Resine e Cellulosa

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European Polymer Journal
journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
PolyDiethyleneglycol–bisallyl carbonate matrix transparent
nanocomposites reinforced with bacterial cellulose microfibrils
Sanosh Kunjalukkal Padmanabhan
⁎
, Carola Esposito Corcione, Rossella Nisi,
Alfonso Maffezzoli, Antonio Licciulli
Department of Engineering for Innovation, University of Salento, Lecce 73100, Italy
A R T I C L E I N F O
Keywords:
Transparent
Bacterial cellulose
Polycarbonate
Nanocomposite
A B S T R A C T
Transparent nanocomposite films were prepared using bacterial cellulose (BC) as reinforcement
and diethylene glycol bis(allyl carbonate) polymer (DEAC) as matrix by vacuum infiltration and
UV polymerization. The BC/DEAC nanocomposites exhibit excellent transparency up to 88% at
wavelength of 550 nm. The uniform dispersion of resin in BC 3D network was evidenced from
SEM and ATR-FTIR analyses, confirms the complete photo-polymerization of diethylene glycol
bis(allyl)carbonate monomer to Poly (diethylene glycol bis(allyl carbonate) in BC network. The
BC/DEAC composites have good mechanical properties, reaching a tensile strength of 130 MPa
and a Young’s Modulus of 6.4 GPa. Applying a micromechanic modeling approach, the elastic
modulus of the composite was used in order to determine the average aspect ratio of BC fibers.
These flexible transparent BC/DEAC composite films can be considered as functional films for
optoelectronics application.
1. Introduction
Bacterial cellulose BC, is an extracellular product of Acetobacter xylinum bacteria, structured in a web-like network. It consists of
ribbon-shaped nanofibers with typical diameter from 10 to 50 nm [1,2]. BC is high purity cellulose without any contaminant such as
hemicellulose and lignin as found in wood cellulose [3]. Bacterial cellulose, a renewable natural nanomaterial having excellent
physical properties, has been proposed for many applications such as tissue engineering, electronics industry, food packaging and
cosmetics [4–6]. Bacterial cellulose is under investigation as reinforcing agent for the design of environmentally friendly nano-
composites due to its high degree of polymerization (14,400 Da), crystallinity (89%) and specific area (37 m2
/g). These structural
properties are accompanied by an excellent moldability and high mechanical properties: a dried BC film can reach a tensile strength
of 200 MPa [7–11].
The development of new transparent films for electronic devices and packaging applications represents a promising field of
research [12–14]. Flexible substrates for optoelectronic applications are requested to be transparent and with a low thermal ex-
pansion coefficient to match that of printed electronic circuits.
During the last few years many researches have been devoted to the use of bacterial cellulose nanofibers as reinforcement in the
preparation of optically transparent materials. These materials were characterized by a low thermal-expansion coefficient, as small as
0.1 × 10−6
1/K, which is an important property in optoelectronic devices [15].
Many different approaches have been explored for the fabrication of transparent nanocomposites based on BC and different
polymer matrices. Among others, chitosan polyhydroxybutirate, polyvinyl alcohol, boehmite-epoxi-siloxane, poly-(L-lactic acid), and
http://dx.doi.org/10.1016/j.eurpolymj.2017.05.037
Received 22 April 2017; Received in revised form 18 May 2017; Accepted 21 May 2017
⁎
Corresponding author.
E-mail addresses: Sanosh.padmanabhan@unisalento.it, Sanosh2001@gmail.com (S. Kunjalukkal Padmanabhan).
European Polymer Journal 93 (2017) 192–199
Available online 31 May 2017
0014-3057/ © 2017 Elsevier Ltd. All rights reserved.
MARK

poly urethane resins were proposed [16–20]. Yano et al. developed highly transparent composites based on BC membranes im-
pregnated with epoxy, acrylic and phenol-formaldehyde resins having a high fiber content (70 wt%) and outstanding mechanical
strength [21]. Pinto et al. synthesized flexible and transparent composite of BC and castor oil based polyurethane [22].
