sub.2] (g), was held on a hotplate equipped with electromagnetic stirring and eventually evacuated. The temperature was set at 135[degrees]C and the reaction conditions maintained for 3 h.

Transesterification of PBS. After the second synthetic reaction step, the prepolymer was placed in a three-necked flask and heated to 150[degrees]C for 2 h. Then, for transesterification, titanium butoxide (Ti[(OBu).sub.4]) (0.1 mL) was added as a catalyst and the temperature gradually increased to 200[degrees]C and held for 3 h.

Characterization

Transmission electron microscopy (TEM; JEM 2100 F electron microscope, JEOL) observations were performed at 100 kV. And morphology of PBS-g-NCC nanocomposites was observed on a GeminiSEM 300 (ZEISS, Germany) scanning electron microscope (SEM) with an accelerating voltage of 3 kV. The fractured surfaces were used for observation and were sputtered with a layer of gold prior to measurement.

Nuclear magnetic resonance (NMR) analyses were implemented at ambient temperature in CD[Cl.sub.3] solutions with help of a AC-P 400 MHz spectrometer (Bruker, Germany), using tetramethysilane as the internal standard.

Crystallization behavior of PBS-g-NCC composites was examined in 3-10 mg samples, using a Netzsch DSC 214 (Netzsch-Geratebau GmbH, Selb, Germany). Measurements were implemented at a scanning rate of 10[degrees]C/min in a sealed aluminum pan under nitrogen atmosphere with an indium standard. Initially, samples were held at 140[degrees]C for 5 min to remove the thermal history and then cooled from 140 to -30[degrees]C, as the first cooling run. At last, the samples were again heated from -30 to 130[degrees]C for the second heating scan.

Two crystallization parameters, melt crystallization temperature ([T.sub.c]) and the initial exothermic temperature ([T.sub.on]) were obtained from the exothermic crystallization peak from the first cooling scan. [T.sub.on] was determined by extrapolation of the exothermic peak to baseline. The crystallinity[[chi].sub.c] was calculated using Eq. 1

[[chi].sub.c] (%) = [DELTA][H.sub.m]/[DELTA][H.sup.0.sub.m] x 100% (1)

where [DELTA][H.sub.m] is the melting enthalpy of the polymer sample (J/g) and [DELTA][H.sup.0.sub.m] is the theoretical value of the melting enthalpy of 100% crystallized PBS (110.3 J/g).

Thermogravimetric analyses (TGA) were implemented using TA Instruments TGA Q500 V20.13 Build 39 thermogravimetric analyzer (TA Instruments, New Castle, DE). Samples were heated from 25 to 600[degrees]C using a heating rate of 10[degrees]C/min under a nitrogen flow (40 mL/min).

X-ray diffraction (XRD) patterns of PBS-g-NCC samples were obtained using a DX-1000 diffractometer (Dandong Fangyuan Instrument Co., Ltd, Dandong, China) and Cu-[K.sub.[alpha]] radiation. The voltage and current were set to 40 kV and 25 mA, respectively, and diffraction patterns recorded for 2[theta] values ranging from 5 to 50[degrees] at a scanning rate of 0.067s.

RESULTS AND DISCUSSION

Transmission Electron Microscope

TEM was performed to image NCC morphology, which was found to have a rod-like morphology (Fig. 1). The NCC length was ~200-300 nm, indicating that it was nanosize.

Morphology of PBS-G-NCC Nanocomposites

To evaluate the possibility of polymerization in situ in synthesis of NCC filled PBS nanocomposite, the morphology of the prepared PBS-g-NCC nanocomposites was studied by SEM. SEM images for the fractured surfaces of pure PBS and PBS-g-NCC nanocomposites with different NCC loadings (Fig. 2). Pure PBS revealed a smooth surface (Fig. 2a). NCC particles, the dots directed by blue arrows. There was no obvious observation of NCC when added 1 wt% NCC in the PBS matrix, which was not much different from pure PBS (Fig. 2(b)). The picture clearly showed a large amount of NCC particles on the PBS surface, when the PBS-g-NCC content was 3 wt% NCC (Fig. 2c). Meanwhile, PBS-g-NCC nanocomposites showed rough surfaces and NCC dispersed very well in PBS. There was no pulling out of NCC, indicating a strong interfacial adhesion between the dispersed NCC and the PBS matrix. In addition, from Fig. 2c, the NCC particle size and distribution were obtained by ImageJ software for approximately 70 individual particles (Fig. 2g). The mean diameter of the NCC is 232.9 [+ or -] 56.5 nm. It could obviously indicate that there was no significant change from the NCC particle size before the reaction. However, aggregation was detected on surface with NCC loadings increased to 5 wt% (Fig. 2d). From Fig. 2e, most of the NCC was agglomerated, and only a small fraction of the small particles are dispersed in the PBS matrix, as indicated by the blue arrow. There was pulling out of NCC, indicating a weak interfacial adhesion between the dispersed NCC and the PBS matrix. Similarly, NCC agglomeration was also very serious in Fig. 2f.

