Session B24: Physics in Polymer Processing

Some factors affecting inter-layer weld strength in material extrusion 3D-printed amorphous and semicrystalline polymers

Material extrusion (MatEx) 3D printing is an innovative polymer processing route which allows fabrication of parts with complex and customized geometries. Being based on a layer-by-layer deposition, the extent of welding occurring between the layers largely affects the mechanical response of the printed material.

To gain a molecular understanding of this process, we conducted experiments and molecular modeling on both amorphous and semicrystalline polymers. It is expected that the macromolecular diffusion process leading to inter-layer welding is arrested by the glass transition, for amorphous polymers, or by crystallization, for semicrystalline ones.

As reference polymers, poly(lactic acid) (PLA) and polypropylene (PP) are chosen. Experimentally, via birefringence and polarization modulated infra-red microspectroscopy measurements, it is revealed that some molecular orientation is frozen-in for PLA, particularly at the weld region. Such orientation increases with more severe printing conditions (higher printing speed and lower nozzle temperatures) and with polymer molar mass. Moreover, it is shown that the presence of residual alignment of the chains at the weld is detrimental for the weld strength, even though the modeling indicates that full macromolecular interdiffusion between the layers has occurred during cooling.

For a series of PP based materials, it is shown that the weld strength is limited with increasing molar mass and crystallization rate of the polymers. The modeling of the process demonstrates that, to capture the hindered interdiffusion between the layers, the role of flow-induced crystallization must be taken into account.

These results, spanning both the effect of molecular variables and processing conditions for amorphous and semicrystalline polymers, expands our understanding on the weld formation process in 3D printing, paving the way for improved control and optimization of polymer additive manufacturing.   

Bottlebrush polymers, networks, and devices

A bottlebrush polymer consists of a long linear backbone densely grafted with many relatively short linear side chains. Analogous to sausages versus spaghetti, a bottlebrush polymer, in some cases, essentially resembles a 'fat' linear polymer and is difficult to entangle. The unique molecular architecture of bottlebrush polymers makes them a platform for designing soft (bio)materials with properties that are inaccessible to their linear counterparts. In this talk, I will describe our recent efforts to understand and apply bottlebrush polymers. First, I will introduce a new theoretical framework for the molecular structure of bottlebrush polymers. Corroborated with experiments, we discover that, in some instances, the bottlebrush backbone can fold to store length, a phenomenon opposite to all previous understandings of bottlebrush polymers. Second, I will demonstrate how the use of foldable bottlebrush polymers as network strands provides a universal strategy for decoupling stiffness and extensibility—two fundamental mechanical properties that are inherently negatively correlated in polymer networks. Finally, I will discuss the applications of bottlebrush-based polymers as a new class of soft materials for additive manufacturing of functional structures and devices.   

Designing Advanced Macromolecules for Advanced Manufacturing: Balancing Reactivity, Rheology, and Resolution

Additive manufacturing (AM) offers the promise for addressing looming concerns for materials sustainability with chemical processes that require less energy and result in less material consumption. However, research must impose a lens of sustainability earlier in the innovation process, where processes and materials are designed to adhere to the principles of green chemistry and strive for more sustainable engineering. Novel materials for AM continue to emerge at a feverish rate, and it is imperative that we begin to consider end-of-life (EOL) earlier in the material and process design. Our research has focused on light-based AM modalities due to the unique combination of low energy consumption, exceptional resolution that enables less material consumption in latticed architectures, and the opportunity to print high molecular weight polymers with low viscosities. An important motivation is the opportunity to intensify chemical processes, wherein the synthetic chemistry occurs in a printed shape that serves as the reactor that ultimately results in a printed object. Advanced macromolecular materials for advanced manufacturing require a precisely tailored balance of reactivity and rheological performance that collectively ensure precise resolution from diverse additive manufacturing modalities. This lecture will exemplify our most recent efforts involving vat photopolymerization of fully aromatic polyimide hydrogels for printing carbonaceous objects, aqueous polymeric latexes for printing ABA triblock copolymer elastomers, and printing unsaturated polyesters in the absence of solvent with the opportunity for subsequent recycling. Polyimide polymeric salts offer desirable solution viscosity, and recent efforts have involved the tuning of crosslink density with mixed polysalts wherein the photo-active diacid concentration is precisely controlled.  Sulfonation of diamines renders all-aromatic polyimide precursors in water, thus enabling the vat photopolymerization of polyimides in water.  SIS triblock copolymers are prepared as a colloid and printed in the presence of an aqueous scaffold.  Removal of water allows for the formation of nanostructured ABA triblock copolymers with tensile elongations exceeding 800%.   

Directed self-assembly of thermoplastic elastomers via 3D printing for mechanically tailored soft architectures

Many biological systems utilize self-assembled hierarchically ordered structures to achieve complex functional properties. However, current methods cannot scalably achieve this level of control over structure and function across multiple length scales in synthetic systems. Here, we make progress towards bridging this gap by demonstrating the use of material extrusion 3D printing to induce tunable alignment of a commercial cylinder-forming polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) thermoplastic elastomer along a controlled print path. We show that the extent of nanostructure alignment and resulting anisotropy can be tuned via the shear and extensional forces applied to the material during 3D printing. In addition, we show that post-printing thermal annealing plays a critical role in maximizing domain alignment via relaxation of trapped stresses. Ultimately, we have demonstrated the ability to induce up to 85x greater tensile modulus along the print direction compared to perpendicular to the print direction. By designing custom print paths for these soft and mechanically anisotropic materials, we enable fabrication of soft architectures with tailored macroscopic mechanical behavior such as controlled localization of strain and bending upon deformation.   

Heating at a Patterned Photothermal Interface

Direct additive manufacturing (AM) of commercial silicones is an unmet need with high demand. We report a new technology, heating at a patterned photothermal interface (HAPPI), which achieves AM of commercial thermoset resins without any chemical modifications. HAPPI integrates desirable aspects of stereolithography with the thermally driven chemical modalities of commercial silicone formulations. In this way, HAPPI combines the geometric advantages of vat photopolymerization with the materials properties of, for example, injection molded silicones. We describe the realization of the new technology, HAPPI printing using a commercial Sylgard 184 polydimethylsiloxane resin, comparative analyses of material properties, demonstration of HAPPI in targeted applications, and commercialization of HAPPI technology to prosthetic and orthotic markets.