3D printing of shape memory polymers via stereolithography process

• Main Author: Choong, Yu Ying Clarrisa
• Other Authors: Su Pei-Chen
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2018
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/75861
• Institution: Nanyang Technological University

Additive manufacturing (AM), also known as 3D printing, with the innovative combination of smart responsive materials such as shape memory polymers (SMPs) has brought about 4D printing as an emerging technology for creation of more dynamic devices. However, its applications have been impeded by the limited printable materials and inferior properties in terms of curing speed, mechanical strength and thermomechanical shape memory properties of currently available 4D printing materials. In recognition of these drawbacks, the motivation of this work is to develop photo-curable thermoset SMP resins that exhibit enhanced shape memory properties with rapid curing characteristics. A tight coupling exists between material development and process development, hence the interaction between material properties of the developed SMPs and process parameters of the stereolithography (SLA) process was examined. While the SLA process can be divided into two major categories – projection and scanning type, the SMPs fabricated via these two systems were compared and found to have distinct curing characteristics. Theoretical calculations on critical energy density and threshold penetration depth were derived for the developed SMPs to enable the material to be successfully printable in any types of UV based 3D printing systems. Following which, characterizations and analysis of tailoring shape memory properties were carried out and the durability of the 4D printed structures was also evaluated. By tuning the material compositions, the flexibility of the developed SMPs allows tailorable thermomechanical properties including glass transition temperatures (from 54.9 ˚C to 74.1 ˚C), high shape recovery (from 90 to 100%) and prolonged shape memory durability (up to 22 cycles). The ability to freely tune the thermomechanical properties of 4D printed parts presents a huge advancement for 4D printing technology to broaden the selection of suitable materials. The robustness of the developed SMPs also addresses the issue of thermomechanical durability of the materials to perform as engineering materials for wide industry adoption. Moreover, for AM to be viable in mass production, print speeds must increase by at least an order of magnitude while maintaining excellent part accuracy. A shape memory polymer composite (SMPC) using nanosilica particles was developed to enhance the speed and performance of 4D printed parts. The nanosilica particles were discovered to promote remarkably fast curing due to nucleation enhancing activity. The curing time of each layer was reduced to 0.7s which effectively shorten the total printing time. The presence of nanosilica particles with high specific surface area promotes stress transfer, hence improving the tensile strength in the rubbery state by 2.4 - 3.6 times higher and the elongation in rubbery state reaches 85.2%. In particular, the shape memory durability was enhanced which offers a promising material for more robust applications. By comprehensively analysing and discussing the approach of process optimization and material evaluation, this work has enabled the use of the stereolithography technology to fabricate high performance responsive SMP components.