Cellulose (2015) 22:2341–2348 DOI 10.1007/s10570-015-0659-2
ORIGINAL PAPER
Influence of solvent polarity on surface-fluorination of cellulose nanofiber by ball milling Xianmeng Rao . Shigenori Kuga . Min Wu . Yong Huang
Received: 19 March 2015 / Accepted: 13 May 2015 / Published online: 22 May 2015 Ó Springer Science+Business Media Dordrecht 2015
Abstract Ball mill-assisted surface-fluorination of cellulose nanofiber was studied for two solvents with different polarity as dispersion/reaction medium. Milling cellulose in neat toluene gave irregular-shaped decrystallized cellulose particles; addition of pentafluorobenzoyl chloride (PFBC) to the system gave partially fluorinated cellulose as thin flakes with smooth surfaces, which maintained original crystallinity. Milling in neat dimethyl formamide (DMF) caused partial dispersion of nanofibers without decrystallization; milling in PFBC/DMF gave more enhanced dispersion of surface-fluorinated nanofibers. Both fluorinated materials were hydrophobic, with water contact angles of 103°–113°. Bulk degree of esterification was 0.20 for toluene and 0.57 for DMF systems. These results show characteristic influences of solvent species in reactive ball milling of cellulose in terms of fibrillation and surface esterification of nanofibers.
X. Rao S. Kuga M. Wu (&) Y. Huang (&) Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, People’s Republic of China e-mail:
[email protected] Y. Huang e-mail:
[email protected] X. Rao University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Keywords Nanocellulose Fluorination Solvent polarity Ball milling Hydrophobicity
Introduction Cellulose nanofibers have attracted much interest in recent years owing to their large surface area ([100 m2 g-1), low density (ca. 1.6 g cm-3), high strength and stiffness (Young’s modulus of 100–140 GPa) (Nishiyama 2009; Nishino et al. 2004), low thermal expansivity (ca. 10-7 K-1, longitudinally) (Nishino et al. 2004) as well as biodegradability and sustainable availability. These features have led to many studies on potential applications such as sorption media (Korhonen et al. 2011; Jiang and Hsieh 2014), gas barrier films (Spence et al. 2010), drug delivery (Roman et al. 2009), bioimaging (Dong and Roman 2007) and reinforcement of nanocomposites (Fujisawa et al. 2013; Eyholzer et al. 2010; Ten et al. 2012; Pei et al. 2011; Peresin et al. 2014). One challenge in utilization of cellulose nanofiber is the control of their surface properties. The hydroxyl groups of native cellulose are exposed on the surface of nanofibers, giving rise to its hydrophilic nature. The hydrophilicity is often detrimental to material applications through poor water resistance and weak adhesion with hydrophobic matrices (Joly et al. 1996; Li et al. 2000). To overcome this problem many attempts have been made, such as using surfactant treatment (Ljungberg et al. 2006), polymer grafting (Araki et al. 2001),
123
2342
fluorination (Cunha et al. 2007; Vaswani et al. 2005) and esterification/etherification of surface hydroxyls (Yuan et al. 2006; Harrisson et al. 2011). While most of the studies employed pre-individualized cellulose nanofibers, recent work by our group showed effectiveness of reactive ball milling as a one-step dispersion-esterification of nanofiber (Huang et al. 2012). While the type of ester group employed so far is limited to hexanoyl and succinyl, this approach is potentially applicable to many other esterifying reagents. To expand the scope of ball mill-assisted physicochemical modification, we here attempted fluorination, which is expected to provide improvements in water resistance, thermal and antioxidative stability, and compatibility with synthetic polymers via low surface energy (Pagliaro and Ciriminna 2005). We also used two types of reaction medium, toluene and N, N-dimethylformamide (DMF), to examine the influence of polarity of the environment of ball milling. The results will be assessed in comparison with those of surface fluorination in earlier studies (Cunha et al. 2007; Vaswani et al. 2005; Maity et al. 2010).