Diethyleneglycol – bisallylcarbonate (DEAC) monomer, primarily used to produce Poly(diethylene glycol bis(allyl carbonate) and
commercially known as CR-39 resins for optical application, provides exceptional clarity and durability [23,24]. DEAC thermal cured
products are typically water white, highly transparent plastic that resemble glass, but are safer, lighter and tougher widely used for
manufacturing lenses, safety shields filters, sensors and touch screens. It has a refractive index of 1.50, an excellent resistance to
chemicals and UV light, better scratch resistance than other transparent plastics [25].
Till now there have been no reports on the composites of bacterial cellulose reinforced with UV curable DEAC resins. In this paper,
we prepared BC/Poly (Diethyleneglycol – bisallylcarbonate) nanocomposite by impregnating BC sheets with Diethyleneglycol –
bisallylcarbonate resin and then inducing crosslinking by UV. The obtained transparent sheets were characterized by X-ray diffraction
(XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), UV – visible spec-
troscopy and mechanical analysis and compared with neat BC and resin sheets.
2. Experimental
2.1. Materials and method
Bacterial cellulose (BC) in the form of hydrogel pellicles (7 × 6 cm, 5 mm thick) were produced from cultures of Acetobacter
strains by Biofaber srl (Italy). They are composed of 99% water and 1% cellulose. The hydrated membranes were kept in between two
glass plates and dried at 60 C to get dried BC membranes (7 × 6 cm, 40um thick).
As photocurable monomer Diethyleneglycol – bisallylcarbonate (DEAC, Sigma Aldrich), was adopted. IRGACURE® 184, supplied
by Ciba was used as a highly efficient non-yellowing photo-initiator.
BC/DEAC composite films were prepared as follows. Firstly, 3% by weight IRGACURE® 184 photoinitiator was dissolved in DEAC
stirring for 30 min at room temperature. Dried BC membranes were impregnated in this photo curable resin in a vacuum desiccator
under reduced pressure for 24 h. After impregnation, the excess resin on the surface of the membrane was carefully wiped out and the
membrane placed between two glass plates at a preset distance and cured using a pressure Hg UV lamp (UV HG 200 ULTRA, Ultra
Electronics, London, UK), with a radiation intensity on the surface of the sample of 9.60 μW/mm2
at 365 nm working in air atmo-
sphere for 1 h. Pure DEAC resin sheets were fabricated in similar manner to be used as reference material.
2.2. Characterization
Light transmittance was evaluated in the wavelength range 200–800 nm using a Cary 5000 UV–Vis-NIR spectrophotometer
(Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a 150 mm PTFE-coated integrating sphere. Fourier transform
infrared spectroscopy (Attenuated Total Reflectance; Perkin Elmer) with diamond crystal as a probe was used to evaluate the re-
activity of the liquid mixtures to complete the polymerization reaction. X-ray Diffraction patterns were obtained with Rigaku Ultima
diffractometer, with Cu Kα radiations generated at 40 kV and 20 mA. The morphology and microstructure of BC and BC/DEAC
nanocomposite films were investigated by a Field Emission Scanning Electron Microscope (FESEM) (Zeiss Sigma VP, Carl Zeiss
Microscopy GmbH, Jena, Germany). The total surface area of BC and BC/nanocomposites were measured by nitrogen adsorption
using an NOVA 2000e (Quantachrome Instruments, USA) apparatus. The samples were degassed for 3 h at 80 °C. Specific surface area
(SSA) was determined by multipoint Brunauer Emmett–Teller (BET) method using the adsorption data in the relative pressure range
of 0.05–0.35. The pore size distribution and pore volume were calculated from the desorption isotherm using Barret–Joyner–Halenda
(BJH) method. Tensile tests were performed on specimens of 20 mm length and 5 mm width [26] at room temperature using a Lloyd
LR50 K dynamometer equipped with a load cell of 1kN and imposing a crosshead speed of 0.5 mm/min. Tensile strength, Young’s
modulus, and strain to failure were calculated as an average of five test specimen data.
