4D-Printing with Cellulose Nanocrystal Thermoplastic Nanocomposites: Mechanical Adaptivity and Thermal Influence

This thesis is concerned with fused filament fabrication (FFF) of cellulose nanocrystal (CNC) and thermoplastic polyurethane (TPU) nanocomposites, focusing on preliminary optimization of a processing window for 3D printing of mechanically responsive composites and the influence of temperature on mechanical adaptivity, thermal stability, and rheology. CNC thermoplastic nanocomposites are a water responsive, mechanically adaptive material that has been gaining interest in additive manufacturing for 4D-printing applications. Using a desktop FlashForge Pro 3D printer, we first established a viable processing window for a nanocomposite comprising 10 wt% CNCs in a thermoplastic urethane (TPU) matrix, formed into a filament through the combination of masterbatch solvent casting and single screw extrusion. Printing temperatures of 240, 250, and 260°C and printing speeds of 600, 1100, and 1600 mm/min instituted a consistent 3D-printing process that produced characterizable CNC/TPU nanocomposite samples. To distinguish the effects of these parameters on the mechanical properties of the printed CNC/TPU samples, a design of experiments (DOE) with two factors and three levels was implemented for each combination of printing temperature and speed. Dynamic mechanical analysis (DMA) highlighted 43 and 66% increases in dry-state storage moduli values as printing speed increases for 250 and 260°C, respectively. 64 and 23% increases in dry-state storage moduli were also observed for 600 and 1100 mm/min, respectively, as temperature decreased from 260 to 250°C. For samples printed at 240°C and 1600 mm/min, it was determined that that parameter set may have fallen out of the processing window due to inconsistent deposition and lower dry-state storage moduli than what the slower speeds exhibited. As a result, the samples printed at 240°C did not follow the same trends as 250 and 260°C. Further analysis helped determine that the thermal energy experienced at the higher end printing temperatures coupled with the slower speeds decreased the dry-state storage moduli by nearly 50% and lead to darker colored samples, suggesting CNC degradation. Isothermal thermogravimetric analyses (TGA) demonstrated that the CNC/TPU filament would degrade at relative residence times in the nozzle for all the chosen printing temperatures. However, degradation did not eliminate the samples' ability to mechanically adapt to a moisture-rich environment. DMA results verified that mechanical adaptivity was persistent for all temperature and speed combinations as samples were immersed in water. However, for the higher temperatures and slower speeds, there was about a 15% decrease in adaptability. Optimal parameters of 250°C and 1600 mm/min provided the highest dry-state storage modulus of 49.7 +/- 0.5 MPa and the highest degree of mechanical adaptivity of 51.9%. To establish the CNC/TPU nanocomposite's use in 4D printing applications, shape memory analysis was conducted on a sample printed at the optimal parameters. Multiple wetting, straining, and drying steps were conducted to highlight 76% and 42% values for shape fixity and shape recovery, respectively. Furthermore, a foldable box was printed to serve as an example of a self-deployable structure application. The box displayed shape fixity and recovery values of 67% and 26%, respectively, further illustrating significant promise and progress for CNC/TPU nanocomposites in 4D-printed, shape adaptable structures.

Further analysis of the effect of degradation during FFF of the CNC/TPU nanocomposite was conducted using rotational rheometry, Fourier-Transform Infrared Spectroscopy (FTIR), and polymer swelling experiments. A temperature ramp from 180 to 270°C showed a significant increase in complex viscosity (h*) at the chosen printing temperatures (240, 250, and 260°C). Moreover, h* of neat TPU suddenly increases at 230°C, indicating a potential chemical crosslinking reaction taking place. 20-minute time sweeps further verified that h* increases along with steady increases in storage (G') and loss (G'') moduli. From these results, it was hypothesized that crosslinking is occurring between CNCs and TPU. Preliminary characterization with FTIR was used to probe the molecular structure of thermally crosslinked samples. At 1060 and 1703 cm-1, there are significant differences in intensities (molecular vibrations) as the temperature increases from 180 to 260°C related to primary alcohol formation and hydrogen bonded carbonyl groups, respectively. The hypothesis is the disassociation of TPU carbamate bonds into soft segments with primary alcohols and hard segments with isocyanate groups. The subsequent increasing peaks at 1060 and 1703 cm-1 may indicate crosslinking of CNCs with these disassociated TPU segments. To quantify potential crosslinking, polymer swelling experiments were implemented. After being submerged in dimethylformamide (DMF) for 24 hours, CNC/TPU samples thermally aged for 15 minutes at 240, 250, and 260°C retained their filament shape and did not dissolve. The 240 and 250°C aged samples had relatively similar crosslink densities close to 900 mole/cm3. However, from 250 to 260°C, there was about a 36% increase in crosslink density. These results suggest that crosslinking is occurring at these printing temperatures because both CNCs and TPU are thermally degrading into reactive components that will lead to covalent crosslinks degradation. Additional characterization is needed to further verify the chemical structure of these CNC/TPU nanocomposites which would provide significant insight for CNC/TPU processing and 3D printing into tunable printed parts with varying degrees of crosslinking.