All-Cellulose Nanocomposites Reinforced with in Situ Retained

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All-cellulose Nanocomposites Reinforced with in-situ Retained Cellulose Nanocrystals during Selective Dissolution of Cellulose in an Ionic Liquid JinMing Zhang, Nan Luo, Xiaoyu Zhang, Lili Xu, Jin Wu, Jian Yu, Jiasong He, and Jun Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01034 • Publication Date (Web): 16 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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ACS Sustainable Chemistry & Engineering

All-cellulose Nanocomposites Reinforced with in-situ Retained Cellulose Nanocrystals during Selective Dissolution of Cellulose in an Ionic Liquid Jinming Zhang,1 Nan Luo,1 Xiaoyu Zhang,2 Lili Xu,2 Jin Wu,1 Jian Yu,1 Jiasong He,1 Jun Zhang1∗ 1 Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing, 100190, China 2 Shandong Henglian New Materials Co., Ltd., Weifang, 261061, Shandong, China

Abstract All-cellulose nanocomposites, with cellulose nanocrystals as the reinforcing phase and regenerated cellulose as the matrix, are prepared by a partial dissolution method in 1-ally-3-methylimidazolium chloride (AmimCl), followed by solution casting. The direct images of many undissolved nanocrystals in cellulose/AmimCl solutions have been observed clearly by conventional transmission electron microscopy (TEM). X-ray diffraction (XRD) also proves that there are original cellulose I crystals in regenerated cellulose films. The nanocomposite films are compact, isotropic and transparent to visible light, and show good mechanical properties as a result of the nanocrystals reinforcement. Using microcrystalline cellulose (MCC) as the raw material, the optimal tensile strength and elastic modulus of nanocomposite films have reached 135 MPa and 8.1 GPa, respectively, by controlling the dissolution temperature and time. This work provides an easy and effective pathway to prepare all-cellulose composites. ∗

Corresponding authors.

E-mail address: [email protected] (J. Zhang).

ACS Paragon Plus Environment


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Keywords: All-cellulose composite; nanocomposite; cellulose nanocrystal; ionic liquid; self-reinforcing materials

Introduction Bioresource is a precious treasure for the mutual development of human society and natural environment due to its biodegradability, renewability and inexhaustibility. Nowadays much attention has been paid to the preparation of biocomposites and biocompatible composites to ease the burden of non-biodegradable petroleum-based materials. Cellulose, representing about 1.5 trillion tons of the totally annual biomass production, is widely considered as the most abundant biopolymer resource on the earth and the almost inexhaustible raw material, which can meet the increasing demand for environmentally friendly and biocompatible products.1,2 Natural cellulose-based materials (wood, cotton, linen, etc.) have been used in our lives as engineering materials for thousands of years and their use continues today as corroborated by the enormity of the worldwide industries in forest products, paper, textiles, and so on.3 Moreover, there is constantly searching for the development of value-added cellulose-based products that could compete with other high-tech industries.4 Strong interest has been attracted in recent years to the properties and utilization of cellulose nanocrystals, a kind of crystalline cellulose particles (cellulose I) in nanometer dimensions with aspect ratios in the range of 30-100 depending on their source. Thanks to the complex network of inter- and intra-molecular hydrogen bonds,


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natural cellulose nanocrystals have excellent mechanical properties and the ability to assemble themselves into a tight, high-strength and high-stiffness crystalline structure that is among the most resilient organic ones produced in nature.5,6 In the direction parallel to the chain axis, the elastic modulus of the cellulose I crystallite, the major composition by the nanocrystal and also the typical form of plant fibers, can reach 138 GPa,7-9 which is comparable with the values of high performance synthetic fibers such as poly(p-phenylene terephthalamide) (156 GPa, Kevlar, Twaron), Vectran (126 GPa), Technora (88 GPa), and Ekonol (130 GPa).10 In addition, the maximum macroscopic Young’s modulus is up to 128 GPa,11 which is higher than those of aluminum (70 GPa) and glass fibers (76 GPa). The ultimate tensile strength of the cellulose I crystallite is estimated to be 17.8 GPa,12 which is 7 times higher than that of steel. When the cellulose I crystallite is regenerated, i. e. dissolved and coagulated from solution, its crystalline structure is transformed to cellulose II and/or amorphous cellulose, which have lower tensile strengths of ~ 9 and 0.8 ± 0.1 GPa, respectively.13 Intrinsically, the excellent mechanical properties imply that the cellulose nanocrystal possesses the potential to replace glass fiber to act as an effective reinforcing agent for polymer nanocomposites. Further, it can be a promising reinforcing fiber for composites where the density is a concern. Therefore, recently, there has been a growing interest in utilizing the high-strength cellulose I to produce high performance all-cellulose composites, in which the matrix is dissolved and then precipitated cellulose, while the reinforcement is undissolved or partly dissolved cellulose microstructures or nanostructures. Since both the reinforcing phase and reinforced