3. Result and discussion
Homogeneous and transparent BC/DEAC nanocomposite films of thickness 70–80 m were obtained without any visible porosity
and defects by our process. Fig. 1a shows the optical transmission spectra of BC, BC /DEAC and pure DEAC films. The transmittances
at 550 nm are 44%, 88% and 92% for BC, BC/DEAC and neat resin, respectively. The high transparency of composite film compared
to neat BC film was obtained thanks to the matrix, which has a refractive index (1.5) slightly lower than refractive index of BC (1.581)
[21,25]. Fig. 1b and c shows the images of opaque dried BC and transparent BC/resin sheets respectively.
Fig. 2 shows XRD patterns of DEAC, BC and BC/DEAC films. XRD pattern of DEAC shows a very broad peak around 20° revealing
the amorphous nature of the resin. For pure BC, broad diffraction peaks observed at 15° and 23°, are characteristics of cellulose Ia and
Ib phases’, showing semi crystalline nature of the cellulose polymer. The peak at 15° corresponds to contribution of reflection from
monoclinic (110) and triclinic (100) planes and peak at 22.5° corresponds to contribution of reflection from monoclinic (002) and
triclinic (110) planes [27]. BC/DEAC nanocomposite show similar diffraction profile, suggesting that crystalline structure of BC is
not affected by experimental procedure adopted for the composite preparation. The relative crystallinity of BC and BC/Resin com-
posite was calculated using equations proposed by Segal [28] and indicates a slight decrease in crystallinity being around 76% for
BC/DEAC nanocomposite and 78% for neat BC. This negligible decrease in crystallanity for BC/DEAC nanocomposite probably
S. Kunjalukkal Padmanabhan et al. European Polymer Journal 93 (2017) 192–199
193

occurred as a result of breakdown of inter-chain hydroxyl hydrogen bonds during penetration of resin into the cellulose chains [29].
SEM images of the surface of the dried BC and BC/DEAC nanocomposite are shown in Figs. 3a and b, respectively. The BC
morphology as evidenced in Fig. 3a, is a compact 3D network of BC nano fibrils clutch into flat ribbon and filamentary-shaped fibers
with a diameter ranging from 50 to 100 nm with an adequate porosity for resin infiltration. The surface of BC/DEAC in Fig. 3b shows
the BC reinforcement fully impregnated by resin, and the 3D network of cellulose nanofibers on the surface completely disappeared
after impregnation with resin. Figs. 3c and 3d shows the cross section images of BC and BC/DEAC nanocomposite respectively. The
thickness of BC sheet was around 35–40 μm and the nano fibers were stacked tightly (Fig. 3c). Fig. 3d shows the cross section of the
BC/DEAC nanocomposite. A composite layer, 40 μm thick was sandwiched between two layers of 15 μm of resin. The fiber content in
the sandwich-like sample, estimated by weight difference, was 40%, corresponding to a volume fraction Vf = 0,63% in the composite
(calculate using theoretical density of cellulose (1.25 g/cm3
) and resin (1.1 g/cm3
) by volume. Fig. 3e and f shows the fracture surface
images of BC and BC/DEAC composite respectively. Fracture surface of BC shows ribbon shaped fibers loosely spaced (Fig. 3e). In the
case of composite (Fig. 3f), the resin penetrated through the ribbon network structure of BC, resulting in tightly compacted layers of
BC nanofibers impregnated by resin, i.e. fibrillation is not observed.
Fig. 1. (A) Optical transmission spectra of DEAC, BC and BC/DEAC, Images of opaque dried BC sheet (B) and transparent BC/DEAC composite (C).
Fig. 2. X-ray diffraction pattern of DEAC, Neat BC and BC/DEAC composite film.
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Fig. 3. SEM image of BC (A, C and E) and BC/DEAC composite (B, D, F).
Fig. 4. (A) N2 Adsorption/desorption isotherm of BC and BC/DEAC composite, (B) pore size distribution of BC and BC/DEAC composite.
