Mechanically adaptive thermoplastic polyurethane/cellulose nanocrystal composites: Process‐driven structure–property relationships

Abstract

To enable filament extrusion additive manufacturing of mechanically adaptive nanocomposites, the effect of melt extrusion on the tensile modulus and mechanical adaptiveness of cellulose nanocrystal (CNC)/thermoplastic polyurethane (TPU) composites was investigated. TPU (Texin RxT70A) was processed with 10 wt % CNC in multiple formats, including solvent‐cast films and melt extrusion‐produced filaments. CNC orientation is characterized by polarized Raman spectroscopy and small‐angle X‐ray scattering (SAXS) and found to be uniaxially oriented in extruded filaments, and randomly oriented in solvent‐cast films. Dynamic mechanical analysis results show the addition of 10 wt % CNC increases the pure TPU storage modulus from 10 to 60–70 MPa in the dry state for solvent‐cast and extruded nanocomposites. Following water saturation, all CNC‐containing samples reach a similar wet storage modulus of approximately 26 MPa, regardless of CNC orientation and thermal history. Fickian diffusion scaling factors (6.8 to 11.6) scale closely to the time‐dependent empirical scaling factors (4.9 to 7.2), confirming the rate of mechanical adaptivity is driven by water diffusion through sample thickness. Our results suggest that mechanical adaptivity is retained following processing, enabling use as a feedstock for extrusion‐based additive manufacturing of CNC/TPU composites for mechanically adaptive parts. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46992.

Replication of annulus fibrosus through fabrication and characterization of polyurethane and cellulose nanocrystal composite scaffolds

“…As well, the minute variability within the mechanical properties obtained from tension testing led to the assumption of a high level of dispersion, as seen in the similar study by Rueda et al 52 Furthermore, research has shown the benefit of adding CNCs to a polymeric matrix for mechanical tunability. 40,53,54 Consequently, when tested dry, the samples demonstrated a drastic increase in modulus as shown in previous work, 40,[55][56][57] but significantly out of the accepted range. To mitigate this increase, the samples were swollen to reach the needed range of moduli, since the presence of CNCs in the composite increased the hydrophilicity, allowing for the absorption of water.…”

“…The ability of the composites to retain the same equilibrium point of water absorption is crucial for maintaining ideal mechanical properties and sample dimensions throughout the entirety of its use. 40,58,59 Although the CNCs in the wet state act individually, there may still be small swollen aggregates of CNCs within the composite. Therefore, if the composites begin to dry out from loss of moisture, the swollen CNC aggregates within the matrix will start to bond together through hydrogen bonding, causing the scaffold to become stiffer and contain defects, as well as decrease in height and cross-sectional area.…”

“…Data acquired from DMA tension testing for each composite composition, both dry and swollen, agreed with predicted correlations between the effects of CNC content and moisture uptake on the resulting mechanical properties, Figures 2 and 3, and Table 4. [40][41][42] As the CNC concentration increased, the composites showed an increase in moduli and yield strength, with a decrease in yield strain. This results from the CNCs within the composite contributing reinforcement from a filler-filler network, instead of solely following the rule of mixtures, leading to higher mechanical reinforcement, specifically stiffness.…”

Melt-Spun Nanocomposite Fibers Reinforced with Aligned Tunicate Nanocrystals

“…To examine the effect of molecular orientation on the mechanical properties of nanocomposites, PU/P-tCNC melt-spun fibers, and solvent-cast PU/P-tCNC nanocomposite films were compared. Note that while slight differences in thermal history between nanocomposite fibers and films were observed in DSC measurements ( Figure S4), such differences were previously shown to have no impact on their mechanical properties [38]. The mechanical properties of fibers and films were then evaluated using the stress-strain measurements with representative curves shown in Figure 4.…”

Shear melting and recovery of crosslinkable cellulose nanocrystal–polymer gels

“…Cellulose nanocrystals (CNCs) are rod-like nanostructures consisting of repeating glucose units, and are a subject of growing interest as key ingredients of green composites due to their mechanical and chemical properties, as well as their sustainability and biocompatibility [36][37][38][39]. CNCs are used as fillers for a variety of thermoplastics, thermosets and hydrogels, resulting in improved mechanical properties and temperature stability [40,41]. Methods used for processing CNC composites include solvent-casting, coagulation and spinning, as well as melt extrusion [42][43][44].…”