Cellulose (2015) 22:2341–2348
cellulose, 1.327 mL (9 mmol) of PFBC, 0.837 mL of pyridine and 20 mL of solvent (toluene or DMF). Mill was driven at 200 rpm starting from room temperature under programmed punctuated operation (2 min pause after every 20 min) for 12 h. This milling time was chosen based on the previous work, which showed near saturation in esterification by ball milling after 12 h (Huang et al. 2012). The milled sample was collected by rinsing with the same solvent. The sample ball-milled with toluene was washed and re-centrifuged with toluene (3 times), then with dichloromethane (3 times) and ethanol (3 times). The sample milled in DMF was washed with DMF (3 times), toluene (3 times) and ethanol (3 times). The washed samples were dried at 80 °C in vacuum for 8 h. The dried samples were marked as F-cell-T (toluene solvent) and F-cell-D (DMF solvent), respectively. Unreacted PFBC and catalyst pyridine should be removed by the toluene wash step because of their high solubility in toluene. Two control samples, cell-T and cell-D, were prepared by ballmilling cellulose in the solvents without reagent. The reaction scheme is shown below: (Scheme 1).
Experimental Characterization Materials Determination of degree of substitution The cellulose material was Sigma cellulose powder (Sigma-Aldrich Company). Cellulose was dried in vacuum at 104 °C for 4 h before use. The fluorinating agent was pentafluorobenzoyl chloride (PFBC) (Aladdin-reagent). Pyridine, toluene, N, N-dimethylformamide (DMF), ethanol and dichloromethane were purchased from Beijing Chemical Reagent Company, China. Distilled water was used throughout.
A 100 mg portion of fluorinated cellulose sample was accurately weighed and dispersed in 10 mL 0.5 M NaOH solution and stirred at room temperature for 8 h for hydrolyzing ester groups. Excess NaOH was titrated with 0.1 N HCl. The degree of substitution (DS) was calculated from the amount of NaOH consumed by ester hydrolysis, taking into account the increase in molecular weight of glucopyranosyl unit.
Pretreatment Fourier transform infrared spectrum (FT-IR) According to recent finding (Huang et al. 2013), cellulose was first soaked in 4 % NaOH at room temperature for 12 h, then washed with water three times. The wet cellulose was solvent-exchanged to toluene or DMF by vacuum filtration.
The samples were ground with KBr (1:100, w/w) to form compressed pellets. The spectra were taken in transmission mode in the range of 400–4000 cm-1, with accumulation of 32 scans and a resolution of 4 cm-1.
Ball mill esterification
X-ray diffraction analysis
A 40 mL zirconia cylinder of Fritsch Pulverisette 7 containing seven zirconia balls (10 mm diameter) was loaded with 500 mg (3 mmol of glucopyranosyl unit) of
The samples were formed into pellets and analyzed with a D8 Focus X-ray Diffractometer (Bruker AXS GmbH) using Cu Ka radiation.
123
Cellulose (2015) 22:2341–2348
2343
Scheme 1 Scheme of ball mill esterification
Scanning electron microscopy (SEM) The morphology of samples were analyzed using a JEOL JSM-4800 at acceleration voltage of 10 kV. All products were sputter-coated with gold to avoid charging. Transmission electron microscopy (TEM) A drop of ethanol suspension of each sample was placed on a carbon film-coated copper grid, dried up and examined by a JEOL E-2100 at 200 kV.
Fig. 1 IR spectra of products by ball milling in DMF and toluene, with and without PFBC
Contact angle Water contact angle on the compressed pellets of samples were measured by a Data-Physics OCA-20.
