SOLID-STATE PROPERTIES OF KONJAC GLUCOMANNAN

3.6 SOLID-STATE PROPERTIES OF KONJAC GLUCOMANNAN

3.6.1 D IELECTRIC ,V ISCOELASTIC , AND B ROAD -L INE NMR S TUDIES OF K ONJAC G LUCOMANNAN F ILMS

KGM films were prepared by a conventional casting method and the dielectric, viscoelastic, and broad-line NMR measurements were carried out to elucidate the

relationship between the chemical structure of KGM and its physicochemical prop- erties. 34 The molecular motion of solid KGM is discussed in comparison with amylose, 35 pullulan, 36 dextran, 37 and cellulose derivatives. 38 The experimental finding showed that the dielectric relaxation strength, Δε (ca. 2.2), of KGM is smaller than that of amylose (3.0) 35 but larger than those of pullulan (1.5) 37 and dextran (0.83). 37 It is considered that the magnitude of Δε is mainly determined by the rotational motion of hydroxymethyl groups because the sample has no acetyl groups (no signal

of the carbonyl carbon was perceived at 180 ppm in the 13 C cross-polarization (CP) and magic angle spinning (MAS) CP/MAS NMR spectrum). The number of

Functional Food Carbohydrates

CS plus water CS and KGM mixture

CS/KGM 3.50% 9.9/0.1

h/h) Δ 0.3 h/h) Δ 0.3

0 5 10 15 20 25 0 5 10 15 20 25 Storage time/day

Storage time/day (a)

(b) FIGURE 3.18 Syneresis of a cornstarch (CS) dispersion (a) and a CS–KGM mixture (b). C,

concentration of cornstarch: ●, 3.50 wt%; 䡲, 3.15 t%; ⽧, 2.80 wt%; 䉱, 2.45 wt%; and 䉲, 2.10 wt%. CS/KGM mixing ratio: -䡩-, 3.50 wt% 9.9/0.1; -▫-, 3.50 wt% 9.75/0.25; and -●-, 3.50 wt% 9/1, 8/2, 7/3, 6/4. (From Yoshimura, M. et al., Carbohydr. Polym., 35, 71, 1998.)

number in pullulan is about two thirds of that. Dextran has very few hydroxymethyl groups. Therefore, the order of magnitude of the dielectric relaxation strength for these polysaccharides can be understood in terms of the content of hydroxymethyl

groups, as discussed above. 38 According to these experimental results, the rotational motion of the hydroxymethyl groups in KGM is slightly hindered in comparison with the motion of the hydroxymethyl groups in amylose.

The complex viscoelastic constant c* = c' + ic'' at 10 Hz was determined by detecting the sinusoidal strain and stress at both ends of the film. The results from

temperature dependence of the viscoelastic coefficients c' and c'' showed that the real part c' decreased monotonically with increasing temperature. The value of c''

for the nondried sample showed a large peak at about –50˚C, and this peak decreased in height when the sample was dried, just as in the case of the dielectric

coefficient results. For dry KGM, a peak appeared at about –100˚C, which is attributed to the rotation of hydroxymethyl groups attached to the C-5 atom in

mannose and glucose residues. As the moisture content decreased, the value of c' became larger at temperatures below about –20˚C, but it became smaller at higher

temperatures. The water plays the role of a plasticizer at temperatures higher than the ambient temperature.

Results from temperature dependence of the second moment of KGM, amylose, pullulan, and dextran showed that the second moment of KGM at lower temperatures is smaller than those of the other three polysaccharides. 39 The second moment decreased rapidly at about 0 and 60˚C. The temperatures at which the second moment

Konjac Glucomannan

decreases rapidly are higher than those in the case of the other polysaccharides. This suggested that the movements of hydroxymethyl groups in KGM are slightly hin- dered in comparison with the other three polysaccharides.

