Effect of temperature on the intrinsic viscosity and conformation of chitosans in dilute HCl solution

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Abstract

The effects of temperature on the intrinsic viscosity and on the conformation of chitosans in dilute HCl solution were studied. Ten chitosans with the same degree of deacetylation but different molecular weights were produced by alkali deacetylation of chitin which was prepared from red shrimp wastes. The degree of deacetylation at 83% and weight average molecular weight of the chitosans ranging 78–914 kDa were determined by infrared spectroscopy and static light scattering, respectively. The intrinsic viscosities ([η]) of these 10 chitosans in 0.01 M hydrochloric acid were measured at 10, 20, 30, 40, and 50°C. Then, d ln [η]/d(1/T) and the Mark-Houwink exponents were calculated as the indices for chain flexibility and molecule conformation, respectively. These results showed: the intrinsic viscosities decreased linearly with increasing temperature, therefore, a temperature-induced conformational transition did not occur for all 10 different molecular weight chitosans in the temperature range studied. Values of d ln [η]/d(1/T) were between 633 and 1334 and increased with decreasing molecular weight, indicating that higher molecular weight chitosans are more flexible. Between 10° and 50°C, the Mark-Houwink exponents ranged 0.64–0.76 and increased with increasing temperature, indicating that the conformation of these chitosans were all in random coil, and a temperature-induced conformational transition did not occur. The a* and a** Mark-Houwink exponents represent those chitosans whose molecular weights are larger and smaller than 223 kDa, respectively, and were obtained by using 223 kDa as the break point in the double logarithmic plots of the intrinsic viscosities and weight average molecular weight. Values of a** were between 0.41 and 0.54, while the a* values were from 0.96 to 1.07. These values for a** and a* indicate that larger and smaller molecular weight chitosans were in random coil and rod shape, respectively.

Introduction

Chitosan is a series of different deacetylated (higher than 50%) chitin derivatives [1]and is the only amino polysaccharide which is widely distributed in huge amounts in nature [2]. Chitosan is a cationic polyelectrolyte because the amino group in the backbone of the molecule will be protonated in acidic solution. The cationic polyelectrolyte properties of chitosan are different from those of most other neutral or anionic polysaccharides. The industrial applications of chitosan have been studied intensely 3, 4, 5, 6, 2, 7, 8.

The Mark-Houwink exponent is usually used as a conformation indicator of chitosan in solution. The Mark-Houwink equation is as follows:[η]=kMawhere [η] is the intrinsic viscosity, M is the molecular weight, and a and k are the Mark-Houwink exponent and constant, respectively. Values of a equal to 0, 0.5∼0.8, and 1.8 indicate polymers in sphere, random coil, and rod shape, respectively 9, 10, 11, 12, 13, 14, 15. The gross conformation of chitosan in solution may be spherical shape [16], random coil 17, 10, 1, 18, 19, 20, and rod shape 17, 10, 21, 22, 15, 23, due to differences in ionic strength, pH, temperature, concentration of urea in the chitosan solution, and molecular weight and degree of deacetylation of the chitosan used.

Tsaih and Chen [15]reported on chitosan dissolved in different ionic strength (0.01∼0.30 M) solutions of 0.01 M HCl–NaCl solvent systems. The Mark-Houwink exponent a* values were between 0.65 and 1.01 for those chitosans whose weight average molecular weights (Mw) were less than 223 kDa. However, for those chitosans whose Mws were greater than 223 kDa, the a** values were between 0.40 and 0.60. Tsaih and Chen [18]reported ε* values between 0.59 and 0.78 for those chitosans whose Mws were less than 223 kDa. However, for those chitosans whose Mws were greater than 223 kDa, the ε** values were between 0.43 and 0.52 for chitosans dissolved in 3 different ionic strengths (0.01, 0.10, and 0.20 M), but the same pH (2.10) HCl–NaCl solution, and in 3 different pH (2.30, 3.10, and 4.14) but the same ionic strength (0.05 M) HOAc–NaOAc solution, respectively. Mark-Houwink exponent ε is derived from a double logarithmic plot of the diffusion coefficients determined by dynamic light scattering, and Mw. The above results indicate that molecular weight-induced conformational change occurred and the break point was around 223 kDa. Molecular weight-induced conformational change was attributed to different spatial distribution forms of the chain molecule unit among bigger and smaller molecular weight chitosans, and/or may be due to differences in intra-molecular hydrogen bonds and/or differences in charge distribution.

The effect of temperature on the viscosity of the polymer is manifest. The rheological properties of a Newtonian fluid or polymer liquid are consistent with the Arrhenius equation when the temperature is higher than the glass transition temperature (Tg) or melting point [24]. The Arrhenius equation is as follows:η=AeEa/RTwhere η is the apparent viscsoity, A is the characteristic constant for polymers of a specific shear rate and molecular weight, Ea is the activation energy for the flow process, R is the gas constant, and T is the absolute temperature (°K). The slope (Ea/R) of the plot of natural logarithmic apparent viscosity (ln η) and the inverse of absolute temperature is the Ea of the polymer. Usually Ea values are between 2.09×107 and 2.09×108 J/(kg mol) [24]. Stiffer polymers have larger Eas 25, 24. If the apparent viscosity is replaced with the intrinsic viscosity or relative viscosity, the slope (d ln [η]/d(1/T) or d ln ηrel/d(1/T)) can also be used as an index for stiffness of the polymer because it relates to Ea. Stiffer polymers have larger slopes [26].