Results and discussion The FT-IR spectra of F-cell-T and F-cell-D (Fig. 1) both show new bands at 1749 cm-1 from C = O stretching of ester group, and 1500 cm-1 from C–C stretching of aromatic carbon of esterified cellulose. The new peak at 1333 cm-1 is typical of C-F stretching modes. These features indicate successful introduction of pentafluorobenzoyl (PFB) groups to cellulose for both F-cell-T and F-cell-D. The degree of substitution (DS) of PFB group onto cellulose determined by acid–base titration was 0.20 for F-cell-T, and 0.57 for F-cell-D. This difference obviously arose from higher polarity of DMF, which swells cellulose better, making more hydroxyl groups accessible than toluene. DS (0.57) of F-cell-D is close to DS (0.60) of the hexanoyl ester obtained by ball milling cellulose with hexanoyl chloride in DMF (Huang et al. 2012). This DS level is higher than those obtained in stirring condition under heating (DS 0.16–0.39 by
PFBC/toluene) (Cunha et al. 2007), and corresponds to full esterification of surface hydroxyls of cellulose nanofiber of approx. 2 nm wide (Huang et al. 2012, Supporting Information). Therefore, the product F-cell-D can be assumed to consist of fully surfaceesterified and individualized cellulose nanofibers. Here, ‘‘Individualization’’ does not necessarily mean spatial separation of nanofibers, but separation of mutually hydrogen-bonded nanofibers by surface esterification. In contrast, the DS of 0.2 for F-cell-T indicates incomplete surface esterification/individualization. This is likely to result from poor swelling of cellulose by toluene, which has definitely lower polarity than DMF. Scanning electron microscopy showed characteristic differences in morphology depending on the type of solvent and absence/presence of PFBC in ball milling (Fig. 2). The sample milled in toluene (cell-T) consisted of 10 lm *30 um irregular-shaped aggregates with rough surfaces (Fig. 2a); the cell-D sample (Fig. 2c), milled in DMF, consisted of partially dispersed nanofibers, which formed interconnected networks of hierarchical aggregation. With addition of PFBC, F-cell-T (Fig. 2b) consisted of large platelets with smooth-surfaces, with seemingly dense structure. The typical platelets were 2 lm *10 um in wideness
123
2344
Cellulose (2015) 22:2341–2348
Fig. 2 SEM images of ball milling products by two solvents with and without PFBC. a cell-T, b F-cell-T, c cell-D, d F-cell-D
Fig. 3 TEM images of ball milling products by two solvents with PFBC. a F-cell-T, b F-cell-D
and 2 nm *40 nm in thickness, the latter estimated by atomic force microscopy (data not shown). This morphology is likely to result from compression of the partially esterified cellulose in ball milling. The morphology of F-cell-D (Fig. 2d) was similar to that of cell-D, but the former contained better-dispersed nanofibers forming intricate networks. TEM observation (Fig. 3) confirmed the features of F-cell-T and F-cell-D, showing that the individual particles of F-cell-T are dense flakes, but F-cell-D consists of lightly aggregated nanofibers. The dispersion into nanofibers of F-cell-D are similar to that observed for the products of ball mill-esterification by hexanoyl chloride in DMF (Huang et al. 2012). Thus the morphological differences indicate that the
123
mechanical action and the reaction mode with PFBC in ball milling are very different for toluene and DMF as medium. Influence of ball milling on the crystalline order of cellulose also attracts attention. Figure 4 shows the XRD diagrams of the products compared with that of starting cellulose. The previous study showed that ball mill-assisted esterification/dispersion in DMF nearly preserved the original crystallinity (Huang et al. 2012). The same behavior was observed for F-cellD, indicating that ball milling and surface esterification do not cause decrystallization or disintegration of the native nanofiber units. Also, milling in neat DMF was found to preserve original crystallinity (cell-D). In contrast, milling in toluene showed a characteristic
Cellulose (2015) 22:2341–2348
Fig. 4 X-ray diffraction of ball milling products. Original crystallinity is maintained except for milling in pure toluene
difference for absence/presence of PFBC. While milling in pure toluene caused complete decrystallization after 12 h (cell-T), addition of PFBC to the system caused near preservation of crystallinity, as well as partial esterification (F-cell-T, DS 0.2). It is well known that crystalline cellulose is converted to amorphous form by dry ball milling (Hermans and Weidinger 1946a, b; Caulfield and Steffes Caulfiel and Steffes 1969; Ouajai and Shanks, 2006; Ago et al. 2004, 2007; Paes et al. 2010; Sun et al. 2014); therefore, the behavior of cellulose decrystallization by toluene-milling can be understood as extension of the category of milling in inert media. In contrast, the addition of PFBC to the system (1.34 mL PFBC/20 mL toluene) changed the situation completely, nearly preserving the original crystallinity, which in turn means that the microfibrillar entity is also preserved. Therefore, the flake-like particles seen in SEM actually are partially surface-esterified and tightly compacted nanofibers. Ago et al. (2007) studied the influence of low-level liquid addition to ball milling system (40 % weight of dry cellulose, for water, toluene, or butanol) on decrystallization behavior of cellulose. They showed that the toluene addition caused preservation of crystallinity, while the other two were barely effective in that way. Our present observation seems to contradict Ago et al’s results in the influence of toluene, but the amount of added toluene is very different for their case and ours (40 % of cellulose vs. 20 mL to 0.5 g cellulose). Because the action of milling to cellulose can be affected strongly by presence of liquid and its amount, the influence of toluene needs further study in terms of the solid–liquid ratio.
