4D printable thermally stimulated (meth)acrylate shape memory polymers with tailorable shape recovery force for vat photopolymerization

• Main Author: Ng, Chin Siang
• Other Authors: Su Pei-Chen
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2025
• Subjects: Engineering; 4D printing; Shape memory polymer; Vat photopolymerization
• Online Access: https://hdl.handle.net/10356/182273
• Institution: Nanyang Technological University

4D printing combines additive manufacturing and smart materials to produce dynamic parts. Using time as the 4th dimension, these 4D printed parts can change their properties when triggered with an external stimulus. This property-changing capability allows for 4D printing to open up numerous opportunities for new devices in the biomedical and robotics industries. Among the smart materials for 4D printing, thermally simulated shape memory polymers (SMPs) are the most used and studied for their unique ability to change, retain, and recover their shapes. The shape changing occurs upon the softening/hardening of the SMP, which is stimulated using the polymer’s phase-transition temperatures, such as glass transition temperature (Tg). SMP also retains a memory of its original shape due to its polymer network. This enables the SMP to recover back to its original shape by itself, just like most elastomers. The most common additive manufacturing technology to 4D print these SMPs is vat photopolymerization. As a light-based printer, vat photopolymerization can print thermoset SMPs with Tg near body temperature, which is difficult to achieve for other heat-based additive manufacturing technologies that print thermoplastics. Moreover, vat photopolymerization has one of the highest resolutions, capable of printing small features with great dimensional accuracy. The majority of developed SMPs that are printable with vat photopolymerization focused on achieving full shape recovery ratio and high shape fixity, which are key performance indicators of the shape memory effect. However, there is very little development on 4D printable SMPs that focus on shape recovery force as their key performance indicator. Having sufficient shape recovery force allows the SMP to overcome any external resistance that prevents its shape recovery. It can also create a continuous pressure onto the object that constrains the shape recovery of the SMP. Yet, there is a lack of understanding on how the chemical composition of the 4D printable SMP, its thermomechanical properties, and the temperature used during the shape memory cycle affect the behaviour of the shape recovery force. This thesis focuses on underpinning the fundamental mechanism of the shape recovery force by considering the constitution of the 4D printable SMP. A qualitative concept of shape-holding bonds consisting of “reactive” and “nonreactive” bonds was hypothesized to describe the dynamic behaviour of the polymer network during the shape memory cycle. This concept would facilitate the understanding of the effects of chemical composition, thermomechanical properties, and temperature stimulus on the SMP’s shape recovery force. Through the in-depth investigations, the rubbery modulus (Er) of the SMP was found to correlate to the baseline shape recovery force when the SMP is recovered above its Tg. The Er is the product of all the nonreactive bonds in the SMP, which consist mainly of the chemical crosslinks formed by crosslinkers. A higher concentration of shorter crosslinkers resulted in a higher Er and lower Tg due to increased crosslinking density. However, longer crosslinkers with low concentrations exhibited the same trend of higher Er and lower Tg. This led to a hypothesis that the physical entanglements between the lengthy crosslinkers and the side groups of the polymerized monomer were acting as nonreactive bonds, similar to the chemical crosslinks. Another important aspect of this thesis is the printability of the formulated SMP in vat photopolymerization. Studies were conducted to showcase the effects of crosslinker’s length on the curing behaviour of the resin mixture. It was found that a longer crosslinker resulted in a faster rate of cure depth growth, due to its increased mobility range to propagate and crosslink with the radicalised monomers. To improve on the printability of the SMP resin, zinc oxide nanoparticles (ZnONPs) were investigated for their effects on dimensional accuracy and thermomechanical integrity of the printed SMP. Due to its photocatalytic and photoabsorbing properties, ZnONPs were able to reduce the unwanted overcuring of resin due to excessive light penetration during the printing process. Additionally, it was found that the mechanical and thermomechanical properties of the SMP remained the same when ZnONPs were added. This was unlike the deterioration in mechanical and thermomechanical properties observed when a common photoabsorber, Sudan I, was used instead. Overall, the comprehensive research in this thesis covered the critical areas in developing 4D printable SMPs for vat photopolymerization. The fundamental knowledge generated across all the investigations has provided the necessary understanding to develop SMPs with the desired shape recovery force for many potential applications, especially in the biomedical industry.