Conformation and dn/dc determination of cellulose in N,N-dimethylacetamide containing lithium chloride
Introduction
Size-exclusion chromatography (SEC) is the technique of choice to evaluate composition and molar mass distribution (MMD) of polymers. Extremely sensitive to early changes, SEC has been widely used to characterise cellulose and monitor its degradation in the pulp and paper industry, and more recently the technique has been applied in the field of paper conservation science (Dupont, 2003a, Jerosch, 2002).
For a biopolymer such as cellulose, which has limited solubility in most common solvents used in chromatography, the polymer solubility/system compatibility (columns packing material) is not a simple issue. Derivatisation can be a solution but direct dissolution offers a better alternative being faster, easier and more reproducible. Solvent systems such as lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) gained considerable popularity since they were first described to dissolve cellulose, by McCormick (1981) and by Turbak (1981). Moreover, the possible use of LiCl/DMAc as SEC mobile phase with column packing such as poly(styrene-divinyl benzene) (PSDVB) simplifies the procedure. A review article by Strlič and Kolar (2003) summarises the research in the field of SEC of cellulose in LiCl/DMAc since it was carried out for the first time (Ekmanis and Turbak, 1986, Ekmanis, 1987).
The work here-described constituted a preliminary step in the study of the degradation upon time of cellulose from historic and modern papers of diverse origins. Recent publications give a review of existing dissolution methods of cellulose in LiCl/DMAc (Dupont, 2003a, Dupont, 2003b). The development of the dissolution method of paper for specific applications to historic prints and archival documents, where naturally and artificially aged paper were chosen as the source for cellulose, as well as the non-aggressiveness of the LiCl/DMAc solvent towards cellulose are also reported in the aforementioned publications.
SEC coupled with multiangle light scattering (MALS) and differential refractive index (DRI) detectors was used in the characterisation of cellulose of these papers. Precision and reproducibility of the method were evaluated for validation. In order to calculate absolute values for the molar mass (Mr) averages and dimensional parameters, such as the root mean square (rms) radii averages, the specific refractive index increment dn/dc of cellulose in the solvent and mobile phase used, 0.5% LiCl/DMAc, needs to be known. It appeared necessary to experimentally determine this parameter since rather different values were found in the literature. The conformation of cellulose in 0.5% LiCl/DMAc was characterised, which provided information on the solvation efficiency of the solvent system. It should be stressed that, with this study being carried out at SEC concentration, the results are representative of the properties of dilute cellulose solutions in low salt concentration LiCl/DMAc, and may differ from the properties of concentrated polymer solutions in higher ionic strength solvent.
Background theory on light scattering can be found in reference work (Flory, 1953, Wyatt, 1993). Briefly, the Rayleigh-Debye-Gans model for dilute polymers (Zimm formalism), which includes both intermolecular and intramolecular effects, embodies the principles of light scattering (Eq. (1)).where (cm−1) is the Rayleigh ratio, A2 (mol cm3 g−2) the second virial coefficient, c (g cm−3) the concentration of the solute molecules, the form factor (also called particle scattering factor), a function of the mass distribution inside the molecule (defined in Eq. (2)), and K* an optical parameter related to the polymer in the solvent (defined in Eq. (3)).where λ (cm) is the measuring wavelength.where n0 is the refractive index of the solvent, N (mol−1) is Avogadro's number, and λ0 (cm) is the vacuum wavelength of the incident light.
In SEC/MALS/DRI, the unknown parameters are Mw, and A2. The dn/dc needs to be accurately measured or otherwise obtained from the literature. In SEC, with polydisperse polymers, each slice of a peak can be considered as having constant Mw and c. Debye plots, represented by the function(Wyatt, 1993), can be built for each data slice in the chromatogram. These plots yield 1/Mw (intercept) and (gradient) and can be constructed using Zimm, Debye, or Berry detector fit methods, which correspond to mathematical transformations of the same function. Zimm formalism ( as a function of ) is most widely used for mid-sized polymers (rms radius 10–100 nm) and was found the most appropriate fit in the present study.
Section snippets
Sample preparation
The paper used as cellulose source was Whatman No.1 (pure cellulose). Papers were artificially aged at 80 °C and 50% relative humidity (rH) by suspending the sheets individually in a climate chamber Versatenn (Tenney Environmental, Parsippany, NJ, USA) for thirty-five and ninety-four days. These samples were labelled Wt35 and Wt94, respectively. The unaged samples were designated as Wt0.
Two to 2.5 g of paper were sampled from different areas in separate sheets and were defibrillated by
dn/dc of cellulose in 0.5% LiCl/DMAc
From the change in the DRI detector voltage ΔV, the difference in refractive index between the pure solvent and the polymer solution (Δn) for each different concentration can be calculated from Δn=αΔV (where α is the DRI calibration constant). Fig. 1 represents the plot of the average Δn obtained in the three experiments as a function of c. The gradient in this plot equals d(Δn)/dc or dn/dc, a unique variable of the studied polymer in the working solvent at the working temperature and working
Conclusions
SEC/MALS/DRI analyses led to a mean determined value of 0.077 ml g−1 for the dn/dc of cellulose in 0.5% LiCl/DMAc, in the experimental conditions of this study. The conformational study showed that with the dissolution method used, the cellulose in dilute solutions at low salt concentration adopted random coil conformation, with q values comprised between 0.5 and 0.6 for both unaged and aged cellulose. Thus, from the dilute solution behaviour, it seems that LiCl/DMAc is a thermodynamically good
Acknowledgements
This research was carried out during the Advanced Training Fellowship in Conservation Science at the National Gallery of Art, Washington DC, US, and was financially supported by the Charles E. Culpeper Foundation. Warm thanks to Prof. Dr E.R. de la Rie, Scientific Research department, National Gallery of Art, for his support and guidance, and to Dr C. Shahani, Preservation Research and Testing division, Library of Congress, Washington DC, for providing the instrumental support. We are also
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