195

The nitrogen adsorption–desorption isotherms of BC and BC/DEAC composite measured at −196 °C, shown in Fig. 4a, BC have a
typical type IV adsorption behavior corresponding to the mesoporous structure of the material. In case of BC/DEAC composite the
isotherm shows type II adsorption behavior indicating the non-porous nature of the material. The specific surface area (SSA) obtained
by BET method and pore volume and pore size calculated by BJH method are given in Table 1. Neat BC shows a specific surface area
of 13 m2
/g whereas the surface area of DEAC impregnated BC film drastically changed to 0.7 m2
/g after resin infiltration. Fig. 4b
represents the pore size distribution of BC and BC/DEAC composite films. BC shows a pore volume of 0.04 cc/g and a pore diameter
of 4 nm in the mesoporous range, whereas BC/DEAC shows a very low pore volume of 0.001 cc/g and without any significant pore
size distribution. These results are supported by the SEM observation.
In order to analyze the photo-polymerization conversion of DEAC, in presence or absence of BC, FTIR spectrum of the samples
were measured. Fig. 5 shows the FT-IR-ATR spectra of photo-initiated DEAC monomer, UV cured DEAC, BC and BC/DEAC composite.
After UV curing the peak of CH]CH2e stretching vibration (3074 and 1650 cm−1
) has completely disappeared and the peak
intensity of CeH asymmetric and symmetric stretching vibration (2952 and 2912 cm−1
) increases. This confirms that after 1 h, UV
treatment, polymerization of resin monomer was completed. For BC/resin composite, characteristic peaks of BC and cured resin were
identified. To the best of our knowledge, photo-polymerized conversion of DEAC is being carried out for the first time and it presents
the advantage of a very fast rate of photo-polymerization to obtain a complete cure of the resin, by avoiding the use of high
temperature and oven compared to thermal curing process [30].
Typical tensile stress–strain curves for BC, BC/resin composites and resin are shown in Fig. 6. The average stress at failure (MPa),
Young’s modulus (GPa) and strain at failure (%) of neat resin (DEAC), BC and BC/DEAC nanocomposite films are presented in
Table 2. The mechanical tests on BC/DEAC composites show a slight decrease of tensile strength (130 MPa) and Young’s modulus
(6.4 GPa) compared to pure BC sheet (160 MPa and 9.5 GPa). On the other hand, a prominent increase of both tensile strength and
Table 1
Specific surface area, pore volume and porse size of BC and BC/DEAC composite.
Specific surface area (m2
/g) Pore volume (cc/g) Pore diameter (nm)
BC 13 0.04 4
BC/resin 0.7 0.001 –
Fig. 5. ATR-FTIR spectra of DEAC monomer, DEAC cured, neat BC and BC/DEAC composite.
196

Young’s modulus was obtained for composite samples in comparison to neat resin (31 MPa and 1 GPa, respectively). The elastic
modulus of BC, although depending on the mechanical properties of cellulose nanofibers and their volume fraction, also results from
their orientation and mainly from the deformability of links among fibers. It is evident from Fig. 3a that bacteria are capable to
produce a complex network of cellulose fibers whose morphology strongly affects modulus and strength of neat BC.
As reported in previous studies on nanocomposites, the measurements of macroscopic properties, such as elastic modulus, gas
permeability, and thermal conductivity, of a nanocomposite can be effectively used to infer some average morphological features. In
particular, the micromechanic analysis can lead to the aspect ratio of nanofiller reinforcements [31,32].
In this case, the composite can be regarded as a laminate made of infinite unidirectional plies each one containing aligned 37 vol
% of BC fibers and characterized by a longitudinal modulus E1 and transversal modulus E2. With these assumptions, the composite
modulus Ec is given by Eq. (1) [33]:
= +E 1/5E 4/5Ec 1 2 (1)
The modulus of the composite Ec (equal to 10.05 GPa) was calculated starting from the measured modulus of the sandwich – like
sample (see Fig. 3b) reported in Table 2 (i.e. Ecm = 6.4 GPa), according to Eq. (2):
= + −Ecm EcVc Em Vc(1 ) (2)
where Vc is the composite volume fraction calculated from Fig. 3b (i.e. 0.37) and Em is the modulus of the resin (i.e. 1.54 GPa),
according to technical data sheet of the resin.
