Films Prepared from Electrosterically Stabilized Nanocrystalline Cellulose Han Yang, Alvaro Tejado, Nur Alam, Miro Antal, and Theo G. M. van de Ven* Pulp & Paper Research Centre, Department of Chemistry, McGill University, 3420 University Street, H3A 2A7 Montreal, Quebec, Canada * S Supporting Information ABSTRACT: Electrosterically stabilized nanocrystalline cellu- lose (ENCC) was modiﬁed in three ways: (1) the hydroxyl groups on C2 and C3 of glucose repeat units of ENCC were converted to aldehyde groups by periodate oxidation to various extents; (2) the carboxyl groups in the sodium form on ENCC were converted to the acid form by treating them with an acid-type ion-exchange resin; and (3) ENCC was cross- linked in two diﬀerent ways by employing adipic dihydrazide as a cross-linker and water-soluble 1-ethyl-3-[3-(dimethylami- nopropyl)] carbodiimide as a carboxyl-activating agent. Films were prepared from these modiﬁed ENCC suspensions by vacuum ﬁltration. The eﬀects of these three modiﬁcations on the properties of ﬁlms were investigated by a variety of techniques, including UV-visible spectroscopy, a tensile test, thermogravimetric analysis (TGA), the water vapor transmission rate (WVTR), and contact angle (CA) studies. On the basis of the results from UV spectra, the transmittance of these ﬁlms was as high as 87%, which shows them to be highly transparent. The tensile strength of these ﬁlms was increased with increasing aldehyde content. From TGA and WVTR experiments, cross-linked ﬁlms showed much higher thermal stability and lower water permeability. Furthermore, although the original cellulose is hydrophilic, these ﬁlms also exhibited a certain hydrophobic behavior. Films treated by trichloromethylsilane become superhydrophobic. The unique characteristics of these transparent ﬁlms are very promising for potential applications in ﬂexible packaging and other high-technology products. ■ INTRODUCTION Cellulose is a natural carbohydrate polymer consisting of repeating β-D-glucose monomer units 1 and is considered to be an almost inexhaustible raw material. 2 With the increasing demand for environmentally friendly products, over the last two decades a large amount of research has been focused on natural cellulose ﬁbers. 3 Recently, nanomaterials science has attracted a great amount of attention because of the large surface area to volume ratio and the unique properties of nanoparticles. 4 Two main cellulose nanostructures are becoming more and more important in this category because they are “green” and a renewable biomass material: cellulose nanoﬁbers (CNF) and nanocrystalline cellulose (NCC). CNF consists of alternating crystalline and amorphous domains, and NCC consists of rodlike crystalline nanoparticles. 5,6 The lateral dimension of CNF and NCC is between 3 and 10 nm, and the length is a few micrometers 5 for CNF and 100-200 nm for NCC. 7 In addition, nanocellulose has an extremely high tensile strength of 2 to 3 GPa 8,9 and high stiﬀness with an elastic modulus of up to 138 GPa. 10,11 They also possess very low coeﬃcients of thermal expansion (CTE of 0.1 ppm K -1 ), 12 which is comparable to that of quartz. 13 These unique properties make nanocellulosic materials promising candidates for future electronic devices, such as ﬂexible displays, 14 and for oxygen- barrier layers, 15,16 such as food packaging. To utilize the advantage of nanocelluloses completely, it is important to ﬁnd an eﬀective method to prepare them. However, each individual nanocellulose ﬁber inside a pulp ﬁber is ﬁrmly attached to others by hydrogen bonds 17 because of the large numbers of hydroxyl groups on the natural ﬁbers. 18 Thus, it is not easy to achieve this goal. Many proposed methods have been developed recently. Fibrillation of cellulose ﬁbers has been performed by mechanical disintegration, 19,20 but most of the obtained ﬁbers contain large bundles of nanoﬁbers despite the huge amounts of energy consumed. The separation of cellulose nanoﬁbers was also performed by treating the cellulose with sulfuric acid. 7,21 However, the acid hydrolysis leads to a decrease in the length of nanoﬁbers of up to 100-200 nm, resulting in NCC, as well as a decrease in the ﬁnal yield to 30- 50%. The combination of enzymatic pretreatment of cellulose and then mechanical disintegration enables the preparation of CNF with reduced energy consumption, 22 and the ﬁber length of CNF was well preserved by this method. 23 Received: December 16, 2011 Revised: March 26, 2012 Published: April 5, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 7834 dx.doi.org/10.1021/la2049663 | Langmuir 2012, 28, 7834-7842