Nuclear Magnetic Resonance

[sup.1]H-NMR spectra to examine PBS-g-NCC structure showed that PBS samples, produced through direct esterification in a closed system equipped with nitrogen feed and transesterification, possessed methylene protons of 1,4-butanediol units with peaks at 1.65 and 4.1 ppm (Fig. 3a). Peaks at 2.6 ppm indicated methylene protons in succinic acid units [29]. Further evidence of esterification between PBS and NCC was provided by [sup.13]C-NMR spectroscopy (Fig. 3b). The [sup.13]C-NMR spectrum of PBS, which exhibited peaks at [delta]'s of 64.2, 25.0, 172.1, and 28.9 ppm [30]. was compared with the PBS-g-CNC spectrum, which contained four additional peaks, at [delta]'s of 64.18, 25.25, 172.27, and 29.06 ppm, which attributed to the NCC component (Table 1).

Differential Scanning Calorimetry

The effects of NCC content on the crystallization properties of PBS were examined using DSC to describe the crystallization behaviors of these PBS-g-NCC composites. DSC cooling traces of selected samples during nonisothermal crystallization, at a cooling rate of 10[degrees]C/min, are shown in Fig. 4a and in Table 2. When the added NCC was 3 wt%, the [T.sub.c] of PBS-g-NCC composites samples moved from 74.6 to 77.0[degrees]C, and the [T.sub.on] of samples improved from 84.4 to 87.7[degrees]C (Table 2). When NCC was at >3 wt%, the [T.sub.c] and [T.sub.on] of the composites decreased with the increase of NCC content. The crystallization enthalpy ([delta][H.sub.c]) of PBS-g-NCC composites was slightly enhanced when 3 wt% NCC was added in PBS, which suggested a limitation in the crystallization process. This indicated that the crystalline qualities of PBS-g NCC composites were greatly influenced by the 3 wt% NCC content.

The thermal behavior of the selective samples was further analyzed by heating the samples again at a heating rate of 10[degrees]C/min after nonisothermal crystallization at the cooling rate of 10[degrees]C/ min. DSC heating curves of pure PBS and PBS-g-NCC composites are shown in Fig. 4(b) and melting temperature ([T.sub.m]) and enthalpy ([DELTA][H.sub.m]) data generalized in Table 2. Similarly, the [DELTA][H.sub.m] of composites enhanced with NCC was in good agreement with the nonisothermal crystallization behavior of the specimens. Although adding 3 wt% NCC synthesized samples had little change in [DELTA][H.sub.c] and the lowest [T.sub.m] than the others, and such results could be explained by the reason that the presence of NCC had a crosslink effect on PBS. Moreover, first cooling down and then heating up, composite was characterized the thermal properties by DSC, usually at the same temperature change rate. Crosslink affect restricted the movement of molecular segments and made it very difficult to accomplish the crystallization at the same cooling rate. Thus, during the heating process, the [T.sub.m] of PBS-g-NCC would be lower than pure PBS, if NCC was well dispersed in the PBS matrix. However as the temperature of PBS-g-NCC increased, the crosslinked segments had sufficient time to orderly align around the NCC particles and to give composites higher crystallinity.

