Entanglement network of agarose in various solvents

INTRODUCTION The molecular structure of polysaccharides has led to the belief that these polymers are stiff. The characteristic ratio (CN) for polysaccharides is rather high, although reported CN values are limited to typical polysaccharides such as cellulose1 and gellan.2 If we can prepare concentrated solutions of polysaccharides, we can determine the molecular weights between entanglements in the molten state (Me,melt) for these polysaccharides,3–5 which reflects the chain properties, that is, the flexibility of the chain. Since the first report that cellulose has good solubility in ionic liquids,6 many studies have been conducted to determine the solution properties of polysaccharides at high concentrations. We prepared concentrated solutions of several kinds of polysaccharides in an ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) and carried out rheological measurements for the solutions to determine Me,melt for the polysaccharides. 7–9 The experiments yielded rather small values of Me,melt for the polysaccharides; for example,8 Me,melt was 2.3 103 for agarose, which is composed of (1,3)-b-D-galactopyranose and (1,4)-3,6-anhydro-a-L-galactopyranose.10 This value appears to be much smaller than expected for a stiff polymer and is actually comparable to the Me,melt for the simplest flexible polymer polyethylene (Me,melt1⁄41.3 103),3 although no reported value of CN for agarose is available. Why the Me,melt for polysaccharides is so small is not clear at present. The Me,melt for amylose was also determined and was much larger than that for cellulose.7 To explain the Me,melt values, it is necessary to examine whether the use of an ionic liquid as a solvent affects the estimation of Me,melt. It has been proposed that ionic liquids form three-dimensional structures even in the liquid state because of cation–anion hydrogen bonding and cation–cation ring stacking.11–13 The threedimensional structure may contribute to the modulus of the solutions; if the elasticity due to the network of solvent molecules contributes to the plateau modulus (GN 0), then Me,melt is reduced. The aim of this study is to clarify whether or not the network formed by the ionic liquid molecules contributes to GN 0. Concentrated solutions of agarose were prepared by using formamide (FA), N-methylformamide (MFA) and BmimCl as solvents. Angular frequency (o) dependence curves of the storage and loss moduli (G¢ and G00, respectively) were compared among these solutions. EXPERIMENTAL PROCEDURE Agarose (Research Organics, Cleveland, OH, USA) was used without further purification. FA (Wako, Osaka, Japan) and MFA (Aldrich, St Louis, MO, USA) were used as received. The ionic liquid BmimCl (Aldrich) was the same as that described in the previous studies.7,8 Agarose solutions were prepared as follows: agarose was added to each solvent (for BmimCl, the solvent was preheated to convert it into the liquid state) in a dry glass vessel, and the mixture was quickly stirred with a stainless spatula on a hot plate at B80 1C. Then, the vessel was sealed and left on the hot plate for complete melting. The concentration of agarose (c) was 1.0 102 or 2.0 102 kg m 3, that is, ca 10 or 20 wt %. In the calculation of c, 1.0 103 kg m 3 was assumed for the density of agarose, and 1.14 103, 1.00 103 and 1.08 103 kg m 3 were used for the densities of FA,14 MFA14 and BmimCl,7,8 respectively. To minimize the effect of moisture absorption, a fresh bottle of solvent was used every time agarose samples were prepared, and the rheological measurements were made just after finishing the sample preparation. Rheological measurements (dynamic viscoelasticity measurements in this study) were carried out with an ARES rheometer (now TA Instruments, New Castle, DE, USA) using the dynamic operation-testing mode under a nitrogen atmosphere. A cone–plate geometry with a diameter of 25 mm and a cone angle of 0.1 rad was used. The o-dependence of G¢ and G00 for the solutions was measured with a strain amplitude (g) of 0.1. Because the melting temperatures (Tm) of FA 14 and MFA14 are reported to be 2.6 and 5.4 1C, respectively, the temperature (T) ranged from 5 to 40 1C for the FA solution and from 0 to 30 1C for the MFA solution. The T-range for the BmimCl solution was 20– 120 1C. For BmimCl, Tm is ca 70 1C, but the supercooled state of the BmimCl solutions was rather stable. Thus, viscoelasticity measurements were successfully taken even at 20 1C. The master curves in each solvent were prepared using only a horizontal shift with the shift factor aT.