Evaluating the state of dispersion on cellulosic biopolymer by rheology Tapasi Mukherjee, Nhol Kao, Rahul K. Gupta, Nurul Quazi, Sati Bhattacharya Department of Chemical Engineering, Rheology and Materials Processing Centre, School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne Victoria 3001, Australia Correspondence to: T. Mukherjee (E - mail: t.mukherjee@uq.edu.au) ABSTRACT: The key challenge in the development of cellulose bio-nanocomposites lies in the spatial distribution of the cellulose fibre, as the presence of surface hydroxyl groups initiates self-agglomeration, thereby resulting in crack or failure of the composites. In this study, nanocrystalline cellulose (NCC) is here effectively surface acetylated to reduce agglomeration. Poly(lactic acid) PLA based cellu- lose bio- nanocomposites were then prepared by solvent casting technique. A rheological percolation threshold is calculated to quan- tify the level of dispersion and the optimal loading. Moreover, high frequency linear viscoelastic behavior is analyzed and the data is fitted to the Krieger-Dougherty equation to determine the maximum packing fraction. Maximum packing fraction value is then used as a mean to rank the quality of dispersion. The value for maximum packing fraction is compared with microcrystalline cellulose (MCC) and nanofibrillated cellulose (NFC), to show how shape and structure affects the quality of dispersion. VC 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43200. KEYWORDS: biomaterials; biopolymers and renewable polymers; cellulose and other wood products; rheology; surfaces and interfaces Received 24 June 2015; accepted 2 November 2015 DOI: 10.1002/app.43200 INTRODUCTION Development of cellulosic biopolymer is an active area of research over the last decade. 1–7 Some of the attractive features that explain its meaningful improvement as a reinforcing filler include its high aspect ratio, 8 their high crystallinity degree, 9 and high Young’s modulus. 10,11 Cellulose as a filler is well known to set up an entangled network held through strong hydrogen bonding. 12 The key challenge in the development of such biocomposites lies in the control of the particulate struc- ture, i.e., the spatial distribution of the cellulose fibre in a com- pletely dispersed and stable state. Similar to other nanocomposite research, filler size, loading, and distribution dictate the amount of affected polymer, while the surface struc- ture and chemistry of the particles dictate the intensity of inter- action of the particle/polymer interphase. Literature review in this area clearly demonstrates that optimum performance requires an effective distribution of the nanoparticles within the matrix. However, dispersion of the nanofiller restricts the for- mation of interface, which makes the nanoparticle as an attrac- tive choice for reinforcement. These particles have natural tendency to “stick” together and are difficult to separate or dis- perse due to high specific surface area and energy. 13 There is a constant citation by the review articles, referring to the chal- lenges involved in nanoparticle distribution for the advance- ment of polymer nanocomposite technology. 14,15 Despite the broadly recognized importance of nanoparticle dispersion, the characterization of dispersion remains largely qualitative and mostly based on subjective interpretations of standard optical and electron microscopy images. Rheology potentially offers a mean to assess the state of disper- sion of nanocomposites directly in the melt state. The rheologi- cal properties, both linear and nonlinear ones, are sensitive to changes in the particulate microstructure, particle size, shape and surface characteristics of the dispersed phase, integrated over length scales. 26 The level of linear properties such as the storage and loss moduli, G 0 and G 00 , are always raised with filler addition. G 0 and G 00 curves can be used diagnostically to assess the state of dispersion, since a flocculated system will show up as an extra low G 0 plateau. This usually takes the form of a pro- gressive increase in the level of properties as more filler is added. However, sometimes, a secondary mechanism can be seen such as the development of a plateau usually from a pseudo network set up between flocs or chains of particles. 16 Different approaches have been proposed to describe the disper- sion quality of the polymer nanocomposites by linear visco- elastic behavior. Establishment of the power law relationship between elastic property and volume fraction assuming filler as the fractal aggregate is very popular. 17–19 The basic concept of the scaling theory for polymer gels is to relate the elastic VC 2015 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.43200 43200 (1 of 9)