Flexibility and Color Monitoring of Cellulose Nanocrystal Iridescent Solid Films Using Anionic or Neutral Polymers Raphael Bardet, Naceur Belgacem, and Julien Bras* Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France CNRS, LGP2, F-38000 Grenoble, France LGP2/Grenoble INP-Pagora/CNRS - 461 rue de la papeterie, Domaine Universitaire, BP 65, 38402 Saint Martin d’He ̀ res cedex, France * S Supporting Information ABSTRACT: One property of sulfated cellulose nanocrystals (CNCs) is their ability to self-assemble from a concentrated suspension under specific drying conditions into an iridescent film. Such colored films are very brittle, which makes them difficult to handle or integrate within an industrial process. The goal of this study is (i) to produce flexible films using neutral poly(ethylene glycol) (PEG) and (ii) to modulate their coloration using an anionic polyacrylate (PAAS). The first part is dedicated to studying the physicochemical interactions of the two polymers with CNCs using techniques such as zeta potential measurements, dynamic light scattering (DLS), quartz crystal microbalance (QCM), and atomic force microscopy (AFM). Iridescent solid films were then produced and characterized using scanning electron microscopy (SEM) and UV-visible spectroscopy. The mechanical and thermal properties of films incorporating CNC were measured to evaluate improvements in flexibility. The addition of 10 wt % of PEG makes these films much more flexible (with a doubling of the elongation), with the coloration being preserved and the temperature of degradation increasing by almost 35 °C. Up to 160 μmol/g CNC PAAS can be added to tune the coloration of the CNC films by producing a more narrow, stronger coloration in the visible spectrum (higher absorption) with a well-pronounced fingerprint texture. KEYWORDS: cellulose nanocrystal, self-assembly, structural color, flexible iridescent films, polymer additives ■ INTRODUCTION In contrast to common pigment coloration, some surfaces appear colored only because of the interaction between light and their microstructure. This is known as structural coloration, and iridescence associated with this phenomenon is a major field of investigation. 1 Iridescence is an optical phenomenon that has fascinated artists, inventors, and scientists including Aristotle, Newton, and Darwin, for millennia and through the current day. 2 This amazing property of surfaces that change color with illumination or observation angle is found in nature. It can be observed in some animals, 3,4 such as the figeater beetle, turquoise emperor butterfly, and satin bowerbird, as well as some plants, 5-7 such as hibiscus trionum flowers, the tulipa species, and pollia condensata. Progress in engineering nanostructures has enabled the mimicry of nature for applications within many fields, such as display technologies, painting, printing, textiles, and cosmetics. 8,9 Since 2010, the use of biobased nanomaterials for the design of photonic nanostructures has become a promising prospect in the field of optical encryption technology. 10,11 Cellulose nanocrystals (CNCs) are one of the most promising biobased nanomaterials, as exhibited by growing interest at the industrial scale following the recent construction of the first CNC processing plants in 2011 and the growth of the patent portfolio since 2008. 12 These rod-like nanoparticles (approximately 5 nm wide and 250 nm in length) extracted from vegetal biomass (e.g., wood pulp) feature an attractive combination of properties including being biobased, biodegradable, and biocompatible. They also have a low density (∼1.6), high stiffness (∼150 GPa), high thermal stability (∼300 °C), and high density of hydroxyl groups that enables chemical modification and self-organiza- tion. 13 This study will focus only on the self-organization properties of CNC; more detailed information on the production, characterization, and utilization of CNC can be found in recent books 14 and reviews. 15-17 Recent works not only describe the characterization and production of CNC but also address its end use in smart applications, including Received: October 13, 2014 Accepted: December 31, 2014 Published: December 31, 2014 Research Article www.acsami.org © 2014 American Chemical Society 4010 DOI: 10.1021/am506786t ACS Appl. Mater. Interfaces 2015, 7, 4010-4018