Article pubs.acs.org/Langmuir
Formation of Nanosized Islands of Dialkyl β‑Ketoester Bonds for Efficient Hydrophobization of a Cellulose Film Surface Quanling Yang, Miyuki Takeuchi, Tsuguyuki Saito, and Akira Isogai* Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan S Supporting Information *
ABSTRACT: The efficient hydrophobization mechanism of a hydrophilic cellulose film surface with alkylketene dimer (AKD) was studied in terms of formation of β-ketoester bonds at AKD/cellulose interfaces and their nanosized distribution analysis. AKD-treated cellulose and nanocellulose films were sequentially extracted with chloroform, hot water, and dioxane/water. Atomic force microscopy and highresolution secondary-ion mass spectrometry were used to analyze the surface structures of the AKD-treated cellulose films and those after the sequential extraction. The results showed that the AKD molecules had melted and transformed into spherical nanoparticles, ∼37 nm in diameter, on the film surface during heat treatment, forming “sea/island”-like structures; the film surface projection area comprised 99% hydrophilic cellulose and 1% hydrophobic AKD nanoparticles. Determination of the AKD contents in the films revealed that an extremely small amount of AKD/cellulose β-ketoester bonds were likely to form at the AKD/cellulose interfaces during heating, clearly contributing to the hydrophobic nature of the sequentially extracted cellulose films.
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(37 MJ m−3). Fourier transform infrared (FTIR) spectroscopy analysis showed that AKD molecules were still present as major species on the AKD-treated film surfaces without hydrolysis after 2 months of conditioning the films at 23 °C and 50% RH, indicating that AKD molecules are stable and that these AKD molecules contribute to the hydrophobic nature of the AKDtreated AUC films. AKD components retained in cellulosic materials may have four chemical structures, depending on the treatment conditions and subsequent conditioning process: the original AKD molecules, hydrolyzed AKD (dialkyl ketones), and βketoesters with hydroxyl groups of cellulose and starch present in the original AKD dispersions as one of the stabilizers (Figure 1). Although the efficient hydrophobization mechanism of cellulose fibers and paper with AKD has been extensively studied since the 1980s, it is still under debate.11−33 As shown in Figure 1, it may be possible that AKD/cellulose β-ketoester bonds are formed to some extent between AKD molecules and cellulose hydroxyls (particularly primary C6hydroxyls, which are less sterically hindered than C2/C3hydroxyls) in the AKD-treated and then heated AUC films. On the other hand, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-oxidized cellulose nanofibril (TOCN) films developed in our group have almost entirely sodium C6-carboxylate groups rather than C6-OH groups on the nanofibril
INTRODUCTION Cellulose is the most abundant biopolymer on earth, and quantitative and qualitative expansion of utilization of cellulosebased materials has been expected for establishment of a sustainable society.1,2 In advanced use of cellulose, green, highly efficient, and water-based conversion processes from cellulosic biomass to end products with leading-edge functions are required. In recent years, transparent, flexible, strong, and highgas barrier films have been prepared from cellulose and nanocelluloses,3−5 which have attracted a lot of attention. However, the presence of abundant hydroxyl groups causes the hydrophilic nature of cellulose materials, resulting in low resistance to water or humidity and poor compatibility with hydrophobic polymers. Therefore, improvement of water resistance or efficient hydrophobization is important for practical application of cellulose-based materials. In our previous work, regenerated cellulose films prepared from an aqueous alkali/urea solvent system (AUC)5−9 were efficiently hydrophobized using a simple method of soaking the AUC films in cationic alkylketene dimer (AKD) dispersions under various conditions.10 The AKD-treated AUC films had oxygen transmission rates of