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matrix are composed of cellulose, all-cellulose composites show excellent interfacial compatibility and are fully biocompatible and biodegradable, combined with light weight, high strength and high stiffness. Significantly this route is attractive for the development of sustainability and green chemistry.14-16 In the preparation of all-cellulose composites two distinct strategies are used: (i) two-step method: complete dissolution of a portion of cellulose to form a solution that is then combined with additional reinforcing cellulosic material;17-25 and (ii) one-step method: partial (or selective) dissolution of cellulosic material to form a matrix phase in situ around the remaining fibre core.26-38 Via both strategies, a large body of work has been published on all-cellulose composites derived from various sources of cellulose. For the second strategy, the key factor is the control of dissolution process of native cellulose, which owes a unique structure composed of the crystal unit, elementary nanofibrils and microfibrils in multilayer. However, it is a big challenge to estimate directly the existence of cellulose nanocrystals and control exactly their amount and status in solutions, corresponding to those in the final composites. In our previous report, based on the non-volatility of ionic liquids, the solution morphology and dissolution process of cellulose in ionic liquids have been visualized directly by conventional transmission electron microscopy (TEM).39 Therefore, in this work, we observed the amount and status of remaining cellulose nanocrystals in cellulose/ionic liquid solutions visually by TEM observation. Then, the relationship of mechanical properties and microstructure was investigated to reach the optimum mechanical performance by adjusting the dissolution conditions.


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Experimental Materials Microcrystalline cellulose (MCC, Vivapur 101) with degree of polymerization (DP) of 220 was purchased from Beijing Fengli Jingqiu Commerce and Trade Co., Ltd. It was dried at 105 °C for 3 h under vacuum before use. The ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl) was synthesized in our laboratory by the method described in our previous work,40 and the water content in AmimCl was less than 0.3 wt% as measured by Carl-Fischer method. Self-reinforced cellulose composites preparation A typical process for preparation of self-reinforced cellulose composites with the help of ionic liquids was as follows. First, MCC powder was added into a round bottom flask containing AmimCl. The mixture of MCC/AmimCl with a cellulose mass fraction of 4-6 wt% was mechanically stirred at the appointed temperature for different lengths of time. Then, the ʻsolutionʼ with undissolved nanocrystals was degassed by a vacuum pump for 0.5 h. After that, a ʻsolutionʼ of highly optical transparence was obtained and cast onto a glass plate to give a thickness of about 0.50 mm, then an immediate coagulation in the deionized water resulted in a transparent cellulose nanocomposite gel. To remove residual ionic liquid in the regenerated samples, these gels were further washed with distilled water at least three times until no Cl- ions were detectable by the AgNO3 test. After drying in vacuum at 80 ºC for 12 h, these cellulose nanocomposite films were kept in a desiccator prior to characterization.



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Characterization TEM analysis was performed by using a JEOL JEM-2200FS transmission electron microscope with accelerating voltage of 200 kV. A thin droplet of solution was spread on copper grids and observed directly without staining. Wide-angle X-ray diffractograms were recorded with an X-ray diffractometer (Rigaku D/max 2500). The X-ray radiation used was Cu Kα with a wavelength of 1.5406 Å, and generated at 40 kV and 200 mA. The radiation was passed perpendicular to the surface of films. Scanning electron micrographs (SEM) were recorded by using a Hitachi S-4800 scanning electron microscope at an accelerating voltage of 10 kV. The cross-section of films was coated with platinum before observation. Tensile testing was performed on an Instron 3365 universal testing machine with a 5 kN load cell at a crosshead speed of 2 mm/min. The specimens were cut into rectangular-shaped strips with 10 mm width and 50 mm length. The average values and standard deviations were calculated for five samples at least.

