Application-oriented development of thermoplastic polyurethanes with shape memory properties
Shape memory polymers (SMPs) are smart materials, that are capable of changing their shape in response to an external stimulus, typically a change in (ambient) temperature. This change in shape is commonly triggered after a thermomechanical treatment, enabling them to return to their original shape, so-called “one-way shape memory effect” (1W-SME). The switching between two shapes as a function of temperature, is known as the “two-way shape memory effect” (2W-SME). This thermoresponsiveness makes them highly interesting for the use as system components in e.g. programmable materials. Programmable materials act self-sufficiently, combining sensory and actuation properties in a single material. Thus, SMPs are providing the entire system a responsiveness to react actively to temperature changes. This makes them suitable for a wide range of applications with a high degree of functionality. Thermoplastic polyurethanes (TPUs) can exhibit such shape memory properties. This is due to their copolymer structure and phase-segregated morphology, which impart shape memory characteristics to the TPUs. TPUs consist of a soft segment (SS) and a hard segment (HS). The SS is composed of an oligomeric diol, while the HS consists of a diisocyanate and a short diol. Various combinations of these three building blocks allow for a wide range of molecular compounds to be synthesized. Every resulting TPU exhibits a unique property profile that can be used to address different applications. This thesis deals with the application-oriented synthesis of TPUs with shape memory properties. Motivated by the numerous variation possibilities, TPUs with diverse property profiles can be produced and customized through the appropriate selection of monomers, stoichiometry and synthesis parameters. This opens up opportunities to extend the functionality of existing applications or unlock entirely new ones. The aim was to synthesize TPUs efficiently and to investigate their structure-property and structure-functionality relationships. Therewith, the tailoring of TPUs according to requirements given by various applications became possible. By evaluating suitable synthesis conditions, different TPUs were synthesized. The synthesis procedure was scalable and transferable to various TPUs. The HS, consisting of 1,4-butanediol and 4,4’-methylene diphenyl diisocyanate, was kept constant in the study, while the SS and the HS/SS ratio were systematically varied to gain insights into the structure-property relationships. At the same time, the precondition was to adjust the phase transition of the SS, which is crucial for the 1W- and 2W-SME, above 23 °C to trigger a shape change "on demand". The focus was placed on polyester- and polyether-based TPUs and to compare their structure-property relationships. For polyester-based TPUs, higher molecular weights of the SS and a lower HS/SS ratio led to elastic, semi-crystalline materials with a phase transition of the SS above 23 °C. Due to high crystallinity, these TPUs were capable of a 2W-SME once thermomechanically treated. It was shown how the 2W-SME could be influenced by factors such as monomer selection, changes in deformation temperature and strain during thermomechanical treatment, electron radiation or actuation under constant load conditions. For polyether-based TPUs, low molecular weights of the SS and high HS contents resulted in amorphous TPUs with a glass transition above room temperature. Unlike the polyester-based TPUs, these materials were stiffer and capable of showing a 1W-SME directly after additive manufacturing (4D-printing) without the need of an additional programming step. The structure-property relationships and material characteristics allowed for targeted customization of TPUs. The 2W-SME of polyester-based TPUs enabled actuation in soft robotics, for instance. A polydecylene adipate-based TPU exhibiting a SS molecular weight of 3300 g × mol−1 and a HS content of 15 wt.% was developed and processed into actuators. Incorporating these into a gripper system enabled lifting and lowering a hen’s egg when the temperature was cycled between 23 °C and 64 °C. Additionally, the actuators were combined with mechanical unit cells to reversibly change their mechanical state as a function of temperature or to open and close small apertures in the unit cell structure. Actuators made from synthesized polydecylene sebacate-TPU with a SS molecular weight of 2800 g × mol−1 and a HS content of 20 wt.% were developed for use in latent heat storage systems for a programmable heat release. A demonstrator showed how the material synergistically interacted with a switchable phase change material (sPCM). The 2W-SME of the TPU was able to induce repeatedly crystallization of the sPCM. For polyether-based TPUs, a synthesized polypropylene glycol-based TPU with a SS molecular weight of 430 g × mol−1 and a HS content of 60 wt.% had a pronounced 1W-SME after 4D-printing, high mechanical stiffness and high adaptability due to its sharp decrease in storage modulus when exceeding the glass transition temperature. This enabled adaptive stiffness functionality, demonstrated by an at room temperature mechanically stable hands-free door opener that could be adaptively applied to a door handle through the activation of the 1W-SME after 4D-printing. Upon heating afterwards, the material became soft for detaching the item. Furthermore, it was non-cytotoxic, transparent and water-responsive. This allowed for the use in medical applications such as orthodontics. Aligners were fabricated from the material, demonstrating both water- and thermo-responsive 1W-SME on dental models. These aligners imply the adjustments of precise force as well as reduction of correction steps for superior functional therapeutic methods in the future. Further potential applications were discussed and referenced, including functional car-wrapping foils, smart textiles, intelligent microfluidics and components for smart assembly and disassembly, among others. This illustrates the versatility and adaptability of TPUs in their application spectrum. The prerequisite and basis for this is an application-oriented synthesis and tailoring of material characteristics based on their structure-property relationships, which were investigated and evaluated in this thesis.
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