1. Art and Diseño

OPTICALLY TRANSPARENT NANOCOMPOSITE REINFORCED WITH MODIFIED
CELLULOSE NANOFIBERS
Yaser Dahman* and Amir Sani
Department of Chemical Engineering, Ryerson University, Toronto, ON, Canada
Abstract: In this work we report a novel example of optically transparent nano-structured composite
which consists of surface-modified bacterial cellulose (B.C.) nanofibers reinforced in poly(hydroxyethyl
methacrylate) (PHEMA) hydrogel matrix. To promote the interfacial strength between the nanofibers and
the PHEMA phases, surface of the cellulose nanofibers was modified by fibrous acetylation to preserve
the B.C. nanofibrillar morphology. The modified nanofibers were then graft copolymerized with acrylic
based PHEMA hydrogel via free-radical mechanisms using a benzoyl-peroxide initiator. Several samples
of grafted nanofibers were prepared with different degrees of acetylation and graft yields, which were
characterized using 13C solid-state NMR, FTIR, and gravimetric method. The maximum degree of
acetylation was quantified to be 2.3, while the gravimetrical graft yield was 82.35 %. The modified
nanofibers were then reinforced into a polymeric matrix of PHEMA to form the final designed
biocomposite.
The nanofiber-network-reinforced PHEMA polymer composite maintained its transparency up to more
than 1% nanofibrillar content. The biocomposite material transmitted more than 80 % of the light when
the nanofiber network content was 1 wt%. The light transmittance was higher with samples with lower
nanofibrillar content. The loss of transparency in B.C.-based nanocomposite was small, despite the
differences in B.C. and PHEMA refractive indices. This clearly indicates the size effect of the nanoscaled fibres. These results clearly indicate the advanced properties of the nanocomposite which allow it
to be combined with various optically functional materials having significantly different refractive indices
and maintain the transparency of the final material. This novel nanocomposite is light in weight, highly
flexible, and easy to be mould. Considering the unique properties of the cellulose nanofibers, in addition
to the optically transparency of the final material, the nanocomposite is expected to have significant
improvements in the thermal and mechanical properties, which will allow a wide range of applications.
Keywords: nanobiomaterials, nanofibers, cellulose, fibrous acetylation, surface grafting, transmittance,
refractive index.
*
Corresponding author, email: [email protected], Tel: (416) 979-5000 ext. 4080, Fax: (416) 979-5083
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1.0 INTRODUCTION
Bacterial cellulose (B.C.) is a nanomaterial produced by Acetobactor xylinum, with a diameter of less than
50 nm and a high degree of crystallinity. It is a linear polymer of glucose linked by β–(1-4)-glycosidic
linkages that are similar to plant cellulose in chemical structure and has a degree of polymerization of
2000-6000. B.C. is produced in the form of pure nanofibres with unique physical properties. High
surface:volume ratio combined with its unique poly-functionality, hydrophilicity and biocompatibility
make B.C. a potential material for a range of biomedical applications. With its high elastic modulus of
78±17 GPa and breaking strengths of up to 1GN/m2 (10,000 MPa), B.C. can be used to form nanocomposites (Fontana et al., 1990; Deinema et al., 1971; Ross et al., 1991; Römling, 2002).
Although, B.C. nanofibers have useful properties, it lacks properties of synthetic polymers, hence,
drawing the attention to be used in “biocomposites”. Cellulose has been investigated for possible
biocomposites with acrylic based polymers including poly(hydroxyethyl methacrylate) (PHEMA).
PHEMA is a porous, cross-linked inert polymer hydrogel with excellent swelling behaviour in water and
solvents. It has a little structural value, being ripped and deformed extremely easily. Optically clear
PHEMA hydrogels are used as soft contact lenses. HEMA has also been tested for pharmaceutical
applications (Karlson and Gatenholm, 1997; Seidel and Malmonge, 2000; Koo et al., 2002; Tsou et al.,
2005).
