2. poly urethane resins were proposed [16ā20]. Yano et al. developed highly transparent composites based on BC membranes im-
pregnated with epoxy, acrylic and phenol-formaldehyde resins having a high ļ¬ber content (70 wt%) and outstanding mechanical
strength [21]. Pinto et al. synthesized ļ¬exible and transparent composite of BC and castor oil based polyurethane [22].
Diethyleneglycol ā 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 ļ¬lters, 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 ā bisallylcarbonate) nanocomposite by impregnating BC sheets with Diethyleneglycol ā
bisallylcarbonate resin and then inducing crosslinking by UV. The obtained transparent sheets were characterized by X-ray diļ¬raction
(XRD), ļ¬eld emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), UV ā 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 Ć 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 Ć 6 cm, 40um thick).
As photocurable monomer Diethyleneglycol ā bisallylcarbonate (DEAC, Sigma Aldrich), was adopted. IRGACUREĀ® 184, supplied
by Ciba was used as a highly eļ¬cient non-yellowing photo-initiator.
BC/DEAC composite ļ¬lms were prepared as follows. Firstly, 3% by weight IRGACUREĀ® 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 Ī¼W/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 200ā800 nm using a Cary 5000 UVāVis-NIR spectrophotometer
(Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a 150 mm PTFE-coated integrating sphere. Fourier transform
infrared spectroscopy (Attenuated Total Reļ¬ectance; 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 Diļ¬raction patterns were obtained with Rigaku Ultima
diļ¬ractometer, with Cu KĪ± radiations generated at 40 kV and 20 mA. The morphology and microstructure of BC and BC/DEAC
nanocomposite ļ¬lms 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 Ā°C. Speciļ¬c surface area
(SSA) was determined by multipoint Brunauer EmmettāTeller (BET) method using the adsorption data in the relative pressure range
of 0.05ā0.35. The pore size distribution and pore volume were calculated from the desorption isotherm using BarretāJoynerāHalenda
(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, Youngās
modulus, and strain to failure were calculated as an average of ļ¬ve test specimen data.
3. Result and discussion
Homogeneous and transparent BC/DEAC nanocomposite ļ¬lms of thickness 70ā80 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 ļ¬lms. The transmittances
at 550 nm are 44%, 88% and 92% for BC, BC/DEAC and neat resin, respectively. The high transparency of composite ļ¬lm compared
to neat BC ļ¬lm 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 ļ¬lms. XRD pattern of DEAC shows a very broad peak around 20Ā° revealing
the amorphous nature of the resin. For pure BC, broad diļ¬raction peaks observed at 15Ā° and 23Ā°, are characteristics of cellulose Ia and
Ib phasesā, showing semi crystalline nature of the cellulose polymer. The peak at 15Ā° corresponds to contribution of reļ¬ection from
monoclinic (110) and triclinic (100) planes and peak at 22.5Ā° corresponds to contribution of reļ¬ection from monoclinic (002) and
triclinic (110) planes [27]. BC/DEAC nanocomposite show similar diļ¬raction proļ¬le, suggesting that crystalline structure of BC is
not aļ¬ected 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) 192ā199
193
3. 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 ļ¬brils clutch into ļ¬at ribbon and ļ¬lamentary-shaped ļ¬bers
with a diameter ranging from 50 to 100 nm with an adequate porosity for resin inļ¬ltration. The surface of BC/DEAC in Fig. 3b shows
the BC reinforcement fully impregnated by resin, and the 3D network of cellulose nanoļ¬bers 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 35ā40 Ī¼m and the nano ļ¬bers were stacked tightly (Fig. 3c). Fig. 3d shows the cross section of the
BC/DEAC nanocomposite. A composite layer, 40 Ī¼m thick was sandwiched between two layers of 15 Ī¼m of resin. The ļ¬ber content in
the sandwich-like sample, estimated by weight diļ¬erence, 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 ļ¬bers 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 nanoļ¬bers impregnated by resin, i.e. ļ¬brillation 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 diļ¬raction pattern of DEAC, Neat BC and BC/DEAC composite ļ¬lm.