2345
Besides the introduction of ester groups for F-cellT, the difference in degree of decrystallization between F-cell-T and cell-T is remarkable (Fig. 4). Because of the polar nature of PFBC, its addition to toluene may alter the mode of milling action via change in polarity of the medium. To examine such behavior, we performed milling of cellulose in DMF/toluene solution with varied composition. Therefore we examined decrystallization of cellulose by ball milling in DMF-toluene mixtures as a model system. Figure 5 shows that the minimum DMF content for preventing decrystallization is approx. 5 %, which translates to cellulose: DMF ratio of approx. 1:2. Thus the addition of a small amount of polar liquid to the nonpolar medium changes decrystallization behavior fundamentally. This phenomenon might be understood as lubricating effect of polar liquids that can swell cellulose, i.e. penetrate to interstitial space between neighboring, originally tightly bound cellulose microfibrils. In the present case of milling in PFBC/toluene, the medium contained 20 mL of toluene, 1.327 mL of PFBC and 0.837 mL of pyridine against 0.5 g of cellulose; because of high polarity of PFBC and pyridine, they would interact with cellulose in the similar manner as DMF. Such a situation could be the cause surface esterification of cellulose nanofiber, though to a limited extent, maintaining its crystallinity. The possible situation is illustrated as Fig. 6. Fluorination of cellulose is expected to alter its surface properties as bulk material. Table 1 shows water contact angles of the original and the ball milltreated cellulose as compressed pellets. The PFBCtreated materials showed contact angle as high as 102.7 ± 0.4° (F-cell-D) and 113 ± 0.7° (F-cell-T), comparable with that of PTFE, 114°. While such
Fig. 5 Cellulose-DMF ratio is weight-based
123
2346
Cellulose (2015) 22:2341–2348
Fig. 6 Schematic drawing of ball milling of cellulose in DMF and toluene
Table 1 Water contact angle of ball milling products Sample Original cellulose cell-D cell-T
CA/° 69.5 ± 0.8 35.2 ± 0.2 51.3 ± 1
F-cell-D
102.7 ± 0.4
F-cell-T
113 ± 0.7
hydrophobization has been reported for bulk cellulose materials obtained by solution process (Cunha et al. 2007) or vapor-phase fluorination (Vaswani et al. 2005; Maity et al. 2010), the present method of reactive ball milling is characteristic in giving surfacefluorinated nanoparticles/fibers of cellulose. Also interesting is the difference between the samples milled in neat toluene and DMF. While the contact angle of cell-T was slightly lower than the original cellulose (51.3 ± 1° vs. 69.5 ± 0.8°), that of cell-D was much lower, 35.2 ± 0.2°, i.e. more hydrophilic. This behavior can be understood by the amphiphilic nature of cellulose molecule as shown in Fig. 7 (Nishiyama et al. 2002). The native cellulose microfibril is formed by mainly exposing the hydroxyl groups at the edge of glucopyranoside rings, but partial exposure of the ring plane (200 plane) can contribute to certain hydrophobicity, resulting in the observed contact angle of 69.5 ± 0.8° of the original cellulose. Then, milling in pure DMF seems to cause slight rearrangements of cellulose molecules to hide the ring
123
Fig. 7 Schematic view of the layered structure of cellulose I microfibril
plane by violent collisions with polar DMF molecules. In contrast, milling in pure toluene causes decrystallization of cellulose (Figs. 4, 6), which would expose both hydrophilic and hydrophobic parts of cellulose, leading to the observed value of 51.3 ± 1°. Such influences of the polarity of medium on surface properties of ball mill products seem to be analogous to the influence of liquid species used in swelling or coagulation of regenerated cellulose (Sato et al. 2004; Isobe et al. 2011).
