Elsevier

Carbohydrate Polymers

Volume 58, Issue 3, 25 November 2004, Pages 233-243
Carbohydrate Polymers

Conformation and dn/dc determination of cellulose in N,N-dimethylacetamide containing lithium chloride

https://doi.org/10.1016/j.carbpol.2004.07.016Get rights and content

Abstract

In order to characterise cellulose dissolved in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) using size-exclusion chromatography with online multiangle laser light scattering and differential refractive index detection, a number of parameters were determined. One of them in particular, the specific refractive index increment (dn/dc) of cellulose in 0.5% LiCl/DMAc, is required in order to calculate the molar mass from the light scattering signal. Pure cellulose paper was used as sample source. The precision and reproducibility of the SEC/MALS/DRI method were evaluated. Molar mass and root mean square radii averages, molar mass distribution as well as conformation of cellulose in 0.5% LiCl/DMAc, from both unaged and artificially aged papers, were studied. The latter was determined to be random coil for cellulose and vary slightly with the polymer molar mass.

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 (rg21/2) 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)).K*cRθ=1MwPθ+2A2cwhere Rθ (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, Pθ 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)).1Pθ=1+16π23λ2rg2sin2(θ2)+where λ (cm) is the measuring wavelength.K*=4π2(dn/dc)2n02N1λ04where 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, rg2 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 functionK*cRθ=f(sin2(θ2))(Wyatt, 1993), can be built for each data slice in the chromatogram. These plots yield 1/Mw (intercept) and rg2 (gradient) and can be constructed using Zimm, Debye, or Berry detector fit methods, which correspond to mathematical transformations of the same function. Zimm formalism (K*c/Rθ as a function of sin2(θ/2)) 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

References (37)

  • R. Berggren et al.

    Improved methods for evaluating the molar mass distributions of cellulose in kraft pulp

    Journal of Applied Polymer Science

    (2003)
  • S. Chrapava et al.

    The investigation of the influence of water and temperature on the LiCl/DMAc/cellulose system

    Physical Chemistry Chemical Physics

    (2003)
  • T.R. Dawsey et al.

    Journal of Macromolecular Science-Reviews in Macromolecular Chemistry and Physics

    (1990)
  • Debzi, El. M. (1992). Celluloses issues du traitement à la vapeur: Évolution des masses moléculaires moyennes,...
  • P.G. De Gennes

    Scaling concepts in polymer physics

    (1979)
  • Dupont, A. -L. (2003a). Gelatine sizing of paper and its impact on the degradation of cellulose during aging, a study...
  • J.L. Ekmanis

    Gel permeation chromatographic analyses of cellulose

    American Laboratory. News

    (1987)
  • J.L. Ekmanis et al.

    GPC analysis of cellulose

    (1986)
  • Cited by (33)

    • Solution-state structures of the cellulose model pullulan in lithium chloride/N,N-dimethylacetamide

      2018, International Journal of Biological Macromolecules
      Citation Excerpt :

      Nevertheless, the solvent systems of LiCl/DMAc and LiCl/DMI are widely used for molecular mass analyses of celluloses by SEC/MALLS [8–16]. This is because most cellulose samples are soluble in LiCl/DMAc or LiCl/DMI without undergoing marked depolymerization, when combined with mild pretreatment to enhance dissolution [3,10]. In particular, most highly crystalline algal, tunicate, and bacterial celluloses, regenerated celluloses, and plant holocelluloses dissolved completely (at the individual molecule level) in 8% (w/w) LiCl/DMAc after soaking the cellulose samples in ethylenediamine (EDA) followed by solvent exchange from EDA to DMAc through methanol [17–20].

    • Transition from microcellular to nanocellular PLA foams by controlling viscosity, branching and crystallization

      2017, European Polymer Journal
      Citation Excerpt :

      The refractive index values were measured by a Wyatt Optilab DSP refractometer at 40 °C and 690 nm calibrated with sodium chloride. Five samples of 3–18 mg mL−1 were prepared in THF for each polymer and injected sequentially to construct a curve with slope dn.dc−1 [24]. The refractive index values for PLA, PLA-TAM, and PLA-GMA were determined to be 0.0482 ml gm−1, close to values reported in the literature [25].

    • Studies on cellulose nanocrystals isolated from groundnut shells

      2017, Carbohydrate Polymers
      Citation Excerpt :

      Static light scattering (SLS) studies have been made in number of ways on the solution behaviour of cellulose samples including molecular weight determination, with LiCl/DMAc system. As a non-derivatising & non-degrading solvent, LiCl/DMAc has potential applications in cellulose chemistry ranging from analytical studies to the preparation of certain derivatives in organic synthesis (Dupont & Harrison, 2004). The formation of super or supramolecular structures such as smaller and larger aggregates have been reported in several studies (Potthast, Rosenau, Buchner, & Röder, 2002; Sjoholm, Gustafsson, Eriksson, Brown, & Colmsjo, 2000) which might responsible for higher molecular weight of different cellulose samples in LiCl/DMAc system (Röder, Morgenstern, Schelosky, & Glatter, 2001).

    View all citing articles on Scopus
    View full text