E1 and E2 can be obtained by Halpin-Tsai equations:
=
+
−
=
−
+
E
E
ξηV
ηV
η η
ξ
1
1
given by
1
m
f
f
E
E
E
E
f
m
f
m (3)
where E can be either E1 or E2 of an unidirectional composite ply, Em is the modulus of the matrix equal to 1.56 GPa, according to
technical data sheet. The parameter = 2 l/d depends on the aspect ratio of the reinforcing fibers, i.e. the ratio between the length, l,
of linear segments in the entangled network of BC fibers of Fig. 3a, and the fiber diameter, d. Vf is the volume fraction of the cellulose,
equal to 0.63, and Ef represents the Young’s modulus of the cellulose nanofibers.
The value of Ef was obtained using again a micromechanic approach: the model proposed by Eichhorn et al. [34], which assumed
that a cellulose fiber is again a composite consisting of cellulose crystals as reinforcement in an amorphous cellulose matrix.
Eichorn et al. compared several literature data with the parallel and series arrangement of matrix (amorphous cellulose) and
Fig. 6. Stress-strain curve of BC, BC/DEAC and neat Resin (DEAC) films tested in tensile mode.
Table 2
Mechanical properties of BC, BC/DEAC composite and neat resin tested in tensile mode.
Stress at failure (MPa) Young’s modulus (GPa) Strain at failure (%)
BC 160 ± 12 9.5 ± 1 4.5 ± 0.5
BC/resin 130 ± 9 6.4 ± 0.8 3.7 ± 0.3
Neat resin 31 ± 5 1 ± 0.1 6.4 ± 0.5
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reinforcement (cellulose crystals) and with Eq. (3) model. The latter well fits the modulus of cellulose fibers with different crystal
content, considering a typical aspect ratio found in microcrystalline cellulose extracted from vegetal. More recently Guhados et al.
[35] found a better agreement between measured modulus of BC fibers and the mentioned parallel model for a BC with 60%
crystallinity.
The Young’s modulus (Ef) of the BC fibers used in this study, characterized by a degree of crystallinity of 78%, resulted equal to
60 GPa or 100 GPa, by applying the series model or Eq. (3), respectively.
The transverse modulus E2 appearing in Eq. (3), can be calculated using ξ = 2 in Eq. (3). By combining Eqs. (1) and (3) and
assuming Ef equal to 60 GPa and to 100 GPa, it was possible to calculate the parameter ξ as the only unknown, i.e. to determine the
average aspect ratio, l/d, of linear segments of the cellulose fibers in the BC web, ranging from 8 to 10. Further the average length of
the fiber, obtained from ξ using the average diameter of BC fiber from several SEM images, 45 ± 10, was in the range 225–180 nm.
The ultimate properties of such composites cannot be reliably obtained from micromechanic theories. However, it should be noted
that the tensile properties of this BC/DEAC nanocomposite films are much higher than the transparent BC/polyurethane composite
films whose tensile strength are usually in a range of 65–69 MPa and used as substrates for flexible OLEDs [22]. The strain at failure
for transparent sheet was 3.7%, indicating an adequately ductile behavior for a composite material.
4. Conclusions
Novel transparent composite films were obtained by infiltrating with diethylene glycol bis(allyl carbonate) resin on a nanos-
tructured fibrous preform of Bacterial cellulose (BC) produced by Acetobacter. SEM analysis shows that uniform and completely filled
composite was obtained after UV curing. Specific surface area was also commendably decreased after resin infiltration. The photo-
polymerization was very fast compared to conventional thermal curing of DEAC monomer and ATR FTIR confirms that poly-
merization reaction is completed after UV irradiation. The obtained BC/DEAC composite film is transparent (88% at 550 nm) and
shows significant improvement of mechanical properties compared to neat resin films. BC/resin composite have a tensile strength of
130 MPa, Young’s Modulus of 6.4 GPa and strain at failure of 3.5%. These outstanding properties enable this material to be in-
troduced as a promising candidate for applications in transparent packaging and electronic industry.
Acknowledgments
Mr. Donato Cannoletta is kindly acknowledged for XRD measurements, Dr. Fabio Marzo for SEM analysis and Dr Sudipto Kumar
Pal for Optical transmission measurements. Authors also thanks to Dr Mariangela Stoppa (Biofaber srl) for bacterial cellulose samples.
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