Structural features and properties of BC em meio agitado

Structural features and properties of BC em meio agitado

(Parte 2 de 3)

Table 1 also shows the difference in Bragg angle between peak 1 and peak 2 in Fig. 2. The difference for Ag-BC is less than that for St-BC. This suggests that Ag-BC has a lower content of cellulose Iá than St-BC. The reason is as follows: it was reported that the heat treatment induces the irreversible crystal transformation from cellulose Iá to cellulose Iâ accompanying the decrease in d-spacing and the shift of peak 1 to the

Peak 3 (a)

(b)

Dif fraction intensity (a.u.)

Peak 1 Peak 2

FIGURE 2. X-ray diffraction patterns of bacterial celluloses in static and agitated cultures. (a) Agitated culture; (b) static culture.

TABLE 1. Structural features of bacterial celluloses produced in agitated and static culture conditions

Structural feature Agitated culture Static culture

aDetermined by X-ray diffractometry. bEstimated as crystallite size of a crystallographic plane (110) by X-ray diffractometry. cDifference in Bragg angle between peak 1 and peak 2 in the X-ray diffraction curve (Fig. 2) dDetermined by CP=MAS 13C NMR.

192 WATANABE ET AL.

wider angle on the X-ray diffraction pattern (Yamamoto et al., 1989). Peak 2 must also shift to the smaller angle when the content of cellulose Iá decreases because of the difference in the unit cells of cellulose Iá and Iâ (Sugiyama et al., 1991). CP=MAS 13C NMR specra of Ag-BC and St-BC are shown in Fig. 3. In the spectrum of St-BC, the enhanced down®eld resonance line for the C4 triplet at 91± 89 ppm and the strong central resonance line for the C1 triplet at 106 ppm indicates that cellulose Iá is dominant in this sample. In the case of Ag-BC, there is a slight decrease in the intensities of the corresponding resonance lines for the C4 and C1 triplets, suggesting the decrease in the mass fraction of cellulose Iá. Broader up®eld resonance lines for C4 at about 84 ppm and for C6 at about 62 ppm, which were assigned to the non-crystalline component of cellulose or to the contribution from the surface of cellulose crystallites (Horii et al., 1982; VanderHart and Attala, 1984) increase in intensity for Ag-BC compared with those for St-BC. This result can be interpreted in terms of the decrease in degree of crystallinity and=or the decrease in crystallite size for Ag-BC, which were revealed by the X-ray diffraction study described above.

To determine the mass fractions of cellulose Iá and cellulose Iâ, line shape analysis was performed for the CP=MAS 13C NMR spectra according to the method of

Yamamoto et al. (1993) and Yamamoto and Horii (1994). The mass fraction of the down®eld component in the C4 line was determined as the degree of crystallinity by the integration of the C4 up®eld lines because this fraction was reported to agree approximately with the degree of crystallinity determined by the X-ray diffraction method (Horii et al, 1982).

The crystallinity and the content of cellulose Iá for Ag-BC were found to be lower than those for St-BC (Table 1). It was also found that the difference in cellulose Iá content was larger than that in crystallinity (Table 1). It was reported that the mass fraction of cellulose Iá was closely related to the crystallite size of micro®brils of bacterial cellulose (Yamamoto et al., 1993, 1996). In the agitated culture, the interference in the crystallization process of bacterial cellulose may lead to the formation of crystallites of a smaller size; as a result of this the formation process of cellulose Iâ may be preferentially induced.

FIGURE 3. CP=MAS 13C NMR spectra of bacterial celluloses produced in static and agitated cultures. (a) Agitated culture; (b) static culture.

(a) C1 C2,3,5

C4 C6 ppm

Intensity (a.u.)

Intensity (a.u.)

(b)

STRUCTURE AND PROPERTIES OF BACTERIAL CELLULOSE 193

Fig. 4 shows DP distribution patterns of the bacterial celluloses from agitated and static cultures. Ag-BC shows an obvious subpeak on the lower SP side. This subpeak corresponds to cellulose molecules composed of 10±1000 glucosidic residues. In contrast, there is no subpeak in St-BC. As a result, Ag-BC has a lower DPw than St- BC as shown in Table 1.

On the basis of the above ®ndings,. As a result, Ag-BC has a lower DPw than St-BC as shown in Table 1.

On the basis of the above ®ndings, it was revealed that Ag-BC had a somewhat disordered structure at different hierarchical levels, such as morphology of cellulose

®brils forming reticulated structure, crystallinity, cellulose Iá content and degree of polymerization.

