4488
J. Phys. Chem. B 2008, 112, 4488-4495
Investigation of the Scaling Law on Cellulose Solution Prepared at Low Temperature Ang Lue and Lina Zhang* Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China ReceiVed: September 25, 2007; In Final Form: December 11, 2007
Cellulose was dissolved rapidly in a 9.5 wt % NaOH/4.5 wt % thiourea aqueous solution pre-cooled to -5 °C to prepare its concentrated solution, in which inclusion complexes (ICs) associated with cellulose, NaOH, thiourea, and water clusters were created. Physical gels could form in the cellulose solution at either high temperature or after long storage time, because of aggregation between the ICs. To clarify whether the Winter and Chambon theory could describe the gelation process of this complex system, we have investigated carefully the viscoelastic behavior of the cellulose solution with the advanced rheological expanded system (ARES). In the temperature range from 10 to 25 °C, we have successfully measured the loss tangent (tan δ) at the gel point according to the Winter and Chambon theory, showing the independence of tan δ on the frequency for the cellulose solution. The exponents of the scaling laws η0 ∝ -γ and Ge ∝ z for the cellulose solution at 10 °C before and beyond the gel point were confirmed to be in agreement with the predicted values based on the percolation theory. The high sensitivity of the cellulose solution on temperature poses a limit for the application of the scaling law for the wide temperature range. The gel formed from the cellulose solution at 30 °C at long storage time could undergo a transition to a transparent liquid state after stirring at -5 °C. At the same time, the loss modulus (G′′) exceeds the storage modulus (G′), indicating a partially reversible sol-gel transition, as a result of the reconstruction of the hydrogen-bond networks between the solvent and cellulose.
Introduction Cellulose is a promising chemical material for the 21st century, and has attracted much recent attention in a wide range of applications. The cellulose industry faces tremendous challenges in providing a viable, economical, and environmentally friendly chemical processing scheme.1-4 The search of a new solvent for cellulose has long attracted the interests of the scientific and industrial communities. For example, N-methylmorpholine-N-oxide (NMMO),5 lithium chloride/N,N-dimethylacetamide (LiCl/DMAc),6 1-allyl-3-methylimidazolium chloride,7 and 1-butyl-3-methylimidazolium chloride,8 which dissolve cellulose at high temperature, have been investigated. In our laboratory, new solvent systems, including NaOH/thiourea, NaOH/urea, and LiOH/urea aqueous solutions, have been used to dissolve cellulose at low temperature (-5 to -12 °C) rather than at high temperature, and novel fibers have been spun from the cellulose dopes.9,10 Compared with traditional dissolution by heating, this low-temperature dissolution phenomenon is much more interesting and puzzling. It has been proven that a new hydrogen-bonding network structure between the solvent and the cellulose macromolecules could form to destroy the inter- and intramolecular hydrogen-bonding of cellulose at low temperature, leading to good dissolution.11,12 Facing the complexity of the cellulose solution systems, a basic understanding of the rheological behavior of the cellulose solution as well as the sol-gel transition is essential for the successful application and industrialization. Polymeric gel is a three-dimensional network, formed from flexible chains through either chemical cross-linking or physical phase transformation. The gelation is a phenomenon in which * Corresponding author. E-mail:
[email protected]. Tel: +8627-87219274. Fax: +86-27-68754067.
a polymeric liquid dramatically becomes solid-like at a critical point in polymer concentration, temperature, storage time, and so forth. To determine the gel point, traditionally, the crossover of the storage modulus G′(ω) curve and the loss modulus G′′(ω) curve has been used as an indicator.13 This method is simple and convenient, but is frequency dependent. Valuable contributions to this area were made by the studies of chemical and physical gelations in Winter’s laboratory.14-16 They first experimentally established a scaling law of G′(ω) ) G′′(ω) ∼ ω1/2 at the gel point, and later generalized it to be
G′(ω) ) G′′(ω) ∼ ωn
0