There has been special interest in grafting acrylic based monomers onto plant cellulose, while a little
information is available on research using HEMA. Karlson et al. (1997) have controlled the degree, the
place and uniformity of grafting PHEMA onto plant cellulose under swollen conditions. Nho et al. (2003)
found an application for HEMA-grafted plant cellulose in hemi-dialysis using
irradiation. They
attached heparin to HEMA strands on the cellulose and measured anti-thermobic properties. Nishioka et
al. (1992) found the thermal stability to be about that of the constituents in graft polymers with a low
weight in grafts, and the thermal stability to be heavily impaired in the case of extensive grafting. They
also found that a blend of graft polymer and homopolymer actually had a higher water capacity than any
of the constituents of this blend. The only research involved in B.C. based biocomposite with PHEMA
has produced a transparent composite with a low thermal-expansion coefficient similar to that of silicon
crystal with a mechanical strength of five times that of engineered plastics, being an excellent material for
a variety of applications (Iwamoto et al., 2005; Nogi et al., 2005; Yano et al., 2005).
There has been no material that combined B.C. and PHEMA hydrogels or little work has done towards
grafting of HEMA onto the cellulose. On this background, we postulated the importance of developing a
novel method to combine B.C. and PHEMA in a pre-designed way to produce a stable biocomposite
material with advanced properties. An approach was in surface/fibrous modification of the nanofibers in
addition to grafting copolymerization, in which PHEMA can be polymerized as side chains of the B.C.
hydroxyl groups. The technique allow for surface modifications of cellulose, which would increase water
holding capacity, tear and temperature resistance. With this wealth of possible modifications, the
applications of B.C. would grow from the commercially produced plastic celluloid to contact lenses, drug
delivery systems, biosensors, pervaporation membranes and even the removal of heavy metals from water
systems.
2
2.0 MATERIALS AND METHODS
2.1 Bacterial Cellulose Biosynthesis
B.C. was produced in shake flasks using a fructose based medium using A. xylinum BPR 2001 (ATCC #
700178) at an optimum temperature of 28oC, in the form of beads (Joseph et al., 2003). B.C. was then
treated with 1% (w/w) of NaOH in a bath of boiling water for 30 min and washed repeatedly with
deionized water. The extracted B.C. has been kept in deionized water at 4oC.
2.2 Surface Acetylation of Bacterial Cellulose
B.C. was partially acetylated to preserve the nanofibrillar morphology and improves the transparency of
the targeted final biocomposites. It was transferred into acetic acid by stepwise solvent exchange and then
was swollen for 72 h. B.C. was acetylated in a solution consisting acetic acid (20 parts), acetic anhydride
(20 parts), and 97% sufuric acid (0.04 parts). The mixture was shaken vigorously for 1 h, and thereafter
was allowed to stand for 1 h at room temperature (~25oC). B.C. was washed several times with methanol
and water while being transferred back in aqueous solution by stepwise solvent exchange.
2.3 Graft Copolymerization
The graft copolymerization of HEMA monomers onto cellulose acetate (CA) nanofibers was carried out
with a benzoyl-peroxide (BPO) free-radical initiator. The aqueous solution of CA of known concentration
was transferred to 100 mL of acetone by stepwise solvent exchange. Polymerization was carried out in a
500 mL glass reactor equipped with a reflux condenser and an overhead stirrer which was operated at 300
rpm under controlled temperature and N2. CA was continuously stirred in the 500-mL reaction vessel
under N2 while adding 5% (wt dry cellulose) of BPO initiator. The initiator was allowed to interact with
the cellulose acetylated fibres for 15 min at 60oC to form free radicals. This was followed by the addition
of monomer(s). Free-radical graft polymerization was continued for 6 h at 60oC under N2 and the reactor
temperature was brought to 25oC. The un-reacted monomer was removed by washing the fibres with a
methanol/water (30:70 v/v), by stepwise solvent exchange. The un-reacted acrylic homopolymers were
removed with benzene through repeated extraction. Finally, the CA grafted with PHEMA (CA-gPHEMA) was stored in water at 25oC.
2.4 Reinforcement Composites of the Grafted B.C.
PHEMA crosslinked matrix was synthesized by free-radical mechanism at presence of CA-g-PHEMA
nanofibers at different initial concentrations (0-1.0 % by weight). The homopolymerization reactions were
carried out in a 500 mL glass reactor with BPO as initiator and ethylene glycol dimethacrylate as
crosslinking agent. The final biocomposite material was soaked in water for several weeks before
examining its transparency.
2.5 Characterization Experiments
Nanostructured biocomposite samples were quantified for the amount of grafting using gravimetric
methods. Compositions were characterized using FTIR and 13C Solid-State NMR. FTIR spectra recorded
in Diffuse Reflectance mode performed on the nanofibers before and after modification. Spectra were
analyzed using a Spectra-Tech Baseline Drifts accessory. The 13C Solid-State NMR spectra were recorded
using a 600 MHz Varian Inova-600 spectrometer.