3.6.2 B IODEGRADABLE M ATERIAL

The widespread use of synthetic polymer film materials has caused serious pollution problems and, as a result, biodegradable films from renewable resources have attracted much attention. Blend films were prepared by casting the mixtures of 7 wt% KGM aqueous solution and 2 wt% chitosan (CH) in acetic acid aqueous

solution. 40 Crystallinities of the blend films measured by wide-angle X-ray diffrac- tion decreased with the increase of KGM. The thermostability, tensile strength, and breaking elongation of the blend films in dry state were obviously higher than those of both pure KGM and chitosan films. The tensile strength of the dry blend film achieved the maximum of 73.0 MPa when the weight ratio of chitosan to KGM was 7:3, which is in line with the observation from scanning electron microscopy of a homogenous morphology of the blend film with the KGM content of 30 wt%. The structure analysis indicated that there is a strong interaction between KGM and chitosan that results from intermolecular hydrogen bonds. The water solubility of the blend films was improved by blending with KGM, in contrast to pure chitosan film. Therefore, KGM/CH blend films have promising application as coatings of pills, because they have good mechanical properties in the dry state and can be dissolved in aqueous medium.

To enhance the mechanical properties of konjac glucomannan film in the dry state and to find an application of konjac glucomannan in the food preservation domain, blend transparent film was prepared by blending 3 wt% sodium alginate aqueous solution with 4.5 wt% konjac glucomannan aqueous solution and coagu-

lating in a mixture of water–ethanol–sodium hydroxide (9:10:1 by weight). 41 Crys- tallinities of blend films increased with the increase of sodium alginate. The tensile

strength and breaking elongation of the blend films in the dry state were 3 ~ 4 times of those for both pure sodium alginate and konjac glucomannan films. The tensile strength of the dry blend film achieved 77.8 MPa when the retention of sodium alginate in the film was 27.9 wt%, as shown in Figure 3.19, indicating the high miscibility of the two components at this composite ratio, as confirmed by the observation from scanning electron microscopy. The structure analysis indicated that there was a strong interaction between konjac glucomannan and sodium alginate, resulting from the intermolecular hydrogen bonds. The moisture content and degree of water swelling of the blend films were increased due to the introduction of sodium alginate. Results from the film-coating preservation experiment on litchi and honey peach, two kinds of traditional fruits abundant in China, both having a high content of syrup, showed that this blend film had a certain degree of water-holding ability. Both the fruit weight loss rate and the rot rate of the experimental fruit groups decreased by various degrees in comparison with those of the control groups.

Abundant hydroxyl groups in konjac glucomannan facilitate the chemical mod- ification in konjac glucomannan by replacing the hydrogen atoms of the hydroxyl

groups with others. Nitro-konjac glucomannan (NKGM) was synthesized by inho-

Functional Food Carbohydrates

60 th/MPa

12 ation/% eng

long 40

10 ing e ensile str

eak T

Tensile strength

Br

20 Breaking elongation

0 20 40 60 Re SA (%)

FIGURE 3.19 Tensile strength and breaking elongation of the pure and blend films against the content of sodium alginate (Re SA ). (From Xiao, C. et al., J. Appl. Polym. Sci., 77, 617,

2000.) mogeneous reaction in the presence of KGM as the raw material and the mixture

of fuming acid–sulfuric acid–phosphorous pentoxide (10:1:4, by weight) as the nitration agent. Results from elemental analysis and light-scattering measurements showed that its degree of substitution and weight-average molecular weight are 2.4

and 4.75 × 10 4 , respectively. 42 Interpenetrating polymer network (IPN) materials, a kind of mutual polymer blend held together by permanent entanglement between two or more distinctly cross-linked polymers, have drawn much attention due to the special properties brought about by interlocking of polymer chains. Normally, the IPN technique is applied to synthetic polymers to obtain composite materials meeting various demands. Semi-IPN materials were successfully synthesized from castor oil- based polyurethane (PU) prepolymer and 10 ~ 40 wt% NKGM. Structural analysis showed that the intermolecular interaction of hydrogen bonding between NKGM and PU exists in the semi-IPN sheets, resulting in the miscibility. When NKGM content in an IPN sheet was 20 wt%, the mechanical properties, thermostability, and light transmittance were significantly higher than in the PU sheet. The NKGM in the IPN sheets plays an important role in the enhancement of the tensile strength and in accelerating the cure. This new material has not only performance similar to that of PU but also biodegradability. 42

