Elsevier

Polymer

Volume 51, Issue 12, 28 May 2010, Pages 2748-2754
Polymer

Effects of carbon nanotubes on rheological behavior in cellulose solution dissolved at low temperature

https://doi.org/10.1016/j.polymer.2010.03.059Get rights and content

Abstract

Multi-walled carbon nanotubes (MWNTs) were dispersed, for the first time, in cellulose solution in 9.5 wt% NaOH/4.5 wt% thiourea aqueous system pre-cooled to −5 °C. Dynamic light scattering and transmission electron microscopy results revealed a relatively strong interaction existed between MWNTs and the cellulose macromolecules, leading to a good dispersion of MWNTs in the cellulose solution. Their rheological behaviors, especially the sol–gel transition were investigated by using the advanced rheological expanded system on the basis of Winter and Chambon theory. The gel point and gel concentration of the cellulose/MWNTs solution system were determined, indicating a regularly rheological behavior. The data of loss tangent and relaxation exponent (n) indicated an enhancement in the viscoelasticity of the MWNTs/cellulose system. The results from scaling law before and beyond the sol–gel transition in the MWNTs/cellulose system confirmed that the cluster formation and alteration of the gelation structure occurred at the gel point. Interestingly, the n values calculated by both the Winter and Chambon theory and scaling law were coincident only at relatively low temperature. The predicted gel strength values of the MWNTs/cellulose system were significantly larger than the pure cellulose solution, suggesting a relatively high strength, supported by the mechanical strength of the cellulose/MWNTs material.

Introduction

As the most abundant renewable resource, cellulose can be converted into derivatives and regenerated fibers and films, as well as various functional materials [1]. As a result of inter- and intra-molecular hydrogen bonds, cellulose is not soluble in common solvent [2]. There are excellent solvents such as N-methylmorpholine-N-oxide (NMMO) [3], lithium chloride/N, N-dimethylacetamide (LiCl/DMAc) [4], and 1-butyl-3-methylimidazolium chloride [5], which can dissolve cellulose at high temperature. In our laboratory, a new solvent system including NaOH/thiourea, NaOH/urea and LiOH/urea aqueous solutions has been developed to dissolve cellulose at low temperature (−5  −12 °C) rather than high temperature, creating a new concept and “green” approach for polymer dissolution. In the system a new hydrogen-bonding network structure between solvent molecules and cellulose macromolecules can be created to destroy the inter- and intra-molecular hydrogen-bonding in the native cellulose at low temperature, leading to good dissolution [6]. It is worth noting that novel fibers have been spun from the cellulose dopes [7], and a series of functional materials has been created. Thus, the cellulose solution system is an attractive and interesting research on the preparation of nano-materials, especially inorganic nano-particles or carbon nanotubes in the aqueous system.

Carbon nanotubes (CNTs) are known as examples of nano-materials with high aspect ratio and many exciting properties. These properties lead to a wide variety of possible applications, such as incorporation into composites for high-strength applications, formation of composites with high conductivity, use as field emitters for high-power microwave transmitter applications, low-voltage electromechanical actuators, electrostatic paint and nanoscale probe tips [8]. Nowadays, CNTs hybrids have attracted great attention, by introducing the remarkable properties of CNTs into flexible polymer materials. However, the difficulty of CNTs dispersing in common solvents confines its application and research. It has been reported that CNTs were introduced in cellulose materials to improve their mechanical properties and transport properties [9], [10], [11]. However, the dispersion of CNTs in NaOH/thiourea aqueous system has been published scarcely. Recently, we found that cellulose/MWNTs composite could be prepared by dispersing MWNTs in the NaOH/thiourea solvent to enhance the mechanical properties. To create cellulose/MWNTs materials, it is important to clarify the influence of MWNTs on the rheological properties and sol–gel transition of cellulose solution to explore the interactions between MWNTs and cellulose chains. In this condition, rheometer is a suitable tool for the investigation of the sol–gel systems [12], [13]. In this work, the influences of MWNTs on the rheological behaviors of cellulose solution dissolved in NaOH/thiourea aqueous solution at low temperature were investigated. We hope to provide useful information on the interaction between two components and sol–gel transition behavior of the cellulose containing MWNTs to promote the development of MWNTs/cellulose functional materials. This work can supply a novel pathway and useful data for fabricating cellulose based materials containing MWNTs.

Section snippets

Materials

Cellulose (cotton linter pulps) with a α-cellulose content of more than 95% was supplied by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China). Its weight-average molecular weight (Mw) was determined in 4.6 wt% LiOH/15.0 wt% urea aqueous solution with laser light scattering to be 12.0 × 104 g/mol [14]. The cellulose sample was vacuum-dried at 55 °C for 24 h to remove any moisture before use. NaOH and thiourea of analytical grade (Shanghai Chemical Reagent Co. Ltd., China) were used without further

Interaction between MWNTs and cellulose in the solution

Our previous work [6] has revealed that cellulose forms inclusion complexes (ICs) which easily aggregate, and cellulose exists in NaOH/thiourea mainly as single ICs and sphere-like IC aggregates. To clarify the dispersion of MWNTs in the cellulose solution and the interactions between the two components, DLS and TEM were applied to study the dilute solution. Fig. 1 shows the apparent hydrodynamic radius (Rh,app) distribution of pure cellulose solution and MWNTs/cellulose solution. There were

Conclusions

MWNTs have been dispersed in the cellulose solution through mixture in NaOH/thiourea aqueous solution at low temperature. The TEM and DLS results indicated that a strong interaction between MWNTs and cellulose chains led to the bonding between MWNTs and cellulose chains in the cellulose IC aggregates. The MWNTs/cellulose solution could be described by the Winter and Chambon theory from 10 to 25 °C. Furthermore, the scaling law before and beyond the gel point: n=z/(z+γ) for the MWNTs/cellulose

Acknowledgments

This work was supported by National Basic Research Program of China (973 Program, 2010CB732203), National Supporting Project for Science and Technology (2006BAF02A09), the National High Technology Research and Development Program of China (863 Program, 2003AA333040 and 2006AA02Z102), major grants of the National Natural Science Foundation of China (30530850 and 59933070), and the National Natural Science Foundation of China (20474048 and 20874079).

We acknowledge the Center of Nanoscience and

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