The molecular flexibility of cellulose trinitrate, hydroxyethylcellulose, and amylose increase with increasing temperature and cause the intrinsic viscosity to decrease 27, 13. The effects of temperature on the Mark-Houwink exponent are very significant when medium temperatures are close to the θ-temperature, and also are very sensitive when the polymer is dissolved in a good solvent. However, if the polymer is a stiff chain, the effects of temperature on the Mark-Houwink exponent are limited [9].

The effects of temperature on chitosan conformation have rarely been studied. Most of the studies focused on the effect of temperature on viscosity. Filar and Wirick [28]reported that apparent viscosities of chitosan decreased with increasing temperature between 25 and to 60°C. Pogodina et al. [21]reported that the intrinsic viscosity of chitosan decreased with increasing temperature and d ln [η]/dT was calculated to be −5.3×103. Rinaudo and Domard [26]transformed this to d ln [η]/d(1/T) and it became 488. This indicates that chitosan is a stiff molecule in 0.33 M acetic acid–0.30 M sodium chloride. Rinaudo and Domard [26]reported that the Ln ηrel of chitosan increased with an increase in the inverse of temperature, i.e. ηrel decreased with increasing temperature. The slope (d ln ηrel/d(1/T)) of the plot was calculated to be 2300. The value was larger than the stiffest polysaccharides, e.g. xanthan and cellulose diacetate. They attributed this to the concentration of the chitosan solution used being too high. Chen and Lin [25]reported that the apparent viscosity of chitosan (1%) decreased with increasing temperature, and Ea=2.005×107 J/(kg mol). This indicates that chitosan is quite stiff in 0.1 M acetic acid. Adding 0.05, 0.10, or 0.30 M sodium chloride in solution, caused Ea to decrease to 1.84×107, 1.83×107, and 1.81×107 J/(kg mol), respectively. This indicates that the flexibility of chitosan increases with increasing solution ionic strength. Wang and Xu [8]reported that Eas of 91 and 75% DD chitosan in 0.2 M acetic acid were 2.5×107 and 1.5×107 J/(kg mol), respectively. They attributed the Ea of 91% DD chitosan being greater than that of the 75% DD one to the ease of entanglement for higher DD chitosan. Desbrieres et al. [29]reported that Huggin's constant for alkylated chitosan derivative increased from 1.50 to 1.92 with increasing temperature from 20 to 22°C. This was attributed to an increase in hydrophobic interactions of alkylated chitosan derivatives with increasing temperature.

The effect of temperature on the conformation of chitosan has rarely been studied. Herein, intrinsic viscosities of ten 83% DD chitosans with different molecular weights were determined at temperatures between 10 and 50°C. The plot of ln [η] versus 1/T was made and d ln [η]/d(1/T), an index of stiffness of chitosan molecules, was determined. The effects of temperature on the intrinsic viscosity and conformation of chitosan were elucidated.

Section snippets

Preparation of the same DD but different molecular weight chitosans

Chitin was prepared from shrimp (Solemocera prominenitis) waste. Chitosan was prepared by alkali deacetylation with 50% NaOH at 100°C for 3 h with a solid/alkali solution ratio of 1: 20 [15]. Infrared spectroscopy (Bio-Rad, FTS 155, USA) was used to determine DDs of chitosans [30], which were 83% ±1%.

The same DD but different molecular weight chitosans were then obtained by ultrasonic degradation (CREST, 950E, U.S.A.) of solutions of 1% chitosan in 5% (v/v) acetic acid aqueous solution for

Effect of molecular weight of chitosan on (d ln [η]/d(1/T))

Fig. 1 is the plot of the natural logarithmic intrinsic viscosity (d ln [η]) versus the inverse of absolute temperature (1/T, K−1) of different molecular weight chitosans (83% DD) in 0.01 M HCl. The slopes of d ln [η]/d(1/T) were obtained by regression analysis and are listed in Table 1. The values of the slope are between 666 and 1334 and decrease with increasing weight average molecular weight of the chitosan used.

Effect of temperature on the intrinsic viscosity of chitosan

Fig. 2 shows the effect of temperature on the intrinsic viscosity of 83% DD

Effect of molecular weight on chain stiffness of chitosan

Table 1 shows d ln [η]/d(1/T) increasing with decreasing weight average molecular weight of the chitosans used. This indicates that the chain flexibility of larger molecular weight chitosans are more flexible than are the smaller molecular weight ones. The results are in accordance with the study of Tsiah and Chen [15], who reported that the relative stiffness parameter, B, was between 0.143 and 0.152 for those chitosans whose molecular weights are equal to or greater than 223 kDa, whereas the

Acknowledgements

The authors wish to thank the National Science Council, Republic of China. Project No: NSC 84-2321-B-019-034 for financial support.

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