S. Kunjalukkal Padmanabhan et al. European Polymer Journal 93 (2017) 192ā199
194
4. 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.
S. Kunjalukkal Padmanabhan et al. European Polymer Journal 93 (2017) 192ā199
195
5. The nitrogen adsorptionādesorption isotherms of BC and BC/DEAC composite measured at ā196 Ā°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 speciļ¬c 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 speciļ¬c surface area
of 13 m2
/g whereas the surface area of DEAC impregnated BC ļ¬lm drastically changed to 0.7 m2
/g after resin inļ¬ltration. Fig. 4b
represents the pore size distribution of BC and BC/DEAC composite ļ¬lms. 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 signiļ¬cant 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 cmā1
) has completely disappeared and the peak
intensity of CeH asymmetric and symmetric stretching vibration (2952 and 2912 cmā1
) increases. This conļ¬rms that after 1 h, UV
treatment, polymerization of resin monomer was completed. For BC/resin composite, characteristic peaks of BC and cured resin were
identiļ¬ed. To the best of our knowledge, photo-polymerized conversion of DEAC is being carried out for the ļ¬rst 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 stressāstrain curves for BC, BC/resin composites and resin are shown in Fig. 6. The average stress at failure (MPa),
Youngās modulus (GPa) and strain at failure (%) of neat resin (DEAC), BC and BC/DEAC nanocomposite ļ¬lms are presented in
Table 2. The mechanical tests on BC/DEAC composites show a slight decrease of tensile strength (130 MPa) and Youngās 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
Speciļ¬c surface area, pore volume and porse size of BC and BC/DEAC composite.
Speciļ¬c surface area (m2
/g) Pore volume (cc/g) Pore diameter (nm)
BC 13 0.04 4
BC/resin 0.7 0.001 ā
Fig. 5. ATR-FTIR spectra of DEAC monomer, DEAC cured, neat BC and BC/DEAC composite.
S. Kunjalukkal Padmanabhan et al. European Polymer Journal 93 (2017) 192ā199
196
6. Youngās 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 nanoļ¬bers and their volume fraction, also results from
their orientation and mainly from the deformability of links among ļ¬bers. It is evident from Fig. 3a that bacteria are capable to
produce a complex network of cellulose ļ¬bers whose morphology strongly aļ¬ects 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 eļ¬ectively used to infer some average morphological features. In
particular, the micromechanic analysis can lead to the aspect ratio of nanoļ¬ller reinforcements [31,32].
In this case, the composite can be regarded as a laminate made of inļ¬nite unidirectional plies each one containing aligned 37 vol
% of BC ļ¬bers 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 ā like
sample (see Fig. 3b) reported in Table 2 (i.e. Ecm = 6.4 GPa), according to Eq. (2):
= + āEcm 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:
=
+
ā
=
ā
+
E
E
Ī¾Ī·V
Ī·V
Ī· Ī·
Ī¾
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 ļ¬bers, i.e. the ratio between the length, l,
of linear segments in the entangled network of BC ļ¬bers of Fig. 3a, and the ļ¬ber diameter, d. Vf is the volume fraction of the cellulose,
equal to 0.63, and Ef represents the Youngās modulus of the cellulose nanoļ¬bers.
The value of Ef was obtained using again a micromechanic approach: the model proposed by Eichhorn et al. [34], which assumed
that a cellulose ļ¬ber 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) ļ¬lms tested in tensile mode.
Table 2
Mechanical properties of BC, BC/DEAC composite and neat resin tested in tensile mode.
Stress at failure (MPa) Youngās modulus (GPa) Strain at failure (%)
BC 160 Ā± 12 9.5 Ā± 1 4.5 Ā± 0.5
BC/resin 130 Ā± 9 6.4 Ā± 0.8 3.7 Ā± 0.3
Neat resin 31 Ā± 5 1 Ā± 0.1 6.4 Ā± 0.5
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