Structure and Low Concentration Self Assembly of Food Grade Bacterial Cellulose Nano-Whiskers
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Structure and Low Concentration Self Assembly of Food Grade Bacterial Cellulose NanoWhiskers
Mudrika Khandelwal, Alan H. Windle Department of Materials Science and Metallurgy University of Cambridge, CB2 3QZ ABSTRACT Nata-de-coco (bacterial cellulose) forms nanowhiskers with a larger aspect ratio owing to higher crystalline, longer and thinner microfibrils than those formed by cellulose from other sources. Suspension of these nanowhiskers undergoes transition from isotropic to liquid crystalline phase at a very low concentration of about 0.05 wt% in water and through a broad biphasic region. Formation of nematic as well as cholesteric phase is presented in this work. INTRODUCTION Cellulose is produced in nature as long, slender and almost endless rod like microfibrils. These microfibrils are supramolecular organisation of cellulose chains which typically display an elastic modulus of 120-150 GPa along their axis [1, 2]. In order to exploit these natural properties for man-made applications (such as fibres), production of an aligned phase of these cellulose microfibrils becomes essential. A lot of previous work has shown that controlled acid hydrolysis of cellulose microfibrils liberates rigid rod like nanowhiskers, which self organise to form lyotropic liquid crystalline suspension. The transition occurs across a bi-phasic concentration range, which depends on the aspect ratio of the nanowhiskers obtained after hydrolysis, which in turn is influenced by the source of cellulose. For example, cotton and wood yield nanowhiskers of dimensions ranging 210 nm wide and 100-300 nm long, which undergo isotropic to liquid crystalline transition between 4-14 wt% [3, 4, 5] whereas liquid crystalline behaviour of bacterial cellulose has been reported at much lower concentration of 0.45 wt% owing to larger aspect ratio [6]. In this work, the liquid crystalline behaviour of food grade bacterial cellulose (Nata-decoco) nanowhiskers is studied in detail with some insight into the structural aspect of the nanowhiskers behaving as the liquid crystal. The transition from an isotropic to liquid crystalline phase is investigated. Also this work illustrates the usage of polarised optical microscopy (POM) to study orientation of cellulose nanowhiskers in a liquid crystalline suspension. MATERIALS AND METHODS Food grade bacterial cellulose (BC) was obtained as nata-de-coco (NdC) from Yeguofood Co. Ltd. as cubes of 1cm. The cubes were thoroughly washed and dried. The dried NdC was treated with 40% (v/v) sulphuric acid (approximately 65 wt/ vol %) at 45oC to obtain a suspension of nanowhiskers. The ratio of solids to acid was 1:100 (weight to volume). No undissolved particles were observed. The suspension was washed with water until the pH reaches close to 7. A concentrate suspension of cellulose is obtained by centrifuging. Suspensions of various concentrations were made by step-wise dilution of this concentrate.
The XRD was performed on Philips generator with 40 KV and 40 mA from 10 to 50 degrees to estimate the crystallinity. Polarise
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