Two different incorporation routes of cellulose nanocrystals in waterborne polyurethane nanocomposites

• Waterborne polyurethane (WBPU) with narrow particle size distribution was synthesized.
• Cellulose nanocrystals (CNC) were isolated and used for nanocomposites preparation.
• WBPU–CNC nanocomposites were prepared by different CNC incorporation routes.
• Alternative CNC addition process lead to different CNC disposition in nanocomposites.
• CNC disposition in films lead to different mechanical properties and hydrophilicity.

The renewability, availability and low-cost of eco-friendly cellulose nanocrystals (CNC), have gaining attention for nanocomposites preparation due to their unique properties in the nanoscale and their water dispersibility, becoming a suitable reinforcement in waterborne polyurethane (WBPU) dispersions. Thereby, a WBPU matrix with a high hard segment content (about 48 wt%) was synthesized resulting in a dispersion of low particle size with a narrow distribution analyzed by means of dynamic light scattering and visually stable over 6 months. The CNC reinforcement isolated from microcrystalline cellulose via acid hydrolysis lead to CNC with a high length/diameter aspect ratio of about 31, determined by atomic force microscopy. In the nanocomposites preparation, two incorporation routes were designed for analyzing the influence of CNC disposition in the nanocomposites films: the classical mixing by sonication or in-situ adding CNC in water during particles formation step. The influence of CNC addition route and their disposition in the final properties of nanocomposites were analyzed by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, atomic force microscopy and dynamic water contact angle, observing considerable variations by adding 1 and 3 wt% of CNC. The reinforcement addition route influenced the WBPU–CNC interactions, which resulted more effective by the alternative in-situ incorporation method. The CNC incorporation restricted the crystallization of soft domains, in a higher extend in nanocomposites prepared by in-situ route, and improved the thermomechanical stability. The studied CNC incorporation routes lead to different dispositions of CNC in the matrix, resulting in different mechanical performance, providing a suitable stress transfer in the nanocomposite and diverse hydrophilic behavior, comparing with the WBPU matrix.

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