Effect of Water Soluble Polymer on Structure and Mechanical Properties of Bacterial Cellulose Composites

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.

Methods
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 Segal’s 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:

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.

Fig. 1:	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 Segal’s 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.

Fig. 2:	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.

Table 1:	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 and durable.

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.