Basically, native cellulose is mixture of two forms, Iα and Iβ celluloses. The Iα cellulose form corresponds to a single chain triclinic crystal while Iβ cellulose form corresponds to two parallel chain monoclinic crystal. The ratio of Iα and Iβ is vary depending on the cellulose source. The existence of 2 crystallographic phases for native cellulose, Iα and Iβ was demonstrated by Atalla and Vanderhart (1984) using 13C Solid State NMR. The other methods in determination of two phases of cellulose were elucidated by using infrared spectroscopy (Sugiyama et al., 1991a), electron microdifraction (Sugiyama et al., 1991b) and atomic force microscope (Baker et al., 1998).
Bacterial Cellulose (BC) produced by Acetobacter xylinum at the top of growing pellicle where cellulose production takes place. Cellulose is produced at medium surface originated from growing the pelicle in downward direction. Any pellicle placed on the surface and grown around and embedded into the cellulose matrics. Cellulose markedly different from cellulose produced from trees and cotton, high purity which free of hemicellulose, lignin and other non-cellulosic compounds. Most attractive characteristics of BC are extremely high crystallinity and ability to hold hundreds of times its weight in water. Cellulosic compound produced by Acetobacter xylinum has a linear β-1,4 glucose polymer. BC is composed of nanofibers determining properties of products. Nanofiber is constructed to sheet of cellulose chain, held together by hydrogen bonding within each sheet and by non-bonded interaction between sheet (Hirai et al., 1998). The unique properties provided by the nanofiber structure have lead to a number of commercial products. Recently, material based on this type of cellulosic compound become increasingly important, including tires, headphone membrane, special papers and textiles, electrical application including conductive membrane (Yoon et al., 2006).
In recent study, formation of Bacterial Cellulose Composites (BCC) prepared by Water Soluble Polymers (WSP) incorporation into short fiber matrix of homogenized BC. WSP including Carboxymethyl Cellulose (CMC) and glycerol solution were added to homogenized BC in which water contributes importantly for CMC and glycerol penetration. Balancing achievement of concentration of CMC and glycerol as important step inducing hardening and softening effects, respectively.
Therefore, structure and mechanical properties of the composites should be evaluated by comparison with untreated BC as control. Structure of composites including crystallinity index and crystal size was evaluated from X-ray diffraction profiles. Crystallinity index determined from the ratio of crystalline scattering versus total scattering which the amorphous contributed. Presence of WSP within BC structure influences mechanical properties of BCC as well. The measurement of mechanical properties was conducted by using Tensile Tester. Monitoring of mechanical properties including elastic modulus, tensile strength and elongation significantly changed.
MATERIALS AND METHODS
Materials: BC gel produced from fermentation of coconut water by Acetobacter xylinum under static condition and incubated at room temperature for several days to reach 5-6 mm in thickness. After incubation, the BC gel on the surface of media were harvested and extensively washed with water, boiled in 1% sodium hydroxide to remove protein and non-cellulosic compounds and neutralized by running tap water. BC gel was homogenized by blender operated at full speed for 5 min providing pseudoplastic behaviour which contains 0.6% w/v cellulosic fibers. The homogenized BC was stored at 5°C in distillated water.
Carboxymethylcellulose (CMC) used in this study soluble in water at room temperature with substitution degree (DS) of 0.7 producing viscosity 500-900 cps at concentration of 1% (Brookfield LV 30 rpm). Homogenized BC poured to CMC solution slowly under heating 50°C. The concentrations of CMC in homogenized BC are 0% (BC), 0.25% (BCC C1), 0.5% (BCC C2), 0.75% (BCC C3) and kept on temperature of 50°C to get a homogenous solution. Subsequently, 0.25 and 0.5% glycerol concentrations were added to BC, BCC C1, BCC C2, BCC C3 solutions as BCC G, BCC C1G, BCC C2G, BCC C3G and BCC G1, BCC C1G1, BCC C2G1, BCC C3G1, respectively. Air removal of the solution was conducted by ultrasonic vibration. The solution was casted onto teflon tray and dried in gear oven at temperature of 40°C.
X-ray diffraction: X-ray diffraction profiles of Bacterial Cellulose
(BC) and Bacterial Cellulose Composites (BCC) were recorded by using X-Ray diffraction
Rigaku RINT 2500, using Ni filtered Cu-Kα radiation (λ = 0.1542 nm).
The operating voltage and current were 35 kV and 40 mA, respectively. The scanning
of 2θ (10-40°) was employed with speed of 5° min-1.
The relative crystallinity index was estimated by Segals method using
the following equation:
Where, I(002) is the peak intensity corresponding to the (110) plane at 2θ = 22.7° for the overall intensity and Iam is the peak intensity of amorphous fraction at 2θ = 18°.
The apparent lateral width of the crystals was calculated for most intens peak at 2θ = 22.7°, using the Scherrer formula:
||Diffraction line-width at half peak size
||Constant usually taken as 0.9 for cellulose (Klug and Alexander, 1954)
Tensile testing: Specimens were mounted into 5 N load cell of Universal Tensile Machine, Tensilon, Orientec, type UCT-5T. Dumbell type used for specimen cutting is ISO 527 type 5B. One load cell statically fixed with the tensile machine frame and the second one fixed with the moveable part of the machine. Specimens were strained at displacement rate of 1 mm min-1. All specimens were deformed under controlled room condition at temperature of 23±1°C and humidity of 50±5%.