2.6 Transmittance Testing
The final nanobiocomposite samples were cast on flat surfaces to form thin sheets of ~ 1 - 2 mm
thickness. The transmittance of the final nanobiocomposite sheets was measured at wavelengths from
200 to 800 nm using a UV/ Visible Spectrophotometer (DU 520, Beckman Coulter). Regular
transmittance was measured by placing the polymer specimens at ~20 cm from the entrance port of the
UV lamp.
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3.0 RESULTS AND DISCUSSIONS
3.1 Surface Acetylation of Bacterial Cellulose.
Figure 1: (a) and (b) show the FTIR spectra of B.C. nanofibers before and after conducting the fibrous
acetylation, respectively. Examination of this Figure reveals a monotonous decrease in the OH band at
3350 cm-1 and an increase in the three major bands of cellulose triacetate, i.e. the C=O stretching band at
1750 cm-1, the C-O band at 1240 cm-1 and the C-CH3 bands at 1375 and 1450 cm-1 respectively. The
degree of substitution (DS) was quantified using back titration method. Results obtained showed that the
DS had initially a steep rise as the additions of acetic anhydride were increased. Thereafter, DS gradually
leveled off as the amount of reagent was increased, reaching the maximum DS obtained. The nonlinearity
mainly indicates the fibrous heterogeneous surface acetylation. Moreover, this represents an initial rapid
surface acetylation and a slow inside acetylation with the collapsing crystal structure of cellulose I. This
proves that the reaction proceeds from the surface to the core of the semicrystalline nanofibers. Degrees
of acetylation of the nanofibers for the different samples were quantified using the back titration method
and found to be in the range of 1 – 2.3.
C-CH3
C-O
C=O
(b)
O-H
(a)
3500
3000
2500
2000
Wavenumbers (cm-1)
1500
1000
Figure 1. FTIR spectra of B.C. nanofibers before (a) and after (b) acetylation (DS by titration was 2.3).
3.2 Graft Copolymerization of CA Nanofibers
Graft copolymerization reactions were initiated by free-radical mechanism as follows:
C6H5OCO–OCO6H5
2C6H5OCO*
60 o C
2C6H5OCO*
2C6H5*+2CO2
CA–OH + C6H5*
CA–O* + C6H6
CA–O* + M
CA–O–M*
CA–O–M* + nM
CA–O–M*(n+1)
CA–O–M*(n+1) + CA–O–M*(m+1)
Grafted Polymer
where, CA-OH is cellulose acetate, CA-O* are cellulose acetate radicals, CA-OM* is the graft copolymer
radical, and M is HEMA monomer.
4
The mechanism shows that when BPO initiator molecules were heated to 60oC, they were decomposed
yielding phenyl radicals. The OH groups on CA nanofibers are the targeted grafting sites. The C5H5*
radicals interact with the OH on CA surface that were not acetylated producing CA macroradicals, which
initiate grafting on the fibres surface. A weight increase was observed in the benzene extracted grafted
fibres compared to the weight of the CA before grafting. This indicates possible grafting of the B.C.
nanofibers. Three samples of the grafted CA were prepared with different HEMA to CA weight ratios of
0.5, 1.0, and 2.0 in the feed; see Table 1.
All the prominent peaks pertaining to polyHEMA arose clearly in the FTIR spectra (not shown) compared
to what is in Figure 1. These peaks are: OH stretching at 3300 cm-1, ester methyl stretching vibrations at
2900 cm-1, asymmetric and symmetric CH2 stretching vibrations at 2800 cm-1, and carbonyl vibrations at
1740 cm-1. In addition to these peaks, some of the characteristic peaks of CA were also observed
confirming the grafting. Quantitative analysis of the polyHEMA grafted on the nanofibres was done
based on the characteristic carbonyl groups arose at 1740 cm-1. Figure 2 shows the 13C solid-state NMR of
the nanofibers after surface graft-copolymerization with HEMA compared with the unmodified BC. Peak
assignment of the 13C solid-state NMR spectra of PHEMA and CA-g-polyHEMA fibres that had initial
weight ratios of 2:1 of HEMA:CA are summarized in Table 1.