Thermogravimetric Analysis

The thermal stability of the PBS-g-NCC composites was examined by TGA under [N.sub.2] atmosphere at a heating rate of 10[degrees]C/min. TGA thermograms of samples are shown in Fig. 5a and the relevant data, based on TG curves, in Table 3. TGA thermograms showed that all PBS-g-NCC composite samples demonstrated the same decomposition outline as that of neat PBS, which indicated that the decomposition process of PBSg-NCC composites was defined primarily by the PBS matrix. The maximum thermal decomposition temperatures ([T.sub.max], peak temperature of the DTG curve) are shown in Fig. 5b and the relevant data in Table 3, There was no significant change in [T.sub.onset.sup.a], [T.sub.50%], and [T.sub.max] of PBS when 1 wt% NCC was added. This indicated that a small amount of NCC had no effective improvement in the thermal properties of PBS. However, for the effect of NCC on the thermal stability of PBS matrix, the inset of Fig. 5a indicated that the thermal stability observably enhanced when the mass fraction of NCC was 3%, as evidenced by the shifting of weight loss to higher temperatures. Likewise, other data were summarized in Table 3, which also illustrated the enhancement of thermal stability. If the content of NCC was too high, NCC would agglomerate and carbonize during the reaction, so that it was not well dispersed in the PBS matrix. This indicated that high content of NCC did not necessarily improve the thermal stability of PBS.

To further study the thermal stability of PBS, we calculated the thermal degradation activation energy of PBS. The activity energy for thermal disintegration was calculated from the TGA curves by the integrated method devised by Horowitz and Metzger [31]. using the following Eq. (3)

ln[ln[(1 - [alpha]).sup.-1]] = [E.sub.[alpha]][theta]/R[T.sup.2.sub.max] (3)

where [alpha] is the decomposed proportion of the specimen, [E.sub.a] is the activity energy for disintegration, [theta] the T-[T.sub.max] (where T is the temperature), and R is the gas constant. [E.sub.a] was calculated from the slope of the straight line of ln[[ln(1-[alpha]).sup.-1]] versus[theta] (Table 3). The PBS modified with 3 wt% NCC exhibit slightly higher [E.sub.a] than other samples. This was ascribed to the formation of branching structures, which reduced segment migration and inhibited chain release during the extension of the degeneration process [32]. Composites with PBS-g-NCC at 9 wt% NCC displayed lower [E.sub.a] values, which had indicated a large number of carbonized NCC resulting from polymerization in situ. This indicated corresponding with the results of the thermal stability analysis of data [T.sub.onset.sup.a], [T.sub.50%], and [T.sub.max].

Wide-Angle XRD Analysis

XRD patterns of PBS-g-NCC composites in Fig. 6, all samples were stored at 40[degrees]C in a vacuum oven for 48 h before testing. PBS had narrow and sharp peaks at 2[theta] = 19.5, 21.5, and 22.5[degrees] [33]. which clearly showed that there were no significant differences in the diffractograms of pure PBS and PBS-g-NCC composites. This indicated that addition of NCC did not influence the crystal structure of PBS matrix. The peak intensities of PBS-g-NCC composites were not changed significantly at NCC <3 wt%. However, the peak of the composite was very narrow and the intensity was the largest, when the mass fraction of NCC was 3%. It showed that 3 wt% NCC effectively improved the crystallinity of PBS. When NCC was at >3 wt%, the intensity of the composites decreased. This indicated that the higher the content of NCC, the easier it was agglomerated, so that it could not be well dispersed and improve the crystallinity in the PBS matrix. These results further confirmed the conclusions of SEM images and DSC analysis.

CONCLUSIONS

The reactants 1,4-butanediol, succinic anhydride, and NCC were used to successfully synthesize PBS-g-NCC nanocomposites via polymerization in situ. Nanocomposites consisting of PBS matrix and well dispersed NCC nanoparticles, with the addition of 3 wt% NCC. The dispersed 3 wt% NCC and PBS matrix showed good interfacial adhesion as evidenced by the absence of pulling out of NCC. [sup.1]H and [sup.13]C-NMR analyses indicated the product to possess peaks characteristic of PBS, and there were no peaks characteristic of NCC in PBS-g-NCC, because PBS-g-NCC was almost insoluble in CD[Cl.sub.3] because of its crosslinked structure. Crystallization behaviors of PBS-g-NCC composites, examined by DSC, revealed that NCC introduction exerted clear effects on the thermal stability of the PBS-g-NCC composites. With NCC content from 3 wt%, the composite of PBS-g-NCC had the highest crystallinity. Such results could be explained by the reason that the presence of NCC had a crosslink effect on PBS, increased the regularity of the arrangement of molecular segments. PBS-g-NCC composites and their products appeared capable of being processed and used at relatively higher temperatures, before their thermal degradation. In particular, the [T.sub.onset] of PBS-g-NCC composites increased to 274.2[degrees]C with the addition of 3 wt% NCC. The PBS modified with 3 wt% NCC exhibit slightly higher Ea than other samples. This was ascribed to the formation of branching structures, which reduced segment migration and inhibited chain release during the extension of the degeneration process. XRD patterns of these composites were in good agreement with the DSC results. It was concluded that, during the crystallization process of PBS matrix, the 3 wt% NCC was added as a good nucleation catalyst, which improved the PBS crystallization behavior.