RESULTS AND DISCUSSION
The X-ray diffraction profiles of Bacterial Cellulose (BC) and Bacterial Cellulose
Composites (BCC) are shown in Fig. 1. BC established as Iα
cellulose, indicated in presence of d spacing 6.17, 5.235 and 3.91 nm as calculated
by Sugiyama et al. (1991b). Addition of CMC (0, 0.25, 0.5, 0.75%) and
glycerol (0, 0.25, 0.5%) to BC forms a slight different structure compared with
BC structure as control. Basically, BC and BCC have similar profiles arranged
in three peaks at 2θ = 15°, 17° and 22.7° as assigned to (100),
(010), (110), respectively. Apart from a slight shoulder at scattering angle
of (100) (2θ = 15°) and (110) (2θ = 22.7°) indicates the presence
of high crystalline portion of Iα cellulose in BC structure. Most composites
pointed out conversion d spacing of (100) at 0.617 to 0.607 nm. According to
Sugiyama et al. (1991b), the presence of d spacing of 0.607 nm assigned
as Iβ cellulose contribution. Scattering plane of (100) is most closely
occupied by hydroxyl group as studied by Takai et al. (1975). Hence
the scattering plane is most sensitive if any disturbance within structure.
In the crystal structure of cellulose, the scattering plane has the largest
number of hydroxyl cellulose since the hydroxyl groups are largely responsible
for the sorption of water.
X-Ray diffraction profiles of BC (control) and BCC (a) Variation
of CMC concentrations (0-0.75%) at glycerol concentration of 0%, (b) Variation
of CMC concentrations (0-0.75%) at glycerol concentration of 0.25% and (c)
Variation of CMC concentrations (0-0.75%) at glycerol concentration of 0.5%
Low concentrations of Water Soluble Polymers (WSP), CMC and glycerol were able
to make a broadening of scattering planes (110) and (100). Broadening peaks
indicate a change of crystalline portion. Low concentrations of CMC (0-0.75%)
interpenetrating to BC network are unable to change structure conformation by
shift in scattering plane (010) at 2θ = 17° to 2θ = 20.4°
as scattering plane of cellulose II.
Change of the composite profiles after incorporation indicates a slight shoulder at scattering planes of (100) and (110). The evaluation of crystallinity index was estimated by Segals method leading to crystalline portion of Iα cellulose in the composite decreased from the highest crystallinity index of 81.0% to the lowest crystallinity index of 51.2%, as shown in Fig. 2. Increasing CMC concentrations from 0 to 0.75% gives considerably crystallinity index reduction from 81% (control) to 61% (BCC C3), 76% (BCC G) to 53% (BCC C3G), 70% (BCC G1) to 50% (BCC C3G1) corresponding to glycerol concentrations of 0, 0.25 and 0.5%, respectively.
Crystallinity index of BC (control) and BCC, (•)
Variation of CMC concentrations (0-0.75%) at glycerol concentration of 0%;
(ο) Variation of CMC concentrations (0-0.75%) at glycerol concentration
of 0.25%; (▲) Variation of CMC
concentrations (0-0.75%) at glycerol concentration of 0.5%
Contribution of CMC and glycerol in BCC formation is also monitored by conversion of 5-37.5% crystalline to amorphous portion which affinity of CMC and glycerol have important role in amorphous structure formation of BCC.
Mechanical properties of BC (control) and BCC
Based on the X-ray diffraction profiles is determined crystal size of the composites
using Sherer formula. Evaluation in crystal size of (110) as the most intense
scattering plane corresponded to sheet of hydrogen bonded chains, approximated
from a half peak width of (110) at 2θ = ~22.7°. Addition of CMC and
glycerol shows a little effect on alteration crystal size from 1.4 nm for BC
to 1.1 nm for the composites. Hydrogen bonding present in homogenized BC forms
weak inter and intramolecular force due to short fibers formation. CMC act as
deflocculating agents causing cellulose fibers disperse in suspension and establish
a network of solution. Consequently, less aggregation of a crystallized cellulose
is probable to arise during drying. Weak hydrogen bonding between cellulose
and CMC predicted to emerge as crystalline prevention. Solubilization of glycerol
in water could easily penetrate cellulose fiber to prevent crystalline.
Addition of CMC and glycerol to homogenized BC formed composite structures
with a change of mechanical properties compared with BC as control. The increasing
CMC concentrations to the composites of BCC to BCC C3, BCC G to BCC C3G, BCC
G1 to BCC C3G1 exhibit increasing tensile strength from 64.8 to 167.7, 52.7
to 78.1, 17.5 to 41.3 MPa and Elastic modulus from 2907 to 6245, 180.4 to 1581
and 30.5 to 212.5 MPa, respectively. Treatment of glycerol (0 to 0.5%) has a
trend toward reducing tensile strength from 167.7 to 17.5 MPa and Elastic modulus
from 6245.0 to 30.5 MPa (Table 1). Otherwise, glycerol treatment
gradually increased elongation from 2.5 to 29.6%. The probable cause of high
elongation caused by presence of hydrophilic alcohol hydroxyl group. In any
case, CMC builds-up cellulose backbond inducing line in order of structure to
be more crystalline. CMC reduces elongation of composites to be more breakable.
Dispersion of cellulose fiber in colloid solution interact between themselves
in order to build up a network which provides a geling structure. CMC within
homogenized BC ensure easier dispersion providing good bonded BC fibers. CMC
as deflocculating agent can be useful in employing homogenized BC in uniform
distribution inducing enhancement fiber to fiber bonding to make stronger sheet
Referred to mechanical properties and structure evaluation, CMC and glycerol play important role in modification of BC characteristic. Balancing CMC and glycerol concentrations within homogenized BC open the opportunity to improve BC characteristic to be friendly water soluble plastic. The importance of water contribution is emphasized in composite formation. Therefore, further study relating to improvement of BC characteristic should be conducted.