Results and discussion After the mixing process of MCC/AmimCl at relatively low temperatures for selected lengths of time and the thorough vacuum degassing, highly transparent cellulose/AmimCl ‘solutions’ are obtained, as shown in Fig. 1A. However, TEM images reveal the presence of many nanocrystals in these solutions as shown in Fig. 2. The width and length of nanocrystals are in the range of 5-50 nm and 50-100 nm, respectively, which are consistent with the dimension of elementary fibrils and


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microfibrils of cellulose.5,39,41,42 As the dissolution time and temperature increase, the aspect ratio and the content of nanocrystals decrease, while the size distribution of nanocrystals gets narrower than the initial state. As the dissolution time is prolonged further, the nanocrystals are dissolved, and relatively uniform solutions are formed. Because of the weak contrast between cellulose molecular chains and the background, the cellulose molecular chains, corresponding to the black fibrils, become difficultly distinguishable in TEM micrographs. Based on the TEM observation, we can expect that, with the increase of the dissolution time, the mechanical performance of cellulose nanocomposites will increase initially, reach a maximum in the middle of dissolution process, and then decrease.

(D) 100

90

90

Transmittance (%)

(C)100 Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 1h 3h 5h 7h

60 50 40 200

300

400

500

600

700

80 70 1h 3h 5h 7h

60 50

800

40 200

300

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Fig. 1. Photographs of (A) 4 wt% cellulose/AmimCl solution prepared at 50 ºC for 3 h and (B) the corresponding all-cellulose nanocomposite film; light transmittance graphs of all-cellulose nanocomposite films prepared from 4 wt% cellulose/AmimCl solution obtained at (C) 50 ºC and (D) 60



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ºC with different dissolution times.

Fig. 2. TEM micrographs of 4 wt% cellulose/AmimCl solutions prepared at different dissolution conditions: (A) 50 ºC, 1 h; (B) 50 ºC, 3 h; (C) 50 ºC, 5 h; (D) 50 ºC, 6 h; (E) 60 ºC, 1 h; (F) 60 ºC, 2 h; (G) 60 ºC, 5 h; (H) 60 ºC, 6 h.

The results of mechanical testing of cellulose nanocomposite films follow our prediction, as shown in Table 1. For example, when the dissolution temperature is 50 ºC and the concentration of cellulose/AmimCl solution was 4 wt%, the tensile strength of the resultant all-cellulose composite film is only 55 MPa after the dissolution of cellulose for 1 h (sample RC-4-50-1). As the dissolution time is prolonged (Fig. 3A and S1), the tensile strength has increased to 76 MPa for 2 h (sample RC-4-50-2), reached a maximum of 135 MPa for 3 h (sample RC-4-50-3), and then decreased to 126 MPa for 5 h (sample RC-4-50-5) and 114 MPa for 6 h (sample RC-4-50-6). This trend fully accords with the TEM observation. In other words, the amount and size distribution of the cellulose nanocrystals, have changed with the increase of the dissolution time, result in the enhanced properties at first, but,


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as the dissolution completed, the enhancement was weakened. In addition, the state of remaining cellulose nanocrystals is also affected by the dissolution temperature, so are the mechanical performances as well. As dissolution temperature is elevated, the dissolution rate of cellulose is also increased, so the time for reaching the maximum mechanical property is reduced (Fig. 3C and 3D). For instance, for the 4 wt% cellulose/AmimCl solution, the time to reach the optimum mechanical property is 3 h at 50 ºC (sample RC-4-50-3), 2 h at 60 ºC (sample RC-4-60-2) and only 0.5 h at 80 ºC (sample RC-4-80-0.5), respectively. The more content, uniform distribution and high aspect ratio of nanocrystals in all-cellulose nanocomposite result in better mechanical properties. The increase of cellulose concentration has been considered as one of important factors to enhance the mechanical properties of resultant films. However, the dissolution of more cellulose nanocrystals demands a longer time or higher temperature for getting the strongest cellulose nanocomposites. For 6 wt% cellulose/AmimCl solution, at 50 ºC, the time to reach the optimum mechanical property is 6 h, which is twice of that for 4 wt% cellulose/AmimCl solution (Fig. 3A and 3B). In summary, using MCC as the raw material, the optimal tensile strength and elastic modulus of cellulose nanocomposite films are 135 MPa and 8.1 GPa, respectively, which are prepared from 4 wt% cellulose concentration at 50 ºC for 3 h (sample RC-4-50-3). Table 1. Mechanical properties of all-cellulose nanocomposite films prepared at different preparation conditions. MCC

