Session F34: 3D Printing of Polymers and Soft Materials I

Re-Printable, Self-Healing Polymer Networks

We report a versatile approach to designing 3D printable, re-processable dynamic covalent networks with controllable mechanical properties. The approach involves synthesis of epoxy-based oligomeric building blocks of controllable length which are then reversibly crosslinked using the Diels-Alder (DA) reaction. The networks demonstrate several features which are beneficial for additive manufacturing, such as reversible dissociation to liquids at temperatures above 110 ^oC and fast curing during fused deposition modeling. Moreover, DA reactivity of the precursor material improved interlayer adhesion during printing and enabled self-healing. Using this approach, a series of networks with glass transition temperatures controlled between -10 and 35 ^oC and the elastic moduli between 10 MPa and several GPa were demonstrated. Finally, the 3D printed networks exhibited shape-memory effects which are based on the stereochemical characteristics of DA adducts.
  

Reactive Processing in Extrusion based Polymeric 3D Printing with Surface Segregating Additives

Fabricating complex geometries with isotropic, robust mechanical properties by fused deposition modeling (FDM) remains a key target in expanding additive manufacturing towards the production of large scale commercially relevant structures. Due to the large size of polymer chains and the complex thermal environment experienced by the printed filament in FDM, entanglement of polymers between layers is incomplete, resulting in weak inter-layer interfaces and unsatisfactory Z-strength. Recently, our group has addressed these issues by developing novel polymer materials for FDM that revolve around the introduction of low molecular weight surface segregating additives (LMW-SuSAs) to the filament. We have recently expanded this concept to reactive additives, including methacrylate terminated linear and 3-arm PLA LMW-SuSAs. These reactive additives can now form inter-layer crosslinks by rational introduction of UV photo-initiators and fiber-optic based UV illumination. In situ reactive processing of the printed layers results in drastic increases in the transverse tensile stresses of the printed layers up to ~140% and ~200% for the linear and 3-arm LMW-SuSAs respectively to create essentially isotropic materials.   

3D Printing Polylactic Acid: modelling residual alignment, annealing and templated crystallinity

Polylactic acid is a semi-crystalline polymer melt commonly used in extrusion-based 3D printing applications. This involves extruding molten polymer through a nozzle and depositing filaments layer-by-layer. Rapid cooling and solidification ensures the structural integrity of the printed part.

Since the crystallization kinetics are typically slow, there is insufficient time above the glass transition for crystallinity to develop. Thus, deposited filaments usually have an amorphous micro-structure upon solidification. Moreover, the deposition flow stretches and orients the polymer molecules, and residual alignment becomes trapped in the weld regions between deposited filaments at the glass transition. We propose that this micro-structure leads to reduced weld strength.

Post-processing thermal annealing may be employed to increase the crystal content and improve mechanical properties. However, we find that non-uniform properties are inevitable. Resdiual polymer stretch can “template” flow-induced nuclei into the weld regions. Upon annealing these nuclei grow into much smaller spherulites, whose size depend on both print speed and temperature.   

Determination of polymer chain orientation in 3D printed filaments using Polarized Raman spectroscopy and Birefringence

To study orientation and alignment of molecular chains in 3D printed polycarbonate filaments, we used a combination of polarized Raman spectroscopy and birefringence measurements. By changing the orientation of the sample with respect to polarization of incident radiation, we probe changes in the ratio between orientation-dependent vibration modes and orientation-independent modes. Raman measurements show that in 3D printed filaments, little to no orientation at the monomer level was detected, supporting molecular dynamics simulation results [1]. Birefringence measurements were compared to the intrinsic birefringence of 0.2, to estimate the degree of orientation. Measured values of birefringence suggest minimal orientation in agreement with the results of Raman measurements. 3D printed filaments were pulled at different draw ratios after annealing to estimate the limit of Raman measurements’ sensitivity to chain orientation in 3D printed fibers through comparison with birefringence measurements.

References
[1] Private communications with Marco Galvani and Mark Robbins   

Mechanical Properties of an Additively Manufactured Cyanate Ester

Additive manufacturing is a recent processing method that allows facile fabrication of complex geometries. Additive manufacturing of thermoset composites is commonly done using an extrusion based method termed direct ink writing. Most direct ink writing materials systems have been epoxy based. Epoxy has good mechanical properties but poor thermal stability. To improve the thermal stability a high temperature thermoset resin, bisphenol e cyanate ester, was selected for 3d printing. The cyanate ester was filled with fumed silica to enable direct ink writing. The high temperature thermoset is a brittle material. Inorganic nanoparticles were used to toughen the cyanate ester composite.   