Figure 2. 13C solid-state NMR of surface grafted CA nanofibers with HEMA with initial weight ratios of
2:1 of HEMA:CA compared to unmodified B.C. nanofibers.
The degree of acetylation and the amount of grafted PHEMA were obtained by comparing the peaks
arising from the fast rotating methyl carbons (a8) at 28.8 ppm and carbonyl carbon (b1) at 178.8 ppm,
with the peak arising from the carbons adjacent of ether linkage in cellulose at 71.07 ppm (Rana et al.,
2006; Lim et al., 1999; Udhardt et al., 2005; Princi et al., 2005). The maximum degree of acetylation of
cellulose nanofibers was found to be 2.5, which was quantified as 2.3 using the surface back titration
method as indicated above. The PHEMA graft yield obtained gravimetrically was 82.35 %. Table 1 lists
the different graft parameter calculated from the weight results. The diameter of the fibres was calculated
from SEM pictures (not shown) results of which are summarized in Table 1. Examining results reveals
that cellulose nanofibers’ diameter increased as the amount of HEMA used initially for the graft
copolymerization increased.
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Table 1. Characterization results for the samples of modified cellulose nanofibers based nanocomposite.
Monomer/
Monomer
Graft
Graft
HomoFibre
Cellulose ratio Conversation
yield
Efficiency polymer
diameter
(w/w)
(%) a
(%)b
(%)c
(%)c
(nm) d
CA-g-PHMEA-0.5
0.5
96
11.76
11.76
88.24
75 - 90
CA-g-PHMEA-1.0
1.0
88
25.49
13.0
87.00
90 – 200
CA-g-PHMEA-2.0
2.0
86
82.35
21.0
79.00
150 - 200
a
Calculated from the change in the weight after removal of un-reacted monomer by washing with methanol/ water
mixture, b Calculated from the change in the weight after removal of homopolymers by washing with benzene
through repeated extraction, c Calculated from FTIR spectra based on the carbonyl peaks, d Diameter of pure
cellulose fibres ~ 27 - 40 nm.
Sample ID
3.3 Optical Transmittance of Biocomposites
Results for the light transmittance versus wavelength measurements (not shown) of the nanocomposite
sheets of PHEMA reinforced with CA-g-polyHEMA in the wavelength range of 500-800 nm showed that
the biocomposite material transmitted more than 80 % of the light, including surface reflection (Fresnel’s
reflection). When comparing the light transmittance of the nanocomposite material against that of pure
PHEMA, the degradation in light transmission due to the nanofiber network content of 1% (by weight) is
~ 10%, while it was less than 5% for the sample that had 0.05%. It is well-known that composite
materials suffer from increased light scattering, resulting in a loss of transparency, caused by differences
in the refractive indices of the materials in the composites. For the B.C. based nanocomposite, this loss of
transparency is small, despite the differences in BC and PHEMA refractive indices (refractive index of
cellulose fibre is 1.618 along the fibre and 1.544 in the transverse direction, and that of PHEMA is 1.49)
(Stickler and Rhein, 1992). These results clearly indicate the size effect of the nano-scaled fibres, which
allows it to combine with various optically functional materials having significantly different refractive
indices.
CONCLUSIONS
Novel optically transparent nano-structured composites were synthesized based on PHEMA hydrogel
matrix reinforced with surface modified B.C. nanofibers. This novel class of biocomposite has many
improved thermal and mechanical characteristics such as lightness, high flexibility and high transparency.
These characteristics will allow for a wide range of applications.
BC nanofibers were first acetylated heterogeneously (i.e., without dissolution) to improve the
transparency. Then, the acetylated fibers were then graft copolymerized with HEMA, and finally the
modified nanofibers were reinforced into continuous matrix of PHEMA hydrogel with different
nanofibrous contents. The nanofiber-network-reinforced PHEMA polymer composite maintained its
transparency up to 1wt. % of the modified cellulose nanofiber content, as it transmitted more than 80 % or
the light. For the BC based nanocomposite, this loss of transparency is small, which clearly indicates the
size effect of the nano-scaled fibres. This characteristic allows the cellulose nanofibers to combine with
various optically functional materials having significantly different refractive indices keeps the optical
transparency of the final composites.
ACKNOWLEDGMENT
This work was financially supported by awards from the Natural Sciences and Engineering Research
Council of Canada (NSERC).
