Fused filament fabrication to manufacture three- and four-dimensional objects made of shape memory polymers

Programmable materials can perform specific tasks with the function of stimuli, like temperature, where the programming of material is understood as the programming of a functionality. The internal structure of the programmable materials enables reversible material properties, behavior, or shape changes according to a program. As the programmable materials require neither control electronics nor technical devices or cables, the self-sufficient behavior makes them fulfill sensor and actuator functionality. Shape memory polymers (SMPs) are smart materials that qualify as functional base materials to design programmable materials. SMPs can retain an imposed, temporary shape after thermomechanical treatment, also called programming. The initial, permanent shape can be recovered when applying an external stimulus like heat. In the last decade, thermoplastic polyurethanes (TPUs) belonged to the most researched SMPs. The thermoplastic nature of TPUs permits them to be molded using classical melt-based processing techniques like extrusion, injection molding, etc. Additive manufacturing (AM), alias three-dimensional (3D) printing, is an effective layer-by-layer technique to process thermoplastic polymers into 3D objects. Amidst various AM technologies, fused filament fabrication (FFF) is a hot-melt extrusion-based 3D printing process and is widely prevalent. The doctoral thesis aims to utilize self-synthesized and commercially available TPUs for FFF and to specifically influence the printing technology to produce either non-thermoresponsive or thermoresponsive objects or structures and open up new material and system functionalities. The primary hurdle of this doctoral study was to process SMPs using a standard commercially available FFF machine. Motivated by the fact that previously presented manufacturing processes of thermoresponsive quick response (QR) codes were too time-consuming for production, QR codes were initially developed as anti-counterfeiting technology. The work introduces a novel manufacturing method for the same, thereby also addressing the AM of TPU-based SMP using standard FFF machines. Following this, the layer deposition pattern of tensile bars of TPU with shape memory properties was modified to achieve printing either in vertical or horizontal orientation. After processing commercially available polyester urethane and characterization, the mechanical and shape memory properties of the 3D printed samples were studied, and the results were compared to its injection-molded analogs and other materials manufactured via FFF. The results showed that the direction of loading and printing pattern orientation could be utilized to control the shape recovery stress and mechanical properties. Subsequently, the filigree printing and the smallest structure that can be obtained from FFF were explored by printing Arial fonts of the letter “A” in different sizes. Afterward, the potential application of SMP as thermally activatable and de-activatable gears and innovative smart keyboard keys was developed by utilizing the one-way (1W) shape memory effect (SME). The second part of the work concentrates on four-dimensional (4D) printing employing FFF that enables the production of thermoresponsive objects directly in AM process. The work presents a facile FFF printing strategy for commercially available polylactic acid (PLA) material and an in-house synthesized thermoplastic polyether urethane to obtain highly shrinkable objects, which allowed to show how to achieve precise control over the shapes after printing and heating. Later, the thermoresponsiveness after 4D printing of other objects in the form of solid cuboid, hollow cuboid, and hollow cylinder, with heights along the z-axis bigger than 30 mm, was explored. One of the applications of the developed highly shrinkable objects is active assembly. The concept is demonstrated by developing a lightweight, hands-free door opener for healthcare applications to counteract the spread of smear infections. After triggering the 4D effect for assembly, the device can be disassembled by heating the TPU over its glass transition temperature once it reaches its end-of-use. After removal from the door handle, the device can be mechanically recycled, and the material can be reused for 4D-printing. Successively, the know-how of 4D-printing was applied to address novel applications in active assembly, disassembly, programming tools, and as thermally deactivating gear. Thirdly, FFF was utilized to produce elements that can undergo thermomechanical treatment to develop thermoresponsive two-way (2W) actuating objects that can bridge the gap toward manufacturing programmable materials. After developing a poly(1,10-decylene adipate) (PDA) based polyester urethane, processing it via FFF and thermomechanical treatment, a novel approach was developed to identify the ideal actuating temperature conditions using dynamic mechanical analysis. Once characterized, the polymer was found to actuate reliably under stress-free conditions by expanding on cooling and shrinking on heating with a maximum thermoreversible strain of ̴16%. Later, allowing the 2W programmed TPU-based SMP to actuate in its ideal actuating temperature range between 15 °C and 64 °C for 100 heating-cooling cycles revealed that the reversible strain change stabilizes after about 25 cycles at 12%. The developed actuating elements were then integrated into a mechanical linkage system to form a thermally activatable gripper. This way, a hen’s egg could be picked up, safely transported, and deposited, qualifying for soft robotic purposes. Further, actuating elements were combined with two types of unit cells to obtain programmable materials that can actuate on temperature variation. Afterward, the development of programmable actuating structures using TPU with a 2W-SME was studied. The primary aim was to enable a better actuation for physically crosslinked SMPs under stress-free conditions. Therefore, a novel gear design was first developed and processed using PDA-based TPU as functional base material. After thermomechanical treatment, the programmable gear actuated efficiently between two metastable states with a reversibility length change of ̴42%. In order to prove that the thermoreversible actuation can be reproduced in other structural motifs, another actuating element was developed and programmed similarly. Once allowed to actuate under the same condition as the programmable gear, the element showed a reversible length change of ̴44%. Lastly, the work develops a novel FFF approach for semicrystalline SMPs to directly obtain objects in their thermoresponsive state, which can arbitrarily actuate between two metastable states by varying the temperature. Here, evidence is shown of how semicrystalline TPU is suitable for 4D-printing. During FFF, a cold air stream was used to cool the SMP strongly. Upon its removal from the build platform, it can be activated under stress-free conditions by shrinking on heating to 62 °C and expanding on cooling to 15 °C with a maximum thermoreversible strain of ̴7%. Later, a 4D printed actuator was integrated into a lever mechanism qualified to witness highly complex shape changes. Subsequently, self-sufficient actuators in the form of a cylinder were fabricated, and their actuation behavior was studied. The work concludes that the functional integration of SMP via FFF to achieve 2W-SME by an “in-situ” programming method is a promising step to produce inherent thermoresponsive programmable materials.

Author dc:creator

• Chalissery, Dilip

Advisors dc:contributor.advisor

• Pretsch, Thorsten
• Böker, Alexander

Licence dc:rights.uri

• https://creativecommons.org/licenses/by/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-17051

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/18258