Development of Stimuli-responsive photopolymers for the 3D-printing of functional active devices

Usman Shaukat

Research output: ThesisDoctoral Thesis

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Abstract

The goal of the thesis was to develop new stimuli-responsive materials processable by digital light processing (DLP) 3D-printing and streamline their applicability for soft-robotic devices. In order to meet the objectives, a primary focus was on the development of photo-curable resins that are 3D-printable and offered a wide variety of thermo-mechanical properties. Secondly, dynamic covalent bonds were introduced undergoing thermally induced transesterification reactions, which rendered robotic systems malleable and healable.
As a first study, a library of dynamic thiol-acrylate networks was developed and structure-property relationships between resin composition, crosslink density, mechanical performance and bond exchange kinetics were established. For this, an acidic organic phosphate was introduced which did not only stabilize the resins but also served as an efficient catalyst, promoting dynamic bond exchange reactions. This breakthrough opened the path to thermally mendable, reprocessable and malleable dynamic photopolymers. The results showcased that the manipulation of the network structure allowed for effective control of the thermo-mechanical characteristics across a broad range. Rapid bond exchange reactions were facilitated by the abundance of –OH and ester groups, in addition to the high polymer chain mobility. Soft devices with thermal mendability and desirable stretchability were successfully DLP 3D-printed. Additionally, multi-material DLP 3D-printing was pursued to fabricate more advanced soft active devices, by combining the advantages of the developed resin formulations. Two different thiol-acrylate resins, with a varying degree of crosslinking, and widely different thermo-mechanical properties were employed to print prototypes, which exhibited great versatility along with shape memory and thermo-activated healing properties.
As a next step, photolatent transesterification catalysts were exploited as they provide a unique method to spatially control dynamic bond exchange reactions in dynamic photopolymers. In non-irradiated condition, the polymer networks containing the latent catalysts behaved like a permanently crosslinked duromer. However, when subjected to UV-light (365 nm), Brønsted acids were released, which efficiently catalyzed transesterification at elevated temperature and allowed for rearrangement of the network's topology above the networks topological freezing temperature (Tv). Moreover, the insensitivity of the photolatent catalysts to visible light (405 nm) enabled the DLP 3D-printing of sophisticated architectures without premature activation of the catalyst. The investigations demonstrated that the speed at which bonds are exchanged depends on the liberated Brønsted acid’s stability, the quantum yield of the disintegration reaction and the counter-anion’s size. In areas exposed to UV light, the dynamic thiol-acrylate networks exhibited triple shape memory, while non-UV exposed domains functioned as simple Tg-based shape memory materials. This controlled locking of specific areas during thermal programming geared up exciting possibilities for additive manufacturing of functional devices.
Furthermore, to reduce the environmental foot print of the polymeric materials, bio-based vegetable oils were exploited as building block for dynamic photopolymers. As a sustainable building block, acrylated linseed oil was synthesized and combined with selected thiol crosslinkers. An organic transesterification phosphate catalyst was employed in the resin formulations which catalyzed efficient dynamic exchange reactions of the hydroxy and ester moieties present in the network. Excellent storage stability and photocuring kinetics facilitated the DLP 3D-printing of soft-active actuators with shape memory properties and malleability.
To further reduce the dependence on fossil fuel, a bio-based phosphate ester catalyst along with bio-based reactive diluents was synthetized. This target was attained by using eugenol, which is abundantly available from biologically renewable green sources, as starting material. The resin formulations were optimized towards the processability with DLP 3D-printing, and soft active structures with shape memory and healing properties were fabricated.
The outputs of the current thesis underscored the substantial promise of highly functional materials that respond to an external stimulus, whilst novel material concepts and manufacturing approaches have been integrated which set the stage for various future applications in smart active soft-robotics.
Translated title of the contributionEntwicklung Stimuli-responsiver Fotopolymere für den 3D-Druck von funktionalen aktiven Geräten
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Kern, Wolfgang, Co-Supervisor (internal)
  • Schlögl, Sandra, Supervisor (internal)
  • Griesser, Thomas, Assessor A (internal)
  • Sangermano, Marco, Assessor B (external), External person
DOIs
Publication statusPublished - 2023

Bibliographical note

no embargo

Keywords

  • 3D-printing
  • Digital light processing
  • vitrimer
  • transesterification
  • soft-robotics
  • healing
  • bio-based resins
  • photolatent catalysts
  • stimuli-responsive materials
  • soft active devices

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