Conclusions This study showed feasibility of fluorination of cellulose by reactive ball milling with a fluorinated
Cellulose (2015) 22:2341–2348
esterifying reagent. The degree of esterification and microscopic morphology of the products were found to depend strongly on the polarity of milling media, DMF and toluene. While milling in DMF with PFBC gave dispersion of highly surface-esterified nanofibers, milling in toluene with PFBC gave compacted platelets with low level of esterification. Despite the difference in DS, both DMF- and toluene systems gave high degree of hydrophobization. Influence of solvent polarity on decrystallization of cellulose is also remarkable. While milling in neat toluene caused decrystallization similarly to dry milling, milling in DMF, with or without reagents, and polar-doped toluene maintained cellulose crystallinity. These findings will serve as useful knowledge in chemical modification of cellulose by ball milling. Acknowledgments The work was supported by the National Program on Key Basic Research Project (973 Program, No. 2011CB933700), the National Natural Science Foundation of China (51373191, 51043003), and the Chinese Academy of Sciences Visiting Professorships.
References Ago M, Endo T, Hirotsu T (2004) Crystalline transformation of native cellulose from cellulose I to cellulose II polymorph by a ball-milling method with a specific amount of water. Cellulose 11:163–167 Ago M, Endo T, Okajima K (2007) Effect of solvent on morphological and structural change of cellulose under ballmilling. Polym J 39:435–441 Araki J, Wada M, Kuga S (2001) Steric stabilization of a cellulose microcrystal suspension by poly(ethylene glycol) grafting. Langmuir 17:21–27 Caulfiel DF, Steffes RA (1969) Water-induced recrystallization of cellulose. Tappi 52:1361–1368 Cunha AG, Carmen SRF, Armando JDS, Carlos PN, Alessandro G, Elina O, Fardim Pedro (2007) Highly hydrophobic biopolymers prepared by the surface pentafluorobenzoylation of cellulose substrates. Biomacromolecules 8:1347–1352 Dong S, Roman M (2007) Fluorescently labeled cellulose nanocrystals for bioimaging applications. J Am Chem Soc 129:13810–13811 Eyholzer C, Lopez SF, Tingaut P, Zimmermann T, Oksman K (2010) Reinforcing effect of carboxymethylated nanofibrillated cellulose powder on hydroxypropyl cellulose. Cellulose 17:793–802 Fujisawa S, Tsuguyuki S, Satoshi K, Tadahisa I, Isogai A (2013) Surface engineering of ultrafine cellulose nanofibrils toward polymer nanocomposite materials. Biomacromolecules 14:1541–1546 Harrisson S, Drisko GL, Malmstrom E, Hult A, Wooley KL (2011) Hybrid rigid/soft and biologic/synthetic materials:
2347 polymers grafted onto cellulose Microcrystals. Biomacromolecules 12:1214–1223 Hermans PH, Weidinger A (1946a) On the recrystallization of amorphous cellulose. J Am Chem Soc 68:2547–2552 Hermans PH, Weidinger A (1946b) Recrystallization of amorphous cellulose. J Am Chem Soc 68:1138 Huang P, Wu M, Kuga S, Huang Y (2013) Aqueous pretreatment for reactive ball milling of cellulose. Cellulose 20:2175–2178 Huang P, Wu M, Kuga S, Wang D, Wu D, Huang Y (2012) Onestep dispersion of cellulose nanofibers by mechanochemical esterification in an organic solvent. Chemsuschem 5:2319–2322 Isobe N, Kim U-J, Kimura S, Wada M, Kuga S (2011) Internal surface polarity of regenerated cellulose gel depends on the species used as coagulant. J Colloid Interface Sci 359:194–201 Jiang F, Hsieh YL (2014) Super water absorbing and shape memory