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Tao Tang (iD), (1) Jiang Zhu, (1) Wentao Wang, (2) Haitao Ni (2)

(1) Chongqing Key Laboratory of Environmental Materials & Remediation Technology, Chongqing University of Arts and Sciences, Chongqing, China

(2) College of Materials and Chemical Engineering, Chongqing University of Arts and Sciences, Chongqing, China

Correspondence to: J. Zhu; e-mail: jiangzhu415@163.com

Contract grant sponsor: Innovative Training Program for College Students of Chongqing Municipal Education Department; contract grant number: 201610642278. contract grant sponsor: Technology Project Affiliated to Chongqing Municipal Education Department; contract grant number: KJ1601121. contract grant sponsor: Natural Science Foundation of Yongchuan District; contract grant number: Ycstc2016ncl001.

DOI 10.1002/pen.25038

Caption: FIG. 1. TEM image of NCC.

Caption: FIG. 2. SEM images for the fractured surfaces of pure PBS (a), 1% NCC (b), 3% NCC (c), 5% NCC (d), 7% NCC (e), 9% NCC (f) and the histograms of NCC obtained according to the measurement results from SEM image by ImageJ software (g). [Color figure can be viewed at wileyonlinelibrary.com!

Caption: FIG. 3. (a) H-NMR and (b), [sup.13]C-NMR spectra of PBS-g-NCC.

Caption: FIG. 4. DSC curves of pure PBS and PBS-g-NCC composites: (a) crystallization curves at a cooling rate of 10[degrees]C/min and (b) melting curves at a heating rate of 10[degrees]C/min. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. (a) TGA and (b) DTG thermograms of PBS-g-NCC composites. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. XRD patterns of neat PBS and PBS-g-NCC composites. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Chemical shifts for [sup.13]C and
[sup.1]H species in PBS-g-NCC.

[sup.13]C     Chemical     [sup.1]H   Chemical shift
             shift (ppm)                  (ppm)

a               64.18         a            1.71
b               25.25         b            4.12
c              172.27         c            2.63
d               29.06

TABLE 2. Thermal properties of neat
PBS and PBS-g-NCC composites.

                           Cooling
Samples     [T.sub.c]      [DELTA]       [T.on]
           ([degrees]C)   [H.sub.c]   ([degrees]C)
                            (J/g)

Neat PBS       74.6         85.8          84.4
1%             75.8         82.5          85.1
3%             77.0         87.3          87.7
5%             75.2         79.8          83.2
7%             74.0         79.3          82.4
9%             74.1         78.6          81.9

                           Heating

Samples     [T.sub.m]      [DELTA]    [[chi].sub.c]
           ([degrees]C)   [H.sub.m]        (%)
                            (J/g)

Neat PBS      114.7         85.9          77.9
1%            114.5         87.0          78.9
3%            111.0         92.5          83.7
5%            111.4         85.9          77.9
7%            112.0         83.6          75.8
9%            113.0         83.1          75.3

TABLE 3. Thermal stability parameters
of the composites in [N.sub.2].

Samples                          Neat PBS    1%      3%

[T.sub.onset] (a) ([degrees]C)    266.9     265.6   274.2
[T.sub.50%] ([degrees]C)          362.5     366.7   370.5
[T.sub.max] ([degrees]C)          382.3     381.1   392.7
[E.sub.[alpha]] (kj/mol)          51.48      53.2    60.5

Samples                           5%      7%      9%

[T.sub.onset] (a) ([degrees]C)   265.7   262.6   258.7
[T.sub.50%] ([degrees]C)         358.8   358.0   350.6
[T.sub.max] ([degrees]C)         385.5   380.1   378.9
[E.sub.[alpha]] (kj/mol)          58.6    58.4    41.9

[T.sub.onset] (a) : initial temperature at 5 wt% mass loss.
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Author:Tang, Tao; Zhu, Jiang; Wang, Wentao; Ni, Haitao
Publication:Polymer Engineering and Science
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Date:May 1, 2019
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