Poly(N-isopropylacrylamide) Brushes Grafted from Cellulose Nanocrystals via Surface-Initiated Single-Electron Transfer Living Radical Polymerization Justin O. Zoppe, † Youssef Habibi, † Orlando J. Rojas,* ,†,‡ Richard A. Venditti, † Leena-Sisko Johansson, ‡ Kirill Efimenko, † Monika O ¨ sterberg, ‡ and Janne Laine ‡ Departments of Forest Biomaterials and Chemical and Biomolecular Engineering, North Carolina State University, Campus Box 8005, Raleigh, North Carolina, and Department of Forest Products Technology, School of Science and Technology, Aalto University, P.O. Box 16300, Aalto FIN-00076 Finland Received June 25, 2010; Revised Manuscript Received August 28, 2010 Cellulose nanocrystals (CNCs) or nanowhiskers produced from sulfuric acid hydrolysis of ramie fibers were used as substrates for surface chemical functionalization with thermoresponsive macromolecules. The CNCs were grafted with poly(N-isopropylacrylamide) brushes via surface-initiated single-electron transfer living radical polymerization (SI-SET-LRP) under various conditions at room temperature. The grafting process was confirmed via Fourier transform IR spectroscopy and X-ray photoelectron spectroscopy and the different molecular masses of the grafts were quantified and found to depend on the initiator and monomer concentrations used. No observable damage occurred to the CNCs after grafting, as determined by X-ray diffraction. Size exclusion chromatography analyses of polymer chains cleaved from the cellulose nanocrystals indicated that a higher degree of polymerization was achieved by increasing initiator or monomer loading, most likely caused by local heterogeneities yielding higher rates of polymerization. It is expected that suspension stability, interfacial interactions, friction, and other properties of grafted CNCs can be controlled by changes in temperature and provide a unique platform for further development of stimuli-responsive nanomaterials. Introduction Recent advances in the field of lignocellulosics have stream- lined the development of renewable biomaterials for advanced applications. 1,2 Cellulose, being the most abundant biopolymer in the biosphere, is an attractive material source due not only to its biodegradability and renewability, but also because of its low density, high strength properties, and potential for chemical modification. Cellulose fibers, containing crystalline and amor- phous domains, can be broken down into crystallite building blocks that exhibit even more unique surface, optical, and mechanical properties. 3,4 These rod-like cellulose nanocrystals (CNCs) or nanowhiskers can be produced after acid hydrolysis of the amorphous regions. Typically, sulfuric acid hydrolysis is employed to yield aqueous suspensions electrostatically stabilized by the negatively charged sulfate ester groups installed on the surface of CNCs, which promote uniform dispersion in water. 5 At a certain solids content, typically between 1 and 10%, this homogeneous suspension self-organizes spontaneously into spectacular liquid crystalline arrangements with a chiral nematic pitch ranging from 20 to 80 µm. 6 However, electrostatic forces affect colloidal stability, making suspensions of CNCs sensitive to the ionic strength of the medium. 7 To prevent aggregation in aqueous and organic media, steric stabilization could be achieved via polymer brushes grafted from CNCs. 8 Importantly, the destabilization effect brought about by the presence of electrolytes can be minimized when nonionic polymer chains are grafted. 9 The use of responsive polymer grafts has been of interest in recent years in the design of “smart” materials that can respond to changes in their environment, such as light, heat, ionic strength, and pH. 10-12 Related materials are especially interest- ing for drug-delivery and sensing applications. Thermorespon- sive polymers may have an upper or a lower critical solution temperature (UCST and LCST), depending on entropic changes and phase behaviors associated with their structural features. Poly(N-isopropylacrylamide) (poly(NiPAAm)), one of the most studied thermoresponsive polymers, has an LCST in aqueous solution ranging between 30 and 35 °C, depending on its detailed molecular architecture. Above the LCST, poly(NiPAAm) phase- separates due to thermal-driven chain dehydration. Because the LCST of poly(NiPAAm) is near the physiological temperature of about 37 °C, it has been extensively used in applications involving controlled drug release. 13,14 The utility of nanocel- lulosic substrates as nanocarriers of responsive functionalities for such applications has an obvious advantage related to their biocompatibility and cost. Polymer grafting on cellulose can be accomplished by two main strategies, namely, the “grafting-onto” and “grafting- from”. 15 The grafting-onto method requires attachment of presynthesized and well characterized polymer chains onto cellulose hydroxyl groups. However, steric hindrance can prevent optimal attachment because the polymer chains must diffuse through the grafted “brushes” to reach the reactive sites. Therefore, the grafting-onto method is limited to low surface density grafts. To increase the grafting density, the “grafting from” approach can be employed. In this method, polymer chains are formed by in situ polymerization from substrate- immobilized initiators, and molecular growth can be achieved by conventional radical, ionic, and ring-opening polymeriza- tions. 16 Since the mid 1990s, 17 living radical polymerizations (LRP) such as atom transfer radical polymerization (ATRP) have been * To whom correspondence should be addressed. E-mail: ojrojas@ncsu.edu. † North Carolina State University. ‡ Aalto University. Biomacromolecules 2010, 11, 2683–2691 2683 10.1021/bm100719d  2010 American Chemical Society Published on Web 09/15/2010