Dissolution

Dissolution

Tensile

Tensile

Elongation at

concentration

temperature

time (h)

strength

modulus

break

Samples*



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(wt%)

(ºC)

RC-4-50-1

4

50

RC-4-50-2

4

RC-4-50-3

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(MPa)

(GPa)

(%)

1

55 ± 3.3

3.6 ± 0.2

3.5 ± 1.3

50

2

76 ± 1.7

4.7 ± 0.3

4.7 ± 1.5

4

50

3

135 ± 5.0

8.1 ± 0.5

5.1 ± 1.5

RC-4-50-5

4

50

5

126 ± 3.9

7.3 ± 0.3

6.0 ± 1.2

RC-4-50-6

4

50

6

114 ± 5.6

6.9 ± 0.5

5.8 ± 1.1

RC-4-60-1

4

60

1

74 ± 2.4

4.4 ± 0.2

6.0 ± 1.3

RC-4-60-2

4

60

2

111 ± 4.8

7.1 ± 0.5

3.5 ± 1.1

RC-4-60-3

4

60

3

100 ± 3.2

7.6 ± 0.2

2.2 ± 0.2

RC-4-60-5

4

60

5

83 ± 1.7

7.5 ± 0.3

1.2 ± 0.1

RC-4-60-6

4

60

6

85 ± 2.9

6.7 ± 0.4

1.7 ± 0.1

RC-4-80-0.5

4

80

0.5

123 ± 3.9

7.8 ± 0.9

4.0 ± 0.9

RC-4-80-1

4

80

1

113 ± 9.3

7.3 ± 0.4

1.9 ± 0.3

RC-4-80-2

4

80

2

67 ± 10

7.4 ± 0.4

0.9 ± 0.1

RC-4-80-3

4

80

3

34 ± 5.5

5.8 ± 1.9

0.7 ± 0.1

RC-6-50-1

6

50

1

45 ± 2.7

2.6 ± 0.5

5.4 ± 1.2

RC-6-50-2

6

50

2

67 ± 2.7

4.3 ± 0.2

5.6 ± 0.9

RC-6-50-3

6

50

3

96 ± 5.1

5.7 ± 1.0

6.2 ± 1.6

RC-6-50-5

6

50

5

114 ± 12

4.8 ± 1.1

3.3 ± 1.4

RC-6-50-6

6

50

6

126 ± 7.0

6.3 ± 1.1

5.3 ± 2.4

*: Samples were named as RC-x-y-z according to their prepared conditions. The x stands for the cellulose concentration, the y for the dissolution temperature, and the z for the dissolution time.


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(B) 9 Tensile modulus /GPa

Tensile strength /MPa

(A) 140 120 100 80 60

4%, 50°C 6%, 50°C

40

8 7 6 5 4

4%, 50°C 6%, 50°C

3 2

0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

t /h

(C) 160

(D)

140

Tensile modulus /GPa

t /h

Tensile strength /MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100 80

80°C 60°C 50°C

60 40

10

8

6

80℃ 60℃ 50℃

4

2 0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

t /h

t /h

Fig. 3. The tensile strength (A and C) and tensile modulus (B and D) of all-cellulose composites prepared at different concentrations of cellulose/AmimCl solutions, temperature and dissolution times.

Since both the reinforcing phase and reinforced matrix are derived from the same material, the cellulose nanocomposites show excellent interfacial compatibility, which has been proved by SEM observation. In Fig. 4, the cross-section of composites is uniform, compact and relatively smooth without any aggregate. Meanwhile, we are unable to find individual cellulose nanocrystals in these SEM micrographs, due to the nanoscale dimension of nanocrystals. These phenomena indicate the good dispersion of nanocrystals in the matrix.



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Fig. 4. SEM micrographs of the cross-sections of all-cellulose composite films: (A) RC-4-50-1, (B) RC-4-50-3, (C) RC-4-50-5, (D) RC-4-50-6, (E) RC-4-60-1, (F) RC-4-60-2, (G) RC-4-60-5, (H) RC-4-60-6.