Effect of Chain Alignment and Entanglements on Thermal Welding in Fused Filament Fabrication

3D printed structures are often substantially weaker than those made with conventional techniques, but the reason has been unclear. Indeed alignment from flow during deposition was expected to reduce the number of entanglements and thus accelerate welding of filaments through interdiffusion. To address this puzzle we performed molecular dynamics simulations of welding between polymer melts with different degrees of initial alignment. Standard measures of the entanglement density, such as primitive path analysis, indicate significant entanglement loss with increasing alignment, but contrary to existing theories there is no change in the rate of interdiffusion or reorientation. The distance polymers have diffused and the number of entanglements formed across the interface are independent of initial alignment, and chain reorientation always occurs on the equilibrium disentanglement time. Despite this, mechanical tests show that parts welded from aligned states are weaker. We show that this is due to residual alignment in regions near the weld that yield before the weld. The maximum shear strength and fracture energy of systems containing a weld are the same as systems starting from uniform melts with the same alignment.   

3D Printing of Polymers and Soft Materials

Soft matter 3D printing (3DP) is a rich and diverse research area with new technologies, materials, and processes entering the literature at a rapid pace. Advancements in “traditional” soft 3DP and materials are giving way to hybrid or entirely new processes and materials designed for 3DP. These advancements have been supported by improvement of in-situ process characterization, a more complete understanding of critical material physics, and focused modeling efforts. Soft 3DP processes can now create function from structure and new materials allow the combination of 3DP and self-assembly, producing features at unprecedented length scales. Models, tuned with in-situ process measurements, predict voxel-by-voxel changes in material properties and combining modeling and multi-material printing has enabled advancements in soft robotics, stimuli-responsive materials, and 4D printing. Further, these advancements are pushing the field toward realizing one of the original promises, complete design freedom. This talk will cover “traditional” and emerging soft 3DP processes, material physics, measurement and modeling challenges, and opportunities.   

Using Eigenvector Centrality to Predict the Mechanical Properties of Structured Materials

We seek to link mesoscale organization to macroscopic mechanical response using a combination of 3D printing, mechanical testing, and theory. Our ultimate objectives are to provide a simple reduced-order model for predicting mechanical parameters for tailored structures and to better inform engineering models. Here, we report our efforts toward accomplishing these objectives by examining a set of lattice structures with controlled strut deletion. Octet lattice structures overlaying body-centered cubic unit cells were 3D-printed at two different length scales, using either the two-photon polymerization (2PP) method or the vat photopolymerization method with commercially available acrylate-based resins. A set of 10 lattice structures were printed in which struts were randomly removed to give a fraction of deleted struts up to 0.35, and each sample was compression-tested to obtain the Young's modulus. We applied graph theoretical tools typically used in complex network theory to analyze this set of samples. In particular, we propose that the bulk mechanical properties are dictated by a network free energy calculated from the principle eigenvector of the adjacency matrix encoding the mesoscopic structure.   

Mechanical Enhancement of Polydopamine Nano-Coatings via Thermal Annealing

Inspired by the adhesive proteins of mussels, polydopamine (pDA) has become one of the most widely employed methods for functionalizing material surfaces, powered in part by the versatility and simplicity of pDA film deposition. Despite the widespread adoption of pDA as a multifunctional coating for surface modification, it exhibits poor mechanical performance and it still remains a challenge to improve its mechanical properties without sacrificing functionality and versatility of pDA. Here, we demonstrate thermal annealing at a moderate temperature (130 C) as a facile route to enhance mechanical robustness of pDA coatings. Chemical spectroscopy, x-ray scattering, molecular force spectroscopy and bulk mechanical analyses indicate that monomeric and oligomeric species undergo further polymerization during thermal annealing, leading to fundamental changes in molecular and bulk mechanical behavior of pDA. Considerable improvements in scratch resistance and elastic modulus were noted for the annealed pDA coating, indicating the enhanced ability of the annealed coating to resist mechanical deformations, possibly due to cross-linking and increased intermolecular and cohesive interactions in the pDA structure.   

The Role of Ionization in Thermal Transport of Solid Polyelectrolytes

Amorphous polymers are known as thermal insulators, but increasing their thermal conductivity has not been guided by fully understood physics. In this work, we use molecular dynamics simulations to study the thermal transport mechanism of solid polyelectrolytes, poly(acrylic acid) (PAA) and its ionized forms. The thermal conductivity of PAA increases monotonically with the ionization strength. Although stronger ionization induces larger Coulombic interactions, the Coulombic interaction does not directly contribute to the thermal conductivity enhancement. Instead, it enhances thermal transport through the Lennard-Jones (LJ) interaction. The strong Coulombic force between the counterion and the ionized carboxylic group shifts the LJ force to the stronger LJ repulsive regime, which is mainly responsible for the improved thermal conductivity.