To investigate the effect of the molecular weight of NKGM on the properties of this IPN material, eight NKGM products with different weight-average molecular weights, from 2.86 × 10 4 to 14.1 × 10 4 , determined by light-scattering measurements were synthesized by controlling the nitration reaction time. 43 Semi-IPN sheets with the content of NKGM of 20 wt% were prepared from castor oil-based PU and the NKGM products. A relatively broad mechanical loss peak appeared on the thermo- grams of dynamic mechanical analysis for the IPN sheets, and a single glass tran- sition (Tg) appeared on the DSC curves for the IPN sheets. The optical transmittances of the IPN sheets in the wavelength of 400 ~ 800 nm were all higher than that of the PU sheet. These results indicated the good miscibility between the two polymers. It is worth noting that the tensile strengths of the IPN composite sheets were

Konjac Glucomannan

120 /MPa 30 b /% ε b 90

(b) FIGURE 3.20 Effect of molecular weights (Mw) of NKGM on (a) tensile strength ( σ b ) and

(b) breaking elongation ( ε b ) of films PU and UNK. (…) represents σ b or ε b of PU. (From Gao, S. and Zhang, L., Macromolecules, 34, 2202, 2001.)

obviously higher than that of pure PU, and the tensile strength and breaking elon- gation of the IPN sheets increased with decreasing NKGM molecular weight from

8.44 × 10 4 to 4.75 ×10 4 , as shown in Figure 3.20. NKGM with relatively low molecular weight plays a role in plasticization, acceleration of curing, and enhance- ment of interaction between PU and NKGM. 43

By changing the synthesis route, namely, first blending NKGM with castor oil in butanone and thereafter adding toluene diisocyanate to promote the polymeriza-

tion reaction, novel semi-IPN materials are synthesized. 44 Compared to pure PU, the α-transition peak on DMA thermograms for each IPN sheet becomes sharper and shifts to a higher temperature position, and Tg on the DSC curves of the IPN sheets increases with increasing the content of NKGM. These experimental results are different from those in the case mentioned above, where PU prepolymer was first synthesized and then NKGM was added in the presence of a chain extender and catalyst. This is because blending NKGM with castor oil before polymerization has hindered the formation of a PU network. As a result, the amount of networks decreased and the microphase separation occurred in the composite sheets, resulting in the decrease of optical transmittance. In this case, NKGM predominantly plays

a role in filling and enhancing PU, resulting in the increase of tensile strength. The stress–strain curves of the composite sheets reflected the transition from elastomer to plastics with an increase of NKGM. 44

Castor oil can be considered a polyether-type polyol, and the polyurethane synthesized from castor oil and isocyanate is called poly(ether urethane). To inves- tigate the effect of the type of PU on the structure and properties of the composite materials, semi-IPN sheets were prepared by mixing poly(ester urethane) synthesized by the polymerization reaction between polyethylene glycol adipate and toluene diisocyanate in the presence of 2,2-bis(hydroxyl methyl) propionic acid as an

extender with NKGM in tetrahydrofuran. 45 These poly(ester urethane)/NKGM semi- IPN sheets showed good miscibility in a NKGM content range of 10 to 90%. It is mainly attributed to the penetration of linear NKGM into poly(ester urethane) and

Functional Food Carbohydrates

UN5 40 UN4 UN1

30 ess (MPa) 20

FIGURE 3.21 Tensile stress–strain curves of the semi-IPN sheets. NKGM contents in the sheets of UN1, UN2, UN3, UN4, UN5, and PU0 are 10, 20, 30, 40, 50 and 0 wt% respectively. (From Gao, S. and Zhang, L., J. Appl. Polym. Sci., 90, 2224, 2003. With permission.)

the intimate entanglement with PU chains. However, this relatively strong interaction restricted the motion of PU soft segment, resulting in the elevation of Tg. Figure

3.21 shows the tensile stress–strain curves of the semi-IPN sheets, where the NKGM contents in the sheets of UN1, UN2, UN3, UN4, UN5, and PU0 are 10, 20, 30, 40,

50 and 0 wt%. The semi-IPN composite sheet showed the best mechanical properties (35 MPa in tensile strength and 960% in breaking elongation) when the NKGM content was 10 wt%. When the NKGM content was more than 40%, the semi-IPN composite films showed the tensile stress–strain behavior of stiff and elastic materials

and had an obviously higher σ b than pure PU film, which can be used as plastics. 45 On the basis of the above results, a semi-IPN system is an effective method to obtain novel biodegradable materials by interpenetrating linear natural polymer into

a polyurethane network, and it provides a novel way for modification and exploitation of natural polymers such as polysaccharides.