Due to the nanoscale dimension, excellent interfacial compatibility and good dispersion of nanocrystals in the matrix, the cellulose nanocomposite films are highly optical transparent (Fig. 1B). In the visible region (400-800 nm), the light transmittance of most films is above 80 %, as shown in Fig. 1. For 4 wt% cellulose/AmimCl solution, the light transmittance of composite film obtained at 50 ºC for 1 h, is only 76 %, owing to the existence of the relatively large nanocrystals at incipient stage. With the increase of dissolution time and temperature, nanocrystals become smaller and dissolved, so the light transmittance of composite films increased appreciably. A comparison of the X-ray diffraction profiles of MCC and regenerated all-cellulose composites prepared from MCC is shown in Fig. 5 and Fig. S2. The X-ray diffractogram of MCC shows a strong crystalline peak at 22.8° for (200) crystal plane, a small peak at 34.6° for (004) crystal plane, and a broad peak at 15.8° overlapped by 15.1° (1-10) crystal plane and 16.8° (110) crystal plane, indicating a typical cellulose Ι crystalline structure.43,44 After the partial dissolution and coagulation from solution, a main and broad diffraction peak appears at 20-22°, which is overlapped mainly by two peaks of cellulose II at 20.1° (110) and 21.9° (020)43,44 and the peak of


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amorphous cellulose at 17.3° 44,45. The peaks at 22.8° and 34.6° assigned to cellulose I crystallite, which is covered and progressively developed into indistinct diffraction peaks at lower diffraction angles, are difficult to be distinguished. These phenomena manifest that the main allomorph of the resultant cellulose composites is cellulose II and amorphous. However, the peaks at 15.1° and 16.8° of original cellulose I crystallite are still noticeable, although they are overlapped seriously with the broad peak at 20-22°. Considering the tiny size and relatively less content of remaining cellulose I crystallite, this phenomenon is reasonable. Therefore, in the regenerated all-cellulose composites, there is a small amount of original cellulose I crystallite,

Intensity(a.u.)

Intensity(a.u.)

which is in accordance with the TEM observation in Fig. 2.

MCC 10min 20min 30min

5

10

15

20

25

30

35

40

50min 60min 80min 120min

5

10

15

2θ ( ° )

150min 180min 210min

10

25

30

35

40

30

35

40

270min 300min 360min 420min

240min 5

20

2θ ( ° )

Intensity(a.u.)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

20

25

30

35

40

2θ ( ° )

5

10

15

20

25

2θ ( ° )

Fig. 5. XRD diffraction profiles of all-cellulose composites prepared from 4 wt% cellulose/AmimCl solutions at 50 ºC with different dissolution times.

Conclusions Self-reinforced all-cellulose nanocomposite films are prepared successfully by a process of partial dissolution of microcrystalline cellulose in ionic liquid AmimCl and



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successive solution casting. The retained nanocrystals reinforce in situ the amorphous and paracrystalline matrix effectively. The optimal tensile strength and elastic modulus reach 135 MPa and 8.1 GPa, respectively, prepared from a solution of 4 wt% cellulose concentration, stirred at 50 ºC for 3 h. The TEM micrographs clearly show un-dissolved cellulose nanocrystals dispersed in solution, which has a significant influence on the mechanical performance of all-cellulose composites. The XRD results also reveal that there is original cellulose I crystal of un-dissolved nanocrystals in these regenerated cellulose films. The excellent interfacial compatibility and good dispersion are shown by SEM micrographs. These features result in nanocomposite films with compactness, isotropy and high transparency to visible light. This work provides an easy and effective pathway to prepare all-cellulose composites, which are expected to be used as biomaterials and food package ingredients. Supporting information Typical

stress-strain

curves,

XRD

diffraction

profiles,

crystallinity

of

all-cellulose

nanocomposite films, and TGA curves.

Acknowledgements This work was supported by National Science Foundation of China (No. 51425307, No. 21374126 and No. 21174151) and Program of Taishan Industry Leading Talents (Shandong Province).

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TOC Graphic (For Table of Contents Use Only)

All-cellulose Nanocomposites Reinforced with in-situ Retained Cellulose Nanocrystals during Selective Dissolution of Cellulose in an Ionic Liquid Jinming Zhang, Nan Luo, Xiaoyu Zhang, Lili Xu, Jin Wu, Jin Yu, Jiasong He, Jun Zhang Strong, transparent and self-reinforced all-cellulose nanocomposite films were prepared by controlling the amount and status of un-dissolved cellulose nanocrystals, which was proved directly by TEM in the solution.


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All-Cellulose Nanocomposites Reinforced with in Situ Retained

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