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REFERENCES
1. Fontana, J., de Souza, A., Fontana, C., Torriani, I., Moreschi, J., Gallotti B., de Souza, S.,
Narcisco, G., Bichara, J. and Farah L. The Future Prospects of Microbial Cellulose in Biomedical
Applications; J Appl Biochem Biotechnol, 25/25 (1990), 253-264.
2. Deinema, M, and Zevehvizen, L. Formation of cellulose fibrils by gram-negative bacteria and
their role in bacterial flocculation, Arch Microbiol, 78 (1971), 42-51.
3. Ross, P., Mayer, R., and Benzimann, M. Cellulose biosynthesis and function in bacteria.
Microbiol, 55 (1991), 35-58.
4. Römling, U. Molecular biology of cellulose production in bacteria. Microbiol Res, 153 (2002),
205-212.
5. Karlson, J. and Gatenholm, P. Preparation and characterization of cellulose-supported HEMA
hydrogels, Polymer, 38/18 (1997), 4727-4731.
6. Seidel, J. and Malmonge S. Synthesis of polyHEMA hydrogels for using on biomaterial. Bulk and
solution radical-initiated polymerization techniques, Mater Res, 3/3 (2000), 79-83.
7. Koo, J., Smith, P. and Williams, R. Synthesis and characterization of methacrylate-based
copolymers for integrated optical applications; Chem Mater, 14 (2002), 5030-5036.
8. Tsou, T., Tang, S., Huang, Y., Wu, J., Young, J., and Wang, H. Poly(2-hydroxyethyl
methacrylate) wound dressing containing ciprofloxacin and its drug release; J Mater Sci: Mater
Med, 16/2 (2005), 95-100.
9. Nho Y. and Kwon, O. Blood compatibility of AAc, HEMA, and PEGMA-grafted cellulose film;
Radiat Phys Chem, 66 (2003), 299-307.
10. Nishioka, N., Yoshida, N. Thermal Decomposition of Cellulose/Synthetic Polymer Blends
Containing Graft Copolymers; Polymer J, 24/10 (1992), 1009-1015.
11. Iwamoto, S., Nakagaito, A. N., Yano, H., Nogi, M. Optically transparent composites reinforced
with plant fiber-based nanofibers; Appl Phys, 81 (2005), 1109 – 1112.
12. Nogi, M., Handa, K., Nakagito, A. N., Yano, H. Optically transparent bionanofiber composites
with low sensitivity to refractive index of the polymer matrix; Appl Phys Lett, 87, 243110 (2005);
DOI:10.1063/1.2146056
13. Yano, H.; Sugiyama, J., Nakagaito, A. N.; Nogi, M.; Matsuura, T.; Hikita, M.; Handa, K.;
Optically transparent composites reinforced with networks of bacterial nanofibres. Adv Mater,
17 (2005), 153 – 155.
14. Rana, D., Matsuurra, T., Khulbe, K. C.; Feng, C. Study on the spin probe added polymeric dense
membranes by 13C solid-state nuclear magnetic resonance spectroscopy; J Appl Polym Sci, 99
(2006), 3062-3069.
15. Lim, A., Schueneman, G. T., Novak, B. M. The T1ρ13C spin-lattice relaxation time of
interpenetrating networks by solid state NMR; Solid State Commun, 109 (1999), 465-470.
16. Udhardt, U., Hesse, S., Klemm, D. Analytical Investigations of Bacterial Cellulose; Macromol
Symp, 223 (2005), 201–202.
17. Princi, E., Vicini, S., Proietti, N., Capitani, D. Grafting polymerization on cellulose based
textiles; A 13C solide state NMR caractérisation; Eur Polym J, 41 (2005), 1196 – 1203.
18. Guruprasad, K. H., Shashidhara, G. M. Grafting, blending, and biodegradability of cellulose
acetate; J Appl Polym Sci, 91 (2004), 1716–1723.
19. Stickler, M.; Rhein, T., “Polymethacrylates” in Ullmann’s Encyclopedia of Industrial Chemistry,
5thed., Elvers, B., Hawkins, S.; Schultz, G. Eds., VHS: New York, 1992, A21, 473"
20. Joseph G., Rowe G., Margaritis A., Wan, W. Effects of polyacrylamide on bacterial cellulose production; J
Chem Technol Biotechnol, 78 (2003), 964 - 970.
21. Kim, D., Nishiyama, Y., Kuga, S. Surface acetylation of bacterial cellulose; Cellulose, 9 (2002),
361–367.
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