These structural features of Ag-BC are thought to relate to CMCase, which is produced by Acetobacter xylinum as reported previously (Tahara et al., 1997). The CMCase was found to induce a decrease in DPw of bacterial cellulose leading to the appearance of the subpeak on the lower DP side (Tahara et al., 1997). In our study, the CMCase activity in the medium at the end of culture was measured to be 2300 U=ml for the agitated culture and 200 U=ml for the static culture. These results suggest that the higher CMCase activity in the agitated culture relates to the presence of the lower DPw fraction in Ag-BC.

Some properties relating to commercial applications

In order to clarify some properties relating to the commercial applications, we measured the Young's modulus of the dried sheets, water holding capacity and viscosity of disintegrated products.

The Young's modulus of the sheet prepared from Ag-BC was 28.3 GPa, which was slightly lower than the value of St-BC, 3.3 GPa (Table 2). The low Young's modulus of Ag-BC may be due to the structural disorder of cellulose shown in Table 1.

(a) (b)

Degree of polymerization

Weight distribution (a.u.)

FIGURE 4. Degree of polymerization distribution patterns of bacterial celluloses produced in static and agitated cultures. (a) Agitated culture; (b) static culture.

194 WATANABE ET AL.

Fig. 5 shows the water holding capacity (WHC) of disintegrated BC. WHC means the weight of water held per unit weight of cellulose ®brils forming a reticulated structure. WHC decreases with an increase in the centrifugal force. This result indicates that water is trapped physically at the surface and on the inside of the particles composed of the reticulated ®brils. In fact, the amount of the bound water in bacterial cellulose was reported to be negligible (Okiyama et al., 1992). Under any of the conditions examined, the WHC of Ag-BC is found to be higher than that of St-BC.

Fig. 6 represents the viscosity of the suspension containing disintegrated Ag-BC or

St-BC as a function of the cellulose concentration. The viscosity of disintegrated Ag- BC suspension is signi®cantly higher than that of St-BC.

TABLE 2. Structural features of bacterial celluloses produced in agitated or static culture conditions

Property Agitated culture Static culture

Young's modulus of sheet (GPa) 28.3 3.3 Water holding capacity (g water=g cellulose) 170 45 Viscosity (Pas) 0.52 0.04 Filler retention aid functiona (%) 43 38 Emulsion stability indexb (%) 83 60 aReported previously (Hioki et al., 1995). bReported previously (Ougiya et al., 1997).

Centrifugation (G)

Water Holding Capacity (g-water/g-cellulose)

FIGURE 5. Water holding capacity of disintegrated bacterial cellulose as a function of the centrifugal force. s, Agitated culture; d, static culture.

STRUCTURE AND PROPERTIES OF BACTERIAL CELLULOSE 195

As described above, Ag-BC has higher WHC and viscosity, although Ag-BC has relatively disordered structure and a slightly lower Young's modulus of the sheet than St-BC (Table 2).

Relationship between structural features and properties

Fig. 7 shows the particle size of bacterial cellulose as determined by the laser light scattering method as a function of disintegration time. The average levelling-off sizes after 1 min disintegration are 21.2 ìm for Ag-BC and 478.0 ìm for St-BC. Figs 8 and 9 show typical polarized optical and scanning electron micrographs of the disintegrated products, respectively. These data indicate that Ag-BC is more easily disintegrated into smaller particles by a conventional homogenizer than St-BC.

In the case of St-BC, the average levelling-off size (Fig. 7, ®lled symbols) agrees with sizes of individual particles observed by polarized optical microscopy (Fig. 8b) and by scanning electron microscopy (Fig. 9b1). The average levelling-off size of Ag- BC particles (Fig. 7, open symbols) also agrees with the size of particles observed by scanning electron microscopy (Fig. 9a1). However, the size (about 200 ìm) of ocs of Ag-BC observed by optical microscopy is signi®cantly larger than the size of particles observed by scanning electron microscopy (Fig. 8a). Therefore, the particle of disintegrated Ag-BC observed by optical microscopy must be composed of many small aggregated fragments which are individually observed by scanning electron microscopy.

The above ®ndings suggest that the smaller particle size of disintegrated Ag-BC leads to a higher WHC than disintegrated St-BC as shown in Fig. 5. Fukui et al. (1986) reported that higher WHC related to higher viscosity in micro®brillated cellulose

Conc. (%)

(Parte 2 de 3)

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