nanocellulose aerogels from TEMPO-oxidized cellulose nanofibrils via cyclic freezing-thawing. J. Mater. Chem. A 2:350–359 Joly C, Gauthier R, Chabert B (1996) Physical chemistry of the interface in polypropylene/cellulosic-fibre composites. Compos Sci Technol 56:761–765 Korhonen JT, Kettunen M, Ras RHA, Ikkala O (2011) Hydrophobic nanocellulose aerogels as floating, sustainable, reusable, and recyclable oil absorbents. ACS Appl Mater Inter 3:1813–1816 Li Y, Mai YW, Ye L (2000) Sisal fibre and its composites: a review of recent developments. Compos Sci Technol 60:2037–2055 Ljungberg N, Cavaille JY, Heux L (2006) Nanocomposites of isotactic polypropylene reinforced with rod-like cellulose whiskers. Polymer 47:6285–6292 Maity J, Kothary P, O’Rear EA, Jacobi C (2010) Preparation and comparison of hydrophobic cotton fabric obtained by direct fluorination and admicellar polymerization of fluoromonomers. Ind Eng Chem Res 49:6075–6079 Nishino T, Matsuda I, Hirao K (2004) All-cellulose composite. Macromolecules 37:7683–7687 Nishiyama Y (2009) Structure and properties of the cellulose microfibril. J Wood Sci 55:241–249 Nishiyama Y, Langan P, Chanzy H (2002) Crystal structure and hydrogen-bonding system in cellulose 1 beta from synchrotron X-ray and neutron fiber diffraction. J Am Chem Soc 124:9074–9082 Ouajai S, Shanks RA (2006) Solvent and enzyme induced recrystallization of mechanically degraded hemp cellulose. Cellulose 13:31–44 Paes SS, Sun S, MacNaughtan W, Ibbett R, Ganster J, Foster TJ, Mitchell JR (2010) The glass transition and crystallization of ball milled cellulose. Cellulose 17:693–709 Pagliaro M, Ciriminna R (2005) New fluorinated functional materials. J Mater Chem 15:4981–4991 Pei A, Malho JM, Ruokolainen J, Zhou Q, Berglund LA (2011) Strong nanocomposite reinforcement effects in polyurethane elastomer with low volume fraction of cellulose nanocrystals. Macromolecules 44:4422–4427 Peresin MS, Vesterinen AH, Habibi Y, Johansson LS, Pawlak JJ, Nevzorov AA, Rojas OJ (2014) Crosslinked PVA nanofibers reinforced with cellulose nanocrystals: water
123
2348 interactions and thermomechanical properties. J Appl Polym Sci. doi:10.1002/app.40334 Roman M, Dong S, Hirani A, Lee YW (2009) Cellulose Nanocrystals for Drug Delivery. In: Edgar KJ, Heinze T, Buchanan CM (eds) Polysaccharide materials: performance by design. ACS, Washington, DC, pp 81–91 Sato K, Mochizuki H, Okajima K, Yamane C (2004) Effects of hydrophobic solvents on X-ray diffraction patterns of regenerated cellulose membrane. Polym J 36:478–482 Spence KL, Venditti RA, Rojas OJ, Habibi Y, Pawlak JJ (2010) The effect of chemical composition on microfibrillar cellulose films from wood pulps: water interactions and physical properties for packaging applications. Cellulose 17:835–848
123
Cellulose (2015) 22:2341–2348 Sun P, Kuga S, Wu M, Huang Y (2014) Exfoliation of graphite by dry ball milling with cellulose. Cellulose 21:2469–2478 Ten E, Bahr DF, Li B, Jiang L, Wolcott MP (2012) Effects of cellulose nanowhiskers on mechanical, dielectric, and rheological properties of Poly(3-hydroxybutyrate-co-3hydroxyvalerate)/cellulose nanowhisker composites. Ind Eng Chem Res 51:2941–2951 Vaswani S, Koskinen J, Hess DW (2005) Surface modification of paper and cellulose by plasma-assisted deposition of fluorocarbon films. Surf Coat Technol 195:121–129 Yuan HH, Nishiyama Y, Wada M, Kuga S (2006) Surface acylation of cellulose whiskers by drying aqueous emulsion. Biomacromolecules 7:696–700
